Patent Publication Number: US-2021179893-A1

Title: Shaping-stage adhesive sheet and lamination shaping device

Description:
TECHNICAL FIELD 
     The present invention relates to technologies for the attachment and detachment (separation) between a build stage (shaping stage) of an additive manufacturing apparatus and an object to be formed on or over the build stage. 
     BACKGROUND ART 
     Additive manufacturing apparatuses, which are also called as 3D printers, are developed more and more. The additive manufacturing apparatuses each include a build stage on which a three-dimensional object is to be formed. In the additive manufacturing apparatuses, the object is formed on the build stage by a so-called additive manufacturing technique such as fused deposition modeling (fused filament fabrication), material jetting, stereolithography (vat polymerization), powder bed fusion, binder jetting, or sheet lamination. The additive manufacturing apparatuses as above are described typically in following Patent Literature (PTL) 1 to 4. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication (JP-A) No. H09-24552 
     PTL 2: JP-A No. 2011-101834 
     PTL 3: JP-A No. 2015-212042 
     PTL 4: JP-A No. 2016-5869 
     SUMMARY OF INVENTION 
     Technical Problem 
     An object during its formation may be attached onto a build stage in a process of forming the object on the build stage, in some manufacturing techniques employed in additive manufacturing apparatuses. In this case, the object has to be detached or separated from the build stage after the completion of the object forming process on the build stage. In a technique often employed for the detachment, the build stage and the object are separated from each other by pushing a scraper or another tool having a sharp edge in between the build stage and the object attached to the build stage. 
     However, the technique as above tends to cause significant damage to the object and/or the build stage by the tool having a sharp edge, such as a scraper, because the object and/or the build stage is often acted upon by relatively large force through the tool. 
     The present invention has been made under these circumstances and has an object to provide a pressure-sensitive adhesive sheet for build stage use that is suitable for allowing an object to be attached or be positionally secured to a build stage of an additive manufacturing apparatus during building up of the object, where the object is to be formed over the build stage and can be attached onto the stage; and is suitable for facilitating the detachment of the object from the build stage after the building up. The present invention has another object to provide an additive manufacturing apparatus that is suitable for employing the pressure-sensitive adhesive sheet for build stage use as above. 
     Solution to Problem 
     The present invention provides, according to a first aspect, a pressure-sensitive adhesive sheet for build stage use. The pressure-sensitive adhesive sheet for build stage use is to be used by affixing the same to an object-forming surface of a build stage of an additive manufacturing apparatus. The pressure-sensitive adhesive sheet for build stage use includes at least one pressure-sensitive adhesive layer (adhesive strength-reducible layer) including a pressure-sensitive adhesive, where the pressure-sensitive adhesive layer has an adhesive strength capable of being reduced or decreased by an adhesive strength-reducing action (adhesive strength-reducing treatment). The adhesive strength-reducing action is an action, which is performed to the adhesive strength-reducible layer, such as heating, cooling, electromagnetic irradiation, voltage application, liquid supply, stretching or deformation, magnetic field application, or pressure change. The adhesive strength-reducible layer has a configuration capable of reducing adhesive strength thereof in response to the adhesive strength-reducing action as above. The category pressure-sensitive adhesive sheet for build stage use includes pressure-sensitive adhesive sheets each including a single pressure-sensitive adhesive layer; and pressure-sensitive adhesive sheets each having a multilayer structure including a pressure-sensitive adhesive layer for constituting an adhesive face to the build stage (stage-facing pressure-sensitive adhesive layer), and at least one pressure-sensitive adhesive layer and/or substrate. The pressure-sensitive adhesive sheet for build stage use may be a so-called double-sided (double-coated) pressure-sensitive adhesive sheet or a so-called single-sided pressure-sensitive adhesive sheet. Assume that the pressure-sensitive adhesive sheet for build stage use includes two or more pressure-sensitive adhesive layers. In this case, the adhesive strength-reducible layer may be the stage-facing pressure-sensitive adhesive layer, or may be another pressure-sensitive adhesive layer in the multilayer structure than the stage-facing pressure-sensitive adhesive layer. The reduction in adhesive strength by the adhesive strength-reducing action upon the adhesive strength-reducible layer includes, in a state where the pressure-sensitive adhesive sheet for build stage use is disposed on the object-forming surface of the build stage, reduction in adhesive strength of an adhesive face of the adhesive strength-reducible layer, where the adhesive face faces the build stage; reduction in adhesive strength of an adhesive face of the adhesive strength-reducible layer, where the adhesive face faces the object; and reduction in adhesive strength due to fracture in the adhesive strength-reducible layer, where the adhesive strength has been actually provided by the pressure-sensitive adhesive layer (adhesive strength-reducible layer). In addition, the pressure-sensitive adhesive sheet for build stage use may be in the form of a pressure-sensitive adhesive tape, which is typically wound. 
     Specifically, the pressure-sensitive adhesive sheet for build stage use can be used typically by a procedure as follows. Initially, the pressure-sensitive adhesive sheet for build stage use is affixed, through the stage-facing pressure-sensitive adhesive layer thereof, to an object-forming surface of a build stage of an additive manufacturing apparatus for the formation of an object. In operation of the apparatus, the target object is gradually built up or formed on the pressure-sensitive adhesive sheet disposed on the object-forming surface of the build stage, through a predetermined process. In the process of forming the object over the build stage, the pressure-sensitive adhesive sheet for build stage use adheres onto the build stage, and the object during its formation is attached onto the pressure-sensitive adhesive sheet on the build stage. Namely, the object during its formation is in a state of being attached to the build stage through the pressure-sensitive adhesive sheet. After the completion of the object forming process, the adhesive strength-reducing action is performed to the pressure-sensitive adhesive sheet on the build stage. This reduces the adhesive strength of the adhesive strength-reducible layer of the pressure-sensitive adhesive sheet for build stage use and frees or loosens the attached state of the object to the build stage through the pressure-sensitive adhesive sheet. The object, whose attached state to the build stage has been freed or loosened, is then detached from the build stage. The pressure-sensitive adhesive sheet for build stage use enables detachment of the object from the build stage typically by performing an operation of removing (peeling) the pressure-sensitive adhesive sheet for build stage use at a predetermined interface defined by the pressure-sensitive adhesive layer whose adhesive strength has been reduced. Assume that the pressure-sensitive adhesive sheet for build stage use includes an adhesive strength-reducible layer that defines an object-facing adhesive face in an object forming process. In this case, the object can be detached from the build stage typically by separating (pulling away) the object from the adhesive strength-reducible layer whose adhesive strength has been reduced by the adhesive strength-reducing action. 
     In contrast, for example, assume that another pressure-sensitive adhesive sheet is used instead of the pressure-sensitive adhesive sheet for build stage use, where the other pressure-sensitive adhesive sheet has a multilayer structure including a substrate and a stage-facing pressure-sensitive adhesive layer having an adhesive strength that is sufficient to a build stage and is incapable of being intentionally reduced after formation of an object. In this case, even when an operation of removing the used pressure-sensitive adhesive sheet from the build stage is performed so as to detach the object after formation from the build stage, the pressure-sensitive adhesive sheet resists removal in a region where the object is attached onto the pressure-sensitive adhesive sheet. In an additive manufacturing apparatus, a build stage of the apparatus and an object to be formed are both hard and resistant to bending deformation. Between the build stage and the object, both of which are hard and resistant to bending deformation, it is difficult to pull the pressure-sensitive adhesive sheet in a direction that forms a significant angle to the build stage plane, and this impedes the removal (peeling) of the pressure-sensitive adhesive sheet. With the alternative technique, therefore, the object after formation resists detachment from the build stage. In contrast to this, the pressure-sensitive adhesive sheet for build stage use according to the first aspect of the present invention includes a pressure-sensitive adhesive layer (adhesive strength-reducible layer) having an adhesive strength capable of being reduced by an adhesive strength-reducing action, as described above. The pressure-sensitive adhesive sheet for build stage use, as having the configuration as above, can be a pressure-sensitive adhesive sheet that allows an object to be attached to a build stage during building up and to be readily detached from the build stage after building up. 
     As described above, the pressure-sensitive adhesive sheet for build stage use according to the first aspect of the present invention is suitable for allowing an object to be formed on or over a build stage of an additive manufacturing apparatus to be attached to or positionally secured to the build stage during building up and to be readily detached from the build stage after building up. Namely, the pressure-sensitive adhesive sheet for build stage use is suitable for achieving both satisfactory attachability or positional securability of the object to, and good detachability of the object from, the build stage, where the two properties are possibly incompatible with each other. The demand for combination or accommodation of the possibly incompatible properties, i.e., satisfactory attachability or positional securability and good detachability of the object to and from the build stage is particularly strong in the case where the object to be formed is a fine, delicate one typically including a large proportion of finely structured portions and being susceptible to significant damage. The pressure-sensitive adhesive sheet for build stage use is suitable for meeting the demand as above. 
     In addition, the pressure-sensitive adhesive sheet for build stage use, when used by the procedure as described above in the formation of an object using an additive manufacturing apparatus, can avoid or minimize the use of a scraper or another tool having a sharp edge for the detachment of the object from the build stage. Assume that a tool such as a scraper is used to detach the object from the build stage. Even in this case, the pressure-sensitive adhesive sheet for build stage use, when employed, facilitates the separation between the build stage and the object by allowing the tool to be inserted typically into the predetermined interface in the adhesive strength-reducible layer with relatively small force. Accordingly, the pressure-sensitive adhesive sheet for build stage use contributes to avoiding or minimizing significant damage of the object and the build stage upon detachment of the object from the build stage. 
     Preferably, the pressure-sensitive adhesive layer, which is the adhesive strength-reducible layer, is a thermally releasable pressure-sensitive adhesive layer, and the adhesive strength-reducing action is heating of the pressure-sensitive adhesive layer. The thermally releasable pressure-sensitive adhesive layer typically includes a thermal expansion agent. The thermal expansion agent may be, for example, thermally expandable microspheres each of which includes a component that gasifies by heating, and a shell that includes the component in an encapsulating manner and is capable of undergoing expansion and/or rupture as a result of the gasification of the component. The configurations as above can suitably allow the object to be actually attachable to, and detachable from, the build stage, using the thermal releasing technology relating to pressure-sensitive adhesive layers. 
     Preferably, the pressure-sensitive adhesive layer, which is the adhesive strength-reducible layer, is a cold-releasable pressure-sensitive adhesive layer, and the adhesive strength-reducing action is cooling of the pressure-sensitive adhesive layer. The cold-releasable pressure-sensitive adhesive layer may typically include a side-chain crystalline polymer. The configurations as above can actually suitably provide the attachment and detachment of the object to and from the build stage, using the cooling releasing technology relating to pressure-sensitive adhesive layers. 
     Preferably, the pressure-sensitive adhesive layer, which is the adhesive strength-reducible layer, is an electromagnetically releasable pressure-sensitive adhesive layer, and the adhesive strength-reducing action is application of electromagnetic radiation to the pressure-sensitive adhesive layer. The electromagnetically releasable pressure-sensitive adhesive layer may typically include an electromagnetic radiation-curable component. The configurations as above can suitably allow the object to be actually attachable to, and detachable from, the build stage, using the electromagnetic releasing technology relating to pressure-sensitive adhesive layers. 
     Preferably, the pressure-sensitive adhesive layer, which is the adhesive strength-reducible layer, is an electrically releasable pressure-sensitive adhesive layer, and the adhesive strength-reducing action is voltage application to the pressure-sensitive adhesive layer. The electrically releasable pressure-sensitive adhesive layer may typically include an electrolyte. The electrolyte is preferably an ionic liquid. The electrically releasable pressure-sensitive adhesive layer may contain the electrolyte in a content of preferably 0.5 to 30 parts by mass, per 100 parts by mass of the pressure-sensitive adhesive. The configurations as above can suitably allow the object to be actually attachable to, and detachable from, the build stage, using the electric releasing technology relating to pressure-sensitive adhesive layers. 
     Preferably, the pressure-sensitive adhesive layer, which is the adhesive strength-reducible layer, is a liquid-releasable pressure-sensitive adhesive layer, and the adhesive strength-reducing action is liquid supply to the pressure-sensitive adhesive layer. The liquid-releasable pressure-sensitive adhesive layer may typically include a water-soluble pressure-sensitive adhesive. The configurations as above can suitably allow the object to be actually attachable to, and detachable from, the build stage, using the liquid releasing technology relating to pressure-sensitive adhesive layers. 
     Preferably, the pressure-sensitive adhesive sheet for build stage use has a multilayer structure including the pressure-sensitive adhesive layer, which is the adhesive strength-reducible layer, and a stretchable substrate, and the adhesive strength-reducing action is stretching of the pressure-sensitive adhesive sheet for build stage use. The pressure-sensitive adhesive sheet for build stage use has an elongation upon rupture (elongation at break) of preferably 300% or more, where the elongation is measured in conformity to the Testing Method for “Elongation” described in JIS K 7311:1995. The configurations as above can suitably allow the object to be actually attachable to, and detachable from, the build stage, using the stretching releasing technology relating to pressure-sensitive adhesive sheets or pressure-sensitive adhesive tapes. 
     The present invention provides, according to a second aspect, an additive manufacturing apparatus. The additive manufacturing apparatus is an additive manufacturing apparatus capable of employing the pressure-sensitive adhesive sheet for build stage use according to the first aspect of the present invention. The additive manufacturing apparatus includes a build stage, and adhesive strength-reducing means for performing an adhesive strength-reducing action upon the pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet for build stage use as disposed on the build stage. The additive manufacturing apparatus as above is suitable for using the pressure-sensitive adhesive sheet for build stage use according to the first aspect of the present invention. 
     In the second aspect of the present invention, preferably, the pressure-sensitive adhesive layer, which is an adhesive strength-reducible layer of the pressure-sensitive adhesive sheet for build stage use, is an electromagnetically releasable pressure-sensitive adhesive layer, and the adhesive strength-reducing means is means for applying electromagnetic radiation to the pressure-sensitive adhesive layer. The electromagnetically releasable pressure-sensitive adhesive layer may typically include an electromagnetic radiation-curable component. The additive manufacturing apparatus, which has the configuration(s) as above, can suitably allow the object to be actually attachable to, and detachable from, the build stage, using the electromagnetic releasing technology relating to pressure-sensitive adhesive layers. 
     In the second aspect of the present invention, preferably, the pressure-sensitive adhesive layer, which is an adhesive strength-reducible layer of the pressure-sensitive adhesive sheet for build stage use, is an electrically releasable pressure-sensitive adhesive layer, and the adhesive strength-reducing means is means for applying a voltage to the pressure-sensitive adhesive layer. The electrically releasable pressure-sensitive adhesive layer may typically include an electrolyte. The electrolyte is preferably an ionic liquid. The electrically releasable pressure-sensitive adhesive layer may contain the electrolyte in a content of preferably 0.5 to 30 parts by mass per 100 parts by mass of the pressure-sensitive adhesive. The additive manufacturing apparatus, which has the configuration(s) as above, can suitably allow the object to be actually attachable to, and detachable from, the build stage, using the electric releasing technology relating to pressure-sensitive adhesive layers. 
     In the first and second aspects of the present invention, the pressure-sensitive adhesive in the adhesive strength-reducible layer preferably includes an acrylic pressure-sensitive adhesive, and the pressure-sensitive adhesive layer has a thickness of preferably 1 to 1000 nm. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a local sectional view of a pressure-sensitive adhesive sheet for build stage use according to an embodiment of the present invention; 
         FIGS. 2( a ), 2( b ), and 2( c )  illustrate how to use a pressure-sensitive adhesive sheet for build stage use according to an embodiment of the present invention, in which  FIGS. 2( a ), 2( b ), and 2( c )  illustrate states, respectively, before the start of formation of an object, during the object forming process, and after the completion of the object forming process; 
         FIGS. 3( a ), 3( b ), 3( c ), and 3( d )  illustrate modifications of the pressure-sensitive adhesive sheet for build stage use illustrated in  FIG. 1 ; 
         FIG. 4  is a local sectional view of a pressure-sensitive adhesive sheet for build stage use according to an embodiment of the present invention; 
         FIGS. 5( a ), 5( b ), 5( c ), and 5( d )  illustrate modifications of the pressure-sensitive adhesive sheet for build stage use illustrated in  FIG. 4 ; 
         FIG. 6  is a local sectional view of a pressure-sensitive adhesive sheet for build stage use according to an embodiment of the present invention; 
         FIGS. 7( a ), 7( b ), 7( c ), and 7( d )  illustrate modifications of the pressure-sensitive adhesive sheet for build stage use illustrated in  FIG. 6 ; 
         FIG. 8  is a local sectional view of a pressure-sensitive adhesive sheet for build stage use according to an embodiment of the present invention; 
         FIGS. 9( a ), 9( b ), 9( d ), and 9( d )  illustrate modifications of the pressure-sensitive adhesive sheet for build stage use illustrated in  FIG. 8 ; 
         FIG. 10  is a local sectional view of a pressure-sensitive adhesive sheet for build stage use according to an embodiment of the present invention; 
         FIGS. 11( a ), 11( b ), 11( c ), and 11( d )  illustrate modifications of the pressure-sensitive adhesive sheet for build stage use illustrated in  FIG. 10 ; 
         FIG. 12  is a local sectional view of a pressure-sensitive adhesive sheet for build stage use according to an embodiment of the present invention; 
         FIG. 13  illustrates a modification of the pressure-sensitive adhesive sheet for build stage use illustrated in  FIG. 12 ; 
         FIG. 14  is a schematic partial view of an additive manufacturing apparatus according to an embodiment of the present invention; 
         FIG. 15  is a schematic partial view of an additive manufacturing apparatus according to an embodiment of the present invention; 
         FIG. 16  is a schematic partial view of an additive manufacturing apparatus according to an embodiment of the present invention; 
         FIG. 17  is a schematic partial view of an additive manufacturing apparatus according to an embodiment of the present invention; 
         FIG. 18  is a schematic partial view of an additive manufacturing apparatus according to an embodiment of the present invention; 
         FIG. 19  is a schematic partial view of an additive manufacturing apparatus according to an embodiment of the present invention; 
         FIG. 20  is a schematic partial view of an additive manufacturing apparatus according to an embodiment of the present invention; and 
         FIG. 21  is a perspective view of an object prepared for the measurement of release force, for pressure-sensitive adhesive sheets according to Examples 6 and 7 and Comparative Example 1. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a local sectional view of a pressure-sensitive adhesive sheet X 1 , which is a pressure-sensitive adhesive sheet for build stage use according to an embodiment of the present invention. The pressure-sensitive adhesive sheet X 1  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 1  and pressure-sensitive adhesive layers  11  and  12 . 
     The substrate S 1  of the pressure-sensitive adhesive sheet X 1  is a portion that functions as a carrier (support) in the pressure-sensitive adhesive sheet X 1 . Examples of a material to constitute the substrate S 1  include, but are not limited to, paper materials, nonwoven fabrics, and plastic films. Non-limiting examples of the paper materials to constitute the substrate S 1  include Japanese paper, crepe paper, kraft paper, glassine paper, and synthetic paper. Non-limiting examples of materials for the nonwoven fabrics to constitute the substrate S 1  include pulps such as hemp pulp and wood pulp; as well as rayon, acetate fibers, polyester fibers, polyamide fibers, polyolefin fibers, and poly(vinyl alcohol) fibers. To bond nonwoven fabric fibers with each other, the fibers are typically impregnated with a binder resin. Non-limiting examples of the plastic films to constitute the substrate S 1  include polyolefin films, polyester films, polyurethane films, polyimide resin films, polyamide resin films, plasticized vinyl chloride resin films, and vinyl acetate resin films. Non-limiting examples of the polyolefin films include polyethylene films, polypropylene films, and films made from ethylene-propylene copolymers. Non-limiting examples of the polyester films include poly(ethylene terephthalate) films. The substrate S 1  of the pressure-sensitive adhesive sheet X 1  may be made from each of different materials alone or in combination. The substrate S 1  may be a multilayer assembly including layers differing in constituent material from each other. In the substrate S 1 , at least one of a surface facing the pressure-sensitive adhesive layer  11  and a surface facing the pressure-sensitive adhesive layer  12  may have undergone a surface treatment so as to have better adhesion with the adjacent pressure-sensitive adhesive layer. Non-limiting examples of the surface treatment include physical treatments such as corona treatment and plasma treatment; and chemical treatments such as primer coating. The substrate S 1  as above has a thickness of typically 10 to 300 μm. 
     The pressure-sensitive adhesive layer  11  of the pressure-sensitive adhesive sheet X 1  is a thermally releasable pressure-sensitive adhesive layer (adhesive strength-reducible layer) having an adhesive strength capable of being reduced by heating (adhesive strength-reducing action). The pressure-sensitive adhesive layer  11  has an adhesive face  11   a . The pressure-sensitive adhesive layer  11 , which is the thermally releasable pressure-sensitive adhesive layer as above, is made from a pressure-sensitive adhesive composition whose adhesive strength is significantly reduced by heating typically from room temperature. The pressure-sensitive adhesive composition or the pressure-sensitive adhesive layer  11  in the embodiment contains a pressure-sensitive adhesive and a thermal expansion agent (thermal blowing agent). The thermal expansion agent herein is a microgranule or microparticle that can undergo at least one of expansion and foaming as a result of heating. The pressure-sensitive adhesive layer  11  containing the thermal expansion agent as above, when subjected to heating for the expansion and/or foaming of the thermal expansion agent, expands and forms asperities (depressions and protrusions) at a surface including the adhesive face  11   a . Assume that the pressure-sensitive adhesive layer  11  undergoes such heating while the adhesive face  11   a  adheres to an adherend. In this case, the pressure-sensitive adhesive layer  11  expands to form asperities at the adhesive face  11   a , and this reduces the adhesive area of the pressure-sensitive adhesive layer  11  or the adhesive face  11   a  and thereby reduces or lowers the adhesive strength of the pressure-sensitive adhesive layer  11  to the adherend. 
     Non-limiting examples of the pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  11  include acrylic polymers as acrylic pressure-sensitive adhesives, natural rubbers, various synthetic rubbers, vinyl alkyl ether pressure-sensitive adhesives, silicone pressure-sensitive adhesives, polyester pressure-sensitive adhesives, polyamide pressure-sensitive adhesives, and urethane pressure-sensitive adhesives. The pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  11  is preferably an acrylic pressure-sensitive adhesive or adhesives. This is preferred from the viewpoint of actually providing satisfactory adhesive strength, cost minimization, and high productivity, for the pressure-sensitive adhesive layer  11 . As used herein, the term “acrylic pressure-sensitive adhesive” or “acrylic polymer” refers to a polymer that includes, as a principal monomer unit present in a largest mass proportion, a monomer unit derived from at least one of an alkyl acrylate and an alkyl methacrylate. 
     Assume that the pressure-sensitive adhesive layer  11  includes an acrylic polymer as an acrylic pressure-sensitive adhesive. In this case, the acrylic polymer typically includes, as a principal monomer unit, a unit derived from an alkyl (meth)acrylate containing a C 22  or lower alkyl. The term “principal monomer unit” as used herein refers to a monomer unit that is present in a largest mass proportion in the acrylic polymer. The term “(meth)acryl” refers to “acryl” and/or “methacryl”. Non-limiting examples of the C 22  or lower alkyl include methyl, ethyl, propyl, butyl, amyl, hexyl, heptyl, 2-ethylhexyl, isooctyl, isodecyl, dodecyl, lauryl, tridecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, icosyl, and docosyl. Each of different alkyl (meth)acrylates may be used alone or in combination to form the acrylic polymer to be contained in the pressure-sensitive adhesive layer  11 . 
     Assume that the pressure-sensitive adhesive layer  11  includes an acrylic polymer as an acrylic pressure-sensitive adhesive. In this case, the acrylic polymer may include a monomer unit or units derived from a monomer or monomers other than alkyl (meth)acrylates, where the monomer or monomers are copolymerizable with such alkyl (meth)acrylates. This may be employed from the viewpoint of modifying properties, such as cohesive force and heat resistance, of the pressure-sensitive adhesive layer  11 . Examples of such copolymerizable monomers include, but are not limited to, carboxy-containing monomers, hydroxy-containing monomers, sulfonate-containing monomers, amide monomers, aminoalkyl (meth)acrylate monomers, maleimide monomers, vinyl monomers, cyanoacrylate monomers, epoxy-containing acrylic monomers, and multifunctional monomers. Non-limiting examples of the carboxy-containing monomers include acrylic acid, methacrylic acid, carboxyethyl acrylate, carboxypentyl acrylate, itaconic acid, maleic acid, fumaric acid, and crotonic acid. Non-limiting examples of the hydroxy-containing monomers include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, hydroxyoctyl (meth)acrylate, hydroxydecyl (meth)acrylate, hydroxylauryl (meth)acrylate, and (4-hydroxymethylcyclohexyl)methyl (meth)acrylate. Non-limiting examples of the sulfonate-containing monomers include styrenesulfonic acid, allylsulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, (meth)acrylamidopropanesulfonic acid, sulfopropyl (meth)acrylate, and (meth)acryloyloxynaphthalenesulfonic acid. Non-limiting examples of the amide monomers include (meth)acrylamide, N,N-dimethyl(meth)acrylamide, N-butyl(meth)acrylamide, N-methylol(meth)acrylamide, and N-methylolpropane(meth)acrylamide. Non-limiting examples of the aminoalkyl (meth)acrylate monomers include aminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, and t-butylaminoethyl (meth)acrylates. Non-limiting examples of the maleimide monomers include N-cyclohexylmaleimide, N-isopropylmaleimide, N-laurylmaleimide, and N-phenylmaleimide. Non-limiting examples of the vinyl monomers include vinyl acetate, vinyl propionate, N-vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperazine, vinylpyrazine, vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholine, N-vinylcarboxamides, styrene, α-methylstyrene, and N-vinylcaprolactam. Non-limiting examples of the cyanoacrylate monomers include acrylonitrile and methacrylonitrile. Non-limiting examples of the epoxy-containing acrylic monomers include glycidyl (meth)acrylate. Non-limiting examples of the multifunctional monomers include hexanediol di(meth)acrylate, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol hexa(meth)acrylate. Each of different copolymerizable monomers may be used alone or in combination to form the acrylic polymer to be contained in the pressure-sensitive adhesive layer  11 . 
     The acrylic polymer as above can be obtained by polymerization of a material monomer component or components. Examples of the polymerization technique include, but are not limited to, solution polymerization, emulsion polymerization, and bulk polymerization. The solution polymerization, when performed, may employ a solvent; and non-limiting examples of the solvent include aromatic hydrocarbons, aliphatic hydrocarbons, alicyclic hydrocarbons, esters, and ketones. The polymerization of the material monomer component(s) to form the acrylic polymer may be performed using a polymerization initiator. The polymerization initiator may be selected typically from photoinitiators and thermal initiators, according to the type of the polymerization reaction. The polymerization of the material monomer component(s) to form the acrylic polymer may be performed using a chain transfer agent. 
     Examples of the thermal expansion agent (thermal blowing agent) that is capable typically of expanding by heating and is usable herein include inorganic blowing agents and organic blowing agents. Non-limiting examples of the inorganic blowing agents include water, ammonium carbonate, ammonium hydrogencarbonate, sodium hydrogencarbonate, ammonium nitrite, sodium borohydride, and graphite. Non-limiting examples of the organic blowing agents include chlorofluoroalkanes, azo compounds, hydrazine compounds, semicarbazide compounds, triazole compounds, and N-nitroso compounds. Non-limiting examples of the chlorofluoroalkanes include trichloromonofluoromethane and dichloromonofluoromethane. Non-limiting examples of the azo compounds include azobisisobutyronitrile, azodicarbonamide, and barium azodicarboxylate. Non-limiting examples of the hydrazine compounds include p-toluenesulfonyl hydrazide, diphenylsulfone-3,3′-disulfonyl hydrazide, 4,4′-oxybis(benzenesulfonyl hydrazide), and allylbis(sulfonyl hydrazide). Non-limiting examples of the semicarbazide compounds include p-toluylenesulfonyl semicarbazide and 4,4′-oxybis(benzenesulfonyl semicarbazide). A non-limiting example of the triazole compounds is 5-morpholyl-1,2,3,4-thiatriazole. Non-limiting examples of the N-nitroso compounds include N,N′-dinitrosopentamethylenetetramine and N,N′-dimethyl-N,N′-dinitrosoterephthalamide. 
     The thermal expansion agent capable typically of expanding by heating for use herein is preferably thermally expandable microspheres each including a component that gasifies by heating, and a shell including the component in an encapsulating manner and being capable of undergoing expansion and/or rupture by the gasification of the component. The thermally expandable microspheres typically have a configuration including a shell and a low-boiling liquid or another substance, where the substance can readily gasify and expand by heating and is encapsulated in the shell, which is elastic. Non-limiting examples of the substance encapsulated in the shell include isobutane, propane, and pentane. For example, the shell may be made from a thermofusible (hot-melt) substance that can be molten by heating for allowing the thermal expansion agent to expand and/or foam. Alternatively, the shell may be made from such a substance and has such a thickness as to be able to expand or be ruptured by the expansion or gasification of the encapsulated substance as a result of heating. Non-limiting examples of the substance to form the shell include vinylidene chloride-acrylonitrile copolymers, poly(vinyl alcohol)s, poly(vinyl butyral)s, poly(methyl methacrylate)s, polyacrylonitriles, poly(vinylidene chloride)s, and polysulfones. The thermally expandable microspheres can be produced typically by coacervation technique or interfacial polymerization technique. The thermally expandable microspheres as above have an average particle diameter of typically 5 to 100 μm. The thermally expandable microspheres have a magnification of cubical expansion of typically 5 times or more, preferably 7 times or more, and more preferably 10 times or more, from the viewpoint of allowing the pressure-sensitive adhesive layer including the thermally expandable microspheres to develop satisfactory thermal releasability. The thermally expandable microspheres may be present in the pressure-sensitive adhesive layer  11  in a content of typically 5 to 200 parts by mass per 100 parts by mass of the pressure-sensitive adhesive. 
     In addition to the components, the pressure-sensitive adhesive layer  11  may further include one or more other components. Non-limiting examples of such other components include crosslinkers, tackifiers, plasticizers, softeners, fillers, age inhibitors, silane coupling agents, surfactants, and colorants such as pigments and dyes. The allowance of these components in the pressure-sensitive adhesive layer  11  is also applicable to other pressure-sensitive adhesive layers described later. 
     The pressure-sensitive adhesive layer  11  of the pressure-sensitive adhesive sheet X 1  has a thickness of preferably 1 to 1000 μm. This is preferred from the viewpoint of allowing the pressure-sensitive adhesive layer  11  to actually have satisfactory tackiness. The lower limit of the thickness of the pressure-sensitive adhesive layer  11  is more preferably 3 μm, furthermore preferably 5 μm, and still more preferably 8 μm. The upper limit of the thickness of the pressure-sensitive adhesive layer  11  is more preferably 500 μm, furthermore preferably 100 μm, and still more preferably 30 μm. The ranges of the thickness of the pressure-sensitive adhesive layer are also applicable to the other pressure-sensitive adhesive layers described later. 
     The adhesive face  11   a , which constitutes or defines one adhesive face of the pressure-sensitive adhesive sheet X 1 , is preferably designed to have a 180-degree peel strength from a SUS 304 sheet of 0.1 N/10 mm or more, in terms of adhesive strength before the adhesive strength-reducing action, where the 180-degree peel strength is measured at a tensile speed of 300 mm/min and a peel temperature of 23° C. This is preferred from the viewpoint of allowing the adhesive face  11   a  to actually have satisfactory bond strength or adhesive strength. The 180-degree peel strength can be measured according to JIS Z 0237. The adhesive strength level as above is also applicable to the adhesive strengths of the after-mentioned other adhesive strength-reducible layers before the adhesive strength-reducing action; and the adhesive strengths of after-mentioned pressure-sensitive adhesive layers that are not adhesive strength-reducible layers. 
     In the configuration, the pressure-sensitive adhesive layer  11  contains a thermal expansion agent capable of undergoing expansion and/or foaming by heating and renders heating-induced reduction in adhesive strength feasible. Instead of or in combination of this configuration, the pressure-sensitive adhesive layer  11 , which is the thermally releasable pressure-sensitive adhesive layer (adhesive strength-reducible layer) in the pressure-sensitive adhesive sheet X 1 , may employ any of configurations as follows: a configuration where the pressure-sensitive adhesive layer  11  contains a component capable of undergoing glass crystallization by heating and thereby renders heating-induced reduction in adhesive strength feasible; a configuration where the pressure-sensitive adhesive layer  11  contains, in addition to a pressure-sensitive adhesive polymer principal component, a crystalline component capable of melting by heating and reducing the cohesive force of the pressure-sensitive adhesive bulk, and thereby renders heating-induced reduction in adhesive strength feasible; a configuration where the pressure-sensitive adhesive layer  11  contains a component capable of causing a crosslinking reaction or curing reaction by heating, and thereby renders heating-induced reduction in adhesive strength feasible; a configuration where the pressure-sensitive adhesive layer  11  has such a chemical composition as to cause phase separation by heating, and thereby renders heating-induced reduction in adhesive strength feasible; and a configuration where the pressure-sensitive adhesive layer  11  has such a chemical composition as to form a phase-separated structure including phases, part of which phases is fusible or meltable by heating, and thereby renders heating-induced reduction in adhesive strength feasible. Alternatively, the pressure-sensitive adhesive layer  11  may employ configurations selected from these configurations in combination. These points relating to the configurations of the pressure-sensitive adhesive layer  11  are also applicable to pressure-sensitive adhesive layers  11  in the after-mentioned modifications of the pressure-sensitive adhesive sheet X 1 . 
     The pressure-sensitive adhesive layer  12  of the pressure-sensitive adhesive sheet X 1  includes a pressure-sensitive adhesive as a base component and has an adhesive face  12   a . As used herein, the term “base component” refers to a component that is present in a largest mass proportion among constitutive components. Non-limiting examples of the pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  12  include acrylic polymers as acrylic pressure-sensitive adhesives, natural rubbers, various synthetic rubbers, vinyl alkyl ether pressure-sensitive adhesives, silicone pressure-sensitive adhesives, polyester pressure-sensitive adhesives, polyamide pressure-sensitive adhesives, and urethane pressure-sensitive adhesives. The pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  12  is preferably an acrylic pressure-sensitive adhesive or adhesives. This is preferred from the viewpoint of actually providing satisfactory adhesive strength, cost minimization, and high productivity, for the pressure-sensitive adhesive layer  12 . The pressure-sensitive adhesive layer  12  may have a multilayer structure including pressure-sensitive adhesive layers differing from each other in chemical composition. 
     The pressure-sensitive adhesive sheet X 1  may have a multilayer structure further including one or more other layers, in addition to the substrate S 1  and the pressure-sensitive adhesive layers  11  and  12 . A non-limiting example of the other layers is an under coat for providing higher adhesive strength between the substrate S 1  and the thermally releasable pressure-sensitive adhesive layer  11 . Non-limiting examples of a constituent material of the under coat include synthetic rubbers; and synthetic resins having rubber elasticity. Non-limiting examples of the synthetic rubbers include nitrile synthetic rubbers, diene synthetic rubbers, and acrylic synthetic rubbers. Non-limiting examples of the synthetic resins having rubber elasticity include polyolefin resins, polyester resins, thermoplastic elastomers, ethylene-vinyl acetate copolymers, polyurethanes, polybutadienes, and plasticized poly(vinyl chloride)s. The under coat has a thickness of typically 0.1 to 100 μm. The configuration, where the under coat as above is disposed between the substrate S 1  and the pressure-sensitive adhesive layer  11 , is suitable for allowing the pressure-sensitive adhesive layer  11  to undergo the heating-induced reduction in adhesive strength at the adhesive face  11   a . In contrast to this, a configuration where no under coat is disposed between the substrate S 1  and the pressure-sensitive adhesive layer  11 ; and a configuration where a release treatment is applied to a surface of the substrate S 1 , where the surface faces the pressure-sensitive adhesive layer  11 , are suitable for allowing the pressure-sensitive adhesive layer  11  to undergo heating-induced reduction in adhesive strength at the surface facing the substrate S 1 . In the pressure-sensitive adhesive sheet X 1 , sufficient breaking strength as a pressure-sensitive adhesive sheet for build stage use may be surely provided even without the substrate S 1 , in some set thickness and/or hardness of each of the pressure-sensitive adhesive layer  11  and the pressure-sensitive adhesive layer  12 ; and in this case, the configuration of including no substrate S 1  may be employed. 
     The pressure-sensitive adhesive sheet X 1  as above can be produced typically by a procedure as follows. Initially, a first pressure-sensitive adhesive composition and a second pressure-sensitive adhesive composition are independently prepared. The first pressure-sensitive adhesive composition is a pressure-sensitive adhesive composition for the formation of the pressure-sensitive adhesive layer  11 . The second pressure-sensitive adhesive composition is a pressure-sensitive adhesive composition for the formation of the pressure-sensitive adhesive layer  12 . The first pressure-sensitive adhesive composition contains a pressure-sensitive adhesive and a thermal expansion agent. The second pressure-sensitive adhesive composition contains a pressure-sensitive adhesive. Next, the first pressure-sensitive adhesive composition is applied onto a release-treated surface of a first release liner or release film to form a first pressure-sensitive adhesive composition layer, followed by drying of the layer. This gives a pressure-sensitive adhesive layer  11  on the first release liner. On the other hand, the second pressure-sensitive adhesive composition is applied onto a release-treated surface of a second release liner or release film to form a second pressure-sensitive adhesive composition layer, followed by drying the layer. This gives a pressure-sensitive adhesive layer  12  on the second release liner. The pressure-sensitive adhesive layer  11  and the pressure-sensitive adhesive layer  12  are then applied respectively onto one side and the other side of the substrate S 1 . The pressure-sensitive adhesive layers  11  and  12  are then further dried, or crosslinking reactions are allowed to proceed, as needed. Pressure-sensitive adhesive sheets for build stage use according to the after-mentioned embodiments can also be produced typically through: preparation of pressure-sensitive adhesive composition(s) for pressure-sensitive adhesive layer(s) to be formed; formation of pressure-sensitive adhesive composition layer(s) from the composition(s); formation of target pressure-sensitive adhesive layer(s) from the pressure-sensitive adhesive composition layer(s); and affixation of the pressure-sensitive adhesive layer(s) to a substrate. 
     The pressure-sensitive adhesive sheet X 1 , which is produced by the procedure as described above, may be in the form of a wound roll with a separator (release liner) provided so as to cover the surface (adhesive face  11   a ) of the pressure-sensitive adhesive layer  11 , or with a separator (release liner) provided so as to cover the surface (adhesive face  12   a ) of the pressure-sensitive adhesive layer  12 . Alternatively, the pressure-sensitive adhesive sheet X 1  may bear a pair of separators (release liners) so as to cover the adhesive faces  11   a  and  12   a . The separator(s) will be removed from the pressure-sensitive adhesive sheet X 1  when the pressure-sensitive adhesive sheet X 1  is affixed to an adherend. These configurations with release liner(s) are also applicable to the pressure-sensitive adhesive sheets for build stage use according to the after-mentioned embodiments. 
     The pressure-sensitive adhesive sheet X 1  is to be used for a build stage  100  of an additive manufacturing apparatus for the formation of an object, typically as illustrated in  FIGS. 2( a ), 2( b ), and 2( c ) . Specifically, the pressure-sensitive adhesive sheet X 1  is affixed to an object-forming surface  101  of the build stage  100  before the start of object formation, as illustrated in  FIG. 2( a ) . The pressure-sensitive adhesive sheet X 1  is applied, through a pressure-sensitive adhesive layer thereof, to the object-forming surface  101 . Specifically, of the pressure-sensitive adhesive sheet X 1 , one surface defined by the pressure-sensitive adhesive layer  11 , or the other surface defined by the pressure-sensitive adhesive layer  12 , adheres to the object-forming surface  101 . During an object forming process with the apparatus being operated, the target object W is gradually formed (built up) on the pressure-sensitive adhesive sheet X 1  affixed to the build stage  100 , through a predetermined process according to an additive manufacturing technique employed in the additive manufacturing apparatus, as illustrated in  FIG. 2( b ) . Non-limiting examples of the additive manufacturing technique include fused deposition modeling, material jetting, stereolithography, and sheet lamination. In the object forming process as above, the pressure-sensitive adhesive sheet X 1  adheres to the object-forming surface  101  of the build stage  100 , and the object W during its formation is attached to the pressure-sensitive adhesive sheet X 1  on the build stage  100 . Specifically, the object W during its formation is in a state of being attached to the build stage  100  through the pressure-sensitive adhesive sheet X 1 . After the completion of the object forming process as illustrated in  FIG. 2( c ) , an adhesive strength-reducing action is performed to the pressure-sensitive adhesive sheet X 1  on the build stage  100 . In the embodiment, the adhesive strength-reducing action is heating of the pressure-sensitive adhesive layer  11 , which is an adhesive strength-reducible layer of the pressure-sensitive adhesive sheet X 1 . For example, assume that the build stage  100  has a built-in heater. In this case, the pressure-sensitive adhesive sheet X 1  on the build stage  100  can be heated by the heater. The heating temperature is typically such a temperature as to allow the thermal expansion agent in the pressure-sensitive adhesive layer  11  to expand and/or foam, and is 70° C. to 200° C. The heating as above, which is the adhesive strength-reducing action, reduces the adhesive strength of the pressure-sensitive adhesive layer  11  and frees or loosens the attached state of the object W to the build stage  100  through the pressure-sensitive adhesive sheet X 1 . This results in facilitating the detachment of the object W from the build stage  100 . Assume that the pressure-sensitive adhesive sheet X 1  is applied, at the surface defined by the pressure-sensitive adhesive layer  11 , onto the build stage  100  before the start of object formation. In this case, the object W can be detached (demounted) from the build stage  100  with an operation of removing the pressure-sensitive adhesive sheet X 1  from the build stage  100 , typically at the adhesive face  11   a  of the pressure-sensitive adhesive layer  11  whose adhesive strength has been reduced by heating. Assume that the pressure-sensitive adhesive sheet X 1  is applied, at the surface defined by the pressure-sensitive adhesive layer  12 , onto the build stage  100  before the start of object formation. In this case, the object W is formed on the pressure-sensitive adhesive layer  11  of the pressure-sensitive adhesive sheet X 1 ; and the object W can be detached from the build stage  100  by pulling off the object W from the pressure-sensitive adhesive layer  11  whose adhesive strength has been reduced by heating. 
     The pressure-sensitive adhesive sheet X 1  for build stage use is suitable for allowing the object W, which is to be formed over the build stage  100  of the additive manufacturing apparatus, to be attached to the build stage  100  during building up, and to be readily detached from the build stage  100  after building up. In addition, the pressure-sensitive adhesive sheet X 1  as above, when used for the formation of the object W using the additive manufacturing apparatus by the procedure as described above, can avoid or minimize the use of a scraper or another tool having a sharp edge for the detachment of the object W from the build stage  100 . Accordingly, the use of the pressure-sensitive adhesive sheet X 1  for build stage use contributes to avoiding or minimizing significant damage on the object W and the build stage  100  upon detachment of the object W from the build stage  100 . 
       FIG. 3( a )  illustrates a pressure-sensitive adhesive sheet X 1   a , which is a modification of the pressure-sensitive adhesive sheet X 1  for build stage use illustrated in  FIG. 1 . The pressure-sensitive adhesive sheet X 1   a  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 1  and pressure-sensitive adhesive layers  11  and  11 , and differs from the pressure-sensitive adhesive sheet X 1  in including a thermally releasable pressure-sensitive adhesive layer  11  instead of the pressure-sensitive adhesive layer  12 .  FIG. 3( b )  illustrates a pressure-sensitive adhesive sheet X 1   b , which is a modification of the pressure-sensitive adhesive sheet X 1  for build stage use. The pressure-sensitive adhesive sheet X 1   b  is a single-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 1  and a pressure-sensitive adhesive layer  11 , and differs from the pressure-sensitive adhesive sheet X 1  in including no pressure-sensitive adhesive layer  12 . The pressure-sensitive adhesive sheet X 1   b  as above is used by allowing its surface defined by the pressure-sensitive adhesive layer  11  to adhere to the object-forming surface  101  of the build stage  100 . Assume that the object to be additively manufactured by the additive manufacturing apparatus is capable of being attached, at its end face facing the stage, to the object-forming surface typically through fusion bonding. In this case, the pressure-sensitive adhesive sheet X 1   b , which includes no pressure-sensitive adhesive layer on the side facing the object, may be employed.  FIG. 3( c )  illustrates a pressure-sensitive adhesive sheet X 1   c , which is a modification of the pressure-sensitive adhesive sheet X 1  for build stage use. The pressure-sensitive adhesive sheet X 1   c  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including pressure-sensitive adhesive layers  11  and  12  and differs from the pressure-sensitive adhesive sheet X 1  in including no substrate S 1 . The pressure-sensitive adhesive sheet X 1   c  as above is used by allowing one surface defined by the pressure-sensitive adhesive layer  11 , or the other surface defined by the pressure-sensitive adhesive layer  12 , to adhere to the object-forming surface  101  of the build stage  100 . Sufficient breaking strength as a pressure-sensitive adhesive sheet for build stage use may be surely provided even without the substrate S 1 , in some set thickness and/or hardness of each of the pressure-sensitive adhesive layer  11  and the pressure-sensitive adhesive layer  12 ; and in this case, the configuration of including no substrate S 1  may be employed.  FIG. 3( d )  illustrates a pressure-sensitive adhesive sheet X 1   d , which is a modification of the pressure-sensitive adhesive sheet X 1  for build stage use. The pressure-sensitive adhesive sheet X 1   d  is a double-sided pressure-sensitive adhesive sheet that includes a thermally releasable pressure-sensitive adhesive layer  11 . The pressure-sensitive adhesive sheet X 1   d  differs from the pressure-sensitive adhesive sheet X 1  in including neither of the substrate S 1  and the pressure-sensitive adhesive layer  12 . Sufficient breaking strength as a pressure-sensitive adhesive sheet for build stage use may be surely provided even without the substrate S 1  and the pressure-sensitive adhesive layer  12 , in some set thickness and/or hardness of the pressure-sensitive adhesive layer  11 ; and in this case, the configuration of including neither of the substrate S 1  and the pressure-sensitive adhesive layer  12  may be employed. These pressure-sensitive adhesive sheets X 1   a , X 1   b , X 1   c , and X 1   d  for build stage use are each usable by a procedure similar to that described above for the pressure-sensitive adhesive sheet X 1  and are therefore suitable for allowing the object W, which is to be formed over the build stage  100  of the additive manufacturing apparatus, to be attached to the build stage  100  during building up, and to be readily detached from the build stage  100  after building up. 
       FIG. 4  is a local sectional view of a pressure-sensitive adhesive sheet X 2 , which is a pressure-sensitive adhesive sheet for build stage use according to an embodiment of the present invention. The pressure-sensitive adhesive sheet X 2  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 1  and pressure-sensitive adhesive layers  21  and  22 . 
     The pressure-sensitive adhesive layer  21  of the pressure-sensitive adhesive sheet X 2  is a cold-releasable pressure-sensitive adhesive layer (adhesive strength-reducible layer) having an adhesive strength reducible by cooling (adhesive strength-reducing action) and has an adhesive face  21   a . The pressure-sensitive adhesive layer  21 , which is a cold-releasable pressure-sensitive adhesive layer, is made from a pressure-sensitive adhesive composition having an adhesive strength that is significantly reduced by cooling typically from room temperature. The pressure-sensitive adhesive composition or the pressure-sensitive adhesive layer  21  in the embodiment contains a side-chain crystalline polymer as, or in addition to, a base component pressure-sensitive adhesive. Assume that the pressure-sensitive adhesive layer  21  is cooled down typically to a temperature lower than the melting point of the side-chain crystalline polymer in the pressure-sensitive adhesive layer  21 , while the adhesive face  21   a  adheres to an adherend. In this case, the side-chain crystalline polymer crystallizes in the pressure-sensitive adhesive layer  21  to reduce the adhesive strength to the adherend. The pressure-sensitive adhesive layer  21  has an adhesive strength of typically 6.5 N at −30° C. The cold-releasable pressure-sensitive adhesive layer as above can be formed typically from a pressure-sensitive adhesive composition as described typically in Japanese Unexamined Patent Application Publication (JP-A) No. 2013-173912. The pressure-sensitive adhesive composition just mentioned above includes an acrylic polymer or a partial polymer, where the acrylic polymer is a polymer of a material monomer component including, for example, an alkyl (meth)acrylate containing linear or branched C 10 -C 13  alkyl and including approximately no carboxy-containing monomer; and the partial polymer is one derived from the material monomer component. Non-limiting examples of a coolant for cooling the cold-releasable pressure-sensitive adhesive layer include carbon dioxide gas and nitrogen gas each of which is cooled at −30° C. to 10° C.; and water and so-called antifreezes each of which is set at a cooling temperature. 
     In the configuration, the pressure-sensitive adhesive layer  21  contains a side-chain crystalline polymer capable of crystallizing by cooling and renders cooling-induced reduction in adhesive strength feasible. In stead of, or in combination of, this configuration, the pressure-sensitive adhesive layer  21 , which is the cold-releasable pressure-sensitive adhesive layer (adhesive strength-reducible layer) in the pressure-sensitive adhesive sheet X 2 , may employ any of configurations as follows: a configuration where the pressure-sensitive adhesive layer  21  has such a chemical composition as to contract by cooling and renders cooling-induced reduction in adhesive strength feasible; and a configuration where the pressure-sensitive adhesive layer  21  has such a chemical composition as to cause phase separation by cooling and renders cooling-induced reduction in adhesive strength feasible. Alternatively, the pressure-sensitive adhesive layer  21  may employ configurations selected from these configurations in combination. These points relating to configurations of the pressure-sensitive adhesive layer  21  are also applicable to pressure-sensitive adhesive layers  21  in the after-mentioned modifications relating to the pressure-sensitive adhesive sheet X 2 . 
     The pressure-sensitive adhesive layer  22  of the pressure-sensitive adhesive sheet X 2  includes a pressure-sensitive adhesive as a base component and has an adhesive face  22   a . Non-limiting examples of the pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  22  include acrylic polymers as acrylic pressure-sensitive adhesives, natural rubbers, various synthetic rubbers, vinyl alkyl ether pressure-sensitive adhesives, silicone pressure-sensitive adhesives, polyester pressure-sensitive adhesives, polyamide pressure-sensitive adhesives, and urethane pressure-sensitive adhesives. The pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  22  is preferably an acrylic pressure-sensitive adhesive or adhesives. This is preferred from the viewpoint of actually providing satisfactory adhesive strength, cost minimization, and high productivity for the pressure-sensitive adhesive layer  22 . The pressure-sensitive adhesive layer  22  may have a multilayer structure including pressure-sensitive adhesive layers differing from each other in chemical composition. 
     The pressure-sensitive adhesive sheet X 2  is to be used for the build stage  100  of the additive manufacturing apparatus for the formation of an object, typically as illustrated in  FIGS. 2( a ), 2( b ), and 2( c ) , as with the pressure-sensitive adhesive sheet X 1 . Specifically, the pressure-sensitive adhesive sheet X 2  is affixed to the object-forming surface  101  of the build stage  100  before the start of object formation, as illustrated in  FIG. 2( a ) . The pressure-sensitive adhesive sheet X 2  is applied through its pressure-sensitive adhesive layer onto the object-forming surface  101 . Specifically, of the pressure-sensitive adhesive sheet X 2 , one surface defined by the pressure-sensitive adhesive layer  21  or the other surface defined by the pressure-sensitive adhesive layer  22  adheres to the object-forming surface  101 . During an object forming process with the apparatus being operated, the target object W is gradually formed or built up on the pressure-sensitive adhesive sheet X 2  affixed to the build stage  100 , as illustrated in  FIG. 2( b ) . In the object forming process as above, the object W during its formation is in a state of being attached to the build stage  100  through the pressure-sensitive adhesive sheet X 2 . After the completion of the object forming process as illustrated in  FIG. 2( c ) , an adhesive strength-reducing action is performed to the pressure-sensitive adhesive sheet X 2  on the build stage  100 . The adhesive strength-reducing action in the embodiment is cooling of the pressure-sensitive adhesive layer  21 , which is an adhesive strength-reducible layer of the pressure-sensitive adhesive sheet X 2 . A non-limiting example of the cooling technique is supply of the coolant to the pressure-sensitive adhesive sheet X 2  or the pressure-sensitive adhesive layer  21  typically by spraying. The cooling temperature is typically such a temperature as to crystallize the side-chain crystalline polymer in the pressure-sensitive adhesive layer  21  and is −60° C. to −20° C. The cooling as above, which is the adhesive strength-reducing action, reduces the adhesive strength of the pressure-sensitive adhesive layer  21  and frees or loosens the attached state of the object W to the build stage  100  through the pressure-sensitive adhesive sheet X 2 . This results in facilitating the detachment of the object W from the build stage  100 . Assume that the pressure-sensitive adhesive sheet X 2  is applied, at the surface defined by the pressure-sensitive adhesive layer  21 , onto the build stage  100  before the start of object formation. In this case, the object W can be detached from the build stage  100  with an operation of removing the pressure-sensitive adhesive sheet X 2  from the build stage  100 , typically at the adhesive face  21   a  of the pressure-sensitive adhesive layer  21  whose adhesive strength has been reduced by cooling. Assume that the pressure-sensitive adhesive sheet X 2  is applied, at the surface defined by the pressure-sensitive adhesive layer  22 , onto the build stage  100  before the start of object formation. In this case, the object W is formed on the pressure-sensitive adhesive layer  21  of the pressure-sensitive adhesive sheet X 2 , and the formed object W can be detached from the build stage  100  by pulling off the object W from the pressure-sensitive adhesive layer  21  whose adhesive strength has been reduced by cooling. 
     As described above, the pressure-sensitive adhesive sheet X 2  for build stage use is suitable for allowing the object W, which is to be formed over the build stage  100  of the additive manufacturing apparatus, to be attached to the build stage  100  during building up, and to be readily detached from the build stage  100  after building up. In addition, the pressure-sensitive adhesive sheet X 2  as above, when used by the procedure as described above in the formation of the object W using the additive manufacturing apparatus, can avoid or minimize the use of a scraper or another tool having a sharp edge for the detachment of the object W from the build stage  100 . Accordingly, the use of the pressure-sensitive adhesive sheet X 2  for build stage use contributes to avoiding or minimizing significant damage on the object W and the build stage  100  upon detachment of the object W from the build stage  100 . 
       FIG. 5( a )  illustrates a pressure-sensitive adhesive sheet X 2   a , which is a modification of the pressure-sensitive adhesive sheet X 2  for build stage use illustrated in  FIG. 4 . The pressure-sensitive adhesive sheet X 2   a  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 1  and pressure-sensitive adhesive layers  21  and  21 , and differs from the pressure-sensitive adhesive sheet X 2  in including a cold-releasable pressure-sensitive adhesive layer  21 , instead of the pressure-sensitive adhesive layer  22 .  FIG. 5( b )  illustrates a pressure-sensitive adhesive sheet X 2   b , which is a modification of the pressure-sensitive adhesive sheet X 2  for build stage use. The pressure-sensitive adhesive sheet X 2   b  is a single-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 1  and a pressure-sensitive adhesive layer  21 , and differs from the pressure-sensitive adhesive sheet X 2  in including no pressure-sensitive adhesive layer  22 . The pressure-sensitive adhesive sheet X 2   b  as above is used by allowing its surface defined by the pressure-sensitive adhesive layer  21  to adhere to the object-forming surface  101  of the build stage  100 . Assume that the object to be additively manufactured by the additive manufacturing apparatus is capable of being attached, at its end face facing the stage, to the object-forming surface typically through fusion bonding. In this case, the pressure-sensitive adhesive sheet X 2   b , which includes no pressure-sensitive adhesive layer on the side facing the object, may be employed.  FIG. 5( c )  illustrates a pressure-sensitive adhesive sheet X 2   c , which is a modification of the pressure-sensitive adhesive sheet X 2  for build stage use. The pressure-sensitive adhesive sheet X 2   c  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including pressure-sensitive adhesive layers  21  and  22 , and differs from the pressure-sensitive adhesive sheet X 2  in including no substrate S 1 . The pressure-sensitive adhesive sheet X 2   c  as above is used by allowing one surface defined by the pressure-sensitive adhesive layer  21 , or the other surface defined by the pressure-sensitive adhesive layer  22 , to adhere to the object-forming surface  101  of the build stage  100 . Sufficient breaking strength as a pressure-sensitive adhesive sheet for build stage use may be surely provided even without the substrate S 1 , in some set thickness and/or hardness of each of the pressure-sensitive adhesive layer  21  and the pressure-sensitive adhesive layer  22 ; and in this case, the configuration of including no substrate S 1  may be employed.  FIG. 5( d )  illustrates a pressure-sensitive adhesive sheet X 2   d , which is a modification of the pressure-sensitive adhesive sheet X 2  for build stage use. The pressure-sensitive adhesive sheet X 2   d  is a double-sided pressure-sensitive adhesive sheet including a cold-releasable pressure-sensitive adhesive layer  21  and differs from the pressure-sensitive adhesive sheet X 2  in including neither of the substrate S 1  and the pressure-sensitive adhesive layer  22 . Sufficient breaking strength as a pressure-sensitive adhesive sheet for build stage use may be surely provided even without the substrate S 1  and the pressure-sensitive adhesive layer  22 , in some set thickness and/or hardness of the pressure-sensitive adhesive layer  21 ; and in this case, the configuration of including neither of the substrate S 1  and the pressure-sensitive adhesive layer  22  may be employed. These pressure-sensitive adhesive sheets X 2   a , X 2   b , X 2   c , and X 2   d  for build stage use are each usable by a procedure similar to that described above for the pressure-sensitive adhesive sheet X 2 . Accordingly, these pressure-sensitive adhesive sheets are suitable for allowing the object W, which is to be formed over the build stage  100  of the additive manufacturing apparatus, to be attached to the build stage  100  during building up, and to be readily detached from the build stage  100  after building up. 
       FIG. 6  is a local sectional view of a pressure-sensitive adhesive sheet X 3 , which is a pressure-sensitive adhesive sheet for build stage use according to an embodiment of the present invention. The pressure-sensitive adhesive sheet X 3  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 1  and pressure-sensitive adhesive layers  31  and  32 . 
     The pressure-sensitive adhesive layer  31  of the pressure-sensitive adhesive sheet X 3  is an electromagnetically releasable pressure-sensitive adhesive layer (adhesive strength-reducible layer) having an adhesive strength capable of being reduced by electromagnetic radiation application (adhesive strength-reducing action), where the electromagnetic radiation is exemplified typically by ultraviolet radiation. The pressure-sensitive adhesive layer  31  has an adhesive face  31   a . The pressure-sensitive adhesive layer  31  is made from a pressure-sensitive adhesive composition that cures by electromagnetic irradiation. The pressure-sensitive adhesive composition or the pressure-sensitive adhesive layer  31  in the embodiment contains a pressure-sensitive adhesive polymer and an electromagnetic radiation-curable component. The electromagnetic radiation-curable component refers typically to a monomer or oligomer capable of allowing a curing reaction to initiate and to proceed upon electromagnetic irradiation. Assume that the pressure-sensitive adhesive layer  31  is irradiated with electromagnetic radiation while the adhesive face  31   a  adheres to an adherend. In this case, the reaction of the electromagnetic radiation-curable component induced by the irradiation proceeds in the pressure-sensitive adhesive layer  31  to cure the pressure-sensitive adhesive layer  31 . The cured pressure-sensitive adhesive layer  31  contracts and gives shearing force at the interface between the pressure-sensitive adhesive layer  31  and the adherend. In the embodiment, the pressure-sensitive adhesive layer  31  is configured to be reduced in adhesive strength to the adherend, using the shearing force occurring as above at the interface with the adherend. The pressure-sensitive adhesive layer  31  as above contains the electromagnetic radiation-curable component in a content of typically 5 to 500 parts by mass, preferably 10 to 200 parts by mass, and more preferably 10 to 100 parts by mass, per 100 parts by mass of the pressure-sensitive adhesive polymer in the pressure-sensitive adhesive layer  31 . 
     In the above-mentioned configuration, the pressure-sensitive adhesive layer  31  contains an electromagnetic radiation-curable monomer and/or oligomer capable of allowing a curing reaction to initiate and to proceed by electromagnetic irradiation and renders electromagnetic radiation-induced reduction in adhesive strength feasible. Instead of, or in combination with, this configuration, the pressure-sensitive adhesive layer  31 , which serves as an electromagnetically releasable pressure-sensitive adhesive layer (adhesive strength-reducible layer) in the pressure-sensitive adhesive sheet X 3 , may employ any of configurations as follows: a configuration where the pressure-sensitive adhesive layer  31  contains a component, such as an azo compound or an azide compound, that is capable of undergoing a photocatalytic reaction upon electromagnetic irradiation, evolving a gas, and foaming, and renders electromagnetic radiation-induced reduction in adhesive strength feasible; and a configuration where the pressure-sensitive adhesive layer  31  contains a component, such as a liquid crystal, that responds to electromagnetic irradiation and changes in structure, and renders electromagnetic radiation-induced reduction in adhesive strength feasible. Alternatively, the pressure-sensitive adhesive layer  31  may employ configurations selected from these configurations in combination. These points relating to configurations of the pressure-sensitive adhesive layer  31  are also applicable to pressure-sensitive adhesive layers  31  of the after-mentioned modifications of the pressure-sensitive adhesive sheet X 3 . 
     The pressure-sensitive adhesive layer  32  of the pressure-sensitive adhesive sheet X 3  includes a pressure-sensitive adhesive as a base component and has an adhesive face  32   a . Non-limiting examples of the pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  32  include acrylic polymers as acrylic pressure-sensitive adhesives, natural rubbers, various synthetic rubbers, vinyl alkyl ether pressure-sensitive adhesives, silicone pressure-sensitive adhesives, polyester pressure-sensitive adhesives, polyamide pressure-sensitive adhesives, and urethane pressure-sensitive adhesives. The pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  32  is preferably an acrylic pressure-sensitive adhesive or adhesives. This is preferred from the viewpoint of actually providing satisfactory adhesive strength, cost minimization, and high productivity for the pressure-sensitive adhesive layer  32 . The pressure-sensitive adhesive layer  32  may have a multilayer structure including pressure-sensitive adhesive layers differing from each other in chemical composition. 
     The pressure-sensitive adhesive sheet X 3  is to be used for the build stage  100  of the additive manufacturing apparatus for the formation of an object, typically as illustrated in  FIGS. 2( a ), 2( b ), and 2( c ) , as with the pressure-sensitive adhesive sheet X 1 . Specifically, the pressure-sensitive adhesive sheet X 3  is affixed to the object-forming surface  101  of the build stage  100  before the start of object formation, as illustrated in  FIG. 2( a ) . The pressure-sensitive adhesive sheet X 3  is applied, through one of pressure-sensitive adhesive layers thereof, to the object-forming surface  101 . Specifically, of the pressure-sensitive adhesive sheet X 3 , one surface defined by the pressure-sensitive adhesive layer  31 , or the other surface defined by the pressure-sensitive adhesive layer  32 , adheres to the object-forming surface  101 . The target object W is gradually formed or built up on the pressure-sensitive adhesive sheet X 3  affixed to the build stage  100  during an object forming process with the apparatus being operated, as illustrated in  FIG. 2( b ) . In the object forming process as above, the object W during its formation is in a state of being attached to the build stage  100  through the pressure-sensitive adhesive sheet X 3 . After the completion of the object forming process as illustrated in  FIG. 2( c ) , an adhesive strength-reducing action is performed to the pressure-sensitive adhesive sheet X 3  on the build stage  100 . In the embodiment, the adhesive strength-reducing action is electromagnetic irradiation of the pressure-sensitive adhesive layer  31 , which is an adhesive strength-reducible layer of the pressure-sensitive adhesive sheet X 3 . The electromagnetic radiation of the pressure-sensitive adhesive layer  31  induces a chemical reaction and allows the chemical reaction to proceed in the pressure-sensitive adhesive layer  31  to thereby cure the pressure-sensitive adhesive layer  31 . The cured pressure-sensitive adhesive layer  31  contracts and gives shearing force at the interface between the pressure-sensitive adhesive layer  31  and an adherend thereof. This reduces the adhesive strength of the pressure-sensitive adhesive layer  31  to the adherend, and frees or loosens the attached state of the object W to the build stage  100  through the pressure-sensitive adhesive sheet X 3 . This results in facilitating the detachment of the object W from the build stage  100 . Assume that the pressure-sensitive adhesive sheet X 3  is applied, at the surface defined by the pressure-sensitive adhesive layer  31 , onto the build stage  100  before the start of object formation. In this case, the object W can be detached from the build stage  100  with an operation of removing the pressure-sensitive adhesive sheet X 3  from the build stage  100 , typically at the adhesive face  31   a  of the pressure-sensitive adhesive layer  31  whose adhesive strength has been reduced by electromagnetic irradiation. Also assume that the pressure-sensitive adhesive sheet X 3  is applied, at the surface defined by the pressure-sensitive adhesive layer  32 , onto the build stage  100  before the start of object formation. In this case, the object W is formed on the pressure-sensitive adhesive layer  31  of the pressure-sensitive adhesive sheet X 3 ; and the formed object W can be detached from the build stage  100  by pulling off the object W from the pressure-sensitive adhesive layer  31  whose adhesive strength has been reduced by electromagnetic irradiation. 
     As described above, the pressure-sensitive adhesive sheet X 3  for build stage use is suitable for allowing the object W, which is to be formed over the build stage  100  of the additive manufacturing apparatus, to be attached to the build stage  100  during building up, and to be readily detached from the build stage  100  after building up. In addition, the pressure-sensitive adhesive sheet X 3  as above, when used by the procedure as described above in the formation of the object W using the additive manufacturing apparatus, can avoid or minimize the use of a scraper or another tool having a sharp edge for the detachment of the object W from the build stage  100 . Accordingly, the use of the pressure-sensitive adhesive sheet X 3  for build stage use contributes to avoiding or minimizing significant damage on the object W and the build stage  100  upon detachment of the object W from the build stage  100 . 
       FIG. 7( a )  illustrates a pressure-sensitive adhesive sheet X 3   a , which is a modification of the pressure-sensitive adhesive sheet X 3  illustrated in  FIG. 6 . The pressure-sensitive adhesive sheet X 3   a  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 1  and pressure-sensitive adhesive layers  31  and  31 , and differs from the pressure-sensitive adhesive sheet X 3  in including the electromagnetically releasable pressure-sensitive adhesive layer  31 , instead of the pressure-sensitive adhesive layer  32 .  FIG. 7( b )  illustrates a pressure-sensitive adhesive sheet X 3   b , which is a modification of the pressure-sensitive adhesive sheet X 3 . The pressure-sensitive adhesive sheet X 3   b  is a single-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 1  and a pressure-sensitive adhesive layer  31 , and differs from the pressure-sensitive adhesive sheet X 3  in including no pressure-sensitive adhesive layer  32 . The pressure-sensitive adhesive sheet X 3   b  as above is used by allowing the surface defined by the pressure-sensitive adhesive layer  31  to adhere to the object-forming surface  101  of the build stage  100 . Assume that the object to be additively manufactured by the additive manufacturing apparatus is capable of being attached, at its end face facing the stage, to the object-forming surface typically through fusion bonding. In this case, the pressure-sensitive adhesive sheet X 3   b , which includes no pressure-sensitive adhesive layer on the side facing the object, may be employed.  FIG. 7( c )  illustrates a pressure-sensitive adhesive sheet X 3   c , which is a modification of the pressure-sensitive adhesive sheet X 3 . The pressure-sensitive adhesive sheet X 3   c  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including pressure-sensitive adhesive layers  31  and  32 , and differs from the pressure-sensitive adhesive sheet X 3  in including no substrate S 1 . The pressure-sensitive adhesive sheet X 3   c  as above is used by allowing the surface defined by the pressure-sensitive adhesive layer  31  or the surface defined by the pressure-sensitive adhesive layer  32  to adhere to the object-forming surface  101  of the build stage  100 . Sufficient breaking strength as a pressure-sensitive adhesive sheet for build stage use may be surely provided even without the substrate S 1 , in some set thickness and/or hardness of each of the pressure-sensitive adhesive layer  31  and the pressure-sensitive adhesive layer  32 ; and in this case, the configuration of including no substrate S 1  may be employed.  FIG. 7( d )  illustrates a pressure-sensitive adhesive sheet X 3   d , which is a modification of the pressure-sensitive adhesive sheet X 3 . The pressure-sensitive adhesive sheet X 3   d  is a double-sided pressure-sensitive adhesive sheet including an electromagnetically releasable pressure-sensitive adhesive layer  31  and differs from the pressure-sensitive adhesive sheet X 3  in including neither of the substrate S 1  and the pressure-sensitive adhesive layer  32 . Sufficient breaking strength as a pressure-sensitive adhesive sheet for build stage use may be surely provided even without the substrate S 1  and the pressure-sensitive adhesive layer  32 , in some set thickness and/or hardness of the pressure-sensitive adhesive layer  31 ; and in this case, the configuration of including neither of the substrate S 1  and the pressure-sensitive adhesive layer  32  may be employed. These pressure-sensitive adhesive sheets X 3   a , X 3   b , X 3   c , and X 3   d  for build stage use are each usable by a procedure similar to that described above for the pressure-sensitive adhesive sheet X 3 . Accordingly, these are suitable for allowing the object W, which is to be formed over the build stage  100  of the additive manufacturing apparatus, to be attached to the build stage  100  during building up, and to be readily detached from the build stage  100  after building up. 
       FIG. 8  is a local sectional view of a pressure-sensitive adhesive sheet X 4 , which is a pressure-sensitive adhesive sheet for build stage use according to an embodiment of the present invention. The pressure-sensitive adhesive sheet X 4  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 2  and pressure-sensitive adhesive layers  41  and  42 . 
     The substrate S 2  is a portion that function as a carrier (support) in the pressure-sensitive adhesive sheet X 4  and also functions as one terminal for voltage application described later. The substrate S 2  typically has a multilayer structure including a base material, and a conductive layer (electroconductive layer) disposed on the base material. Non-limiting examples of the base material include plastic substrates, fibrous substrates, and paper substrates. The conductive layer is a layer having conductivity (electrical conductivity) and is made typically from or of a metal or a conductive polymer. The conductive layer has a thickness of typically 10 to 1000 μm. The conductive layer as above can be formed on the base material by a technique of metal plating, chemical vapor deposition, or sputtering. The pressure-sensitive adhesive layer  41  adheres to, of the substrate S 2 , the surface defined by the conductive layer and is electrically coupled to the conductive layer. The substrate S 2  in the embodiment includes an extending portion that extends in an in-plane direction of the substrate S 2  and is exposed from (protruded from) the pressure-sensitive adhesive layer  41 . The configuration as above actually facilitates electrical coupling, through the extending portion, between one of terminals of a voltage application device or direct-current power supply device and the substrate S 2  or the conductive layer thereof. Instead of such a substrate having a multilayer structure including a base material and a conductive layer, the substrate S 2  in the embodiment may be a conductive substrate. The substrate S 2  just mentioned above may be made typically of or from a metal or a conductive polymer. The substrate S 2 , which has any of the configurations as above, has a thickness of typically 10 to 1000 μm, preferably 30 to 500 μm, and more preferably 50 to 300 μm. 
     The pressure-sensitive adhesive layer  41  of the pressure-sensitive adhesive sheet X 4  is an electrically releasable pressure-sensitive adhesive layer (adhesive strength-reducible layer) having an adhesive strength capable of being reduced by voltage application (adhesive strength-reducing action). The pressure-sensitive adhesive layer  41  has an adhesive face  41   a . The pressure-sensitive adhesive layer  41  typically contains a pressure-sensitive adhesive and an electrolyte. Assume that a direct-current voltage is applied to the pressure-sensitive adhesive layer  41  so as to cause potential difference in a thickness direction of the pressure-sensitive adhesive layer  41 . In this case, the electrolyte in the pressure-sensitive adhesive layer  41  undergoes orientation modulation and/or migration within a thickness direction of the layer, and this may change the chemical composition of the surface of the pressure-sensitive adhesive layer  41 . Assume that the pressure-sensitive adhesive layer  41  receives the applied voltage while the adhesive face  41   a  adheres to an adherend. In this case, the adhesive strength of the adhesive face  41   a  can be reduced using the chemical composition change occurring at the adhesive face  41   a.    
     Non-limiting examples of the pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  41  include acrylic polymers as acrylic pressure-sensitive adhesives, rubber pressure-sensitive adhesives, vinyl alkyl ether pressure-sensitive adhesives, silicone pressure-sensitive adhesives, polyester pressure-sensitive adhesives, polyamide pressure-sensitive adhesives, urethane pressure-sensitive adhesives, fluorocarbon pressure-sensitive adhesives, and epoxy pressure-sensitive adhesives. The pressure-sensitive adhesive layer  41  may include each of different pressure-sensitive adhesives alone or in combination. The pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  41  is preferably an acrylic pressure-sensitive adhesive or adhesives, from the viewpoint of actually providing cost minimization and high productivity. The chemical composition and polymerization technique of the acrylic polymers as acrylic pressure-sensitive adhesives are as with those described above for the acrylic polymer as an acrylic pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  11  of the pressure-sensitive adhesive sheet X 1 . 
     The electrolyte contained in the pressure-sensitive adhesive layer  41  is a substance that can be ionized into an anion and a cation. Non-limiting examples of such electrolyte include ionic liquids, alkali metal salts, and alkaline earth metal salts. The electrolyte contained in the pressure-sensitive adhesive layer  41  is preferably selected from ionic liquids, from the viewpoint of allowing the pressure-sensitive adhesive layer  41  to actually have satisfactory electric releasability (electrically induced releasability). Such an ionic liquid is a salt that is liquid at room temperature (about 25° C.) and includes an anion and a cation. The ionic liquid, when contained in the pressure-sensitive adhesive layer  41 , may include, as the anion, at least one selected from the group consisting typically of (CF 3 SO 2 ) 2 N − , (CF 3 CF 2 SO 2 ) 2 N − , (CF 3 SO 2 ) 3 C − , Br − , AlCl 4   − , Al 2 Cl 7   − , NO 3   − , BF 4   − , PF 6   − , CH 3 COO − , CF 3 COO − , CF 3 CF 2 CF 2 COO − , CF 3 SO 3   − , CF 3 (CF 2 ) 3 SO 3   − , AsF 6   − , and SbF 6   − . The ionic liquid, when contained in the pressure-sensitive adhesive layer  41 , may include, as the cation, at least one selected from the group consisting typically of imidazolium cations, pyridinium cations, pyrrolidinium cations, and ammonium cations. Non-limiting examples of commercial products of the ionic liquid to be contained in the pressure-sensitive adhesive layer  41  include IL-A series, IL-P series, IL-C series, IL-IM series, and IL-AP series, each supplied by KOEI CHEMICAL COMPANY, LIMITED. 
     The electrolyte may be present in the pressure-sensitive adhesive layer  41  in a content of typically 0.1 mass percent or more, so as to impart electric releasability to the pressure-sensitive adhesive layer  41 . The content of the electrolyte in the pressure-sensitive adhesive layer  41  is preferably 0.5 part by mass or more, more preferably 0.6 part by mass or more, furthermore preferably 0.8 part by mass or more, still more preferably 1.0 part by mass or more, and still furthermore preferably 1.5 parts by mass or more, per 100 parts by mass of the pressure-sensitive adhesive in the pressure-sensitive adhesive layer  41 . These are preferred from the viewpoint of allowing the pressure-sensitive adhesive layer  41  to actually have satisfactory electric releasability. The content of the electrolyte in the pressure-sensitive adhesive layer  41  is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, furthermore preferably 15 parts by mass or less, still more preferably 10 parts by mass or less, and still furthermore preferably 5 parts by mass or less, per 100 parts by mass of the pressure-sensitive adhesive in the pressure-sensitive adhesive layer  41 . These are preferred from the viewpoint of allowing the pressure-sensitive adhesive layer  41  to actually have adhesive strength and electric releasability in good balance at satisfactory levels. 
     In the above-mentioned configuration, the pressure-sensitive adhesive layer  41  contains an electrolyte capable of being unevenly distributed by the application of a direct-current voltage, in addition to a pressure-sensitive adhesive polymer, and renders voltage application-induced reduction in adhesive strength feasible. Instead of, or in combination with, this configuration, the pressure-sensitive adhesive layer  41 , which is an electrically releasable pressure-sensitive adhesive layer (adhesive strength-reducible layer) in the pressure-sensitive adhesive sheet X 4 , may employ any of configurations as follows: a configuration where the pressure-sensitive adhesive layer  41  contains charged particles and/or molecules capable of undergoing electrophoresis by the application of a direct-current voltage, in addition to a pressure-sensitive adhesive polymer and renders voltage application-induced reduction in adhesive strength feasible; and a configuration where pressure-sensitive adhesive layer  41  contains an electro-sensitive low-molecular-weight liquid crystal component in addition to a pressure-sensitive adhesive polymer, is thereby capable of undergoing phase separation by the application of an alternating voltage, and renders voltage application-induced reduction in adhesive strength feasible. Alternatively, the pressure-sensitive adhesive layer  41  may employ configurations selected from these configurations in combination. These points relating to configurations of the pressure-sensitive adhesive layer  41  are also applicable to pressure-sensitive adhesive layers  41  in the after-mentioned modifications of the pressure-sensitive adhesive sheet X 4 . 
     The pressure-sensitive adhesive layer  42  of the pressure-sensitive adhesive sheet X 4  includes a pressure-sensitive adhesive as a base component and has an adhesive face  42   a . Non-limiting examples of the pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  42  include acrylic polymers as acrylic pressure-sensitive adhesives, natural rubbers, various synthetic rubbers, vinyl alkyl ether pressure-sensitive adhesives, silicone pressure-sensitive adhesives, polyester pressure-sensitive adhesives, polyamide pressure-sensitive adhesives, and urethane pressure-sensitive adhesives. The pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  42  is preferably an acrylic pressure-sensitive adhesive or adhesives. This is preferred from the viewpoint of actually providing satisfactory adhesive strength, cost minimization, and high productivity for the pressure-sensitive adhesive layer  42 . The pressure-sensitive adhesive layer  42  may have a multilayer structure including pressure-sensitive adhesive layers differing from each other in chemical composition. 
     The pressure-sensitive adhesive sheet X 4  is to be used for the build stage  100  of the additive manufacturing apparatus for the formation of an object, typically as illustrated in  FIGS. 2( a ), 2( b ), and 2( c ) , as with the pressure-sensitive adhesive sheet X 1 . Specifically, the pressure-sensitive adhesive sheet X 4  is affixed to the object-forming surface  101  of the build stage  100  before the start of object formation, as illustrated in  FIG. 2( a ) . In the embodiment, at least one of the object-forming surface  101  and the object has conductivity. The build stage  100  can have an object-forming surface  101  that is conductive, typically by securing a conductive plate made from a conductive material to the surface of a build stage body made from a non-conductive material, or by employing a build stage body made from a conductive material. An object having conductivity can be formed typically when a modeling material (object-forming material) to be fed to the build stage  100  in an object forming process is a resin composition containing a conductive filler. The pressure-sensitive adhesive sheet X 4  is applied, through a pressure-sensitive adhesive layer thereof, to the object-forming surface  101  before the start of object formation. For example, when the object-forming surface  101  has conductivity, the pressure-sensitive adhesive layer  41 , which is electrically releasable, adheres to the object-forming surface  101 ; whereas, when the object has conductivity, the pressure-sensitive adhesive layer  42  adheres to the object-forming surface  101 . During an object forming process with the apparatus being operated, the target object W is gradually formed on the pressure-sensitive adhesive sheet X 4  affixed to the build stage  100 , as illustrated in  FIG. 2( b ) . In the object forming process as above, the object W during its formation is in a state of being attached to the build stage  100  through the pressure-sensitive adhesive sheet X 4 . After the completion of the object forming process as illustrated in  FIG. 2( c ) , an adhesive strength-reducing action is performed to the pressure-sensitive adhesive sheet X 4  on the build stage  100 . The adhesive strength-reducing action in the embodiment is application of a voltage to the pressure-sensitive adhesive layer  41 , which is an adhesive strength-reducible layer of the pressure-sensitive adhesive sheet X 4 . Specifically, the voltage is applied to the pressure-sensitive adhesive layer  41  so as to give a potential difference in a thickness direction of the layer. Assume that the object-forming surface  101  has conductivity. In this case, the voltage application to the pressure-sensitive adhesive layer  41  can be performed after electrically coupling a pair of terminals of a voltage application device or direct-current power supply device respectively to the substrate S 2 , or the conductive layer thereof, of the pressure-sensitive adhesive sheet X 4 , and the object-forming surface  101  of the build stage  100 . Assume that the object has conductivity. In this case, the voltage application to the pressure-sensitive adhesive layer  41  can be performed after electrically coupling a pair of terminals of a voltage application device or direct-current power supply device respectively to the substrate S 2 , or the conductive layer thereof, of the pressure-sensitive adhesive sheet X 4 , and the object. These voltage applications are performed typically by bringing a positive electrode terminal of the voltage application device into contact with the substrate S 2  or the conductive layer thereof; and bringing a negative electrode terminal into contact with the object-forming surface  101  having conductivity or the object having conductivity, each of which serves as the other conductive part (assume that the voltage application can be performed to an electrically releasable pressure-sensitive adhesive layer  41  through an object-forming surface  101  having conductivity and an object having conductivity, in the after-mentioned modifications of the pressure-sensitive adhesive sheet X 4 ; in this case, the voltage application may be performed after electrically coupling a pair of terminals of a voltage application device respectively to the object-forming surface  101  and the object). The voltage application as above gives a potential difference in a thickness direction of the pressure-sensitive adhesive layer  41 , and causes the electrolyte in the pressure-sensitive adhesive layer  41  to change in orientation and/or to migrate in a thickness direction of the layer to change the chemical composition of the surface (electrically releasable adhesive face) of the pressure-sensitive adhesive layer  41 . This reduces the adhesive strength of the surface of the pressure-sensitive adhesive layer  41 . Then, when the pressure-sensitive adhesive layer  41  contains the ionic liquid as the electrolyte, the cation and the anion of the ionic liquid in the pressure-sensitive adhesive layer  41  undergoes orientation modulation and/or migration to a thickness direction of the layer. The cation migrates toward the negative electrode, which has a lower potential; and the cation migrates toward the positive electrode, which has a higher potential. In the ionic liquid, the cation tends to have a higher diffusion coefficient and to migrate faster as compared with the anion. As a result of the voltage application in the direction to the pressure-sensitive adhesive layer  41  containing the ionic liquid, in the pressure-sensitive adhesive layer  41 , the chemical composition change (cation uneven distribution) at the surface facing the negative electrode precedes the chemical composition change (anion uneven distribution) at the surface facing the positive electrode. Accordingly, in the pressure-sensitive adhesive layer  41 , the adhesive strength to the object-forming surface  101  is precedently significantly reduced. With an increasing voltage application time, the adhesive strength of the pressure-sensitive adhesive layer  41  or the adhesive face  41   a  thereof tend to be reduced. The voltage application to the pressure-sensitive adhesive layer  41  may be performed at a voltage of preferably 1 to 100 V for a time of typically 60 seconds or shorter. The voltage application as above, which is an adhesive strength-reducing action, frees or loosens the attached state of the object W to the build stage  100  through the pressure-sensitive adhesive sheet X 4 . This results in facilitating the detachment of the object W from the build stage  100 . Assume that the pressure-sensitive adhesive sheet X 4  is applied, at the surface defined by the pressure-sensitive adhesive layer  41 , onto the object-forming surface  101  having conductivity before the start of object formation. In this case, the object W can be detached from the build stage  100  with an operation of removing the pressure-sensitive adhesive sheet X 4  from the build stage  100 , typically at the adhesive face  41   a  of the pressure-sensitive adhesive layer  41  whose adhesive strength has been reduced by the voltage application. Also assume that the pressure-sensitive adhesive sheet X 4  is applied, at the surface defined by the pressure-sensitive adhesive layer  42 , to the object-forming surface  101  before the start of object formation; and an object W having conductivity is then formed on the pressure-sensitive adhesive layer  41  of the pressure-sensitive adhesive sheet X 4 . In this case, the object W can be detached from the build stage  100  by pulling off the object W from the pressure-sensitive adhesive layer  41  whose adhesive strength has been reduced by the voltage application. 
     As described above, the pressure-sensitive adhesive sheet X 4  for build stage use is suitable for allowing the object W, which is to be formed over the build stage  100  of the additive manufacturing apparatus, to be attached to the build stage  100  during building up, and to be readily detached from the build stage  100  after building up. In addition, the pressure-sensitive adhesive sheet X 4  as above, when used by the procedure as described above in the formation of the object W using the additive manufacturing apparatus, can avoid or minimize the use of a scraper or another tool having a sharp edge for the detachment of the object W from the build stage  100 . Accordingly, the use of the pressure-sensitive adhesive sheet X 4  for build stage use contributes to avoiding or minimizing significant damage on the object W and the build stage  100  upon detachment of the object W from the build stage  100 . 
       FIG. 9( a )  illustrates a pressure-sensitive adhesive sheet X 4   a , which is a modification of the pressure-sensitive adhesive sheet X 4  for build stage use illustrated in  FIG. 8 . The pressure-sensitive adhesive sheet X 4   a  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 2  and pressure-sensitive adhesive layers  41  and  41 , and differs from the pressure-sensitive adhesive sheet X 4  in including the pressure-sensitive adhesive layer  41 , which is electrically releasable, instead of the pressure-sensitive adhesive layer  42 .  FIG. 9( b )  illustrates a pressure-sensitive adhesive sheet X 4   b , which is a modification of the pressure-sensitive adhesive sheet X 4  for build stage use. The pressure-sensitive adhesive sheet X 4   b  is a single-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 2  and a pressure-sensitive adhesive layer  41 , and differs from the pressure-sensitive adhesive sheet X 4  in including no pressure-sensitive adhesive layer  42 . The pressure-sensitive adhesive sheet X 4   b  as above is used by allowing the surface defined by the pressure-sensitive adhesive layer  41  to adhere to the object-forming surface  101  of the build stage  100 . The object-forming surface  101  of the build stage  100  for use with the pressure-sensitive adhesive sheet X 4   b  has conductivity. Specifically, assume that the object to be additively manufactured by the additive manufacturing apparatus is capable of being attached, at its end face facing the stage, to the object-forming surface typically through fusion bonding; and in this case, the pressure-sensitive adhesive sheet X 4   b , which includes no pressure-sensitive adhesive layer on the side facing the object, may be employed.  FIG. 9( c )  illustrates a pressure-sensitive adhesive sheet X 4   c , which is a modification of the pressure-sensitive adhesive sheet X 4  for build stage use. The pressure-sensitive adhesive sheet X 4   c  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including pressure-sensitive adhesive layers  41  and  42  and differs from the pressure-sensitive adhesive sheet X 4  in including no substrate S 2 . The pressure-sensitive adhesive sheet X 4   c  as above is used by allowing the surface defined by the pressure-sensitive adhesive layer  41  to adhere to the object-forming surface  101  of the build stage  100 . In this case, the object-forming surface  101  has conductivity. Alternatively, the pressure-sensitive adhesive sheet X 4   c  is used by allowing the surface defined by the pressure-sensitive adhesive layer  42  to adhere to the object-forming surface  101 . In this case, the object W to be formed on the pressure-sensitive adhesive sheet X 4   c  or the pressure-sensitive adhesive layer  41  has conductivity. Sufficient breaking strength as a pressure-sensitive adhesive sheet for build stage use may be surely provided even without the substrate S 2 , in some set thickness and/or hardness of each of the pressure-sensitive adhesive layer  41  and the pressure-sensitive adhesive layer  42 ; and in this case, the configuration of including no substrate S 2  may be employed.  FIG. 9( d )  illustrates a pressure-sensitive adhesive sheet X 4   d , which is a modification of the pressure-sensitive adhesive sheet X 4  for build stage use. The pressure-sensitive adhesive sheet X 4   d  is a double-sided pressure-sensitive adhesive sheet including a pressure-sensitive adhesive layer  41 , which is electrically releasable. The pressure-sensitive adhesive sheet X 4   d  differs from the pressure-sensitive adhesive sheet X 4  in including neither of the substrate S 2  and the pressure-sensitive adhesive layer  42 . The object-forming surface  101  of the build stage  100  for use with the pressure-sensitive adhesive sheet X 4   d  has conductivity, and the object W to be formed on the pressure-sensitive adhesive sheet X 4   d  or the pressure-sensitive adhesive layer  41  also has conductivity. Sufficient breaking strength as a pressure-sensitive adhesive sheet for build stage use may be surely provided even without the substrate S 2  and the pressure-sensitive adhesive layer  42 , in some set thickness and/or hardness of the pressure-sensitive adhesive layer  41 ; and, in this case, the configuration of including neither of the substrate S 2  and the pressure-sensitive adhesive layer  42  may be employed. These pressure-sensitive adhesive sheets X 4   a , X 4   b , X 4   c , and X 4   d  for build stage use are each usable by a procedure similar to that described above for the pressure-sensitive adhesive sheet X 4 . Accordingly, these are suitable for allowing the object W, which is to be formed over the build stage  100  of the additive manufacturing apparatus, to be attached to the build stage  100  during building up, and to be readily detached from the build stage  100  after building up. 
       FIG. 10  is a local sectional view of a pressure-sensitive adhesive sheet X 5 , which is a pressure-sensitive adhesive sheet for build stage use according to an embodiment of the present invention. The pressure-sensitive adhesive sheet X 5  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 1  and pressure-sensitive adhesive layers  51  and  52 . 
     The pressure-sensitive adhesive layer  51  of the pressure-sensitive adhesive sheet X 5  is a liquid-releasable pressure-sensitive adhesive layer (adhesive strength-reducible layer) having an adhesive strength capable of being reduced by liquid supply (adhesive strength-reducing action). The pressure-sensitive adhesive layer  51  has an adhesive face  51   a . In the embodiment, the pressure-sensitive adhesive layer  51 , which is a liquid-releasable pressure-sensitive adhesive layer, includes a pressure-sensitive adhesive having high affinity for a liquid to be used in the adhesive strength-reducing action. When the liquid to be used in the adhesive strength-reducing action is water or an aqueous solution, the pressure-sensitive adhesive layer  51  includes a water-soluble pressure-sensitive adhesive. Examples of the water-soluble pressure-sensitive adhesive include, but are not limited to, water-soluble acrylic pressure-sensitive adhesives and water-soluble polyvinyl ether pressure-sensitive adhesives. Non-limiting examples of the water-soluble acrylic pressure-sensitive adhesives include acrylic pressure-sensitive adhesives each resulting from copolymerization among a (meth)acrylic ester, a carboxy-containing (meth)acrylic monomer, and a basic monomer (such as N-vinylpyrrolidone). Non-limiting examples of the water-soluble polyvinyl ether pressure-sensitive adhesives include polyvinyl ether pressure-sensitive adhesives each including a monomer unit derived from any of vinyl ethyl ether, vinyl butyl ether, and vinyl isobutyl ether. Assume that the pressure-sensitive adhesive layer  51  includes a water-soluble pressure-sensitive adhesive. In this case, when water is supplied to the pressure-sensitive adhesive layer  51  with the adhesive face  51   a  adhering to an adherend, at least part of the water-soluble pressure-sensitive adhesive in the pressure-sensitive adhesive layer  51  is dissolved in and/or swollen by, the water. This causes volume change in the pressure-sensitive adhesive layer  51 , impairs the adhesive face  51   a , and reduces the adhesive strength of the pressure-sensitive adhesive layer  51  to the adherend. 
     In the above-mentioned configuration, the pressure-sensitive adhesive layer  51  has such a chemical composition as to allow part of the component of the pressure-sensitive adhesive layer to be dissolved in, and/or swollen by, liquid supply and renders liquid supply-induced reduction in adhesive strength feasible. Instead of, or in combination with, this configuration, the pressure-sensitive adhesive layer  51 , which serves as a liquid-releasable pressure-sensitive adhesive layer (adhesive strength-reducible layer) in the pressure-sensitive adhesive sheet X 5 , may have a configuration where the pressure-sensitive adhesive layer  51  has such a chemical composition as to cause volume change through drying after the liquid supply and renders liquid supply-induced reduction in adhesive strength feasible. Alternatively, the pressure-sensitive adhesive layer  51  may employ these configurations in combination. These points relating to configurations of the pressure-sensitive adhesive layer  51  are applicable also to pressure-sensitive adhesive layers  51  of the after-mentioned modifications of the pressure-sensitive adhesive sheet X 5 . 
     The pressure-sensitive adhesive layer  52  of the pressure-sensitive adhesive sheet X 5  includes a pressure-sensitive adhesive as a base component and has an adhesive face  52   a . Non-limiting examples of the pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  52  include acrylic polymers as acrylic pressure-sensitive adhesives, natural rubbers, various synthetic rubbers, vinyl alkyl ether pressure-sensitive adhesives, silicone pressure-sensitive adhesives, polyester pressure-sensitive adhesives, polyamide pressure-sensitive adhesives, and urethane pressure-sensitive adhesives. The pressure-sensitive adhesive contained in the pressure-sensitive adhesive layer  52  is preferably an acrylic pressure-sensitive adhesive or adhesives. This is preferred from the viewpoint of actually providing satisfactory adhesive strength, cost minimization, and high productivity for the pressure-sensitive adhesive layer  52 . The pressure-sensitive adhesive layer  52  may have a multilayer structure including pressure-sensitive adhesive layers differing from each other in chemical composition. 
     The pressure-sensitive adhesive sheet X 5  is to be used for the build stage  100  of the additive manufacturing apparatus for the formation of an object, typically as illustrated in  FIGS. 2( a ), 2( b ), and 2( c ) , as with the pressure-sensitive adhesive sheet X 1 . Specifically, the pressure-sensitive adhesive sheet X 5  is affixed to the object-forming surface  101  of the build stage  100  before the start of object formation, as illustrated in  FIG. 2( a ) . The pressure-sensitive adhesive sheet X 5  is applied, through a pressure-sensitive adhesive layer thereof, to the object-forming surface  101 . Specifically, of the pressure-sensitive adhesive sheet X 5 , the surface defined by the pressure-sensitive adhesive layer  51 , or the surface defined by the pressure-sensitive adhesive layer  52 , adheres to the object-forming surface  101 . During an object forming process with the apparatus being operated, the target object W is gradually formed or built up on the pressure-sensitive adhesive sheet X 5  affixed to the build stage  100 , as illustrated in  FIG. 2( b ) . In the object forming process as above, the object W during its formation is in a state of being attached to the build stage  100  through the pressure-sensitive adhesive sheet X 5 . After the completion of the object forming process as illustrated in  FIG. 2( c ) , an adhesive strength-reducing action is performed to the pressure-sensitive adhesive sheet X 5  on the build stage  100 . The adhesive strength-reducing action in the embodiment is supply of a liquid to the pressure-sensitive adhesive layer  51 , which is an adhesive strength-reducible layer of the pressure-sensitive adhesive sheet X 5 . When a liquid is supplied to the pressure-sensitive adhesive layer  51 , at least part of the pressure-sensitive adhesive in the pressure-sensitive adhesive layer  51  is dissolved in, and/or swollen by, the liquid, to cause volume change of the pressure-sensitive adhesive layer  51 . This impairs the adhesive face  51   a , reduces the adhesive strength of the pressure-sensitive adhesive layer  51 , and thereby frees or loosens the attached state of the object W to the build stage  100  through the pressure-sensitive adhesive sheet X 5 . This results in facilitating the detachment of the object W from the build stage  100 . Assume that the pressure-sensitive adhesive sheet X 5  is applied, at the surface defined by the pressure-sensitive adhesive layer  51 , onto the build stage  100  before the start of object formation. In this case, the object W can be detached from the build stage  100  with an operation of removing the pressure-sensitive adhesive sheet X 5  from the build stage  100  typically at the adhesive face  51   a  of the pressure-sensitive adhesive layer  51  whose adhesive strength has been reduced by the liquid supply. Assume that the pressure-sensitive adhesive sheet X 5  is applied, at the surface defined by the pressure-sensitive adhesive layer  52 , onto the build stage  100  before the start of object formation. In this case, the object W is formed on the pressure-sensitive adhesive layer  51  of the pressure-sensitive adhesive sheet X 5 ; and the formed object W can be detached from the build stage  100  by pulling off the object W from the pressure-sensitive adhesive layer  51  whose adhesive strength has been reduced by the liquid supply. 
     As described above, the pressure-sensitive adhesive sheet X 5  for build stage use is suitable for allowing the object W, which is to be formed over the build stage  100  of the additive manufacturing apparatus, to be attached to the build stage  100  during building up, and to be readily detached from the build stage  100  after building up. In addition, the pressure-sensitive adhesive sheet X 5  as above, when used by the procedure as described above in the formation of the object W using the additive manufacturing apparatus, can avoid or minimize the use of a scraper or another tool having a sharp edge for the detachment of the object W from the build stage  100 . Accordingly, the use of the pressure-sensitive adhesive sheet X 5  for build stage use contributes to avoiding or minimizing significant damage on the object W and the build stage  100  upon detachment of the object W from the build stage  100 . 
       FIG. 11( a )  illustrates a pressure-sensitive adhesive sheet X 5   a , which is a modification of the pressure-sensitive adhesive sheet X 5  for build stage use illustrated in  FIG. 10 . The pressure-sensitive adhesive sheet X 5   a  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 1  and pressure-sensitive adhesive layers  51  and  51 , and differs from the pressure-sensitive adhesive sheet X 5  in including the pressure-sensitive adhesive layer  51 , which is releasable by liquid supply, instead of the pressure-sensitive adhesive layer  52 .  FIG. 11( b )  illustrates a pressure-sensitive adhesive sheet X 5   b , which is a modification of the pressure-sensitive adhesive sheet X 5  for build stage use. The pressure-sensitive adhesive sheet X 5   b  is a single-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 1  and a pressure-sensitive adhesive layer  51  and differs from the pressure-sensitive adhesive sheet X 5  in including no pressure-sensitive adhesive layer  52 . The pressure-sensitive adhesive sheet X 5   b  as above is used by allowing the surface defined by the pressure-sensitive adhesive layer  51  to adhere to the object-forming surface  101  of the build stage  100 . Assume that the object to be additively manufactured by the additive manufacturing apparatus is capable of being attached, at its end face facing the stage, to the object-forming surface typically through fusion bonding. In this case, the pressure-sensitive adhesive sheet X 5   b , which includes no pressure-sensitive adhesive layer on the side facing the object, may be employed.  FIG. 11( c )  illustrates a pressure-sensitive adhesive sheet X 5   c , which is a modification of the pressure-sensitive adhesive sheet X 5  for build stage use. The pressure-sensitive adhesive sheet X 5   c  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including pressure-sensitive adhesive layers  51  and  52 , and differs from the pressure-sensitive adhesive sheet X 5  in including no substrate S 1 . The pressure-sensitive adhesive sheet X 5   c  as above is used by allowing the surface defined by the pressure-sensitive adhesive layer  51 , or the surface defined by the pressure-sensitive adhesive layer  52 , to adhere to the object-forming surface  101  of the build stage  100 . Sufficient breaking strength as a pressure-sensitive adhesive sheet for build stage use may be surely provided even without the substrate S 1 , in some set thickness and/or hardness of each of the pressure-sensitive adhesive layer  51  and the pressure-sensitive adhesive layer  52 ; and in this case, the configuration of including no substrate S 1  may be employed.  FIG. 11( d )  illustrates a pressure-sensitive adhesive sheet X 5   d , which is a modification of the pressure-sensitive adhesive sheet X 5  for build stage use. The pressure-sensitive adhesive sheet X 5   d  is a double-sided pressure-sensitive adhesive sheet including a pressure-sensitive adhesive layer  51 , which is releasable by liquid supply. The pressure-sensitive adhesive sheet X 5   d  differs from the pressure-sensitive adhesive sheet X 5  in including neither of the substrate S 1  and the pressure-sensitive adhesive layer  52 . Sufficient breaking strength as a pressure-sensitive adhesive sheet for build stage use may be surely provided even without the substrate S 1  and the pressure-sensitive adhesive layer  52 , in some set thickness and/or hardness of the pressure-sensitive adhesive layer  51 ; and in this case, the configuration of including neither of the substrate S 1  and the pressure-sensitive adhesive layer  52  may be employed. These pressure-sensitive adhesive sheets X 5   a , X 5   b , X 5   c , and X 5   d  for build stage use are each usable by a procedure similar to that described above for the pressure-sensitive adhesive sheet X 5 . Accordingly, these are suitable for allowing the object W, which is to be formed over the build stage  100  of the additive manufacturing apparatus, to be attached to the build stage  100  during building up, and to be readily detached from the build stage  100  after building up. 
       FIG. 12  is a local sectional view of a pressure-sensitive adhesive sheet X 6 , which is a pressure-sensitive adhesive sheet for build stage use according to an embodiment of the present invention. The pressure-sensitive adhesive sheet X 6  is a double-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 3  and pressure-sensitive adhesive layers  61  and  62 . 
     The substrate S 3  of the pressure-sensitive adhesive sheet X 6  is a portion that functions as a carrier (support) in the pressure-sensitive adhesive sheet X 6  and is a substrate in the form of a stretchable film (stretchable substrate). The film substrate S 3  as above has such stretchability or extensibility as to offer an elongation at break of typically 300% or more, preferably 500% or more, and more preferably 700% or more. The elongation at break can be measured in conformity to the Measuring Method of “Elongation” described in JIS K 7311:1995. Non-limiting examples of the film substrate S 3  include resin films, woven fabric films, and nonwoven fabric films. Non-limiting examples of a material to constitute the substrate S 3 , when being a resin film, include polyurethanes, polyolefins, polyesters, and polycarbonates. Non-limiting examples of the polyurethanes include ether polyurethanes, ester polyurethanes, and carbonate polyurethanes. Non-limiting examples of the polyolefins include polyethylenes, polypropylenes, and ethylene-propylene copolymers. Non-limiting examples of the polyesters include poly(ethylene terephthalate), poly(butylene terephthalate), poly(ethylene naphthalate), and poly(butylene naphthalate). Preferably, the substrate S 3  is selected from polyurethane resin films each including a polyurethane as a base component. Most of polyurethane resin films are made from materials offering approximately no yield point. The use of such a polyurethane resin film as the substrate S 3  is advantageous for actually providing a substrate, and consequently a pressure-sensitive adhesive sheet for build stage use, that has an elongation at break and/or breaking strength at relatively high levels. The substrate S 3  of the pressure-sensitive adhesive sheet X 6 , as described above, may be made from (formed from) each of different materials alone or in combination. The substrate S 3  may be a multilayer assembly including layers differing from each other in constituent material. Of the substrate S 3 , at least one of the surface facing the pressure-sensitive adhesive layer  61  and the surface facing the pressure-sensitive adhesive layer  62  has been subjected to a surface treatment for better adhesion with the adjacent pressure-sensitive adhesive layer. Non-limiting examples of such surface treatment include physical treatments such as corona treatment and plasma treatment; and chemical treatments such as primer coating. The substrate S 3  as above has a thickness of typically 10 to 300 μm. 
     The pressure-sensitive adhesive layers  61  and  62  of the pressure-sensitive adhesive sheet X 6  are stretching-releasable pressure-sensitive adhesive layers (adhesive strength-reducible layers) having an adhesive strength capable of being reduced by the stretching of the pressure-sensitive adhesive sheet X 6  (adhesive strength-reducing action). The pressure-sensitive adhesive layer  61  includes a pressure-sensitive adhesive as a base component and has an adhesive face  61   a  which adheres typically to the build stage. The pressure-sensitive adhesive layer  62  includes a pressure-sensitive adhesive as a base component and has an adhesive face  62   a . Non-limiting examples of the pressure-sensitive adhesives contained individually in the pressure-sensitive adhesive layers  61  and  62  include acrylic polymers as acrylic pressure-sensitive adhesives, natural rubbers, various synthetic rubbers, vinyl alkyl ether pressure-sensitive adhesives, silicone pressure-sensitive adhesives, polyester pressure-sensitive adhesives, polyamide pressure-sensitive adhesives, and urethane pressure-sensitive adhesives. The pressure-sensitive adhesives contained in the pressure-sensitive adhesive layers  61  and  62  are preferably selected from acrylic pressure-sensitive adhesives. This is preferred from the viewpoint of actually providing satisfactory adhesive strength, cost minimization, and high productivity for the pressure-sensitive adhesive layers  61  and  62 . At least one of the pressure-sensitive adhesive layer  61  and the pressure-sensitive adhesive layer  62  may have a multilayer structure including pressure-sensitive adhesive layers differing from each other in chemical composition. Assume that the pressure-sensitive adhesive sheet X 6  as above is stretched while the pressure-sensitive adhesive layer  61  or the adhesive face  61   a  in the pressure-sensitive adhesive sheet X 6  adheres to an adherend. This gives shearing force in the stretching direction at the interface between the pressure-sensitive adhesive layer  61  and the adherend, and loosens or mitigates the previous adhering state of the adhesive face  61   a  to the adherend. This reduces the adhesive strength of the adhesive face  61   a  to the adherend. Also assume that the pressure-sensitive adhesive sheet X 6  is stretched while the pressure-sensitive adhesive layer  62  or the adhesive face  62   a  adheres to an adherend. This gives shearing force in the stretching direction at the interface between the pressure-sensitive adhesive layer  62  and the adherend, loosens or mitigates the previous adhering state of the adhesive face  62   a  to the adherend, and reduces the adhesive strength of the adhesive face  62   a  to the adherend. 
     The pressure-sensitive adhesive sheet X 6  is to be used for the build stage  100  of the additive manufacturing apparatus for the formation of an object, typically as illustrated in  FIGS. 2( a ), 2( b ), and 2( c ) , as with the pressure-sensitive adhesive sheet X 1 . Specifically, the pressure-sensitive adhesive sheet X 6  is affixed to the object-forming surface  101  of the build stage  100  before the start of object formation, as illustrated in  FIG. 2( a ) . Of the pressure-sensitive adhesive sheet X 6  in the embodiment, the surface defined by the pressure-sensitive adhesive layer  61  adheres to the object-forming surface  101 . During an object forming process with the apparatus being operated, the target object W is gradually formed or built up on the pressure-sensitive adhesive sheet X 6  affixed to the build stage  100 , as illustrated in  FIG. 2( b ) . In the object forming process as above, the object W during its formation is in a state of being attached to the build stage  100  through the pressure-sensitive adhesive sheet X 6 . After the completion of the object forming process as illustrated in  FIG. 2( c ) , an adhesive strength-reducing action is performed to the pressure-sensitive adhesive sheet X 6  on the build stage  100 . The adhesive strength-reducing action in the embodiment is stretching of the pressure-sensitive adhesive sheet X 6 . The pressure-sensitive adhesive sheet X 6  can be stretched by pulling, of the pressure-sensitive adhesive sheet X 6 , an end or another region where the object W is not attached, where the pulling is performed in an in-plane direction of the pressure-sensitive adhesive sheet X 6 . The stretching of the pressure-sensitive adhesive sheet X 6  typically by the above procedure gives shearing force in the stretching direction at the interface between the pressure-sensitive adhesive layer  61  of the pressure-sensitive adhesive sheet X 6  and the build stage  100 , loosens or mitigates the previous adhering state of the adhesive face  61   a  to the build stage  100 , and reduces the adhesive strength of the adhesive face  61   a  to the build stage  100 . With this, the stretching of the pressure-sensitive adhesive sheet X 6  gives shearing force in the stretching direction at the interface between the pressure-sensitive adhesive layer  62  of the pressure-sensitive adhesive sheet X 6  and the object W attached to the layer  62 , loosens or mitigates the previous adhering state of the adhesive face  62   a  to the object W, and reduces the adhesive strength of the adhesive face  62   a  to the object W. Thus, the attached state of the object W to the build stage  100  through the pressure-sensitive adhesive sheet X 6  is freed. This results in facilitating the detachment of the object W from the build stage  100 . Specifically, the object W can be detached from the build stage  100  with an operation of removing the pressure-sensitive adhesive sheet X 6  from the build stage  100  at the adhesive face  61   a  of the pressure-sensitive adhesive layer  61  whose adhesive strength has been reduced. The object W can also be detached from the build stage  100  by pulling off the object W from the adhesive face  62   a  of the pressure-sensitive adhesive layer  62  whose adhesive strength has been reduced. 
     As described above, the pressure-sensitive adhesive sheet X 6  for build stage use is suitable for allowing the object W, which is to be formed over the build stage  100  of the additive manufacturing apparatus, to be attached to the build stage  100  during building up, and to be readily detached from the build stage  100  after building up. In addition, the pressure-sensitive adhesive sheet X 6  as above, when used by the procedure as described above in the formation of the object W using the additive manufacturing apparatus, can avoid or minimize the use of a scraper or another tool having a sharp edge for the detachment of the object W from the build stage  100 . Accordingly, the use of the pressure-sensitive adhesive sheet X 6  for build stage use contributes to avoiding or minimizing significant damage on the object W and the build stage  100  upon detachment of the object W from the build stage  100 . 
       FIG. 13  illustrates a pressure-sensitive adhesive sheet X 6   a , which is a modification of the pressure-sensitive adhesive sheet X 6  for build stage use. The pressure-sensitive adhesive sheet X 6   a  is a single-sided pressure-sensitive adhesive sheet having a multilayer structure including a substrate S 3  and a pressure-sensitive adhesive layer  61 , and differs from the pressure-sensitive adhesive sheet X 6  in including no pressure-sensitive adhesive layer  62 . The pressure-sensitive adhesive sheet X 6   a  as above is used by allowing the surface defined by the pressure-sensitive adhesive layer  61  to adhere to the object-forming surface  101  of the build stage  100 . Specifically, assume that the object to be additively manufactured by the additive manufacturing apparatus is capable of being attached, at its end face facing the stage, to the object-forming surface typically through fusion bonding; and in this case, the pressure-sensitive adhesive sheet X 6   a , which includes no pressure-sensitive adhesive layer on the side facing the object, can also be employed. The pressure-sensitive adhesive sheet X 6   a  for build stage use as above is usable by a procedure similar to that described above for the pressure-sensitive adhesive sheet X 6  and is therefore suitable for allowing the object W, which is to be formed over the build stage  100  of the additive manufacturing apparatus, to be attached to the build stage  100  during building up, and to be readily detached from the build stage  100  after building up. 
     In an embodiment, the pressure-sensitive adhesive sheet for build stage use according to the present invention may include a magnetically releasable pressure-sensitive adhesive layer (adhesive strength-reducible layer) having an adhesive strength capable of being reduced by the application of a magnetic field (adhesive strength-reducing action). The magnetically releasable pressure-sensitive adhesive layer as above typically contains a magnetic liquid such as a magnetic ionic liquid, in addition to a pressure-sensitive adhesive. A magnetic field, when applied to the magnetically releasable pressure-sensitive adhesive layer as above, causes orientation modulation and/or migration of the magnetic liquid in the pressure-sensitive adhesive layer and can thereby change the chemical composition at the surface of the pressure-sensitive adhesive layer. Assume that the pressure-sensitive adhesive layer undergoes magnetic field application while an adhesive face of the pressure-sensitive adhesive layer adheres to an adherend. In this case, the adhesive strength of the adhesive face of the magnetically releasable pressure-sensitive adhesive layer can be reduced using the chemical composition change as above occurring in the adhesive face. 
     In an embodiment, the pressure-sensitive adhesive sheet for build stage use according to the present invention may include a pressure-releasable pressure-sensitive adhesive layer (adhesive strength-reducible layer) having an adhesive strength capable of being reduced by pressure change. The pressure-releasable pressure-sensitive adhesive layer can employ any of configurations as follows: a configuration where the pressure-sensitive adhesive layer has such a chemical composition as to cause phase separation by pressure rise (compression) and renders pressurization-induced reduction in adhesive strength feasible; and a configuration where the pressure-sensitive adhesive layer has such a chemical composition as to be capable of foaming by pressure rise or pressure reduction and renders pressure change-induced reduction in adhesive strength feasible. 
     In an embodiment, the pressure-sensitive adhesive sheet for build stage use according to the present invention may have a configuration including different pressure-sensitive adhesive layers selected from the group consisting of the thermally releasable pressure-sensitive adhesive layer, the cold-releasable pressure-sensitive adhesive layer, the electromagnetically releasable pressure-sensitive adhesive layer, the electrically releasable pressure-sensitive adhesive layer, the liquid-releasable pressure-sensitive adhesive layer, the stretching-releasable pressure-sensitive adhesive layer, the magnetically releasable pressure-sensitive adhesive layer, and the pressure-releasable pressure-sensitive adhesive layer. For example, the pressure-sensitive adhesive sheet for build stage use according to the present invention may include, with or without a substrate, one selected from the group consisting of these releasable pressure-sensitive adhesive layers as a pressure-sensitive adhesive layer to constitute or define an adhesive face facing the build stage; and another one as a pressure-sensitive adhesive layer to constitute or define an adhesive face facing the object, where the releasable pressure-sensitive adhesive layers are: the thermally releasable pressure-sensitive adhesive layer, the cold-releasable pressure-sensitive adhesive layer, the electromagnetically releasable pressure-sensitive adhesive layer, the electrically releasable pressure-sensitive adhesive layer, the liquid-releasable pressure-sensitive adhesive layer, the stretching-releasable pressure-sensitive adhesive layer, the magnetically releasable pressure-sensitive adhesive layer, and the pressure-releasable pressure-sensitive adhesive layer. 
       FIG. 14  is a schematic partial view of an additive manufacturing apparatus Y 1  according to an embodiment of the present invention. The additive manufacturing apparatus Y 1  is an apparatus for the formation of an object by an additive manufacturing technique such as fused deposition modeling, material jetting, stereolithography, or sheet lamination. The additive manufacturing apparatus Y 1  includes a build stage  100  and a heating unit  110  (adhesive strength-reducing means). The build stage  100  is made from a heat-transferable material and has an object-forming surface  101  over which a three-dimensional object is to be formed. The heating unit  110  is a unit for heating a pressure-sensitive adhesive sheet for build stage use affixed to the build stage  100  and typically includes a heater  111  that is integrated inside the build stage  100  and is capable of generating heat. Assume that the additive manufacturing apparatus Y 1  is capable of performing fused deposition modeling, material jetting, or another additive manufacturing technique that supplies a modeling material (object-forming material) sequentially onto the build stage  100  in an object forming process. In this case, the additive manufacturing apparatus Y 1  further includes a supply unit (not shown) that includes a nozzle capable of discharging the modeling material. Such a configuration where the additive manufacturing apparatus further includes a supply unit that includes a nozzle capable of discharging a modeling material is applicable also to after-mentioned additive manufacturing apparatuses. 
     The additive manufacturing apparatus Y 1  enables the formation of a three-dimensional object over the build stage  100  using the pressure-sensitive adhesive sheet X 1 , which includes the thermally releasable pressure-sensitive adhesive layer  11 . The use procedure of the pressure-sensitive adhesive sheet X 1  is as with that described above for the pressure-sensitive adhesive sheet X 1  with reference to  FIGS. 2( a ), 2( b ), and 2( c ) . Specifically, in an object forming process during operation of the additive manufacturing apparatus Y 1 , the object W during its formation is in a state of being attached to the build stage  100  through the pressure-sensitive adhesive sheet X 1 , as illustrated in  FIG. 2( b ) . After the completion of the object forming process as typically illustrated in  FIG. 2( c )  during operation of the additive manufacturing apparatus Y 1 , the additive manufacturing apparatus Y 1  can perform heating as an adhesive strength-reducing action to the pressure-sensitive adhesive sheet X 1  on the build stage  100 . Specifically, when the heating unit  110  operates to generate heat after the completion of the object forming process, the heat propagates through the build stage  100  and reaches the pressure-sensitive adhesive sheet X 1  or the pressure-sensitive adhesive layer  11  thereof. The pressure-sensitive adhesive layer  11 , when undergoes the heating, is reduced in adhesive strength, and this frees or loosens the attached state of the object W to the build stage  100  through the pressure-sensitive adhesive sheet X 1 , as described above. This results in facilitating the detachment of the object W from the build stage  100 , as described above for the usage mode of the pressure-sensitive adhesive sheet X 1  with reference to  FIGS. 2( a ), 2( b ), and 2( c ) . For example, the object W can be detached from the build stage  100  with an operation of removing the pressure-sensitive adhesive sheet X 1  at a predetermined interface defined by the pressure-sensitive adhesive layer  11  whose adhesive strength has been reduced. As described above, the additive manufacturing apparatus Y 1  can suitably actually provide the attachment and detachment of the object W to and from the build stage  100 , using the thermal releasing technology relating to the pressure-sensitive adhesive layer  11  of the pressure-sensitive adhesive sheet X 1 . The use of any of the pressure-sensitive adhesive sheets X 1   a , X 1   b , X 1   c , and X 1   d , instead of the pressure-sensitive adhesive sheet X 1 , can give technical advantageous effects similar to this. 
       FIG. 15  is a schematic partial view of an additive manufacturing apparatus according to an embodiment of the present invention Y 2 . The additive manufacturing apparatus Y 2  is an apparatus for the formation of an object by an additive manufacturing technique such as fused deposition modeling, material jetting, stereolithography, or sheet lamination. The additive manufacturing apparatus Y 2  includes a build stage  100  and a cooling unit  120  (adhesive strength-reducing means). The build stage  100  has an object-forming surface  101  over which a three-dimensional object is to be formed. The cooling unit  120  is a unit for cooling a pressure-sensitive adhesive sheet for build stage use affixed to the build stage  100  and includes a cooling tube  121  (partially not shown) that is embedded in the build stage  100 . The cooling tube  121  is coupled to a coolant reservoir (out of the figure) and is configured so that a coolant fed from the coolant reservoir can pass through the cooling tube  121 . Non-limiting examples of the coolant usable herein include carbon dioxide gas, nitrogen gas, water, and antifreezes. 
     The additive manufacturing apparatus Y 2  enables the formation of a three-dimensional object over the build stage  100  using the pressure-sensitive adhesive sheet X 2 , which includes the cold-releasable pressure-sensitive adhesive layer  21 . The use procedure of the pressure-sensitive adhesive sheet X 2  is as with that described above for the pressure-sensitive adhesive sheet X 2  with reference to  FIGS. 2( a ), 2( b ), and 2( c ) . In an object forming process during operation of the additive manufacturing apparatus Y 2 , the object W during its formation is in a state of being attached to the build stage  100  through the pressure-sensitive adhesive sheet X 2 , as illustrated in  FIG. 2( b ) . After the completion of the object forming process during operation of the additive manufacturing apparatus Y 2  as typically illustrated in  FIG. 2( c ) , the additive manufacturing apparatus Y 2  can preform cooling, as an adhesive strength-reducing action, to the pressure-sensitive adhesive sheet X 2  on the build stage  100 . Specifically, when the cooling unit  120  operates so as to allow the coolant to pass through the cooling tube  121  after the completion of the object forming process, the build stage  100  and, consequently, the pressure-sensitive adhesive sheet X 2  or the pressure-sensitive adhesive layer  21  thereof are decreased in temperature and cooled. As described above, the cooling, when performed to the pressure-sensitive adhesive layer  21 , reduces the adhesive strength of the layer and frees or loosens the attached state of the object W to the build stage  100  through the pressure-sensitive adhesive sheet X 2 . This results in facilitating the detachment of the object W from the build stage  100 , as described above for the usage mode of the pressure-sensitive adhesive sheet X 2  with reference to  FIGS. 2( a ), 2( b ), and 2( c ) . For example, the object W can be detached from the build stage  100  with an operation of removing the pressure-sensitive adhesive sheet X 2  at a predetermined interface defined by the pressure-sensitive adhesive layer  21  whose adhesive strength has been reduced. As described above, the additive manufacturing apparatus Y 2  can suitably actually provide the attachment and detachment of the object W to and from the build stage  100 , using the cooling releasing technology relating to the pressure-sensitive adhesive layer  21  of the pressure-sensitive adhesive sheet X 2 . The use of any of the pressure-sensitive adhesive sheets X 2   a , X 2   b , X 2   c , and X 2   d , instead of the pressure-sensitive adhesive sheet X 2 , can give technical advantageous effects similar to this. 
       FIG. 16  is a schematic partial view of an additive manufacturing apparatus Y 3  according to an embodiment of the present invention. The additive manufacturing apparatus Y 3  is an apparatus for the formation of an object by an additive manufacturing technique such as fused deposition modeling, material jetting, stereolithography, or sheet lamination. The additive manufacturing apparatus Y 3  includes a build stage  100  and an electromagnetic irradiation unit  130  (adhesive strength-reducing means). The build stage  100  is made from a material being transparent to electromagnetic radiation and has an object-forming surface  101  over which a three-dimensional object is to be formed. The electromagnetic irradiation unit  130  is a unit for applying electromagnetic radiation to a pressure-sensitive adhesive sheet for build stage use affixed to the build stage  100 , and includes an electromagnetic radiation source capable of applying electromagnetic radiation at a predetermined time or timing. Non-limiting examples of the electromagnetic radiation source include ultraviolet lamps. 
     The additive manufacturing apparatus Y 3  enables the formation of a three-dimensional object over the build stage  100  using the pressure-sensitive adhesive sheet X 3 , which includes the electromagnetically releasable pressure-sensitive adhesive layer  31 . The use procedure of the pressure-sensitive adhesive sheet X 3  is as with that described above for the pressure-sensitive adhesive sheet X 3  with reference to  FIGS. 2( a ), 2( b ), and 2( c ) . In an object forming process during operation of the additive manufacturing apparatus Y 3 , the object W during its formation is in a state of being attached to the build stage  100  through the pressure-sensitive adhesive sheet X 3 , as illustrated in  FIG. 2( b ) . After the completion of the object forming process as typically illustrated in  FIG. 2( c )  during operation of the additive manufacturing apparatus Y 3 , the additive manufacturing apparatus Y 3  can perform electromagnetic application as an adhesive strength-reducing action to the pressure-sensitive adhesive sheet X 3  on the build stage  100 . Specifically, when the electromagnetic irradiation unit  130  operates after the completion of the object forming process, electromagnetic radiation is emitted from the electromagnetic radiation source of the unit, passes through the build stage  100 , and is applied to the pressure-sensitive adhesive sheet X 3  or the pressure-sensitive adhesive layer  31  thereof. Assume that the electromagnetic radiation source is an ultraviolet lamp. In this case, ultraviolet radiation passes through the build stage  100  and is applied to the pressure-sensitive adhesive sheet X 3  or the pressure-sensitive adhesive layer  31  thereof. As described above, the electromagnetic application to the pressure-sensitive adhesive layer  31  reduces the adhesive strength of the pressure-sensitive adhesive layer  31 , and thereby frees or loosens the attached state of the object W to the build stage  100  through the pressure-sensitive adhesive sheet X 3 . This facilitates the detachment of the object W from the build stage  100 , as described above for the usage mode of the pressure-sensitive adhesive sheet X 3  with reference to  FIGS. 2( a ), 2( b ), and 2( c ) . For example, the object W can be detached from the build stage  100  by performing an operation of removing the pressure-sensitive adhesive sheet X 3  at a predetermined interface defined by the pressure-sensitive adhesive layer  31  whose adhesive strength has been reduced. As described above, the additive manufacturing apparatus Y 3  can suitably actually provide the attachment and detachment of the object W to and from the build stage  100 , using the electromagnetic releasing technology relating to the pressure-sensitive adhesive layer  31  of the pressure-sensitive adhesive sheet X 3 . The use of any of the pressure-sensitive adhesive sheets X 3   a , X 3   b , X 3   c , and X 3   d , instead of the pressure-sensitive adhesive sheet X 3 , can give technical advantageous effects similar to this. 
       FIG. 17  is a schematic partial view of an additive manufacturing apparatus Y 4  according to an embodiment of the present invention. The additive manufacturing apparatus Y 4  is an apparatus for the formation of an object by an additive manufacturing technique such as fused deposition modeling, material jetting, stereolithography, or sheet lamination. The additive manufacturing apparatus Y 4  includes a build stage  100  and a voltage application unit  140  (adhesive strength-reducing means). The build stage  100  has an object-forming surface  101  over which a three-dimensional object is to be formed. For the additive manufacturing apparatus Y 4 , the build stage  100  or the object-forming surface  101  thereof, and/or the object to be formed has conductivity. The voltage application unit  140  is a unit for applying a voltage to an electrically releasable pressure-sensitive adhesive layer of a pressure-sensitive adhesive sheet for build stage use affixed to the build stage  100  and includes a power source that can apply a voltage at a predetermined time. The voltage application unit  140  is configured so as to be able to apply a voltage to the electrically releasable pressure-sensitive adhesive layer via two conductive portions selected from: the object-forming surface  101  when the object-forming surface  101  has conductivity; a conductive portion when the pressure-sensitive adhesive sheet for build stage use to be used as disposed (applied) on the build stage  100  further includes the conductive portion in addition to the electrically releasable pressure-sensitive adhesive layer, where the conductive portion is electrically coupled to the electrically releasable pressure-sensitive adhesive layer; and the object when the object has conductivity.  FIG. 17  exemplarily illustrates the case where the voltage application is performed via the object-forming surface  101  and the conductive portion in the pressure-sensitive adhesive sheet. 
     The additive manufacturing apparatus Y 4  enables the formation of a three-dimensional object over the build stage  100  using the pressure-sensitive adhesive sheet X 4 , which includes the electrically releasable, pressure-sensitive adhesive layer  41 . The use procedure of the pressure-sensitive adhesive sheet X 4  is as with that described above for the pressure-sensitive adhesive sheet X 4  with reference to  FIGS. 2( a ), 2( b ), and 2( c ) . In an object forming process during operation of the additive manufacturing apparatus Y 4 , the object W during its formation is in a state of being attached to the build stage  100  through the pressure-sensitive adhesive sheet X 4 , as illustrated in  FIG. 2( b ) . After the completion of the object forming process as typically illustrated in  FIG. 2( c )  during operation of the additive manufacturing apparatus Y 4 , the additive manufacturing apparatus Y 4  can perform voltage application as an adhesive strength-reducing action to the pressure-sensitive adhesive sheet X 4  on the build stage  100 . Assume that the voltage application unit  140  operates after the completion of the object forming process. This causes a potential difference in the pressure-sensitive adhesive layer  41  of the pressure-sensitive adhesive sheet X 4  in a thickness direction of the layer, then reduces the adhesive strength of the surface of the pressure-sensitive adhesive layer  41  as described above, and frees or loosens the attached state of the object W to the build stage  100  through the pressure-sensitive adhesive sheet X 4 . This results in facilitating the detachment of the object W from the build stage  100 , as described above for the usage mode of the pressure-sensitive adhesive sheet X 4  with reference to  FIGS. 2( a ), 2( b ), and 2( c ) . For example, the object W can be detached from the build stage  100  by performing an operation of removing the pressure-sensitive adhesive sheet X 4  at a predetermined interface defined by the pressure-sensitive adhesive layer  41  whose adhesive strength has been reduced. As described above, the additive manufacturing apparatus Y 4  can suitably actually provide the attachment and detachment of the object W to and from the build stage  100 , using the electric releasing technology relating to the pressure-sensitive adhesive layer  41  of the pressure-sensitive adhesive sheet X 4 . The use of any of the pressure-sensitive adhesive sheets X 4   a , X 4   b , X 4   c , and X 4   d , instead of the pressure-sensitive adhesive sheet X 4 , can give technical advantageous effects similar to this. 
       FIG. 18  is a schematic partial view of an additive manufacturing apparatus Y 5  according to an embodiment of the present invention. The additive manufacturing apparatus Y 5  is an apparatus for the formation of an object by an additive manufacturing technique such as fused deposition modeling, material jetting, stereolithography, or sheet lamination. The additive manufacturing apparatus Y 5  includes a build stage  100  and a liquid supply unit  150  (adhesive strength-reducing means). The build stage  100  has an object-forming surface  101  over which a three-dimensional object is to be formed. The liquid supply unit  150  is a unit for supplying a liquid to a pressure-sensitive adhesive sheet for build stage use affixed to the build stage  100  and includes a liquid supply main pipe  151  (partially not shown) embedded in the build stage  100 . The main pipe  151  has side discharge openings (not shown) at the wall thereof. The build stage  100  has a porous structure that extends from the wall or side discharge openings of the main pipe  151  to the object-forming surface  101  and allows the liquid to pass through the build stage  100 . Instead of this configuration as above, the liquid supply unit  150  may employ (may be) a spray unit capable of spraying a liquid. 
     The additive manufacturing apparatus Y 5  enables the formation of a three-dimensional object over the build stage  100  using the pressure-sensitive adhesive sheet X 5 , which includes the liquid-releasable, pressure-sensitive adhesive layer  51 . The use procedure of the pressure-sensitive adhesive sheet X 5  is as with that described above for the pressure-sensitive adhesive sheet X 5  with reference to  FIGS. 2( a ), 2( b ), and 2( c ) . In an object forming process during operation of the additive manufacturing apparatus Y 5 , the object W during its formation is in a state of being attached to the build stage  100  through the pressure-sensitive adhesive sheet X 5 , as illustrated in  FIG. 2( b ) . After the completion of the object forming process as typically illustrated in  FIG. 2( c )  during operation of the additive manufacturing apparatus Y 5 , the additive manufacturing apparatus Y 5  can preform liquid supply, as an adhesive strength-reducing action, to the pressure-sensitive adhesive sheet X 5  on the build stage  100 . Specifically, the liquid supply unit  150  operates after the completion of the object forming process to allow a liquid capable of reducing the adhesive strength of the pressure-sensitive adhesive layer  51  to pass through the main pipe  151 . Part of the flowing liquid overflows via the side discharge openings of the main pipe  151  out of the main pipe  151 , travels in the porous structure in the build stage  100 , and reaches the pressure-sensitive adhesive sheet X 5 , or the pressure-sensitive adhesive layer  51  thereof, on the build stage  100 . Assume that the spray unit is employed as the liquid supply unit  150 . In this case, the spray unit, which serves as the liquid supply unit  150 , operates after the completion of the object forming process to spray a liquid on the pressure-sensitive adhesive layer  51 , where the liquid is capable of reducing the adhesive strength of the pressure-sensitive adhesive layer  51 . As described above, the liquid supply, when performed, reduces the adhesive strength of the pressure-sensitive adhesive layer  51 , and this frees or loosens the attached state of the object W to the build stage  100  through the pressure-sensitive adhesive sheet X 5 . This results in facilitating the detachment of the object W from the build stage  100 , as described above for the usage mode of the pressure-sensitive adhesive sheet X 5  with reference to  FIGS. 2( a ), 2( b ), and 2( c ) . For example, the object W can be detached from the build stage  100  by performing an operation of removing the pressure-sensitive adhesive sheet X 5  at a predetermined interface defined by the pressure-sensitive adhesive layer  51  whose adhesive strength has been reduced. As described above, the additive manufacturing apparatus Y 5  can suitably actually provide the attachment and detachment of the object W to and from the build stage  100 , using the liquid releasing technology relating to the pressure-sensitive adhesive layer  51  of the pressure-sensitive adhesive sheet X 5 . The use of any of the pressure-sensitive adhesive sheets X 5   a , X 5   b , X 5   c , and X 5   d , instead of the pressure-sensitive adhesive sheet X 5 , can give technical advantageous effects similar to this. 
       FIG. 19  is a schematic partial view of an additive manufacturing apparatus Y 6  according to an embodiment of the present invention. The additive manufacturing apparatus Y 6  is an apparatus for the formation of an object by an additive manufacturing technique such as fused deposition modeling, material jetting, stereolithography, or sheet lamination. The additive manufacturing apparatus Y 6  includes a build stage  100  and a stretching unit  160  (adhesive strength-reducing means). The build stage  100  has an object-forming surface  101  over which a three-dimensional object is to be formed. The stretching unit  160  is a unit for automatically stretching a pressure-sensitive adhesive sheet for build stage use affixed to the build stage  100 . 
     The additive manufacturing apparatus Y 6  enables the formation of a three-dimensional object over the build stage  100  using the stretching-releasable, pressure-sensitive adhesive sheet X 6  including the substrate S 3 , which is a stretchable substrate. The use procedure of the pressure-sensitive adhesive sheet X 6  is as with that described above for the pressure-sensitive adhesive sheet X 6  with reference to  FIGS. 2( a ), 2( b ), and 2( c ) . In an object forming process during operation of the additive manufacturing apparatus Y 6 , an object W during its formation is in a state of being attached to the build stage  100  through the pressure-sensitive adhesive sheet X 6 , as illustrated in  FIG. 2( b ) . After the completion of the object forming process as typically illustrated in  FIG. 2( c )  during operation of the additive manufacturing apparatus Y 6 , the additive manufacturing apparatus Y 6  can perform stretching, as an adhesive strength-reducing action, of the pressure-sensitive adhesive sheet X 6  on the build stage  100 . Specifically, the stretching unit  160  operates after the completion of the object forming process to automatically stretch the pressure-sensitive adhesive sheet X 6  in an in-plane direction. As described above, stretching of the pressure-sensitive adhesive sheet X 6  gives shearing force in the stretching direction at interfaces between each adherend and each pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet X 6 , and this loosens or mitigates the previous adhering state of the surface (adhesive face) of the pressure-sensitive adhesive layer to the adherend and reduces the adhesive strength of the adhesive face to the adherend. Assume that the surface defined by the pressure-sensitive adhesive layer  61  of the pressure-sensitive adhesive sheet X 6  adheres to the object-forming surface  101  before the start of object formation. In this case, for example, stretching of the pressure-sensitive adhesive sheet X 6  causes shearing force in the stretching direction at the interface between the pressure-sensitive adhesive layer  61  of the pressure-sensitive adhesive sheet X 6  and the build stage  100 , loosens or mitigates the previous adhering state of the adhesive face  61   a  to the build stage  100 , and reduces the adhesive strength of the adhesive face  61   a  to the build stage  100 . This frees or loosens the attached state of the object W to the build stage  100  through the pressure-sensitive adhesive sheet X 6 , and results in facilitating the detachment of the object W from the build stage  100 , as described above for the usage mode of the pressure-sensitive adhesive sheet X 6  with reference to  FIGS. 2( a ), 2( b ), and 2( c ) . For example, the object W can be detached from the build stage  100  by performing an operation of removing the pressure-sensitive adhesive sheet X 6  from the build stage  100  at the adhesive face  61   a  of the pressure-sensitive adhesive layer  61  whose adhesive strength has been reduced. As described above, the additive manufacturing apparatus Y 6  can suitably actually provide the attachment and detachment of the object W to and from the build stage  100 , using the stretching releasing technology relating to the pressure-sensitive adhesive layers of the pressure-sensitive adhesive sheet X 6 . The use of the pressure-sensitive adhesive sheet X 6   a , instead of the pressure-sensitive adhesive sheet X 6 , can give technical advantageous effects similar to this. 
       FIG. 20  is a schematic partial view of an additive manufacturing apparatus Y 7  according to an embodiment of the present invention. The additive manufacturing apparatus Y 7  is an apparatus for the formation of an object by an additive manufacturing technique such as fused deposition modeling, material jetting, stereolithography, or sheet lamination. The additive manufacturing apparatus Y 7  includes a build stage  100  and a magnetic field application unit  170  (adhesive strength-reducing means). The build stage  100  has an object-forming surface  101  over which a three-dimensional object is to be formed. The magnetic field application unit  170  is a unit for applying a magnetic field to a pressure-sensitive adhesive sheet for build stage use affixed to the build stage  100  and typically includes coils  171  that are embedded in the build stage  100 , where an electric current can pass through the coils  171 . 
     The additive manufacturing apparatus Y 7  enables the formation of a three-dimensional object over the build stage  100  using the pressure-sensitive adhesive sheet including a magnetically releasable pressure-sensitive adhesive layer (in  FIG. 20 , the pressure-sensitive adhesive sheet X 7 ). In an object forming process during operation of the additive manufacturing apparatus Y 7 , the object W during its formation is in a state of being attached to the build stage  100  through the pressure-sensitive adhesive sheet X 7 , as illustrated in  FIG. 2( b ) . After the completion of the object forming process during operation of the additive manufacturing apparatus Y 7  as typically illustrated in  FIG. 2( c ) , the additive manufacturing apparatus Y 7  can perform magnetic field application, as an adhesive strength-reducing action, to the pressure-sensitive adhesive sheet X 7  on the build stage  100 . Specifically, the magnetic field application unit  170  operates after the completion of the object forming process, to apply a magnetic field to the magnetically releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet X 7 . The magnetic filed application to the magnetically releasable pressure-sensitive adhesive layer reduces the adhesive strength of the pressure-sensitive adhesive layer as described above, and this frees or loosens the attached state of the object W to the build stage  100  through the pressure-sensitive adhesive sheet X 7 , and results in facilitating the detachment of the object W from the build stage  100 . For example, the object W can be detached from the build stage  100  by performing an operation of removing the pressure-sensitive adhesive sheet X 7  at a predetermined interface defined by the magnetically releasable pressure-sensitive adhesive layer whose adhesive strength has been reduced. As described above, the additive manufacturing apparatus Y 7  can suitably actually provide the attachment and detachment of the object W to and from the build stage  100 , using the magnetic releasing technology relating to the magnetically releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet X 7 . 
     EXAMPLES 
     Synthetic Example 1: Acrylic Polymer A1 
     A mixture was subjected to a polymerization reaction at 60° C. for 6 hours with nitrogen gas introduction, in a reactor equipped with a reflux condenser, a nitrogen gas inlet tube, a stirrer, and a thermometer, where the mixture included 60 parts by mass of ethyl acrylate, 30 parts by mass of 2-ethylhexyl acrylate, 5 parts by mass of methyl methacrylate, 5 parts by mass of hydroxyethyl acrylate, 0.2 part by mass of benzoyl peroxide as a polymerization initiator, and 100 parts by mass of toluene as a polymerization solvent. This gave a solution (acrylic polymer solution S1) containing an acrylic polymer (acrylic polymer A1) for the formation of a thermally releasable pressure-sensitive adhesive layer in Example 1. 
     Synthetic Example 2: Acrylic Polymer A2 
     A mixture was subjected to a polymerization reaction at 60° C. for 6 hours with nitrogen gas introduction, in a reactor equipped with a reflux condenser, a nitrogen gas inlet tube, a stirrer, and a thermometer, where the mixture included 92 parts by mass of butyl acrylate, 3 parts by mass of methyl methacrylate, 5 parts by mass of acrylic acid, 0.2 part by mass of 2,2′-azobisisobutyronitrile as a polymerization initiator, and 100 parts by mass of ethyl acetate as a polymerization solvent. This gave a solution (acrylic polymer solution S2) containing an acrylic polymer (acrylic polymer A2) for the formation of a pressure-sensitive adhesive layer. 
     Synthetic Example 3: Acrylic Polymer A3 
     A mixture was subjected to a polymerization reaction at 60° C. for 6 hours with nitrogen gas introduction, in a reactor equipped with a reflux condenser, a nitrogen gas inlet tube, a stirrer, and a thermometer, where the mixture included 55 parts by mass of 2-ethylhexyl acrylate, 40 parts by mass of methyl acrylate, 5 parts by mass of 2-hydroxyethyl acrylate, 0.2 part by mass of benzoyl peroxide as a polymerization initiator, and 100 parts by mass of toluene as a polymerization solvent. This gave a solution (acrylic polymer solution S3) containing an acrylic polymer (acrylic polymer A3) for the formation of a thermally releasable pressure-sensitive adhesive layer in Example 2. 
     Synthetic Example 4: Acrylic Polymer A4 
     A mixture was subjected to a polymerization reaction at 80° C. for 6 hours with nitrogen gas introduction, in a reactor equipped with a reflux condenser, a nitrogen gas inlet tube, a stirrer, and a thermometer, where the mixture included 40 parts by mass of behenyl acrylate, 35 parts by mass of stearyl acrylate, 20 parts by mass of methyl acrylate, 5 parts by mass of acrylic acid, 5 parts by mass of dodecyl mercaptan as a chain-transfer agent, 0.5 part by mass of benzoyl peroxide as a polymerization initiator, and 100 parts by mass of toluene as a polymerization solvent. This gave a solution (acrylic polymer solution S4) containing a side-chain-crystalline acrylic polymer (acrylic polymer A4) for the formation of a thermally releasable pressure-sensitive adhesive layer in Example 2. 
     Synthetic Example 5: Acrylic Polymer A5 
     A mixture was subjected to a polymerization reaction at 60° C. for 6 hours with nitrogen gas introduction, in a reactor equipped with a reflux condenser, a nitrogen gas inlet tube, a stirrer, and a thermometer, where the mixture included 60 parts by mass of behenyl acrylate, 35 parts by mass of isobolnyl methacrylate, 5 parts by mass of methacrylic acid, 0.5 part by mass of benzoyl peroxide as a polymerization initiator, and 100 parts by mass of ethyl acetate as a polymerization solvent. This gave a solution (acrylic polymer solution S5) containing a side-chain-crystalline acrylic polymer (acrylic polymer A5) for the formation of a cold-releasable pressure-sensitive adhesive layer in Example 3. 
     Synthetic Example 6: Acrylic Polymer A6 
     A mixture was subjected to a polymerization reaction at 60° C. for 6 hours with nitrogen gas introduction, in a reactor equipped with a reflux condenser, a nitrogen gas inlet tube, a stirrer, and a thermometer, and yielded an acrylic polymer A6′, where the mixture included 35 parts by mass of 2-ethylhexyl acrylate, 45 parts by mass of butyl acrylate, 20 parts by mass of 2-hydroxyethyl acrylate, 0.2 part by mass of 2,2′-azobisisobutyronitrile as a polymerization initiator, and 100 parts by mass of toluene as a polymerization solvent. Next, the reaction solution containing the acrylic polymer A6′ was combined with 20 parts by mass of 2-methacryloyloxyethyl isocyanate, and the mixture was subjected to an addition reaction at 50° C. in an air atmosphere for 48 hours. This gave a solution (acrylic polymer solution S6) containing an electromagnetic radiation-curable acrylic polymer (acrylic polymer A6, with carbon-carbon double bonds being introduced into side chains in the polymer molecule) for the formation of an electromagnetic radiation-curable pressure-sensitive adhesive layer in Example 4. 
     Synthetic Example 7: Acrylic Oligomer A7 
     A mixture was subjected to a polymerization reaction at 60° C. for 8 hours with nitrogen gas introduction, in a reactor equipped with a reflux condenser, a nitrogen gas inlet tube, a stirrer, and a thermometer, where the mixture included 50 parts by mass of 2-ethylhexyl acrylate, 45 parts by mass of butyl acrylate, 5 parts by mass of 2-hydroxyethyl acrylate, and 100 parts by mass of toluene as a polymerization solvent. This gave a solution (acrylic polymer solution S7) containing an electromagnetic radiation-curable acrylic oligomer (acrylic oligomer A7) for the formation of an electromagnetically releasable pressure-sensitive adhesive layer in Example 5. 
     Synthetic Example 8: Acrylic Polymer A8 
     A mixture was stirred for one hour with nitrogen gas introduction, in a reactor equipped with a reflux condenser, a nitrogen gas inlet tube, a stirrer, and a thermometer, where the mixture included 90 parts by mass of n-butyl acrylate (BA), 5 parts by mass of methyl methacrylate (MMA), 5 parts by mass of acrylic acid (AA), and 186 parts by mass of ethyl acetate as a polymerization solvent. After removing oxygen from the polymerization system by the above procedure, the solution in the reactor was subjected to a reaction at 63° C. for 10 hours. The solution was then cooled and yielded a solution (acrylic polymer solution S8) containing an acrylic polymer (acrylic polymer A8, a copolymer among BA, MMA, and AA) for the formation of a pressure-sensitive adhesive layer in Comparative Example 1. The acrylic polymer solution S8 had a solids concentration of 35 mass percent. 
     Primer U1 
     The acrylic polymer solution S1 was combined with and mixed with, per 100 parts by mass of solid matter (the acrylic polymer A1) in the solution S1, 4 parts by mass of an isocyanate crosslinker CORONATE L (trade name, a polyisocyanate compound, supplied by TOSOH CORPORATION), and yielded a primer U1. 
     Primer U2 
     A primer U2 was prepared by mixing 100 parts by mass of a natural rubber MMA-Grafted Natural Rubber MG30 (trade name, supplied by Rubber Research Institute of Malaysia) in which methyl methacrylate is polymerized in a natural rubber, 50 parts by mass of an isocyanate crosslinker CORONATE L (trade name, a polyisocyanate compound, supplied by TOSOH CORPORATION), and 900 parts by mass of toluene. 
     PET Substrate B1 
     The primer U1 was applied to one side of a 50-μm thick poly(ethylene terephthalate) (PET) film LUMIRROR S-10 (trade name, supplied by Toray Industries Inc.), and the applied primer was dried to form an under coat (5 μm thick). Thus, a PET substrate (PET substrate B1) bearing, on one side, the under coat formed from the primer U1 was prepared. 
     PET Substrate B2 
     A 50-μm thick poly(ethylene terephthalate) (PET) film LUMIRROR S-10 (trade name, supplied by Toray Industries Inc.) was prepared as a PET substrate B2. 
     Example 1 
     Production of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     The acrylic polymer solution S1 was combined with, per 100 parts by mass of solid matter (the acrylic polymer A1) in the solution S1, 4 parts by mass of an isocyanate crosslinker CORONATE L (trade name, a polyisocyanate compound, supplied by TOSOH CORPORATION) and 30 parts by mass of thermally expandable microspheres Matsumoto Microsphere F-50D (trade name, expansible at 120° C., supplied by Matsumoto Yushi-Seiyaku Co., Ltd.) and yielded a pressure-sensitive adhesive composition C1, which is a composition for the formation of a thermally releasable pressure-sensitive adhesive layer. On the other hand, the acrylic polymer solution S2 was combined with, per 100 parts by mass of solid matter (the acrylic polymer A2) in the solution S2, 5 parts by mass of an isocyanate crosslinker CORONATE L (trade name, a polyisocyanate compound, supplied by TOSOH CORPORATION) and yielded a pressure-sensitive adhesive composition C2, which is a composition for the formation of a pressure-sensitive adhesive layer. Next, the pressure-sensitive adhesive composition C2 was applied to, of the PET substrate B1, a side to which the primer U1 had not been applied (namely, a side bearing no under coat) to form a pressure-sensitive adhesive composition layer. A release film was laminated so as to cover the pressure-sensitive adhesive composition layer on the PET substrate B1. Next, the pressure-sensitive adhesive composition C1 was applied to, of the PET substrate B1, the side coated with the primer U1 (namely, the side bearing the under coat) to form a pressure-sensitive adhesive composition layer. Next, these pressure-sensitive adhesive composition layers were heated to be dried and to undergo a crosslinking reaction, to form a 25-μm thick pressure-sensitive adhesive layer derived from sensitive adhesive composition C2, and a 30-μm thick thermally releasable pressure-sensitive adhesive layer derived from the pressure-sensitive adhesive composition C1. The heating herein was performed at a temperature of 100° C. for a time of 5 minutes. Thus, a pressure-sensitive adhesive sheet for build stage use (double-sided pressure-sensitive adhesive sheet) according to Example 1 was prepared. This had a multilayer structure including the pressure-sensitive adhesive layer (non-thermally-releasable), the PET substrate B1, and the thermally releasable pressure-sensitive adhesive layer disposed in the specified sequence. 
     Use of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     The pressure-sensitive adhesive sheet produced by the procedure as described above was affixed, through the thermally releasable pressure-sensitive adhesive layer thereof, to an object-forming surface of a build stage of a 3D printer BS01+(trade name, supplied by BONSAI LAB, Inc.), in such a manner as to avoid wrinkling and partial gap of the pressure-sensitive adhesive sheet. Then, the 3D printer was operated to form an object. Specifically, a modeling material was discharged and fed from a modeling material-discharging nozzle of the 3D printer toward the build stage and formed a planar object (100 mm by 100 mm, 2 mm thick) on the pressure-sensitive adhesive sheet for build stage use disposed on the build stage. The modeling material used herein was ABS resin filaments Plasil (trade name, white color, supplied by PLASIL) having a cross-sectional diameter of 1.75 mm. The object forming process was performed at a temperature of the object-forming surface of the build stage of 90° C. and a heating temperature of the modeling material in the modeling material-discharging nozzle of 240° C. The object was formed in a state of being firmly attached to the non-thermally-releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet on the build stage. After the object forming process was completed and the object was naturally cooled, the object-forming surface of the build stage was raised in temperature up to 120° C. to heat the thermally releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet on the build stage and to thereby expand the thermally expandable microspheres in the pressure-sensitive adhesive layer. This reduced the adhesive strength of the pressure-sensitive adhesive layer (and, in turn, freed or loosened the attached state of the object to the build stage through the pressure-sensitive adhesive sheet). Subsequently, a removing operation of the pressure-sensitive adhesive sheet on the build stage was performed by separating the thermally releasable pressure-sensitive adhesive layer, whose adhesive strength had been reduced by heating, from the build stage surface, and with this operation, the object was detached from the build stage. The pressure-sensitive adhesive sheet was then removed from the object. 
     Example 2 
     Production of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     The acrylic polymer solution S3 was combined with the acrylic polymer solution S4 and an aluminum chelate crosslinker ALCH-TR (trade name, supplied by Kawaken Fine Chemicals Co., Ltd.) in such proportions that solid matter (the acrylic polymer A4 as a side-chain-crystalline acrylic polymer) in the solution S4 and the aluminum chelate crosslinker be present respectively in amounts of 5 parts by mass and 1 part by mass per 100 parts by mass of solid matter (the acrylic polymer A3) in the solution S3. This gave a pressure-sensitive adhesive composition C3, which is a composition for the formation of a thermally releasable pressure-sensitive adhesive layer. Next, the pressure-sensitive adhesive composition C2 was applied to one side of the PET substrate B2 to from a pressure-sensitive adhesive composition layer, and a release film was laminated so as to cover the pressure-sensitive adhesive composition layer on the PET substrate B2. Next, the pressure-sensitive adhesive composition C3 was applied to the other side of the PET substrate B2 to form a pressure-sensitive adhesive composition layer. Next, these pressure-sensitive adhesive composition layers were heated to be dried and to undergo a crosslinking reaction, to form a 25-μm thick pressure-sensitive adhesive layer derived from the pressure-sensitive adhesive composition C2, and a 30-μm thick thermally releasable pressure-sensitive adhesive layer derived from the pressure-sensitive adhesive composition C3. The heating herein was performed at a temperature of 130° C. for a time of 3 minutes. Thus, a pressure-sensitive adhesive sheet for build stage use (double-sided pressure-sensitive adhesive sheet) according to Example 2 was produced. This had a multilayer structure including the pressure-sensitive adhesive layer (non-thermally-releasable), the PET substrate B2, and the thermally releasable pressure-sensitive adhesive layer disposed in the specified sequence. 
     Use of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     The pressure-sensitive adhesive sheet produced by the procedure as described above was affixed, through the thermally releasable pressure-sensitive adhesive layer thereof, to an object-forming surface of a build stage of a 3D printer BS01+ (trade name, supplied by BONSAI LAB, Inc.), in such a manner as to avoid wrinkling and partial gap of the pressure-sensitive adhesive sheet. The 3D printer was then operated to form an object. Specifically, a modeling material was discharged and fed from a modeling material-discharging nozzle of the 3D printer toward the build stage, to form a planar object (100 mm by 100 mm, 2 mm thick) on the pressure-sensitive adhesive sheet for build stage use disposed on the build stage. The modeling material used herein was polylactic acid filaments Polyplus (trade name, natural color, supplied by Polymaker) having a cross-sectional diameter of 1.75 mm. The object forming process was performed at a temperature of the object-forming surface of the build stage of 30° C. and a heating temperature of the modeling material in the modeling material-discharging nozzle of 220° C. The object was formed while being firmly attached to the non-thermally-releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet on the build stage. After the object forming process was completed and the object was naturally cooled, the object-forming surface of the build stage was raised in temperature up to 60° C. to heat the thermally releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet disposed on the surface to thereby loosen the crystallized state of the crystalline side chains of the acrylic polymer A4 (side-chain crystalline polymer) in the pressure-sensitive adhesive layer. This reduced the adhesive strength of the pressure-sensitive adhesive layer (and, in turn, freed or loosened the attached state of the object to the build stage through the pressure-sensitive adhesive sheet). Subsequently, a removing operation of the pressure-sensitive adhesive sheet on the build stage was performed by separating the thermally releasable pressure-sensitive adhesive layer, whose adhesive strength had been reduced by heating, from the build stage, and with this operation, the object was detached from the build stage. The pressure-sensitive adhesive sheet was then removed from the object. 
     Example 3 
     Production of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     The acrylic polymer solution S5 was combined with, per 100 parts by mass of solid matter (the acrylic polymer A5) in the solution S5, 1 part by mass of an aluminum chelate crosslinker ALCH-TR (trade name, supplied by Kawaken Fine Chemicals Co., Ltd.) and yielded a pressure-sensitive adhesive composition C4, which is a composition for the formation of a cold-releasable pressure-sensitive adhesive layer. Next, the pressure-sensitive adhesive composition C2 was applied to one side of the PET substrate B2 to form a pressure-sensitive adhesive composition layer, and a release film was laminated so as to cover the pressure-sensitive adhesive composition layer on the PET substrate B2. Next, the pressure-sensitive adhesive composition C4 was applied to the other side of the PET substrate B2 to form a pressure-sensitive adhesive composition layer. Next, these pressure-sensitive adhesive composition layers were heated to be dried and to undergo a crosslinking reaction, to form a 25-μm thick pressure-sensitive adhesive layer derived from the pressure-sensitive adhesive composition C2, and a 30-μm thick cold-releasable pressure-sensitive adhesive layer derived from the pressure-sensitive adhesive composition C4. The heating herein was performed at a temperature of 130° C. for a time of 3 minutes. Thus, a pressure-sensitive adhesive sheet for build stage use (double-sided pressure-sensitive adhesive sheet) according to Example 3 was produced. This had a multilayer structure including the pressure-sensitive adhesive layer (non-thermally-releasable), the PET substrate B2, and the cold-releasable pressure-sensitive adhesive layer disposed in the specified sequence. 
     Use of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     The pressure-sensitive adhesive sheet produced by the procedure as described above was affixed, through the cold-releasable pressure-sensitive adhesive layer thereof, to an object-forming surface of a build stage of a 3D printer BS01+ (trade name, supplied by BONSAI LAB, Inc.), in such a manner as to avoid wrinkling and partial gap of the pressure-sensitive adhesive sheet. The 3D printer was then operated under conditions similar to those described above for Example 1, to form a planar object (100 mm by 100 mm, 2 mm thick) over the build stage. The object was formed while being firmly attached to the non-thermally-releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet on the build stage. After the object forming process was completed and the object was naturally cooled, the object-forming surface of the build stage was cooled down to 5° C. to cool the cold-releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet on the surface and to crystallize the crystalline side chains of the acrylic polymer A5 (side-chain crystalline polymer) in the pressure-sensitive adhesive layer. This reduced the adhesive strength of the pressure-sensitive adhesive layer (and, in turn, freed or loosened the attached state of the object to the build stage through the pressure-sensitive adhesive sheet). Subsequently, a removing operation of the pressure-sensitive adhesive sheet on the build stage was performed by separating the cold-releasable pressure-sensitive adhesive layer, whose adhesive strength had been reduced by cooling, from the build stage, and, with this operation, the object was detached from the build stage. The pressure-sensitive adhesive sheet was then removed from the object. 
     Example 4 
     Production of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     The acrylic polymer solution S6 was combined with, per 100 parts by mass of solid matter (the acrylic polymer A6) in the solution S6, 5 parts by mass of an isocyanate crosslinker CORONATE L (trade name, a polyisocyanate compound, supplied by TOSOH CORPORATION) and 5 parts by mass of a photoinitiator Irgacure 651 (trade name, supplied by BASF SE) and yielded a pressure-sensitive adhesive composition C5, which is a composition for the formation of an electromagnetically releasable pressure-sensitive adhesive layer. Next, the pressure-sensitive adhesive composition C2 was applied to one side of the PET substrate B2 to form a pressure-sensitive adhesive composition layer, and a release film was laminated so as to cover the pressure-sensitive adhesive composition layer on the PET substrate B2. Next, the pressure-sensitive adhesive composition C5 was applied to the other side of the PET substrate B2 to form a pressure-sensitive adhesive composition layer. Next, these pressure-sensitive adhesive composition layers were heated to be dried and to undergo a crosslinking reaction by the isocyanate crosslinker, to thereby form a 20-μm thick electromagnetically releasable pressure-sensitive adhesive layer derived from the pressure-sensitive adhesive composition C5, and a 25-μm thick pressure-sensitive adhesive layer derived from the pressure-sensitive adhesive composition C2. The heating herein was performed at a temperature of 130° C. for a time of 3 minutes. Thus, a pressure-sensitive adhesive sheet for build stage use (double-sided pressure-sensitive adhesive sheet) according to Example 4 was produced. This had a multilayer structure including the electromagnetically releasable pressure-sensitive adhesive layer, the PET substrate B2, and the pressure-sensitive adhesive layer (non-electromagnetically-releasable) disposed in the specified sequence. 
     Use of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     The pressure-sensitive adhesive sheet produced by the procedure as described above was affixed, through the non-electromagnetically-releasable pressure-sensitive adhesive layer thereof, to an object-forming surface of a build stage of a 3D printer BS01+ (trade name, supplied by BONSAI LAB, Inc.), in such a manner as to avoid wrinkling and partial gap of the pressure-sensitive adhesive sheet. The 3D printer was then operated under conditions similar to those described above for Example 1, to form a planar object (100 mm by 100 mm, 2 mm thick) over the build stage. The object was formed while being firmly attached to the electromagnetically releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet on the build stage. After the object forming process was completed and the object was naturally cooled, ultraviolet radiation was applied via the build stage to the pressure-sensitive adhesive sheet disposed on the build stage (capable of transmitting ultraviolet radiation). This reduced the adhesive strength of the electromagnetically releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet (and, in turn, freed or loosened the attached state of the object to the build stage through the pressure-sensitive adhesive sheet). The ultraviolet application was performed using a high-pressure mercury lamp as an ultraviolet irradiator, at an output of 75 W, an irradiation intensity of 150 mW/cm 2 , and an ultraviolet cumulative irradiance of 500 mJ/cm 2 . The object was then detached from the build stage by puling off the object from the electromagnetically releasable pressure-sensitive adhesive layer whose adhesive strength had been reduced by the electromagnetic irradiation. After the detachment of the object, the pressure-sensitive adhesive sheet was removed from the build stage surface. 
     Example 5 
     Production of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     The acrylic polymer solution S7 was combined with, per 100 parts by mass of solid matter (the acrylic oligomer A7) in the solution S7, 30 parts by mass of an electromagnetic radiation-curable urethane acrylate KAYARAD DPHA (trade name, supplied by Nippon Kayaku Co., Ltd.), 5 parts by mass of an isocyanate crosslinker CORONATE L (trade name, a polyisocyanate compound, supplied by TOSOH CORPORATION), and 5 parts by mass of a photoinitiator Irgacure 651 (trade name, supplied by BASF SE), and yielded a pressure-sensitive adhesive composition C6, which is a composition for the formation of an electromagnetically releasable pressure-sensitive adhesive layer. Next, the pressure-sensitive adhesive composition C2 was applied to one side of the PET substrate B2 to form a pressure-sensitive adhesive composition layer, and a release film was laminated so as to cover the pressure-sensitive adhesive composition layer on the PET substrate B2. Next, the pressure-sensitive adhesive composition C6 was applied to the other side of the PET substrate B2 to form a pressure-sensitive adhesive composition layer. Next, these pressure-sensitive adhesive composition layers were heated to be dried and to undergo a crosslinking reaction by the isocyanate crosslinker, to thereby form a 20-μm thick electromagnetically releasable pressure-sensitive adhesive layer derived from the pressure-sensitive adhesive composition C6, and a 25-μm thick pressure-sensitive adhesive layer derived from the pressure-sensitive adhesive composition C2. The heating herein was performed at a temperature of 130° C. for a time of 3 minutes. Thus, a pressure-sensitive adhesive sheet for build stage use (double-sided pressure-sensitive adhesive sheet) according to Example 5 was produced. This had a multilayer structure including the electromagnetically releasable pressure-sensitive adhesive layer, the PET substrate B2, and the pressure-sensitive adhesive layer (non-electromagnetically-releasable) disposed in the specified sequence. 
     Use of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     The pressure-sensitive adhesive sheet produced by the procedure as described above was affixed, through the non-electromagnetically-releasable pressure-sensitive adhesive layer thereof, to an object-forming surface of a build stage of a 3D printer BS01+ (trade name, supplied by BONSAI LAB, Inc.), in such a manner as to avoid wrinkling and partial gap of the pressure-sensitive adhesive sheet. The 3D printer was then operated under conditions similar to those described above for Example 1, to form a planar object (100 mm by 100 mm, 2 mm thick) over the build stage. The object was formed while being firmly attached to the electromagnetically releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet on the build stage. After the object forming process was completed and the object was naturally cooled, ultraviolet radiation was applied via the build stage (capable of transmitting ultraviolet radiation) to the pressure-sensitive adhesive sheet on the build stage, by a procedure similar to that described above for Example 4. This reduced the adhesive strength of the electromagnetically releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet (and, in turn, freed or loosened the attached state of the object to the build stage through the pressure-sensitive adhesive sheet). The object was then detached from the build stage by pulling off the object from the electromagnetically releasable pressure-sensitive adhesive layer whose adhesive strength had been reduced by the electromagnetic application. After the detachment of the object, the pressure-sensitive adhesive sheet was removed from the build stage surface. 
     Example 6 
     Preparation of Stretchable Substrate 
     A poly(vinyl chloride) (PVC) composition was prepared by mixing 100 parts by mass of a poly(vinyl chloride) TH-1300 (trade name, supplied by Shin-Etsu Chemical Co., Ltd.), 10 parts by mass of a chlorinated polyethylene Elaslen 302NA (trade name, supplied by Showa Denko K.K.), 50 parts by mass of an adipic acid polyester plasticizer Polycizer W-2310 (trade name, supplied by DIC Corporation), 0.5 part by mass of zinc laurate (supplied by Mitsuwa Chemicals Co., Ltd.) as a zinc soap, 0.5 part by mass of calcium stearate (supplied by KISHIDA CHEMICAL Co., Ltd.) as an alkaline earth metal soap, 2 parts by mass of hydrotalcite ALCAMIZER (trade name, supplied by Kyowa Chemical Industry Co., Ltd.), and 2 parts by mass of a black pigment. From the PVC composition, a PVC film (100-μm thick) was prepared using a calender. The primer U2 was applied to one side of the PVC film, followed by drying to give an under coat (about 1 g/m 2 ). Thus, a stretchable substrate bearing an under coat on one side thereof was prepared. 
     Production of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     A composition (pressure-sensitive adhesive composition C7) having a solids concentration of 20 mass percent for the formation of a pressure-sensitive adhesive layer was prepared by mixing 50 parts by mass of a styrene-butadiene rubber SBR1013 (trade name, binding styrene amount: 43 mass percent, supplied by JSR Corporation), 50 parts by mass of a natural rubber Grade RSS1 (trade name, supplied by Nomura Trading Co., Ltd.), 50 parts by mass of a terpene resin YS RESIN PX 1150 (trade name, having a softening temperature of 115° C., supplied by Yasuhara Chemical Co., Ltd.), 10 parts by mass of an alkylphenol resin TACKROL 201 (trade name, supplied by Taoka Chemical Co., Ltd.), 7 parts by mass of an organometallic compound (aluminum chelate) ALCH (trade name, supplied by Kawaken Fine Chemicals Co., Ltd.), 1 part by mass of a phenolic age inhibitor Irganox 1010 (trade name, supplied by BASF SE), and toluene as a solvent. Next, the pressure-sensitive adhesive composition C7 was applied to the under-coat-bearing surface of the stretchable substrate, dried, and yielded a 10-μm thick pressure-sensitive adhesive layer. Thus, a pressure-sensitive adhesive sheet for build stage use (single-sided pressure-sensitive adhesive sheet) according to Example 6 was produced. This had a multilayer structure including the stretchable substrate and the rubber pressure-sensitive adhesives layer disposed on each other. 
     Use of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     The pressure-sensitive adhesive sheet produced by the procedure as described above was affixed, through the pressure-sensitive adhesive layer thereof, to an object-forming surface of a build stage of a 3D printer BS01+(trade name, supplied by BONSAI LAB, Inc.), in such a manner as to avoid wrinkling and partial gap of the pressure-sensitive adhesive sheet. The 3D printer was then operated under conditions similar to those described above for Example 1, to form a planar object (100 mm by 100 mm, 2 mm thick) over the build stage. The object was formed while being attached to the stretchable substrate of the pressure-sensitive adhesive sheet on the build stage. After the object forming process was completed and the object was naturally cooled, the pressure-sensitive adhesive sheet was stretched by pulling ends of the pressure-sensitive adhesive sheet in an in-plane direction of the pressure-sensitive adhesive sheet. This reduced the adhesive strength of the pressure-sensitive adhesive layer to the build stage surface (and, in turn, freed or loosened the attached state of the object to the build stage through the pressure-sensitive adhesive sheet). Subsequently, a removing operation of the pressure-sensitive adhesive sheet on the build stage was performed by separating the pressure-sensitive adhesive layer, whose adhesive strength had been reduced, from the build stage surface, and with this operation, the object was detached from the build stage. The pressure-sensitive adhesive sheet was then removed from the object. 
     Example 7 
     Production of Pressure-Sensitive Adhesive Sheet for Build Stage Use. The acrylic polymer solution S2 was combined with, per 100 parts by mass of solid matter (the acrylic polymer A2) in the solution S2, 5 parts by mass of an isocyanate crosslinker CORONATE L (trade name, a polyisocyanate compound, supplied by TOSOH CORPORATION), 20 parts by mass of a tackifier SUMILITERESIN PR-12603 (trade name, supplied by Sumitomo Durez Co., Ltd.), and 20 parts by mass of a rosin ester ESTER GUM AT (trade name, supplied by Arakawa Chemical Industries, Ltd.) and yielded a pressure-sensitive adhesive composition C8, which is a composition for the formation of a pressure-sensitive adhesive layer. The pressure-sensitive adhesive composition C8 was applied to both sides of a Manila hemp nonwoven fabric substrate (having a thickness of 50 μm and a basis weight of 20 g/m 2 ) and dried, to form 55-μm thick pressure-sensitive adhesive layers. Thus, a pressure-sensitive adhesive sheet was produced. This had a multilayer structure including the pressure-sensitive adhesive layer, the nonwoven fabric substrate, and the pressure-sensitive adhesive layer disposed in the specified sequence. On the other hand, the acrylic polymer solution S2 was combined with, per 100 parts by mass of solid matter (the acrylic polymer A2) in the solution S2, 5 parts by mass of an isocyanate crosslinker CORONATE L (trade name, a polyisocyanate compound, supplied by TOSOH CORPORATION) and 100 parts by mass of thermally expandable microspheres Matsumoto Microsphere F-50D (trade name, expandable at 120° C., supplied by Matsumoto Yushi-Seiyaku Co., Ltd.) and yielded a pressure-sensitive adhesive composition C9, which is a composition for the formation of a thermally releasable pressure-sensitive adhesive layer. Next, one side of a 25-μm thick polyester film was subjected to a release treatment to give a release film, and the pressure-sensitive adhesive composition C9 was applied to the release-treated surface of the release film to form a pressure-sensitive adhesive composition layer. Next, the pressure-sensitive adhesive composition layer on the release film was heated to be dried and to undergo a crosslinking reaction, and yielded a 50-μm thick thermally releasable pressure-sensitive adhesive layer. The heating herein was performed at a temperature of 100° C. for a time of 5 minutes. This thermally releasable pressure-sensitive adhesive layer was affixed to one of the pressure-sensitive adhesive layers of the pressure-sensitive adhesive sheet including, on both sides thereof, the pressure-sensitive adhesive layers each derived from the pressure-sensitive adhesive composition C8. Thus, a pressure-sensitive adhesive sheet for build stage use (double-sided pressure-sensitive adhesive sheet) according to Example 7 was produced. This had a multilayer structure including the pressure-sensitive adhesive layer (non-thermally-releasable), the nonwoven fabric substrate, the pressure-sensitive adhesive layer (non-thermally-releasable), and the thermally releasable pressure-sensitive adhesive layer disposed in the specified sequence. 
     Use of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     The pressure-sensitive adhesive sheet produced by the procedure as described above was affixed, through the thermally releasable pressure-sensitive adhesive layer thereof, to an object-forming surface of a build stage of a 3D printer BS01+ (trade name, supplied by BONSAI LAB, Inc.), in such a manner as to avoid wrinkling and partial gap of the pressure-sensitive adhesive sheet. The 3D printer was then operated under conditions similar to those described above for Example 1, to form a planar object (100 mm by 100 mm, 2 mm thick) over the build stage. The object was formed while being firmly attached to the non-thermally-releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet on the build stage. After the object forming process was completed and the object was naturally cooled, the object-forming surface of the build stage was raised in temperature up to 120° C. to heat the thermally releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet on the surface to thereby expand the thermally expandable microspheres in the pressure-sensitive adhesive layer. This reduced the adhesive strength of the pressure-sensitive adhesive layer (and, in turn, freed or loosened the attached state of the object to the build stage through the pressure-sensitive adhesive sheet). Subsequently, a removing operation of the pressure-sensitive adhesive sheet on the build stage was performed by separating the thermally releasable pressure-sensitive adhesive layer, whose adhesive strength had been reduced by heating, from the build stage, and with this operation, the object was detached from the build stage. The pressure-sensitive adhesive sheet was then removed from the object. 
     Example 8 
     Production of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     The pressure-sensitive adhesive composition C5 for the formation of an electromagnetically releasable pressure-sensitive adhesive layer was applied to one side (the side bearing no under coat) of the PET substrate B1 to form a pressure-sensitive adhesive composition layer. Next, the pressure-sensitive adhesive composition layer was heated to be dried and to undergo a crosslinking reaction by the isocyanate crosslinker, and thereby yielded a 20-μm thick electromagnetically releasable pressure-sensitive adhesive layer. The heating herein was performed at a temperature of 130° C. for a time of 3 minutes. On the other hand, one side of a 25-μm thick polyester film was subjected to a release treatment to give a release film, and the pressure-sensitive adhesive composition C9 for the formation of a thermally releasable pressure-sensitive adhesive layer was applied to the release treated surface of the release film to form a pressure-sensitive adhesive composition layer. Next, the pressure-sensitive adhesive composition layer on the release film was heated to be dried and to undergo a crosslinking reaction, and thereby yielded a 50-μm thick thermally releasable pressure-sensitive adhesive layer, where the heating herein was performed at a temperature of 100° C. for a time of 5 minutes. This thermally releasable pressure-sensitive adhesive layer was affixed to the under-coat-bearing side of the PET substrate B1 bearing, on the other side, the electromagnetically releasable pressure-sensitive adhesive layer formed from the pressure-sensitive adhesive composition C5. Thus, a pressure-sensitive adhesive sheet for build stage use (double-sided pressure-sensitive adhesive sheet) according to Example 8 was produced. This had a multilayer structure including the electromagnetically releasable pressure-sensitive adhesive layer, the PET substrate B1, and the thermally releasable pressure-sensitive adhesive layer disposed in the specified sequence. 
     Use of Pressure-Sensitive Adhesive Sheet for Build Stage Use 
     The pressure-sensitive adhesive sheet produced by the procedure as described above was affixed, through the thermally releasable pressure-sensitive adhesive layer thereof, to an object-forming surface of a build stage of a 3D printer BS01+ (trade name, supplied by BONSAI LAB, Inc.), in such a manner as to avoid wrinkling and partial gap of the pressure-sensitive adhesive sheet. The 3D printer was then operated under conditions similar to those described above for Example 1, to form a planar object (100 mm by 100 mm, 2 mm thick) over the build stage. The object was formed while being firmly attached to the electromagnetically releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet on the build stage. After the object forming process was completed and the object was naturally cooled, ultraviolet radiation was applied via the build stage (capable of transmitting ultraviolet radiation) to the pressure-sensitive adhesive sheet on the build stage. This reduced the adhesive strength of the electromagnetically releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet (and, in turn, freed or loosened the attached state of the object to the build stage through the pressure-sensitive adhesive sheet). The ultraviolet application was performed using a high-pressure mercury lamp as an ultraviolet irradiator, at an output of 75 W, an irradiation intensity of 150 mW/cm 2 , and an ultraviolet cumulative irradiance of 500 mJ/cm 2 . The object was then detached from the build stage by pulling off the object from the electromagnetically releasable pressure-sensitive adhesive layer whose adhesive strength had been reduced by the electromagnetic application. Next, the object-forming surface of the build stage was raised in temperature up to 120° C. to heat the thermally releasable pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet on the surface and to expand the thermally expandable microspheres in the pressure-sensitive adhesive layer. This reduced the adhesive strength of the pressure-sensitive adhesive layer. Thus, the pressure-sensitive adhesive sheet was removed from the build stage surface. 
     Comparative Example 1 
     Production of Pressure-Sensitive Adhesive Sheet A pressure-sensitive adhesive composition C11, which is a composition for the formation of a pressure-sensitive adhesive layer, was prepared by blending the acrylic polymer solution S8, a tetrafunctional epoxy crosslinker TETRAD-C (trade name, supplied by MITSUBISHI GAS CHEMICAL COMPANY, INC.), and a polymerized rosin ester PENSEL D-135 (trade name, supplied by Arakawa Chemical Industries, Ltd.) in proportions respectively of 100 parts by mass, 0.02 part by mass, and 40 parts by mass. Next, the pressure-sensitive adhesive composition C11 was applied onto a release-treated surface of a 38-μm thick polyester film (release film) to form a pressure-sensitive adhesive composition layer. Next, the pressure-sensitive adhesive composition layer on the release film was heated to be dried and to undergo a crosslinking reaction, and yielded a 65-μm thick pressure-sensitive adhesive layer. The heating herein was performed at a temperature of 100° C. for a time of 3 minutes. By the technique as above, two pressure-sensitive adhesive layers (65-μm thick) with release film were formed. Next, the pressure-sensitive adhesive layers were affixed to both sides of a Manila hemp nonwoven fabric substrate (having a thickness of 50 μm and a basis weight of 20 g/m 2 ). Thus, a pressure-sensitive adhesive sheet (double-sided pressure-sensitive adhesive sheet) according to Comparative Example 1 was produced. This had a multilayer structure including the pressure-sensitive adhesive layer, the nonwoven fabric substrate, and the pressure-sensitive adhesive layer disposed in the specified order. 
     Use of Pressure-Sensitive Adhesive Sheet 
     The pressure-sensitive adhesive sheet produced by the procedure as described above was affixed, through one pressure-sensitive adhesive layer thereof, to an object-forming surface of a build stage of a 3D printer BS01+(trade name, supplied by BONSAI LAB, Inc.) in such a manner as to avoid wrinkling and partial gap of the pressure-sensitive adhesive sheet. The 3D printer was then operated under conditions similar to those described above for Example 1, to form a planar object (100 mm by 100 mm, 2 mm thick) over the build stage. The object was formed while being attached to the other pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet on the build stage. After the object forming process was completed and the object was naturally cooled, there was attempted a removing operation of the pressure-sensitive adhesive sheet on the build stage by separating the pressure-sensitive adhesive layer from the build stage surface and, with this operation, the object was detached from the build stage. The pressure-sensitive adhesive sheet was then removed from the object. 
     Comparative Example 2 
     Production of Pressure-Sensitive Adhesive Sheet 
     A pressure-sensitive adhesive sheet (double-sided pressure-sensitive adhesive sheet) according to Comparative Example 2 was prepared by a procedure similar to that in Comparative Example 1, except for using the PET substrate B1 instead of the nonwoven fabric substrate; and forming a 30-μm thick pressure-sensitive adhesive layer, instead of the 65-μm thick pressure-sensitive adhesive layer, from the pressure-sensitive adhesive composition C11. The pressure-sensitive adhesive sheet according to Comparative Example 2 had a multilayer structure including the pressure-sensitive adhesive layer, the PET substrate B1, and the pressure-sensitive adhesive layer disposed in the specified sequence. 
     Use of Pressure-Sensitive Adhesive Sheet 
     The pressure-sensitive adhesive sheet produced by the procedure as described above was affixed, through one pressure-sensitive adhesive layer thereof, to an object-forming surface of a build stage of a 3D printer BS01+ (trade name, supplied by BONSAI LAB, Inc.) in such a manner as to avoid wrinkling and partial gap of the pressure-sensitive adhesive sheet. The 3D printer was then operated under conditions similar to those described above for Example 1, to form a planar object (100 mm by 100 mm, 2 mm thick) over the build stage. The object was formed while being attached to the other pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet on the build stage. After the object forming process was completed and the object was naturally cooled, there was attempted a removing operation of the pressure-sensitive adhesive sheet on the build stage by separating the pressure-sensitive adhesive layer from the build stage and, with this operation, the object was detached from the build stage. The pressure-sensitive adhesive sheet was then removed from the object. 
     Detachment of Object after Forming 
     The pressure-sensitive adhesive sheets according to Examples 1 to 8 and Comparative Examples 1 and 2 were evaluated for detachability of the object from the build stage, after the completion of the object forming process in which the 3D printer was operated to form (build up) the planar object. For each of the pressure-sensitive adhesive sheets according to Examples 1 to 3, 6, and 7, the object after undergoing the adhesive strength-reducing action corresponding to the target pressure-sensitive adhesive sheet was detached from the build stage with the removing operation of the pressure-sensitive adhesive sheet from the build stage, as described above. For each of the pressure-sensitive adhesive sheets according to Examples 4, 5, and 8, the object, after undergoing the adhesive strength-reducing action corresponding to the target pressure-sensitive adhesive sheet, was detached from the build stage by separating the object from the pressure-sensitive adhesive sheet on the build stage, as described above. For each of the pressure-sensitive adhesive sheets according to Comparative Examples 1 and 2, the object was detached from the build stage by attempting the removing operation of the pressure-sensitive adhesive sheet from the build stage, as described above. The detachment of the object from the build stage was evaluated as follows: a sample in which the object can be easily detached was evaluated as having very good detachability (Very good); a sample in which the detachment requires significantly larger force as compared with the sample of very good detachability was evaluated as having somewhat poor detachability (Somewhat poor); and a sample in which the detachment is hardly performed without the use of a scraper or another tool having a sharp edge was evaluated as having poor detachability (Poor). The results are given in Tables 1 to 3. 
     Release Force Required of Object Detachment 
     The pressure-sensitive adhesive sheet according to Example 6 was evaluated, by measurement, for release force required of detachment of the object from the build stage, namely, for external force required of removal of the pressure-sensitive adhesive sheet, with the object, from the build stage surface. Specifically, initially, the pressure-sensitive adhesive sheet was affixed to an object-forming surface of a build stage of a fused deposition modeling 3D printer BS01+ (trade name, supplied by BONSAI LAB, Inc.), which is an additive manufacturing apparatus. In the affixation, the surface defined by the pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet was affixed to the object-forming surface in such a manner as not to cause the pressure-sensitive adhesive sheet to suffer from wrinkling and partial gap from the build stage. Next, the 3D printer was operated to form an object. Specifically, a modeling material ABS resin filaments Plasil (trade name, having a cross-sectional diameter of 1.75 mm, white color, supplied by PLASIL), which was heated at 240° C. in a modeling material-discharging nozzle of the 3D printer, was discharged and fed from the nozzle toward the build stage to form the object on the pressure-sensitive adhesive sheet for build stage use on the build stage. The object-forming surface of the build stage was held at a temperature of 100° C. during the object forming process. The formed object includes a body  201  and a wall  202  as illustrated in  FIG. 21 . The body  201  has a planar shape (100 mm by 100 mm, 2 mm thick) which is square in plan view. The wall  202  is positioned at an outside edge of the body  201 , extends along throughout one side of the body  201 , and has a height of 7 mm and a thickness of 2 mm. The wall  202  is a portion with a 2-mm diameter hole (not shown). The hole is positioned at the center in the extending direction of the wall  202  and penetrates the wall  202  in the thickness direction. Next, when the temperature of the build stage after the completion of the object forming process fell down to 30° C., the pressure-sensitive adhesive sheet was stretched by pulling an end portion of the pressure-sensitive adhesive sheet in an in-plane direction of the sheet at a force of 4.5 kgf. This reduced the adhesive strength of the pressure-sensitive adhesive layer to the build stage surface. A string was bound, at one end thereof, to the wall  202  of the object using the hole and then pulled in the direction shown by the arrow D in  FIG. 21 . In this process, an external force (maximum value) required of the detachment of the object from the build stage was measured as a release force using a push-pull gauge PS (Max. Capacity 5 kg) (trade name, supplied by IMADA SEISAKUSHO CO., LTD.) to which the other end of the string was bound. The measured value in the measurement was 0 kgf. Specifically, the object to be detached was approximately in a state where the object had been detached from the build stage by the stretching of the pressure-sensitive adhesive sheet. 
     The pressure-sensitive adhesive sheet according to Example 7 was evaluated, by measurement, for release force required of detachment of the object from the build stage, namely, for external force required of removal of the pressure-sensitive adhesive sheet, with the object, from the build stage surface. Specifically, the release force was measured by a procedure similar to that described above for Example 6, except for performing the object forming process at a temperature of the object-forming surface of the build stage of 90° C., instead of 100° C.; and except for performing heating of the thermally releasable pressure-sensitive adhesive layer as the adhesive strength-reducing action, instead of the stretching of the pressure-sensitive adhesive sheet. The heating of the thermally releasable pressure-sensitive adhesive layer was performed by raising the temperature of the object-forming surface of the build stage up to 120° C. The heating caused expansion of the thermally expandable microspheres in the thermally releasable pressure-sensitive adhesive layer, and thereby reduced the adhesive strength of the pressure-sensitive adhesive layer. The measured value in the measurement was 0.05 kgf. 
     The pressure-sensitive adhesive sheet according to Comparative Example 1 was evaluated, by measurement, for release force required of detachment of the object from the build stage, namely, for external force required of removal of the pressure-sensitive adhesive sheet, with the object, from the build stage surface. Specifically, the release force was measured by a procedure similar to that described above for Example 6, except for performing the removing operation when the temperature of the build stage after the completion of the object forming process fell down to 30° C. The measured value in the measurement was equal to or greater than the measurement limit (5 kgf) of the push-pull gauge used. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Pressure-sensitive 
                 Pressure-sensitive 
                 Pressure-sensitive 
                 Pressure-sensitive 
                 Electromagnetically 
               
               
                 adhesive layer 
                 adhesive layer 
                 adhesive layer 
                 adhesive layer 
                 releasable 
               
               
                 facing object 
                 (formed from 
                 (formed from 
                 (formed from 
                 pressure-sensitive 
               
               
                   
                 pressure-sensitive 
                 pressure-sensitive 
                 pressure-sensitive 
                 adhesive layer 
               
               
                   
                 adhesive composition C2) 
                 adhesive composition C2) 
                 adhesive composition C2) 
                 (formed from 
               
               
                   
                   
                   
                   
                 pressure-sensitive 
               
               
                   
                   
                   
                   
                 adhesive composition C5) 
               
               
                 Substrate 
                 PET substrate B1 
                 PET substrate B2 
                 PET substrate B2 
                 PET substrate B2 
               
               
                 Pressure-sensitive 
                 Thermally releasable 
                 Thermally releasable 
                 Cold-releasable 
                 Pressure-sensitive 
               
               
                 adhesive layer 
                 pressure-sensitive 
                 pressure-sensitive 
                 pressure-sensitive 
                 adhesive layer 
               
               
                 facing build stage 
                 adhesive layer 
                 adhesive layer 
                 adhesive layer 
                 (formed from 
               
               
                   
                 (formed from 
                 (formed from 
                 (formed from 
                 pressure-sensitive 
               
               
                   
                 pressure-sensitive 
                 pressure-sensitive 
                 pressure-sensitive 
                 adhesive composition C2) 
               
               
                   
                 adhesive composition C1) 
                 adhesive composition C3) 
                 adhesive composition C4) 
               
               
                 Object detachability 
                 Very good 
                 Very good 
                 Very good 
                 Very good 
               
               
                 after forming 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Example 5 
                 Example 6 
                 Example 7 
                 Example 8 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Pressure-sensitive 
                 Electromagnetically 
                 — 
                 Pressure-sensitive 
                 Electromagnetically 
               
               
                 adhesive layer 
                 releasable 
                   
                 adhesive layer 
                 releasable 
               
               
                 facing object 
                 pressure-sensitive 
                   
                 (formed from 
                 pressure-sensitive 
               
               
                   
                 adhesive layer 
                   
                 pressure-sensitive 
                 adhesive layer 
               
               
                   
                 (formed from 
                   
                 adhesive composition C8) 
                 (formed from 
               
               
                   
                 pressure-sensitive 
                   
                   
                 pressure-sensitive 
               
               
                   
                 adhesive composition C6) 
                   
                   
                 adhesive composition C5) 
               
               
                 Substrate 
                 PET substrate B2 
                 Stretchable substrate 
                 Nonwoven fabric substrate 
                 PET substrate B1 
               
               
                   
                   
                 (PVC) 
                 (Manila hemp) 
               
               
                 Pressure-sensitive 
                 Pressure-sensitive 
                 Rubber pressure-sensitive 
                 Thermally releasable 
                 Thermally releasable 
               
               
                 adhesive layer 
                 adhesive layer 
                 adhesive layer 
                 pressure-sensitive 
                 pressure-sensitive 
               
               
                 facing build stage 
                 (formed from 
                 (formed from 
                 adhesive layer 
                 adhesive layer 
               
               
                   
                 pressure-sensitive 
                 pressure-sensitive 
                 (formed from 
                 (formed from 
               
               
                   
                 adhesive composition C2) 
                 adhesive composition C7) 
                 pressure-sensitive 
                 pressure-sensitive 
               
               
                   
                   
                   
                 adhesive composition C9) 
                 adhesive composition C9) 
               
               
                 Object detachability 
                 Very good 
                 Very good 
                 Very good 
                 Very good 
               
               
                 after forming 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Comparative Example 1 
                 Comparative Example 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Pressure-sensitive adhesive layer 
                 Pressure-sensitive adhesive layer 
                 Pressure-sensitive adhesive layer 
               
               
                 facing object 
                 (formed from pressure-sensitive 
                 (formed from pressure-sensitive 
               
               
                   
                 adhesive composition C11) 
                 adhesive composition C11) 
               
               
                 Substrate 
                 Nonwoven fabric substrate 
                 PET substrate B1 
               
               
                   
                 (Manila hemp) 
               
               
                 Pressure-sensitive adhesive layer 
                 Pressure-sensitive adhesive layer 
                 Pressure-sensitive adhesive layer 
               
               
                 facing build stage 
                 (formed from pressure-sensitive 
                 (formed from pressure-sensitive 
               
               
                   
                 adhesive composition C11) 
                 adhesive composition C11) 
               
               
                 Object detachability after forming 
                 Poor 
                 Poor 
               
               
                   
               
            
           
         
       
     
     Evaluations 
     The pressure-sensitive adhesive sheets for build stage use according to Examples 1 to 8, which have the configuration(s) according to the present invention, allowed the object to be readily detached from the build stage after the completion of the object forming process, as demonstrated by the evaluation results in “Object detachability after forming” in Tables 1 and 2. The pressure-sensitive adhesive sheet according to Example 6 and the pressure-sensitive adhesive sheet according to Example 7 required very low forces respectively of 0 kgf and 0.05 kgf for the object detachment, where the forces were measured by the measuring method. In contrast, the pressure-sensitive adhesive sheets according to Comparative Examples 1 and 2 failed to allow the object to be readily detached from the build stage after the completion of the object forming process, as demonstrated by the evaluation results in “Object detachability after forming” in Table 3. The pressure-sensitive adhesive sheet according to Comparative Example 1 required a force equal to or greater than the measurement limit, namely, equal to or greater than 5 kgf, for the object detachment, where the force was measured by the measuring method. 
     REFERENCE SIGNS LIST 
     
         
         
           
             X 1 , X 2 , X 3 , X 4 , X 5 , X 6 , X 7  pressure-sensitive adhesive sheet (pressure-sensitive adhesive sheet for build stage use) 
             S 1 , S 2 , S 3  substrate 
               11 ,  12 ,  21 ,  22 ,  31 ,  32 ,  41 ,  42 ,  51 ,  52 ,  61 ,  62  pressure-sensitive adhesive layer 
               100  build stage 
               101  object-forming surface 
               110  heating unit 
               120  cooling unit 
               130  electromagnetic irradiation unit (electromagnetic radiation applying means, adhesive strength-reducing means) 
               140  voltage application unit (voltage applying means, adhesive strength-reducing means) 
               150  liquid supply unit 
               160  stretching unit 
               170  magnetic field application unit