Patent Publication Number: US-11034083-B2

Title: Three dimensional printing system that automatically removes particles from build plane

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This non-provisional patent application is a continuation-in-part of U.S. application Ser. No. 15/926,148, Entitled “THREE DIMENSIONAL PRINTING SYSTEM THAT AUTOMATICALLY REMOVES PARTICLES FROM BUILD PLANE”, by Christopher Tanner et al., filed on Mar. 20, 2018 which is hereby incorporated by reference. U.S. application Ser. No. 15/926,148 claims priority to U.S. Provisional Application Ser. No. 62/477,747, Entitled “THREE DIMENSIONAL PRINTING SYSTEM THAT AUTOMATICALLY REMOVES PARTICLES FROM BUILD PLANE” by Christopher Tanner et al., filed on Mar. 28, 2017, incorporated herein by reference under the benefit of U.S.C. 119(e). 
    
    
     FIELD OF THE INVENTION 
     The present disclosure concerns an apparatus and method for fabrication of solid three dimensional (3D) articles of manufacture from radiation curable (photocurable) resins. More particularly, the present disclosure improves the quality of a three dimensional (3D) article of manufacture through the removal of particles extending into a build plane. 
     BACKGROUND 
     Three dimensional (3D) printers are in rapidly increasing use. One class of 3D printers includes stereolithography printers having a general principle of operation including the selective curing and hardening of radiation curable (photocurable) liquid resins. A typical stereolithography system includes a containment vessel holding the photocurable resin, a movement mechanism coupled to a support surface, and a controllable light engine. The stereolithography system forms a three dimensional (3D) article of manufacture by selectively curing layers of the photocurable resin. 
     In one system embodiment the vessel includes a transparent sheet that forms part of a lower surface of the vessel. The support surface is positioned above and in facing relation with the transparent sheet. The following steps take place: (1) The movement mechanism positions the support surface whereby a thin layer of the photocurable resin resides between the support surface and the transparent sheet. (2) The light engine transmits pixelated light up through the transparent sheet to selectively cure a layer of the photocurable resin proximate to and onto the support surface. The focus of the pixelated light curing is referred to a “build plane.” (3) The movement mechanism then incrementally raises the support surface. Steps (2) and (3) are repeated to form a three dimensional (3D) article of manufacture having a lower face in facing relation with the transparent sheet. 
     One difficulty is an accumulation of particles on the transparent sheet and/or within the photocurable resin. The particles are formed from the photocurable resin and are the result of portions of a fabricated article that may break off and settle into the resin. A required gap between the build plane and the transparent sheet is very small. The lower face of the support surface or three dimensional article of manufacture must therefore be positioned very close (a small fraction of a millimeter typically) to the transparent sheet in order to perform step (2) above. During this positioning, the accumulated particles can become compressed between the transparent sheet and the lower face. These particles can be compressed and can damage the transparent sheet and become embedded in the three dimensional article of manufacture, possibly creating a defect. Damage to the transparent sheet will affect its light transmissive properties and therefore impact the quality of all subsequent fabrication. The transparent sheet is also expensive and disruptive to replace. What is needed is a system and method to facilitate particle removal. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a block diagram schematic of an exemplary embodiment of a three dimensional printing system for forming a three dimensional article of manufacture. 
         FIG. 2A  is a block diagram schematic depicting a print engine and a first embodiment of a configuration whereby the print engine traps deleterious particles. 
         FIG. 2B  is a block diagram schematic depicting a print engine and a second embodiment of a configuration whereby the print engine traps deleterious particles. 
         FIG. 2C  is a block diagram schematic depicting a print engine and a third embodiment of a configuration whereby the print engine traps deleterious particles. 
         FIG. 3  is a plan view schematic of an exemplary fixture for supporting a particle trapping sheet. 
         FIG. 4  is a sectional view taken from AA′ of  FIG. 3  with an exemplary particle trapping sheet included. 
         FIG. 5  is a schematic plan view of an exemplary particle trapping sheet. 
         FIG. 5A  is a cross sectional view of an exemplary particle trapping sheet taken through BB′ of  FIG. 5 . This particle trapping sheet embodiment corresponds to  FIG. 2A . 
         FIG. 5B  is a cross sectional view of an exemplary particle trapping sheet taken through BB′ of  FIG. 5 . This particle trapping sheet embodiment corresponds to  FIG. 2B . 
         FIG. 6A  is a flowchart depicting a first embodiment of a process for operating printing system  2  which corresponds to  FIG. 2A . 
         FIG. 6B  is a flowchart depicting a second embodiment of a process for operating printing system  2  which corresponds to  FIG. 2B . 
         FIG. 6C  is a flowchart depicting a third embodiment of a process for operating printing system  2  which corresponds to  FIG. 2C . 
         FIG. 7A  is a schematic block diagram depicting an embodiment of a standalone print engine forming a particle trapping sheet. 
         FIG. 7B  is a schematic block diagram depicting an embodiment of a standalone print engine manufacturing a three dimensional article. 
         FIG. 8  is a flowchart depicting an embodiment of a method of manufacturing a three dimensional article. 
         FIG. 9A  is an isometric drawing of an alternative embodiment of a support fixture having a central opening. 
         FIG. 9B  is a side cutaway view of the support fixture of  FIG. 9A . 
         FIG. 10A  is a cross-sectional view of the support fixture of  FIG. 9A . 
         FIG. 10B  is a cross-sectional view of the support fixture of  FIG. 10A  with an addition of a particle trapping sheet. 
         FIG. 10C  is a cross-sectional view of the support fixture of  FIG. 10A  with additions of a particle trapping sheet and a three dimensional article. 
         FIG. 11  is a flowchart that represents an embodiment of a method for manufacturing a three dimensional article that utilizes the support fixture  40  of  FIGS. 9A and 9B . 
         FIG. 12  is a cross-sectional view of a support fixture with a first embodiment of a particle trapping sheet that can be formed according to steps  152 - 156  of method  150  of  FIG. 11 . 
         FIG. 13  is a cross-sectional view of a support fixture with a second embodiment of a particle trapping sheet that can be formed according to steps  152 - 156  of method  150  of  FIG. 11 . 
         FIG. 14  is cross sectional view of a support fixture with a third embodiment of a particle trapping sheet that can be manufactured according to the method  150  of  FIG. 11 . 
         FIG. 15A  depicts a peripheral support that has been formed onto a support fixture. 
         FIG. 15B  depicts an upper layer of a particle trapping sheet. 
         FIG. 15C  depicts a middle layer of a particle trapping sheet. 
         FIG. 15D  depicts a lower layer of a particle trapping sheet. 
     
    
    
     SUMMARY 
     In a first aspect of the disclosure, a three dimensional printing system is for manufacturing a three dimensional article. The three dimensional printing system includes a resin vessel, a light engine, a movement mechanism, a support fixture, and a controller. The resin vessel is for containing a photocurable resin and has a lower portion with a transparent sheet. The light engine is configured to selectively project radiation up through the transparent sheet and over a build plane. The support fixture is coupled to the movement mechanism and includes an upper portion for coupling to the movement mechanism, a lower portion with a rim surrounding a central opening, and a side wall coupling the upper and lower portions. The controller is configured to: (1) operate the movement mechanism to position the rim at an operating distance from the transparent sheet, (2) operate the light engine and the movement mechanism to form a particle trapping sheet that spans the central opening, and (3) operate the light engine and the movement mechanism to form the three dimensional article attached to and below the particle trapping sheet. 
     In one implementation the controller is further configured to operate the light engine to form a peripheral support along the rim after positioning the rim at the operating distance but before forming the particle trapping sheet. The peripheral support can have a shape much like a rectangular frame. Alternatively, the peripheral support can have a zig-zag shape alternating shape along the rim. 
     In another implementation, the particle trapping sheet includes tapering features that taper between the particle trapping sheet and the three dimensional article to facilitate removal of the three dimensional article from the particle trapping sheet. 
     In yet another implementation, the particle trapping sheet defines an array of conduits to allow resin to flow through the particle trapping sheet and through the central opening as the movement mechanism raises and lowers the support fixture. The conduits individually are at least partially oriented at an oblique angle with respect to a vertical axis to facilitate complete trapping of particles. 
     In a further implementation, the particle trapping sheet defines an array of conduits to allow resin to flow through the particle trapping sheet and through the central opening as the movement mechanism raises and lowers the support fixture. The conduits individually define an upper opening on an upper side of the particle trapping sheet and a lower opening on a lower side of the particle trapping sheet, the upper and lower openings are laterally offset to facilitate trapping of particles. 
     In a yet further implementation, the particle trapping sheet has at least a vertical thickness of one millimeter to provide structural support for the three dimensional article. 
     In another implementation, the particle trapping sheet includes a plurality of support ribs to provide structural support for the three dimensional article. The support ribs can be defined between an upper sheet and a lower sheet that are thinner vertically than the support ribs. 
     In a second aspect of the disclosure, a three dimensional printing system includes a resin vessel, a light engine, a movement mechanism, a fixture, and a controller. The resin vessel is for containing a photocurable resin and has a lower portion with a transparent sheet. The light engine is configured to selectively project radiation up through the transparent sheet and over a build plane. The fixture is coupled to the movement mechanism and has a lower face that faces the transparent sheet. The controller is configured to operate the light engine to solidify a particle trapping sheet proximate to the build plane thereby trapping particles that extend upwardly from the transparent sheet into the build plane. 
     In one implementation the particle trapping sheet is solidified and formed while the fixture is in a raised position above the resin vessel in which the lower face of the fixture is not in contact with the resin. 
     In another implementation the controller is configured to pause operation of the print engine to allow removal of the particle trapping sheet from the resin vessel before the lower face of the fixture is lowered into the resin. 
     In yet another implementation the three dimensional printing system includes a user interface device. The controller is configured to display instructions on a user interface that instruct a user to remove the particle trapping sheet from the resin vessel. 
     In a further implementation the three dimensional printing system includes a user interface device. The controller is configured to pause operation of the print engine to allow removal of the particle trapping sheet from the resin vessel before the lower face of the fixture is lowered into the resin. The controller is also configured to receive instructions from a user interface to restart operation of the print engine. The controller is then configured to lower the fixture until the lower face is positioned within the resin and proximate to the build plane and operate the light engine and movement mechanism to manufacture the three dimensional article onto the lower face. 
     In a third aspect of the disclosure, a three dimensional printing system includes a print engine, a fixture, and a controller. The print engine further includes a vessel, a light engine, and a movement mechanism. The vessel is for containing a photocurable resin and has a lower portion with a transparent sheet defining at least part of a lower surface of the vessel. The light engine is configured to project radiation up through the transparent sheet over a lateral build plane which defines a maximum addressable lateral range of the light engine. The fixture has a lower face that faces downwardly. The controller is configured to: (1) position the lower face of the fixture at the build plane which is at an operating distance from the transparent sheet, and (2) operate the light engine and movement mechanism to solidify a particle trapping sheet proximate to the transparent sheet and substantially spanning the build plane and to thereby trap particles that are present along the build plane. 
     In one implementation the controller includes a processor coupled to an information storage device. The information storage device includes a non-transient or non-volatile storage device storing instructions that, when executed by the processor, control the light engine and the movement mechanism. The controller can be at one location or distributed among a plurality of locations in the printing system. In a first embodiment the controller is entirely within the print engine which operates as a standalone three dimensional printer. In a second embodiment the controller includes a control server that controls the overall printing system and a print engine controller that is located within the print engine. In the second embodiment the three dimensional printing system includes various modules including one or more of a fixture cassette, a post processing station, an inspection station, and a robotic transport mechanism for transporting the fixture between the modules. 
     In another implementation the fixture has a lower end defining a recessed surface from which a plurality of projections extend downwardly from the recessed surface to distal tips. The particle trapping sheet defines an upper surface coupled to the distal tips and an opposed lower surface. The opposed lower surface further defines a plurality of tapering features. The controller is further configured to operate the light engine and the movement mechanism to form a three dimensional article of manufacture that is coupled to the tapering features. The tapering features minimize a surface area of connection between the three dimensional article of manufacture and the particle trapping sheet in order to facilitate the later physical separation of the particle trapping sheet from the three dimensional article of manufacture. 
     In yet another implementation the printing system includes a transport mechanism and the controller is further configured to: (3) unload the fixture from the print engine, (4) load a new fixture into the printing system, and (5) operate the light engine and the movement mechanism to form a three dimensional article of manufacture. The fixture used in steps (1) and (2) is a disposable fixture that is used exclusively for forming the particle trapping sheet and removing particles. The disposable fixture has a lower end defining a recessed surface from which a plurality of projections extend downwardly from the recessed surface to distal tips. An upper surface of the particle trapping sheet is formed onto the distal tips. The fixture used in steps (3) to (5) is a reusable fixture that is used entirely for forming three dimensional articles of manufacture. 
     In a further implementation the controller is configured to operate the light engine and the movement mechanism to form a three dimensional article of manufacture onto the fixture before forming the particle trapping sheet. The particle trapping sheet includes a plurality of upwardly extending extensions that form a framework for supporting the particle trapping sheet onto the three dimensional article of manufacture. 
     In a yet further implementation the particle trapping sheet has an upper surface and an opposed lower surface. The particle trapping sheet defines at least one opening passing from the upper surface to the opposed lower surface to provide flow of photocurable resin therethrough when the movement mechanism is raising or lowering the fixture. Preferably the at least one opening includes an array or plurality of openings that are laterally distributed across the particle trapping sheet. 
     In another implementation the particle trapping sheet includes a thin parallelepiped portion covering most or essentially all of the lateral area of the build plane. The thickness is minimized so as to minimize a required amount of resin for fabricating the particle trapping sheet. The particle trapping sheet also includes a framework of ribs or thickened portions to provide mechanical support for the thin parallelepiped portion. In some embodiments the particle trapping sheet includes tapering features and/or extensions for coupling to a three dimensional article of manufacture. The coupling occurs at a narrowed distal tip. The tapering features and/or extensions can be laterally aligned with the ribs so as to improve structural integrity. The thin parallelepiped portion can define openings therethrough to allow a flow of photocurable resin therethrough to facilitate vertical movement of the fixture. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a block diagram schematic of an exemplary embodiment of a three dimensional printing system  2  for forming three dimensional articles of manufacture. Three dimensional printing system  2  includes fixture cassettes  4 , print engines  6 , post-processing stations  8 , inspection stations  10 , a transport mechanism  12 , and a control server  14 . 
     A fixture cassette  4  stores a stack of fixtures that are utilized in print engine  6 , post-process stations  8 , and inspection station  10 . In some embodiments there are different fixtures stored in different fixture cassettes  4 . One stack of fixtures can be disposable and utilized for a particle removal process. Another stack of fixtures can be reusable and utilized for the formation and transport of a three dimensional article of manufacture. 
     An embodiment of print engine  6  will be described in further detail with respect to  FIGS. 2A, 2B, and 2C . Post process stations  8  are for added processes for a three dimensional article of manufacture after it is formed. Post processing stations can include rinsing and cleaning stations, drying stations, and curing stations, to name some examples. Inspection stations  10  can be utilized to inspect for defects and/or to measure critical dimensions for a three dimensional article of manufacture after fabrication and post processing is complete. 
     Transport mechanism  12  is configured to pick up a fixture from a fixture cassette  4  and to transfer it to a print engine  6 . Transport mechanism  12  also transfers the fixture to the post process stations  8  and to the inspection stations  10 . In one embodiment, the transport mechanism includes a robotic gripper that can move in three axes. 
       FIG. 2A  is a schematic block diagram depicting print engine  6  and a first embodiment through which print engine  6  removes deleterious particles. In this and other figures, mutually perpendicular axes X, Y and Z will be used. Axes X and Y are lateral axes. In some embodiments X and Y are also horizontal axes. Axis Z is a central axis. In some embodiments Z is a vertical axis. In some embodiments the direction +Z is generally upward and the direction −Z is generally downward. 
     Print engine  6  includes a vessel  16  containing photocurable resin  18 . Vessel  16  includes a transparent sheet  20  that defines at least a portion of a lower surface  22  of vessel  16 . A light engine  24  is disposed to project light up through the transparent sheet  20  to selectively cure the photocurable resin  18  during formation of a three dimensional article of manufacture  26 . Light engine  24  includes light source  28  and spatial light modulator  30 . 
     Between a lower face  32  of the three dimensional article of manufacture  26  and the transparent sheet  20  is a thin layer  34  of photocurable resin  18 . As the light engine  24  operates, a portion of the thin layer  34  of photocurable resin  18  is cured and solidified at and proximate to a build plane  36 . Build plane  36  defines a lateral extent (along X and Y) of a layer of photocure resin that the light engine  24  is capable of curing when forming the three dimensional article of manufacture  26 . 
     Print engine  6  also includes a vertical movement mechanism  38  coupled to a fixture  40 . Fixture  40  is for supporting the three dimensional article of manufacture  26 . Fixture  40  includes a lower end  42  having an upwardly recessed surface  44  and projections  46  that extend downwardly from the recessed surface  44 . 
     Print engine  6  includes print engine controller  48  that is under control of control server  14  and is coupled to light engine  24  and to vertical movement mechanism  38 . In the illustrated embodiment, the print engine controller  48  controls the light engine  24  and the movement mechanism  38  to form a particle trapping sheet  50  before forming the three dimensional article of manufacture  26 . The particle trapping sheet  50  is first formed at and proximate to the build plane  36  and just above the transparent sheet  20 . During formation of the particle trapping sheet  50  the deleterious particulates are “bound up” in the particle trapping sheet  50  so that they are removed from the vicinity of the transparent sheet  20  to prevent build-up of particles and subsequent damage. The lateral extent of the particle trapping sheet  50  is preferably substantially the entire build plane  36 . 
     The particle trapping sheet  50  includes tapering features  52  that taper between the particle trapping sheet  50  and the three dimensional article of manufacture  26 . These tapering feature  52  facilitate removal of the particle trapping sheet  50  from the three dimensional article of manufacture  26  after processing is complete. 
       FIG. 2B  is a schematic block diagram depicting print engine  6  and a second embodiment through which print engine  6  removes deleterious particles. In comparing  FIGS. 2A and 2B , like reference numerals indicate like elements. The discussion for  FIG. 2B  will be limited to those features that necessarily make it different than  FIG. 2A . 
     In the illustrated embodiment of  FIG. 2B , the print engine controller  48  controls the light engine  24  and the movement mechanism  38  to form a three dimensional article of manufacture  26  before forming a particle trapping sheet  50 . The particle trapping sheet  50  includes extensions  54  that form a framework for coupling the particle trapping sheet  50  to the three dimensional article of manufacture  26 . As with  FIG. 2A , the particle trapping sheet  50  preferably covers the entire build plane  36  of light engine  24 . 
       FIG. 2C  is a schematic block diagram depicting print engine  6  and a third embodiment through which print engine  6  removes deleterious particles. In comparing  FIGS. 2A, 2B, and 2C , like reference numerals indicate like elements. The discussion for  FIG. 2C  will be limited to those features that necessarily make it different than  FIGS. 2A and 2C . 
     In the illustrated embodiment of  FIG. 2C , the fixture  40  is a disposable fixture  40  that is used entirely for forming the particle trapping sheet  50  to remove the deleterious particles. Other than being disposable, the fixture  40  is similar to the fixture  40  illustrated with respect to  FIG. 2A  and includes the upwardly recessed surface  44  from which the projections  46  extend downwardly. 
       FIG. 3  is a plan view schematic of fixture  40  looking upwardly in the +Z direction. The fixture  40  is shown having a recessed surface  44  from which projections  46  extend in the downward −Z direction. While the illustrated embodiment depicts nine projections  46  it is to be understood that any number of projections  46  can be employed. The use of closely spaced projections  46  can allow a reduction in the thickness and rigidity of the particle trapping sheet  50  because an unsupported distance is thereby reduced. The fixture  40  also includes openings  56  for allowing the photocurable resin  18  to pass through the fixture  40  as it is raised and lowered in the vessel  16 . 
       FIG. 4  is a cross-sectional view of fixture  40  taken through AA′ of  FIG. 3 .  FIG. 4  also includes the particle trapping sheet  50  which has been formed onto the projections  46  of the fixture  40 . The projections  46  taper in downward −Z direction toward a distal end  58 . Having a distal end  58  with a smaller cross sectional area reduces the impact of the projections  46  upon particles that are proximate to the transparent sheet  20 . 
       FIG. 5  is a schematic plan view of an exemplary particle trapping sheet  50 .  FIG. 5A  is a cross section of a first embodiment of the particle trapping sheet  50  that corresponds to the embodiment depicted in  FIG. 2A . The particle trapping sheet  50  includes a thin parallelepiped portion  60  covering most or essentially all of a lateral area of the build plane  36 . Minimizing the thickness of the thin parallelepiped portion  60  minimizes an amount of photocurable resin required to fabricate the particle trapping sheet  50 . The particle trapping sheet  50  also includes thicker sections or ribs  62  that form a frame for supporting the thin parallelepiped portion  60 . The particle trapping sheet  50  also includes tapering features  52  for attachment to the three dimensional article of manufacture  26 . The tapering geometry of the tapering features  52  minimizes a lateral area of contact between the particle trapping sheet  50  and the three dimensional article of manufacture  26  to facilitate their later separation. The tapering features  52  are preferably laterally aligned with ribs  62  to improve structural integrity. The thin parallelepiped portion  60  also defines openings  64  that allow the flow of the photocurable resin  18  as the particle trapping sheet  50  is raised or lowered in the vessel  16 . In this embodiment the particle trapping sheet  50  is attached to the fixture  40  at an upper side and to the three dimensional article of manufacture at a lower side defined by the tapering features  52  as in  FIG. 2A . 
     While only a few openings  64  are shown, it is to be understood that a large number of such openings  64  can be defined. The openings can be angled or stepped whereby particles are trapped at the lateral positions of the openings  64 . With a large number of openings  64 , they can have a relatively small lateral dimensions to further enhance particle trapping in their vicinity. 
       FIG. 5B  is a cross section of a second embodiment of the particle trapping sheet  50  that corresponds to the embodiment depicted in  FIG. 2B . In comparing the embodiment of  FIGS. 5A and 5B , like element numbers corresponding to like features. Therefore this discussion will focus on differences. The illustrated particle trapping sheet  50  includes extensions  54  for coupling the particle trapping sheet  50  to a lower side of the three dimensional article of manufacture  26 . The extensions  54  may vary greatly in a vertical extent in Z according to a geometry of the three dimensional article of manufacture  26 . Distal ends  66  of the extensions  54  are of a minimal cross sectional area to facilitate the separation of the particle trapping sheet  50  from the three dimensional article of manufacture  26 . Preferably the extensions  54  are laterally aligned with ribs  62  to improve structural integrity. 
       FIG. 6A  is a flowchart depicting a process  70  for operating printing system  2  to fabricate a three dimensional article of manufacture  26 . All steps of this process are executed by control server  14  and print engine controller  48  that control portions of printing system  2  and print engine  6 . Process  70  corresponds to the description of  FIG. 2A . 
     According to step  72 , the transport mechanism  12  retrieves a fixture  40  from a fixture cassette  4  and loads it into a print engine  6 . The fixture  40  is an embodiment similar to that discussed with respect to  FIG. 2A ,  FIG. 3 , and/or  FIG. 4 . Upon loading the fixture  40  into print engine  6 , the movement mechanism  38  can engage and vertically position the fixture  40 . 
     According to step  74 , the movement mechanism  38  lowers and positions the fixture  40  whereby the distal ends  58  of projections  46  are positioned at build plane  36 . Build plane  36  is at an operating distance from the transparent sheet  20 . 
     According to step  76 , the print engine controller  48  operates the light engine  24  and the vertical movement mechanism  38  to form a particle trapping sheet  50  as is illustrated in  FIG. 2A ,  FIG. 4 , or  FIG. 5A . The particle trapping sheet  50  preferably spans essentially the entire build plane  36  and is coupled to all of the projections  46 . Formation of the particle trapping sheet traps loose particles that are primarily solidified photocurable resin. Also as part of step  76 , the connecting features  52  are formed that taper downwardly. 
     According to step  78 , the print engine controller  48  operates the light engine  24  and the vertical movement mechanism  38  to form a three dimensional article of manufacture  26  that couples to the connecting features  52 . A lateral cross sectional area over which the connecting features  52  couple to the three dimensional article of manufacture  26  is preferably minimized to facilitate later separation. 
     According to step  79 , the transport mechanism unloads the fixture  40  from the print engine  6  and additional processes are performed. These additional processes can include post processing, inspection, and removal of the particle trapping sheet  50  from the three dimensional article of manufacture  26 . In one embodiment the transport mechanism  12  sequentially transfers the fixture  40  to different post process stations  8  and inspection stations  10 . When the particle trapping sheet  50  is removed it separates along the lateral area between the connecting features  52  and the three dimensional article of manufacture  26 . 
     In an alternative embodiment the print engine  6  is a standalone unit and steps  72  and  79  are performed manually. This includes manual loading and unloading of fixture  40  as well as cleaning, drying, UV curing, inspection, and removal of the particle trapping sheet  50 . 
       FIG. 6B  is a flowchart depicting a process  80  for operating printing system  2  to fabricate a three dimensional article of manufacture  26 . All steps of this process are executed by control server  14  and print engine controller  48  that control portions of printing system  2  and print engine  6 . Process  80  corresponds to the description of  FIG. 2B . 
     According to step  82 , the transport mechanism  12  retrieves a fixture  40  from a fixture cassette  4  and loads it into a print engine  6 . The fixture  40  has a lower face  45  that faces downwardly. 
     According to step  84 , the movement mechanism lowers and positions the lower face  45  of fixture  40  at the build plane  36 . The build plane  36  is at an operating distance from the transparent sheet  20 . According to step  86 , the print engine controller  48  operates the light engine  24  and the vertical movement mechanism  38  to form a three dimensional article of manufacture  26 . 
     According to step  88 , the print engine controller  48  operates the light engine  24  and the vertical movement mechanism  38  to form a particle trapping sheet  50  as is illustrated in  FIG. 2B  or  FIG. 5B . The particle trapping sheet  50  preferably spans the entire build plane  36 . The particle trapping sheet  50  is coupled to the three dimensional article of manufacture  26  via extensions  54 . The extensions  54  form a framework for properly supporting the particle trapping sheet  50  in the vessel  24 . 
     According to step  89 , the transport mechanism unloads the fixture  40  from the print engine  6  and additional processes are performed. These additional processes can include post processing, inspection, and removal of the particle trapping sheet  50  from the three dimensional article of manufacture  26 . In one embodiment the transport mechanism  12  sequentially transfers the fixture  40  to different post process stations  8  and inspection stations  10 . When the particle trapping sheet  50  is removed it separates along an interface between the distal ends  66  of extensions  54  and the three dimensional article of manufacture  26 . 
     In an alternative embodiment the print engine  6  is a standalone unit and steps  82  and  89  are performed manually. This includes manual loading and unloading of fixture  40  as well as cleaning, drying, UV curing, inspection, and removal of the particle trapping sheet  50 . 
       FIG. 6C  is a flowchart depicting a process  90  for operating printing system  2  to fabricate a three dimensional article of manufacture  26 . All steps of this process are executed by control server  14  and print engine controller  48  that control portions of printing system  2  and print engine  6 . Process  90  corresponds to the description of  FIG. 2C . 
     According to step  92 , the transport mechanism  12  retrieves a disposable fixture  40  from a fixture cassette and loads it into a print engine  6 . The fixture can be an embodiment that is similar to that discussed with respect to  FIG. 2C ,  FIG. 3 , or  FIG. 4 . Upon loading the fixture  40  into print engine  6 , the movement mechanism  38  can engage and vertically position the fixture  40 . 
     According to step  94 , the movement mechanism  38  lowers and positions the fixture  40  whereby the distal ends  58  of projections  46  are positioned at build plane  36 . Build plane  36  is at an operating distance from the transparent sheet  20 . 
     According to step  96 , the print engine controller  48  operates the light engine  24  and the vertical movement mechanism  38  to form a particle trapping sheet  50  as is illustrated in  FIG. 2C . The particle trapping sheet  50  preferably spans essentially the entire build plane  36  and is coupled to all of the projections  46 . Formation of the particle trapping sheet traps loose particles that are primarily solidified photocurable resin. 
     According to step  98 , the transport mechanism unloads the disposable fixture  40  from the print engine  6 . According to step  100 , the transport mechanism loads a new fixture  40  into the print engine  6 . According to step  102 , the print engine controller  48  operates the light engine  24  and the vertical movement mechanism  38  to form a three dimensional article of manufacture  26 . 
     According to step  104 , the transport mechanism unloads the fixture  40  from the print engine  6  and additional processes are performed. These additional processes can include post processing, and inspection. In one embodiment the transport mechanism  12  sequentially transfers the fixture  40  to different post process stations  8  and inspection stations  10 . 
       FIGS. 7A and 7B  are schematic block diagrams depicting an embodiment of a standalone print engine  6  for manufacturing a three dimensional article  26 .  FIGS. 7A and 7B  illustrate two stages of a manufacturing process. The element numbers shown in  FIGS. 7A and 7B  are the same as those depicted in  FIG. 2A  except as discussed infra. 
     A controller  106  is configured to control the light engine  24 , the vertical movement mechanism  38 , and to control and/or receive signals from other portions of the print engine  6 . The controller  106  is coupled to a user interface device  108 . User interface device (referred hereinafter as UID  108 ) can be a touchscreen permanently coupled to the standalone print engine  6 . Alternatively, the UID  108  can have either a wireless or wired connection to the controller  106 . The UID can also be a laptop computer, a smartphone, a personal digital assistant (PDA), a tablet computer, a desktop computer, a floor standing computer, or any client device accessible by a user. 
       FIG. 7A  depicts a first stage of a manufacturing process during which the light engine  24  is forming a particle trapping sheet  50  at the build plane  36 . This is illustrated as taking place while the lower face  45  of fixture  40  is not immersed in the photocurable resin  18 .  FIG. 7B  depicts a second stage of the manufacturing process during which operation of the light engine  24  and the movement mechanism  38  are manufacturing a three dimensional article  26 . 
       FIG. 8  depicts a method  110  of manufacturing a three dimensional article  26 . The steps of method  110  are generally performed by the controller  106 . According to element  112 , the method starts with the fixture  40  raised above the resin vessel  16 . Alternatively, the fixture  40  is positioned whereby the build plane  36  is disconnected from the fixture  40  and is not connected to the lower face  45  of fixture  40 . 
     According to step  114 , the light engine is operated to solidify a particle trapping sheet  50  proximate to the transparent sheet  20  as depicted in  FIG. 7A . Any particles that project up from the transparent sheet are “captured” by the particle trapping sheet  50 . The particle trapping sheet is disconnected from the transparent sheet  20  and disconnected from fixture  40 . 
     According to step  116 , operation of the print engine  6  is paused. According to step  118 , instructions are sent to UID  108  whereby the UID displays instructions to the user of print engine  6  to remove the particle trapping sheet  50  from the resin vessel  16 . After removing the particle trapping sheet, the user provides an input to the UID  108 . According to step  120 , the input is received from the UID which signals the controller  106  to restart operation of print engine  6 . 
     According to step  122 , the movement mechanism  38  is operated to move the lower face  45  of fixture  40  to the build plane  36 . According to step  124 , the light engine  24  and the movement mechanism  38  are operated to manufacture the three dimensional article  26  as depicted in  FIG. 7B . 
     Variations and/or portions of method  110  can be used. In one embodiment, steps  112 ,  114 , and  118  can be utilized after a particle-based failure is diagnosed. In another embodiment, steps  112 ,  114 , and  118  can be performed as preventative maintenance that can have a fixed periodic schedule. Other variations are possible that can include more or fewer steps of method  110 . 
       FIGS. 9A and 9B  are isometric and side cutaway views of an alternative embodiment of a support fixture  40  having a central opening  130 . Support fixture  40  includes an upper portion  132  coupled to a lower rim  134  by a sidewall  136 . The upper portion  132  defines interface features  138  for interfacing (aligning and coupling) the support fixture  40  with the vertical movement mechanism  38 . The sidewall  136  can include openings (not shown) for facilitating movement and draining of the photocurable resin  18  as the lower rim  134  is raised out of the vessel  16 . 
     The lower rim  134  includes an inner edge  142  that bounds the central opening  130 . The central opening  130  spans most or all of the area of the build plane  36 . The lower rim  134  defines a downwardly facing planar surface  144  that overlays an outer peripheral portion of the build plane  36 . 
       FIGS. 10A-C  are a sequence of diagrams representing a vertical cross-section through the support fixture  40  and the three-dimensional article  26  during a manufacturing process.  FIG. 10A  depicts the support fixture  40  with central opening  130  before the process has started. As part of the process, the lower planar surface  144  of lower rim  134  is positioned at or proximate to the build plane  36 . 
     According to  FIG. 10B , a particle trapping sheet  50  is formed that spans the central opening  130  and is attached to the lower planar surface  144 . The particle trapping sheet  50  is preferably at least one millimeter thick in the vertical direction to provide adequate support for the subsequent formation of the three dimensional article  26 . Thus, sheet  50  had a dual role of trapping particles and providing support for the three dimensional article  26 . Sheet  50  preferably includes an array of conduits that pass vertically therethrough to allow resin to pass vertically through the conduits. The conduits can define openings on an upper surface  146  and a lower surface  148  of the sheet  50 . The openings can have varying shapes such as circular, square, rectangular, polygonal, or irregular. In one embodiment, the openings are hexagonal and can form a hexagonal close packed (HCP) pattern across the upper  146  and/or lower  148  surface of the sheet  50 . 
     According to  FIG. 10C , a three dimensional article  26  is formed onto and downward from the lower surface  148  of the sheet  50 . After the three dimensional article  26  is formed, the sheet  50  can be separated from the surface  144  and then the article  26  can be separated from the sheet  50 . 
       FIG. 11  is a flowchart that represents an embodiment of a method  150  for manufacturing a three dimensional article  26  that utilizes the support fixture  40  depicted in  FIGS. 9A and 9B . The particle trapping sheet depicted by method  150  is depicted in any of  FIGS. 12-14 . 
     According to  152 , the vertical movement mechanism  38  is operated to position the planar surface  144  at the build plane  36 . According to  154 , movement mechanism  38  and the light engine  24  are operated to form a peripheral support upon the planar surface  144 . The peripheral support provides two functions: (1) an attachment support for sheet  50  and (2) adjust for possible inaccuracies in vertical tolerances. The latter is a result of the difficultly in accurately positioning the planar surface  144  exactly at the build plane  36 . 
     According to  156 , the movement mechanism  38  and the light engine  24  are operated to form a particle trapping sheet  50  attached to the peripheral support and spanning the opening  130 . The particle trapping sheet  50  includes fluid passages having lateral geometric aspects that enhance particle trapping. 
     According to  158 , the movement mechanism  38  and the light engine  24  are operated to form tapering supports  52  onto the lower planar support. According to  160 , the movement mechanism  38  and the light engine  24  are operated to form the three dimensional article  26  attached to the tapering supports. Steps  158  and  160  can be performed sequentially and/or concurrently. 
       FIG. 12  is a cross-sectional view of a support fixture  40  with a first embodiment of a particle trapping sheet  50  that can be formed according to steps  152 - 156  of method  150 . A peripheral support  162  has been formed onto the planar surface  144  according to step  154  of method  150 . The particle trapping sheet  50  has been formed to span opening  130  and onto the peripheral support  162 . 
     In the illustrated embodiment, the particle trapping sheet  50  defines an array of conduits  164  that allow resin to flow through the particle trapping sheet  50  as the movement mechanism  38  raises and lowers the support fixture  40 . The illustrated conduits  164  are oriented at an oblique angle with respect to vertical axis Z in order to more completely trap particles. The conduits  164  define openings  166  on the upper  146  and lower  148  surfaces of sheet  50 . 
       FIG. 13  is a cross-sectional view of a support fixture  40  with a second embodiment of a particle trapping sheet  50  that can be formed according to steps  152 - 156  of method  150 . A peripheral support  162  has been formed onto the planar surface  144  according to step  154  of method  150 . The particle trapping sheet  50  has been formed to span opening  130  and onto the peripheral support  162 . 
     In the illustrated embodiment, the particle trapping sheet  50  is formed from three layers including an upper sheet  50 U, a middle portion  50 M, and a lower sheet  50 L. The upper sheet  50 U defines an array of upper openings  166 U. The lower sheet defines an array of lower openings  166 L. The upper  166 U and lower  166 L openings are offset from each other to reduce a chance that particles can escape from being trapped in one of the upper  50 U and lower  50 L sheets. The middle portion  50 M defines ribs or beams  168  that help to rigidify the sheet  50  to allow it to support a three dimensional article  26 . The middle portion  50 M also fluidically couples the upper  166 U and lower  166 L openings. Thus, the three layers of sheet  50  define conduits  164  that pass from the upper surface  146  to the lower surface  148 . 
       FIG. 14  is a cross-sectional view of the support fixture  40  with a third embodiment of a particle trapping sheet  50  formed according to method  150 . In this illustrated embodiment, sheet  50  includes tapering supports  52  coupled to the three dimensional article  26 . Otherwise, the third embodiment of  FIG. 14  is similar to the second embodiment of  FIG. 13 . 
       FIGS. 15A-D  illustrate steps  154  and  156  of method  150  and correspond to the particle trapping sheet  50  of either  FIG. 13  or  FIG. 14 .  FIG. 15A  depicts the peripheral support  162  that has been formed upon the lower planar surface  144  of the lower rim  134 . While the peripheral support  162  is illustrated as being a rectangular frame in shape, it may be more effective if it has a stepped or a zig-zag geometry around the edge  142 . 
       FIG. 15B  depicts an upper layer  50 U of particle trapping sheet  50  with upper openings  166 U.  FIG. 15C  depicts the middle layer  50 M of sheet  50 .  FIG. 15D  depicts the lower layer with lower openings  166 L. The upper  166 U and lower  166 L openings are laterally offset from each other to enhance particle trapping. The middle layer defines fluid passages  170  that couple the upper  166 U and lower  166 L openings. Also, support ribs  168  are defined between the fluid passages  170 . 
     The specific embodiments and applications thereof described above are for illustrative purposes only and do not preclude modifications and variations encompassed by the scope of the following claims.