Patent Publication Number: US-6706353-B1

Title: Image forming substrate

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image-forming substrate coated with a layer of microcapsules filled with dye or ink, on which an image is formed by selectively breaking or squashing the microcapsules in the layer of microcapsules. 
     2. Description of the Related Art 
     In a conventional type of image-forming substrate coated with a layer of microcapsules filled with dye or ink, a shell of each microcapsule is formed from a suitable photo-setting resin, and an optical image is recorded and formed as a latent image on the layer of microcapsules by exposing it to light rays in accordance with image-pixel signals. Then, the latent image is developed by exerting a pressure on the layer of microcapsules. Namely, the microcapsules, which are not exposed to the light rays, are broken and squashed, whereby the dye or ink seeps out of the broken and squashed microcapsules, and thus the latent image is visually developed by the seepage of the dye or ink. 
     Of course, each of the conventional image-forming substrates must be packed so as to be protected from being exposed to light, resulting in wastage of materials. Further, the image-forming substrates must be handled such that they are not subjected to excess pressure due to the softness of unexposed microcapsules, resulting in an undesired seepage of the dye or ink. 
     Also, a color-image-forming substrate coated with a layer of microcapsules filled with different color dyes or inks, is known. In this substrate, the respective different colors are selectively developed on an image-forming substrate by applying specific temperatures to the layer of color microcapsules. Nevertheless, for fixing, it is necessary to irradiate a developed color using a light of a specific wavelength. Accordingly, a color-image-forming system for forming a color image on the color-image forming substrate is costly, because an additional radiation apparatus for the fixing of a developed color is needed, which in turn increases electric power consumption. Also, since a heating process for the color development and an irradiation process for the fixing of a developed color must be carried out with respect to each color, this hinders a quick formation of a color image on the color-image-forming substrate. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide an easy-to-handle image-forming substrate coated with a layer of microcapsules filled with dye or ink, in which an image can be quickly formed on the image-forming substrate at a low cost. 
     In accordance with a first aspect of the present invention, there is provided an image-forming substrate comprising: a base member; and a layer of microcapsules, coated over the base member, that contains at least one type of microcapsules filled with a liquid dye, a shell wall of each of the microcapsules being composed of a resin that exhibits a temperature/pressure characteristic such that, when each of the microcapsules is squashed under a predetermined pressure at a predetermined temperature, the liquid dye seeps from the squashed microcapsule, wherein a viscosity of the liquid dye varies in accordance a degree of surface roughness of the base member such that the seeped liquid dye securely and finely fixes on the base member. 
     The base member may comprise a printing paper, and as the degree of surface roughness of the printing paper decreases, the viscosity of the liquid dye increases. For example, when the base member comprises an ordinary printing paper exhibiting a high degree of surface roughness, the viscosity of the liquid dye may be approximately 10 cP. Also, when the base member comprises a calendered printing paper exhibiting an intermediate degree of surface roughness, the viscosity of the liquid dye may be approximately 100 cP. Further, when the base member comprises a coated or ferrotype printing paper exhibiting a low degree of surface roughness, and the viscosity of the liquid dye may be approximately 1000 cP. 
     In accordance with a second aspect of the present invention, there is provided an image-forming substrate comprising: a base member; and a layer of transparent microcapsules, coated over the base member, that contains at least one type of transparent microcapsules filled with a transparent liquid dye such a liquid leuco-pigment, a shell wall of each of the transparent microcapsules being composed of a resin that exhibits a temperature/pressure characteristic such that, when each of the transparent microcapsules is squashed under a predetermined pressure at a predetermined temperature, the transparent liquid dye seeps from the squashed microcapsule and reacts with a transparent color developer to produce a given single color. 
     In the second aspect of the present invention, the base member may comprise a transparent plastic sheet. In this case, a layer of the transparent color developer is formed on a surface of the transparent plastic sheet formed on a surface thereof, and the transparent microcapsule layer is coated over the transparent color developer layer. Thus, the image-forming substrate can be advantageously utilized to produce a transparency film for an overhead projector. Optionally, the transparent color developer is contained in a transparent binder solution used to form the transparent microcapsule layer. 
     Also, in the second aspect of the present invention, the base member may comprise a sheet of paper. In this case, a layer of the transparent color developer is formed on a surface of the paper sheet, and the transparent microcapsule layer is coated over the transparent color developer layer. Thus, when the microcapsule is broken or compacted, so that a single color is exhibited due to a seepage of the dye or ink from the broken and compacted microcapsule, the exhibited single color cannot be influenced by the shell of the broken and compacted microcapsule, due to the transparency of the microcapsule shell. Optionally, the transparent color developer may be contained in a binder solution used to form the transparent microcapsule layer. 
     In accordance with a third aspect of the present invention, there is provided an image-forming substrate comprising: a base member; and a layer of microcapsules, coated over the base member, that contains at least one type of microcapsules filled with a dye, a shell wall of each of the microcapsules being composed of resin that exhibits a temperature/pressure characteristic such that, when each of the microcapsules is squashed under a predetermined pressure at a predetermined temperature, the liquid dye is seeped from the squashed microcapsule, wherein at least one layer of function is incorporated in the image-forming substrate for achieving a given purpose. 
     The function layer may comprise a sheet of transparent ultraviolet barrier film covering the microcapsule layer. In this case, a preservation of a color image, formed on the image-forming substrate, can be considerably improved due to the existence of the ultraviolet barrier film sheet. Namely, by the ultraviolet barrier film sheet, the formed color image can be prevented from deteriorating due to ultraviolet light. Preferably, the transparent ultraviolet barrier film sheet is covered with a sheet of heat-resistant transparent protective film. 
     The function layer may comprise a white coat layer formed on a surface of the base member to give a desired white quality to the surface. In this case, the microcapsule layer is formed over the surface of the white coat layer. Also, the function layer may comprise an electrical conductive layer formed on another surface of the base member. 
     In the third aspect of the present invention, the base member may comprise a sheet of paper, and the function layer may comprise a layer of adhesive formed on another surface of the paper sheet, and a sheet of release paper applied to the adhesive layer. In this case, the image-forming substrate is produced in a form of a seal sheet, a piece of which may be utilized as a seal adapted to be adhered to a post card, an envelop, a package or the like. 
     The base member may comprise a sheet of film composed of a suitable synthetic resin, and the function layer may comprise a peeling layer formed over a surface of the film sheet, and a layer of transparent ultraviolet barrier formed on the peeling layer. In this case, the image-forming substrate is produced in a form of a transfer film sheet, and is used together with a printing sheet of paper. Namely, an image is once formed on the transfer film sheet, and is then transferred from the transfer film sheet to the printing paper sheet. Further, a preservation of the transferred image can be considerably improved because the transferred image is coated with a thermally-fused transparent material, derived from the ultraviolet barrier layer. 
     The base member also may comprise a sheet of film composed of a suitable transparent synthetic resin, and the function layer may comprise a peeling layer formed on a surface of the transparent film sheet, and a layer of transparent ultraviolet barrier formed on the peeling layer, the microcapsule layer being coated over the transparent ultraviolet barrier layer. In this case, the image-forming substrate is also produced in a form of a transfer film sheet, and is used together with a printing sheet of paper. Similar to the above-mentioned transfer film sheet, an image is once formed on the transfer film sheet, and is then transferred from the transfer film sheet to the printing paper sheet. Nevertheless, after the transfer of the image from the transfer film sheet to the printing paper sheet, the remaining transfer film sheet can be utilized as a transparency film carrying a negative image. Also, a preservation of the transferred image can be considerably improved because the transferred image is coated with a thermally-fused transparent material, derived from the ultraviolet barrier layer. 
     The base member may comprise a sheet of board paper, and the function layer may comprise a heat-sensitive recording layer formed on another surface of the board paper sheet. In this case, the image-forming substrate can be advantageously utilized as a post card. 
     The base member may comprise a sheet composed of a suitable transparent synthetic resin, and the function layer may comprise a heat-sensitive recording layer formed on another surface of the transparent sheet. In this case, the heat-sensitive recording layer is used for producing a black dot on the image-forming substrate. 
     In accordance with a fourth aspect of the present invention, there is provided an image-forming substrate which is produced in a form of a duplicating-paper sheet or a double-recording-paper sheet. Namely, the image-forming substrate comprises: a first image-forming substrate element that includes a first sheet of paper and a first layer of microcapsules coated over a surface of the first paper sheet, the first microcapsule layer containing at least one type of microcapsules filled with a dye, a shell of wall of each of the microcapsules being composed of a resin that exhibits a temperature/pressure characteristic such that, when each of the microcapsules is squashed under a first predetermined pressure at a first predetermined temperature, the dye seeps from the squashed microcapsule; a second image-forming substrate element that includes a second sheet of paper and a second layer of microcapsules coated over a surface of the second paper sheet, the second microcapsule layer containing at least one type of microcapsules filled with a dye, a shell of wall of each of the microcapsules being composed of a resin that exhibits a temperature/pressure characteristic such that, when each of the microcapsules is squashed under a second predetermined pressure at a second predetermined temperature, the dye seeps from the squashed microcapsule; and an peeling layer interposed between the first and second image-forming substrate elements, wherein the first and second predetermined pressures and the first and second predetermined temperatures are simultaneously applied to the first and second image forming substrate elements, and the second image-forming substrate is peelable from the peeling layer. 
     In the above-mentioned aspects of the present invention, the resin of the shell wall may be a shape memory resin that exhibits a glass-transition temperature corresponding to the predetermined temperature. 
     Optionally, the shell wall may comprise a double-shell wall. In this case, one shell wall element of the double-shell wall is composed of a shape memory resin, and another shell wall element of the double-shell wall is composed of a resin not exhibiting a shape memory characteristic, such that the temperature/pressure characteristic is a resultant temperature/pressure characteristic of both the shell wall elements. 
     Also, the shell wall may comprise a composite-shell wall including at least two shell wall elements formed of different types of resin not exhibiting a shape memory characteristic, such that the temperature/pressure characteristic is a resultant temperature/pressure characteristic of the shell wall elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The object and other objects of the present invention will be better understood from the following description, with reference to the accompanying drawings in which: 
     FIG. 1 is a schematic conceptual cross sectional view showing a first embodiment of an image-forming substrate, according to the present invention, comprising a layer of microcapsules including a first type of cyan microcapsules filled with a cyan ink, a second type of magenta microcapsules filled with a magenta ink and a third type of yellow microcapsules filled with a yellow ink; 
     FIG. 2 is a graph showing a characteristic curve of a longitudinal elasticity coefficient of a shape memory resin; 
     FIG. 3 is a graph showing temperature/pressure breaking characteristics of the respective cyan, magenta and yellow microcapsules shown in FIG. 1, with respective hatched area indicating each of a cyan-producing area, a magenta producing area and a yellow-producing area; 
     FIG. 4 is a schematic cross-sectional view showing different shell wall thicknesses of the respective cyan, magenta and yellow microcapsules shown in FIG. 1; 
     FIG. 5 is a schematic conceptual cross-sectional view similar to FIG. 1, showing only a selective breakage of one of the cyan microcapsules in the layer of microcapsules; 
     FIG. 6 is a schematic cross-sectional view of a color printer for forming a color image on the image-forming substrate shown in FIG. 1; 
     FIG. 7 is a partial schematic block diagram of three line-type thermal heads and three driver circuits therefor incorporated in the color printer of FIG. 6; 
     FIG. 8 is a schematic block diagram of a control board of the color printer shown in FIG. 6; 
     FIG. 9 is a partial block diagram representatively showing a set of an AND-gate circuit and a transistor included in each of the thermal head driver circuits of FIGS. 7 and 8; 
     FIG. 10 is a timing chart showing a strobe signal and a control signal for electronically actuating one of the thermal head driver circuits for producing a cyan dot on the image-forming substrate of FIG. 1; 
     FIG. 11 is a timing chart showing a strobe signal and a control signal for electronically actuating another one of the thermal head driver circuits for producing a magenta dot on the image-forming substrate of FIG. 1; 
     FIG. 12 is a timing chart showing a strobe signal and a control signal for electronically actuating the remaining thermal head driver circuit for producing a yellow dot on the image-forming substrate of FIG. 1; 
     FIG. 13 is a conceptual view showing, by way of example, the production of color dots of a color image in the color printer of FIG. 6; 
     FIG. 14 is a schematic conceptual cross-sectional view showing a second embodiment of an image-forming substrate, according to the present invention, comprising a layer of microcapsules including a first type of microcapsules filled with a first transparent liquid leuco-pigment, a second type of microcapsules filled with a second transparent liquid leuco-pigment, and a third type of microcapsules filled with a third transparent liquid leuco-pigment; 
     FIG. 15 is a schematic cross-sectional view showing different shell wall thicknesses of the respective first, second and third types of microcapsules shown in FIG. 14; 
     FIG. 16 is a schematic conceptual cross-sectional view similar to FIG. 14, showing a modification of the second embodiment of the image-forming substrate, according to the present invention; 
     FIG. 17 is a schematic conceptual cross-sectional view showing a third embodiment of an image-forming substrate, according to the present invention; 
     FIG. 18 is a schematic conceptual cross sectional view showing a fourth embodiment of an image-forming substrate, according to the present invention; 
     FIG. 19 is a schematic conceptual cross-sectional view showing a fifth embodiment of an image-forming substrate, according to the present invention; 
     FIG. 20 is a schematic conceptual cross-sectional view similar to FIG. 19, showing the image-forming substrate together with a printing sheet of paper to which a color image should be transferred from the image-forming substrate of FIG. 19; 
     FIG. 21 is a schematic conceptual cross-sectional view similar to FIG. 20, showing a modification of the fifth embodiment of the image-forming substrate shown in FIG. 19; 
     FIG. 22 is a schematic conceptual cross-sectional view showing a sixth embodiment of an image-forming substrate, according to the present invention; 
     FIG. 23 is a schematic conceptual cross-sectional view similar to FIG. 22, showing the image-forming substrate together with a printing sheet of paper to which a color image should be transferred from the image-forming substrate of FIG. 22; 
     FIG. 24 is a schematic conceptual cross-sectional view similar to FIG. 23, showing a modification of the sixth embodiment of the image-forming substrate shown in FIG. 22; 
     FIG. 25 is a schematic conceptual cross-sectional view showing a seventh embodiment of an image-forming substrate, according to the present invention; 
     FIG. 26 is a schematic conceptual cross-sectional view showing an eighth embodiment of an image-forming substrate, according to the present invention; 
     FIG. 27 is a schematic conceptual cross-sectional view showing a ninth embodiment of an image-forming substrate, according to the present invention; 
     FIG. 28 is a graph showing temperature/pressure breaking characteristics of respective cyan, magenta and yellow microcapsules included in a second microcapsule layer shown in FIG. 27; 
     FIG. 29 is a schematic conceptual cross-sectional view showing the ninth embodiment of the image-forming substrate of FIG. 27 at an aspect different from that of FIG. 27; 
     FIG. 30 is a schematic conceptual cross-sectional view transfer showing a tenth embodiment of an image-forming substrate, according to the present invention; 
     FIG. 31 is a schematic conceptual cross-sectional view showing the tenth embodiment of the image-forming substrate of FIG. 30 at an aspect different from that of FIG. 30; 
     FIG. 32 is a cross-sectional view showing three types of cyan, magenta and yellow microcapsules, respectively, as another embodiment of a microcapsule according to the present invention; 
     FIG. 33 is a graph showing temperature/pressure breaking characteristics of the cyan, magenta and yellow microcapsules shown in FIG. 32; 
     FIG. 34 is a cross-sectional view showing three types of cyan, magenta and yellow microcapsules, respectively, as yet another embodiment of a microcapsule according to the present invention; 
     FIG. 35 is a graph showing temperature/pressure breaking characteristics of the cyan, magenta and yellow microcapsules shown in FIG. 34; and 
     FIG. 36 is a schematic plan view showing a further embodiment of an image-forming substrate, according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a first embodiment of an image-forming substrate, generally indicated by reference  10 , according to the present invention. In this first embodiment, the image-forming substrate  10  is produced in a form of paper sheet. In particular, the image-forming substrate  10  comprises a sheet of paper  12 , a layer of microcapsules  14  coated over a surface of the sheet of paper  12 , and a sheet of transparent protective film  16  covering the microcapsule layer  14 . 
     The microcapsule layer  14  is formed from three types of microcapsules: a first type of microcapsules  18 C filled with cyan liquid dye or ink, a second type of microcapsules  18 M filled with magenta liquid dye or ink, and a third type of microcapsules  18 Y filled with yellow liquid dye or ink, and these three types of microcapsules are uniformly distributed in the microcapsule layer  14 . In each type of microcapsule ( 18 C,  18 M,  18 Y), a shell of a microcapsule is formed of a synthetic resin material, usually colored white. Also, each type of microcapsule ( 18 C,  18 M,  18 Y) may be produced by a well-known polymerization method, such as interfacial polymerization, in-situ polymerization or the like, and may have an average diameter of several microns, for example, 5 μ to 10 μ. 
     Note, when the sheet of paper  12  is colored with a single color pigment, the resin material of the microcapsules  18 C,  18 M and  1 BY may be colored by the same single color pigment. 
     For the uniform formation of the layer of microcapsules  14 , for example, the same amounts of cyan, magenta and yellow microcapsules  18 C,  18 M and  18 Y are homogeneously mixed with a suitable binder solution to form a suspension, and the sheet of paper  12  is coated with the binder solution, containing the suspension of microcapsules  18 C,  18 M and  18 Y, by using an atomizer. In FIG. 1, for the convenience of illustration, although the layer of microcapsules  14  is shown as having a thickness corresponding to the diameter of the microcapsules  18 C,  18 M and  18 Y, in reality, the three types of microcapsules  18 C,  18 M and  18 Y overlay each other, and thus the layer of microcapsules  14  has a larger thickness than the diameter of a single microcapsule  18 C,  18 M or  18 Y. 
     In the first embodiment of the image-forming substrate  10 , for the resin material of each type of microcapsule ( 18 C,  18 M,  18 Y), a shape memory resin is utilized. For example, the shape memory resin is represented by a polyurethane-based-resin, such as polynorbornene, trans-1, 4-polyisoprene polyurethane. As other types of shape memory resin, a polyimide-based resin, a polyamide-based resin, a polyvinyl-chloride-based resin, a polyester-based resin and so on are also known. 
     In general, as shown in a graph of FIG. 2, the shape memory resin exhibits a coefficient of longitudinal elasticity, which abruptly changes at a glass-transition temperature boundary T g . In the shape memory resin, Brownian movement of the molecular chains is stopped in a low-temperature area “a”, which is less than the glass-transition temperature T g , and thus the shape memory resin exhibits a glass-like phase. On the other hand, Brownian movement of the molecular chains becomes increasingly energetic in a high-temperature area “b”, which is higher than the glass-transition temperature T g , and thus the shape memory resin exhibits a rubber elasticity. 
     The shape memory resin is named due to the following shape memory characteristic: after a mass of the shape memory resin is worked into a shaped article in the low-temperature area “a”, when such a shaped article is heated over the glass-transition temperature T g , the article becomes freely deformable. After the shaped article is deformed into another shape, when the deformed article is cooled to below the glass-transition temperature T g , the other shape of the article is fixed and maintained. Nevertheless, when the deformed article is again heated to above the glass-transition temperature T g , without being subjected to any load or external force, the deformed article returns to the original shape. 
     In the image-forming substrate or sheet  10  according to this invention, the shape memory characteristic per se is not utilized, but the characteristic abrupt change of the shape memory resin in the longitudinal elasticity coefficient is utilized, such that the three types of microcapsules  18 C,  18 M and  18 Y can be selectively broken and squashed at different temperatures and under different pressures, respectively. 
     As shown in a graph of FIG. 3, a shape memory resin of the cyan microcapsules  18 C is prepared so as to exhibit a characteristic longitudinal elasticity coefficient having a glass-transition temperature T 1 , indicated by a solid line; a shape memory resin of the magenta microcapsules  18 M is prepared so as to exhibit a characteristic longitudinal elasticity coefficient having a glass-transition temperature T 2 , indicated by a single-chained line; and a shape memory resin of the yellow microcapsules  18 Y is prepared so as to exhibit a characteristic longitudinal elasticity coefficient having a glass-transition temperature T 3 , indicated by a double-chained line. 
     Note, by suitably varying compositions of the shape memory resin and/or by selecting a suitable one from among various types of shape memory resin, it is possible to obtain the respective shape memory resins, with the glass-transition temperatures T 1 , T 2  and T 3 . For example, the respective glass-transition temperatures T 1 , T 2  and T 3  may be 70° C., 110° C. and 130° C. 
     As shown in FIG. 4, the microcapsule walls of the cyan microcapsules  18 C, magenta microcapsules  18 M, and yellow microcapsules  18 Y, respectively, have differing thicknesses W C , W M  and W Y . The thickness W C  of cyan microcapsules  18 C is larger than the thickness W M  of magenta microcapsules  18 M, and the thickness W M  of magenta microcapsules  18 M is larger than the thickness W Y  of yellow microcapsules  18 Y. 
     Also, the wall thickness W C  of the cyan microcapsules  18 C is selected such that each cyan microcapsule  18 C is broken and compacted under a breaking pressure that lies between a critical breaking pressure P 3  and an upper limit pressure P UL  (FIG.  3 ), when each cyan microcapsule  18 C is heated to a temperature between the glass-transition temperatures T 1  and T 2 ; the wall thickness W M  of the magenta microcapsules  18 M is selected such that each magenta microcapsule  18 M is broken and compacted under a breaking pressure that lies between a critical breaking pressure P 2  and the critical breaking pressure P 3  (FIG.  3 ), when each magenta microcapsule  18 M is heated to a temperature between the glass-transition temperatures T 2  and T 3 ; and the wall thickness W Y  of the yellow microcapsules  18 Y is selected such that each yellow microcapsule  18 Y is broken and compacted under a breaking pressure that lies between a critical breaking pressure P 1  and the critical breaking pressure P 2  (FIG.  3 ), when each yellow microcapsule  18 Y is heated to a temperature between the glass-transition temperature T 3  and an upper limit temperature T UL . 
     Note, the upper limit pressure P UL  and the upper limit temperature T UL  are suitably set in view of the characteristics of the used shape memory resins. 
     As is apparent from the foregoing, by suitably selecting a heating temperature and a breaking pressure, which should be exerted on the image-forming sheet  10 , it is possible to selectively break and squash the cyan, magenta and yellow microcapsules  18 C,  18 M and  18 Y. 
     For example, if the selected heating temperature and breaking pressure fall within a hatched cyan area C (FIG.  3 ), defined by a temperature range between the glass-transition temperatures T 1  and T 2  and by a pressure range between the critical breaking pressure P 3  and the upper limit pressure P UL , only the cyan microcapsules  18 C are broken and squashed, as shown in FIG.  5 . Also, if the selected heating temperature and breaking pressure fall within a hatched magenta area M, defined by a temperature range between the glass-transition temperatures T 2  and T 3  and by a pressure range between the critical breaking pressures P 2  and P 3  only the magenta microcapsules  18 M are broken and squashed. Further, if the selected heating temperature and breaking pressure fall within a hatched yellow area Y, defined by a temperature range between the glass-transition temperature T 3  and the upper limit temperature T UL  and by a pressure range between the critical breaking pressures P 1  and P 2  only the yellow microcapsules  18 Y are broken and squashed. 
     Accordingly, if the selection of a heating temperature and a breaking pressure, which should be exerted on the image-forming sheet  10 , are suitably controlled in accordance with digital color image-pixel signals: digital cyan image-pixel signals, digital magenta image-pixel signals and digital yellow image-pixel signals, it is possible to form a color image on the image-forming sheet  10  on the basis of the digital color image-pixel signals. 
     FIG. 6 schematically shows a thermal color printer, which is constituted as a line printer so as to form a color image on the image-forming sheet  10 . 
     The color printer comprises a rectangular parallelopiped housing  20  having an entrance opening  22  and an exit opening  24  formed in a top wall and a side wall of the housing  20 , respectively. The image-forming sheet  10  is introduced into the housing  20  through the entrance opening  22 , and is then discharged from the exit opening  24  after the formation of a color image on the image-forming sheet  10 . Note, in FIG. 6, a path  26  for movement of the image-forming sheet  10  is indicated by a chained line. 
     A guide plate  28  is provided in the housing  20  so as to define a part of the path  26  for the movement of the image-forming sheet  10 , and a first thermal head  30 C, a second thermal head  30 M and a third thermal head  30 Y are securely attached to a surface of the guide plate  28 . Each thermal head ( 30 C,  30 M,  30 Y) is formed as a line thermal head perpendicularly extended with respect to a direction of the movement of the image-forming sheet  10 . 
     As shown in FIG. 7, the line thermal head  30 C includes a plurality of heater elements or electric resistance elements R c1  to R cn , and these resistance elements are aligned with each other along a length of the line thermal head  30 C. The electric resistance elements R c1  to R cn  are selectively energized by a first driver circuit  31 C in accordance with a single-line of cyan image-pixel signals, and are then heated to a temperature between the glass-transition temperatures T 1  and T 2 . 
     Also, the line thermal head  30 M includes a plurality of heater elements or electric resistance elements R m1  to R m2  and these resistance elements are aligned with each other along a length of the line thermal head  30 M. The electric resistance elements R m1  to R mn  are selectively energized by a second driver circuit  31 M in accordance with a single-line of magenta image-pixel signals, and are then heated to a temperature between the glass-transition temperatures T 2  and T 3 . 
     Further, the line thermal head  30 Y includes a plurality of heater elements or electric resistance elements R y1  to R yn , and these resistance elements are aligned with each other along a length of the line thermal head  30 Y. The electric resistance elements R y1  to R yn  are selectively energized by a third driver circuit  31 Y in accordance with a single-line of yellow image-pixel signals, and are heated to a temperature between the glass-transition temperature T 3  and the upper limit temperature T UL . 
     Namely, the line thermal heads  30 C,  30 M and  30 Y are arranged in sequence so that the respective heating temperatures increase in the movement direction of the image-forming substrate  10 . 
     The color printer further comprises a first roller platen  32 C, a second roller platen  32 M and a third roller platen  32 Y associated with the first, second and third thermal heads  30 C,  30 M and  30 Y, respectively, and each of the roller platens  32 C,  32 M and  32 Y may be formed of a suitable hard rubber material. The first roller platen  32 C is provided with a first spring-biasing unit  34 C so as to be elastically pressed against the first thermal head  30 C at a pressure between the critical breaking-pressure P 3  and the upper limit pressure P UL ; the second roller platen  32 M is provided with a second spring-biasing unit  34 M so as to be elastically pressed against the second thermal head  30 M at a pressure between the critical breaking-pressures P 2  and P 3 ; and the third roller platen  32 Y is provided with a third spring-biasing unit  34 Y so as to be elastically pressed against the second thermal head  30 Y at a pressure between the critical breaking-pressures P 1  and P 2 . 
     Namely, the platens  32 C,  32 M and  32 Y are arranged in sequence so that the respective pressures, exerted by the platens  32 C,  32 M and  32 Y on the line thermal heads  30 C,  30 M and  30 Y, decrease in the movement direction of the image-forming substrate  10 . 
     Note, in FIG. 6, reference  36  indicates a control circuit board for controlling a printing operation of the color printer, and reference  38  indicates an electrical main power source for electrically energizing the control circuit board  36 . 
     FIG. 8 shows a schematic block diagram of the control circuit board  36 . As shown in this drawing, the control circuit board  36  comprises a central processing unit (CPU)  40 , which receives digital color image-pixel signals from a personal computer or a word processor (not shown) through an interface circuit (I/F)  42 , and the received digital color image-pixel signals, i.e. digital cyan image-pixel signals, digital magenta image-pixel signals and digital yellow image-pixel signals, are stored in a memory  44 . 
     Also, the control circuit board  36  is provided with a motor driver circuit  46  for driving three electric motors  48 C,  48 M and  48 Y, which are used to rotate the roller platens  32 C,  32 M and  32 Y, respectively. In this embodiment, each of the motors  48 C,  48 M and  48 Y is a stepping motor, which is driven in accordance with a series of drive pulses outputted from the motor driver circuit  46 , the outputting of drive pulses from the motor driver circuit  46  to the motors  48 C,  48 M and  48 Y being controlled by the CPU  40 . 
     During a printing operation, the respective roller platens  32 C,  32 M and  32 Y are rotated in a counterclockwise direction (FIG. 6) by the motors  48 C,  48 M and  48 Y, respectively, with a same peripheral speed. Accordingly, the image-forming sheet  10 , introduced through the entrance opening  22 , moves toward the exit opening  24  along the path  26 . Thus, the image-forming sheet  10  is subjected to pressure ranging between the critical breaking-pressure P 3  and the upper limit pressure P UL  when passing between the first line thermal head  30 C and the first roller platen  32 C; the image-forming sheet  10  is subjected to pressure ranging between the critical breaking-pressures P 2  and P 3  when passing between the second line thermal head  30 M and the second roller platen  32 M; and the image-forming sheet  10  is subjected to pressure ranging between the critical breaking-pressures P 1  and P 2  when passing between the third line thermal head  30 Y and the third roller platen  32 Y. 
     Note, in this embodiment, the introduction of the image-forming sheet  10  into the entrance opening  22  of the printer is carried out such that the transparent protective film sheet  16  of the image-forming sheet  10  comes into contact with the thermal heads  30 C,  30 M and  30 Y. 
     As is apparent from FIG. 8, the respective driver circuits  31 C,  31 M and  31 Y for the line thermal heads  30 C,  30 M and  30 Y are controlled by the CPU  40 . Namely, the driver circuits  31 C,  31 M and  31 Y are controlled by n sets of strobe signals “STC” and control signals “DAC”, n sets of strobe signals “STW” and control signals “DAM” and n sets of strobe signals “STY” and control signals “DAY”, respectively, thereby carrying out the selective energization of the electric resistance elements R c1  to R cn , the selective energization of the electric resistance elements R m1  to R mn  and the selective energization of the electric resistance elements R y1  to R yn , as stated in detail below. 
     In each driver circuit ( 31 C,  31 M and  31 Y), n sets of AND-gate circuits and transistors are provided with respect to the electric resistance elements (R cn , R mn , R yn ) respectively. With reference to FIG. 9, an AND-gate circuit and a transistor in one set are representatively shown and indicated by references  50  and  52 , respectively. A set of a strobe signal (STC, STM or STY) and a control signal (DAC, DAM or DAY) is inputted from the CPU  40  to two input terminals of the AND-gate circuit  50 . A base of the transistor  52  is connected to an output terminal of the AND-gate circuit  50 ; a corrector of the transistor  52  is connected to an electric power source (V cc ); and an emitter of the transistor  52  is connected to a corresponding electric resistance element (R cn , R mn , R yn ). 
     When the AND-gate circuit  50 , as shown in FIG. 9, is one included in the first driver circuit  31 C, a set of a strobe signal “STC” and a control signal “DAC” is inputted to the input terminals of the AND-gate circuit  50 . As shown in a timing chart of FIG. 10, the strobe signal “STC” has a pulse width “PWC”. On the other hand, the control signal “DAC” varies in accordance with binary values of a digital cyan image-pixel signal. Namely, when the digital cyan image-pixel signal has a value “1”, the control signal “DAC” produces a high-level pulse having the same pulse width as that of the strobe signal “STC”, whereas, when the digital cyan image-pixel signal has a value “0”, the control signal “DAC” is maintained at a low-level. 
     Accordingly, only when the digital cyan image-pixel signal has the value “1”, is a corresponding electric resistance element (R c1 , . . . , R cn ) electrically energized during a period corresponding to the pulse width “PWC” of the strobe signal “STC”, whereby the electric resistance element concerned is heated to the temperature between the glass-transition temperatures T 1  and T 2 , resulting in the production of a cyan dot on the image-forming sheet  10  due to the breakage and compacting of cyan microcapsules  18 C, which are locally heated by the electric resistance element concerned. 
     Similarly, when the AND-gate circuit  50 , as shown in FIG. 9, is one included in the second driver circuit  31 M, a set of a strobe signal “STM” and a control signal “DAM” is inputted to the input terminals of the AND-gate circuit  50 . As shown in a timing chart of FIG. 11, the strobe signal “STM” has a pulse width “PWM”, being longer than that of the strobe signal “STC”. On the other hand, the control signal “DAM” varies in accordance with binary values of a digital magenta image-pixel signal. Namely, when the digital magenta image-pixel signal has a value “1”, the control signal “DAM” produces a high-level pulse having the same pulse width as that of the strobe signal “STM”, whereas, when the digital magenta image-pixel signal has a value “0”, the control signal “DAM” is maintained at a low-level. 
     Accordingly, only when the digital magenta image-pixel signal is “1”, is a corresponding electric resistance element (R m1 , . . . , R mn ) electrically energized during a period corresponding to the pulse width “PWM” of the strobe signal “STM”, whereby the electric resistance element concerned is heated to the temperature between the glass-transition temperatures T 2  and T 3 , resulting in the production of a magenta dot on the image-forming sheet  10  due to the breakage and compacting of magenta microcapsules  18 M, which are locally heated by the electric resistance element concerned. 
     Further, the AND-gate circuit  50 , as shown in FIG. 9, is one included in the first driver circuit  31 Y, a set of a strobe signal “STY” and a control signal “DAY” is inputted to the input terminals of the AND-gate circuit  50 . As shown in a timing chart of FIG. 12, the strobe signal “STY” has a pulse width “PWY”, being longer than that of the strobe signal “STM”. On the other hand, the control signal “DAY” varies in accordance with binary values of a corresponding digital yellow image-pixel signal. Namely, when the digital yellow image-pixel signal has a value “1”, the control signal “DAY” produces a high-level pulse having the same pulse width as that of the strobe signal “STY”, whereas, when the digital yellow image-pixel signal has a value “0”, the control signal “DAY” is maintained at a low-level. 
     Accordingly, only when the digital yellow image-pixel signal is “1”, is a corresponding electric resistance element (R y1 , . . . , R yn ) electrically energized during a period corresponding to the pulse width “PWY” of the strobe signal “STY”, whereby the resistance element concerned is heated to the temperature between the glass-transition temperature T 3  and the upper limit temperature T UL , resulting in the production of a yellow dot on the image-forming sheet  10  due to the breakage and squashing of yellow microcapsules  18 Y, which are locally heated by the electric resistance element concerned. 
     Note, the cyan, magenta and yellow dots, produced by the heated resistance elements R cn , R mn  and R yn , have a dot size of about 50 μ to about 100 μ, and thus three types of cyan, magenta and yellow microcapsules  18 C,  18 M and  1 BY are uniformly included in a dot area to be produced on the image-forming sheet  10 . 
     Of course, a color image is formed on the image-forming sheet  10  on the basis of a plurality of three-primary color dots obtained by selectively heating the electric resistance elements (R c1  to R cn ; R m1  to R mn ; and R y1  to R yn ) in accordance with three-primary color digital image-pixel signals. Namely, a certain dot of the color image, formed on the image-forming sheet  10 , is obtained by a combination of cyan, magenta and yellow dots produced by corresponding electric resistance elements R cn , R mn  and R yn . 
     In particular, for example, as conceptually shown by FIG. 13, in a single-line of dots, forming a part of the color image, if a first dot is white, none of the electric resistance elements R c1 , R m1  and R y1  are heated. If a second dot is cyan, only the electric resistance element R c2  is heated, and the remaining electric resistance elements R m2  and R y2  are not heated. If a third dot is magenta, only the resistance element R m3  is heated, and the remaining resistance elements R c3  and R y3  are not heated. Similarly, if a fourth dot is yellow, only the resistance element R y4  is heated, and the remaining resistance elements R c4  and R m4  are not heated. 
     Further, as shown in FIG. 13, if a fifth dot is blue, the electric resistance elements R c5  and R m5  are heated, and the remaining electric resistance element R y5  is not heated. If a sixth dot is green, the resistance elements R c6  and R y6  are heated, and the remaining resistance element R m6  is not heated. If a seventh dot is red, the resistance elements R m7  and R y7  are heated, and the remaining resistance element R c7  is not heated. If an eighth dot is black, all of the resistance elements Rc 8 , R m8  and R y8  are heated. 
     According to the first embodiment of the image-forming substrate  10 , a viscosity of each of the cyan, magenta and yellow liquid dyes or inks is changed in accordance with a degree of surface roughness of the sheet of paper  12  used, such that a produced dot can be securely and finely fixed on the sheet of paper  12 . 
     In particular, for example, when an ordinary printing paper, exhibiting a high degree of surface roughness, is used as the sheet of paper  12  in the image-forming substrate  10 , each of the cyan, magenta and yellow liquid dyes or inks is prepared so as to exhibit a low viscosity, for example, 10 cp (centipoise) at a temperature at which the corresponding monochromatic microcapsules ( 18 C,  18 M,  18 Y) are broken or compacted. In this case, a liquid dye or ink, which seeps out of the broken and squashed microcapsules, immediately permeates a tissue of the ordinary printing paper  12 , and thus can be securely fixed on the ordinary printing paper due to the immediate permeation of the discharged liquid dye or ink into the tissue thereof. Thus, a dot can be finely and definitely produced on the ordinary printing paper  12  by the seeped liquid dye or ink. 
     Also, when a calendered printing paper, exhibiting an intermediate degree of surface roughness, is used as the sheet of paper  12  in the image-forming substrate  10 , each of the cyan, magenta and yellow liquid dyes or inks is prepared so as to exhibit an intermediate viscosity, for example, 100 cp at a temperature at which the corresponding monochromatic microcapsules ( 18 C,  18 M,  18 Y) are broken or compacted. In this case, a liquid dye or ink, which seeps out of the broken and squashed microcapsules, cannot immediately permeate a tissue of the calendered printing paper, but the discharged liquid dye or ink can be securely fixed on the calendered printing paper  12 , without spreading of the seeped liquid dye or ink due to the intermediate viscosity thereof. Thus, a dot can be finely and definitely produced on the calendered printing paper  12  by the seeped liquid dye and ink. 
     Further, when a coated or ferrotype printing paper, exhibiting a low degree of surface roughness, is used as the sheet of paper  12  in the image-forming substrate  10 , each of the cyan, magenta and yellow liquid dyes or inks is prepared so as to exhibit a high viscosity, for example, 1000 cp at a temperature at which the corresponding monochromatic microcapsules ( 18 C,  18 M,  18 Y) are broken or compacted. In this case, a liquid dye or ink, which seeps out of the broken and squashed microcapsules, does not quickly permeate a tissue of the coated or ferrotype printing paper  12 , but the discharged liquid dye or ink can be securely fixed on the coated or ferrotype printing paper  12 , without spreading of the seeped liquid dye or ink due to the high viscosity thereof. Thus, a dot can be finely and definitely produced on the coated or ferrotype printing paper  12  by the seeped liquid dye and ink. 
     FIG. 14 shows a second embodiment of an image-forming substrate, generally indicated by reference  54 , according to the present invention. In this second embodiment, the image-forming substrate  54  is produced in a form of a transparent sheet. In particular, the image-forming substrate  54  comprises a sheet  56  of suitable transparent resin, a layer of transparent color developer  58  formed on a surface of the transparent sheet  56 , a layer of transparent microcapsules  60  coated over a surface of the transparent color developer layer  58 , and a sheet of transparent protective film  62  covering the microcapsule layer  58 . 
     The transparent microcapsule layer  60  is formed from three types of microcapsules: a first type of microcapsules  64 C filled with a first transparent liquid leuco-pigment, a second type of microcapsules  64 M filled with a second transparent liquid leuco-pigment, and a third type of microcapsules  64 Y filled with a third transparent liquid leuco-pigment, and the respective first, second and third liquid leuco-pigments react with the color developer, included in the color developer layer  58 , to thereby produce cyan, magenta and yellow. 
     Similar to the first embodiment, for the resin material of each type of microcapsule ( 64 C,  64 M,  64 Y), a shape memory resin is utilized, but it is transparent. Of course, the microcapsules  64 C,  64 M and  64 Y, which are filled with leuco-pigments, are produced by one of the well-known polymerization methods mentioned above. 
     The microcapsules  64 C,  64 M and  64 Y are uniformly distributed in the microcapsule layer  60 . To this end, for example, similar to the first embodiment, the same amounts of cyan, magenta and yellow microcapsules  64 C,  64 M and  64 Y are homogeneously mixed with a suitable transparent binder solution to form a suspension, and the transparent sheet  56  is coated with the binder solution, containing the suspension of microcapsules  64 C,  64 M and  64 Y, by using an atomizer. Also, similar to FIG. 1, in FIG. 14, for the convenience of illustration, although the microcapsule layer  60  is shown as having a thickness corresponding to the diameter of the microcapsules  64 C,  64 M and  64 Y, in reality, the three types of microcapsules  64 C,  64 M and  64 Y overlay each other, and thus the microcapsule layer  60  has a larger thickness than the diameter of a single microcapsule  64 C,  64 M or  64 Y. 
     Further, similar to the first embodiment, the cyan microcapsules  64 C, magenta microcapsules  64 M, and yellow microcapsules  64 Y, respectively, have differing thicknesses W C , W M  and W Y  as shown in FIG.  15 . Namely, the thickness W C  of cyan microcapsules  64 C is larger than the thickness W M  of magenta microcapsules  64 M, and the thickness W M  of magenta microcapsules  64 M is larger than the thickness W Y  of yellow microcapsules  64 Y. 
     Accordingly, the respective microcapsules  64 C,  64 M and  64 Y also exhibit the temperature/pressure characteristics, as shown in FIG.  3 . Namely, by suitably selecting a heating temperature and a breaking pressure, which should be exerted on the image-forming substrate  54 , it is possible to selectively break and squash the cyan, magenta and yellow microcapsules  64 C,  64 M and  64 Y, and thus a color image can be formed on the image-forming substrate  54  by the thermal color printer as shown in FIG.  6 . 
     Especially, the second embodiment of the transparency image-forming substrate, according to the present invention, can be advantageously utilized to produce a transparency film for a well-known overhead projector (OHP). Namely, when a color image is formed on the image-forming substrate  54 , it is possible to directly use this transparency-type substrate  54 , carrying the color image, as a transparency film for the overhead projector. 
     FIG. 16 shows a modification of the second embodiment of the image-forming substrate, generally indicated by reference  54 ′, according to the present invention. In the modified image-forming substrate  54 ′, a sheet of paper  56 ′ is substituted for the transparent sheet  56 , and thus the image-forming substrate  54 ′ cannot be utilized to produce a transparency film for the overhead projector. Nevertheless, the image-forming substrate  54 ′ is useful and advantageous in view of another aspect. 
     In particular, when a monochromatic dye or ink is encapsulated in a microcapsule as the case of the first embodiment, a shell of the microcapsule cannot be transparent. 
     Namely, the microcapsule shell must be colored with the same single color pigment as a color (usually, white) of the sheet of paper  56 ′. In this case, when the microcapsule is broken or compacted, so that a single color is exhibited due to a seepage of the monochromatic dye or ink from the broken and compacted microcapsule, the exhibited single color may be influenced by the single color pigment of the shell of the broken and compacted microcapsule, because the shell of the broken and compacted microcapsule cannot necessarily be completely hidden by the seeped monochromatic dye or ink, as shown by way of example in FIG.  5 . For example, when the single color pigment of the microcapsule shell is white, the exhibited single color is thinned. 
     Nevertheless, in the modified embodiment shown in FIG. 16, although a liquid leuco-pigment, seeped from a broken and compacted microcapsule ( 64 C,  64 M,  64 Y), reacts with the color developer to thereby produce a single color, this produced single color cannot be influenced by the transparent shell of the broken and compacted microcapsule ( 64 C,  64 M,  64 Y). 
     In the embodiments shown in FIGS. 14 and 16, the transparent binder solution may contain the transparent color developer which reacts on the first, second and third transparent liquid leuco-pigments to produce cyan, magenta and yellow. Also, when a sufficient amount of transparent color can be contained in the transparent binder solution, the transparent color developer layer  58  may be omitted from the image-forming substrate ( 54 ,  54 ′). 
     FIG. 17 shows a third embodiment of an image-forming substrate, generally indicated by reference  66 , according to the present invention. Similar to the first embodiment, the image-forming substrate  66  is produced in a form of paper sheet. Namely, the image-forming substrate  66  comprises a sheet of paper  68 , a white-coat layer  70  formed on a surface of the paper sheet  68 , a layer of microcapsules  72  coated over a surface of the white-coat layer  70 , a sheet of transparent ultraviolet barrier film  74  covering the microcapsule layer  72 , and a sheet of transparent protective film  76  applied to the transparent ultraviolet barrier film  74 . 
     The white-coat layer  70  is composed of a suitable white-pigment, and gives a desired white quality to the surface of the paper sheet  68 . The microcapsule layer  72  may be identical to the microcapsule layer  14  of the first embodiment shown in FIG.  1 . Namely, the cyan, magenta and yellow microcapsules, included in the microcapsule layer  72 , exhibit the temperature/pressure characteristics as shown in FIG.  3 . Accordingly, by suitably selecting a heating temperature and a breaking pressure, which should be exerted on the image-forming substrate  66 , the cyan, magenta and yellow microcapsules can be selectively broken and squashed, and thus a color image can be formed on the image-forming substrate  66  by the thermal color printer as shown in FIG.  6 . 
     Also, in the third embodiment, it is possible to considerably improve a preservation of a color image, formed on the image-forming substrate  66 , due to the existence of the ultraviolet barrier film sheet  74 . Namely, by the ultraviolet barrier film sheet  74 , the formed color image can be prevented from deteriorating due to ultraviolet light. While the color image is formed on the image-forming substrate  66  by the thermal printer shown in FIG. 6, the ultraviolet barrier film sheet  74  may be thermally fused by the thermal heads ( 30 C,  30 M and  30 Y). Nevertheless, due to the existence of the protective film sheet  76 , the thermally-fused ultraviolet barrier film sheet  74  is prevented from being stuck to the thermal heads. 
     Further, in the third embodiment, the image-forming substrate  66  features an electrical conductive layer  78  formed on the other surface or back surface of the paper sheet  68 , and the electrical conductive layer  78  may be composed of a suitable electrical conductive coating material. In general, an image-forming substrate is susceptible to an electrical charge due to triboelectrification, and the electrically-charged image-forming substrate may be entangled by a platen ( 32 C,  32 M,  32 Y), due to the generation of an electrostatic attractive force between the platen and the charged image-forming substrate during a formation of a color image by the printer shown in FIG.  6 . Nevertheless, in the third embodiment, the electrostatic entanglement of the image-forming substrate  66  by a platen can be prevented due to the existence of the electrical conductive layer  78 . 
     In particular, although the image-forming substrate  66  is electrostatically charged, the electrostatic charge can be easily dissipated from the image-forming substrate  66  through the electrical conductive layer  78 , during the formation of the color image by the printer, because the electrical conductive layer  78  can be in electrical contact with a conductive part of the printer. 
     In the third embodiment, a leuco-pigment may be utilized. In this case, a color developer, which reacts with the leuco-pigment, may be contained in a binder solution, which is used for the formation of the microcapsule layer  72 . Optionally, the color developer may be contained in the white-coat layer  70 . 
     FIG. 18 shows a fourth embodiment of an image-forming substrate, generally indicated by reference  80 , according to the present invention. In this fourth embodiment, the image forming substrate  80  is produced in a form of a seal sheet, a piece of which may be utilized as a seal adapted to be adhered to a post card, an envelop, a package or the like. Namely, the image-forming substrate  80  comprises a sheet of paper  82 , a layer of microcapsules  84  coated over a surface of the paper sheet  82 , a sheet of transparent protective film  86  covering the microcapsule layer  84 , a layer of adhesive  88  formed on the other surface of the paper sheet  82 , and a sheet of release paper  90  applied to the adhesive layer  88 . 
     The microcapsule layer  84  may be identical to the microcapsule layer  14  of the first embodiment shown in FIG.  1 . Namely, the cyan, magenta and yellow microcapsules, included in the microcapsule layer  84 , exhibit the temperature/pressure characteristics as shown in FIG.  3 . Accordingly, by suitably selecting a heating temperature and a breaking pressure, which should be exerted on the image-forming substrate  80 , the cyan, magenta and yellow microcapsules can be selectively broken and squashed, and thus a color image can be formed on the image-forming substrate  80  by the thermal color printer as shown in FIG.  6 . 
     Preferably, the image-forming substrate  80  is provided with crosswise perforated lines (not shown) so as to enable division into a plurality of rectangular sections, and respective identical or different images are formed on the rectangular sections of the image-forming substrate  80 . Thereafter, one of the rectangular sections is cut off from the image-forming substrate  80 , and a piece of the release paper sheet  90  is peeled therefrom, whereby the rectangular section concerned can be adhered to a post card, an envelop, a package, or the like. 
     Similar to the third embodiment, in the fourth embodiment, a leuco-pigment may be utilized as an ink to be encapsulated in the microcapsules. In this case, a color developer, which reacts with the leuco-pigment, may be contained in a binder solution, which is used for the formation of the microcapsule layer  84 . Optionally, a layer of color developer may be interposed between the paper sheet  82  and the microcapsule layer  84 . 
     FIG. 19 shows a fifth embodiment of an image-forming substrate, generally indicated by reference  92 , according to the present invention. In this fifth embodiment, the image-forming substrate  92  is produced in a form of a transfer film sheet. Namely, the image-forming substrate  92  comprises a sheet of film  94  composed of a suitable synthetic resin, such as polyethylene terephthalate, a peeling layer  96  composed of a teflon-based coating material or a silicone-based coating material and formed over a surface of the film sheet  94 , a layer of a transparent ultraviolet barrier  98  formed on the peeling layer  96 , and a layer of microcapsules  100  coated over the ultraviolet barrier layer  98 . 
     The microcapsule layer  100  may be identical to the microcapsule layer  14  of the first embodiment shown in FIG.  1 . Namely, the cyan, magenta and yellow microcapsules, included in the microcapsule layer  100 , have the temperature/pressure characteristics, as shown in FIG.  3 . Accordingly, by suitably selecting a heating temperature and a breaking pressure, which should be exerted on the image-forming substrate  92 , the cyan, magenta and yellow microcapsules can be selectively broken and squashed, and thus a color image can be formed on the image-forming substrate  92  by the thermal color printer as shown in FIG.  6 . 
     Further, the image-forming substrate  92  may optionally comprise an electrical conductive layer  102  formed on the other surface or back surface of the film sheet  94 , and a sheet of protective film  104  is applied to the electrical conductive layer  102 . 
     As shown in FIG. 20, the image-forming substrate  92  is used together with a printing sheet of paper P. Namely, the image-forming substrate  92 , overlaid with the printing paper sheet P, is fed in the printer as shown in FIG. 6, such that the protective film sheet  104  contacts the thermal heads ( 30 C,  30 M and  30 Y), and the cyan, magenta and yellow microcapsules are selectively broken and squashed in accordance with respective digital color image-pixel signals. Thus, as conceptually shown in FIG. 20, ink, seeped from the broken and squashed microcapsule, is transferred from the image-forming substrate  92  to the printing paper sheet P,. Namely, a color image is once formed on the image-forming substrate  92 , and then the formed color image is transferred to the printing paper sheet P. 
     On the other hand, when the image-forming substrate  92  is heated by the thermal heads ( 30 C,  30 M and  30 Y), the transparent ultraviolet barrier layer  98  is thermally fused locally in accordance with the digital color image-pixel signal. Thus, as shown in FIG. 20, the ink, transferred from the image-forming substrate  92  to the printing sheet paper P, is covered with a thermally-fused transparent ultraviolet barrier material  98  ′, derived from the transparent ultraviolet barrier layer  98  which separates from the film sheet  94  due to the existence of the peeling layer  96 . Accordingly, it is possible to considerably improve the preservation of a transferred color image, formed on the printing paper sheet P, due to the existence of the thermally-fused transparent ultraviolet barrier material  98 ′. 
     Similar to the third embodiment, in the fifth embodiment, during a formation of a color image on the printing sheet paper P by the printer shown in FIG. 6, an electrostatic entanglement of the image-forming substrate  92  by a platen can be prevented due to the existence of the electrical conductive layer  102 . Namely, during the formation of the color image by the printer, a side edge of the image-forming substrate  92  is in contact with a grounded conductive element of the printer (not shown in FIG.  6 ), whereby an electrostatic charge can be easily dissipated from the image-forming substrate  92  through the electrical conductive layer  102 . Also, during the formation of the color image by the printer, although the electrical conductive layer  102  may be thermally fused by the thermal heads ( 30 C,  30 M,  30 Y), the thermally-fused electrical conductive layer  102  is prevented from being stuck to the thermal heads, due to the existence of the protective film sheet  104 . 
     In the fifth embodiment, optionally, as an ink to be encapsulated in the microcapsules, a leuco-pigment may be utilized. In this case, as shown in FIG. 21, a layer of color developer  106  is formed over the paper sheet P. 
     FIG. 22 shows a sixth embodiment of an image-forming substrate, generally indicated by reference  108 , according to the present invention. In this six embodiment, the image-forming substrate  108  is also produced in a form of a transfer film sheet. Namely, the image-forming substrate  108  comprises a sheet of transparent film  110  composed of a suitable synthetic resin, such as polyethylene terephthalate, a transparent peeling layer  112  composed of a teflon-based coating material or a silicone-based coating material and formed over a surface of the film sheet  110 , a layer of transparent ultraviolet barrier  114 , and a layer of microcapsules  116  coated over the ultraviolet barrier layer  114 . 
     The microcapsule layer  116  may be identical to the microcapsule layer  14  of the first embodiment shown in FIG. 1, except that a shell of the cyan, magenta and yellow microcapsules is formed of a transparent shape memory resin. Namely, the cyan, magenta and yellow microcapsules, included in the microcapsule layer  114 , have the temperature/pressure characteristics as shown in FIG.  3 . Accordingly, by suitably selecting a heating temperature and a breaking pressure, which should be exerted on the image-forming substrate  108 , the cyan, magenta and yellow microcapsules can be selectively broken and squashed, and thus a color image can be formed on the image-forming substrate  108  by the thermal color printer as shown in FIG.  6 . 
     As shown in FIG. 23, the image-forming substrate  108  is used together with a printing sheet of paper P. Namely, the image-forming substrate  108 , overlaid with the printing paper sheet P, is fed in the printer, as shown in FIG. 6, such that the printing paper sheet P contacts the thermal heads ( 30 C,  30 M and  30 Y), and the cyan, magenta and yellow microcapsules are selectively broken and squashed in accordance with respective digital color image-pixel signals. Thus, as conceptually shown in FIG. 24, ink, discharged from the broken and squashed microcapsules, is transferred from the image-forming substrate  108  to the printing paper sheet P. Namely, a color image is once formed on the image-forming substrate  108 , and then the formed color image is transferred to the printing paper sheet P. 
     Similar to the fifth embodiment, in this six embodiment, when the image-forming substrate  108  is heated by the thermal heads ( 30 C,  30 M,  30 Y), the transparent ultraviolet barrier layer  114  is thermally fused locally in accordance with the digital color image-pixel signal. Thus, as shown in FIG. 23, the ink, transferred from the image-forming substrate  108  to the printing sheet paper P, is covered with a thermally-fused transparent ultraviolet barrier material  114 ′, derived from the transparent ultraviolet barrier layer  114  which separates from the film sheet  110  due to the existence of the peeling layer  112 . Accordingly, it is possible to considerably improve a preservation of a transferred color image, formed on the printing paper sheet P, due to the existence of the thermally-fused transparent ultraviolet barrier material  114 ′. 
     According to the sixth embodiment, after a frame of color image is completely transferred to the printing paper sheet P, the remaining image-forming substrate  108  can be utilized as a transparency film carrying a frame of negative color image, due to the transparent film sheet  110  and the transparent shells of the cyan, magenta and yellow microcapsules included in the microcapsule layer  116 . 
     On the other hand, in the sixth embodiment, as an ink to be encapsulated in the microcapsules, a transparent leuco-pigment may be utilized. In this case, as shown in FIG. 24, a layer of color developer  118  is be formed over the paper sheet P. Of course, in the embodiment of FIG. 24, after a frame of color image is completely transferred to the printing paper sheet P, the remaining image-forming substrate  108  cannot be utilized as a transparency film carrying a frame of a negative color image, because the leoco-pigments, encapsulated in the microcapsules, are transparent. Nevertheless, the remaining transparent image-forming sheet  108  can be recycled for a certain purpose due to the transparency characteristic thereof. For example, the remaining transparent image-forming substrate  108  can be used as a wrapping sheet. 
     FIG. 25 shows a seventh embodiment of an image-forming substrate, generally indicated by reference  120 , according to the present invention. In this seventh embodiment, the image-forming substrate  120  is produced in a form of a board paper sheet, which may be advantageously utilized as a post card. Namely, the image-forming substrate  120  comprises a sheet of board paper  122 , a layer of microcapsules  124  coated over a surface of the board paper sheet  122 , and a sheet of transparent protective film  126  covering the microcapsule layer  124 . 
     The microcapsule layer  124  may be identical to the microcapsule layer  14  of the first embodiment shown in FIG.  1 . Namely, the cyan, magenta and yellow microcapsules, included in the microcapsule layer  124 , have the temperature/pressure characteristics as shown in FIG.  3 . Accordingly, by suitably selecting a heating temperature and a breaking pressure, which should be exerted on the image-forming substrate  120 , the cyan, magenta and yellow microcapsules can be selectively broken and squashed, and thus a color image can be formed on the image-forming substrate  120  by the thermal color printer as shown in FIG.  6 . Note, of course, the spring-biasing units ( 34 C,  34 M and  34 Y) are adjustable in accordance with a thickness of the image-forming substrate  120 , such that the platens ( 32 C,  32 M,  32 Y) can be elastically pressed against the thermal heads ( 30 C,  30 M,  30 Y) at the required predetermined pressures. 
     Further, in the seventh embodiment, the image-forming substrate  120  features a heat-sensitive recording layer  128  formed on the other surface of the board paper sheet  122 . The heat-sensitive recording layer  128  per se is well known. Namely, the heat-sensitive recording layer  128 , which usually exhibits a white surface, is changed into a black surface when the heat-sensitive recording layer  128  is heated to beyond a predetermined temperature. 
     Accordingly, when the image-forming substrate  120  is fed in the printer, as shown in FIG. 6, such that the transparent protective film contacts the thermal heads ( 30 C,  30 M and  30 Y), the cyan, magenta and yellow microcapsules are selectively broken and squashed in accordance with respective digital color image-pixel signals, whereby a color image is formed on the microcapsule layer  124  of the image-forming substrate  120 . 
     On the other hand, by operating one of the thermal heads ( 30 C,  30 M and  30 Y) of the printer, black images, such as black characters, can be formed and recorded on the heat-sensitive recording layer  128  of the image-forming substrate  120 . Of course, in this case, the image-forming substrate  120  is fed in the printer, such that the heat-sensitive recording layer  128  contacts the thermal heads ( 30 C,  30 M and  30 Y). 
     Note, during the formation of the color image on the microcapsule layer  124  of the image-forming substrate  120  by the thermal heads ( 30 C,  30 M and  30 Y), the heat-sensitive recording layer  128  cannot be thermally influenced by the thermal heads, due to a sufficient thickness of the board paper sheet  122 . Of course, the reverse is true for the microcapsule layer  124  when forming an image on the heat-sensitive recording layer  128 . 
     Similar to the fourth embodiment, in the seventh embodiment, a leuco-pigment may be utilized as an ink to be encapsulated in the microcapsules. In this case, a color developer, which reacts with the leuco-pigment, may be contained in a binder solution, which is used for the formation of the microcapsule layer  124 . Optionally, a layer of color developer may be interposed between the board paper sheet  122  and the microcapsule layer  124 . 
     FIG. 26 shows an eighth embodiment of an image-forming substrate, generally indicated by reference  130 , according to the present invention. In this eighth embodiment, the image-forming substrate  130  is produced in a form of a paper sheet. Namely, the image-forming substrate  130  comprises a sheet of suitable transparent resin  132 , a layer of microcapsules  134  coated over a surface of the transparent resin sheet  132 , and a sheet of transparent protective film  136  covering the microcapsule layer  134 . 
     The microcapsule layer  134  may be identical to the microcapsule layer  14  of the first embodiment shown in FIG.  1 . Namely, the cyan, magenta and yellow microcapsules, included in the microcapsule layer  134 , have the temperature/pressure characteristics as shown in FIG.  3 . Accordingly, by suitably selecting a heating temperature and a breaking pressure, which should be exerted on the image-forming substrate  130 , the cyan, magenta and yellow microcapsules can be selectively broken and squashed, and thus a color image can be formed on the image-forming substrate  130  by the thermal color printer as shown in FIG.  6 . 
     Further, in the eighth embodiment, the image-forming substrate  130  features a heat-sensitive recording layer  138  formed on the other surface of the transparent resin sheet  132 . The heat-sensitive recording layer  138  is identical to the heat-sensitive recording layer  128  of the seventh embodiment. Namely, the heat-sensitive recording layer  138  usually exhibits a white surface, but the white surface is changed into a black surface when the heat-sensitive recording layer  138  is heated to beyond a predetermined temperature, as indicated by the reference T UL  of FIG.  3 . 
     As is apparent from the description made accompanying FIG. 13, a dot area, in which a black dot should be produced on the microcapsule layer  134 , is successively heated by three resistance elements (R cn , R mn  and R yn ) of the thermal heads ( 30 C,  30 M,  30 Y), which correspond to each other. Thus, a temperature of the above-mentioned dot area exceeds the predetermined temperature (T UL ), due to the successive heating by the three resistance elements (R cn , R mn  and R yn ). Accordingly, a white area of the heat-sensitive recording layer  138 , corresponding to the black dot produced on the microcapsule layer  134  is thermally changed into a black area. 
     As is well known, it is possible to produce black by mixing the three primary-colors: cyan, magenta and yellow, but, in reality, it is difficult to generate a true or vivid black by the mixing of the primary colors. Nevertheless, according to the eighth embodiment, it is possible to easily obtain a suitable black, due to the existence of the heat-sensitive recording layer  138 . 
     Similar to the fourth embodiment, in the eighth embodiment, a leuco-pigment may be utilized as an ink to be encapsulated in the microcapsules. In this case, a transparent color developer, which reacts with the leuco-pigment, may be contained in a binder solution, which is used for the formation of the microcapsule layer  134 . Optionally, a layer of transparent color developer may be interposed between the transparent resin sheet  132  and the microcapsule layer  134 . 
     FIG. 27 shows a ninth embodiment of an image-forming substrate, generally indicated by reference  140 , according to the present invention. In this ninth embodiment, the image-forming substrate  140  is produced in a form of a duplicating-paper sheet or a double-recording-paper sheet. Namely, the image-forming substrate  140  comprises a first image-forming substrate element  142 , a second image-forming substrate element  144 , and a peeling layer  146  interposed between the first and second image-forming substrate elements  142  and  144 , which is composed of a teflon-based coating material or a silicone-based coating material. 
     In particular, the first image-forming substrate element  142  includes a first sheet of paper  142 A, a first layer of microcapsules  142 B coated over a surface of the first paper sheet  142 A, and a sheet of transparent protective film  142 C covering the first microcapsule layer  142 B, and the second image forming substrate element  144  includes a second sheet of paper  144 A and a second layer of microcapsules  144 B coated over a surface of the second paper sheet  144 A. The peeling layer  146  is provided between the other surface of the first paper sheet  142 A and the second microcapsule layer  144 B, as shown in FIG. 29, and is formed on and adhered to the other surface of the first paper sheet  142 A with a larger adhesive force than that between the second microcapsule layer  144 B and the peeling layer  146 . Namely, the second image-forming substrate element  144  can be easily peeled from the peeling layer  146  when the image-forming substrate  140  is separated into the two substrate elements  142  and  144 . 
     In the ninth embodiment, the first microcapsule layer  142 B is substantially identical to the microcapsule layer  14  of the first embodiment shown in FIG.  1 . Namely, the cyan, magenta and yellow microcapsules, included in the first microcapsule layer  142 B, exhibit the temperature/pressure characteristics as shown in FIG.  3 . Accordingly, by suitably selecting a heating temperature and a breaking pressure, which should be exerted on the first image-forming substrate element  142 , the cyan, magenta and yellow microcapsules can be selectively broken and squashed, and thus a color image can be formed on the first image-forming substrate element  142 . 
     Similar to the microcapsule layer  14  of the first embodiment, shown in FIG. 1, the second microcapsule layer  144 B is formed from three types of microcapsules: a first type of microcapsules filled with cyan liquid dye or ink, a second type of microcapsules filled with magenta liquid dye or ink, and a third type of microcapsules filled with yellow liquid dye or ink, and these three types of microcapsules are uniformly distributed in the second microcapsule layer  144 B. The respective cyan, magenta and yellow microcapsules, included in the second microcapsule layer  144 B, exhibit temperature/pressure characteristics, indicated by a solid line, a single-chained line and a double-chained line in FIG.  28 . Accordingly, by suitably selecting a heating temperature and a breaking pressure, which should be exerted on the second image-forming substrate element  144 , the cyan, magenta and yellow microcapsules can be selectively broken and squashed, and thus a color image can be formed on the second image-forming substrate element  144 . 
     As is apparent from the graph of FIG. 28, a shape memory resin of the cyan microcapsules is prepared so as to exhibit a characteristic longitudinal elasticity coefficient having a glass-transition temperature T 1 ′, indicated by the solid line; a shape memory resin of the magenta microcapsules is prepared so as to exhibit a characteristic longitudinal elasticity coefficient having a glass-transition temperature T 2 ′, indicated by the single-chained line; and a shape memory resin of the yellow microcapsules is prepared so as to exhibit a characteristic longitudinal elasticity coefficient having a glass-transition temperature T 3 ′, indicated by the double-chained line. Also, the glass-transition temperatures T 1 ′, T 2 ′, and T 3 ′ are lower than the glass-transition temperatures T 1 , T 2  and T 3 , shown in the graph of FIG.  3 . 
     Accordingly, when the image-forming substrate  140  is fed in the printer, as shown in FIG. 6, such that the transparent protective film  142 C contacts the thermal heads ( 30 C,  30 M and  30 Y), the cyan, magenta and yellow microcapsules, included in the first microcapsule layer  142 B, and the cyan, magenta and yellow microcapsules, included in the second microcapsule layer  144 B, are selectively broken and squashed in accordance with respective digital color image-pixel signals, whereby two color images can be simultaneously formed on the first and second microcapsule layer  142 B and  144 B of the image-forming substrate  140 . 
     In particular, when the image-forming substrate  140  is heated by the thermal heads ( 30 C,  30 M and  30 Y), a temperature of the second microcapsule layer  144 B is lower than a temperature of the first microcapsule layer  142 B, due to the interposition of the first paper sheet  142 A and the peeling layer  146  between the first and second microcapsule layers  142 B and  144 B. Nevertheless, since the glass-transition temperatures T 1 ′, T 2 ′ and T 3 ′ are set to be correspondingly lower than the glass-transition temperatures T 1 , T 2  and T 3 , shown in the graph of FIG. 3, the simultaneous formation of the respective color images on the first and second microcapsule layers  142 B and  144 B is made possible. 
     As already stated hereinbefore, the second image-forming substrate element  144  can be easily peeled from the peeling layer  146  when the image-forming substrate  140  is torn into the two substrate elements  142  and  144 . Accordingly, after the simultaneous formation of the respective color images on the first and second microcapsule layers  142 B and  144 B, it is possible to individually obtain the respective first and second image-forming substrate elements  142  and  144  carrying the formed color images, as shown in FIG.  29 . 
     Similar to the fourth embodiment, in the eighth embodiment, a leuco-pigment may be utilized as an ink to be encapsulated in the microcapsules. In this case, a transparent color developer, which reacts with the leuco-pigment, may be contained in two respective binder solutions, which are used for the formation of the first and second microcapsule layers  142 B and  144 B. Optionally, a first layer of color developer may be interposed between the first paper sheet  142 A and the first microcapsule layer  142 B, and a second layer of color developer may be interposed between the second paper sheet  144 A and the second microcapsule layer  144 B. 
     FIG. 30 shows a tenth embodiment of an image-forming substrate, generally indicated by reference  148 , according to the present invention. Similar to the ninth embodiment, in this tenth embodiment, the image-forming substrate  148  is produced in a form of a duplicating-paper sheet or a double-recording-paper sheet. Namely, the image-forming substrate  148  comprises a first image-forming substrate element  150 , a second image-forming substrate element  152 , and a peeling layer  154  interposed between the first and second image-forming substrate elements  150  and  152  and composed of a teflon-based coating material or a silicone-based coating material. 
     In particular, the first image-forming substrate element  150  includes a first sheet of paper  150 A, a first layer of microcapsules  150 B coated over a surface of the first paper sheet  150 A, and a sheet of transparent protective film  150 C covering the first microcapsule layer  150 B, and the second image forming substrate element  152  includes a second sheet of paper  152 A, a layer of color developer formed over the second paper sheet  152 B, and a second layer of microcapsules  152 C coated over the color developer layer  152 B. The peeling layer  154  is provided between the other surface of the first paper sheet  150 A and the second microcapsule layer  152 C, as shown in FIG.  30 . 
     In the tenth embodiment, the first microcapsule layer  150 B is substantially identical to the microcapsule layer  14  of the first embodiment shown in FIG.  1 . Namely, the cyan, magenta and yellow microcapsules, included in the first microcapsule layer  152 B, exhibit the temperature/pressure characteristics as shown in FIG.  3 . Accordingly, by suitably selecting a heating temperature and a breaking pressure, which should be exerted on the first image-forming substrate element  150 , the cyan, magenta and yellow microcapsules can be selectively broken and squashed, and thus a color image can be formed on the first image-forming substrate element  150 . 
     On the other hand, the second microcapsule layer  152 C is formed from three types of microcapsules: a first type of microcapsules filled with a first liquid leuco-pigment, a second type of microcapsules filled with a second liquid leuco-pigment, and a third type of microcapsules filled with a third liquid leuco-pigment, and the respective first, second and third liquid leuco-pigments react with the color developer, included in the color developer layer  152 B, to thereby produce cyan, magenta and yellow. The respective first, second and third microcapsules, included in the second microcapsule layer  152 C, exhibit the temperature/pressure characteristics as shown in the graph of FIG.  28 . Thus, by suitably selecting a heating temperature and a breaking pressure, which should be exerted on the second image-forming substrate element  152 , the first, second and third microcapsules can be selectively broken and squashed, and thus a color image can be formed on the second image-forming substrate element  152 . 
     Accordingly, similar to the ninth embodiment, when the image-forming substrate  148  is fed in the printer, as shown in FIG. 6, such that the transparent protective film  150 C contacts the thermal heads ( 30 C,  30 M and  30 Y), the cyan, magenta and yellow microcapsules, included in the first microcapsule layer  150 B, and the first, second and third microcapsules, included in the second microcapsule layer  152 C, are selectively broken and squashed in accordance with respective digital color image-pixel signals, whereby two color images can be simultaneously formed on the first and second microcapsule layers  150 B and  152 C of the image-forming substrate  148 . 
     In the image-forming substrate  148 , the peeling layer  154  is formed on and adhered to the other surface of the first paper sheet  150 A with a sufficiently large adhesive force. Also, the microcapsule shells of the second microcapsule layer  152 C are adhered to the peeling layer  154  with a larger adhesive force than that which adheres the microcapsule shells of the second microcapsule layer  152 C to the peeling layer  154 . Nevertheless, the leuco-pigment, seeped from a broken or compacted microcapsule, can be easily separated from the peeling layer  154 . Accordingly, after the simultaneous formation of the respective color images on the first and second microcapsule layers  150 B and  152 C, when the image-forming substrate  148  is torn into the two substrate elements  150  and  152 , the second paper sheet  152 A with the color developer layer  152 B carrying the formed color image is peeled from the peeling layer  154 , as shown in FIG.  31 . 
     According to the tenth embodiment, since the second paper sheet  152 A with the color developer layer  152 B carrying the formed color image has no unbroken microcapsules, the formed color image cannot be subjected to damage even if a large external force is exerted on the second paper sheet  152 A and even if the second paper sheet  152 A is carelessly heated. 
     FIG. 32 shows another embodiment of a microcapsule filled with a dye or ink. In this drawing, respective references  156 C,  156 M and  156 Y indicate a cyan microcapsule, a magenta microcapsule, and a yellow microcapsule. A shell wall of each microcapsule is formed as a double-shell wall. The inner shell wall element ( 158 C,  158 M,  158 Y) of the double-shell wall is formed of a shape memory resin, and the outer shell wall element ( 160 C,  160 M,  160 Y) is formed of a suitable resin, which does not exhibit a shape memory characteristic. 
     As is apparent from a graph in FIG. 33, the inner shell walls  158 C,  158 M and  158 Y exhibit characteristic longitudinal elasticity coefficients indicated by a solid line, a single-chained line and a double-chained line, respectively, and these inner shells are selectively broken and compacted under the temperature/pressure conditions as mentioned above. 
     Also, the outer shell wall  160 C,  160 M and  160 Y exhibits temperature/pressure breaking characteristics indicated by reference BPC, BPM and BPY, respectively. Namely, the outer shell wall  160 C is broken and squashed when subjected to a pressure beyond BP 3 ; the outer shell wall  160 M is broken and squashed when subjected to a pressure beyond BP 2 ; and the outer shell wall  160 Y is broken and squashed when subjected to a pressure beyond BP 1 . 
     Thus, as shown in the graph of FIG. 33, a cyan-producing area, a magenta-producing area and a yellow-producing area are defined as a hatched area C, a hatched area M and a hatched area Y, respectively, by a combination of the characteristic longitudinal elasticity coefficients (indicated by the solid line, single-chained line and double-chained line) and the temperature/pressure breaking characteristics BPC, BPM and BPY. 
     Note, by suitably varying compositions of well-known resins and/or by selecting a suitable resin from among well-known resins, it is possible to easily obtain microcapsules that exhibit the temperature/pressure breaking characteristics BPC, BPM and BPY. 
     According to the microcapsules  156 C,  156 M and  156 Y shown in FIG. 32, regardless of the characteristic longitudinal elasticity coefficient of each microcapsule, it is a possible option to accurately determine a critical breaking pressure for each microcapsule. 
     Note, in the embodiment shown in FIG. 32, the inner shell wall element ( 158 C,  158 M,  158 Y) and the outer shell wall element ( 160 C,  160 M,  160 Y) may replace each other. Namely, when the outer shell wall element of the double-shell wall is formed of the shape memory resin, the inner shell wall element is formed of the suitable resin, which does not exhibit the shape memory characteristic. 
     FIG. 34 shows yet another embodiment of a microcapsule filled with a dye or ink. In this drawing, respective references  162 C,  162 M and  162 Y indicate a cyan microcapsule, a magenta microcapsule, and a yellow microcapsule. A shell wall of each microcapsule is formed as a composite shell wall. In this embodiment, each composite shell wall comprises an inner shell wall element ( 164 C,  164 M,  164 Y), an intermediate shell wall element ( 166 C,  166 M,  166 Y) and an outer shell element ( 168 C,  168 M,  168 Y), and these shell wall elements are formed from suitable resins, which do not exhibit shape memory characteristics. 
     In a graph in FIG. 35, the inner shell walls  164 C,  164 M and  164 Y exhibit temperature/pressure breaking characteristics indicated by references INC, INM and INY, respectively. Also, reference IOC indicates a resultant temperature/pressure breaking characteristic of both the intermediate and outer shell walls  166 C and  168 C; reference IOM indicates a resultant temperature/pressure breaking characteristic of both the intermediate and outer shell walls  166 M and  168 M; and reference IOY indicates a resultant temperature/pressure breaking characteristic of both the intermediate and outer shell walls  166 Y and  168 Y. 
     Thus, as shown in the graph of FIG. 35, by a combination of the temperature/pressure breaking characteristics (INC, INM and INY; IOC, IOM and IOY), a cyan-producing area, a magenta-producing area and a yellow-producing area are defined as a hatched area C, a hatched area M and a hatched area Y, respectively. 
     Note, similar to the above-mentioned case, by suitably varying compositions of well known resins, by selecting a suitable resin from among the well-known resins, and/or by suitably regulating a thickness of each shell wall, it is possible to easily obtain resins exhibiting the temperature/pressure breaking characteristics (INC, INM and INY; IOC, IOM and IOY). 
     According to the microcapsules  162 C,  162 M and  162 Y, shown in FIG. 34, both critical breaking temperature and pressure for each microcapsule can be optimally and exactly determined. 
     Although all of the above-mentioned embodiments are directed to a formation of a color image, the present invention may be applied to a formation of a monochromatic image. In this case, a layer of microcapsules ( 14 ,  60 ,  72 ,  84 ,  100 ,  116 ,  124 ,  134 ,  142 B,  144 B,  150 B,  152 C) is composed of only one type of microcapsule filled with, for example, a black ink. Also, as shown in FIG. 36, a cyan microcapsule layer, a magenta microcapsule layer and a yellow microcapsule layer may be formed on divided area sections C, M and Y, respectively, of a single image-forming substrate. When this image-forming substrate is fed in the printer as shown in FIG. 6, a cyan image is formed on the area of section C by the thermal head ( 30 C); a magenta image is formed on the area of section M by the thermal head ( 30 M); and a yellow image is formed on the area of section Y by the thermal head ( 30 Y). 
     Finally, it will be understood by those skilled in the art that the foregoing description is of preferred embodiments of the image-forming substrate, and that various changes and modifications may be made to the present invention without departing from the spirit and scope thereof. 
     The present disclosure relates to subject matters contained in Japanese Patent Applications No. 9-247688 (filed on Aug. 28, 1997) and No. 9-251365 (filed on Sep. 1, 1997) which are expressly incorporated herein, by reference, in their entireties.