When a heat capacity of a recording medium having a base material coated with a foamable resin layer is larger than a normal heat capacity, to form a raised amount equal to that with the normal case with a normal heating quantity, a turned mirror image of the image is generated. The image is transferred onto the front face of the recording medium and fixed. The recording medium is reversed, and conveyed again to the transfer part. The turned mirror image is transferred onto the rear face of the recording medium and fixed. The recording medium is heated with the heat quantity equal to that in the normal case, and combination of heat absorbed by the turned minor image and heat absorbed by the image is transmitted to the foamable resin layer to expand the foamable resin layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application No. 2013-184583 filed on Sep. 6, 2013 and Japanese Patent Application No. 2014-018807 filed on Feb. 3, 2014, which are hereby incorporated by reference herein in their entireties.

BACKGROUND

1. Technical Field

The present invention relates to a three-dimensional printer configured to form a raised image on a thermally expandable sheet, a three-dimensional image forming method, and a three-dimensional image.

2. Related Art

In related art, JP 64-028660 A proposes a method for forming a three-dimensional image by forming a front-back reversed image of a desired image on a rear face, on which no thermally expandable coating layer is formed, of a thermally expandable sheet with an image forming material excellent in light absorption characteristics, irradiating the image forming side of the thermally expandable sheet with light to selectively heat an image part by using the light absorption characteristics of the image forming material to expand the thermally expandable coating layer.

The coating layer formed on the front face (the face on which an image is to be printed) of the thermally expandable sheet disclosed in JP 64-028660 A has microspheres dispersed thereon, the microspheres being obtained by microencapsulating a low-boiling, vaporizable substance with a thermoplastic resin. The microcapsules expand by heat applied to the thermally expandable sheet and forms raised portions on the sheet front face.

With the technology disclosed in JP 64-028660 A, the image part is selectively heated to obtain a three-dimensional image by using the light absorption characteristics of the image forming material. If, however, the thickness of the sheet (sheet base material) before the coating layer is formed is increased, that is, if the basis weight increases, there is a problem that the heat capacity of the sheet increases and thus the quantity of heat applied to the microcapsules decreases, resulting in that the raised amount of the raised portions decreases.

The heat capacity is increased not only by the problem of the increased basis weight of the sheet base material but also when a sheet made of another material is adhered to the sheet base material. In other words, the raised amount of the raised portions resulting from thermal expansion of the coating layer varies widely depending on the magnitude of the heat capacity of the thermally expandable sheet itself.

With a thermally expandable sheet with a large heat capacity, the raised amount of the raised portions can be prevented from decreasing by increasing the quantity of heat applied thereto. A large quantity of heat applied to the thermally expandable sheet, however, causes a failure such as thermal expansion of a non-printed region.

SUMMARY

The present invention solves the aforementioned problems of the related art, and an object thereof is to provide a three-dimensional printer and a three-dimensional image forming method to form images always with desired raised amounts on thermally expandable sheets having different heat capacities, and a three-dimensional image formed by the three-dimensional image forming method.

To solve the aforementioned problems, a three-dimensional printer according to the present invention includes: a first printed image forming unit configured to form a first printed image on one face of a thermally expandable sheet in which a thermally expandable layer is formed; a second printed image forming unit configured to form a second printed image on the other face of the thermally expandable sheet, the second printed image having a higher density as a thermal capacity of the thermally expandable sheet is larger; and a heat applying and expanding unit configured to apply thermal energy to the thermally expandable sheet to expand the thermally expandable layer.

DETAILED DESCRIPTION

First Embodiment

FIG. 1is a cross-sectional view schematically illustrating an internal configuration of a three-dimensional printer according to a first embodiment. As illustrated inFIG. 1, the three-dimensional printer1includes a lowermost black toner printing unit2, a thermal expansion processing unit3thereon, and an uppermost full-color inkjet printer unit (hereinafter simply referred to as an inkjet printer unit)4.

The black toner printing unit2includes an endless transfer belt6extending in the horizontal direction at the center of the inside of the printer. The transfer belt6is looped around a driving roller7and a driven roller8while being tightly stretched by a stretching mechanism that is not illustrated, driven by the driving roller7to move circularly in the counterclockwise direction indicated by an arrow a inFIG. 1.

A photosensitive drum11of an image forming unit9is provided in contact with an upper surface of the transfer belt6moving circularly. The photosensitive drum11is provided with a cleaner, an initialization charger and an optical write head, which are not illustrated, followed by a developing roller12and the like close to and around the peripheral surface thereof.

The developing roller12is placed at a side opening of a toner container13. The toner container13contains black toner K. The black toner K is made of nonmagnetic monocomponent toner or nonmagnetic dual component toner.

The developing roller12carries a thin layer of black toner K contained in the toner container13on the surface thereof and develops an image in the black toner K on an electrostatic latent image formed on the peripheral surface of the photosensitive drum11by the optical write head.

A primary transfer roller14is pressed against a lower portion of the photosensitive drum11with the transfer belt6therebetween and forms a primary transfer part here. A bias voltage is supplied to the primary transfer roller14from a bias supply that is not illustrated.

The primary transfer roller14applies the bias voltage supplied from the bias supply to the transfer belt6at the primary transfer part to transfer the image in the black toner K developed on the peripheral surface of the photosensitive drum11onto the transfer belt6.

A secondary transfer roller15is pressed against the driven roller8around which a right end portion of the transfer belt6illustrated inFIG. 1with the transfer belt6therebetween, and forms a secondary transfer part here. A bias voltage is supplied to the secondary transfer roller15from a bias supply that is not illustrated.

The secondary transfer roller15applies the bias voltage supplied from the bias supply to the transfer belt6at the secondary transfer part to transfer the image in the black toner K primarily transferred on the transfer belt6onto a recording medium17conveyed from below inFIG. 1as indicated by an arrow along an image forming conveyance path20. Note that a thermally expandable sheet is used for the recording medium17of the present embodiment.

The recording medium17is stacked and stored in a recording medium storage part18that is a sheet cassette or the like, one sheet at the top is taken out by a sheet feeding roller19and delivered by a pair of standby rollers16, and conveyed on the conveyance path20, and the image in the black toner K is transferred while the recording medium17passes through the secondary transfer part.

The recording medium17that has passed the secondary transfer part while the image in the black toner K is transferred is conveyed to a fixing part21. A heating roller22and a pressing roller23of the fixing part21hold the recording medium17therebetween and apply heat and pressure to the recording medium17while conveying the recording medium17. As a result, the image in the black toner K secondarily transferred onto the recording medium17is fixed onto the sheet surface thereof.

The recording medium17having the image fixed on the sheet surface thereof and further conveyed by the heating roller22and the pressing roller23is then conveyed by a pair of discharge rollers24, discharged partially to the thermal expansion processing unit3above, and suspended immediately before a back end thereof passes through the pair of discharge rollers24.

Note that the speed at which the recording medium17(thermally expandable sheet) is conveyed at the fixing part21is relatively high, and thus the black toner printed region of the thermally expandable sheet is not expanded by the heat of the heating roller22.

Here, a double-side printing conveyance unit5will be described. The double-side printing conveyance unit5includes a return path25composed of a return start path25athat branches off in the right direction inFIG. 1immediately before the pair of discharge rollers24, a return intermediate path25bcurving downward from the return start path25a, and a return end path25ccurving to the left opposite to the above to finally turn over a sheet to be returned.

In addition, five pairs of return rollers26(26a,26b,26c,26d, and26e) are arranged along the return path25. An exit of the return end path25cmerges into a conveyance path communicating from the sheet feeding roller19of the recording medium storage part18to the pair of standby rollers16.

As described above, the recording medium17suspended immediately before the back end thereof passes through the pair of discharge rollers24is delivered to the return start path25aof the double-side printing conveyance unit5from the back end as a result of switching the conveyance path by a switching mechanism that is not illustrated and starting reverse rotation of the pair of discharge rollers24.

The recording medium17delivered into the return start path25ais reversed front-to-back and turned upside down while passing through the return intermediate path25band the return end path25cand delivered back to the pair of standby rollers16by the successive pairs of return rollers26b,26c,26d, and26e.

The recording medium17delivered back to the pair of standby rollers16is conveyed on the conveyance path20again, and an image in black toner K is transferred onto the rear face of the recording medium17while the recording medium17passes through the secondary transfer part. The recording medium17is then conveyed to the fixing part21, where the image in the black toner K transferred onto the rear face is fixed on the sheet surface.

The recording medium17having the black toner image fixed on the front and rear faces thereof is then conveyed by the pair of discharge rollers24and delivered to the thermal expansion processing unit3without being suspended this time.

The thermal expansion processing unit3is provided with a medium conveyance path27formed in the upper part thereof, along which four pairs of conveying rollers28(28a,28b,28c, and28d) are arranged. In addition, a heat ray emitting unit29is arranged substantially below the center of the medium conveyance path27.

The heat ray emitting unit29includes an infrared lamp29a, and a reflector29bhaving a substantially semicircular cross section surrounding a lower half of the infrared lamp29a. In the present embodiment, the infrared lamp29ais arranged at a position away from the surface of the recording medium17conveyed on the medium conveyance path27by a certain distance.

The conveyance speed of the pair of conveying rollers28conveying the recording medium17is set to an optimum speed (mm/sec), and the recording medium17is heated to a high temperature that does not cause sheet degradation under this condition to thermally expand a black solid printed region of the recording medium17.

Note that the conveyance speed of the recording medium17in the black toner printing unit2is high while the conveyance speed of the recording medium17is low in the thermal expansion processing unit3, and the recording medium17is conveyed one sheet by one sheet from the recording medium storage part18and is not conveyed successively until the conveyance in the thermal expansion processing unit3is completed.

Thus, the recording medium17conveyed to the thermal expansion processing unit3is retained only for a short time in a bent state along a conveyance path b between the pair of discharge rollers24in the black toner printing unit2and the first pair of conveying rollers28ain the thermal expansion processing unit3, which does not cause any inconvenience in the conveyance as a whole.

The recording medium17with the black solid printed region raised as a result of thermal expansion in the thermal expansion processing unit3is then conveyed along a conveyance path c into the inkjet printer unit4.

Note that the pairs of conveying rollers28described above may be pairs of long rollers extending in the width direction of the recording medium17perpendicular to the conveying direction, or may be pairs of short rollers that convey the recording medium17by nipping only both side ends of the recording medium17.

FIG. 2is a perspective view illustrating a configuration of the inkjet printer unit4. The inkjet printer unit4illustrated inFIG. 2includes an inner frame33illustrated inFIG. 2between the conveyance path c and a medium discharge opening32provided with a sheet output tray31outside thereof illustrated inFIG. 1.

The inkjet printer unit4includes a carriage34provided in a manner capable of reciprocating in the direction indicated by a double-headed arrow d perpendicular to the sheet conveying direction. The carriage34has a print head35for printing and ink cartridges36(36w,36c,36m, and36y) containing ink attached thereto.

The cartridges36w,36c,36m, and36ycontain white ink W, cyan ink C, magenta ink M, and yellow ink Y, respectively. The cartridges are provided separately or in a form in which ink compartments are integrated in one housing, and connected to the print head35having nozzles for discharging the respective color inks.

The carriage34is also slidably supported by a guide rail37at one side thereof and securely fixed to a toothed driving belt38at the other side thereof. This allows the print head35and the ink cartridges36(36w,36c,36m, and36y) to be driven to reciprocate together with the carriage34in the direction perpendicular to the sheet conveying direction indicated by the double-headed arrow d inFIG. 2, that is, in the horizontal scanning direction.

A flexible communication cable39is connected between the print head35and a controller, which will be described later, of the three-dimensional printer1with an inner frame33therebetween. Print data and control signals are sent from the controller to the print head35via this flexible communication cable39.

A platen41that is opposed to the print head35, extending in the horizontal scanning direction of the print head35and forming part of the sheet conveyance path is provided at a lower end of the inner frame33.

The recording medium17is intermittently conveyed in contact with the platen41in the vertical scanning direction indicated by an arrow e inFIG. 2by a pair of sheet feeding rollers42(the lower roller of which is behind the recording medium17and thus is not illustrated inFIG. 2) and a pair of discharge rollers43(the lower roller of which is similarly behind the recording medium17and thus is not illustrated).

During a suspension period of the intermittent conveyance of the recording medium17, the print head35is driven by a motor44via the toothed driving belt38and the carriage34to spray ink droplets onto the sheet surface in a state close to the recording medium17. As a result of repeating the intermittent conveyance of the recording medium17and printing during reciprocation of the print head35in this manner, printing is performed on the entire surface of the recording medium17.

For full-color printing over white, which will be described later, the recording medium17printed in white is reversely conveyed in the direction opposite to the vertical scanning direction indicated by the arrow e, and then full-color printing is performed while the recording medium17is conveyed again in the direction of the arrow e.

For full-color printing on a raised front face as a result of thermal expansion by heating the recording medium17, which will be described later, the recording medium17conveyed from the thermal expansion processing unit3via the conveyance path c is turned upside down by using a recording medium turning mechanism, which is not illustrated inFIG. 2, similar to that used in normal double-side printing arranged above the inner frame33.

FIG. 3is a circuit block diagram including the controller of the three-dimensional printer1having the configuration described above. As illustrated inFIG. 3, the circuit block includes a central processing unit (CPU)45at the center, and an interface controller (I/F_CONT)46, a printer controller (PR_CONT)47, and a heat capacity measuring unit48are connected to the CPU45via a data bus.

The PR_CONT47is connected with a printing unit49. The heat capacity measuring unit48is also connected to the I/F_CONT46.

The CPU45is connected with a read only memory (ROM)51, an electrically erasable programmable ROM (EEPROM)52, an operation panel53of a body operation unit, and a sensor unit54to which outputs from sensors arranged in respective parts are input. The ROM51stores system programs. The operation panel53has a touch display screen.

The CPU45reads out a system program stored in the ROM51, controls respective components according to the read system program to perform processes. Thus, among the respective components, the I/F_CONT46first converts print data supplied from a host device such as a personal computer into bitmap data, and develops the data in a frame memory55.

The frame memory55has storage areas set in association with print data of black toner K and print data of respective color inks of white W, cyan C, magenta M, and yellow Y, and the print data of images of the respective colors are developed in the storage areas. The developed data are output to the PR_CONT47, and output to the printing unit49from the PR_CONT47.

The printing unit49is an engine part configured to control drive output to process loads such as applied voltage of the image forming unit9including a rotary drive system including the photosensitive drum11, the primary transfer roller14, etc. of the black toner printing unit2that are illustrated inFIG. 1and a driven part including the initialization charger, the optical write head, etc. that are not illustrated inFIG. 1, and driving of the transfer belt6and the fixing part21under the control of the PR_CONT47.

The printing unit49further controls driving of four pairs of conveying rollers28and driving of emission of the heat ray emitting unit29of the thermal expansion processing unit3illustrated inFIG. 1, and the timing thereof. Furthermore, the printing unit49further controls operation of the respective components of the inkjet printer unit4illustrated inFIGS. 1 and 2.

The image data of black toner K output from the PR_CONT47is then supplied from the printing unit49to the optical write head that is not illustrated of the image forming unit9of the black toner printing unit2illustrated inFIG. 1. The image data of the respective color inks of white W, cyan C, magenta M, and yellow Y output from the PR_CONT47are supplied to the print head35illustrated inFIG. 2.

FIGS. 4A,4B,4C,5,6A,6B, and7are diagrams illustrating a basic concept of forming a three-dimensional plane with a black density of an image printed in black toner K on the recording medium17and an amount of expansion formed by the thermal expansion processing unit3in the three-dimensional printer1.

FIGS. 4A and 4Bare diagrams for explaining the principle of processing for selectively expanding to partially raise the recording medium17, andFIG. 4Cis a diagram schematically illustrating the relation between the amount of expansion (hereinafter simply referred to as a raised amount) including the area and the height of a raised region and the density of an image printed in black toner K (hereinafter simply referred to as a black density).

As illustrated inFIG. 4A, the recording medium17that is a thermally expandable sheet includes a base material56and a foamable resin layer57containing a thermal foaming agent coated on the base material56. The base material56is paper, cloth such as canvas, a panel material such as plastic, or the like, and the material therefor is not particularly limited. A known commercial product can be used for the recording medium17made of the base material56and the foamable resin layer57containing thermal foaming agent.

A black toner K image58, which will be described later, is printed on a region to be made three-dimensional of the foamable resin layer57of the recording medium17in the black toner printing unit2inFIG. 1. As illustrated inFIG. 4B, thermal radiation is then emitted by a heater similar to the heat ray emitting unit29to heat the surface of the foamable resin layer57of the recording medium17on which the black toner K image58is printed.

As a result, the black toner K image58absorbs the thermal radiation, transmits the heat to the thermal foaming agent contained in the foamable resin layer57. This causes thermal expansion reaction of the thermal foaming agent, and the region G of the recording medium17where the black toner K image58is printed is expanded and raised, as illustrated inFIG. 4B.

In this manner, the recording medium17heated by the heater is made three-dimensional as a result of foaming of the foaming agent only in the region G printed in black toner K due to the difference in heat absorptivity between the region G printed in black toner K and the non-printed region F.

Regarding the height H (hereinafter also simply referred to as a raised amount) of the black toner K image58on the printed face made three-dimensional, the height of the black toner K image58is low (the raised amount is small) when the black density is low and the height of the black toner K image58is high (the raised amount is large) when the black density is high, as illustrated inFIG. 4C. The relation between the black density and the raised amount, however, varies depending on the heat capacity of the recording medium17.

FIG. 5is a characteristic chart illustrating the relation between the black density and the raised amount depending on the difference in the heat capacity of the recording medium17.FIG. 5shows the black density (%) on the horizontal axis and the raised amount in height (mm) on the vertical axis. Note thatFIG. 5shows a difference in the sheet basis weight as an example of the difference in the heat capacity.

A curve of plots represented by circles in the characteristic chart illustrated inFIG. 5shows characteristics of a recording medium17with a basis weight of 105 g/m2with a relatively small heat capacity, and a curve of plots represented by crosses shows characteristics of a recording medium17with a basis weight of 157 g/m2with a relatively large heat capacity. In the two cases, the black toner K image of the same black density is heated under the same heating conditions.

As can be seen in the characteristic chart illustrated inFIG. 5, the relation between the black density and the raised amount is such that not only the black density and the raised amount of the black toner K image58are proportional to each other but also the raised amount varies widely with the basis weight of the recording medium17(in addition to the basis weight of the sheet base material, a sheet of another material adhered to the sheet base material increases the heat capacity).

Thus, for obtaining always the same raised amount for a black density of an image, the heat capacity of the recording medium17is measured, and the heating quantity is increased so that the raised amount will not be decreased if the heat capacity is large.

If the heating quantity is increased for the entire recording medium17, however, a failure such as thermal expansion of a region on which no image is printed may occur as described above. Thus, the inventors have sought for a method for increasing the heating quantity only for the region on which an image is printed instead of increasing the heating quantity for the entire recording medium17.

The inventors have focused on the fact that infrared rays have better transmission than other types of light, and attempted to print a mirror image of the black toner K image58as a solid black image on the rear face of the black toner K image58.

FIG. 6Ais a cross-sectional view illustrating a form in which a solid black mirror image59of the black toner K image58is printed on the rear face of the recording medium17, that is, the surface of the base material56, andFIG. 6Bis a view illustrating a state of raised portions as a result of expansion of the foamable resin layer57when the recording medium17is heated from the front face or the rear face.

A stiff sheet material having a relatively large heat capacity and not being deformable is used for the base material56. Since the base material56is stiff and not deformable, the base material56has an effect of suppressing expansion toward the base material56when the foamable resin layer57expands.

Thus, even when either of the front and the rear of the recording medium17on which the solid black mirror image59of the black toner K black image58(58a,58b) is printed is heated, part of the resin layer corresponding to the solid black mirror image59of the recording medium17receives the quantity of heat absorbed by the solid black mirror image59, foams evenly and expands uniformly.

Regions N and L of the resin layer illustrated inFIG. 6Bcorresponding to a black toner K black image58awith a low density and a black toner K black image58bwith a high density, respectively, receive the quantities of heat absorbed in proportion to the densities of the black toner K black images58aand58b, and foam and expand in proportion to the densities of the black toner K black images58aand58b.

Note thatFIG. 6Bshows the raised amounts combining the amount of even and uniform expansion for the solid black mirror image59and the amounts of expansion in proportion to the densities of the black toner K black images58aand58b. In this manner, since the recording medium17can receive transmission of heat and be subjected to expansion limited to the image forming regions from both faces as a result of heating, a desired raised amount can be obtained with a small amount of heating energy.

Specifically, for a recording medium17having a normal heat capacity, a black toner K black image58is formed only on the front face thereof, and the recording medium17is then heated and expanded. For a recording medium17having a larger heat capacity than a normal heat capacity, a black toner K black image58is formed on the front face and a solid black mirror image59is formed on the rear face, and the recording medium17is then heated and expanded. In both cases, desired raised amounts can be obtained.

Although an example in which the black toner K black images58aand58bdifferent in the density (gradation) are formed as the black toner K black image58is illustrated inFIGS. 6A and 6B, a clear difference between a case in which the solid black mirror image59is present on the rear face and a case in which no solid black mirror image59is present can be seen through examination simply using a black toner K black image58of a single scale (such as solid black).

FIG. 7is a characteristic chart illustrating the relation between the black density and the raised amount in a case where the solid black mirror image59is present on the rear face and in a case where no solid black mirror image59is present on the rear face of a recording medium17with a relatively large heat capacity.FIG. 7shows the black density (%) on the horizontal axis and the raised amount (mm) on the vertical axis.

A curve of plots represented by diamonds inFIG. 7shows the relationship characteristics between the black density and the raised amount in the case where the solid black mirror image59is present on the rear face, and a curve of plots represented by squares shows the relationship characteristics between the black density and the raised amount in the case where no solid black mirror image59is present on the rear face.

In the characteristic chart ofFIG. 7, when the black density is 50%, for example, the raised amount in the case where the solid black mirror image is not printed on the rear face is 0.9 mm while the raised amount in the case where the solid black mirror image is printed on the rear face is 1.5 mm. Thus, the raised amount with the printing on the rear face is larger by more than 60% than that without the printing on the rear face. This shows that printing of the solid black mirror image59on the rear face makes the heating effect greater for the recording medium17having a relatively large heat capacity.

FIG. 8is a flowchart illustrating processing operation for forming a desired raised amount in the image printed on the recording medium17regardless of the magnitude of the heat capacity of the recording medium17by the CPU45of the controller according to the present embodiment.

FIGS. 9A to 9D,10A and10B are diagrams for clearly explaining processing operation in steps S3and S5of the flowchart.

First, inFIG. 8, the CPU45measures the basis weight of the sheet (base material56) that is the recording medium17by the heat capacity measuring unit48(step S1). In this process, since the thickness of the foamable resin layer57before foaming is known, the measurement of the basis weight can by replaced by measurement of the thickness of the recording medium17, for example. The thickness of the recording medium17may be measured when the recording medium17is nipped by the pair of standby rollers16, between the heating roller22and the pressing roller23in the fixing part21, or by the pair of discharge rollers24along the path between the pair of standby rollers16and the pair of discharge rollers24, can be measured by using a sensor for thickness measurement that is not illustrated, or in like manner. Alternatively, the thickness may be measured, entered and stored by the user in advance, and the heat capacity measuring unit48may use the entry.

Subsequently, the CPU45stores the measured basis weight data in a predetermined area of the EEPROM52(step S2), and subsequently prints an image to be printed on the front face of the sheet (step S3). In this process, an image illustrated inFIG. 9A, for example, is printed.

The image60illustrated inFIG. 9Aincludes a contour61of a background mountain, a standing image62of a dog under the mountain contour61, and characters ABC63in three different scales under the feet of the dog. The image60is printed as the black toner K image58on the front face of the sheet conveyed in the direction of an arrow f.

Subsequently, inFIG. 8, the CPU45reads out the basis weight data stored in the predetermined area of the EEPROM52, and determines whether or not the read out basis weight data is equal to or larger than 157 m2(step S4).

If the basis weight data is smaller than 157 m2(No as a result of determination in step S4), the base material56of the recording medium17is determined to be a sheet having a normal heat capacity, and a sheet heating process is immediately performed (step S6).

In this process, at the secondary transfer part where the driven roller8and the secondary transfer roller15are opposed to each other as illustrated inFIG. 1, the image60illustrated inFIG. 9Ais printed on the surface of the foamable resin layer57, and the recording medium17on which the image60is fixed at the fixing part21is heated while being conveyed at the thermal expansion processing unit3.

Note that since the processes of color printing and sheet discharge by the inkjet printer unit4that follow are described with reference toFIGS. 1 and 2above, the description is not repeated here.

If the basis weight data is equal to or larger than 157 m2in the determination in step S4(Yes as a result of the determination in step S4), the CPU45prints a minor image of the image60turned by 180° in black solid on the rear face of the recording medium17conveyed to the secondary transfer part via the double-side printing conveyance unit5(step S5).

In this process, first, as illustrated inFIG. 9B, a black solid image (binary image)64of the image60is formed in the frame memory55. The CPU45further reverses the binary image64to form a reversed, mirror image65thereof as illustrated inFIG. 9C. The CPU45further forms a turned mirror image66by turning the reversed, mirror image65by 180°.

The reason for which this complicated image processing is performed will be explained. With the recording medium17having the basis weight data representing a relatively large heat capacity of 157 m2, a solid black minor image59of the image60illustrated inFIG. 9Aneeds to be printed on the rear face of the image60(black toner K image58) to obtain a desired raised amount for the image60(seeFIGS. 6A and 6B).

Since printing is also performed on the rear face, that is, for double-side printing, the recording medium17onto which the image60illustrated inFIG. 9Ais fixed is turned upside down and further reversed front-to-back through the double-side printing conveyance unit5and conveyed to the secondary transfer part from the back end of the image60.

In this process, when the recording medium17is viewed from the secondary transfer roller15, the image60is reversed upside down and left-to-right and faces the secondary transfer roller15while the recording medium17is conveyed as illustrated inFIG. 10A. The outline of the image60reversed upside down and left-to-right illustrated inFIG. 10Aas viewed in a perspective manner form the transfer belt6and the driving roller7is as illustrated inFIG. 10B.

The outline illustrated inFIG. 10Bis the same as the outline of the turned mirror image66illustrated inFIG. 9D. In step S5, the CPU45prints the turned mirror image66onto the rear face of the recording medium17conveyed to the secondary transfer part. As a result, the black solid image that is a reversed, mirror image of the image60is printed on the rear face of the recording medium17(seeFIG. 6A).

Subsequently, the CPU45performs the process in step S6described above. As a result, a raised amount equal to that for a recording medium17having a normal heat capacity can also be obtained for a recording medium17having a large heat capacity without increasing the heating quantity.

Although certain embodiments of the present invention have been described, the present invention is within a scope of the invention defined in the claims and the equivalents. For example, although the heat ray emitting unit29is arranged below the medium conveyance path27in the thermal expansion processing unit3in the embodiment described above, a heat ray emitting unit may be additionally arranged at an upper position depending on the heat capacity of the thermally expandable sheet.

In addition, various further modifications can be made without departing from the scope of the present invention.