Patent Publication Number: US-7911490-B2

Title: Printer

Description:
PRIORITY INFORMATION 
     This application claims priority to Japanese Patent Application No. 2008-202248, filed on Aug. 5, 2008, which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a printer which performs image printing on a print surface of a flat shaped print medium by a thermal transfer method. 
     2. Related Art 
     Conventionally, printers for performing printing on print media which are stiff and shaped like plates, such as optical disks or magnetic cards, have been known (for example, refer to Japanese patent publication JP 2007-301879 A). In some such printers, the print medium is usually placed on a print tray and transported with the print tray. A recessed section which has a shape conforming to an outside shape of the print medium is formed on the print tray. Then, positioning of a recording medium relative to the print tray is achieved by placing and housing the print medium in the recessed section. On the other hand, a print tray for a printer in which an optical disc is used as the print medium is equipped with a plurality of clamping lugs which are brought into intimate contact with a circumferential edge of an opening (center hole) formed in the center of the optical disc, to thereby clamp the optical disc. Then, the optical disc is retained by the clamping lugs and accordingly prevented from unintentional dropping off or lifting up. However, the clamping lugs effectively function only for optical discs having a center hole, while flat shaped print media having no center hole, such as magnetic cards or IC cards, are typically simply placed on the print tray in many cases. 
     In the printer as described above, a sheet material on which a print image is formed is pressed against a print surface of the print medium in a planar manner by a heat roller and a separation roller, to thereby realize print processing. When the print image is pressed against the print surface, the print image is transferred to the print surface. Here, the heat roller is a roller containing in the inside thereof a heat source, and the ink that constitutes the print image is heated by the heat source and accordingly fused. On the other hand, the separation roller is a roller installed at a separating position where the sheet material pressed against the print surface is separated from the print surface. Upon reaching a location of the separation roller, a forwarding direction of the sheet material which has been substantially parallel to the print surface is turned to an obliquely upward direction. Then, the turning of the forwarding direction to the obliquely upward direction causes the sheet material to be separated from the print surface. 
     However, for the print medium in which the center hole is not provided, the above-described separation of the sheet material subsequent to a transfer process could not be realized smoothly and efficiently in some cases. Namely, the print medium having no center hole is simply placed on the print tray as described above without any mechanism to prevent the print medium from being lifted up. Therefore, when the sheet member is forwarded to the obliquely upward direction for separation, the print medium might be lifted obliquely upward while adhering to the sheet member in some cases. Accordingly, there have been problems that the print image is not sufficiently transferred, and that failure is caused by taking up the sheet member with the print medium which remains affixed to the sheet member. 
     Hence, the present invention advantageously provides a printer capable of performing print processing in a smooth and efficient manner even on a print medium in which a center hole is absent. 
     SUMMARY 
     According to the present invention, a printer for performing image printing on a print surface of a plate-shaped print medium using a thermal transfer method comprises a printing unit that transfers a print image formed on a sheet material to the print surface for performing the image printing on the print surface, a print tray on which the print medium is placed with the print surface being exposed, the print tray moving relative to the printing unit along a predetermined print proceeding direction when the print image is transferred. In the printer, the printing unit comprises the sheet material, on which the formed print image is formed, the sheet material being taken up in synchronization with relative movement of the print tray along the print proceeding direction, a transfer assembly for pressing the sheet material against the print surface to transfer the print image to the print surface, and a retaining member which is installed to push the print medium from above and located, in a printing direction, downstream of a separating position where a forwarding direction of the sheet is turned to separate the sheet material having been pressed against the print surface from the print surface. 
     In a preferable embodiment, the printer further comprises a biasing means for exerting a force on the retaining member along a direction of pushing down the print tray. It is further preferable that the print tray has an end edge formed into a bevel (a slanting edge), the end edge being located downstream in the printing direction. 
     Further, in another preferred embodiment, the retaining member may be switched, depending on advancing and retreating conditions of the print tray, between a lowered state where the retaining member has been lowered to push an upper surface of the print tray, and an elevated state where the retaining member has been elevated to keep away from the upper surface of the print tray. In this case, the printer further comprises a cam pin attached to one of the retaining member or the print tray and a cam groove formed in the other of the retaining member or the print tray to receive the cam pin, the cam groove defining a path of relative movement between the cam pin and the cam groove corresponding to advancing and retreating motions of the print tray. Further, the cam groove has a shape such that the path of the relative movement during the advancing motion of the print tray is different from the path of the relative movement during the retreating motion of the print tray. 
     According to the present invention, the print medium is pushed down by the retaining member installed downstream from the separating position in the printing direction, to thereby prevent lifting of the print medium. Then, in this way, because the sheet member is always separated with reliability, print processing can be performed smoothly and efficiently. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a perspective view of a printer according to an embodiment of the present invention; 
         FIG. 2  shows a schematic configuration of the printer; 
         FIG. 3  is an enlarged view of a portion around a transfer assembly; 
         FIG. 4  is an enlarged view of the portion around the transfer assembly; 
         FIG. 5  is a schematic perspective view of a roller unit; 
         FIG. 6  is a schematic side view of a print tray; 
         FIG. 7A  is a schematic side view of the roller unit and the print tray; 
         FIG. 7B  is a schematic side view of the roller unit and the print tray; 
         FIG. 7C  is a schematic side view of the roller unit and the print tray; 
         FIG. 7D  is a schematic side view of the roller unit and the print tray, and 
         FIG. 8  is an enlarged view of a portion around a transfer assembly in a conventional printer. 
     
    
    
     DETAILED DESCRIPTION 
     A preferred embodiment of the present invention will be described hereunder by reference to the drawings.  FIG. 1  is a perspective view of a printer  10  according to the embodiment of this invention, and  FIG. 2  shows a schematic configuration of the printer  10 . The printer  10  is an apparatus for performing print processing of a print medium which is relatively thick and stiff, such as, for example, a magnetic card or an IC card. 
     A print medium  100  is transported while being supported by a print tray  12 . The print tray  12  can advance and retreat along a direction (an X direction in  FIGS. 1 and 2 ) substantially orthogonal to a rotation axis of a heat roller  58  which will be described below. Then, when the print tray  12  advances or retreats, the print medium  100  supported by the print tray  12  is transported to the outside or inside of a housing  16 . A housing recess  30  is formed on an upper surface of the print tray  12  in which the print medium  100  is placed and housed in the housing recess  30 . The housing recess  30  has a shape conforming with an outer peripheral shape of the print medium  100 , so that positioning of the print medium  100  relative to the print tray  12  is appropriately achieved by placing the print medium  100  in the housing recess  30 . 
     In addition, the print tray  12  may be composed of a tray main body attached to the printer  10  and an adapter removably connected to the tray main body in order to enable the printer  10  to accept print media  100  of various shapes. In this case, an adapter is prepared for each shape category of the print media  100 , and a plurality of adapters are accordingly provided. Further, the housing recess  30  whose shape conforms with the outer peripheral shape of the corresponding print medium  100  is formed on each of the adapters. Then, from among the plurality of adapters, a user may select one adapter corresponding to a desired print medium  100  on which printing is desired, and attaches the thus-selected adapter to the tray main body for use. 
     Next, a printing unit  18  installed in the printer  10  will be described with reference to  FIG. 2 . The printing unit  18  according to the present embodiment performs printing using a thermal transfer method in which ink of an ink ribbon  40  is transferred to an intermediate transfer sheet  42 , and then the ink (a print image) having been transferred to the intermediate transfer sheet  42  is subsequently transferred from the intermediate transfer sheet  42  onto a print surface of the print medium  100 . In the ink ribbon  40 , ink of a plurality of colors, for example four, is repeatedly arranged along a longitudinal direction of the ribbon. Inks suitable for the ink ribbon  40  include thermofusible ink and sublimation ink, but the configuration of the printer  10  and the transfer process are similar regardless of the type of ink used. Hence, in the following, an example in which a thermofusible ink is used will be described. This ink ribbon  40  is fed from a ribbon delivery bobbin  46 , guided by a plurality of guide rollers  62 , and sequentially picked up by a ribbon take-up bobbin  44 . At any suitable position along the way of take-up of this ribbon, there is provided a thermal head  48  which brings the ink ribbon  40  into direct contact with the intermediate transfer sheet  42  and fuses the ink on the surface of the ink ribbon  40 . A plurality of heating elements (not shown) are provided in the thermal head  48 . 
     In response to a command from a control unit (not shown), the thermal head  48  selectively heats the plurality of heating elements, to thereby partially fuse the ink of the ink ribbon  40 . When the ink ribbon  40  whose ink is partially fused is pressed against the intermediate transfer sheet  42 , the fused ink is transferred to the intermediate transfer sheet  42 . 
     The intermediate transfer sheet  42  is fed from a sheet feed bobbin  50 , guided by a plurality of guide rollers  62 , and taken up by a sheet take-up bobbin  52 . A platen roller  54  which receives pressure from the thermal head  48  is provided at an arbitrary position along the way of take-up of the intermediate transfer sheet  42 . As a result of the thermal head  48  pressing the ink ribbon  40  toward the platen roller  54 , the ink fused by heat from the thermal head  48  is transferred to the intermediate transfer sheet  42  fed along the platen roller  54 . 
     As described above, ink of the plurality of colors is repeatedly arranged on the surface of the ink ribbon  40  along the longitudinal direction of the ink ribbon  40 . When a full-color image is printed, all the plurality of colors of ink must have been transferred onto the intermediate transfer sheet  42  in advance. Accordingly, in the case of full-color printing, the intermediate transfer sheet  42  is taken up by the sheet feed bobbin  50  every time transfer of ink of one color affixed to the surface of the ink ribbon  40  is completed. The intermediate transfer sheet  42  is again transported from the sheet feed bobbin  50  to the sheet take-up bobbin  52  in order for the next color of ink to be transferred. The number of repetitions of transfer of ink and transportation of the sheet corresponds to the number of colors of ink on the ink ribbon  40  in order to form a full-color print image on the surface of the intermediate transfer sheet  42 . 
     The full-color print image formed on the intermediate transfer sheet  42  is finally transferred onto the print surface of the print medium  100  by means of a transfer assembly  56 . The transfer assembly  56  is a component in which the heat roller  58 , including the heating elements in the inside thereof, is connected to a separation roller  60  located downstream of the heat roller  58 . During the course of transfer of ink from the ink ribbon  40  to the intermediate transfer sheet  42 , the transfer assembly  56  is raised to a height at which the transfer assembly  56  is located away from the print medium  100 . Meanwhile, when the transfer of ink from the ink ribbon  40  to the intermediate transfer sheet  42  is completed and the full-color print image is formed on the intermediate transfer sheet  42 , the transfer assembly  56  descends to press the intermediate transfer sheet  42  against the print surface of the print medium  100 . In this state, the heat roller  58  and the separation roller  60  are arranged in such a manner that the heights of their lower ends are at substantially the same level. Therefore, when the transfer assembly  56  is moved down, the intermediate transfer sheet  42  located between the heat roller  58  and the separation roller  60  is brought into contact with the print surface in the form of plane contact. Because the intermediate transfer sheet  42  is thus contacted, as plane contact, with the print surface for a predetermined period of time, poor transfer or other failure caused by premature separation of the intermediate transfer sheet  42  from the print surface can be prevented. 
     During this pressing process, the ink transferred to the intermediate transfer sheet  42  is fused by the heating elements incorporated into the heat roller  58 . Further, during the pressing process, the intermediate transfer sheet  42  is taken up to the sheet take-up bobbin  52  at a constant speed. The print tray  12  is moved together with the intermediate transfer sheet  42  along the same direction at the same speed. In other words, the print tray  12  moves relative to the transfer assembly  56  during the pressing process. Then, operation of the heat roller  58  and the print tray  12  causes the full-color print image formed on the intermediate transfer sheet  42  to be finally transferred onto the print surface, whereby printing of the image is implemented. 
     Here, as is evident from  FIG. 2 , the intermediate transfer sheet  42  is forwarded between the heat roller  58  and the separation roller  60  along a direction parallel to the print surface, and forwarded in a downstream side of the separation roller  60  along a slanting direction relative to the print surface. In other words, a forwarding direction of the intermediate transfer sheet  42  is turned upon reaching a separating position where the separation roller  50  is installed. Thus, the intermediate transfer sheet  42  will be separated from the print surface by turning the forwarding direction as described above. 
     In addition to the above-described configuration, the printer  10  according to this embodiment further comprises a retaining roller  70  in order to further ensure that the separation of the intermediate transfer sheet  42  is performed with reliability. Before describing the retaining roller  70  in detail, conventional problems will be briefly explained with reference to  FIG. 8 .  FIG. 8  is an enlarged view of a part around the transfer assembly  56  in a conventional printer. In this conventional printer, the print medium  100  is simply placed in the housing recess  30 , and a mechanism to prevent lifting of the print medium  100  is not provided. In such a configuration, it is assumed that the print medium  100  is transported downstream in a state where the intermediate transfer sheet  42  is pressed against the print medium  100 . Here, on reaching the separating position where the separation roller  60  is installed, the intermediate transfer sheet  42  should be independently forwarded along an obliquely upward direction while the print medium  100  should be moved along a horizontal direction, thereby causing separation between the intermediate transfer sheet  42  and the print medium  100  as originally intended. However, in the conventional printer where the mechanism to prevent lifting of the print medium  100  is absent, the fused ink might function in some cases just as an adhesive for affixing the print medium  100  to the intermediate transfer sheet  42 . As a result, the print medium  100 , which remains affixed to the intermediate transfer sheet  42  even after passing through the separating position, might sometimes be lifted obliquely upward together with the intermediate transfer sheet  42 . In this situation, there has been a problem that transfer of the print image to the print surface might be insufficient. In addition, the print medium  100  might be taken up together with the intermediate transfer sheet  42 , resulting in a breakdown of the printer  10 . 
     Here, when a print medium has, at a substantial center thereof, an opening (a center hole) like a CD or a DVD, the above-described problem of lifting up the print medium  100  can be circumvented by installing, in the print tray, clamping lugs to be engaged with a circumferential edge of the center hole. More specifically, for the print medium  100  such as a CD having the center hole, it is possible to retain the print medium  100  by tightly coupling the clamping lugs capable of moving in a radial direction to the circumferential edge of the center hole, which can, in turn, prevent the print medium  100  from lifting up. 
     However, for a magnetic card or an IC card, it is not possible to prevent the lifting by means of the clamping lugs as described above because of the absence of the center hole. Further, it is not inconceivable to have a way of holding the circumferential edge of the card (the print medium  100 ) from above using a lug or other components. In this case, however, printing cannot be performed on the circumferential edge, resulting in a problem that constraint on a design of the print image is increased. 
     Taking this situation into account, in the present embodiment, the retaining roller  70  is provided in order to implement satisfactory printing even on the print medium which is shaped like a flat plate having no center hole. The retaining roller  70  is a roller installed to facilitate smooth and efficient separation of the intermediate transfer sheet  42  from the print surface. As shown in  FIG. 2 , the retaining roller  70  is installed slightly downstream of the separation roller  60  and located below the intermediate transfer sheet  42  being forwarded in the obliquely upward direction and above the print tray  12 . 
     The retaining roller  70  is biased in a downward direction, i.e. a direction of pushing down the upper surface of the print tray  12  by a biasing means such as a spring (not shown), and lifting of the print medium  100  is prevented by this biasing force. Referring to  FIG. 3 , action of the retaining roller  70  will be specifically described.  FIG. 3  is an enlarged view of a portion around the transfer assembly  56  in the printer according to this embodiment. 
     As has been discussed, because the transfer assembly  56  presses the intermediate transfer sheet  42  against the print medium  100 , the print medium  100  is affixed to the intermediate transfer sheet  42  by viscosity (adhesive property) of the fused ink. Then, the print medium  100  tries to lift up while remaining affixed to the intermediate transfer sheet  42  even after passing across the separating position where the intermediate transfer sheet  42  undergoes direction change (an installation position of the separation roller  60 ). The retaining roller  70  which is located slightly downstream of the separation roller  60  downwardly pushes the print medium  100  which is trying to lift up, to thereby prevent lifting of the print medium  100 . As a result, taking up the print medium  100  together with the intermediate transfer sheet  42  is also prevented, thereby ensuring reliable separation of the intermediate transfer sheet  42  from the print medium  100  in the vicinity of the separation roller  60 . In this manner, the print image can be properly transferred on the print surface, so that excellent print quality is obtained. 
     In addition, a forward end of the print tray  12  has a bevel  12   a  for allowing the retaining roller  70  to easily go up onto the print tray  12 . More specifically, the print tray  12  usually starts an advancing motion to perform transfer processing after retreating to an innermost side. During the advancing motion, the retaining roller  70  may impinge upon a forward end surface of the print tray  12  and interfere with the forward end surface in some cases. Then, if there is the bevel  12   a  formed on the forward end of the print tray  12 , the retaining roller  70  will undergo relative movement along a surface of the bevel  12   a  and become able to easily go up onto the print tray  12 . 
     Note that the retaining roller  70  is a member which makes physical contact directly with the print surface of the print medium  100  without intervention by the intermediate transfer sheet  42  or the like. In general, it is desirable to restrict the direct physical contact between the print surface and another component to the minimum necessary, in order to prevent flaws or dirt. For this reason, the retaining roller  70  is designed to have a special structure for suppressing the physical contact between the retaining roller  70  and the print surface in this embodiment. 
     Specifically, the retaining roller  70  is switched between a lowered state in which the retaining roller  70  has been lowered to push the upper surface of the print tray, and an elevated state where the retaining roller  70  has been elevated to keep away from the upper surface of the print tray, depending on advancing and retracting motions of the print tray  12 . More specifically, in this embodiment, while the print tray  12  is advancing from a predetermined printing start position to a printing end position to transfer the print image to the print surface, the retaining roller  70  has been lowered so as to be capable of pushing down the print surface or the upper surface of the print surface. On the other hand, during a pullback operation for retreating the print tray  12  from a medium removal position where the housing recess  30  is exposed to the outside (where the print tray  12  is projected out of the housing  16 ) to the printing start position, and during an ejection operation for advancing the print tray  12  from the printing end position to the medium removal position, the retaining roller  70  has been elevated to prevent the retaining roller  70  from making physical contact with the upper surface of the print tray  12 . 
     Although ascent and descent operations of the retaining roller  70  may be achieved by means of an electrically controllable drive source such as a motor, the operation is performed through mechanical interaction between the retaining roller  70  and the print tray  12  in this embodiment. This will be described with reference to  FIGS. 5 to 7 . 
       FIG. 5  is a schematic perspective view of a roller unit  69  into which the retaining roller  70  used in this embodiment is unitized with other components. The roller unit  69  comprises the retaining roller  70  for pushing the print surface and a pair of support arms  72  for supporting the retaining roller  70  from both sides. The retaining roller  70  is a cylindrical member rotatably held by the pair of support arms  72 . Lifting of the print medium  100  is prevented by the retaining roller  70  pushing the print surface of the print medium  100 . Further, it is desirable that an outer surface of the retaining roller  70  is composed of an elastic material such as rubber to avoid damage to the print medium  100 . 
     The support arm  72  for supporting the retaining roller  70  is, for example, a roughly L-shaped member. A front end of the support arm  72  is provided with a support axis (not shown) for rotatably holding the retaining roller  70 . On the other hand, a rear end of the support arms  72  is suspended and retained by a coil spring  74 . The coil spring  74  functions as a biasing means for exerting a force on the retaining roller  70  along a direction of pushing down the print tray  12 . One end of the coil spring  74  is attached to the rear end of the support arm  72 , while the other end of the coil spring  74  is attached to a fixed member such as a chassis (not shown) of the printer  10 . The rear end of the support arm  72  is biased upward by the coil spring  74 . 
     In addition, a cam pin  76  and a swing shaft  78  are protrudingly formed on the support arm  72 . The swing shaft  78  is a shaft to be inserted into a shaft hole (not shown) provided in the fixed member such as a chassis of the printer  10 . The support arm  72  swings around the swing shaft  78 . Then, a swing of the support arm  72  can cause the retaining roller  70  to ascend or descend. The cam pin  76  is a pin mounted on a rear end side of the swing shaft  78 . The cam pin  76  is inserted into a cam groove  80  formed on a side surface of the print tray  12 . Then, the swing of the support arm  72 , and thus ascending and descending actions of the retaining roller  70 , is implemented by an engagement relationship between the cam pin  76  and the cam groove  80 . 
     Next, the cam groove  80  formed on the side surface of the print tray  12  will be described with reference to  FIG. 6 .  FIG. 6  is a schematic side view of the print tray  12 . Note that in  FIG. 6 , areas other than the cam groove  80  are shown hatched to facilitate understanding. The cam groove  80  in which the cam pin  76  is inserted and engaged is formed on the side surface of the print tray  12 . The cam groove  80  has a shape of a laid down numeral “6” as shown in  FIG. 6 . More specifically, the cam groove  80  generally consists of four grooves. A first groove  80   a  is a groove extending in the form of a straight line along the advancing/retreating direction of the print tray  12 . A second groove  80   b  is a groove formed above the first groove  80   a  and extending in parallel with the first groove  80   a . However, the second groove  80   b  is shorter than the first groove  80   a , and a posterior end of the second groove  80   b  is located in a more forward position than a posterior end of the first groove  80   a . A third groove  80   c  is a groove for connecting the anterior ends of the first groove  80   a  and the second groove  80   b  with each other, and extends along a substantially vertical direction. It should be noted that each groove width of the first, second, and third grooves  80   a ,  80   b , and  80   c  is slightly larger than a diameter of the cam pin  76  so that the cam pin  76  can move within the grooves. A fourth groove  80   d  is a groove for connecting the posterior end of the second groove  80   b  and an approximate middle position of the first groove  80   a , and is gently inclined. A groove width of the fourth groove  80   d , which is smaller than the diameter of the cam pin  76  in an initial state, can be expanded from the initial state by a pivoting motion of a pivot valve  82 . The pivot valve  82  is a valve body for adjusting the groove width of the fourth groove  80   d , and is capable of pivoting around a pivot axis  84 . A biasing member such as a spring (not shown) is attached to the pivot valve  82  for forcing the pivot valve  82  to return to the initial state where the groove width is smaller than the diameter of the cam pin  76 . 
     Next, the mechanical interaction between the roller unit  69  and the print tray  12  will be described with reference to  FIG. 7 .  FIGS. 7A to 7D  are schematic side views showing the roller unit  69  and the print tray  12 . As can be clearly seen from  FIGS. 7A to 7D , a position (height) of the cam pin  76  is changed in response to the advancing and retreating motions of the print tray  12 . On the other hand, because the swing shaft  78  is inserted into the shaft hole formed in the fixed member, the position of the swing shaft  78  remains unchanged. Then, as a relative position between the cam pin  76  and the swing shaft  78  changes, an angle of inclination of the support arms  72  is changed, thereby causing the retaining roller  70  to descend or ascend. This will be described in detail below. 
     To perform print processing on the print medium  100 , the print tray  12  is advanced to the medium removal position where the housing recess  30  is exposed outside the housing  16 .  FIG. 7A  shows the print tray  12  which has reached the medium removal position. As can be seen from  FIG. 7A , when the print tray  12  stays at the medium removal position, the cam pin  76  is located at the posterior end of the first groove  80   a . The angle of the support arms  72 , and thus the height of the retaining roller  70 , is defined by a relative position (height) between the cam pin  76  and the swing shaft  78  whose position remains unchanged. In this embodiment, the cam groove  80  and others are formed into shapes that define a height at which the retaining roller  70  is maintained away from the print tray  12  when the cam pin  76  is engaged in the first groove  80   a.    
     A user places the print medium  100  in the housing recess  30  of the print tray  12  advanced to the medium removal position. Upon placement of the print medium  100 , the print tray  12  performs its pullback operation for retreating to the inner side.  FIG. 7B  shows a way of performing the pullback operation. As shown in  FIG. 7B , the cam pin  76  undergoes relative movement along the first groove  80   a  when the pullback operation is performed (the cam pin  76  does not actually move, while the print tray  12  moves instead). During the pullback operation, the retaining roller  70  remains elevated to the height at which the retaining roller  70  keeps away from the upper surface of the print tray  12 . It should be noted that because the fourth groove  80   d  is closed by the pivot valve  82 , the cam pin  76  is not allowed to slide into the fourth groove  80   d.    
     The pullback operation is performed until the print tray  12  reaches the printing start position.  FIG. 7C  shows the print tray  12  which has arrived at the printing start position. When the print tray  12  arrives at the printing start position, the cam pin  76  reaches the anterior end of the first groove  80   a  (which also constitutes a lower end of the third groove  80   c ). Here, an upward force is exerted on the rear ends of the support arms  76  by the spring. The cam pin  76  that has arrived at the anterior end of the first groove  80   a  is moved by the upward force within the third groove  80   c  extending along the substantially vertical direction to the second groove  80   b  located above the first groove  80   a . When the cam pin  76  moves into the second groove  80   b , i.e. when the cam pin  76  ascends, the retaining roller  70  located on the opposite side of the swing shaft  78  descends, contrary to the cam pin  76 . As a result, it becomes possible for the retaining roller  70  to push the upper surface of the print tray  12  and therefore the print medium  100 . 
     Upon arrival of the print tray  12  at the printing start position, actual transfer processing for transferring the print image to the print surface is started. During the transfer processing, the print tray  12  is gradually advanced simultaneously with the intermediate transfer sheet  42  being taken up. In the course of this advancing motion, the cam pin  76  undergoes relative movement along the second groove  80   b , while the retaining roller  70  is maintained in the lowered state. As a result, lifting of the print medium  100  is prevented by the retaining roller  70  as described above, which makes it possible to obtain excellent print quality. 
     When the transfer processing is completed, the print tray  12  arrives at the printing end position. Upon arrival of the print tray  12  at the printing end position, the cam pin  76  reaches the posterior end of the second groove  80   b  (which also constitutes the anterior end of the fourth groove  80   d ). After the completion of the transfer processing, the print tray  12  further performs its ejection operation in which the print tray  12  advances to the medium removal position to allow removal of the print medium  100  having been printed. The ejection operation (advancing motion) causes relative movement of the cam pin  76  along the fourth groove  80   d .  FIG. 7D  is a diagram showing a situation during the ejection operation. In this situation, the pivot valve  82  is depressed down by the cam pin  76  and accordingly pivoted in a direction of increasing the groove width of the fourth groove  80   d . In this way, the cam pin  76  is allowed to move within the fourth groove  80   d . In other words, the cam pin  76  is gradually lowered along the fourth groove  80   d . As the cam pin  76  is lowered, the retaining roller  70  located on the opposite side of the swing shaft  78  is, contrary to the cam pin  76 , elevated. In this way, the retaining roller  70  is located away from the upper surface of the print tray. Upon arrival at the first groove  80   a , the cam pin  76  undergoes relative movement along the first groove  80   a . Then, after the print tray  12  reaches the medium removal position, a sequence of print processing is completed when the printed print medium  100  is taken out by the user. 
     As will be clearly understood from the above description, a path of relative movement of the cam pin  76  is established in such a manner that the path of relative movement during the retreating motion of the print tray is different from that during the advancing motion of the print tray in this embodiment. In this way, the retaining roller  70  can be lowered at a time of transfer processing and elevated at all other times, to thereby avoid more physical contact between the retaining roller  70  and the print surface than is necessary. It should be noted that the above-described configuration is disclosed by way of illustration, and other configurations may, of course, be implemented as long as the elevated state where the retaining roller  70  is elevated can be switched to the lowered state where the retaining roller  70  is lowered. For example, the cam pin may be formed on the print tray  12 , while the cam groove may be formed in the roller unit  69 . Alternatively or additionally, an electrically controllable drive source such as a motor may be used. In addition, the mechanism for elevating the retaining roller  70  may be omitted, provided that the retaining roller  70  is capable of at least pushing the print surface at the time of transfer processing. Further, although the roller is used as a retaining means for pushing the print surface, the retaining means is not limited to the roller, and a plate may be used as the retaining member when the plate is capable of pushing the print surface. Still further, the retaining means need not be biased by the biasing means as long as the retaining means can push the print medium. Moreover, the support arms  72  for supporting the retaining roller  70  are shaped almost like the letter L in this embodiment, but may be, of course, formed in other shapes. In any case, the print surface can be pushed down on a slightly downstream side of the separation roller  60  according to this embodiment. In this way, it becomes possible to reliably separate the intermediate transfer sheet  42  from the print surface, with a result that excellent print quality can be obtained.