Abstract:
A thermal transfer printer includes: an ink ribbon conveyor unit that conveys an ink ribbon; a sheet conveyor unit that conveys a sheet; a dummy pattern generation unit that generates a dummy pattern; an image data generation unit that generates print image data joining n screens together, the print image data including the dummy pattern inserted between two adjacent screens among the n screens; a thermal head that transfers a dye coated on the ink ribbon in accordance with the print image data; and a peeler unit that peels the ink ribbon from the sheet. An average density of the dummy pattern is equal to an average density of the image over an area equivalent to a distance between a tail end portion of the thermal head and the peeler unit, on one screen that follows the dummy pattern.

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a thermal transfer printer, and particularly to a technique to suppress lateral banding from occurring in a thermal transfer printer that performs multi-screen printing such as two-screen printing or three-screen printing. 
     2. Related Art 
     In a thermal transfer printer, such as a dye sublimation printer, it is preferable to use an ink ribbon having an optimal length for a given print size. For example, to print images having sizes of 6×4 inches and 6×8 inches, it is preferable to use dedicated ink ribbons for the respective sizes. There is known a technique for performing two-screen printing by which consumption of materials and a printing time are reduced. In the known technique, an ink ribbon having a size of, for example, 6×8 inches may be used to perform two-screen printing of two images each having a size of 6×4 inches, in a single process. There is also known a technique of performing three-screen printing by using an ink ribbon having a size of 8×12 inches to perform three-screen printing of three images each having a size of 8×4 inches, in a single process. 
       FIG. 1  shows a structure of an image forming unit in a thermal transfer printer. Dye that is coated on an ink ribbon  120  is heated by a thermal head  130  and transferred onto a print sheet  141 . The ink ribbon  120  and the print sheet  141  after completion of the transfer are separated from one another by a peeler plate  131 . A load necessary to effect peeling (hereinafter, “peel force”) of the ink ribbon  120  from the print sheet  141  varies depending on an image to be printed. In general, a higher a density of an image to be printed, a greater a peel force required. 
     In a case of printing plural screens, as noted above, a roll sheet is used and printing is carried out such that a blank space is provided between adjacent screens so that a printed screen is not influenced by another screen. This blank space is cut out finally upon completion of printing, and as a print result only the printed screens are output. In this case, no images are printed on the blank space, and therefore a peel force at the blank space varies greatly from that at other parts where images are formed. This kind of variation in peel force (hereinafter “load variation”) causes a tension of the ink ribbon  120  to vary. Such variation in tension of the ink ribbon  120  results in lateral banding in printed images. 
     Techniques for preventing lateral banding are described in, for example, Patent documents 1 and 2. Patent document 1 discloses a technique for preventing lateral banding by increasing a tension of an ink ribbon; and Patent document 2 discloses a technique for applying a bias energy to a blank space in an ink ribbon (e.g., energy at a level which does not give rise to coloring) so as to prevent lateral banding. 
     Patent document 1: JP-A-8-197762 
     Patent document 2: JP-A-7-125293 
     SUMMARY 
     The technique disclosed in Patent document 1 is effective for a thermal transfer printer which carries out single-screen printing. However, this technique has a problem that further load variation is caused if a tension is raised between screens in another thermal transfer printer which carries out multi-screen printing. Meanwhile, according to the other technique of applying bias energy, as described in Patent document 2, no sufficient effects to suppress lateral banding are obtained. 
     The present invention has been made in view of the circumstances as described above and provides a thermal transfer printer capable of performing multi-screen printing while suppressing lateral banding. 
     To address the problems noted above, according to an embodiment of the present invention, there is provided a thermal transfer printer including: an ink ribbon conveyor unit that conveys an ink ribbon with a dye coated thereon in a layout corresponding to image formation of an a×b size; a sheet conveyor unit that conveys a sheet compatible with the image formation of the a×b size; a dummy pattern generation unit that generates a dummy pattern; an image data generation unit that generates print image data joining n screens together, the n screens each having 1/n size of the a×b size (where n is an integer not smaller than 2), and the print image data including the dummy pattern generated by the dummy pattern generation unit and inserted between two screens among the n screens, one of the two screens being adjacent to the other one along a sheet conveying direction of the sheet conveyor unit; a thermal head that transfers the dye coated on the ink ribbon conveyed by the ink ribbon conveyor unit to the sheet conveyed by the sheet conveyor unit in accordance with the print image data generated by the image data generation unit; and a peeler unit that peels the ink ribbon from the sheet to which an image has been transferred by the thermal head, wherein an average density of the dummy pattern generated by the dummy pattern generation unit is equal to an average density of the image over an area equivalent to a distance between a tail end portion of the thermal head and the peeler unit, on one of the n screens that follows the dummy pattern. 
     The thermal transfer printer is preferably configured such that a density of the dummy pattern is uniform in the sheet conveying direction of the sheet conveyor unit. 
     Alternatively, the thermal transfer printer may also preferably be configured such that a density of the dummy pattern periodically changes in the sheet conveying direction of the sheet conveyor unit. This thermal transfer printer may also further preferably be configured such that a density of the dummy pattern changes in the form of a sine wave or saw tooth wave in the sheet conveying direction of the sheet conveyor unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described in detail with reference to the following figures wherein: 
         FIG. 1  shows a structure of an image forming unit in a thermal transfer printer; 
         FIG. 2  shows a structure of an ink ribbon  120 ; 
         FIG. 3  is a diagram showing a configuration of functions of the thermal transfer printer  100 ; 
         FIG. 4  shows a structure of image data in two-screen printing; and 
         FIGS. 5A-5C  are graphs exemplifying dummy patterns. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 1  shows a structure of a thermal transfer printer  100  according to an embodiment of the present invention. This thermal transfer printer  100  substantially includes a sheet feed mechanism  110 , an ink ribbon  120 , an ink ribbon feed mechanism  150 , a thermal head  130 , and a sheet cassette  140 . The thermal transfer printer  100  is driven in accordance with image data from a personal computer, not shown, (hereinafter, “PC”). The thermal transfer printer  100  may be constructed so as to include a memory card interface which reads out image data from a memory card or the like, and may be driven in accordance with the image data read out. 
     The sheet feed mechanism  110  is sectioned into two sides by the thermal head  130  as a boundary therebetween, i.e., a sheet feed side and a sheet discharge side. The sheet feed mechanism  110  includes a sheet feed roller  111 , a pinch roller  112 , a platen roller  113 , a sheet discharge roller  114 , and another pinch roller  115 . The sheet feed roller  111  and the pinch roller  112  are located in the sheet feed side. The platen roller  113  is located at a position where the roller  113  faces the thermal head  130 . The color filter  114  and pinch roller  115  are located in the sheet discharge side. The sheet feed mechanism  110  conveys a print sheet  141  between the sheet feed roller  111  and the pinch roller  112 , between the thermal head  130  and the platen roller  113 , and further between the sheet discharge filter  114  and the pinch roller  115 , sequentially. 
     The sheet feed roller  111 , platen roller  113 , and sheet discharge filter  114  are driven to rotate by a drive device (not shown in the figures) such as a stepping motor, for example. When this drive device is driven to rotate the sheet feed roller  111 , platen roller  113 , and sheet discharge filter  114  in a clockwise direction, the print sheet  141  is conveyed in a feed direction F. On the other side, when these rollers are rotated in an anti-clockwise direction, the print sheet  141  is conveyed in a return direction R. 
     The ink ribbon feed mechanism  150  conveys the ink ribbon  120  from a roller  121  in the feed side to a winder roller  122 . Two ends of the ink ribbon  120  are wound about the feeder roller  121  and the winder roller  122 , respectively. The winder roller  122  is rotated in a clockwise direction by a drive device (not shown in the figures) such as a DC motor, to wind up the ink ribbon  120 . As a result, the ink ribbon  120  is conveyed in the feed direction f. 
       FIG. 2  shows structure of the ink ribbon  120 . The ink ribbon  120  is constituted of a thin base film  120   a  and dye layers  120 Y,  120 M, and  120 C. The dye layers are formed by repeatedly coating dyes of Y (yellow), M (magenta), and C (cyan), in that order, in the lengthwise direction of the base film  120   a.    
     Further description will now be made referring again to  FIG. 1 . In the present embodiment, dyes that may be thermally sublimated are used for the ink ribbon  120 . In the external sensor terminal  100  using the ink ribbon  120 , print density levels are changed by temperature adjustment of thermal head  130 , and thus, tone printing may be performed. As a result, high-quality color images are formed on a print sheet  141 . 
     Like the sheet feed mechanism  110 , the ink ribbon  120  is sectioned into two sides by the thermal head  130  as a boundary therebetween, i.e., a feed side and a discharge side. A guide roller  123  in the feed side is located between the thermal head  130  and the feeder roller  121 , as well as another guide roller  124  in the discharge side between the thermal head  130  and the winder roller  122 . 
     The thermal head  130  is constructed by arraying plural heating elements (not shown in the figures) on a board. The thermal head  130  is moved apart from and pressed towards to contact the platen roller  113  by an elevation mechanism not shown. The peeler plate  131  is provided near the thermal head  130 . The peeler plate  131  is provided in one side of the thermal head  130  to which the feed direction F extends. The peeler plate  131  is brought into contact from above with the ink ribbon  120  which has already transferred dyes to the print sheet  141 . In this manner, the peeler plate  131  changes the conveying course of the ink ribbon  120  so that it deviates from the conveying course of the print sheet. In other words, the ink ribbon  120  is peeled off from the print sheet about the peeler plate  131  which acts as a fulcrum. 
     The sheet cassette  140  contains a large number of print sheets  141  having a fixed size (e.g., JIS A4, A5, etc.). One after another, print sheets  141  are picked up by a sheet feeder not shown and conveyed through a sheet conveying path  116 . Onto a print sheet  141  thus conveyed, dyes of respective colors are transferred within an image forming area between the thermal head  130  and the platen roller  113 . 
       FIG. 3  is a diagram showing a configuration of functions of the thermal transfer printer  100 . The sheet feed mechanism  110  and the ink ribbon feed mechanism  150  have already described above with reference to  FIG. 1 . A dummy pattern generation unit  170  generates a dummy pattern, which will be described in detail later. An image data generation unit  180  generates print image data to drive the thermal head  130 . A control unit  160 , the dummy pattern generation unit  170 , and the image data generation unit  180  may be configured such that a processor such as a CPU executes a program to realize functions thereof. Alternatively, circuits respectively dedicated to these functions may be used. 
     The following describes operation of the thermal transfer printer  100 , exemplifying a case of performing two-screen printing to print out images each having a 6×4 inch size by use of an ink ribbon having a 6×8 inch size. 
       FIG. 4  shows a structure of image data in two-screen printing. In this case, two screens  1  and  2  are printed on one print sheet  141  through one process. A blank space M is provided between the two screens so that no blank space might appear at edge parts of each screen and that each screen might not influence the other screen. The image data generation unit  180  inserts a dummy pattern generated by the dummy pattern generation unit  170  into the blank space M. The print sheet  141  is cut along a cutoff line at the position C in  FIG. 4  by a cutting mechanism (not shown in the figures). In  FIG. 4 , the symbol F denotes the conveying direction of the print sheet  141 . 
     The dummy pattern generation unit  170  generates a dummy pattern in the fashion described below. The dummy pattern generation unit  170  calculates an average density of each color of C, M, and Y over an area equivalent to the distance (d in  FIG. 1 ) between the tail end of the thermal head  130  and the peeler plate  131 , on the screen following the dummy pattern. The dummy pattern generation unit  170  generates a dummy pattern as to have average densities equal to the calculated average densities over the area noted above. 
       FIGS. 5A to 5C  are graphs exemplifying dummy patterns. As shown in  FIG. 5A , a dummy pattern may have a uniform density in the conveying direction F of the print sheet  141 . Alternatively, as shown in  FIG. 5B  or  5 C, another dummy pattern may have a density which changes periodically. FIG.  5 B shows a pattern a density of which changes in the form of a sine wave.  FIG. 5C  shows a pattern a density of which changes in the form of a saw tooth wave. Thus, a dummy pattern is printed on the blank space M, and a load variation may thereby be suppressed, i.e., lateral banding may be suppressed. Particularly when using a periodic pattern, as shown in  FIG. 5B  or  5 C, a load variation may be reduced if a pattern to be printed is made periodic. 
     As has been described above, the thermal transfer printer  100  according to the present embodiment is capable of performing multi-screen printing while suppressing lateral banding. The thermal transfer printer  100  is not limited only to performing two-screen printing but may be configured to perform n-screen printing (where n is an integer not smaller than two). For example, where n=6, a screen may be arrayed in a matrix layout of three rows×two columns. In this case, a dummy pattern may be inserted between each adjacent screen in the conveying direction of the print sheet  141 .