Patent Publication Number: US-2007122222-A1

Title: Platen roller holding structure

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a platen roller holding structure of a thermal transfer printer in which a plurality of thermal heads are detachably disposed to a platen roller at predetermined intervals in a circumferential direction of the platen roller, and particularly to a platen roller holding structure capable of preventing bearing plates which hold the platen roller from bending due to a load generated when each of the thermal heads is pressed against the platen roller.  
      2. Description of the Related Art  
      In a related art, a thermal printer in which a plurality of thermal heads are disposed to a platen roller at predetermined intervals in a circumferential direction of the platen roller has been known as a thermal printer which ensures high speed and high quality during printing.  
      In the thermal printer according to the related art, a platen roller is rotatably held by a platen roller holding structure having a pair of bearings disposed facing each other in a case, and the platen roller is rotatably driven due to the rotation of a driving source composed of a stepping motor by a driving gear.  
      On an outer periphery of the platen roller, four thermal heads are detachably disposed to the platen roller, respectively, at constant intervals in a circumferential direction of the platen roller, and ink ribbons on which ink layers of cyan (C), magenta (M), yellow (Y), black (K) are formed and are drawn between the platen roller and each thermal head.  
      By rotatably driving the platen roller, a print medium is conveyed between the platen roller and each thermal head by each ink ribbon, and each thermal head is pressed against the platen roller with each ink ribbon and the print medium interposed therebetween, and then a plurality of heat emitting elements are selectively heated, thus obtaining a full color printing (see JP-A-2003-291388).  
      However, in the thermal printer according to the related art, if printing is repeatedly performed, a load generated when each thermal head is pressed against the platen roller with each ink ribbon and the print medium interposed therebetween causes the platen roller holding structure holding the platen roller to bend. As a result, the platen roller bends, thus deteriorating printing quality.  
     SUMMARY OF THE INVENTION  
      An object of the invention is to provide a platen roller holding structure capable of preventing a platen roller from bending due to a load generated when a plurality of thermal heads is pressed against the platen roller, in a thermal printer in which the plurality of thermal heads is detachably disposed to one platen roller.  
      In order to achieve the above-described object, according to an aspect of the invention, a platen roller holding structure for holding a platen roller of a thermal printer that includes a plurality of thermal heads detachably disposed to the periphery of the platen roller at predetermined intervals in a circumferential direction. Further, the platen roller holding structure includes a plurality of protrusions that are formed on the periphery of the bearing plates for rotatably holding a rotation shaft of the platen roller at both ends of the rotation shaft. The protrusions protrude outward from portions of the bearing plates facing each other. In this case, the protrusions of the bearing plates facing each other are connected with each other by connecting members such that an interval is formed between the platen roller and each of the protrusions.  
      With this structure, rigidity of the bearing plates is enhanced, and it is possible to prevent the bearing plates from bending although a load is generated when each of the thermal heads is pressed against the platen roller. As a result, it is possible to perform high quality printing.  
      In the platen roller holding structure according to the aspect of the invention, each of the thermal heads may be disposed between each pair of the connecting members in a circumferential direction of the platen roller.  
      With this structure, since each of the thermal heads is disposed between a pair of the connecting members in the circumferential direction of the platen roller, it may be possible to prevent displacement of a portion of the platen roller where each thermal head is pressed against the platen roller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view schematically showing a platen roller holding structure according to an embodiment of the invention; and  
       FIG. 2  is a schematic view showing main parts of a thermal printer using the platen roller holding structure of  FIG. 1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Hereinafter, a platen roller holding structure according to an embodiment of the invention will be described, with reference to the accompanying drawings. The same elements or parts throughout the drawings are designated by the same reference characters.  
       FIG. 1  is a perspective view schematically showing a platen roller holding structure according to an embodiment of the invention, and  FIG. 2  is a schematic view showing main parts of a thermal printer using the platen roller holding structure of  FIG. 1 .  
      As shown in  FIG. 2 , according to the embodiment, in a thermal printer  1 , a platen roller  2  is rotatably held in a platen roller holding structure  3 , which is disposed in a case (not shown).  
      On an outer periphery of the platen roller  2 , four thermal heads  10  are detachably disposed to the platen roller  2  at predetermined intervals in a circumferential direction of the platen roller  2 , and an ink ribbon  13  is drawn between each thermal head  10  and the platen roller  2 .  
      The platen roller  2  is geared with a driving source (not shown) like a stepping motor through a driving gear (not shown), which is disposed to a rotation shaft  2   a,  and it can be rotated due to rotation of the driving source. Meanwhile, according to this embodiment, the normal rotation direction of the platen roller  2  is in a counter-clockwise direction.  
      According to this embodiment, the platen roller holding structure  3  includes a pair of bearing plates  4  and  4  formed in a circular shape having a larger outer diameter than the diameter of the platen roller  2 , as shown in  FIG. 1 .  
      A bearing  4   a  is formed at the center of each of the bearing plates  4  and  4 , and both ends of the rotation shaft  2   a  of the platen roller  2  are rotatably held by both bearings  4   a.    
      Five protrusions  5  are formed in a rectangular shape on the periphery of the bearing plates  4  and  4 , respectively, so as to face each other at opposite sides, and to be positioned between a pair of thermal heads  10  in the circumferential direction of the bearing plates  4  and  4 . The five protrusions  5  protrude outward with respect to the radial direction of the platen roller  2 , and the protrusions  5  facing each other are connected to each other by connecting members  6 . Each of the protrusions  5  is disposed to the platen roller  2  with an interval therebetween.  
      Furthermore, on the periphery of a portion of the bearing plates  4  and  4  where the thermal head  10  does not exist, protrusions  7  are formed in a fan shape protruding outward so as to face each other between both bearing plates  4  and  4 . Protrusions  7  are connected to each other by a connecting member  8  that is disposed to the platen roller  2  with an interval therebetween. Meanwhile, each of side surfaces  8   a  and  8   b  of the connecting member  8  is designed to guide a print medium  19  together with a guide roller  9  which is disposed inside the case when the print medium  19  such as recording paper is conveyed by the platen roller  2 .  
      Each of the thermal heads  10  includes a plurality of heat emitting elements  10   a  which are disposed parallel to the axis of the platen roller  2 , and each thermal head  10  is mounted on a carriage  12  which is disposed to a base  11  having a flat shaper so as to be attached to and detached from the platen roller  2  by a movable attaching and detaching cylinder.  
      Each of the ink ribbons  13  is wound around a take-up core  15  and a supply core  14  which are rotatably disposed to the base  11  on which the carriage  12  is mounted, and guided by a plurality of pinch rollers  16 , so as to be drawn between the platen roller  2  and each of the thermal heads  10 .  
      Each of the ink ribbons  13  is provided with an ink layer each having different colors. Ink layers of cyan (C), magenta (M), yellow (Y), and black (K) may be sequentially formed on the ink ribbons, from the ink ribbon  13  located in the uppermost position in a normal rotation direction of the platen roller  2  to the ink ribbon  13  located in the lowermost position.  
      On the outer periphery of the platen roller  2 , the print medium  19  wound around a paper feed roller  17  and a take-up roller  18  is drawn around the platen roller  2 .  
      In a print method using the thermal printer  1  constructed as described above according to this embodiment, the platen roller  2  and the take-up roller  18  are rotatably driven so as to feed the print medium  19 . Then, each thermal head  10  is pressed against the platen roller  2  with each ink ribbon  13  and the print medium  19  interposed therebetween, and the heat emitting element  10   a  of each thermal head  10  is selectively heated, thereby printing out a full color image.  
      In the platen roller holding structure  3  according to the embodiment, the connecting members  6  and the connecting member  8  connect the protrusions  5  and the protrusions  7 , which are formed to the bearing plates  4  and  4  and face each other between the bearing plates  4  and  4 , respectively, which enhances rigidity of the bearing plates  4  and  4 . Thus, even though printing is repeatedly performed, it is possible to prevent the bearing plates  4  and  4  from bending due to a load generated when each of the thermal heads  10  is pressed against the platen roller  2 . As a result, it is possible to prevent the platen roller  2  from bending, thereby performing stable and high quality printing.  
      The invention is not limited to the above-described embodiment, and it can be modified in various forms within the spirit of the invention.  
      In the platen roller holding structure according to an embodiment of the invention, the rigidity of the bearing plates holding the platen roller can be enhanced. Thus, in a thermal printer where a plurality of thermal heads are attached to and detached from the platen roller, even though printing is repeatedly performed, it is possible to prevent the bearing plates from bending due to a load generated when each of the thermal heads is pressed against the platen roller. As a result, it is possible to prevent the platen roller from bending, thereby performing stable and high quality printing.  
      The invention has been explained by the embodiment described herein; however, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiment without materially departing from the novel teachings and advantages of the invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims.