Patent Publication Number: US-2007122223-A1

Title: Printer

Description:
This application claims the benefit of priority to Japanese Patent Application No. 2005-340639, filed on Nov. 25, 2005, which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a printer, and particularly to a printer capable of being preferably used in a line thermal printer or an inkjet printer that prints on a recording medium, such as recording paper or the like, having a predetermined size, for example, a size of A4 or long roll paper with ink while pressing the recording medium against a roller-shaped platen.  
      2. Description of the Related Art  
      In general, a printer, such as a line thermal printer or an inkjet printer, applies or thermally transfers ink onto a recording medium, such as roll paper or recording paper, and may be used as an output device of a computer.  
      As shown in  FIG. 8 , for example, a printer  101  in the related art includes a roller-shaped platen  103 , recording units  104 , a feed roller  102 , and a discharge roller  111 . While applying tension to the recording medium  110  wound around the outer periphery of the platen, the roller-shaped platen  103  is rotated in the same direction as a feed direction FD of a recording medium  110 . Each of the recording units  104  includes a thermal head or a recording head  104   a,  such as an ink nozzle. The feed roller  102  is disposed at a front position before the platen  103  in the feed direction FD so as to feed the recording medium  110 , and the discharge roller  111  is disposed at a back position after the platen  103  in the feed direction FD so as to discharge the recording medium  110 .  
      In this case, four recording units  104  are provided in a circumferential direction of the platen  103  so as to correspond to four colors, such as yellow, magenta, cyan, and black (or overcoat). Further, the recording medium  110  is interposed between the recording head  104   a  of each recording unit  104  and the platen  103 . Accordingly, when the feed roller  102  and the discharge roller  111  are rotated, the recording medium  110  is wound around the outer peripheral surface of the platen  103  and fed in the predetermined feed direction FD. As a result, each of the recording units  104  records desired letters or images on the recording medium  110  (see, for example, JP-A-2003-251840).  
      However, the printer  101  in the related art does not include a unit for removing slack SS from the recording medium  110 . Accordingly, as shown in  FIG. 8 , when the recording medium  110  does not come in close contact with the platen  103  and is fed with the slack SS, it is not possible to make the recording head  104   a  of each recording unit  104  appropriately come in contact with a portion of the recording medium  110  on which the slack SS occurs. As a result, it is not possible to record desired letters or images on the recording medium  110 .  
      In addition, according to the printer  101  in the related art, until the recording medium  110  is fed to the discharge roller  111 , the recording medium  110  cannot come in close contact with the outer peripheral surface of the platen  103 , as a result, the recording units cannot begin recording on the recording medium  110 .  
     SUMMARY  
      An object of the invention is to provide a printer in which a recording medium can come in close contact with the surface of the platen.  
      Further, it is another object of the invention to provide a printer that does not perform unnecessary feeding of the recording medium and makes the recording medium come in close contact with the platen from the leading end of the recording medium.  
      Furthermore, it is another object of the invention to provide a printer in which each of the recording units can perform recording at a desired position on the recording medium even if the recording medium slides on the platen to remove slack from the recording medium.  
      In order to achieve the above-mentioned objects, according to a first aspect of the invention, a printer includes a feed roller that feeds a recording medium in a predetermined feed direction, a roller-shaped platen that is rotated in the feed direction of the recording medium, recording units that face an outer peripheral surface of the platen and approach or recede from the platen so as to record desired letters or images on the recording medium, a pressure auxiliary roller that faces the outer peripheral surface of the platen at a front position before the recording units in the feed direction and presses the recording medium against the outer peripheral surface of the platen, a pressure unit that includes pressure rollers, pushing means, and moving means, and a control unit that makes a circumferential speed of the feed roller become temporarily slower than a circumferential speed of the platen when the pressure rollers press the recording medium against the platen. In this case, each of the pressure rollers is formed in the shape of a roller that has a diameter smaller than a maximum distance between each of the recording units and the platen, and faces the outer peripheral surface of the platen at a back position after the pressure auxiliary roller in the feed direction. Further, when the recording medium is provided in a predetermined gap between each pressure roller and the platen, the pushing means pushes the pressure rollers against the outer peripheral surface of the platen with a pressing force larger than a pressing force of the pressure auxiliary roller. In addition, when the pressure rollers are pressed against the outer peripheral surface of the platen, the moving means moves the pressure rollers from front positions to the back positions in the feed direction of the recording medium in conjunction with the rotation of the platen.  
      According to the first aspect of the invention, the recording medium comes in close contact with the platen, and the platen and the feed roller for feeding the recording medium apply tension to the recording medium so as to remove the slack from the recording medium. As a result, it is possible to make the recording medium come in close contact with the platen.  
      According to a second aspect of the invention, in the printer according to the first aspect of the invention, the control unit may temporarily stop the platen and temporarily rotate the feed roller in a reverse direction.  
      According to the second aspect of the invention, since the recording medium is temporarily stopped to apply tension to the recording medium, it is possible to make the recording medium come in close contact with the platen from the leading end of the recording medium without over feeding the recording medium. Further, since the platen is stopped and the feed roller  2  is rotated in the reverse direction, it is possible to more quickly remove the slack from the recording medium as compared to when the feed roller and the platen are rotated in the same direction and the slack is removed by using a difference between the rotational speeds thereof.  
      According to a third aspect of the invention, in the printer according to the first or the second aspect of the invention, until each of the recording units begins to perform recording, the control unit may make the circumferential speed of the feed roller be slower than the circumferential speed of the platen. Further, after each of the recording units begins to perform the recording, the control unit may make the circumferential speed of the feed roller be equal to the circumferential speed of the platen.  
      According to the third aspect of the invention, after each of the recording units begins to perform the recording, the recording medium does not perform a sliding motion, which is performed to apply tension to the recording medium, on the outer peripheral surface of the platen. For this reason, the recording units can perform the recording at a desired position on the recording medium. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a front view of a printer according to an embodiment of the invention as seen in an axial direction of a platen;  
       FIG. 2  is a perspective view showing the printer of  FIG. 1 . Elements are mirror symmetric with respect to the platen, and only elements on one side of the platen in the axial direction of the platen are shown; and  
       FIG. 3  is a perspective view of a ring gear of  FIG. 2 ;  
       FIGS. 4A and 4B  are perspective views showing a roller unit of  FIG. 2 ;  FIG. 4A  is a perspective view of the roller unit as seen from a front side of a base plate, and  FIG. 4B  is a perspective view of the roller unit as seen from a backside of the base plate;  
       FIG. 5  is a front view showing a platen, a first roller unit, a stationary cam disk, an actuator of the ring gear, and a first high pressure unit according to the embodiment of the invention, and is a front view schematically showing when a leading end of a recording medium is provided in a predetermined gap between the platen and a first pressure roller;  
       FIG. 6  is a front view of a detail of  FIG. 1  schematically showing the platen, the first roller unit, the stationary cam disk, the actuator of the ring gear, and the first high pressure unit, showing that the first pressure roller is interposed between the first high pressure unit and the platen;  
       FIG. 7  is a schematic front view of a detail of  FIG. 1 ; and  
       FIG. 8  is a front view of a printer in the related art. 
    
    
     DETAILED DESCRIPTION  
      Reference will now be made in detail to embodiments. While the invention will be described in conjunction with these embodiments, it will be understood that it is not intended to limit the invention to such embodiments. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention which, however, may be practiced without some or all of these specific details. The same or equivalent elements or parts throughout the drawings are designated by the same reference characters.  
       FIG. 1  is a front view of a printer, and  FIG. 2  is a perspective view partially showing the printer.  FIG. 3  is a perspective view of a ring gear.  FIGS. 4A and 4B  are perspective views showing a roller unit.  FIGS. 5 and 6  are front views showing a platen, a first roller unit, a stationary cam disk, an actuator of the ring gear, and a first high pressure unit.  FIG. 5  is a front view schematically showing when a leading end of a recording medium is provided in a predetermined gap between the platen and a first pressure roller.  FIG. 6  is a front view schematically showing that the first pressure roller is interposed between the first high pressure unit and the platen. Since elements are mirror symmetric with respect to the platen,  FIG. 2  shows only elements on one side of the platen in the axial direction of the platen.  
      As shown in  FIG. 1 , the printer  1  includes a feed roller  2 , a roller-shaped platen  3 , recording units  4 (reference numerals  4 A,  4 B,  4 C, and  4 D of four recording units provided at four positions in the circumferential direction of the platen  3  are generalized by reference numeral  4 ), a pressure auxiliary roller  12 , a pressure unit  8 , and a control unit (not shown) in a case (not shown). The feed roller  2  feeds a recording medium  10 , such as recording paper having a size of A4 or long roll paper, in a predetermined feed direction FD, and the platen  3  is rotated in the feed direction FD of the recording medium  10 . The recording units  4  face the outer peripheral surface of the platen  3 . The recording units  4  slide to approach or recede from the platen  3  in a radial direction of the platen  3 , and record desired letters or images on the recording medium  10 . The pressure auxiliary roller  12  faces the outer peripheral surface of the platen  3  at a front position before the recording units in the feed direction, and presses the recording medium  10  against the outer peripheral surface of the platen  3 . The pressure unit  8  presses the recording medium  10  against the outer peripheral surface of the platen  3  by using pressure rollers  5  (reference numerals  5 A,  5 B,  5 C, and  5 D of four pressure rollers provided at four positions in the circumferential direction of the platen  3  are generalized by reference numeral  5  in this embodiment), and moves together with the platen  3 . When the pressure rollers  5  press the recording medium  10  against the platen  3 , the control unit controls a circumferential speed Vs of the feed roller  2  such that the circumferential speed Vs becomes temporarily slower than a circumferential speed Vp of the platen  3 .  
      In addition, the pressure unit  8  includes the pressure rollers  5 , pushing means  6 , and moving means  7 . Each of the pressure rollers  5  is formed in the shape of a roller that has a diameter smaller than a maximum distance between each recording unit  4  and the platen  3  and a length equal to or longer than an axial length of the platen  3 . Further, the pressure rollers  5  face the outer peripheral surface of the platen  3  at positions after the pressure auxiliary roller  12  in the feed direction FD. When the recording medium  10  is provided in a predetermined gap between each pressure roller and the platen  3 , the pushing means  6  pushes the pressure rollers  5  against the outer peripheral surface of the platen  3  with a pressing force F 1  larger than a pressing force F 2  of the pressure auxiliary roller  12 . When the pressure rollers  5  are pressed against the outer peripheral surface of the platen  3 , the moving means  7  moves the pressure rollers  5  from front positions to the back positions in the feed direction FD of the recording medium  10  in conjunction with the rotation of the platen  3 .  
      Specifically, the feed roller  2  includes a cylindrical rod and a synthetic rubber, such as EPDM (ethylene propylene diene monomer) or the like, having a roller shape. The cylindrical rod is made of a material having high rigidity, such as metal or the like, and the synthetic rubber is attached on the side surfaces of the cylindrical rod. Further, the feed roller  2  is rotated in the same direction as the feed direction FD of the recording medium  10  by a rotational force from a stepper motor (not shown).  
      The platen  3  includes a cylindrical rod and a synthetic rubber, such as EPDM or the like, having a roller shape. The cylindrical rod is made of a material having high rigidity, such as metal or the like, and the synthetic rubber is attached on the side surfaces of the cylindrical rod. Further, the platen includes a mechanism that is rotated independently of the feed roller  2  in the same direction (counterclockwise direction in  FIG. 1 ) as the feed direction FD of the recording medium  10  by a rotational force from a stepper motor (not shown). The diameter of the platen  3  is determined by consideration of the number of the recording units  4  and the size of the printer  1 .  
      The recording units  4  include recording heads  41  (reference numerals  41 A,  41 B,  41 C, and  41 D are generalized by reference numeral  41 ) facing the outer peripheral surface of the platen  3 , and are provided along the outer peripheral surface of the platen  3  so as to correspond to the number of colors to be printed, such as YMCK (yellow, magenta, cyan, and black), so that the colors are printed on the recording medium  10 . Recording units, such as ink cartridge type or thermal head type recording units, may be used as the recording units  4 . However, thermal head type recording units  4  may be used in this embodiment to improve printing quality. The thermal head type recording units  4  includes ink ribbons  42 A,  42 B,  42 C, and  42 D corresponding to the four colors of YMCK, and thermal heads (recording heads)  41 A,  41 B,  41 C, and  41 D that thermally transfer color ink onto the recording medium by using the ink ribbons. In addition, the four recording units  4  according to this embodiment are provided in the feed direction FD of the recording medium  10 , and are referred to as a first recording unit  4 A, a second recording unit  4 B, a third recording unit  4 C, and a fourth recording unit  4 D from a front position to a back position in the feed direction FD.  
      The pressure auxiliary roller  12  includes a cylindrical rod and a synthetic rubber, such as EPDM or the like, having a roller shape. The cylindrical rod has a length equal to or longer than the axial length of the platen  3 , and is made of a material having high rigidity, such as metal or the like. The synthetic rubber is attached on the side surfaces of the cylindrical rod. Further, the pressure auxiliary roller  12  includes a pressure mechanism. When it is determined that the recording medium  10  is interposed between the pressure auxiliary roller  12  and the platen  3  on the basis of a detection result from a detection sensor (not shown) that detects the recording medium  10  in the front vicinity of the pressure auxiliary roller  12  and a feed condition of the recording medium  10  obtained by the number of revolutions of the feed roller  2 , the pressure mechanism pushes the pressure auxiliary roller  12  against the platen  3  toward the center of the platen  3  with a pressing force F 2  by using pushing means (not shown), such as a coil spring or a hydraulic cylinder.  
      As described above, the pressure unit  8  includes the pressure rollers  5 , and the pushing means  6 , and the moving means  7 . As shown in  FIGS. 1 and 2 , the pressure unit  8  includes two ring gears (only one ring gear is shown)  20  that are provided at both sides of the platen  3  in the axial direction of the platen  3 , two pairs of stationary cam disks (only a pair of stationary cam disks is shown)  30  (reference numerals  30 A and  30 B are generalized by reference numeral  30 ) that are provided on the inner sides (both sides of the platen  3 ) and the outer sides of the ring gears  20  in the axial direction, and four roller units  50  (reference numerals  50 A,  50 B,  50 C and  50 D are generalized by reference numeral  50 ). In this case, two of the four roller units  50  are provided to each stationary cam disk  30 . Further, each of the roller units  50 , which has a pressure roller  5  and an elastic member  56 , and stationary cam disks  30  that change pressing forces of the pressure rollers  5  are used as the pushing means  6  in the pressure unit  8  of this embodiment. Further, the ring gears  20  and the stationary cam disks  30  are used as the moving means  7 . The ring gears  20  push the roller units  50 , and the stationary cam disks  30  guide the roller units  50  in the circumferential direction while causing the roller units  50  to slide. Hereinafter, the components  20 ,  30 , and  50  of the pressure unit  8  according to the embodiment will be described in detail.  
      As shown in FIGS.  1  to  3 , each of the ring gears  20  is a spur gear that has a diameter of an addendum circle smaller than an outer diameter of the platen  3 . An actuator  21 , which rotates the pressure rollers  5  or members for supporting the pressure rollers  5  in the circumferential direction of the platen  3 , is provided on each of the ring gears  20  inside a dedendum circle thereof.  
      The actuator  21  includes a sliding part  22  and a contact part  23 . The sliding part  22  is formed in the shape of a rectangular flat plate, and the contact part  23  protrudes from an outer end of the sliding part  22  in the radial direction of the ring gear  20  toward the both sides (front and rear sides) of the sliding part  22  in a direction perpendicular to the sliding part  22 . Further, the sliding part  22  of the actuator  21  is engaged with a sliding hole  25  having an engaging claw. The sliding hole  25  is formed through an inner portion of each ring gear  20  so as to be inclined in a direction (hereinafter, referred to as a “slide direction”) toward the clockwise direction by 30 to 60°. Further, the actuator  21  includes an elastic member  26 , such as a coil spring, which pushes the contact part  23  toward the outside in the slide direction, at the other end of the sliding part  22 .  
      In this example, the contact part  23  includes a circumferential pushing surface  23   a  and a radial pushing surface  23   b . The circumferential pushing surface  23   a  is formed at the back end (left side in the counterclockwise direction) of the contact part  23  in the feed direction FD so as to be parallel to the radial direction of the ring gear  20 . The radial pushing surface  23   b  is formed by chamfering the contact part  23  by 45° at the inner portion, which is disposed at the inner side as compared to the circumferential pushing surface  23   a , of the contact part  23 . When the contact part  23  is pushed toward the most outer portion of the ring gear in the slide direction by the elastic member  26 , the circumferential pushing surface  23   a  of the contact part  23  comes in contact with lateral portions of movable guide posts  52  (to be described below) of the four roller units  50  that are mounted on the stationary cam disks  30  provided at both sides of each ring gear  20  and pushes the movable guide posts  52  in the rotation direction of the ring gear  20 .  
      As shown in  FIG. 2 , each of the roller units  50  includes a pressure roller  5  provided to be parallel to a central axis of the platen  3 , and two base plates (only one base plate is shown)  51  provided at both ends of each pressure roller  5  in the axial direction of the pressure roller.  
      Each of the pressure rollers  5  includes a cylindrical rod and a synthetic rubber, such as EPDM or the like, having a roller shape. The cylindrical rod is made of a material having high rigidity, such as metal or the like, and the synthetic rubber is attached on the side surfaces of the cylindrical rod. Further, since each of the pressure rollers  5  is provided at a back position after each of recording units  4  in the feed direction, the number of pressure rollers  5  is four in total. The recording rollers  5  according of this embodiment are provided in the feed direction FD of the recording medium  10 , and are referred to as a first recording roller  5 A, a second recording roller  5 B, a third recording roller  5 C, and a fourth recording roller  5 D from a front position to a back position in the feed direction FD.  
      As shown in  FIGS. 4A and 4B , each of the two base plates  51  is formed in a lateral T shape. Although being not shown, the two base plates  51  are provided at both ends of each pressure roller  5  so as to be mirror symmetric to each other. Further, the pressure roller  5  is rotatably supported by one of two ends facing each other among three ends of each base plate  51 .  
      In addition, as shown in  FIGS. 4A and 4B , each of the roller units  50  includes a movable guide post  52 , a stationary guide post  53 , a lift-up pin  54 , a connection plate  55 , an elastic member  56 , and a stopper plate  57 . The movable guide post  52  is provided at one end, which faces another end of the base plate supporting the pressure roller  5 , of the base plate  51  on any one (this surface is not limited to a surface on which the pressure roller  5  is not provided) of the surfaces of the base plate  51 . The stationary guide post  53  is fixed at the other end at which the pressure roller  5  and the movable guide post  52  are not provided, on the surface on which the movable guide post  52  is provided (hereinafter, the surface on which the movable guide post  52  is provided is referred to as a “front surface of the base plate”). The lift-up pin  54  is formed in a cylindrical shape, and is fixed on the front surface  51   c  of the base plate  51  between the pressure roller  5  and the movable guide post  52 . A base portion of the connection plate  55  is rotatably supported by the stationary guide post  53  and the movable guide post  52  is fixed by the connection plate  55 , so that both members  52  and  53  are connected to each other on the surface on which the movable guide post  52  is not provided (hereinafter, the surface on which the movable guide post  52  is not provided is referred to as a “back surface of the base plate”). The elastic member  56  pushes the connection plate  55  on the back surface  51   d  of the base plate  51  so that the movable guide post  52  approaches the lift-up pin  54 . The stopper plate  57  includes a stopper part  57   a  and a stopper releasing part  57   b , and is connected to the connection plate  55 .  
      In this case, as shown in  FIGS. 4A and 4B , the movable guide post  52  is formed in the shape of a pin having a flange, and is inserted into an elongated hole  51   a  formed through the base plate  51  in a direction in which the movable guide post  52  approaches or recedes from the pressure roller  5 . That is, the movable guide post  52  is slidably moved about the stationary guide post  53  by the connection plate  55 . Further, the movable guide post  52  is pushed toward the pressure roller  5  and the lift-up pin  54  by the connection plate  55  and the elastic member  56 .  
      The stationary guide post  53  is formed in the shape of a pin having a flange, and is fixed on the back surface  51   d  of the base plate  51  so that the connection plate  55  and the stopper plate  57  are axially supported by the stationary guide post  53 .  
      The connection plate  55  is formed of a thin metal sheet and formed substantially in a lateral L shape. At one end of the connection plate  55 , the connection plate  55  rotates about the stationary guide post  53 . In addition, the movable guide post  52  is provided at the other end of the connection plate  55 , and one end of the elastic member  56  is fixed to the outside of the bend portion of the connection plate  55  that is formed in the lateral L shape.  
      An elastic member, which generates a pressing force F 1  larger than a pressing force F 2  of the pressure auxiliary roller  12 , is used as the elastic member  56 . A coil spring or the like may be used as the elastic member  56 . One end of the elastic member  56  is fixed to a part of the connection plate  55  as described above, and the other end thereof is fixed on the back surface  51   d  of the base plate  51  in the vicinity of the pressure roller  5 .  
      The stopper plate  57  is formed in the shape of a bifurcate thin plate that includes a long leg  57   c  and a short leg  57   d . A base portion, at which the long leg  57   c  and the short leg  57   d  are connected to each other, of the stopper plate  57  is mounted on the back surface  51   d  of the base plate  51  at the back side of the connection plate  55 , and is rotated about the stationary guide post  53 . In this case, the long leg  57   c  includes a stopper releasing part  57   b  that is bent from the end of the long leg  57   c  toward the front surface  51   c  of the base plate  51 , and a stopper part  57   a  that is bent from the vicinity of the stopper releasing part  57   b  toward the front surface  51   c  of the base plate  51  so as to have a length shorter than the stopper releasing part  57   b . Further, the end of the short leg  57   d  is bent toward the front surface  51   c  of the base plate  51 , and inserted into a mounting hole  55   a  of the connection plate  55  and a mounting groove  51   b  of the base plate  51 . In this case, the mounting hole  55   a  of the connection plate  55  is formed in a rectangular shape, and the mounting groove  51   b  of the base plate  51  is formed to be oriented in a direction in which the short leg  57   d  does not prevent the slidable movement of the movable guide post  52 .  
      Further, two roller units  50 , which each include the pressure roller  5  on the back surface  51   d  of the base plate  51  as shown in  FIGS. 4A and 4B , and two roller units  50  (not shown), which each include the pressure roller  5  on the front surface  51   c  of the base plate  51 , are prepared in accordance with the positional relationship between the ring gears  20  and the stationary cam disks  30  (see  FIG. 2 ). Elements of the roller units  50  are mirror symmetric with respect to the pressure roller  5  interposed therebetween so as to correspond to each other.  
      Further, as shown in  FIG. 2 , the roller units  50  including the pressure roller  5  on the back surface  51   d  of the base plate  51  are first provided to the stationary cam disks  30 A and  3 DB, and the four roller units  50  are alternately provided to the stationary cam disks  30 A and  30 B from the front positions to the back positions in the feed direction FD of the recording medium  10 . In this case, the four roller units  50  provided to the stationary cam disks are referred to as a first roller unit  50 A, a second roller unit  50 B, a third roller unit  50 C, and a fourth roller unit  50 D from the front positions to the back positions in the feed direction ED of the recording medium  10 . As shown in  FIG. 2 , the first roller unit  50 A and the third roller unit  50 C (the roller units  50  including the pressure roller  5  on the back surface  51   d  of the base plate  51 ) are mounted on one pair of stationary cam disks  30 A that are provided on the inner sides (both sides of the platen  3 ) of the ring gears  20  in the axial direction. Further, the second roller unit  50 B and the fourth roller unit  50 D (the roller units  50  including the pressure roller  5  on the front surface  51   c  of the base plate  51 ) are mounted on the other pair of stationary cam disks  30 B that are provided on the outer sides of the ring gears  20  in the axial direction.  
      As shown in  FIGS. 5 and 6 , each of the stationary cam disks  30  provided with the roller units  50  is formed in a disk shape and has a diameter smaller than a distance that is obtained by subtracting a distance between the central axis of the pressure roller  5  and the central axis of the lift-up pin  54  from the radius of the platen  3 . Each of the stationary cam disks  30  is concentric with the platen  3 , and is fixed not to rotate together with the platen  3  or the roller units  50 . In addition, each of the stationary cam disks  30  includes movable guide post guiding holes  31 , stationary guide post guiding holes  32 , lift cams  33 , stopper releasing holes  34 , and stopper holes  35 . Each of the movable guide posts  52  is inserted into each of the movable guide post guiding holes  31 , and each of the stationary guide posts  53  is inserted into each of the stationary guide post guiding holes  32 . The lift cams  33  push the lift-up pins  54  toward the outside in the radial direction of the stationary cam disk  30 . The stopper releasing holes  34  are formed on the peripheral surface of each stationary cam disk  30 , and the stopper holes  35  are formed on each stationary cam disk  30  so as to be close to the stopper releasing holes  34 , respectively.  
      Each of the movable guide post guiding holes  31  has a radius obtained by subtracting a distance, which is larger than a minimum distance between the lift-up pin  54  and the movable guide post  52  and smaller than a maximum distance therebetween, from the radius of the stationary cam disk  30 . Each of the movable guide post guiding holes  31  is formed of an arc-shaped through hole that has the same center as the stationary cam disk  30 . A curved portion  31   a  oriented toward the center of the stationary cam disk  30  is formed at the end (left end of the arc shown in  FIG. 2 ) of each movable guide post guiding hole  31  in the rotation direction.  
      Further, each of the stationary guide post guiding holes  32  is formed in the shape of an arc that has the same center as the arc-shaped movable guide post guiding holes  31 . The stationary guide post guiding holes  32  are formed in the stationary cam disks  30  so that the stationary guide posts  53  are inserted into the stationary guide post guiding holes  32  when the movable guide post  52  is inserted into the movable guide post guiding holes  31  and the lift-up pin  54  comes in contact with the peripheral surface of the stationary cam disk  30 .  
      Each of the lift cams  33  is formed in a convex shape on the stationary cam disks  30  so as to push the lift-up pins  54  toward the outside in the radial direction of the stationary cam disk  30  when the movable guide post  52  and the stationary guide post  53  of each roller unit  50  are positioned at the front positions (the most right sides) of the movable guide post guiding holes  31  and the stationary guide post guiding holes  32 , respectively. Further, each of the lift cams  33  has a lift distance such that a distance obtained by adding a distance between the center of the stationary cam disk  30  and the top of the lift cam  33  and a distance between the central axis of the pressure roller  5  and the central axis of the lift-up pin  54  is larger than the radius of the platen  3 .  
      Each of the stopper releasing holes  34  is cut and formed in the stationary cam disks  30  so that the lift-up pins  54  and the stopper releasing parts  57   b  positioned near the peripheral surfaces of the stationary cam disks  30  are fitted into the stopper releasing holes  34  in the radial direction of the stationary cam disks  30  when the movable guide post  52  and the stationary guide post  53  of each roller unit  50  are positioned at the back positions (the most left sides) of the movable guide post guiding holes  31  and the stationary guide post guiding holes  32 , respectively (see  FIG. 6 ).  
      Each of the stopper holes  35  is sized to contain the stopper part  57   a  of the stopper plate  57  therein, and is formed in a rectangular shape. When each of the lift-up pins  54  is moved to the inner portion in the radial direction due to the stopper releasing hole  34 , the stopper part  57   a  of the stopper plate  57  is engaged with the stopper hole  35 .  
      Further, the positions of the movable guide post guiding holes  31 , the stationary guide post guiding holes  32 , the lift cams  33 , the stopper releasing holes  34 , and the stopper holes  35  are different depending on each pair of stationary cam disks  30 . The positions of the movable guide post guiding holes  31 , the stationary guide post guiding holes  32 , the lift cams  33 , the stopper releasing holes  34 , and the stopper holes  35  are determined by one pair of stationary cam disks  30 A in accordance with the mounting positions of the first roller unit  50 A and the third roller unit  50 C. Further, the positions of the movable guide post guiding holes  31 , the stationary guide post guiding holes  32 , the lift cams  33 , the stopper releasing holes  34 , and the stopper holes  35  are determined by the other pair of stationary cam disks  308  in accordance with the mounting positions of the second roller unit  50 B and the fourth roller unit  50 D.  
      As shown in  FIG. 1 , the pressure unit  8  may include four high pressure units  9  (reference numerals  9 A,  9 B,  9 C and  9 D are generalized by reference numeral  9 ). As shown in  FIG. 5 , each of the high pressure units  9  includes a high pressure plate  91 , a base plate restraining plate  92 , a high pressure elastic member  93 , and a cam  94  for high pressure plate.  
      Specifically, the high pressure plate  91  is formed of a metal sheet that has a length longer than the axial length of the platen  3  and a width smaller than the axial length of the pressure roller  5 , so as to have a ladle-shaped cross section in the radial direction of the platen  3 . A portion of the high pressure plate  91  corresponding to a bottom  91   b  of a receiving part  91   a  in the ladle-shaped cross section is rotatably supported by a rotating shaft  96  provided parallel to the axial direction of the platen  3 . The high pressure plate  91  is provided so that a portion (a portion opposite to the high pressure elastic member  93 ) of the high pressure plate  91  corresponding to the end  91   c  of the receiving part  91   a  in the ladle-shaped cross section is separated from the outer peripheral surface of the platen  3  by a distance larger than the diameter of the pressure roller  5  when the high pressure plate  91  is rotated about the rotating shaft  96  in the counterclockwise direction. Further, the high pressure plate  91  is provided so that the portion of the high pressure plate  91  corresponding to the end  91   c  of the receiving part  91   a  in the ladle-shaped cross section is separated from the outer peripheral surface of the platen  3  by a distance smaller than the diameter of the pressure roller  5  when the high pressure plate  91  is rotated about the rotating shaft  96  in the clockwise direction.  
      The base plate restraining plate  92  is formed of a metal sheet that has a width longer than the axial length of the platen  3 , so as to have a ladle-shaped cross section in the radial direction of the platen smaller than that of the high pressure plate  91 . The base plate restraining plate  92  is provided so that the base plate restraining plate  92  comes in contact with only the base plate  51  and prevents the movement of the roller unit  50  when the roller unit  50  is moved in conjunction with the rotation of the platen  3 .  
      The high pressure elastic member  93  is formed to have an elastic force larger than the elastic member  56  of the roller unit  50 . The high pressure elastic member  93  is provided so that an elastic force is generated from a portion of the high pressure plate  91  corresponding to an end  91   e  of a handle  91   d  of the ladle-shaped cross section so as to rotate the high pressure plate  91  in the clockwise direction.  
      The cam  94  for a high pressure plate is formed on a cam shaft  95  having a gear that is connected to a step motor (not shown). The cam  94  for high pressure plate is provided so that a portion of the high pressure plate  91  corresponding to a connection portion  91   f  between the receiving part  91   a  and the handle  91   d  at a position corresponding to the inside of the receiving part  91   a  in the ladle-shaped cross section is pressed due to the rotation of the cam  94  for the high pressure plate and the high pressure plate  91  is rotated in the counterclockwise direction. The control unit for controlling the movement of the recording units  4  controls the rotation of the cam  94  for the high pressure plate.  
      Since each of the high pressure units  9  is provided at the back position after each of the recording units  4 , the high pressure units  9  are referred to as a first high pressure unit  9 A, a second high pressure unit  9 B, a third high pressure unit  9 C, and a fourth high pressure unit  9 D from a front position to a back position in the feed direction FD of the recording medium  10 .  
      Further, as shown in  FIG. 1 , the control unit of the printer  1  independently controls the circumferential speed Vs of the feed roller  2 , the circumferential speed Vp of the platen  3 , the moving speeds of the pressure rollers  5 , and the positions to which the recording units  4  are slidably moved. Accordingly, the control unit is formed of a circuit that receives information from the step motor for rotating the ring gears  20  or the step motor for rotating the feed roller  2  and outputs control signals to the recording units  4  or the cams  94  for the high pressure plate. Specifically, the control unit is formed of a control circuit formed using a storage unit, such as a memory, and a processing device, such as a CPU.  
      In addition, it is preferable that the control unit temporarily stop the platen  3  and temporarily rotate the feed roller  2  in a reverse direction. In this case, when the control unit controls the platen and the feed roller as described above, the control unit may strongly press the pressure rollers  5  against the platen  3  by using the high pressure units  9  and then temporarily stop the platen  3 .  
      Until each of the recording units  4  provided at the front position before each of the pressure rollers  5  in the feed direction FD begins to perform the recording on the recording medium  10 , the control unit according to this embodiment may preferably perform the control in which the circumferential speed Vs of the feed roller  2  becomes slower than the circumferential speed Vp of the platen  3 . Further, after each of the recording units  4  begins to perform the recording, the control unit according to this embodiment may perform the control in which the circumferential speed Vs of the feed roller  2  becomes equal to the circumferential speed Vp of the platen  3 .  
      In addition, the printer  1  may include a calculating device (not shown) and a pressure unit controlling device (not shown). The calculating device calculates that the leading end of the recording medium  10  is provided in a predetermined gap  5  between each of the pressure rollers  5  and the platen  3 , on the basis of the detection result from the detection sensor (not shown) for detecting the recording medium  10  in the front vicinity of the pressure auxiliary roller  12  and the feed condition of the recording medium  10  obtained by the number of revolutions of the feed roller  2 . The pressure unit controlling device controls the moving means  7  of the pressure unit  8  on the basis of the calculation results.  
      A sensor that reacts to the characteristics of the recording medium  10  other than the color or material of the recording medium  10  may be used as the detection sensor. Further, a CPU (Central Processing Unit), which performs a calculation process on the basis of the detection results from the detection sensor and the number of steps of the step motor for the feed roller  2 , or a circuit that is formed using other processing devices may be used as the calculating device.  
      Furthermore, the pressure unit controlling device may be formed of a circuit that outputs control signals to the moving means  7 . The pressure unit controlling device may be formed of a circuit formed using a storage unit, such as a memory, or a processing device, such as a CPU.  
      The calculating device and the pressure unit controlling device may also be formed to serve as the above-mentioned control unit.  
      The operations of the printer  1  according to this embodiment will be described with reference to FIGS.  1  to  7 .  FIG. 7  schematically shows the operations of the printer.  
      As shown in  FIG. 1 , according to the printer  1  of this embodiment, the recording medium  10  is pressed against the outer peripheral surface of the platen  3  by the pressure rollers  5  and the pressure auxiliary roller  12  of the pressure unit  8  that faces the outer peripheral surface of the platen  3 . When the recording medium  10  begins to be fed due to the rotation of the feed roller  2 , the recording medium  10  passing between the pressure auxiliary roller  12  and the platen  3  is pressed against the platen  3  and the leading end of the recording medium  10  enters the predetermined gap  5  between the pressure rollers  5  and the platen  3 . When the calculating device calculates that the leading end of the recording medium  10  is provided in the predetermined gap  5 , the ring gear  20  is rotated at a speed equal to the circumferential speed of the platen  3  and the contact part  23  of the actuator  21  of the ring gear  20  pushes the movable guide post  52  of the first roller unit  50 A in the rotation direction of the platen  3  as shown in  FIG. 5 . Since the stationary cam disk  30  on which the first roller unit  50 A is mounted is rotated, the lift-up pin  54  is moved down from the lift cam  33  of the stationary cam disk  30  onto the outer periphery of the stationary cam disk  30  and moved inward in the radial direction of the stationary cam disk  30  due to the tension of the elastic member  56  of the roller unit  50 . Accordingly, the pressure rollers  5  press the leading end of the recording medium  10  against the outer peripheral surface of the platen  3 . In addition, the movable guide post  52  pushed by the actuator  21  is moved along the movable guide post guiding hole  31  in conjunction with the rotation of the platen  3 . That is, as shown in  FIGS. 1 and 5 , the pressure auxiliary roller  12  and the pressure rollers  5  press the recording medium  10  against the platen  3 , and the pressure rollers  5  are moved toward the back position in the feed direction in conjunction with the rotation of the platen  3  while pressing the recording medium  10  against the platen  3 .  
      Further, as shown in  FIG. 7  the circumferential speed Vs of the feed roller  2  is controlled to be temporarily slower than the circumferential speed Vp of the platen  3 . In this case, when the circumferential speed Vs of the feed roller  2  is slower than the circumferential speed Vp of the platen  3 , the speed of the recording medium  10  fed by the platen  3  is relatively faster than the speed of the recording medium  10  fed by the feed roller  2 . In addition, the pressing force F 2  of the pressure auxiliary roller  12  is smaller than the pressing force F 1  of each pressure roller  5 . Accordingly, while being interposed between the platen  3  and the first pressure roller  5 A, the recording medium  10 , which is pressed against the platen  3  by the pressure auxiliary roller  12  and the first pressure roller  5 A, is stretched by the platen  3  and the first pressure roller  5 A. As a result, tension is applied to the recording medium  10  by the feed roller  2  and the pressure rollers  5 .  
      Since the tension is applied to the recording medium  10  to remove the slack SS (see  FIG. 8 ) of the recording medium  10 , the recording medium  10  can come in close contact with the surface of the platen  3 . As shown in  FIG. 7 , the pressure roller  5  passes by the lower portion of the first recording unit  4 A. Accordingly, even though the recording medium  10  has not been yet fed to a discharge roller (not shown) provided at a back position after the platen  3  in the feed direction FD, the first recording unit  4 A slidably approaches the platen  3  to begin to perform the recording on the recording medium  10  (hereinafter, “the slidable approach of the first recording unit  4 A to the platen  3 ” is referred to as “the head down” of the first recording unit  4 A”).  
      The calculating device of the printer  1  can calculate the correct position of the leading end of the recording medium  10 , which is detected by the detection sensor, on the feed path. Accordingly, the pressure unit controlling device can make the pressure rollers  5  come in close contact with the leading end of the recording medium  10 . For this reason, since the tension is applied to the recording medium  10  from the leading end thereof, the first recording unit  4 A can begin to perform the recording on the recording medium  10  without forming an unnecessary margin in the vicinity of the leading end of the recording medium  10 . Therefore, while being pressed against the platen  3 , the leading end of the recording medium  10  is fed to the back position in the feed direction in conjunction with the rotation of the platen  3 . As a result, it is possible to prevent the leading end of the recording medium  10  from returning while being bumped against the recording units  4 , and to easily control the feed direction FD of the recording medium  10 .  
      Further, as shown in  FIG. 6 , when the pressure rollers  5  press the recording medium  10  against the platen  3  by the actuator  21  of the ring gear  20  and are moved in conjunction with the rotation of the platen  3 , the high pressure plates  91  of the high pressure units  9  press the pressure rollers  5  against the platen  3 .  
      Specifically, as shown in  FIG. 6 , the base plate  61  of the first roller unit  50 A comes in contact with the base plate restraining plate  92  of the first high pressure unit  9 A, thereby preventing the movement of the first roller unit  5 CA. In this case, the movable guide post  52  pushed in the circumferential direction by the contact part  23  of the actuator  21  reaches the curved portion  31   a  of the movable guide post guiding hole  31  formed in the stationary cam disk  30 . Accordingly, the movable guide post  52  is moved to the inner portion in the radial direction along the movable guide post guiding hole  31 , and is pushed to the inner portion in the radial direction by the radial pushing surface  23   b  of the contact part  23 . As a result, the movable guide post  52  is separated from the contact part  23  along the circumferential pushing surface  23   a  and the radial pushing surface  23   b  in this order, so that the force that moves the pressure roller  5  of the first roller unit  50 A in conjunction with the rotation of the platen  3  is removed.  
      In addition, when the movable guide post  52  is pushed to the inner portion in the radial direction, the stopper part  57   a  of the stopper plate  57  that is interlocked with the connection plate  55  provided to the movable guide post  52  is moved to the inner portion in the radial direction. Further, the stopper part  57   a  is engaged with the stopper hole  35  of the stationary cam disk  30 . For this reason, the movable guide post  52  is separated from the contact part  23  of the actuator  21 . The stopper part  57   a  and the stopper hole  35  are engaged with each other and the movable guide post  52  is not moved due to the curved portion  31   a  of the guide hole  31 . As a result, even though the elastic member  56  of the first roller unit  50 A pulls the movable guide post  52  to the outside in the radial direction, the first roller unit  50 A is positioned.  
      When the first recording unit  4 A begins to perform the head down operation on the basis of the feed distance of the recording medium  10 , the cam  94  for the high pressure plate of the first high pressure unit  9 A is rotated so as to reduce the lift distance thereof and the high pressure elastic member  93  pushes the high pressure plate  91  so that the high pressure plate  91  is rotated about the rotating shaft  96  in the clockwise direction.  
      That is, since the first high pressure unit  9 A operates to apply a high pressing force, which pushes the pressure roller  5  against the platen  3 , to the pressure roller  5 , the recording medium  10  interposed between the pressure roller  5  and the platen  3  is strongly pressed against the platen  3 . As a result, even though the first recording unit  4 A comes in contact with the recording medium  10  so as to perform the recording, the first recording unit  4 A can perform the recording on the recording medium  10 .  
      Even after the first high pressure unit  9 A applies the high pressing force to the pressure roller  5 , the pressure roller  5  is driven to rotate due to the rotation of the platen  3  while applying the high pressing force to the recording medium  10 . For this reason, the printer  1  can feed the recording medium  10  in the predetermined feed direction FD.  
      When a pressing force, which is larger than the pressing force F 1  applied to the pressure roller  5  by the high pressure unit  9 A, is applied to the recording medium  10 , the control unit may temporarily stop the platen  3  and temporarily rotate the feed roller  2  in a reverse direction. For this reason, even though the recording medium  10  is not fed, tension is applied to the recording medium  10 . As a result, even though the recording medium  10  is over fed to remove the slack SS, it is possible to make the recording medium  10  come in close contact with the platen  3 . Further, when the platen  3  is stopped and the feed roller is rotated in the reverse direction, it is possible to quickly and easily remove the slack SS as compared to when the feed roller  2  and the platen  3  are rotated in the same direction and the slack SS is removed by using a difference between the rotational speeds thereof. However, when the feed roller  2  is rotated in the reverse direction and the leading end of the recording medium  10  does not slide and is interposed between the platen  3  and the pressure roller  5  due to the pressing force F 1  applied to the platen  3  by the pressure roller  5 , the platen  3  may be temporarily stopped and the feed roller  2  may be temporarily rotated in the reverse direction before the high pressure unit  9  strongly presses the pressure roller  5  against the platen  3 .  
      In some cases, while the recoding medium  10  slides on the outer peripheral surface of the platen  3 , the above-mentioned operations may be performed to remove the slack SS. For this reason, when the recording medium  10  slides on the outer peripheral surface of the platen, the recording units  4  may not correctly record desired letters or images on the recording medium  10 . Accordingly, until each of the recording units  4  provided at the front position before each of the pressure rollers  5  in the feed direction FD begins to perform the recording on the recording medium  10 , the control unit may perform the control in which the circumferential speed Vs of the feed roller  2  becomes slower than the circumferential speed Vp of the platen  3 . Further, after each of the recording units  4  begins to perform the recording, the control unit may perform the control in which the circumferential speed Vs of the feed roller  2  becomes equal to the circumferential speed Vp of the platen  3 . For this reason, after each of the recording units  4  begins to perform the recording, the recording medium  10  does not slide on the outer peripheral surface of the platen  3 . As a result, it is possible to perform the recording at desired positions on the recording medium  10 .  
      In addition, the roller units  50  and the high pressure units  9  are provided in the printer  1  so as to correspond to the number of the recording units  4 . Accordingly, after the first roller unit  50 A strongly presses the recording medium against the platen, it is possible to sequentially perform the pressing operations in which the first recording unit  4 A begins to perform the recording and the second roller unit  50 B then presses the recording medium  10  against the platen and is moved during the recording performed by the first recording unit  4 A.  
      As shown in  FIG. 6 , when the movable guide post  52  of the first roller unit  50 A is separated from the contact part  23  of the actuator  21  and the stopper part  57   a  is engaged with the stopper hole  35  of the stationary cam disk  30 , the actuator  21  is positioned so as to push the movable guide post  52  of the second roller unit  50 B in the circumferential direction.  
      In this case, as shown in  FIG. 2 , since the first roller unit  50 A and the second roller unit  50 B are provided to different stationary cam disks  30 , the movable guide post  52  of the second roller unit  50 B protrudes from the base plate  51  in a direction different from a direction in which the movable guide post  52  of the first roller unit  50 A protrudes from the base plate  51 . However, as shown in  FIG. 2 , since the movable guide post  52  of the first roller unit  50 A and the movable guide post  52  of the second roller unit  50 B are provided to face the ring gear  20 , the contact part  23  of the actuator  21  protrudes toward both sides of the ring gear  20  in the axial direction of the ring gear  20 . Accordingly, as shown in  FIG. 6 , the actuator  21  pushes the movable guide post  52  of the second roller unit  50 B next to the positioned first roller unit  50 A and moves down the lift-up pin  54 , so that the second pressure roller  50 B can press the leading end of the recording medium  10  against the platen  3 . Since this is the same as in the relationship between the second roller unit  50 B and the third roller unit  50 C and in the relationship between the third roller unit  50 C and the fourth roller unit  50 D, it is possible to apply the correct pressing force to the recording medium  10  so as to correspond to each of the recording units  4 .  
      That is, even though the feed path between the feed roller  2  and the discharge roller  11  is increased due to the increase in the number of recording units  4  to be provided and the diameter of the platen  3 , the plurality of pressure rollers  5 A,  5 B,  5 C, and  5 D strongly press the recording medium  10  against the platen at positions corresponding to the recording units  4 . For this reason, even though the recording units  4 , which come in contact with the recording medium  10  to perform the recording thereon, apply impact to the recording medium  10  several times, it is possible to prevent the desired letters or images from being incorrectly recorded on the recording medium  10  due to misalignment of the recording medium  10  with the platen  3 .  
      The pressure rollers sequentially press the recording medium  10  from the first pressure roller  5 A to correctly apply the pressing force, which is necessary to perform the recording, to the recording medium  10 . Accordingly, unlike a printer  101  in the related art, even though the leading end of the recording medium  10  has not been yet fed to a discharge roller provided at a back position after the platen  3  in the feed direction, the recording units can sequentially record desired letters or images on the recording medium  10  from the first recording unit  4 A.  
      That is, even though the slack occurs on the recording medium, it is possible to make the recording medium preferably come in close contact with the platen until the recording units begin to perform the recording. Therefore, it is possible to improve the recording quality of the printer.  
      Further, it is possible to make the recording medium come in close contact with the platen from the leading end of the recording medium. Accordingly, it is possible to perform the recording on the recording medium from the leading end thereof without forming the unnecessary margin, on which the recording cannot be performed, on the recording medium.  
      Even though slack occurs on the recording medium, it is possible to make the recording medium come in close contact with the platen until the recording units begin to perform the recording. Therefore, it is possible to improve the recording quality of the printer.  
      Further, it is possible to make the recording medium come in close contact with the platen from the leading end of the recording medium. Accordingly, it is possible to perform the recording on the recording medium from the leading end thereof without forming the unnecessary margin, on which the recording cannot be performed, on the recording medium.  
      Although only a few examples of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible thereto 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.