Abstract:
A printer includes a transporting belt, a liquid ejecting head, and a negative pressure generator. The transporting belt transports a printing medium while electrostatically attracting the printing medium. The liquid ejecting head ejects liquid droplets onto the printing medium transported by the transporting belt and thereby performs printing. The negative pressure generator is in contact with the inner surface of the transporting belt and supports the transporting belt. The negative pressure generator has a suction opening for generating negative pressure formed in a contact surface thereof in contact with the transporting belt. The transporting belt is a substantially airtight member.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a printer that performs printing by ejecting liquid droplets from a liquid ejecting head onto a printing medium electrostatically attracted and transported by a transporting belt. 
         [0003]    2. Related Art 
         [0004]    Examples of this type of printer include a fixed head type ink jet printer that electrostatically attracts a charged printing medium onto a transporting belt and transports the printing medium, that ejects (discharges) liquid (ink droplets) from nozzle rows of a liquid ejecting head (ink jet head) formed in a direction perpendicular to the transportation direction and across the width of the transporting belt, and that thereby performs printing on the printing medium. 
         [0005]    In such an ink jet printer, when the transporting belt rotates at high speed, the transporting belt flutters at its natural frequency, thereby fluctuating the distance between the ink jet head and the printing medium. In the nozzle rows, the time between discharge of an ink droplet and impact thereof on the printing medium can fluctuate, and therefore the impact position can fluctuate in the direction of transportation of the printing medium. 
         [0006]    Fluctuation in the impact position of an ink droplet can cause deterioration in printing quality. For example, when an image in a desired color is formed by superposing ink droplets discharged from a plurality of nozzles, the dot superposition accuracy can deteriorate, and therefore a color different from the desired color can be produced, or color can change along the direction of transportation of the printing medium, and therefore color shading can occur in the printed image. 
         [0007]    In order to improve the quality of printed image, for example, JP-A-2002-145474 discloses a printer that has a transporting belt attractor disposed inside a transporting belt to electrostatically attract the transporting belt and that suppresses the flutter of the transporting belt by attracting the transporting belt. 
         [0008]    In the above known ink jet printer, a charged printing medium is electrostatically attracted onto the transporting belt, and the inner surface of the transporting belt is electrostatically attracted onto the transporting belt attractor. Therefore, when the charge amount of the printing medium is changed according to the material or humidity, the electrostatic attraction of the transporting belt attractor on the transporting belt is also changed. Decreased electrostatic attraction on the transporting belt can decrease the flutter-suppressing effect on the transporting belt. Increased electrostatic attraction on the transporting belt can increase the frictional resistance between the transporting belt and the transporting belt attractor and therefore can deteriorate the accuracy of transportation of the printing medium. 
       SUMMARY 
       [0009]    An advantage of some aspects of the invention is to provide a printer in which the attraction on a transporting belt can be prevented from changing when the amount of charge for attracting a printing medium is changed. 
         [0010]    According to an aspect of the invention, a printer includes at least one transporting belt, a liquid ejecting head, and a negative pressure generator. The at least one transporting belt transports a printing medium while electrostatically attracting the printing medium. The liquid ejecting head ejects liquid droplets onto the printing medium transported by the transporting belt and thereby performs printing. The negative pressure generator is in contact with the inner surface of the transporting belt and supports the transporting belt. The negative pressure generator has a suction opening for generating negative pressure formed in a contact surface thereof in contact with the transporting belt. The transporting belt is a substantially airtight member. 
         [0011]    Due to this structure, the transporting belt can be sucked by negative pressure. Unlike, for example, electrostatically attracting the transporting belt, the attraction on the transporting belt can be prevented from changing when the amount of charge for attracting the printing medium is changed. 
         [0012]    The contact surface of the negative pressure generator may be provided with a plurality of holes and be flat. 
         [0013]    Alternatively, the portion of the contact surface of the negative pressure generator opposite the nozzles of the liquid ejecting head may be flat. 
         [0014]    In this case, since the transporting belt is sucked against the flat portion at a position where liquid droplets are ejected by the liquid ejecting head, the flatness at the position can be improved, and the distance between the liquid ejecting head and the printing medium can be prevented from changing. 
         [0015]    It is preferable that the negative pressure generator have a low friction member on the contact surface thereof in contact with the transporting belt. 
         [0016]    In this case, the sliding load can be reduced when the transporting belt slides on the contact surface of the negative pressure generator, and the accuracy of transportation of the printing medium can be improved. 
         [0017]    It is also preferable that the transporting belt have a low friction member on a contact surface thereof in contact with the negative pressure generator. 
         [0018]    In this case, the sliding load can be reduced when the transporting belt slides on the contact surface of the negative pressure generator, and the accuracy of transportation of the printing medium can be improved. 
         [0019]    The negative pressure generator may have a plurality of rollers in the contact surface thereof in contact with the transporting belt. The plurality of rollers are disposed at predetermined intervals in the direction of transportation of the printing medium and rotate in the moving direction of the transporting belt. 
         [0020]    In this case, with the rotation of the transporting belt, the rollers in contact with the transporting belt rotate. Therefore, the sliding load can be reduced when the transporting belt slides on the contact surface of the negative pressure generator, and the accuracy of transportation of the printing medium can be improved. 
         [0021]    It is preferable that the rollers be disposed opposite nozzles of the liquid ejecting head. 
         [0022]    In this case, at a position where liquid droplets are ejected by the liquid ejecting head, the flatness of the printing medium transported by the transporting belt can be improved, and the distance between the liquid ejecting head and the printing medium can be prevented from changing. 
         [0023]    The at least one transporting belt may include a plurality of transporting belts disposed at predetermined intervals in a direction perpendicular to the direction of transportation of the printing medium, and the liquid ejecting head may eject liquid droplets onto a region of the printing medium out of contact with the transporting belts. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
           [0025]      FIGS. 1A and 1B  schematically show the structure of an ink jet printer of a first embodiment.  FIG. 1A  is a plan view, and  FIG. 1B  is a side view. 
           [0026]      FIG. 2  is a block diagram showing the internal structure of the ink jet printer. 
           [0027]      FIGS. 3A and 3B  show the structure of a suction unit.  FIG. 3A  is a plan view, and  FIG. 3B  is a side view. 
           [0028]      FIGS. 4A and 4B  illustrate the operation of the ink jet printer. 
           [0029]      FIGS. 5A and 5B  show the structure of a modification of the suction unit.  FIG. 5A  is a plan view, and  FIG. 5B  is a side view. 
           [0030]      FIGS. 6A and 6B  schematically show the structure of an ink jet printer of a second embodiment.  FIG. 6A  is a plan view, and  FIG. 6B  is a side view. 
           [0031]      FIG. 7  shows the structure of a suction unit. 
           [0032]      FIG. 8  shows the structure of a modification of the suction unit. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0033]    Embodiments of the invention will hereinafter be described with reference to the drawings by taking, as an example of a printer of the invention, an ink jet printer that discharges ink and thereby records characters and/or images on a printing medium. 
       First Embodiment 
     Structure 
       [0034]      FIGS. 1A and 1B  schematically show the structure of an ink jet printer of this embodiment.  FIG. 2  is a block diagram showing the internal structure of the ink jet printer. 
         [0035]    As shown in  FIGS. 1A and 1B , the ink jet printer includes a paper transporting unit  1 , a suction unit  2 , a magnetic encoder  3 , and a head unit  4 . 
         [0036]    The paper transporting unit  1  includes an endless transporting belt  6  disposed along the direction of transportation of a printing medium  5 ; a drive roller  7 , an idler roller  8 , and a tension roller  9  around which the transporting belt  6  is stretched; and a transportation section drive motor  10  that rotationally drives the drive roller  7 . 
         [0037]    In the paper transporting unit  1 , the transportation section drive motor  10  rotationally drives the drive roller  7 , and thereby the transporting belt  6  is rotated. The transporting belt  6  is electrostatically charged by a charging roller  11 . The printing medium  5  fed from a paper feed section  12  is electrostatically attracted onto the upper surface of the transporting belt  6  and is transported under the head unit  4  and then to a paper ejection section  13 , that is, in the direction of arrows in  FIGS. 1A and 1B . 
         [0038]    The contact surface of the transporting belt  6  in contact with the suction unit  2  is coated with a material having a low coefficient of friction such as fluoropolymer to reduce the sliding load occurring when the transporting belt  6  slides on the upper surface of the suction unit  2 , and so that the transporting belt  6  can rotate stably. 
         [0039]    An AC voltage of a predetermined frequency is applied to the charging roller  11  from an AC source  14 . With the grounded idler roller  8  serving as a counter electrode, an alternating charge pattern is formed on the transporting belt  6 . 
         [0040]    The suction unit  2  is disposed inside the transporting belt  6 . As shown in  FIGS. 3A and 3B , the suction unit  2  includes a flat plate member  15 , a suction chamber  16 , and a suction fan  17 . 
         [0041]    The flat plate member  15  is disposed opposite the nozzles of the head unit  4 , is in contact with the lower surface of the transporting belt  6  so as to support the transporting belt  6 , and has through-holes  18  provided in the contact surface thereof in contact with the transporting belt  6 . 
         [0042]    The contact surface of the flat plate member  15  in contact with the transporting belt  6  is coated with a material having a low coefficient of friction such as fluoropolymer to reduce the sliding load occurring when the transporting belt  6  slides on the upper surface of the suction unit  2 , and so that the transporting belt  6  can rotate stably. 
         [0043]    The suction chamber  16  is disposed so as to cover the lower surface of the flat plate member  15 . The suction fan  17  sucks in air to generate negative pressure in the through-holes  18  in the contact surface of the flat plate member  15  in contact with the transporting belt  6  and so that the transporting belt  6  is sucked against the contact surface. 
         [0044]    The transporting belt  6  is formed of a substantially airtight member (for example, a member without through-holes) so that the transporting belt  6  can be effectively sucked by the generated negative pressure. Therefore, the negative pressure generated in the suction chamber  16  does not contribute to the suction of the printing medium  5  and contributes only to the suppression of the flutter of the transporting belt  6 . 
         [0045]    The suction fan  17  is rotationally driven by a fan motor (not shown), sucks in the air in the suction chamber  16 , and discharges the air downward. 
         [0046]    The magnetic encoder  3  includes an encoder scale  20  and an encoder sensor  21 . 
         [0047]    The encoder scale  20  is disposed on an edge of the transporting belt  6  so as not to interfere with the printing medium  5 . In the encoder scale  20 , magnetic signals are written at regular pitches along the direction of transportation of the printing medium  5 . 
         [0048]    The encoder sensor  21  detects the magnetic signals written in the encoder scale  20 , at a fixed position, thereby detects positional information of the transporting belt  6 , and outputs the detection result to the head unit  4 . 
         [0049]    The head unit  4  is disposed over the transporting belt  6  and includes yellow (Y), magenta (M), cyan (C), and black (K) ink jet heads  22   1  to  22   4  arranged in the direction of transportation of the printing medium  5 . 
         [0050]    Based on positional information of the transporting belt  6  output from the encoder sensor  21 , the head unit  4  discharges ink droplets downward from nozzle rows of the ink jet heads  22   1  to  22   4 , thereby performing printing on the printing medium  5 . 
         [0051]    Technologies for driving the ink jet heads  22   1  to  22   4  include electrostatic actuator technology, piezoelectric technology, and film boiling technology. 
       Operation 
       [0052]    Next, the operation of the ink jet printer of this embodiment will be described. 
         [0053]    First, upon request to start printing, the transportation section drive motor  10  is driven so as to rotate the transporting belt  6 , and the fan motor is driven so as to suck the lower surface of the transporting belt  6  against the suction unit  2 . 
         [0054]    The paper feed section  12  sends a printing medium  5  to the gate roller  23 . The charging roller  11  electrostatically charges the transporting belt  6 . Thereafter, the gate roller  23  is rotated so as to feed the printing medium  5  onto the upper surface of the transporting belt  6 . The printing medium  5  is electrostatically attracted onto the transporting belt  6  and is transported under the head unit  4  and then to the paper ejection section  13 . 
         [0055]    At the same time, a timer is activated. When a predetermined time has elapsed since activating the timer and the printing medium  5  passes under the head unit  4 , the head unit  4  discharges ink droplets so as to perform printing on the printing medium  5 . Thereafter, the printing medium  5  is ejected into the paper ejection section  13 . 
         [0056]    In this embodiment, the suction unit  2  of  FIG. 1B  serves as a negative pressure generator. 
       Advantages 
       [0057]    (1) As described above, the ink jet printer of this embodiment includes the suction unit  2  that is in contact with the inner surface of the transporting belt  6  so as to support the transporting belt  6  and that has the through-holes  18  for generating negative pressure formed in the contact surface thereof in contact with the transporting belt  6 . Therefore, the transporting belt  6  can be sucked by negative pressure. Unlike, for example, electrostatically attracting the transporting belt  6 , the attraction of the suction unit  2  on the transporting belt  6  can be prevented from changing when the amount of charge for attracting the printing medium  5  is changed. 
         [0058]    In addition, by preventing the change in the attraction of the suction unit  2  on the transporting belt  6 , the flutter of the transporting belt  6  can be suppressed as shown in  FIG. 4A . The dot superposition accuracy can be improved, and therefore the quality of recorded image can be improved. 
         [0059]    The frictional resistance between the transporting belt  6  and the suction unit  2  can be prevented from increasing, and therefore the accuracy of transportation of the printing medium  5  can be improved. 
         [0060]      FIG. 4B  shows the result of an experiment that measures the magnitude of flutter of the transporting belt  6  when the transporting belt  6  is not sucked against the suction unit  2 . It can be confirmed that the magnitude of flutter of the transporting belt  6  when the transporting belt  6  is sucked against the suction unit  2 , that is, the experimental result shown in  FIG. 4A  is smaller. 
         [0061]    In the case of sucking the transporting belt  6  by negative pressure, compared to, for example, the case of electrostatically attracting the transporting belt  6  onto the suction unit  2 , the generation of noise can be restricted, and the magnetic signals for detecting the position of the transporting belt  6  can be stably obtained from the magnetic encoder  3 . 
         [0062]    In the case of electrostatically attracting the transporting belt  6  onto the suction unit  2 , electrostatic noise is generated, the magnetic signals output from the magnetic encoder  3  are destabilized, and therefore the position of the transporting belt  6  cannot be detected normally. 
         [0063]    (2) Since the flat plate member  15  is disposed under the ink jet heads  22   1  to  22   4 , that is, opposite the nozzles of the ink jet heads, the transporting belt  6  is sucked against the flat portion at the positions where ink droplets are discharged by the ink jet heads  22   1  to  22   4 , the flatness at the positions can be improved, and the distance between the ink jet heads  22   1  to  22   4  and the printing medium  5  can be prevented from changing. 
         [0064]    (3) In addition, since the contact surface of the suction unit  2  in contact with the transporting belt  6  is coated with a material having a low coefficient of friction, the sliding load can be reduced when the transporting belt  6  slides on the upper surface of the suction unit  2 , and the accuracy of transportation of the printing medium  5  can be improved. 
         [0065]    (4) In addition, since the contact surface of the transporting belt  6  in contact with the suction unit  2  is coated with a material having a low coefficient of friction, the sliding load can be reduced when the transporting belt  6  slides on the upper surface of the suction unit  2 , and the accuracy of transportation of the printing medium  5  can be improved. 
         [0066]    (5) The suction unit  2  has the flat plate member  15  having the through-holes  18  for generating negative pressure formed in the contact surface thereof in contact with the transporting belt  6 . However, the invention is not limited to this. 
         [0067]    Instead of the flat plate member  15 , the suction unit  2  may have, for example, a plurality of rollers  24   1  to  24   4  provided in the contact surface thereof in contact with the transporting belt  6 , as shown in  FIGS. 5A and 5B . The rollers are disposed at regular intervals in the direction of transportation of the printing medium  5 . During the transportation of the printing medium  5 , the rollers are in contact with the lower surface of the transporting belt  6  so as to support the transporting belt  6  and rotate in the moving direction of the transporting belt  6 . 
         [0068]    With the rotation of the transporting belt  6 , the rollers  24   1  to  24   4  in contact with the transporting belt  6  rotate. Therefore, the sliding load can be reduced when the transporting belt  6  slides on the upper surface of the suction unit  2 , and the accuracy of transportation of the printing medium  5  can be improved. 
         [0069]    (6) The rollers  24   1  to  24   4  may be disposed along and opposite the nozzles of the ink jet heads  22   1  to  22   4 . At the positions where ink droplets are discharged by the ink jet heads  22   1  to  22   4 , the flatness of the printing medium  5  transported by the transporting belt  6  can be improved, and the distance between the ink jet heads  22   1  to  22   4  and the printing medium  5  can be prevented from changing. 
       Second Embodiment 
       [0070]    Next, a second embodiment of an ink jet printer of the present invention will be described with reference to the drawings. 
         [0071]    The second embodiment differs from the first embodiment in that two units each including transporting belts and ink jet heads are tandemly arranged in the transportation direction. 
         [0072]    Specifically, as shown in  FIGS. 6A and 6B , a paper transporting unit  1  includes an upstream transportation unit  25  and a downstream transportation unit  26  disposed downstream of the upstream transportation unit  25 . 
         [0073]    The upstream transportation unit  25  includes a plurality of transporting belts  25   1  to  25   4  disposed at predetermined intervals in a direction perpendicular to the direction of transportation of a printing medium  5 , a drive roller  27  and an upstream idler roller  28  around which the transporting belts  25   1  to  25   4  are stretched, and a transportation section drive motor  29  that rotationally drives the drive roller  27 . 
         [0074]    Rotational driving of the transportation section drive motor  29  rotates the transporting belts  25   1  to  25   4 . A charging roller  11   1  electrostatically charges the transporting belts  25   1  to  25   4 . The printing medium  5  fed from the paper feed section is electrostatically attracted onto the upper surfaces of the transporting belts  25   1  to  25   4  and is transported under an upstream head unit  36  (described below) and then onto the downstream transportation unit  26 . 
         [0075]    The downstream transportation unit  26  includes a plurality of transporting belts  26   1  to  26   5  disposed at predetermined intervals in a direction perpendicular to the direction of transportation of the printing medium  5  and so that the transporting belts  25   1  to  25   4  and  26   1  to  26   5  are staggered in plan view, and the drive roller  27  (shared with the upstream transportation unit  25 ) and a downstream idler roller  30  around which the transporting belts  26   1  to  26   5  are stretched. 
         [0076]    Rotational driving of the transportation section drive motor  29  rotates the transporting belts  26   1  to  26   5 . The printing medium  5  transported from the upstream transportation unit  25  is electrostatically attracted onto the upper surfaces of the transporting belts  26   1  to  26   5  and is transported under a downstream head unit  37  (described below) and then to the paper ejection section  13 . 
         [0077]    Suction units  2  are disposed inside the transporting belts  25   1  to  25   4  of the upstream transportation unit  25  and inside the transporting belts  26   1  to  26   5  of the downstream transportation unit  26 . As shown in  FIG. 7 , each suction unit  2  includes flat plate members  31 , suction chambers  32 , and a suction fan  33 . 
         [0078]    The flat plate members  31  are disposed opposite the nozzles of the upstream head unit  36  (described below) and the nozzles of the downstream head unit  37  (described below) are in contact with the lower surfaces of the transporting belts  25   1  to  25   4  and the lower surfaces of the transporting belts  26   1  to  26   5  so as to support the transporting belts  25   1  to  25   4  and the transporting belts  26   1  to  26   5 , and have through-holes  34  provided in the contact surfaces thereof in contact with the transporting belts  25   1  to  25   4  and  26   1  to  26   5 . 
         [0079]    The suction chambers  32  are disposed so as to cover the lower surfaces of the flat plate members  31 . The suction chambers of each of the upstream and downstream suction units are communicated with each other. The suction fans  33  sucks in air to generate negative pressure in the through-holes  34  in the contact surfaces of the flat plate members  31  in contact with the transporting belts  25   1  to  25   4  and  26   1  to  26   5  and so that the transporting belts  25   1  to  25   4  and  26   1  to  26   5  are sucked against the contact surfaces. 
         [0080]    The suction fans  33  are rotationally driven by fan motors (not shown), suck in the air in the suction chambers  32 , and discharges the air downward. 
         [0081]    A magnetic encoder  3  detects positional information of the transporting belt  26   1 , and outputs the detection result to a head unit  4 . 
         [0082]    The head unit  4  includes the upstream head unit  36  disposed over the upstream transportation unit  25  and the downstream head unit  37  disposed over the downstream transportation unit  26 . 
         [0083]    The upstream head unit  36  is disposed over the transporting belts  25   1  to  25   4  of the upstream transportation unit  25  and includes a plurality of ink jet heads  36   1  to  36   5  each having Y, M, C, and K nozzle rows arranged in the direction of transportation of the printing medium  5 . 
         [0084]    The upstream head unit  36  performs printing by discharging ink droplets downward from the nozzle rows of each of the ink jet heads  36   1  to  36   5  on the basis of positional information of the transporting belt  26   1  of the downstream transportation unit  26  output from the encoder sensor  21 . 
         [0085]    The downstream head unit  37  is disposed over the transporting belts  26   1  to  26   5  of the downstream transportation unit  26  and includes a plurality of ink jet heads  37   1  to  37   4  each having Y, M, C, and K nozzle rows arranged in the direction of transportation of the printing medium  5 . 
         [0086]    The downstream head unit  37  performs printing in the regions where the upstream head unit  36  does not perform printing, by discharging ink droplets downward from the nozzle rows of each of the ink jet heads  37   1  to  37   4  on the basis of positional information of the transporting belt  26   1  of the downstream transportation unit  26  output from the encoder sensor  21 . 
         [0087]    In this embodiment, the flat plate members  31  have the through-holes  34  for generating negative pressure formed in the contact surfaces thereof in contact with the transporting belts  25   1  to  25   4  of the upstream transportation unit  25  and the transporting belts  26   1  to  26   5  of the downstream transportation unit  26 . However, the present invention is not limited to this. 
         [0088]    Instead of the flat plate members  31 , the suction units  2  may have, for example, a plurality of rollers  38  provided in the contact surfaces thereof in contact with the transporting belts  25   1  to  25   4  and  26   1  to  26   5 , as shown in  FIG. 8 . The rollers  38  are disposed at regular intervals in the direction of transportation of the printing medium  5 . The rollers are in contact with the transporting belts  25   1  to  25   4  of the upstream transportation unit  25  and the transporting belts  26   1  to  26   5  of the downstream transportation unit  26  so as to support the transporting belts  25   1  to  25   4  and  26   1  to  26   5 , and rotate in the moving direction of the transporting belts  25   1  to  25   4  and  26   1  to  26   5 . 
         [0089]    With the rotation of the transporting belts  25   1  to  25   4  and  26   1  to  26   5 , the rollers  38  in contact with the transporting belts  25   1  to  25   4  and  26   1  to  26   5  rotate. Therefore, the sliding load can be reduced when the transporting belts  25   1  to  25   4  and  26   1  to  26   5  slide on the upper surfaces of the suction units  2 , and the accuracy of transportation of the printing medium  5  can be improved. 
         [0090]    The rollers  38  may be disposed along and opposite the nozzles of the upstream head unit  36  and the downstream head unit  37 . At the positions where ink droplets are discharged by the upstream and downstream head units  36  and  37 , the flatness of the printing medium  5  transported by the transporting belts can be improved, and the distance between the upstream and downstream head units  36  and  37  and the printing medium  5  can be prevented from changing.