Abstract:
A fixing device includes a tubular member, a nip member, a backup member, and a bias member. The tubular member has an outer peripheral surface, an inner peripheral surface defining an internal space, and an axis defining an axis direction. The nip member is in contact with the inner peripheral surface. The backup member is opposed to the nip member and in contact with the outer peripheral surface to form a nip region for nipping a sheet conveyed in a conveying direction orthogonal to the axis direction. The backup member is configured to move in the conveying direction and the tubular member is configured to circularly move around the axis in accordance with the movement of the backup member due to a friction force generated between the tubular member and the backup member. The bias member protrudes toward the backup member through a bias region.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2010-288620 filed Dec. 24, 2010. The entire content of the priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a fixing device that thermally fixes a developing agent image transferred on a sheet. 
     BACKGROUND 
     A conventional fixing device includes an endless fusing film, a heater disposed in an internal space of the fusing film, and a nip plate defining a nip portion relative to a pressure roller through the fusing film. Due to a friction force that generates between the fusing film and the pressure roller, the fusing film is circularly moved in accordance with a rotation of the pressure roller driven by a motor. 
     SUMMARY 
     However, when a recording sheet passes between the fusing film and the pressure roller, the friction force can be reduced, thereby the fusing film may slip. If the fusing film slips, a developer image cannot be fixed on the recording sheet successfully. 
     It is an object of the invention to provide a fixing device capable of restraining a slip of a fusing film. 
     In order to attain the above and other objects, the present invention provides a fixing device including a flexible tubular member, a nip member, a backup member, and a bias member. The tubular member has an outer peripheral surface, an inner peripheral surface defining an internal space, and an axis defining an axis direction. The nip member is configured to be in contact with the inner peripheral surface. The backup member is opposed to the nip member and configured to be in contact with the outer peripheral surface to form a nip region for nipping a sheet conveyed in a conveying direction orthogonal to the axis direction. The sheet has a width in a width direction orthogonal to the conveying direction. The backup member is configured to move in the conveying direction and the tubular member is configured to circularly move around the axis in accordance with the movement of the backup member due to a friction force that generates between the tubular member and the backup member. The bias member protrudes toward the backup member through a bias region that is only outside of a width region corresponding to the width to bias the tubular member to the backup member. 
     Another aspect of the present invention provides a method for manufacturing a fixing device. The fixing device includes a flexible tubular member, a nip member, a backup member, and a bias member. The tubular member has an outer peripheral surface, an inner peripheral surface defining an internal space, and an axis defining an axis direction. The nip member is configured to be in contact with the inner peripheral surface. The backup member is opposed to the nip member and configured to be in contact with the outer peripheral surface to form a nip region for nipping a sheet conveyed in a conveying direction orthogonal to the axis direction. The sheet has a width in a width direction orthogonal to the conveying direction. The backup member is configured to move in the conveying direction and the tubular member is configured to circularly move around the axis in accordance with the movement of the backup member due to a friction force that generates between the tubular member and the backup member. The bias member protrudes toward the backup member through a bias region that is only outside of a width region corresponding to the width to bias the tubular member to the backup member. The nip member has a first surface and a second surface nearer to the backup member than the first surface. The method includes: determining, based on a desired protruding distance and a possible size of the bias member, a position of the bias member; and disposing the bias member at the determined position. 
     A fixing device includes: a tubular film; a nip plate; a backup roller; and a protruding member. The tubular film has an outer peripheral surface, an inner peripheral surface defining an internal space, and an axis defining an axis direction. The nip plate that is configured to be in contact with the inner peripheral surface. The backup roller that is opposed to the nip member and configured to be in contact with the outer peripheral surface to form a nip region for nipping a sheet conveyed in a conveying direction orthogonal to the axis direction. The sheet has a width in a width direction orthogonal to the conveying direction. The backup roller is configured be driven to rotate and the tubular film is configured to circularly move around the axis in accordance with the rotation of the backup roller. The protruding member protrudes toward the backup roller through a protruding region that is only outside of a width region corresponding to the width. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a schematic cross-sectional view showing a structure of a laser printer having a fixing device according to a first embodiment of the present invention; 
         FIG. 2  is a schematic cross-sectional view showing a structure of the fixing device according to the first embodiment; 
         FIG. 3  is an exploded perspective view showing a halogen lamp, a nip plate, a reflection plate, and a stay; 
         FIG. 4  is a cross-sectional view of the fusing device; 
         FIG. 5  is a perspective view showing a nip plate and a reflection plate according to a variation of the first embodiment; 
         FIG. 6(   a ) is a perspective view showing a nip plate according to a second embodiment of the present invention; 
         FIG. 6(   b ) is a cross-sectional view of the nip plate according to the second embodiment; and 
         FIG. 7  is a perspective view showing a nip plate according to a variation of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     Next, a general structure of a fixing device according to a first embodiment of the present invention will be described with references. A laser printer  1  shown in  FIG. 1  is provided with a fixing device  100  according to the embodiment of the present invention. A detailed structure of the fixing device  100  will be described later. 
     &lt;General Structure of Laser Printer&gt; 
     As shown in  FIG. 1 , the laser printer  1  includes a main frame  2  with a movable front cover  21 . Within the main frame  2 , a sheet supply unit  3  for supplying a sheet P, an exposure unit  4 , a process cartridge  5  for transferring a toner image (a developing agent image) on the sheet P, and the fixing device  100  for thermally fixing the toner image onto the sheet P are provided. 
     Throughout the specification, the terms “above”, “below”, “right”, “left”, “front”, “rear” and the like will be used assuming that the laser printer  1  is disposed in an orientation in which it is intended to be used. More specifically, in  FIG. 1 , a left side and a right side are a rear side and a front side, respectively. 
     The sheet supply unit  3  is disposed at a lower portion of the main frame  2 . The sheet supply unit  3  includes a sheet supply tray  31  for accommodating the sheet P, a lifter plate  32  for lifting up a front side of the sheet P, a sheet supply roller  33 , a sheet supply pad  34 , paper dust removing rollers  35 ,  36 , and registration rollers  37 . Each sheet P accommodated in the sheet supply tray  31  is directed upward to the sheet supply roller  33  by the lifter plate  32 , separated by the sheet supply roller  33  and the sheet supply pad  34 , and conveyed toward the process cartridge  5  passing through the paper dust removing rollers  35 ,  36 , and the registration rollers  37 . 
     The exposure unit  4  is disposed at an upper portion of the main frame  2 . The exposure unit  4  includes a laser emission unit (not shown), a polygon mirror  41 , lenses  42 ,  43 , and reflection mirrors  44 ,  45 ,  46 . In the exposure unit  4 , the laser emission unit is adapted to project a laser beam (indicated by a dotted line in  FIG. 1 ) based on image data so that the laser beam is deflected by or passes through the polygon mirror  41 , the lens  42 , the reflection mirrors  44 ,  45 , the lens  43 , and the reflection mirror  46  in this order. A surface of a photosensitive drum  61  is subjected to high speed scan of the laser beam. 
     The process cartridge  5  is disposed below the exposure unit  4 . The process cartridge  5  is detachable or attachable relative to the main frame  2  through a front opening defined by the front cover  21  at an open position. The process cartridge  5  includes a drum unit  6  and a developing unit  7 . 
     The drum unit  6  includes the photosensitive drum  61 , a charger  62 , and a transfer roller  63 . The developing unit  7  is detachably mounted to the drum unit  6 . The developing unit  7  includes a developing roller  71 , a toner supply roller  72 , a regulation blade  73 , and a toner accommodating portion  74  in which toner (developing agent) is accommodated. 
     In the process cartridge  5 , after the surface of the photosensitive drum  61  has been uniformly charged by the charger  62 , the surface is subjected to high speed scan of the laser beam from the exposure unit  4 . An electrostatic latent image based on the image data is thereby formed on the surface of the photosensitive drum  61 . The toner accommodated in the toner accommodating portion  74  is supplied to the developing roller  71  via the toner supply roller  72 . The toner is conveyed between the developing roller  71  and the regulation blade  73  so as to be deposited on the developing roller  71  as a thin layer having a uniform thickness. 
     The toner deposited on the developing roller  71  is supplied to the electrostatic latent image formed on the photosensitive drum  61 . Hence, a visible toner image corresponding to the electrostatic latent image is formed on the photosensitive drum  61 . Then, the sheet P is conveyed between the photosensitive drum  61  and the transfer roller  63 , so that the toner image formed on the photosensitive drum  61  is transferred onto the sheet P. 
     The fixing device  100  is disposed rearward of the process cartridge  5 . The toner image (toner) transferred onto the sheet P is thermally fixed on the sheet P while the sheet P passes through the fixing device  100 . The sheet P on which the toner image is thermally fixed is conveyed by conveying rollers  23  and  24  so as to be discharged on a discharge tray  22 . 
     &lt;Detailed Structure of Fixing Device&gt; 
     As shown in  FIG. 2 , the fixing device  100  includes a fusing film  110 , a halogen lamp  120 , a nip plate  130 , a reflection plate  140 , a pressure roller  150 , a stay  160 , and a pair of rotating members  170 . 
     The fusing film  110  has an endless (tubular) configuration having heat resistivity and flexibility. Each end portion of the fusing film  110  in an axis direction (left to right direction in  FIG. 2 ) is guided by a guide member  180  ( FIG. 4 ), thereby the fusing film  110  is circularly movable. The fusing film  110  slid-contacts the nip plate  130  through grease. The grease may not be necessarily coated between the fusing film  110  and the nip plate  130  if the material of the fusing film  110  and the nip plate  130  does not need the grease. 
     The halogen lamp  120  is a conventional heater for heating toner on the sheet P by heating the nip plate  130  and the fusing film  110 . The halogen lamp  120  is positioned at an internal space of the fusing film  110  and is spaced away from inner surfaces of the fusing film  110  and the nip plate  130  by a predetermined distance. 
     The nip plate  130  is adapted to receive radiant heat radiated from the halogen lamp  120  and is disposed so as to slide-contact the inner surface of the fusing film  110 . The nip plate  130  transmits the radiant heat radiated from the halogen lamp  120  to toner on a sheet P through the fixing film  110 . 
     The nip plate  130  is formed from, for example, an aluminum plate having a heat conductivity higher than the stay  160  from a stainless. Note that in order to effectively absorb the radiant heat radiated from the halogen lamp  120 , an upper surface  130 A of the nip member  130  may be coated with black or a heat absorbing member. 
     As shown in  FIG. 3 , the nip plate  130  is formed with a pair of through-holes  131  in which the pair of rotating members  170  is disposed, respectively. 
     The pair of through-holes  131  is positioned at both ends of the nip plate  130  in the left-right direction, respectively, outside of a sheet conveying region W in the left-right direction, and at the center of the nip plate  130  in the front-rear direction. The sheet conveying region W means a width in the left-right direction of a maximum size of sheet on which the laser printer  1  can form an image. 
     Further, two pair of supporting members  132  are integrally formed on the nip plate  130  with sheet-metal processing. Each pair of supporting members  132  protrudes upwards from both ends of corresponding through-hole  130  in the left-right direction on the upper surface  130 A of the nip plate  130 , respectively, in order to rotatably supports a rotational shaft  172  (described later) of the corresponding rotating member  170 . 
     As shown in  FIG. 2 , the reflection plate  140  is adapted to reflect radiant heat radiated in the frontward/rearward direction and the upper direction from the halogen lamp  120  toward the nip plate  130  (the upper surface  103 A). The reflection plate  140  is positioned within the fusing film  110  and surrounds the halogen lamp  120 , with a predetermined distance therefrom. 
     Thus, radiant heat radiated from the halogen lamp  120  can be efficiently concentrated onto the nip plate  130  to promptly heat the nip plate  130  and the fusing film  110 . 
     The reflection plate  140  has a U-shape in cross-section and is made from a material such as aluminum having high reflection ratio regarding infrared ray and far infrared ray. Specifically, the reflection plate  140  has a U-shaped reflection portion  141  and a pair of flange portions  142  extending in the front-rear direction from both ends of the reflection portion  141 , respectively. A mirror surface finishing is available on the surface of the aluminum reflection plate  140  for specular reflection in order to enhance heat reflection ratio. 
     The pressure roller  150  that is elastically deformable is positioned below the nip plate  130 . By elastically deforming, the pressure roller  150  nips the fusing film  110  in cooperation with the nip plate  130  to provide a nip region N for nipping the sheet P between the pressure roller  150  and the fusing film  110 . 
     As shown in  FIG. 4 , a driving gear  151  is attached to one shaft of the pressure roller  150 . The pressure roller  150  is rotationally driven with a driving force transmitted to the driving gear  151  from a drive motor (not shown) disposed in the main frame  2 . In accordance with the rotation of the pressure roller  150 , the fusing film  110  is circularly moved due to a friction force that generates between the fusing film  110  and the pressure roller  150  (or the sheet P). Thus, a toner image on the sheet P can be thermally fixed thereto by heat and pressure during passage of the sheet P at the nip region N between the pressure roller  150  and the fusing film  110 . 
     The stay  160  is positioned within the fusing film  110  and surrounds the reflection plate  140  to support both ends of the nip plate  130  in the front-rear direction through the pair of flange portions  142  of the reflection plate  140 . For fabricating the stay  160 , a highly rigid member such as a steel plate is folded into U-shape following the outer surface of the reflection plate  140  (the reflection portion  141 ). Thus, the position shift of the reflection plate  140  in the top-bottom direction is restrained. Further, since the pair of flange portions  142  of the reflection plate  140  is supported by the stay  160  having a high rigidity, the rigidity of the reflection part  140  is also held. 
     The pair of rotating members  170  includes a first rotating member  170 A disposed at the right side in  FIG. 3  and a second rotating member  170 B disposed at the left side in  FIG. 3  to bias both ends of the fusing film  110  in the left-right direction, respectively, to a peripheral surface  150 A of the pressure roller  150 . Each rotating member  170  includes a bias member  171  and a rotational shaft  172 . 
     As shown in  FIGS. 2 and 3 , each bias member  171  is disposed in the corresponding through-holes  131  and protrudes below a lower surface  130 B of the nip plate  130  to bias the fusing film  110 . Since the pair of through-holes  131  is formed outside of the sheet conveying region W in the left-right direction, the pair of bias members  171  results in being also disposed outside of the sheet conveying region W. 
     With this construction, even when the sheet P is passing between the fusing film  110  and the pressure roller  150 , each bias member  171  does not contact the sheet P, that is, each bias member  171  can keep biasing the fusing film  110  to the pressure roller  150 . Therefore, the rotation (the driving force) of the pressure roller  150  is reliably transmitted to the fusing film  110 . Further, since the pair of bias members  171  does not contact the sheet P, it is prevented that the pair of bias members  171  interrupts the thermal fix. 
     Further, the pair of bias member  171  is disposed so as to overlap the nip portion NP as viewed from the left-right direction. With this construction, it becomes possible to reliably bias the fusing film  110  to the peripheral surface  150 A of the pressure roller  150 . 
     Each rotational shaft  172  is rotatably supported by the pair of supporting members  132  protruding upwards from the upper surface  130 A. As the result, each bias member  171  is positioned upwards of the upper surface  130 A. 
     As shown in  FIGS. 3 and 4 , the rotational shaft  172  of the first rotating member  170 A extends leftward through the guide member  180 . Further, the first rotating member  170 A includes a driven gear  173  attached to the left end of the rotational shaft  172 . The driven gear  173  is meshingly engaged with the driving gear  151  of the pressure roller  150 . Thus, the driving force transmitted from the drive motor (not shown) to the pressure roller  150  is also transmitted to the rotational member  170 A. 
     On the other hands, the rotating member  170 B does not include the driven gear  173 . Therefore, the rotating member  170 B is rotated in accordance with the rotation of the fusing film  110  (the pressured roller  150 ) with the help of the friction force that generates between the rotating member  170 B and the fusing film  110 . 
     As described above, in the fusing device  100 , the rotating member  170  protrudes below a lower surface  130 B of the nip plate  130  to bias the fusing film  110  to the peripheral surface  150 A of the pressure roller  150 . Therefore, even if the friction force that generates between the fusing film  110  and the pressure roller  150  is reduced when the sheet P onto which the toner image is transferred passes between the pressure roller  150  and the heated fusing film  110  (nip portion NP), the driving force of the pressure roller  150  is reliably transmitted to the fusing film  110 , thereby the slip of the fusing film  110  is restrained. 
     Further, the rotating member  170 A is coupled with the pressure roller  150 . Therefore, the rotating member  170 A provides the fusing film  110  with the driving force from inside, thereby the rotating of the fusing film  110  can be assisted. 
     Further, the pair of bias members  171  is disposed outside of the sheet conveying region W in the left-right direction. Therefore, it is prevented that the pair of bias members  171  interrupts the thermal fix. 
     Further, the pair of bias member  171  is disposed so as to overlap the nip portion NP as viewed from the left-right direction. Therefore, it becomes possible to reliably bias the fusing film  110  to the peripheral surface  150 A of the pressure roller  150 . Note that the pair of bias member  171  may be not disposed so as to overlap the nip portion NP as viewed from the left-right direction. However, it is preferable that the pair of bias member  171  is disposed so as to overlap the nip portion NP as viewed from the left-right direction, in order to reliably bias the fusing film  110  to the peripheral surface  150 A of the pressure roller  150 . 
     Further, the pair of bias members  171  is isolated from the nip plate  130 . Therefore, it becomes easy to adjust the biasing force of the pair of bias members  171  compared with when a bias member is integrally formed on the nip plate  130 . 
     Further, each bias member  171  is disposed in the corresponding through-hole  131 . Therefore, it becomes easy to adjust the biasing force of the pair of bias members  171 . 
     Further, each rotating member  170  is rotatably supported by the pair of supporting members  132  disposed at both sides of the pair of through-holes  131  in the left-right direction. Therefore, the rotating member  170  can bias the fusing film  110  to the pressure roller  150  without interrupting the movement of the fusing film  110 . 
     Further, each supporting member  132  is integrally formed on the nip plate  130 . Therefore, it is not necessary to provide a supporting member isolated from the nip plate  130 . 
     Note that if a bias member  171  has a small diameter, the bias member  171  contacts the fusing film  110  with a small area. In such case, parts of the fusing film  110  that contacts the bias member  171  can deform compared with parts of the fusing film  110  that does not contact the bias member  171 . Therefore, it is preferable to use a bias member  171  having a large diameter. At a manufacturing stage, the position of each supporting member  132  supporting the bias member  171  can be determined based on a desired protruding distance and a possible diameter of the bias member  171 . 
     For example, if there is a large space to dispose the bias member  171 , it becomes possible to use a bias member  171  having a larger diameter. As the result, a desired protruding distance is available without deforming the fusing film  110 . On the other hands, if there is a small space to dispose the bias member  171 , a bias member  171  having a smaller diameter is used. However, if the fusing film  110  can be deformed by the bias member  171  having the smaller diameter, the protruding diameter may be reduced although the bias force is reduced. 
     In the first embodiment, each rotating member  170  is supported by the nip plate  130 . However, the rotating member  170  may be supported by a member other than the nip plate  130 . 
     For example, as shown in  FIG. 5 , the rotating member  170  may be supported by a reflection plate  240  at the outside of the nip plate  130  in the left-right direction. 
     Specifically, the reflection plate  240  has a pair of deformed parts  241  formed with sheet-metal processing at both ends in the left-right direction. Each deformed part  240  has a through-hole  242  for accepting the bias member  171  of the corresponding rotating member  170  and a supporting member  243  disposed at both ends of the through-hole  242  in the left-right direction to support the rotational shaft  172  of the corresponding rotating member  170 . 
     Thus, even if the rotating member  170  is supported by a member other than the nip plate  130 , the slip of the fusing film  110  can be restrained. 
     Second Embodiment 
     Next, a fixing device according to a second embodiment of the present invention will be described with references. In the present embodiment, as shown in  FIG. 6(   a ), a pair of plate-like members  270  that does not rotate is used instead of the pair of rotating members  170 . Like parts and components are designated by the same reference numerals as the first embodiment to avoid duplicating description. 
     As shown in  FIGS. 6(   a ) and  6 ( b ), in the present embodiment, each plate-like member  270  has a curved bias part  271  disposed in the corresponding through-hole  131  and a pair of flange portions  272  that protrudes in the front-rear direction from both ends of the bias member  271 , respectively. 
     The pair of flange portions  272  is fixedly biased to edges of the through-hole  131  by a compression spring  290 . The bias part  271  protrudes downwards than the lower surface  130 B of the nip plate  130  through the through-hole  131  to bias the fusing film  110  to the peripheral surface  150 A of the pressure roller  150 . 
     Thus, even if the plate-like member  270  that does not rotate is used instead of the rotating member  170 , the bias part  271  can bias the fusing film  110  to the pressure roller  150 , thereby the rotating of the fusing film  110  can be assisted. 
     In the second embodiment, the bias member  271  (plate-like member  270 ) is isolated from the nip plate  130 . However, as shown in  FIG. 7 , a bias member  231  may be integrally formed on a nip plate  230 . 
     Specifically, the bias member  231  is integrally formed on the nip plate  230  with sheet-metal processing and has a convex shape protruding toward the pressure roller  150 . With this construction, the number of the components can be reduced. 
     While the invention has been described in detail with reference to the embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
     A thicker member that does not have a plate shape may be used instead of the nip plate  130 , for example. 
     A belt-like pressure member may be used instead of the pressure roller  150 , for example. 
     An OHP, a plain paper, and a post card sheet can be used as the sheet P, for example. 
     Further, an LED printer that performs an exposure with an LED, a copier, or a multifunction peripheral may be used instead of the laser printer  1 , for example. Further, an image-forming device that forms a color image may be used as the laser printer  1 , for example.