Abstract:
A fixing device includes: a tubular member having an outer peripheral surface, an inner peripheral surface defining an internal space, and an axis defining an axial direction, the tubular member being configured to move around the axis; a heater disposed in an internal space to radiate heat; a nip member that contacts the inner peripheral surface to transmit the heat to the tubular member; a backup member that is in contact with the outer peripheral surface to form a nip region between the backup member and the tubular member, the backup member applying a load to the nip member; and a stay disposed in the internal space to support the nip member against the load. The nip member includes: a first member that contacts the inner peripheral surface; and a second member disposed between the first member and the stay to transmit the load to the stay.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2010-287377 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, a nip member defining a nip portion relative to a pressure roller through the fusing film, and a reflection plate for reflecting radiant heat radiated from the heater to the nip member. The above fixing device thermally fixes a developing agent image transferred on a sheet fed between the fusing film and the pressure roller. 
     SUMMARY 
     It is an object of the invention to provide a fixing device capable of raising a temperature of the nip member to a predetermined fixing temperature in a short time. 
     In order to attain the above and other objects, the present invention provides a fixing device a fixing device including: a flexible tubular member having an outer peripheral surface, an inner peripheral surface defining an internal space, and an axis defining an axial direction, the tubular member being configured to circularly move in a rotational direction around the axis; a heater disposed in the internal space to radiate a radiant heat; a nip member that is in contact with the inner peripheral surface to transmit the radiant heat to the tubular member; a backup member that is in contact with the outer peripheral surface to form a nip region between the backup member and the tubular member, the backup member applying a load to the nip member; and a stay disposed in the internal space to support the nip member against the load. The nip member includes: a first member that is in contact with the inner peripheral surface; and a second member disposed between the first member and the stay to transmit the load to the stay. The first member has a heat conductivity higher than the second member. The second member is formed with an opening for transmitting the radiant heat to the first member. 
    
    
     
       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 one embodiment of the present invention; 
         FIG. 2  is a schematic cross-sectional view showing a structure of the fixing device according to the embodiment; 
         FIG. 3  is an exploded perspective view showing a halogen lamp, a nip member, a reflection plate, and a stay; 
         FIG. 4  is a perspective view showing the nip member; and 
         FIG. 5  is a view showing the nip member and a fusing film as viewed from a top. 
     
    
    
     DETAILED DESCRIPTION 
     Next, a general structure of a fixing device according to one 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 member  130 , a reflection plate  140 , a pressure roller  150 , a stay  160 , and two thermistors  170  ( FIG. 3 ). 
     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 (not shown) so that the fusing film  110  is circularly movable. 
     The halogen lamp  120  is a conventional heater for heating toner on the sheet P by heating a first member  131  (described later) of the nip member  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 member  130  by a predetermined distance. 
     The nip member  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 member  130  (first member  131 ) transmits the radiant heat radiated from the halogen lamp  120  to toner on a sheet P through the fixing film  110 . A detailed structure of the nip member  130  will be described later. 
     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 member  130  (first member  131 ). 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 member  130  (the first member  131 ) to promptly heat the nip member  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 flange portion  142  extending from each end portion of the reflection portion  141  in the front-rear direction. 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  is positioned below the nip member  130  (the first member  131 ) and nips the fusing film  110  in cooperation with the nip member  130  to provide a nip region N for nipping the sheet P between the pressure roller  150  and the fusing film  110 . In order to provide the nip region N, one of the pressure roller  150  and the nip member  130  is biased toward the other by a bias member such as a spring. 
     The pressure roller  150  is rotationally driven by a drive motor (not shown) disposed in the main frame  2 . By the rotation of the pressure roller  150 , the fusing film  110  is circularly moved along the nip plate  130  because of a friction force generated therebetween or between the sheet P and the fusing film  110 . 
     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 a second member  132  (described later) of the nip member  130  via the flange portion  142  of the reflection plate  140  against a load applied from the pressure roller  150 . Note that when the nip member  130  biases the pressure roller  150 , a reaction force of the bias force corresponds to the load. 
     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 upper-lower direction is restrained by sandwiching the flange portion  142  of the reflection plate  140  between the stay  160  and the nip member  130  (the second member  132 ). Further, since the flange portion  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 thermistor  170  that is a conventional temperature sensor is disposed inside the fusing film  110  to detect the temperature of the nip member  130  (the first member  131 ). The detail configuration of the thermistor  170  is described later. 
     The detection result by the two thermistors  170  is inputted into a controller (not shown) provided at the fixing device  100  or the laser printer  1 . The controller controls the temperature of the nip portion N by controlling the output and the ON/OFF of the halogen lamp  120 . The detail description of the above control is omitted since the above control is well known. 
     &lt;Detail Construction of Nip Member&gt; 
     The nip member  130  mainly includes the plate-like first member  131  that slide-contacts the inner surface of the fusing film  110  and the second member  132  disposed between the first member  131  and the stay  160  (the flange portion  142  of the reflection part  140 ). 
     As shown in  FIGS. 3 and 4 , the first member  131  is formed from an aluminum plate having a heat conductivity higher than the second member  132  formed from a stainless plate as described later. The first member  131  mainly includes a main body  133  having a rectangular shape extending in the left right direction as viewed from the top, a pair of supporting convex parts  134 , three engaging convex parts  135 , and two protruding parts  136 . 
     The main body  133  has a plate shape having an upper surface opposed to the halogen lamp  120  via an opening  137 A of the second member  132  described later ( FIG. 2 ) and a lower surface that slide-contacts the inner surface of the fusing film  110 . With this construction, the radiant heat radiated from the halogen lamp  120  is transmitted to the fusing film  110 . 
     The main body  133  has a front side end surface  133 A (positioned at an upstream side in the rotational direction of the fusing film  110 ) extending in the left-right direction (the axis direction of the fusing film  110 ). A portion that causes an interference with the inner surface of the fusing film  110  rotating does not exist on the front side end surface  133 A of the main body  133 . Therefore, the fusing film  110  can rotate successfully, thereby it being restrained that the inner surface of the fusing film  110  is worn and damaged. 
     Note that in order to effectively absorb the radiant heat radiated from the halogen lamp  120 , the upper surface of the main body  133  may be coated with black or a heat absorbing member. 
     Each of the pair of supporting convex parts  134  has a plate shape. One supporting convex part  134  extends from one end of the main body  133  in the left-right direction toward the outside of the main body  133 , while the other supporting convex part  134  extends from the other end of the main body  133  in the left-right direction toward the outside of the main body  133 . The pair of supporting convex parts  134  is supported by a pair of supporting parts  138  described later, respectively. 
     The three engaging convex parts  135  extend, upward, from the rear edge of the main body  133  (the downstream side in the rotational direction of the fusing film  110 ) within a width W ( FIG. 5 ) over which the main body  133  slide-contacts the inner surface of the fusing film  110  in the left-right direction. For fabricating each engaging convex part  135 , a plate protruding from the rear edge of the main body  133  is folded into L-shape in the upper direction ( FIGS. 2 and 3 ). 
     Each oh the two protruding parts  136  has a plate shape protruding from the rear edge of the main body  133  rearward. The two thermistors  170  are disposed inside the fusing film  110  so as to oppose the upper surfaces of the two protruding parts  136 , respectively, to detect the temperature of the two protruding parts  136 , that is, the temperature of the nip member  130  (the first member  131 ). Note that a temperature detecting surface of the thermistor  170  may contact the protruding part  136  or may not contact the protruding part  136  (may be spaced away from the protruding part  136  by a predetermined distance). 
     The second member  132  is formed from a folded stainless plate having a rigidity higher than the first member  131  formed from the aluminum. The second member  132  includes a load transmitting part  137 , the pair of supporting parts  138 , and a guiding part  139 . 
     The load transmitting part  137  has a frame shape extending in the left-right direction as viewed from the upper side, and is disposed so that the lower surface of the load transmitting part  137  contacts the outer circumference of the upper surface of the first member  131  (the main body  133 ). Further, as shown in  FIG. 2 , the load transmitting part  137  is sandwiched between the first member  131  and the stay  160  (the flange portion  142  of the reflection plate  140 ). With this construction, the load that the first member  131  receives from the pressure roller  150  is transmitted to the stay  160  having a high rigidity via the flange portion  142  of the reflection part  140 , thereby the durability of the nip member  130  being held. 
     Returning to  FIGS. 3 and 4 , the load transmitting part  137  is formed with the opening  137 A for transmitting, to the first member  131 , radiant heat radiated downward from the halogen lamp  120  and radiant heat reflected downward by the reflection plate  140 , and three engaging concave parts  137 B. 
     The opening  137 A has a rectangular shape slightly smaller than the outline of the main body  133  of the first member  131 . By the opening  137 A, a frame is formed on the load transmitting part  137 A, and the frame contacts the outer circumference of the upper surface of the main body  133 . 
     The three engaging concave parts  137 B are formed at positions corresponding to the three engaging convex parts  135  of the first member  131 , respectively, so as to engage the three engaging convex parts  135 , respectively, when the second member  132  is mounted on the first member  131 . Further, the inner surface of each concave part  137 B at the rear side of serves as a supporting surface  137 C ( FIG. 5 ). When a friction force is applied to the first member  131  from the front to the rear in accordance with the rotation of the fusing film  110 , each convex part  135  abuts the corresponding supporting surface  137 C. 
     The pair of supporting parts  138  extends downward from both sides of the load transmitting part  137  in the left-right direction, respectively, and is opposed to one another in the left-right direction. Further, each supporting part  138  is formed with an opening  138 A engaged with the corresponding supporting convex part  134  of the first member  131 . The first member  131  is held by engaging the supporting convex part  134  with the opening  138 A. 
     As shown in  FIG. 2 , the guiding part  139  has a plate shape protruding upward from the front edge of the load transmitting part  137  to guide the inner surface of the fusing film  110  in the rotational direction of the fusing film  110  at the upstream of the nip region N. For fabricating the guiding part  139 , a plate protruding from the rear edge of the main body  133  is folded into L-shape in the upper direction. The folded portion of the guiding part  139  forms an R shape (curved shape) part that slide-contacts the inner surface of the fusing film  110   
     With this construction of the guiding part  139 , the fusing film  110  is smoothly fed to the nip region N defined between the first member  131  and the pressure roller  150 . Further, since the guiding part  139  is integrally provided on the second member  132 , the construction of the fusing device  100  becomes simple and mounting facility is improved, compared with the construction in which a guiding part is not integrally provided on the second member, that is, the guiding part is provided as a separated member from the second member. 
     Further, in the present embodiment, a lubricant agent (not shown) is coated between the nip member  130  (the main body  133  and the guiding part  139 ) and the fusing film  110  to reduce the reflection resistance between the fusing film  110  and the nip member  130 . With this construction, the fusing film  110  can be smoothly rotated. 
     As described above, in the present embodiment, the nip member  130  is constructed of the first member  131  that slide-contacts the inner surface of the fusing film  110  and the second member  132  disposed between the first member  131  and the stay  160  to transmit, to the stay  160 , the load applied from pressure roller  150  to the first member  131 . Therefore, it becomes possible to reduce the size (the heat capacity) of the first member  131  for transmitting, to the fusing film  110 , the radiant heat radiated from the halogen lamp  120  by reducing the thickness of the plate-like first member  131  and the dimension of the first member  131  (the main body  133 ). 
     Further, in the present embodiment, the opening  137 A for transmitting, to the first member  131 , the radiant heat radiated from the halogen lamp  120  is formed on the second member  132 . Therefore, it becomes possible to promptly heat the first member  131 . Further, in the present embodiment, the first member  131  has a heat conductivity higher than the second member  132  (the second member  132  has a heat conductivity lower than the first member  131 ). Therefore, it becomes possible to transmit, to the fusing film  110 , most part of heat that the first member  131  has received, without releasing the heat to the reflection plate  140  (the flange portion  142 ) and the stay  160  via the second member  132 . 
     Thus, since the fusing device  100  according to the present embodiment can effectively transmit, to the fusing film  110  (nip portion N), the radiant heat radiated from the halogen lamp  120 , the starting time of the fusing device  100  (the time from when image data is inputted into the laser printer  1  to when the image-forming operation is started) is reduced, thereby the speed-up of the laser printer  1  being achieved. 
     Further, in a fixing unit that adopts the film fixing method, a friction force from the front to the rear is applied to the nip member  130  (the first member  131 ) when the fusing film  110  rotates. If the fixing unit  100  is not provided with the engaging convex part  135  and the engaging concave part  137 B, the first member  131  can be arched (chain line of  FIG. 5 ) when the temperature of the first member  131  can become high due to the radiant heat radiated from the halogen lamp  120 . 
     However, since in the fixing unit  100  according to the present embodiment, the engaging convex part  135  abuts the supporting surface  137 C of the engaging concave part  137 B when the fusing film  110  rotates, it is restrained that the first member  131  deforms. 
     Further, a greater tension is applied to the fusing film  110  at the upstream side of the nip portion N than the downstream side. Therefore, if the engaging convex part  135  is formed at the front end of the main body  133 , the inner surface of the fusing film  110  can be partially worn by strongly slide-contacting the engaging convex part  135 . 
     However, in the present embodiment, the engaging convex part  135  is formed at the rear end (positioned at the downstream side in the rotational direction of the fusing film  110 ) of the main body  133 . Therefore, the engaging convex part  135  can slide-contacts the inner surface of the fusing film  110  with less tension, thereby it being restrained that the inner surface of the fusing film  110  is partially worn. 
     Further, the tension applied to the fusing film  110  is different between a portion of the fusing film  110  that slide-contacts the engaging convex part  135  and a portion of the fusing film  110  that does not slide-contacts the engaging convex part  135 . If the engaging convex part  135  is formed at the front end of the nip portion N where a greater tension is applied to the fusing film  110 , the difference of the tension becomes greater. As the result, the fusing film  110  can be folded. However, in the present embodiment, the engaging convex part  135  is formed at the rear end of the nip portion N where the tension becomes substantially even between a portion of the fusing film  110  that slide-contacts the engaging convex part  135  and a portion of the fusing film  110  that does not slide-contacts the engaging convex part  135 . Therefore, it can be restrained that the fusing film  110  is folded. 
     Further, if the engaging convex part  135  is formed at the front end of the nip portion N in the main body  133 , much lubricant collects on the engaging convex part  135  than the other part of the main body  133 , thereby the amount of the lubricant inserted between the fusing film  110  and the first member  131  becoming uneven between a portion of the fusing film  110  that slide-contacts the engaging convex part  135  and a portion of the fusing film  110  that does not slide-contacts the engaging convex part  135 . However, in the present embodiment, the engaging convex part  135  is formed at the rear end of the nip portion N. Therefore, it can be restrained that the amount of the lubricant inserted between the fusing film  110  and the first member  131  becomes uneven between a portion of the fusing film  110  that slide-contacts the engaging convex part  135  and a portion of the fusing film  110  that does not slide-contacts the engaging convex part  135 . 
     Further, in the present embodiment, the front end of the first member  131  (the front side  133 A of the main body  133 ) has a plate shape extending in the left-right direction. Therefore, the amount of the lubricant inserted between the fusing film  110  and the first member  131  can become even between a portion of the fusing film  110  that slide-contacts the engaging convex part  135  and a portion of the fusing film  110  that does not slide-contacts the engaging convex part  135 . 
     Further, in the present embodiment, the first member  131  is provided with the protruding part  136  opposed to the thermistor  170 . Therefore, it becomes possible to accurately detect the temperature of the nip member  130  (the first member  131 ), thereby accurately controlling the temperature of the nip portion N. 
     Further, in the present embodiment, the protruding part  136  is also disposed at the rear end of the main body  133  as the engaging convex part  135 . Therefore, it can be restrained that the inner surface of the fusing film  110  is partially worn, the amount of the lubricant inserted between the fusing film  110  and the first member  131  becomes uneven between a portion of the fusing film  110  that slide-contacts the protruding part  136  and a portion of the fusing film  110  that does not slide-contacts the protruding part  136 , and the fusing film  110  is folded. 
     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. 
     For example, the first member  131  may be made from an aluminum alloy, a copper, or a copper alloy, instead of the aluminum. Note that it is preferable that the heat conductivity becomes greater in the order of the second member, the tubular flexible fusing member, and the first member. 
     Further, a guide member that is not integrally provided on the second member  132  may be provided on the fixing unit  100 , instead of the guide member  139 . 
     Further, the front side end surface  133 A of the main body  135  may has a plate shape extending in the left-right direction that is slightly longer than a width W ( FIG. 5 ) of the fusing film  110 . 
     Further, a thermostat may be used as a temperature detecting unit, instead of the thermistor  170 , for example. Further, more than two temperature detecting units may be provided on the fixing unit  100 . In this case, both the thermistor and the thermostat may be provided on the fixing unit  100 . 
     Further, the protruding part  136  may not be provided if the temperature detecting unit is disposed outside the tubular flexible fusing member, for example. 
     Further, other members may be used instead of the engaging convex part  135  and the engaging concave part  137 B if the same function and effect are obtained. 
     For example, a convex part that is not penetrated may be used instead of the engaging convex part  135 . 
     Further, the flange portion  160  may not be provided on the fixing unit  100 . In this case, the second member directly contacts the stay. 
     Further, the reflection part  140  and the stay  160  may be integrally formed. For example, a reflection part can be formed at the inner surface of a stay. In this case, the stay includes both a function for receiving a load from a backup member and supporting the nip member  130  and a function of the reflection part  140 . 
     Further, a belt-like pressure member may be used instead of the pressure roller  150 , for example. 
     Further, an infrared heater or a carbon heater may be used instead of the halogen lamp  120 , 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.