Abstract:
Some fixing devices include a nip member, an endless belt, a rotating member and a stay. The stay, in some arrangements, has a first supporting face. Additionally, the first supporting face includes a first downstream edge. The first downstream edge includes a first portion, a third portion, and a second portion. According to various aspects, the second portion is positioned between the first portion and the third portion. The second supporting face includes a second downstream edge. The second downstream edge has a fourth portion, a sixth portion, and a fifth portion. According to further aspects, the second portion and the fifth portion define a first distance while the first portion and the fourth portion define a second distance, and the third portion and the sixth portion define a third distance. The second distance and the third distance is longer than the first distance in some examples.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2013-074369 filed Mar. 29, 2013. 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 transferred developing agent image to a sheet. 
     BACKGROUND 
     Japanese Patent No. 3817482 discloses a fixing device that includes an endless belt, a nip member disposed at an internal space of the endless belt, and a pressure roller that opposes the nip member so as to interpose the endless belt between the pressure roller and the nip member. More specifically, the nip member is subjected to machining to have a convex surface in contact with the endless belt and having a central portion and end portions in an axial direction of the endless belt. The central portion has a protruding amount protruding toward the pressure roller greater than that of the end portions. In this way, wrinkling of recording sheets can be prevented. 
     SUMMARY 
     However, with the conventional technology, the protruding amount of the central portion of the nip member must be directly adjusted by machining the surface of the nip member to be in contact with the endless belt. Here, accurate machining is troublesome, and dimensional error may occur in the amount of protrusion. 
     In view of the foregoing, it is an object of the present invention to provide a fixing device capable of reducing dimensional error in the protrusion amount of the central portion of the nip member. 
     In order to attain the above and other objects, the present invention provides a fixing device that may include a nip member, an endless belt, a rotating member, and a stay. The endless belt may have an inner peripheral surface and an outer peripheral surface. The inner peripheral surface may be configured to be in sliding contact with the nip member in a sliding direction. The rotating member may be configured to nip the endless belt in cooperation with the nip member, and may be configured to constitute a nip region between the endless belt and the rotating member. The rotating member may have an axis defining an axial direction. The stay may be disposed opposite to the nip region with respect to the nip member and may have a first supporting face configured to support the nip member and a second supporting face configured to support the nip member. The second supporting face may be spaced apart from the first supporting face in the sliding direction and may be disposed downstream of the first supporting face in the sliding direction. The first supporting face may have a first upstream edge and a first downstream edge positioned downstream of the first upstream edge in the sliding direction. The first downstream edge may have one side portion as a first portion, another side portion as a third portion, and a central portion as a second portion in the axial direction. The first portion may be positioned opposite to the third portion in the axial direction. The second portion may be positioned between the first portion and the third portion. The second supporting face may have a second upstream edge and a second downstream edge positioned downstream of the second upstream edge in the sliding direction. The second downstream edge may have one side portion as a fourth portion, another side portion as a sixth portion, and a central portion as a fifth portion in the axial direction. The fourth portion may be positioned opposite to the sixth portion in the axial direction. The fifth portion may be positioned between the fourth portion and the sixth portion. The second portion and the fifth portion may define a first distance therebetween in the sliding direction. The first portion and the fourth portion may define a second distance therebetween in the sliding direction. The third portion and the sixth portion may define a third distance therebetween in the sliding direction. The second distance and the third distance may be longer than the first distance. 
     The present invention further provides a fixing device that may include a nip member, an endless belt, a rotating member, and a stay. The endless belt may have an inner peripheral surface and an outer peripheral surface. The inner peripheral surface may be configured to be in sliding contact with the nip member in a sliding direction. The rotating member may be configured to nip the endless belt in cooperation with the nip member, and may be configured to constitute a nip region between the endless belt and the rotating member. The rotating member may have an axis defining an axial direction. The stay may have a first supporting face configured to support the nip member and a second supporting face configured to support the nip member. The second supporting face may be spaced apart from the first supporting face in the sliding direction and may be disposed downstream of the first supporting face in the sliding direction. The first supporting face may have a first upstream edge and a first downstream edge positioned downstream of the first upstream edge in the sliding direction. The first downstream edge may have one side portion as a first portion, another side portion as a third portion, and a central portion as a second portion in the axial direction. The first portion may be positioned opposite to the third portion in the axial direction. The second portion may be positioned between the first portion and the third portion. The second supporting face may have a second upstream edge and a second downstream edge positioned downstream of the second upstream edge in the sliding direction. The second downstream edge may have one side portion as a fourth portion, another side portion as a sixth portion, and a central portion as a fifth portion in the axial direction. The fourth portion may be positioned opposite to the sixth portion in the axial direction. The fifth portion may be positioned between the fourth portion and the sixth portion. The second portion and the fifth portion may define a first distance therebetween in the sliding direction. The first portion and the fourth portion may define a second distance therebetween in the sliding direction. The second distance may be longer than the first distance. 
    
    
     
       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 cross-sectional view of the fixing device; 
         FIG. 3  is an exploded perspective view showing a halogen lamp, a nip plate, a reflection plate, a pressure roller, and a stay; 
         FIG. 4A  is a bottom view showing each position of a first supporting surface and a second supporting surface; 
         FIG. 4B  is a cross sectional view taken along line I-I of  FIG. 4A ; 
         FIG. 4C  is a cross sectional view taken along line II-II of  FIG. 4A ; 
         FIG. 4D  is a cross sectional view taken along line III-III of  FIG. 4A ; 
         FIG. 5  shows a stay according to a first modification of the present invention; 
         FIG. 6  shows a stay according to a second modification of the present invention; 
         FIGS. 7A and 7B  show an end portion of a stay according to a third modification of the present invention; and 
         FIG. 7C  shows a center portion of the stay according to the third modification of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A general structure of a laser printer as an image forming device according to one embodiment of the present invention will be described with reference to  FIG. 1 . 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 while referring to  FIGS. 2 to 4D . 
     &lt;General Structure of Laser Printer&gt; 
     As shown in  FIG. 1 , the laser printer  1  includes a main frame  2 . 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 (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” 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 of the figure are a rear side and a front side of the printer, 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  and  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  and  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  and  43 , and reflection mirrors  44 ,  45  and  46 . In the exposure unit  4 , the laser emission unit is adapted to project a laser beam 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  and  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 doctor blade  73  for regulating toner thickness, and a toner accommodating portion  74  in which toner 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 doctor 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  FIGS. 2 and 3 , the fixing device  100  includes a fusing belt  110 , a halogen lamp  120 , a nip plate  130 , a reflection plate  140 , a pressure roller  150 , and a stay  160 . In  FIG. 3 , for the sake of convenience a length of the pressure roller  150  in a leftward/rightward direction is shown as being shorter than that of the nip plate  130 , but in actuality the length of the pressure roller  150  in the leftward/rightward direction is approximately the same as that of the nip plate  130 . (See  FIG. 4D .) 
     The fusing belt  110  is a heat-resistant and flexible endless belt. The fusing belt  110  has a metallic tube and a fluorocarbon resin layer coated thereover. The metallic tube is made from stainless steel. The fusing belt  110  has an inner peripheral surface  111  in sliding contact with the nip plate  130 , and an outer peripheral surface  112  in sliding contact with the pressure roller  150 . 
     The inner peripheral surface  111  is in sliding contact with the nip member and runs rearward relative to the nip plate  130 . Here, the sliding contact direction of the inner peripheral surface  111  relative to the nip plate  130  refers to an average direction in which the inner peripheral surface  111  is in sliding contact with any points of the nip plate  130  in the frontward/rearward direction. In this embodiment, the sliding contact direction refers to a direction extending in the frontward/rearward direction in  FIG. 2 . In other words, the sliding contact direction refers to a direction that extends from an upstream end to a downstream end of a nip region NP relative to a rotation direction of the pressure roller  150 . 
     As a modification to the fusing belt  110 , a rubber layer can be provided between the metallic tube and the fluorocarbon resin layer. 
     The halogen lamp  120  is a heater to generate a radiant heat to heat the nip plate  130  and the fusing belt  110  for heating toner on the sheet S. The halogen lamp  120  is positioned at the internal space of the fusing belt  110  such that the halogen lamp  120  is spaced away from the inner peripheral surface of the fusing belt  110  as well as an inner (upper) surface of the nip plate  130  by a predetermined distance. 
     The nip plate  130  is an elongated member extending in the leftward/rightward direction, and is formed into a substantially plate-like shape. The nip plate  130  is disposed to be in sliding contact with the inner peripheral surface  111  of the tubular fusing belt  110 . The nip plate  130  is adapted to transfer the radiant heat received from the halogen lamp  120  and onto the toner on the sheet P through the fusing belt  110 . 
     This nip plate  130  is formed into a planar shape and is made from a metal, for example, aluminum, so as to have a thermal conductivity higher than that of a stay  160  made from a steel (described later). This nip plate  130  has a thickness permitting bending deformation thereof. The surface of the nip plate  130  that is in contact with the inner peripheral surface  111  of the fusing belt  110  can be coated with, for example, a metal oxide film or a fluororesin layer. Moreover, the thickness of the nip plate  130  can be ranging from 0.1 to 3.0 mm, or 0.3 to 2.0 mm, or 0.1 to 1.0 mm. 
     The reflection plate  140  is adapted to reflect radiant heat from the halogen lamp  120  toward the nip plate  130 . As shown in  FIG. 2 , the reflection plate  140  is positioned within the fusing belt  110  and surrounds the halogen lamp  120 , with a predetermined distance therefrom. Thus, radiant heat from the halogen lamp  120  can be efficiently concentrated onto the nip plate  130  to promptly heat the nip plate  130  and the fusing belt  110 . 
     The reflection plate  140  is configured into substantially U-shape in cross-section and is made from a material such as aluminum having high reflection ratio for infrared rays or far infrared rays. The reflection plate  140  has substantially a U-shaped reflection portion  141  and a flange portion  142  extending outward from each end portion of the reflection portion  141  in the frontward/rearward direction. A mirror surface finishing is applicable on the surface of the aluminum reflection plate  140  for specular reflection in order to enhance heat reflection ratio. 
     The pressure roller  150  is an elastically deformable member. The pressure roller  150  is disposed downward of the nip plate  130  to vertically oppose the outer peripheral surface  112  of the fusing belt  110 . The pressure roller  150  is rotatable about an axis extending in the leftward/rightward direction. The pressure roller  150  is configured to provide the nip region NP in cooperation with the fusing belt  110 , when the fusing belt  110  is nipped between the pressure roller  150  and the nip plate  130  while the pressure roller  150  is in an elastically deformed state. 
     The pressure roller  150  has a metallic shaft  151  and a rubber layer  152  formed over an outer periphery of the shaft  151 . The shaft  151  is formed into a linear shape, with a radius that is substantially constant across the leftward/rightward direction. 
     The rubber layer  152  has a first end portion  152 A, a central portion  152 B, and a second end portion  152 C, in the axial direction (leftward/rightward direction) of the pressure roller  150 . The rubber layer  152  is formed into a concave shape such that respective outer diameters of the end portions  152 A and  152 C are larger than an outer diameter of the central portion  152 B when fixing operation is not being performed (heat is not being applied) and when fixing operation is being performed. In other words, the rubber layer  152  is formed such that the end portions  152 A and  152 C are thicker than the central portion  152 B. 
     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 belt  110  is circularly moved along the nip plate  130  because of a friction force generated therebetween or between the sheet P and the fusing belt  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 NP between the pressure roller  150  and the fusing belt  110 . 
     The stay  160  is adapted to support the end portions of the nip plate  130  through the flange portion  142  for maintaining rigidity of the nip plate  130 . The stay  160  is positioned on the opposite side of the nip region NP with respect to the nip plate  130 . The stay  160  has a substantially U-shape configuration in conformity with the outer shape of the reflection portion  141  covering the reflection plate  140 . For fabricating the stay  160 , a highly rigid member such as a steel plate is folded into substantially U-shape. 
     The stay  160  is disposed upward of the reflection plate  140 . The stay  160  has a top wall  161 , a front wall  162 , and a rear wall  163 . The top wall  161  is formed into a planar shape. The front wall  162  extends downward from a front end of the top wall  161 . The rear wall  163  extends downward from a rear end of the top wall  161 . As shown in  FIG. 3 , the rear wall  163  is formed into an arcuate shape in cross-sectional view and has a central portion and end portions in the leftward/rightward direction, with the central portion recessed inward (frontward) more than the end portions in the frontward/rearward direction. In addition, the reflection portion  141  of the reflection plate  140  has a rear wall which is also formed into an arcuate shape in cross-sectional view in conformance with the shape of the rear wall  163 . The stay  160  and the reflection plate  140  are formed into these respective shapes using press working. 
     The stay  160  has left and right end portions that are respectively supported by left and right side frames SF (only a left side frame is shown in  FIG. 3 ). The side frames SF are vertically movably supported by a fixing frame (not shown) of the fixing device  100 . In addition, the nip plate  130  and the reflection plate  140  are supported indirectly by the side frames SF through the stay  160 . 
     Coil springs CS (only a left coil spring is shown in  FIG. 3 ) are provided for urging the respective side frames SF downward. Thus, the side frames SF press the nip plate  130  toward the pressure roller  150  through the stay  160  and the reflection plate  140 . Incidentally, as modifications, the halogen lamp  120  can be supported by the side frames SF or by the fixing frame. Further, the stay  160  and the nip plate  130  can be fixed to the fixing frame, whereas the pressure roller  150  is urged toward the nip plate  130  by a urging member. Moreover, instead of the coil spring CS, a combination of an arm and a coil spring is available. 
     As shown in  FIG. 2 , the front wall  162  has a lower end at which is located an end face constituting a first supporting face  164  that supports the nip plate  130  through the flange portion  142  of the reflection plate  140 . The rear wall  163  has a lower end at which is located an end face constituting a second supporting face  165  that supports the nip plate  130  through the flange portion  142  of the reflection plate  140 . 
     As shown in  FIG. 4A , the first supporting face  164  has a first downstream edge  164 A and a first upstream edge  164 B. The first downstream edge  164 A is located at a rear side, i.e. downstream in the sliding direction, of the first supporting face  164 , and the first upstream edge  164 B is located at a front side, i.e. upstream in the sliding direction, of the first supporting face  164 . In other words, the first downstream edge  164 A is located on a downstream of the first supporting face  164 , and the first upstream edge  164 B is located on an upstream of the first supporting face  164 , in a direction of conveyance of the sheets P. 
     The first downstream edge  164 A has a first point (first portion) A 1 , a second point (second portion) A 2 , and a third point (third portion) A 3 . The first point A 1  is located at the first end side; the second point A 2  is positioned at the central portion; the third point A 3  is located at the second end side in the leftward/rightward direction, i.e. the axial direction of the pressure roller  150 . The first point A 1 , the second point A 2 , and the third point A 3  are located on a straight line extending in the leftward/rightward direction, and are thus arrayed in the leftward/rightward direction. The first upstream edge  164 B is formed in conformance with the first downstream edge  164 A. 
     The second supporting face  165  is located downstream in the sliding direction of, and spaced away from, the first supporting face  164 . The second supporting face  165  has a second downstream edge  165 A and a second upstream edge  165 B. The second downstream edge  165 A is located downstream in the sliding direction of the second supporting face  165 , and the second upstream edge  165 B is located upstream in the sliding direction of the second supporting face  165 . The second upstream edge  165 B has a first end side, a central portion, and a second end side in the leftward/rightward direction, at which are respectively located a fourth point (fourth portion) B 4 , a fifth point (fifth portion) B 5 , and a sixth point (sixth portion) B 6 . The fourth point B 4 , the fifth point B 5 , and the sixth point B 6  are located on a convex line protruding toward the sliding direction upstream, such that the fifth point B 5  is located further toward the sliding direction upstream than the fourth point B 4  and the sixth point B 6 . The second downstream edge  165 A is formed in conformance with the convex-shaped second upstream edge  165 B. 
     A first distance L is defined as a distance in the frontward/rearward direction from the second point A 2  to the fifth point B 5 . Meanwhile, a second distance L 2  is defined as a distance in the frontward/rearward direction from the first point A 1  to the fourth point B 4 . The second distance L 2  is larger than the first distance L 1 . In addition, a third distance L 3  is defined as a distance in the frontward/rearward direction from the third point A 3  to the sixth point B 6 . The third distance L 3  is larger than the first distance L 1 , since the second supporting face  165  forms protruding shape. 
     Moreover, the difference between the first distance L 1  and the second distance L 2  (i.e. L 2 −L 1 ) and the difference between the first distance L 1  and the third distance L 3  (i.e. L 3 −L 1 ) can be ranging from 0.5 to 10.0 mm, or 1.0 to 7.0 mm, or 1.5 to 5.0 mm. 
     By making the distance L 2  between the first end sides (the first point A 1  and the fourth point B 4 ) and the distance L 3  between the second end sides (the third point A 3  and the sixth point B 6 ) greater than the distance L 1  between the central portions (the second point A 2  and the fifth point B 5 ) of the first downstream edge  164 A and the first upstream edge  164 B. Accordingly, a pair of ends  131  of the nip plate  130  in the leftward/rightward direction becomes more flexible than the central portion of the nip plate  130  in the leftward/rightward direction, as shown in exaggerated fashion in  FIGS. 4B and 4C . In this way, as shown in  FIG. 4D , the nip plate  130  can be imparted with a convex shape wherein the central portion  132  in the leftward/rightward direction protrudes farther than the ends  131  toward the pressure roller  150 . Incidentally, members such as the reflection plate  140  and the fusing belt  110  have been omitted in  FIG. 4D  for the sake of convenience. 
     Since the distance between the first downstream edge  164 A and the second upstream edge  165 B differs in the axial direction, the amount of protrusion of the central portion  132  can be adjusted properly. Accordingly, errors in the amount of protrusion can be reduced in comparison to conventional technology wherein the amount of protrusion of the central portion of the nip member is adjusted directly by performing machining (press working) on the surface of the nip member in contact with the fixing belt. 
     In addition, the supporting faces  164  and  165  can be easily formed by the aforementioned machining (press working), because the downstream edges  164 A and  165 A are arrayed in parallel as well as the upstream edges  164 B and  165 B. Comparatively, if these edges are not formed in conformance with each other, the machining can be more difficult. 
     The first point A 1 , the second point A 2 , the third point A 3 , the fourth point B 4 , the fifth point B 5 , and the sixth point B 6  are disposed within a sheet width BB. Here, the sheet width BB refers to a width of one of multiple types of sheets P that can be specified for the laser printer  1 . In other words, the fixing device  100  is configured to convey sheets P within a conveyance region having a prescribed width in the leftward/rightward direction (the same width as the sheet width BB shown), and to the nip region NP. Here, the conveyance region can be defined as an area where the nip region NP and the conveyed sheet P overlaps with each other, when viewed in the vertical direction. 
     Incidentally, the sheet width BB for determining respective positions of the points A 1  to B 6  can be 176 mm to conform to B 5  size, 215.9 mm to conform to letter or legal size, or 210 mm to conform to A4 size, of the International Organization for Standardization (ISO). 
     By thus locating the respective points A 1  to B 6  within the sheet width BB, the nip region NP within the applicable sheet width BB can be formed into a convex shape such as that described above, and wrinkling of the sheets P conforming to the sheet width BB can be prevented effectively. 
     The respective points A 1  to B 6  will now be described more specifically. The first point A 1  and the fourth point B 4  are disposed within a range at least 55 mm and at most 107 mm from a conveyance center line CL of the conveyance path of the sheets P in the axial direction. The third point A 3  and the sixth point B 6  are disposed within a range at least 55 mm and at most 107 mm from the conveyance center line CL of the conveyance path of the sheets P in the axial direction. 
     The first point A 1  and the fourth point B 4  are disposed within a range at least 60 mm and at most 95 mm from the conveyance center line CL in the axial direction. The first point A 1  and the fourth point B 4  are disposed within a range at least 60 mm and at most 95 mm from the conveyance center line CL in the axial direction. 
     Here, the conveyance center line CL refers to a line which constitutes a conveyance reference line when respective sheets P of various types differing in width are conveyed without altering the position of the center portion in the leftward/rightward direction, i.e. a line which runs through the center portion of different types of sheets P being conveyed. 
     The first downstream edge  164 A and the second upstream edge  165 B are formed substantially symmetrically relative to the conveyance center line CL of the sheets P. That is, a distribution along the axial direction of distances between the first downstream edge  164 A and the second upstream edge  165 B in the sliding direction is symmetrical with respect to the conveyance center line CL of the sheets P. In other words, a distribution along the axial direction of distances between the first downstream edge  164 A and the second upstream edge  165 B in the sliding direction is symmetrical with respect to a surface which contains the conveyance center line CL and is orthogonal to the leftward/rightward direction. 
     Here, the definition of “symmetrical” includes configurations wherein a difference of up to 0.9 mm exists between two distances: one distance between the edges  164 A and  165 B when measured at a location that is on one side of the conveyance center line CL and separated from the conveyance center line CL by X mm (an arbitrary distance), and another distance between the edges  164 A and  165 B when measured at a location that is on another side of the conveyance center line CL and separated from the conveyance center line CL by X mm (the same distance as the arbitrary distance). 
     In addition, the definition of “symmetrical” also includes configurations wherein a difference of up to 0.6 mm exists between two distances: one distance between the edges  164 A and  165 B when measured at a location that is on one side of the conveyance center line CL and separated from the conveyance center line CL by X mm (an arbitrary distance), and another distance between the edges  164 A and  165 B when measured at a location that is on another side of the conveyance center line CL and separated from the conveyance center line CL by X mm (the same distance as the arbitrary distance) in the leftward/rightward direction. 
     In addition, the definition of “symmetrical” also includes configurations wherein a difference of up to 0.4 mm exists between two distances: one distance between the edges  164 A and  165 B when measured at a location that is on one side of the conveyance center line CL and separated from the conveyance center line CL by X mm (an arbitrary distance) in the leftward/rightward direction, and another distance between the edges  164 A and  165 B when measured at a location that is on another side of the conveyance center line CL and separated from the conveyance center line CL by X mm (the same distance as the arbitrary distance). 
     According to these configurations, the sheets P can be more readily conveyed on a straight path following the conveyance center line CL in comparison to configurations wherein the distances between the first downstream edge  164 A and the second upstream edge  165 B in the sliding direction are not symmetrical. 
     As indicated by a broken line in  FIG. 4A , the nip region NP is defined so as to be located downstream in the sliding direction of the first downstream edge  164 A and upstream of the second upstream edge  165 B. That is, the nip region NP is defined so as to not protrude out from the first downstream edge  164 A and the second upstream edge  165 B in the frontward/rearward direction. The pressure roller  150  is thereby brought into pressing contact with the nip plate  130  between the supporting faces  164  and  165 . Thus the ends  131  and the central portion  132  of the nip plate  130  can be bent by an intended amount, and the amount of protrusion of the central portion  132  can be adjusted effectively. 
     Incidentally, the present invention is not limited to the above-described embodiment, and can be utilized according to a variety of modifications, as will be described below. In the descriptions below, members having a structure substantially identical to that in this embodiment are assigned by the same numerals and characters as those shown in this embodiment. 
     In this embodiment, the first downstream edge  164 A was formed into a straight shape parallel to the leftward/rightward direction, and the second upstream edge  165 B was formed into an arcuate shape protruding frontward. However, the present invention is not limited to this configuration. For example, as shown in  FIG. 5 , a first modification is available wherein the first downstream edge  164 A is formed into a convex shape protruding toward downstream in the sliding direction, and the second upstream edge  165 B is formed into a straight shape parallel to the leftward/rightward direction. 
     More specifically, in this embodiment, the first downstream edge  164 A is formed such that the second point A 2  at the central portion is located downstream in the sliding direction of the first point A 1  and the third point A 3 . In this case as well, the distance between the respective first end sides (the first point A 1  and the fourth point B 4 ) and the distance between the respective second end sides (the third point A 3  and the sixth point B 6 ) of the first downstream edge  164 A and the second upstream edge  165 B can be made larger than the distance between the respective central portions (the second point A 2  and the fifth point B 5 ). Thus, a similar effect as with the above-described embodiment can be achieved. 
     In addition, as shown in  FIG. 6 , a second modification is available wherein the first downstream edge  164 A is formed into a convex shape protruding toward downstream in the sliding direction, and the second upstream edge  165 B is formed into a convex shape protruding toward upstream in the sliding direction. More specifically, in this modification, the first downstream edge  164 A is formed such that the second point A 2  at the central portion is located upstream in the sliding direction of the first point A 1  and the third point A 3 . 
     The second upstream edge  165 B is formed such that the fifth point B 5  at the central portion is located upstream in the sliding direction of the fourth point B 4  and the sixth point B 6 . In this case as well, the distance between the respective first end sides (the first point A 1  and the fourth point B 4 ) and the distance between the respective second end sides (the third point A 3  and the sixth point B 6 ) can be made larger than the distance between the respective central points (the second point A 2  and the fifth point B 5 ), and thus a similar effect as with the above-described embodiment can be achieved. 
     In addition, as shown in  FIG. 6 , the first downstream edge  164 A and the second upstream edge  165 B can also be formed in parallel in the axial direction of the pressure roller  150  within a minimum sheet width BS. Here, the minimum sheet width BS refers to a width of sheets PS having the minimum width that can be specified with the laser printer  1 , in other words a minimum sheet width that can be specified using a width guide of the sheet supply tray  31 . For example, the minimum sheet width BS can be set to postcard width (100 mm). 
     According to this configuration, the minimum width sheets PS can be more readily conveyed on a straight path in the frontward/rearward direction in comparison to configurations wherein a first downstream edge and a second upstream edge are not parallel in the axial direction within the minimum sheet width BS. 
     In the above-described embodiment, the nip plate  130  is formed into a substantially plate-like shape. However, the present invention is not limited to this configuration. For example, as shown in  FIG. 7A  as a third modification, a front portion  231  of a nip plate  230  can be formed into an arcuate shape so as to curve upward. In this case, lower end faces of front walls  262  and  242  can be formed so as to be more upwardly offset than lower end faces of rear walls  263  and  243  of the stay  260  and the reflection plate  240 . 
     That is, in this modification, a first supporting face  264  is disposed at a location that is more upwardly offset than a second supporting face  265 . Because a lower end portion of the front wall  262  is bent frontward, the lower end face of the front wall  262  is formed over a wide area in the frontward/rearward direction, and a portion of this wide lower end face supports a front end face  232  of the nip plate  230  through the reflection plate  240 . 
     In addition, in the third modification, as shown in  FIG. 7B , first supporting face  264  refers to a surface constituting a region wherein a portion of the wide lower end face of the front wall  262  overlaps with the front end face  232  when viewed in the vertical direction. Moreover, by making a distance LE between a pair of end points (A 1  and A 3 ) and another pair of end points (B 4  and B 6 ) larger than a distance LC between two central points A 2  and B 5  as shown in  FIG. 7C , the same effect as with the above-described embodiment can be achieved. 
     In the above-described embodiment, the pressure roller  150  as a rotating member was configured such that, when the fixing operation is not being performed, the respective diameters of the end portions  152 A and  152 C are larger than the diameter of the central portion  152 B. However, the present invention is not limited to this configuration. A pressure roller can be configured such that, at least when fixing operation is being performed, diameters of end portions are larger than a diameter of a central portion. 
     As one example of the above configuration, the pressure roller can be configured to have a shaft, an elastic layer covering the shaft, and a tube over the elastic layer, wherein a first end portion and a second end portion of the tube in the axial direction have wrinkles. In this case, when fixing operation is not being performed, the respective end portions and the central portion of the pressure roller have substantially the same diameter. However, when fixing operation is being performed, i.e. when heat is applied to the pressure roller, the wrinkles expand, and the respective diameters of the end portions of the pressure roller become larger than the diameter of the central portion. 
     As another example, the pressure roller can be configured to have a shaft and an elastic layer coating the shaft, wherein the respective diameters of a first end portion and a second end portion of the shaft are smaller than the diameter of a central portion of the shaft and, in addition, the diameter of the elastic layer is constant in the axial direction. In this case as well, when fixing operation is not being performed, the respective end portions and the central portion of the pressure roller have substantially the same diameter, but the elastic layer is thick at the end portions thereof and thin at the central portion thereof, and when fixing operation is being performed, i.e. when heat is applied to the pressure roller, the end portions of the elastic layer expand more than the central portion of the elastic layer, and the respective diameters of the end portions of the pressure roller become larger than the diameter of the central portion of the pressure roller. 
     In the above-described embodiment, the distances L 2  and L 3  between the pair of end points (A 1  and A 3 ) and the pair of end points (B 4  and B 6 ) are respectively larger than the first distance L 1 . However, the present invention is not limited to this configuration. If at least a distance between end points at one end is made larger than the first distance, distances between respective end points can be configured in any arbitrary manner. 
     In the above-described embodiment, the nip region NP was prescribed to be located downstream in the sliding direction of the first downstream edge  164 A, and upstream in the sliding direction of the second upstream edge  165 B. However, the present invention is not limited to this configuration. The nip region can be located downstream of the first point, and the third point, and upstream of the fourth point and the sixth point. That is, the nip region can protrude toward upstream of the second point and downstream of the fifth point at the central position. 
     In the above-described embodiment, the nip plate  130  supports the stay  160  through the reflection plate  140 . However, the present invention is not limited to this configuration. The nip member may support the stay directly. 
     Further, the sheet P can be an OHP sheet instead of plain paper and a postcard. 
     Further, in the depicted embodiment, the pressure roller  150  is employed as a rotating member. However, a belt like pressure member is also available. 
     Further, in the depicted embodiment, the image forming device is the monochromatic laser printer. However, a color laser printer, an LED printer, a copying machine, and a multifunction device are also available. In this case, the axial direction of one of the rollers supporting the belt constitutes the axial direction of the rotating body. 
     Further, in the depicted embodiment, the nip plate  130  is employed as a nip member. However, a block shaped member or a pad like member is also available. 
     Further, in the depicted embodiment, the halogen lamp  120  is employed as a heater. However, a carbon heater is also available.