Patent Publication Number: US-9423735-B2

Title: Fixing device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2014-074790, filed on Mar. 31, 2014, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     Aspects of invention relate to a fixing device that thermally fixes a developer image on a recording sheet. 
     BACKGROUND 
     A fixing device is known that includes an endless belt, a heating element and a nip member that are disposed in the endless belt, a backup member that nips the endless belt together with the nip member so as to form a nip together with the endless belt, and a reflection member that reflects radiant heat from the heating element towards the nip member (see JP2011095534A). Specifically, in the above technique, the reflection member is configured in a U-shape in cross-sectional view and is in contact with both edge portions of the nip member in the sheet transport direction from the opposite side with respect to the backup member. Furthermore, portions of the reflection member that are in contact with the nip member are formed so as to extend across substantially one end to substantially the other end of the nip member in the longitudinal direction (in detail, an area corresponding to one end to the other end of the nip). 
     SUMMARY 
     However, in the known technique, since the reflection member is in contact with the nip member across substantially one end to substantially the other end of the nip member in the longitudinal direction, when heating the endless belt with the heating element through the nip member at the beginning of printing, heat escapes from the end portions of the nip member to the reflection member; accordingly, temperatures of the edge portions of the endless belt may disadvantageously become insufficient. 
     Aspects of the invention may provide a fixing device that is capable of hindering the temperatures of edge portions of an endless belt from becoming insufficient at the beginning of printing. 
     The fixing device may include an endless belt and a nip member being in contact with an inner peripheral surface of the endless belt. The fixing device may further include a backup member that nips the endless belt together with the nip member forms a nip together with the endless belt. The fixing device may still further include a contact member disposed opposite the backup member with the nip member therebetween. The contact member may be in contact with the nip member. The contact member may include a first portion that extends across a width of a maximum image forming area in an axial direction of the endless belt and a second portion positioned outside the width of the maximum image forming area in the axial direction of the endless belt and positioned inside a width of the nip in the axial direction of the endless belt. A heat transfer coefficient per unit dimension between the nip member and the second portion in the axial direction may be smaller than a heat transfer coefficient per unit dimension between the nip member and the first portion in the axial direction. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a color laser printer including a fixing device according to an embodiment of the present disclosure. 
         FIG. 2  is a cross-sectional view illustrating the fixing device. 
         FIG. 3  is an exploded perspective view in which a nip plate and other components have been disassembled. 
         FIG. 4  is a perspective view in which the two end portions of the reflecting plate are illustrated in enlarged manner. 
         FIG. 5  is a diagram illustrating a relationship between the nip plate, the reflecting plate, and a stay. 
         FIG. 6  is a diagram for describing a relationship between a first portion, a second portion, and a third portion. 
         FIG. 7  is a diagram illustrating a first modification. 
         FIG. 8  is a diagram illustrating a second modification. 
         FIG. 9  is a diagram illustrating a third modification. 
         FIG. 10  is an exploded perspective view in which a heat insulation member and other components have been disassembled. 
         FIG. 11  is a plan view in which the heat insulation member is viewed from below. 
         FIG. 12  is a diagram for describing a relationship between a first portion, a second portion, and a third portion. 
         FIG. 13  is a diagram illustrating a fourth modification. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present disclosure will be described in detail next while referring to the drawings as required. Note that in the description below, if not otherwise specified, directions will be set forth such that the up-down direction illustrated in  FIG. 1  is the up-down direction, the right side in  FIG. 1  is the front direction, the left side is the rear direction, the near side with respect to the sheet surface is the left direction, and the far side with respect to the sheet surface is the right direction. The left and right herein are defined on the basis of the directions seen from a person standing on a front side of a color laser printer  1 . 
     As illustrated in  FIG. 1 , the color laser printer  1  includes, inside a device body  2 , a sheet feeding portion  5  that feeds a sheet  51  (a recording sheet), an image forming portion  6  that forms an image on the sheet  51  that has been fed thereto, and a sheet discharging portion  7  that discharges the sheet  51  on which an image has been formed. 
     The sheet feeding portion  5  in the lower portion inside the device body  2  includes a sheet feed tray  50  that is attached and detached through the front side of the device body  2  with a slide operation and a sheet feed mechanism M 1  that lifts up the front side of the sheet  51  from the sheet feed tray  50 , turns the sheet  51  over to the rear side, and transports the sheet  51 . 
     The sheet feed mechanism M 1  includes a pickup roller  52 , a separation roller  53 , and a separation pad  54  that are provided near the front end portion of the sheet feed tray  50  such that the stack of sheets  51  in the sheet feed tray  50  are separated into separate sheets and are sent upwards. The sheet  51  that has been transported upwards passes between a paper powder removing roller  55  and a pinch roller  56 , passes through a transport path  57  and is turned towards the rear, and is fed onto a transport belt  73  described later. While the sheet  51  is passing between the paper powder removing roller  55  and the pinch roller  56 , paper powder that has adhered to the sheet  51  is removed from the sheet  51  with the paper powder removing roller  55 . 
     The image forming portion  6  includes a scanner portion  61 , a processing portion  62 , a transfer portion  63 , and a fixing device  100 . 
     The scanner portion  61  is provided on the upper portion of the device body  2  and includes, although not shown, a laser emission portion, a polygon mirror, a plurality of lens, and a reflecting mirror. In the scanner portion  61 , laser that corresponds to colors, such as cyan, magenta, yellow, and black and that is emitted from the laser emission portion is scanned in the left and right directions at high speed with the polygon mirror, is passed through the plurality of lens and is reflected on the reflecting mirror, and is irradiated on photosensitive drums  31 . 
     The processing portion  62  includes a photoreceptor unit  3  that is disposed below the scanner portion  61  and above the sheet feeding portion  5  and that is movable in the front-rear direction with respect to the device body  2 . The photoreceptor unit  3  includes drum sub-units  30 , and developing cartridges  40  that are mounted on the drum sub-units  30 . 
     The drum sub-units  30  include known photosensitive drums  31  and known scorotron type electrifiers  32 . The developing cartridges  40  accommodate therein toners serving as examples of the developer and include known feed rollers  41 , known development rollers  42 , and known layer thickness regulating blades  43 . 
     The above processing portion  62  functions in the following manner. Toners inside the developing cartridges  40  are fed to the development rollers  42  with the feed rollers  41 . At this point, the toners are positively electrified by friction between the feed rollers  41  and the development rollers  42 . The toners that have been fed to the development rollers  42  are scraped by the layer thickness regulating blades  43  upon rotation of the development rollers  42  and are carried on the surface of the development rollers  42  as thin layers each with a uniform thickness. 
     Meanwhile, in the drum sub-units  30 , the scorotron type electrifiers  32  positively charge the photosensitive drums  31  in a uniform manner by corona discharge. Laser is irradiated on the charged photosensitive drums  31  from the scanner portion  61  and electrostatic latent images corresponding to the image to be formed on the sheet  51  are formed on the photosensitive drums  31 . 
     Furthermore, upon rotation of the photosensitive drums  31 , the toners carried by the development rollers  42  are supplied to the electrostatic latent images of the photosensitive drums  31 , in other words, in the surfaces of the photosensitive drums  31  positively charged in a uniform manner, the toners are supplied to portions exposed to laser and to where the potentials have been reduced. With the above, the electrostatic latent images of the photosensitive drums  31  are each turned into visible images and toner images each corresponding to a color of the corresponding toner are created by reversal development and are carried on the surfaces of the photosensitive drums  31 . 
     The transfer portion  63  includes a driving roller  71 , a driven roller  72 , the transport belt  73 , transfer rollers  74 , and a cleaning portion  75 . 
     The driving roller  71  and the driven roller  72  are disposed so as to be spaced apart from each other at the front and rear in a parallel manner, and the transport belt  73  formed of an endless belt is wound around the driving roller  71  and the driven roller  72 . The outer surface of the transport belt  73  is in contact with each of the photosensitive drums  31 . Furthermore, the transfer rollers  74  that nip the transport belt  73  together with the photosensitive drums  31  are disposed inside the transport belt  73 . Transfer biases are applied to the transfer rollers  74  from a high voltage substrate (not shown). When forming an image, the sheet  51  that has been transported with the transport belt  73  is nipped between the photosensitive drums  31  and the transfer rollers  74  and the toner images on the photosensitive drums  31  are transferred onto the sheet  51 . 
     The cleaning portion  75  is disposed below the transport belt  73 . The cleaning portion  75  removes the toner adhered to the transport belt  73  and drops the removed toner into a toner reservoir  76  disposed therebelow. 
     The fixing device  100  is provided on the rear side with respect to the transfer portion  63  and thermally fixes the toner image, which has been transferred onto the sheet  51 , on the sheet  51 . Note that a detailed description of the fixing device  100  will be given later. 
     In the sheet discharging portion  7 , a sheet-discharge-side transport path  91  of the sheet  51  is formed so as to extend upwards from the exit of the fixing device  100  and turn over towards the front side. A plurality of transport rollers  92  that transport the sheet  51  are disposed through the sheet-discharge-side transport path  91 . A sheet discharge tray  93 , which accumulates the sheet  51  to which printing has been performed, is formed on the upper surface of the device body  2 . The sheets  51  that have been discharged from the sheet-discharge-side transport path  91  with the transport rollers  92  are accumulated on the sheet discharge tray  93 . 
     Detailed Configuration of the Fixing Device 
     As illustrated in  FIG. 2 , the fixing device  100  mainly includes a fixing belt  110  serving as an example of an endless belt, a halogen lamp  120  serving as an example of a heating element, a nip plate  130  serving as an example of a nip member, a reflecting plate  140  serving as an example of a contact member, a pressure roller  150  serving as an example of a backup member, and a stay  160 . 
     Note that in the following description, the transport direction of the sheet  51  (substantially the front-rear direction) is merely referred to as a “transport direction” and the axial direction of the fixing belt  110  (substantially the left-right direction) is merely referred to as an “axial direction”. Furthermore, the pressing direction of the pressure roller  150  (substantially the up-down direction) is merely referred to as a “pressing direction”. 
     The fixing belt  110  is a heat resistant and flexible endless (tubular) belt. The fixing belt  110  is configured so as to be rotatable and the two edge portions in the axial direction are guided by a guide member (not shown). 
     Note that the fixing belt  110  may be configured as a metal belt including a metal base material and resin coated on the outer periphery of the base material, may be configured so as to have a rubber layer on a surface of a metal, or may be configured so as to further have a protective layer formed of nonmetal, such as a fluorine coating, on the surface of the rubber layer. 
     The halogen lamp  120  is a heating element that heats the toner on the sheet  51  by heating the nip plate  130  and the fixing belt  110  and is disposed inside the fixing belt  110  while being spaced apart at a predetermined distance with the inner surfaces of the fixing belt  110  and the nip plate  130 . 
     The nip plate  130  receives pressing force of the pressure roller  150  and is a plate-shaped member that transmits radiant heat from the halogen lamp  120  to the toner on the sheet  51  through the fixing belt  110 . The nip plate  130  is disposed so as to be in contact with the inner peripheral surface of the tubular fixing belt  110 . 
     The nip plate  130  includes a metal plate. The metal plate may be an aluminum plate or may be an SUS plate. 
     The nip plate  130  is formed by bending, for example, an aluminum plate that has a thermal conductivity that is greater than that of the steel stay  160  described later into a substantially U-shape in cross-sectional view. In more detail, the nip plate  130  in cross-sectional view mainly includes a base portion  131  that extends in the transport direction and sidewall portions  132  that extend upwards from each of the edge portions of the base portion  131  in the front-rear direction. 
     The base portion  131  is bent and formed such that a middle portion  131 A in the transport direction forms a convexity extending towards the pressure roller  150  side (downwards) with respect to the two edge portions  131 B. Note that a black coating or a heat absorption member may be provided on the inner surface (the upper surface) of the base portion  131 . With the above, radiant heat from the halogen lamp  120  can be efficiently absorbed. 
     As illustrated in  FIG. 3 , the nip plate  130  further includes an insertion portion  133  that extends in a tabular manner from the right end portion of the base portion  131  and an engagement portion  134  that is formed at the left end portion of the base portion  131 . The engagement portion  134  is formed in a U-shape in side view and engagement holes  134 B are provided in sidewall portions  134 A that have been formed by being bent upwards. 
     As illustrated in  FIG. 2 , the reflecting plate  140  is a member that reflects the radiant heat (mainly the radiant heat radiated in the front-rear direction and the upper direction) from the halogen lamp  120  towards the nip plate  130  (the inner surface of the base portion  131 ) and is disposed inside the fixing belt  110  so as to surround the halogen lamp  120  while being spaced apart at a predetermined distance from the halogen lamp  120 . 
     With such a reflecting plate  140 , radiant heat from the halogen lamp  120  is collected to the nip plate  130 ; accordingly, the radiant heat from the halogen lamp  120  can be used efficiently and the nip plate  130  and the fixing belt  110  can be heated promptly. 
     Furthermore, the reflecting plate  140  is disposed on the opposite side with respect to the pressure roller  150  with the nip plate  130  therebetween and receives force from the pressure roller  150  by being in contact with the nip plate  130 . Note that in the present embodiment, a pressing mechanism (not shown) presses the stay  160  downwards. With the above, the pressing force from the pressing mechanism is transmitted to the pressure roller  150  through the stay  160 , the reflecting plate  140 , the nip plate  130 , and the fixing belt  110 . Furthermore, reaction force against the pressing force is generated towards the upper direction from the pressure roller  150 . The reaction force is received by the reflecting plate  140  through the fixing belt  110  and the nip plate  130 . 
     Note that opposite to the above, the pressure roller  150  may be biased towards the stay  160 . 
     The reflecting plate  140  includes a metal plate. For example, the metal plate may be an aluminum plate or may be an SUS plate. The thickness of the reflecting plate is 0.3 mm, for example. 
     The reflecting plate  140  is formed by bending, for example, an aluminum plate that has a large reflectivity of infrared rays and far-infrared rays into a substantially U-shape in cross-sectional view. In more detail, the reflecting plate  140  mainly includes a reflecting portion  141  having a curved shape (a substantially U-shape in cross-sectional view) and flange portions  142  that extend in the transport direction from the two edge portions of the reflecting portion  141 . Note that in order to increase the heat reflectivity, the reflecting plate  140  may be formed using an aluminum plate on which mirror finishing has been performed. 
     As illustrated in  FIG. 3 , a total of four flange-shaped lock portions  143  (only three thereof are illustrated) are formed in the two end portions of the reflecting plate  140  in the axial direction. The lock portions  143  are positioned above the flange portions  142  and, as illustrated in  FIG. 5 , are disposed so as to be engaged with lower edges of a front wall  161  and a rear wall  162  of the stay  160  described later when the nip plate  130 , the reflecting plate  140 , and the stay  160  are assembled. 
     As illustrated in  FIGS. 2 and 3 , the reflecting portion  141  includes an arcuate upper wall portion  141 A and a pair of sidewall portions  141 B that extend downwards from the front and rear edges of the upper wall portion  141 A. The lock portions  143  described above are provided at the two end portions of each of the sidewall portions  141 B in the axial direction, and U-shaped cutouts  141 C (a total of four) each open downwards is formed on the inner side of each of the lock portions  143  in the axial direction. The flange portions  142  are provided on the inner sides of the cutouts  141 C in the axial direction. In detail, the reflecting plate  140  includes the following at each of the front and rear portions thereof: a pair of lock portions  143  that are spaced apart from each other in the axial direction, a pair of cutouts  141 C that are disposed on the inner side of the lock portions  143  in the axial direction, and a flange portion  142  that is disposed between the pair of cutouts  141 C. 
     Among the front and rear flange portions  142 , the underside of the flange portion  142  on the front side (on the upstream side in the transport direction) is an upstream supporting surface  142 F that supports the edge portion  131 B on the upstream side of the nip plate  130 . Furthermore, the underside of the flange portion  142  on the rear side (on the downstream side in the transport direction) is a downstream supporting surface  142 R that supports the edge portion  131 B on the downstream side of the nip plate  130 . 
     The downstream supporting surface  142 R is set apart from the upstream supporting surface  142 F and is disposed on the downstream side in the transport direction (the moving direction of the fixing belt  110  relative to the nip) with respect to the upstream supporting surface  142 F. Furthermore, the cutouts  141 C described above are formed in both of the upstream supporting surface  142 F and the downstream supporting surface  142 R. 
     As illustrated in  FIG. 4 , each of the cutouts  141 C is constituted by a first surface C 1  that is disposed so as to be spaced apart from the nip plate  130  in the up-down direction, a second surface C 2  that extends downwards from the end of the first surface C 1  on the outer side in the axial direction, a third surface C 3  that extends downwards from the end of the first surface C 1  on the inner side in the axial direction, a fourth surface C 4  that extends outwardly in the transport direction from the lower end of the second surface C 2 , and a fifth surface C 5  that extends outwardly in the transport direction from the lower end of the third surface C 3 . Note that a length Lc of each of the cutouts  141 C in the axial direction may be 2.0 to 5.0 mm, 5.0 to 10.0 mm, 2.0 to 15.0 mm, or 3.0 to 25.0 mm. 
     As illustrated in  FIG. 2 , the pressure roller  150  nips the fixing belt  110  together with the nip plate  130 , is a member that forms a nip portion together with the fixing belt  110 , and is disposed below the nip plate  130 . In more detail, the pressure roller  150  forms a nip together with the fixing belt  110  by pressing the nip plate  130  through the fixing belt  110 . 
     The pressure roller  150  includes a cylindrical roller body  151  and a shaft  152  that is inserted in the roller body  151  and that is rotatable together with the roller body  151 . The roller body  151  can be elastically deformed. 
     The pressure roller  150  is configured so as to be rotationally driven by transmission of a driving power from a motor (not shown) provided inside the device body  2 . By being rotationally driven, the pressure roller  150 , with the frictional force between the fixing belt  110  (or the sheet  51 ), makes the fixing belt  110  rotate in a driven manner. 
     The sheet  51  on which the toner images have been transferred is transported between the pressure roller  150  and the heated fixing belt  110  (the nip); accordingly, the toner images (toners) are thermally fixed thereon. 
     The stay  160  is a metal member that secures the rigidity of the nip plate  130  by supporting the two edge portions  131 B of the nip plate  130  (the base portion  131 ) in the transport direction. The stay  160  has a shape (a substantially U-shape in cross-sectional view) that extends along the shape of the outer surface of the reflecting plate  140  (the reflecting portion  141 ) and is disposed so as to cover the reflecting plate  140 . Such a stay  160  is formed by bending, for example, a steel plate that has a relatively high rigidity into a substantially U-shape in cross-sectional view. 
     As illustrated in  FIGS. 3 and 5 , a plurality of support portions  163  are provided so as to protrude downwards in the lower edges of the front wall  161  and the rear wall  162  of the stay  160 . Each of the support portions  163  supports the nip plate  130  through the flange portions  142  of the reflecting plate  140 . 
     Furthermore, a lock portion  165  having a substantially L-shape that extends downwards and, further, leftwards is provided in each of the right end portions of the front wall  161  and the rear wall  162  of the stay  160 . The right end portion of the nip plate  130  is supported by the lock portions  165 . Furthermore, a holding portion  167  that extends towards the left from the upper wall  166  and that is bent in a substantially U-shape in side view is provided at the left end of the stay  160 . Engagement bosses  167 B (only the engagement boss  167 B on one side is illustrated) that engage with the engagement holes  134 B of the nip plate  130  described above and that extend towards the inner side are provided on inner surfaces of sidewall portions  167 A of the holding portion  167 . 
     As illustrated in  FIGS. 2 and 3 , abutment bosses  168 , four in total, that protrude towards the inner side are provided at the two end portions of the inner surfaces of the front wall  161  and the rear wall  162  of the stay  160  in the axial direction. The abutment bosses  168  abuts against the reflecting plate  140  (the reflecting portion  141 ) in the transport direction. With the above, even when the reflecting plate  140  is about to be moved in the front-rear direction with the vibration or the like generated when the fixing device  100  is driven, the displacement of the reflecting plate  140  in the transport direction is restricted with the abutting abutment bosses  168 . As a result, the reflecting plate  140  can be prevented from being out of position in the transport direction. 
     Details of the Reflecting Plate 
     A structure of the reflecting plate  140  will be described in detail next with reference to  FIGS. 5 and 6 . Note that in  FIG. 5 , a first plane P 1  illustrated by a virtual line is a plane that passes through the transport center of the sheet  51  and that is orthogonal to the axial direction. Note that the transport center is a center of the sheet  51 , which is transported by the fixing device  100 , in the axial direction. 
     Note that in the present embodiment, a transporting method in which the transport center of the sheet  51  is aligned with the substantially center portion of the nip plate  130  in the left-right direction is adopted as the transporting method of the sheet  51 ; however, the transporting method is not limited to the above method and, for example, a transporting method in which an end of the sheet in the left-right direction is brought near to one end side of the nip plate in the left-right direction may be adopted. 
     Furthermore, referring to  FIG. 6 , a second plane P 2  illustrated by a virtual line is a plane that passes through one edge of a maximum image forming area W 1  and that is orthogonal to the axial direction, and a third plane P 3  illustrated by a virtual line is a plane that passes through the other edge of the maximum image forming area W 1  and that is orthogonal to the axial direction. Note that the maximum image forming area W 1  refers to a width of the image having the largest dimension in the axial direction that can be formed by the color laser printer  1  (that can be fixed by the fixing device  100 ). Note that in a printer that is capable of performing printing without any margin, the value of the maximum image forming area W 1  is the same as the value of a maximum sheet passing width W 2  described later. 
     Furthermore, a fourth plane P 4  illustrated by a virtual line is a plane that passes through one edge of the sheet  51  in the axial direction, the sheet  51  having the maximum sheet passing width W 2 , and that is orthogonal to the axial direction, and a fifth plane P 5  illustrated by a virtual line is a plane that passes through the other edge of the sheet  51  in the axial direction, the sheet  51  having the maximum sheet passing width W 2 , and that is orthogonal to the axial direction. Note that the maximum sheet passing width W 2  refers to a width of the sheet  51  having the largest dimension in the axial direction that can be printed by the color laser printer  1  (that can be fixed by the fixing device  100 ). 
     Furthermore, a sixth plane P 6  illustrated by a virtual line is a plane that passes through one edge of the nip in the axial direction and that is orthogonal to the axial direction, and a seventh plane P 7  illustrated by a virtual line is a plane that passes through the other edge of the nip in the axial direction and that is orthogonal to the axial direction. In other words, the length from the sixth plane P 6  to the seventh plane P 7  is a width W 3  of the nip in the axial direction. Furthermore, in the present embodiment, the relationship between the maximum image forming area W 1 , the maximum sheet passing width W 2 , and the width W 3  of the nip is W 1 &lt;W 2 &lt;W 3 . 
     As illustrated in  FIG. 6 , the reflecting plate  140  includes a first portion  140 A that extends across the whole width of the maximum image forming area W 1  in the axial direction, a pair of second portions  140 B positioned outside the maximum image forming area W 1  in the axial direction and inside the width W 3  of the nip in the axial direction, and a pair of third portions  140 C positioned outside of the width W 3  of the nip in the axial direction. 
     The first portion  140 A is a portion of the reflecting plate  140  between the second plane P 2  and the third plane P 3  and includes the middle portion of the reflecting portion  141 , the flange portions  142 , the third surfaces C 3 , and the fifth surfaces C 5 , which have been described above. A length L 1  of the first portion  140 A in the axial direction is the same as the width of the maximum image forming area W 1 . 
     The second portions  140 B are portions of the reflecting plate  140  between the second plane P 2  and the sixth plane P 6  and between the third plane P 3  and the seventh plane P 7  and include portions of the reflecting portion  141  and portions of the first surfaces C 1 . A length L 2  of each of the second portions  140 B in the axial direction is shorter than the length L 1  of the first portion  140 A in the axial direction and is longer than a length L 3  of each of the third portions  140 C in the axial direction. 
     Furthermore, the second portions  140 B do not come in contact with the nip plate  130 . In other words, a second heat transfer coefficient Q 2  per unit dimension between the nip plate  130  and each of the second portions  140 B in the axial direction is smaller than a first heat transfer coefficient Q 1  per unit dimension between the nip plate  130  and the first portion  140 A in the axial direction. Here, each of the heat transfer coefficients Q 1  and Q 2  is to satisfy the following expression (1) when the length L 2  of the second portions  140 B is given as the unit dimension.
 
 Q 2&lt; Q 1· L 2/ L 1.  (1)
 
     Note that the heat transfer coefficient in the present disclosure indicates the degree of heat transmission per unit length. The unit of the heat transfer coefficient is W/mK, where K is kelvin, m is meter, and W is watt. The larger the heat transfer coefficient, the easier it will be for the heat to be transmitted through objects per unit length in the axial direction. 
     In other words, the contact area per unit dimension between the second portions  140 B and the nip plate  130  in the axial direction is smaller than the contact area per unit dimension between the first portion  140 A and the nip plate  130  in the axial direction. By configuring the first portion  140 A and the second portions  140 B in the above manner, heat can be hindered from escaping from the nip plate  130  to the second portions  140 B; accordingly, lack of temperature in the edge portions of the fixing belt  110  at the beginning of printing can be prevented. 
     The third portions  140 C are portions of the reflecting plate  140  that are on the outside of the sixth plane P 6  or the seventh plane P 7  and include portions of the reflecting portion  141 , the lock portions  143 , the other portions of the first surfaces C 1 , the second surfaces C 2 , and the fourth surfaces C 4 , which have been described above. 
     Furthermore, the cutout  141 C on one of the left and right sides is formed from the second plane P 2  to the outside of the sixth plane P 6  (the middle portion of the corresponding third portion  140 C in the axial direction), and the cutout  141 C on the other of the left and right sides is formed from the third plane P 3  to the outside of the seventh plane P 7  (the middle portion of the corresponding third portion  140 C in the axial direction). 
     With the above configuration, the present embodiment can obtain the following effects. The cutout  141 C is formed from the second plane P 2  to the outside of the sixth plane P 6  (or from the third plane P 3  to the outside of the seventh plane P 7 ), in other words, the entire second portions  140 B do not come in contact with the nip plate  130 ; accordingly, heat can be favorably hindered from escaping from the nip plate  130  to the second portions  140 B. 
     Note that the present disclosure is not limited to the above-described embodiment and may be employed in various forms such as those exemplified below. In the following description, members that have structures that are substantially similar to those of the embodiment described above are attached with the same reference numerals and description thereof is omitted. 
     In the above-described embodiment, the entire second portions  140 B do not come in contact with the nip plate  130 ; however, the present disclosure is not limited to the above configuration and, for example, as illustrated in  FIG. 7 , portions of second portions  140 E (portions in the range of length L 2 ) may be in contact with the nip plate  130 . In other words, in the present form, the second portions  140 E each include a portion of the reflecting portion  141 , a portion of the flange portion  142 , the corresponding third surface C 3 , the corresponding fifth surface C 5 , and a portion of the corresponding first surface C 1 , which have been described above. 
     Furthermore, the undersides of the flange portions  142  of the second portions  140 E are contact surfaces  142 B that are in contact with the nip plate  130 . Furthermore, in the above case, the contact surfaces  142 B are configured so as to include portions of the cutouts  141 C described above. Furthermore, each of the cutouts  141 C extends from an edge of the maximum sheet passing width W 2  (the fourth plane P 4  or the fifth plane P 5 ) to a substantially middle portion of the corresponding third portion  140 C. 
     A similar effect can also be obtained with the above form by having the relationship between the first heat transfer coefficient and the second heat transfer coefficient (between each of the contact areas) be similar to the relationship in the embodiment described above. Note that as illustrated in  FIG. 7 , a plurality of cutouts  141 D may be provided in the flange portions  142  of first portions  140 D as long as the relationship between each of the heat transfer coefficients is similar to that in the embodiment described above. Furthermore, in the present form, the range in which the reflecting plate  140  supports the nip plate  130  in the axial direction is the maximum sheet passing width W 2  and is wider than that in the embodiment described above (the maximum image forming area W 1 ); accordingly, the nip plate  130  can be supported by the reflecting plate  140  in a favorable manner. 
     Note that the size and the position of the cutouts are not limited to those in the embodiment described above and may be set optionally. For example, each of the cutouts may be formed so as to be within the areas of the corresponding second portion, maybe formed so as to extend from the corresponding second portion to a predetermined region of the corresponding first portion, or may be formed from a position outside of and away from the corresponding edge of the maximum sheet passing width to a predetermined region of the corresponding third portion. 
     In the embodiment described above, heat is hindered from escaping from the nip plate  130  to the second portions  140 B by forming the cutouts  141 C in the second portions  140 B; however, the present disclosure is not limited to the above configuration. For example, as illustrated in  FIG. 8 , heat escaping from the nip plate  130  to the second portions  140 F can be hindered by providing heat insulation sheets SH that have a lower heat conductivity than that of the reflecting plate  140  between the second portions  140 F and the nip plate  130 . 
     In detail, in the present form, each heat insulation sheet SH extends from an inner end (the second plane P 2  or the third plane P 3 ) of the corresponding second portion  140 F in the axial direction to an outer end (an outer end of the reflecting plate  140  in the axial direction) of a corresponding third portion  140 G. Furthermore, while the first portion  140 A is in contact with the nip plate  130 , the heat insulation sheets SH are interposed between the second portions  140 F and the third portions  140 G, and the nip plate  130 . In such a case as well, an effect similar to that of the embodiment described above can be obtained by having the relationship between the first heat transfer coefficient and the second heat transfer coefficient be similar to the relationship in the embodiment described above. 
     Note that the heat insulation sheets SH may be adhered to the reflecting plate  140 , may be adhered to the nip plate  130 , or may be merely held between the reflecting plate  140  and the nip plate  130 . Furthermore, the relationship between the first heat transfer coefficient and the second heat transfer coefficient may be made similar to the relationship in the embodiment described above by, instead of providing the heat insulation sheets SH, making the surface roughness of the underside of the second portions  140 F (or the upper surface of the nip plate  130  with which the underside is in contact) coarser than the surface roughness of the underside of the first portion  140 A (or the upper surface of the nip plate  130  with which the underside is in contact). 
     In the embodiment described above, the cutouts  141 C are formed both in the upstream supporting surface  142 F and the downstream supporting surface  142 R; however, the present disclosure is not limited to the above configuration and, for example, cutouts may be formed only in the upstream supporting surface or cutouts may be formed only in the downstream supporting surface. In other words, even if cutouts are formed only on either of the upstream supporting surface and the downstream supporting surface, an effect similar to that of the embodiment described above can be obtained by having the relationship between the first heat transfer coefficient and the second heat transfer coefficient be similar to the relationship in the embodiment described above. 
     In the embodiment described above, the cutouts  141 C are formed from the ends of the flange portions  142  to the sidewall portions  141 B, in other words, among the surfaces constituting the cutouts  141 C, one or some of the surfaces (the first surfaces C 1 , for example) is disposed so as to be spaced apart from the nip plate  130 ; however, the present disclosure is not limited to the above configuration. For example, small cutouts that can be formed within the area of the flange portion may be formed. In other words, an end of each of the surfaces that constitute the cutouts may be in contact with the nip plate. However, as in the embodiment described above, compared to a structure in which the end of each of the surfaces of the cutouts are in contact with the nip plate, the structure in which, among the surfaces constituting the cutouts  141 C, one or some of the surfaces (the first surfaces C 1 , for example) is disposed so as to be spaced apart from the nip plate  130  can favorably hinder heat from escaping from the nip plate  130  to the second portions  140 B. 
     In the present embodiment described above, the reflecting plate  140  is exemplified as the contact member; however, the present disclosure is not limited to the above reflecting plate  140  and the contact member may be any member that is directly in contact with the nip member. For example, the present disclosure can be applied to structures illustrated in  FIGS. 9 to 12 . 
     Specifically, a fixing device  300  according to the present form includes the fixing belt  110 , the halogen lamp  120  disposed inside the fixing belt  110 , a reflection member  330 , a support member  340 , a heat insulation member  350 , a nip plate  360 , and the pressure roller  150 . The nip plate  360 , the heat insulation member  350 , and the support member  340  are each formed in a substantially U-shape in cross-sectional view that open upwards (to the opposite side with respect to the pressure roller  150 ). The heat insulation member  350  is inserted inside the nip plate  360 , and the support member  340  is inserted inside the heat insulation member  350 . 
     The reflection member  330  is disposed above the nip plate  360 , the heat insulation member  350 , and the support member  340  and the halogen lamp  120  is disposed above the reflection member  330 . With the above, radiant heat from the halogen lamp  120  is reflected towards the fixing belt  110  above the halogen lamp  120  with the reflection member  330 . 
     The heat insulation member  350  is an example of a contact member and is configured so as to be in contact directly with the nip plate  360  and to receive the force from the pressure roller  150 . The heat insulation member  350  is formed of resin such as a liquid crystal polymer and hinders heat from the halogen lamp  120  from being directly transmitted to the nip plate  360 . 
     The heat insulation member  350  includes a lower wall portion  351  and a pair of sidewall portions  352  that extend upwards from the two edge portions of the lower wall portion  351  in the transport direction. Furthermore, as illustrated in  FIGS. 9 and 11 , recess  353  that is an example of a cutout and that is recessed upwards from an underside  351 A of the lower wall portion  351  is formed in the underside  351 A. Note that in  FIG. 11 , for convenience, the recess  353  is illustrated by dotted hatching. 
     The bottom surface of the recess  353  is a retreat portion  353 A that is disposed so as to be spaced apart from the nip plate  360 . The underside  351 A is the contact surface. The retreat portion  353 A includes an intermittent portion A 1  that is provided in the substantially middle portion of the lower wall portion  351  in the transport direction and that extends in the axial direction and a pair of end portions A 2  that are provided adjacent to both ends of the intermittent portion A 1  in the axial direction and that extend from one edge to the other edge of the lower wall portion  351  in the transport direction. Furthermore, the underside  351 A that is in contact with the nip plate  360  is formed on both sides of the intermittent portion A 1  in the transport direction and outside of each of the end portions A 2  in the axial direction. 
     As illustrated in  FIG. 12 , the heat insulation member  350  includes a first portion  350 A that extends across the width of the maximum image forming area W 1  in the axial direction, a pair of second portions  350 B positioned outside the width of the maximum image forming area W 1  in the axial direction and inside the width W 3  of the nip in the axial direction, and a pair of third portions  350 C positioned outside the width W 3  of the nip in the axial direction. Furthermore, each of the end portions A 2  of the retreat portion  353 A is formed so as to extend from a position that is outside the corresponding edge (the second plane P 2  or the third plane P 3 ) of the maximum image forming area W 1  in the axial direction and that is inside the corresponding edge (the fourth plane P 4  or the fifth plane P 5 ) of the sheet  51  in the axial direction, the sheet  51  having the maximum sheet passing width W 2 , to the substantially middle portion of the corresponding third portion  350 C. 
     In such a form as well, an effect similar to that of the embodiment described above can be obtained by having the relationship between the first heat transfer coefficient and the second heat transfer coefficient be similar to the relationship in the embodiment described above. Note that in the present form as well, the relationship between the heat transfer coefficients may be made similar to the relationship in the embodiment described above by, instead of providing the recess  353 , providing the heat insulation sheets, such as the ones described above, in the second portion or changing the surface roughness of the first portion and the second portion with respect each other. 
     In the embodiment described above, the plurality of support portions  163  are provided in the lower edges of the front wall  161  and the rear wall  162  of the stay  160 ; however, the present disclosure is not limited to the above configuration and, for example, as illustrated in  FIG. 13 , a single support  164  that protrudes downwards at the substantially middle portion of the front wall  161  and at the substantially middle portion of the rear wall  162  of the stay  160  in the axial direction and that extends in the axial direction may be provided. 
     In the embodiment described above, sheet  51  such as a cardboard, a postcard, or thin paper is exemplified as an example of a sheet; however, the present disclosure is not limited to the above sheet  51  and, for example, may be an OHP sheet. 
     In each of the above-described embodiments, the nip plate is exemplified as an example of the nip member; however, the present disclosure is not limited to the above nip plate and the nip member may be a thick member that does not have a tabular shape, for example. 
     In the embodiment described above, the pressure roller  150  is exemplified as the backup member; however, the present disclosure is not limited to the pressure roller  150  and, for example, the backup member may be a belt-shaped pressure member. 
     In the embodiment described above, the present disclosure is applied to the color laser printer  1 ; however, the present invention is not limited to the above application and may be applied to other image forming apparatuses such as, for example, a copying machine and a multifunction machine. 
     In each of the above-described embodiments, the halogen lamp  120  is exemplified as an example of the heating element; however, the present disclosure is not limited to the halogen lamp  120  and the heating element may be a carbon heater, for example. 
     Note that the fixing belt may be a resin film containing polyimide as the main component. In such a case, the surface of the fixing belt is coated with fluororesin, such as PTFE. 
     In the embodiment described above, support portions of the stay  160  that support the reflecting plate  140  are intermittently formed so as to be protruded and recessed along the axial direction of the fixing belt; however, the support portions may each be formed in a linear manner (in a planar manner) in cross-sectional view that extends from one end to the other end of the stay in the axial direction of the fixing belt.