Patent Publication Number: US-11048194-B2

Title: Fixing device, image forming apparatus, and heat- conducting multilayer body

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2018-164381 filed Sep. 3, 2018. 
     BACKGROUND 
     (i) Technical Field 
     The present disclosure relates to a fixing device, an image forming apparatus, and a heat-conducting multilayer body. 
     (ii) Related Art 
     There is a related-art technique applied to a fixing device that includes a heating member having a heat generating body provided on a substrate, and a film sliding on the heating member. In this technique, the rise of the temperature of a non-sheet-passing portion is suppressed by providing a high-thermal-conductivity member on a side of the heating member opposite a side of contact with the film (see Japanese Unexamined Patent Application Publication No. 5-289555). 
     SUMMARY 
     In the fixing device, for example, a contact portion such as a belt that comes into contact with a recording material is heated by a heat source, and the heated contact portion is brought into contact with the recording material, whereby an image formed on the recording material is fixed. 
     In such a fixing device, when, for example, an image formed on a recording material having a width smaller than the width of the heat source is fixed, heat generated by the heat source is not consumed in non-sheet-passing areas that are at two respective ends of the heat source. Consequently, in the non-sheet-passing areas, the temperature of the contact portion may rise excessively. To suppress the occurrence of such a situation, there are some fixing devices that each include, for example, a high-thermal-conductivity portion having a higher thermal conductivity than the contact portion and so forth and provided over the heat source. 
     In a fixing device including such a high-thermal-conductivity portion, for example, if the length of the area of overlap between the high-thermal-conductivity portion and the heat generator of the heat source in a transport direction is equal between width-direction end portions and a width-direction central portion, it may take a long time to heat the contact portion to a predetermined temperature at the start of heating of the contact portion by the heat source. 
     Aspects of non-limiting embodiments of the present disclosure relate to making the time required for heating the contact portion shorter than in the case where the length of the area of overlap between the high-thermal-conductivity portion and the heat generator of the heat source in the transport direction is equal between the width-direction end portions and the width-direction central portion. 
     Aspects of certain non-limiting embodiments of the present disclosure overcome the above disadvantages and/or other disadvantages not described above. However, aspects of the non-limiting embodiments are not required to overcome the disadvantages described above, and aspects of the non-limiting embodiments of the present disclosure may not overcome any of the disadvantages described above. 
     According to an aspect of the present disclosure, there is provided a fixing device including a contact portion that comes into contact with a recording material transported; a heat source that heats the contact portion and includes a heat generator extending in a width direction intersecting a transport direction in which the recording material is transported, and a support portion supporting the heat generator, the heat source having a counter surface that faces the contact portion, and an opposite surface; and a high-thermal-conductivity portion provided on the opposite surface of the heat source and extending in the width direction such that at least a part of the high-thermal-conductivity portion overlaps the heat generator of the heat source, the high-thermal-conductivity portion having a higher thermal conductivity than at least one of materials forming the support portion and the contact portion. A length of an area of overlap between the high-thermal-conductivity portion and the heat generator of the heat source in the transport direction is shorter in a width-direction central portion than in two width-direction end portions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present disclosure will be described in detail based on the following figures, wherein: 
         FIG. 1  illustrates an overall configuration of an image forming apparatus; 
         FIG. 2  illustrates a configuration of a fixing device; 
         FIGS. 3A and 3B  illustrate the configuration of the fixing device; 
         FIG. 4  illustrates an arrangement of a heat source and a high-thermal-conductivity portion according to a first exemplary embodiment; 
         FIGS. 5A to 5C  illustrate the arrangement of the heat source and the high-thermal-conductivity portion according to the first exemplary embodiment; 
         FIGS. 6A to 6C  illustrate an arrangement of a heat source and a high-thermal-conductivity portion according to a second exemplary embodiment; 
         FIGS. 7A to 7C  illustrate an arrangement of a heat source and a high-thermal-conductivity portion according to a third exemplary embodiment; 
         FIGS. 8A to 8C  illustrate an arrangement of a heat source and a high-thermal-conductivity portion according to a fourth exemplary embodiment; and 
         FIGS. 9A and 9B  illustrate an arrangement of a heat source, a high-thermal-conductivity portion, and a low-thermal-conductivity portion according to a fifth exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     First Exemplary Embodiment 
       FIG. 1  illustrates an overall configuration of an image forming apparatus  1 . 
     The image forming apparatus  1  is a so-called tandem-type color printer. 
     The image forming apparatus  1  includes an image forming section  10  as an exemplary image forming device. The image forming section  10  forms an image on a sheet P as an exemplary recording material in accordance with pieces of image data for different colors. 
     The image forming apparatus  1  further includes a controller  30  and an image processor  35 . 
     The controller  30  controls relevant functional elements included in the image forming apparatus  1 . 
     The image processor  35  processes the pieces of image data received from a device such as a personal computer (PC)  3  or an image reading device  4 . 
     The image forming section  10  includes four image forming units  11 Y,  11 M,  11 C, and  11 K (hereinafter also generally denoted as “image forming units  11 ”) arranged at intervals and in parallel. 
     The image forming units  11  all have the same configuration, but different kinds of toner are stored in respective developing devices  15  (to be described below). The image forming units  11  form toner images (images) in respective colors of yellow (Y), magenta (M), cyan (C), and black (K). 
     The image forming units  11  each include a photoconductor drum  12 , a charger  200  that charges the photoconductor drum  12 , and a light-emitting-diode (LED) printhead (LPH)  300  that exposes the photoconductor drum  12  to light. 
     The photoconductor drum  12  is charged by the charger  200 . Furthermore, the photoconductor drum  12  is exposed to light emitted from the LPH  300 , whereby an electrostatic latent image is formed on the photoconductor drum  12 . 
     The image forming units  11  each further include the developing device  15  that develops the electrostatic latent image formed on the photoconductor drum  12 , and a cleaner (not illustrated) that cleans the surface of the photoconductor drum  12 . 
     The image forming section  10  includes an intermediate transfer belt  20  to which the toner images in the respective colors formed on the respective photoconductor drums  12  are transferred, and first transfer rollers  21  with which the toner images in the respective colors formed on the respective photoconductor drums  12  are transferred sequentially to the intermediate transfer belt  20  (first transfer). 
     The image forming section  10  further includes a second transfer roller  22  with which the toner images transferred to the intermediate transfer belt  20  are collectively transferred to the sheet P (second transfer), and a fixing device  40  that fixes the toner images to the sheet P. 
     The fixing device  40  includes a fixing belt module  50  and a pressing roller  60 . The fixing belt module  50  includes a heat source  52  (see  FIG. 2 ). 
     The fixing belt module  50  is provided on the left side, in  FIG. 1 , of a sheet transport path R 1 . The pressing roller  60  is provided on the right side, in  FIG. 1 , of the sheet transport path R 1  and is pressed against the fixing belt module  50 . 
     The fixing belt module  50  includes a film-type fixing belt  51  that comes into contact with the sheet P. 
     The fixing belt  51  is an exemplary contact portion and includes, for example, a releasing layer forming an outermost layer that comes into contact with the sheet P, an elastic layer provided immediately on the inner side of the releasing layer, and a base layer supporting the elastic layer. 
     The fixing belt  51  has an endless shape and rotates counterclockwise in  FIG. 1 . An inner peripheral surface  51 A of the fixing belt  51  is lubricated with a lubricant so that the sliding resistance between the fixing belt  51  and the heat source  52  and so forth to be described below is reduced. Examples of the lubricant include liquid oils such as silicone oil and fluorine oil, a mixture of a solid substance and liquid such as grease, and a combination of the foregoing materials. These kinds of lubricant are each exemplary heat-conducting viscous liquid. 
     The fixing belt  51  comes into contact with the sheet P that is transported from the lower side in  FIG. 1 , and a portion of the fixing belt  51  that has come into contact with the sheet P moves with the sheet P, whereby the sheet P is nipped between the fixing belt  51  and the pressing roller  60 . Thus, the fixing belt  51  presses and heats the sheet P. 
     The fixing belt module  50  includes the heat source  52  (to be described below) provided on the inner side of the fixing belt  51 . The heat source  52  heats the fixing belt  51 . 
     The pressing roller  60  is an exemplary pressing member and is provided on the right side, in  FIG. 1 , of the sheet transport path R 1 . The pressing roller  60  is pressed against an outer peripheral surface  51 B of the fixing belt  51  and presses the sheet P passing through the nip between the fixing belt  51  and the pressing roller  60  (i.e., the sheet P moving along the sheet transport path R 1 ). 
     The pressing roller  60  is caused to rotate clockwise in  FIG. 1  by a motor (not illustrated). When the pressing roller  60  rotates clockwise, the fixing belt  51  receives a driving force from the pressing roller  60  and rotates counterclockwise. 
     In the image forming apparatus  1 , the image processor  35  processes the pieces of image data received from the PC  3  or the image reading device  4 , and the processed pieces of image data are supplied to the respective image forming units  11 . 
     Then, in the image forming unit  11 K for the black (K) color, for example, the photoconductor drum  12  is charged by the charger  200  while rotating in a direction of arrow A and is exposed to light emitted from the LPH  300  in accordance with a corresponding one of the pieces of image data received from the image processor  35 . 
     Consequently, an electrostatic latent image based on the piece of image data for the black (K) color is formed on the photoconductor drum  12 . The electrostatic latent image formed on the photoconductor drum  12  is then developed by the developing device  15 , whereby a toner image in the black (K) color is formed on the photoconductor drum  12 . 
     Likewise, other toner images in the colors of yellow (Y), magenta (M), and cyan (C) are formed in the image forming units  11 Y,  11 M, and  11 C, respectively. 
     The toner images in the respective colors formed by the respective image forming units  11  are then sequentially electrostatically attracted by the respective first transfer rollers  21  to the intermediate transfer belt  20  rotating in a direction of arrow B, whereby a toner image composed of the toner images having the respective colors and superposed one on top of another is formed on the intermediate transfer belt  20 . 
     With the rotation of the intermediate transfer belt  20 , the toner image on the intermediate transfer belt  20  is transported to a position (a second transfer part T) where the second transfer roller  22  is provided. Then, in accordance with the timing of reaching of the toner image to the second transfer part T, a sheet P is supplied from a sheet container  1 B to the second transfer part T. 
     In the second transfer part T, a transfer electric field generated by the second transfer roller  22  causes the toner image on the intermediate transfer belt  20  to be electrostatically transferred to the sheet P transported thereto. 
     Then, the sheet P having the toner image electrostatically transferred thereto is released from the intermediate transfer belt  20  and is transported to the fixing device  40 . 
     In the fixing device  40 , the sheet P is nipped between the fixing belt module  50  and the pressing roller  60 . Specifically, the sheet P is nipped between the fixing belt  51  rotating counterclockwise and the pressing roller  60  rotating clockwise. 
     Thus, the sheet P is pressed and heated, whereby the toner image on the sheet P is fixed to the sheet P. The sheet P having undergone the fixing is transported to a sheet stacking portion lE by a pair of discharge rollers  500 . 
       FIG. 2  and  FIGS. 3A and 3B  illustrate a configuration of the fixing device  40 .  FIG. 2  is a sectional view of the fixing device  40 , more specifically, a sectional view of the fixing device  40  taken in a central portion of the fixing belt  51  in the width direction to be described below.  FIGS. 3A and 3B  illustrate a configuration of the heat source  52  to be described below. 
     As illustrated in  FIG. 2 , the fixing device  40  includes the fixing belt module  50  and the pressing roller  60 . 
     The fixing belt module  50  includes the fixing belt  51  used for fixing the toner image to the sheet P. The fixing belt  51  is pressed against a side of the sheet P that has the toner image. 
     The pressing roller  60  is pressed against the outer peripheral surface  51 B of the fixing belt  51  and thus presses the sheet P passing through the nip between the fixing belt  51  and the pressing roller  60 . 
     Specifically, the pressing roller  60  is positioned in contact with the outer peripheral surface  51 B of the fixing belt  51  and forms a nip part N in combination with the fixing belt  51 . The nip part N formed between the pressing roller  60  and the fixing belt  51  is an area through which the sheet P passes while being pressed. In the first exemplary embodiment, in the process of the passing of the sheet P through the nip part N, the sheet P is heated and pressed, whereby the toner image is fixed to the sheet P. 
     Hereinafter, the direction in which the fixing belt  51  moves in the nip part N is referred to as the moving direction of the fixing belt  51  or simply the moving direction. The moving direction of the fixing belt  51  in the nip part N and the transport direction in which the sheet P is transported through the nip part N are the same. The width direction of the fixing belt  51  that is orthogonal to the moving direction is referred to as the width direction of the fixing belt  51  or simply the width direction. 
     As illustrated in  FIG. 2 , the fixing belt module  50  includes, on the inner side of the fixing belt  51 , the heat source  52  that heats the fixing belt  51 , and a high-thermal-conductivity portion  53  that receives the heat from the heat source  52 . The fixing belt module  50  further includes, on the inner side of the fixing belt  51 , a pressing member  54  that presses the high-thermal-conductivity portion  53  against the heat source  52 ; and a support member  55  that supports the heat source  52 , the high-thermal-conductivity portion  53 , and the pressing member  54 . The fixing belt module  50  further includes, on the inner side of the fixing belt  51 , a temperature sensor  57  that detects the temperature of the heat source  52 . 
     The heat source  52  has a plate-like shape and extends in the moving direction of the fixing belt  51  and in the width direction of the fixing belt  51 . The heat source  52  has a counter surface  52 A that faces the fixing belt  51 , and an opposite surface  52 B on a side thereof opposite the counter surface  52 A. The heat source  52  also has two side surfaces  52 C that connect the counter surface  52 A and the opposite surface  52 B to each other. In the first exemplary embodiment, the counter surface  52 A of the heat source  52  is in contact with the inner peripheral surface  51 A of the fixing belt  51 . 
     In the first exemplary embodiment, heat is supplied from the heat source  52  to the fixing belt  51 , whereby the fixing belt  51  is heated. Furthermore, in the first exemplary embodiment, the pressing roller  60  is pressed against the counter surface  52 A of the heat source  52  with the fixing belt  51  interposed therebetween. 
     As illustrated in  FIGS. 3A and 3B , the heat source  52  includes a plate-like base layer  521 , and a heat generating layer  522  and power feeding layers  523  that are provided on a side of the base layer  521  nearer to the fixing belt  51  and extend in the width direction of the fixing belt  51  (see  FIG. 2 ) that is orthogonal to the plane of  FIG. 2 . The heat source  52  further includes a protection layer  524  having an insulating characteristic and that covers the heat generating layer  522  and the power feeding layers  523 . 
     The base layer  521  of the heat source  52  is formed of a substrate made of a metal material such as SUS, with an insulating layer made of glass or the like provided thereon. The base layer  521  may alternatively be made of insulating ceramic or the like, such as aluminum nitride or alumina. The base layer  521  has a uniform thickness over the entirety thereof in the width direction of the fixing belt  51 . In other words, the thickness of the base layer  521  is equal between end portions thereof and a central portion thereof in the width direction of the fixing belt  51 . In addition, the heat capacity of the base layer  521  is equal between the end portions thereof and the central portion thereof in the width direction of the fixing belt  51 . 
     The heat generating layer  522  of the heat source  52  is an exemplary heat generator and is a heating resistor that generates heat by receiving electric power. The heat generating layer  522  is made of, for example, AgPd or the like. In the first exemplary embodiment, as illustrated in  FIG. 3A , the heat generating layer  522  extends in the width direction of the fixing belt  51 . In the first exemplary embodiment, the heat generating layer  522  has a uniform thickness over the entirety thereof in the width direction of the fixing belt  51 . Furthermore, the length of the heat generating layer  522  in the moving direction of the fixing belt  51  is uniform over the entirety thereof in the width direction of the fixing belt  51 . 
     If the power supplied to the heat generating layer  522  and the thickness of the heat generating layer  522  are uniform, the amount of heat generated by the heat generating layer  522  is inversely proportional to the length of the heat generating layer  522  in a direction orthogonal to the direction of electrification of the heat generating layer  522  (in the first exemplary embodiment, the moving direction of the fixing belt  51 ). That is, the amount of heat generated by the heat generating layer  522  becomes greater as the length of the heat generating layer  522  in the moving direction of the fixing belt  51  becomes smaller. 
     The power feeding layers  523  of the heat source  52  are exemplary electrode portions and are connected to two width-direction ends of the heat generating layer  522 , respectively, thereby feeding electric power to the heat generating layer  522 . The power feeding layers  523  are made of metal having a lower resistance than the heat generating layer  522 , for example, Ag, or AgPd or the like containing a greater ratio of Ag than the heat generating layer  522 . The power feeding layers  523  generate substantially no heat even if an electric current is supplied thereto, unlike the heat generating layer  522 . 
     In the first exemplary embodiment, as illustrated in  FIG. 3A , one of the power feeding layers  523  includes an extended portion  523 A provided adjacent to and on the upstream side with respect to the heat generating layer  522  in the moving direction of the fixing belt  51  and extending in the width direction of the fixing belt  51 . In the first exemplary embodiment, the extended portion  523 A of the power feeding layer  523  is bent at one width-direction end thereof (the right end in  FIG. 3A ), and the bent end is connected to one end of the heat generating layer  522 . 
     The protection layer  524  of the heat source  52  covers and protects the heat generating layer  522  and the power feeding layers  523  provided on the base layer  521 . The protection layer  524  is made of, for example, baked glass having an insulating characteristic. 
     The pressing member  54  (see  FIG. 2 ) is provided between the high-thermal-conductivity portion  53  (see  FIG. 2 ) and the support member  55  (see  FIG. 2 ) and presses the high-thermal-conductivity portion  53  against the opposite surface  52 B of the heat source  52 . The pressing member  54  brings a plurality of high-thermal-conductivity members  531 , to be described below, included in the high-thermal-conductivity portion  53  into close contact with one another. 
     The pressing member  54  is an elastic member, such as a compression spring or a rubber member, and presses the high-thermal-conductivity portion  53  against the heat source  52  with the elastic restoring force thereof. 
     The high-thermal-conductivity portion  53  is provided on the opposite surface  52 B of the heat source  52  and in contact therewith and receives heat from the heat source  52 . In the description of the first exemplary embodiment, the state where the high-thermal-conductivity portion  53  is provided on the opposite surface  52 B of the heat source  52  and in contact therewith includes not only a state where the high-thermal-conductivity portion  53  is provided directly on the opposite surface  52 B of the heat source  52  but also a state where the high-thermal-conductivity portion  53  is provided on the opposite surface  52 B of the heat source  52  with, for example, heat-conducting grease or the like interposed therebetween. In other words, the heat source  52  is configured to supply heat to the high-thermal-conductivity portion  53 . The heat source  52  is exemplary another member. 
     The high-thermal-conductivity portion  53  according to the first exemplary embodiment includes the plurality of high-thermal-conductivity members  531  each having a plate-like shape and that are stacked one on top of another with heat-conducting grease or the like interposed therebetween. The high-thermal-conductivity portion  53  formed of the stack of the high-thermal-conductivity members  531  generally has a block-like shape. 
     The high-thermal-conductivity members  531  forming the high-thermal-conductivity portion  53  are each made of a material having a higher thermal conductivity than at least one of the materials forming the fixing belt  51  and the base layer  521  and the protection layer  524  of the heat source  52 . The high-thermal-conductivity members  531  may each be made of a material having a higher thermal conductivity than the material forming the fixing belt  51 . 
     The material forming the high-thermal-conductivity members  531  may be, for example, metal such as copper or aluminum, or an alloy such as SUS. The high-thermal-conductivity members  531  may all be made of the same material or different materials. 
     In the first exemplary embodiment, the high-thermal-conductivity portion  53  includes the stack of the high-thermal-conductivity members  531  each having a plate-like shape. Therefore, when the high-thermal-conductivity portion  53  is pressed by the pressing member  54 , the high-thermal-conductivity members  531  deform independently of one another. Hence, the high-thermal-conductivity portion  53  comes into contact with the opposite surface  52 B of the heat source  52  more closely than in a case where, for example, the high-thermal-conductivity portion  53  is formed of a single block-like member. 
     The high-thermal-conductivity portion  53  supplies heat generated in a portion of the heat source  52  that is at a high temperature to another portion of the heat source  52  that is at a low temperature. 
     If the sheet P to be subjected to the fixing process has a small width, the temperature of the heat source  52  tends to rise in non-sheet-passing areas that are at the two width-direction ends of the heat source  52  and do not come into contact with the sheet P. In such a case, temperature nonuniformity in the width direction may occur in the heat source  52  and in the fixing belt  51 . If the fixing process of any sheet P having a larger width is performed after the occurrence of such temperature nonuniformity, fixing nonuniformity may occur. 
     In contrast, if the high-thermal-conductivity portion  53  is provided, the heat of the portion of the heat source  52  that is at a high temperature is supplied to the portion of the heat source  52  that is at a low temperature. Therefore, the temperature nonuniformity in the heat source  52  and in the fixing belt  51  is reduced. 
     In the fixing device  40  including the high-thermal-conductivity portion  53  that receives heat from the heat source  52 , when the fixing belt  51  starts to be heated by the heat source  52 , the heat generated by the heat generating layer  522  of the heat source  52  is conducted not only to the fixing belt  51  but also to the high-thermal-conductivity portion  53 . Therefore, depending on the relationship between the heat generating layer  522  of the heat source  52  and the high-thermal-conductivity portion  53 , the heat conduction from the heat generating layer  522  of the heat source  52  to the fixing belt  51  may be slow, leading to an increase in the time required for heating the fixing belt  51  to a predetermined temperature. For example, if the length of an area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  in the moving direction of the fixing belt  51  is equal between end portions and a central portion in the width direction of the fixing belt  51 , the time required for heating the fixing belt  51  to the predetermined temperature tends to increase. 
     In contrast, in the fixing device  40  according to the first exemplary embodiment, the length of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  in the moving direction of the fixing belt  51  is made shorter in the central portion in the width direction of the fixing belt  51  than in the two end portions in the width direction of the fixing belt  51  (hereinafter, the end portions in the width direction of the fixing belt  51  are also referred to as the width-direction end portions, and the central portion in the width direction of the fixing belt  51  is also referred to as the width-direction central portion). Thus, the increase in the time required for heating the fixing belt  51  is suppressed. 
     Now, the configuration of the high-thermal-conductivity portion  53  and the relationship between the high-thermal-conductivity portion  53  and the heat source  52  will be described in detail. 
       FIG. 4  and  FIGS. 5A to 5C  illustrate an arrangement of the heat source  52  and the high-thermal-conductivity portion  53  according to the first exemplary embodiment.  FIG. 4  is a perspective view illustrating the heat source  52  and the high-thermal-conductivity portion  53 .  FIG. 5A  is a plan view of the heat source  52  and the high-thermal-conductivity portion  53  seen in a direction VA represented in  FIG. 4 .  FIG. 5B  is a sectional view taken along line VB-VB illustrated in  FIG. 5A .  FIG. 5C  is a sectional view taken along line VC-VC illustrated in  FIG. 5A . In  FIGS. 5A to 5C , the plurality of high-thermal-conductivity members  531  (see  FIG. 4 ) are collectively illustrated as the high-thermal-conductivity portion  53 . Hereinafter, the plurality of high-thermal-conductivity members  531  will be collectively described as the high-thermal-conductivity portion  53 , occasionally. 
     As described above, the high-thermal-conductivity portion  53  generally has a block-like shape extending in the width direction of the fixing belt  51 . In the first exemplary embodiment, as illustrated in  FIG. 5A  and others, the length of the high-thermal-conductivity portion  53  in the width direction is equal to the length of the heat generating layer  522  of the heat source  52  in the width direction. 
     Furthermore, as illustrated in  FIGS. 4 and 5A , the high-thermal-conductivity portion  53  has a flat upstream side face  53 C positioned on the upstream side in the moving direction, and a downstream side face  53 D opposite and on the downstream side with respect to the upstream side face  53 C in the moving direction. The upstream side face  53 C and the downstream side face  53 D each extend in the width direction. In the first exemplary embodiment, the distance between the downstream side face  53 D and the upstream side face  53 C in the moving direction is shorter in the width-direction central portion than in the width-direction end portions. 
     That is, the length of the high-thermal-conductivity portion  53  according to the first exemplary embodiment in the moving direction is shorter in the width-direction central portion than in the width-direction end portions. In other words, the high-thermal-conductivity portion  53  according to the first exemplary embodiment includes a narrow portion  53 A positioned in the width-direction central portion thereof, and wide portions  53 B positioned at two respective width-direction ends of the narrow portion  53 A and being wider than the narrow portion  53 A in the moving direction. In the first exemplary embodiment, the length of the narrow portion  53 A in the moving direction gradually increases toward each of the wide portions  53 B at the two respective width-direction ends of the narrow portion  53 A. 
     As described above, the length of the heat generating layer  522  of the heat source  52  in the moving direction of the fixing belt  51  is uniform from one width-direction end thereof to the other width-direction end thereof. 
     Furthermore, in the first exemplary embodiment, as illustrated in  FIGS. 5A to 5C , the length of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  in the moving direction is shorter in the width-direction central portion than in the width-direction end portions. Herein, the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  refers to an area where the high-thermal-conductivity portion  53  and the heat generating layer  522  overlap each other when seen in a direction of stacking of the high-thermal-conductivity portion  53  on the heat source  52  (a direction orthogonal to the plane of  FIG. 5A ). The length of the area in the moving direction includes a length in a case where there is no overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  (i.e., a length of zero). 
     More specifically, as illustrated in  FIGS. 5A and 5B , the narrow portion  53 A of the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  do not overlap each other in the width-direction central portion. In other words, the length of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  in the moving direction is zero in the width-direction central portion. 
     On the other hand, as illustrated in  FIGS. 5A and 5C , each of the wide portions  53 B of the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  overlap each other in a corresponding one of the two width-direction end portions. 
     That is, the length (denoted by D 1  in  FIG. 5C ) of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  in the moving direction is shorter in the width-direction central portion than in the width-direction end portions. 
     In the first exemplary embodiment, the width-direction end portions of the high-thermal-conductivity portion  53  or the heat generating layer  522  refer to regions of the high-thermal-conductivity portion  53  or the heat generating layer  522  that are positioned at two respective ends in the width direction and each have a predetermined length in the width direction. Likewise, the width-direction central portion of the high-thermal-conductivity portion  53  or the heat generating layer  522  refers to a region of the high-thermal-conductivity portion  53  or the heat generating layer  522  that is positioned in the center in the width direction and has a predetermined length in the width direction. 
     In the first exemplary embodiment, since the length of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  in the moving direction is set as described above, the time required for heating the fixing belt  51  to the predetermined temperature at the start of heating of the fixing belt  51  by the heat source  52  is shorter than in a case where, for example, the length of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  in the moving direction is equal between the width-direction end portions and the width-direction central portion. 
     More specifically, since the length of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  in the moving direction is shorter in the width-direction central portion than in the width-direction end portions, the heat generated by the heat generating layer  522  of the heat source  52  is more likely to be conducted to the fixing belt  51  than to the high-thermal-conductivity portion  53 . Consequently, at the start of heating of the fixing belt  51  by the heat source  52 , the temperature of the fixing belt  51  rises more quickly with the heat generated by the heat generating layer  522 . Accordingly, the time required for heating the fixing belt  51  to the predetermined temperature is reduced. 
     As described above, in the first exemplary embodiment, the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  do not overlap each other in the width-direction central portion. Therefore, the heat generated by the heat generating layer  522  of the heat source  52  is less likely to be conducted to the high-thermal-conductivity portion  53  but is more likely to be conducted to the fixing belt  51  than in a case where, for example, the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  overlap each other in the width-direction central portion. Consequently, at the start of heating of the fixing belt  51  by the heat source  52 , the temperature of the fixing belt  51  rises much more quickly with the heat generated by the heat generating layer  522  and the time required for heating the fixing belt  51  to the predetermined temperature becomes shorter than in the case where the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  overlap each other in the width-direction central portion. 
     Meanwhile, as described above, if the sheet P to be subjected to the fixing process has a small width, the temperature tends to rise in the non-sheet-passing areas that are at the width-direction ends of the heat source  52 . 
     To avoid such a situation, in the first exemplary embodiment, the length of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  in the moving direction is made longer in the width-direction end portions than in the width-direction central portion, so that the heat generated by the heat generating layer  522  is more assuredly conducted to the high-thermal-conductivity portion  53  in the width-direction end portions. The heat thus conducted from the width-direction end portions of the heat generating layer  522  to the high-thermal-conductivity portion  53  is conducted throughout the high-thermal-conductivity portion  53  in the width direction and is supplied to the width-direction central portion of the heat source  52  that is at a low temperature. Thus, the temperature nonuniformity in the heat source  52  and in the fixing belt  51  is reduced more assuredly. 
     As illustrated in  FIG. 5C  and others, the width-direction end portions of the heat generating layer  522  of the heat source  52  overlaps the high-thermal-conductivity portion  53  over the entirety thereof in the moving direction. Therefore, the heat generated by the heat generating layer  522  is more assuredly conducted to the high-thermal-conductivity portion  53  in the width-direction end portions than in a case where the width-direction end portions of the heat generating layer  522  of the heat source  52  each include a region that does not overlap the high-thermal-conductivity portion  53 . Consequently, even if the temperature rises in the non-sheet-passing areas at the respective width-direction ends of the heat source  52 , the temperature nonuniformity in the heat source  52  and in the fixing belt  51  is reduced more assuredly. 
     As illustrated in  FIG. 5A , the high-thermal-conductivity portion  53  according to the first exemplary embodiment further includes a region that does not overlap the heat generating layer  522  of the heat source  52  over the entirety from one end to the other end in the width direction. Therefore, the heat conducted from the high-temperature portion of the heat source  52  to the high-thermal-conductivity portion  53  is conducted in the width direction through the region that does not overlap the heat generating layer  522 . Hence, the heat is more assuredly supplied to the low-temperature portion of the heat source  52 . Accordingly, for example, even if the temperature rises in the non-sheet-passing areas corresponding to the width-direction end portions of the heat source  52 , the temperature nonuniformity in the heat source  52  and in the fixing belt  51  is reduced more assuredly. 
     In particular, in the first exemplary embodiment, the region of the high-thermal-conductivity portion  53  that does not overlap the heat generating layer  522  over the entirety from one end to the other end in the width direction corresponds to a region of the high-thermal-conductivity portion  53  that is on the upstream side in the moving direction and adjoins the upstream side face  53 C. Hence, with the presence of the high-thermal-conductivity portion  53 , the temperature nonuniformity in the fixing belt  51  tends to be reduced before the fixing belt  51  reaches the nip part N. 
     Furthermore, in the first exemplary embodiment, the region of the high-thermal-conductivity portion  53  that does not overlap the heat generating layer  522  over the entirety from one end to the other end in the width direction overlaps the extended portion  523 A of one of the power feeding layers  523  included in the heat source  52 . Therefore, while the increase in the size of the heat source  52  in the moving direction is suppressed, the size of the high-thermal-conductivity portion  53  in the moving direction is allowed to be made greater than in a case where the region of the high-thermal-conductivity portion  53  that does not overlap the heat generating layer  522  over the entirety from one end to the other end in the width direction does not overlap the power feeding layer  523 . 
     Second Exemplary Embodiment 
     A second exemplary embodiment of the present disclosure will now be described. Elements that are the same as those described in the first exemplary embodiment are denoted by corresponding ones of the reference numerals, and detailed description of those elements is omitted herein. 
       FIGS. 6A to 6C  illustrate an arrangement of the heat source  52  and the high-thermal-conductivity portion  53  according to the second exemplary embodiment.  FIG. 6A  is a plan view of the heat source  52  and the high-thermal-conductivity portion  53  seen in the direction of stacking of the high-thermal-conductivity portion  53  on the heat source  52  (a direction corresponding to the direction VA represented in  FIG. 4 ).  FIG. 6B  is a sectional view taken along line VIB-VIB illustrated in  FIG. 6A .  FIG. 6C  is a sectional view taken along line VIC-VIC illustrated in  FIG. 6A . In  FIGS. 6A to 6C , the plurality of high-thermal-conductivity members  531  (see  FIG. 4 ) are collectively illustrated as the high-thermal-conductivity portion  53 . 
     The high-thermal-conductivity portion  53  according to the second exemplary embodiment has the same shape as the high-thermal-conductivity portion  53  according to the first exemplary embodiment. That is, the high-thermal-conductivity portion  53  according to the second exemplary embodiment includes the narrow portion  53 A positioned in the width-direction central portion thereof, and wide portions  53 B positioned at two respective width-direction ends of the narrow portion  53 A and being wider than the narrow portion  53 A in the moving direction. 
     The heat generating layer  522  of the heat source  52  according to the second exemplary embodiment has a different shape from the heat generating layer  522  according to the first exemplary embodiment. 
     Specifically, the length of the heat generating layer  522  according to the second exemplary embodiment in the moving direction of the fixing belt  51  is smaller in the width-direction end portions thereof than in the width-direction central portion thereof. As described above, the heat generating layer  522  has a higher resistance and generates a greater amount of heat with a smaller length thereof in the moving direction of the fixing belt  51 . Hence, in the heat source  52  according to the second exemplary embodiment, the amount of heat generated by the heat generating layer  522  when power is supplied thereto is greater in the width-direction end portions than in the width-direction central portion. 
     In the second exemplary embodiment, since the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  have the respective shapes described above, the length of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  in the moving direction is shorter in the width-direction central portion than in the width-direction end portions, as with the case of the first exemplary embodiment. 
     Hence, as with the case of the first exemplary embodiment, the time required for heating the fixing belt  51  to the predetermined temperature at the start of heating of the fixing belt  51  by the heat source  52  is shorter than in the case where the length of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  in the moving direction is equal between the width-direction end portions and the width-direction central portion. 
     Furthermore, as illustrated in  FIGS. 6A and 6C  and others, the width-direction end portions of the heat generating layer  522  of the heat source  52  where the amount of heat generation is greater each overlap the high-thermal-conductivity portion  53  over the entirety thereof in the moving direction. Hence, the heat generated in the width-direction end portions of the heat generating layer  522  is more assuredly conducted to the high-thermal-conductivity portion  53 , and the temperature nonuniformity in the heat source  52  and in the fixing belt  51  is reduced more assuredly with the presence of the high-thermal-conductivity portion  53 . 
     Third Exemplary Embodiment 
     A third exemplary embodiment of the present disclosure will now be described. Elements that are the same as those described in the first exemplary embodiment are denoted by corresponding ones of the reference numerals, and detailed description of those elements is omitted herein. 
       FIGS. 7A to 7C  illustrate an arrangement of the heat source  52  and the high-thermal-conductivity portion  53  according to the third exemplary embodiment.  FIG. 7A  is a plan view of the heat source  52  and the high-thermal-conductivity portion  53  seen in the direction of stacking of the high-thermal-conductivity portion  53  on the heat source  52  (a direction corresponding to the direction VA represented in  FIG. 4 ).  FIG. 7B  is a sectional view taken along line VIIB-VIIB illustrated in  FIG. 7A .  FIG. 7C  is a sectional view taken along line VIIC-VIIC illustrated in  FIG. 7A . In  FIGS. 7A to 7C , the plurality of high-thermal-conductivity members  531  (see  FIG. 4 ) are collectively illustrated as the high-thermal-conductivity portion  53 . 
     In the third exemplary embodiment, the length of the heat generating layer  522  of the heat source  52  in the moving direction of the fixing belt  51  is greater in the width-direction end portions thereof than in the width-direction central portion thereof. Hence, in the heat source  52  according to the third exemplary embodiment, the amount of heat generated by the heat generating layer  522  when power is supplied thereto is smaller in the width-direction end portions than in the width-direction central portion. 
     Meanwhile, the high-thermal-conductivity portion  53  generally has an oblong cuboid shape extending in the width direction. In other words, each of the high-thermal-conductivity members  531  forming the high-thermal-conductivity portion  53  has an oblong rectangular shape extending in the width direction. That is, the length of the high-thermal-conductivity portion  53  according to the third exemplary embodiment in the moving direction is equal between the width-direction central portion and the width-direction end portions. 
     In the third exemplary embodiment, as with the case of the first exemplary embodiment, the length of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  in the moving direction is shorter in the width-direction central portion than in the width-direction end portions. In other words, in the third exemplary embodiment, the length of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  in the moving direction in the width-direction central portion (a length denoted by D 2  in  FIG. 7B ) is shorter than that in the width-direction end portions (a length denoted by D 3  in  FIG. 7C ) (D 2 &lt;D 3 ). 
     Hence, as with the case of the first exemplary embodiment, the time required for heating the fixing belt  51  to the predetermined temperature at the start of heating of the fixing belt  51  by the heat source  52  is shorter than in the case where the length of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  in the moving direction is equal between the width-direction end portions and the width-direction central portion. 
     In the third exemplary embodiment, the heat generating layer  522  of the heat source  52  is shaped such that the length thereof in the moving direction is greater in the width-direction end portions than in the width-direction central portion. Instead, the high-thermal-conductivity portion  53  has a simple shape such as a cuboid as illustrated in  FIG. 7A . 
     Fourth Exemplary Embodiment 
     A fourth exemplary embodiment of the present disclosure will now be described. Elements that are the same as those described in the first exemplary embodiment are denoted by corresponding ones of the reference numerals, and detailed description of those elements is omitted herein. 
       FIGS. 8A to 8C  illustrate an arrangement of the heat source  52  and the high-thermal-conductivity portion  53  according to the fourth exemplary embodiment.  FIG. 8A  is a plan view of the heat source  52  and the high-thermal-conductivity portion  53  seen in the direction of stacking of the high-thermal-conductivity portion  53  on the heat source  52  (a direction corresponding to the direction VA represented in  FIG. 4 ).  FIG. 8B  is a sectional view taken along line VIIIB-VIIIB illustrated in  FIG. 8A .  FIG. 8C  is a sectional view taken along line VIIIC-VIIIC illustrated in  FIG. 8A . In  FIGS. 8A to 8C , the plurality of high-thermal-conductivity members  531  (see  FIG. 4 ) are collectively illustrated as the high-thermal-conductivity portion  53 . 
     As illustrated in  FIG. 8A , the heat source  52  according to the fourth exemplary embodiment includes a plurality of (two in the fourth exemplary embodiment) heat generating layers  522  arranged side by side at intervals in the moving direction of the fixing belt  51  and each extending in the width direction of the fixing belt  51 . Specifically, the heat generating layers  522  according to the fourth exemplary embodiment include an upstream heat generating layer  522 B and a downstream heat generating layer  522 C each extending in the width direction. The upstream heat generating layer  522 B is positioned on the upstream side of the heat source  52  in the moving direction. The downstream heat generating layer  522 C is positioned on the downstream side with respect to the upstream heat generating layer  522 B in the moving direction and at an interval therefrom. The upstream heat generating layer  522 B and the downstream heat generating layer  522 C are each connected at one width-direction end thereof to the extended portion  523 A of one of the power feeding layers  523 . 
     The length of the upstream heat generating layer  522 B included in the heat generating layers  522  in the moving direction of the fixing belt  51  is uniform over the entirety thereof from one end to the other end in the width direction. The length of the downstream heat generating layer  522 C included in the heat generating layers  522  in the moving direction of the fixing belt  51  is greater in two width-direction ends thereof than in a width-direction central portion thereof. 
     Furthermore, as with the case of the third exemplary embodiment, the high-thermal-conductivity portion  53  generally has an oblong cuboid shape extending in the width direction. In other words, each of the high-thermal-conductivity members  531  forming the high-thermal-conductivity portion  53  has an oblong rectangular shape extending in the width direction. That is, the length of the high-thermal-conductivity portion  53  according to the fourth exemplary embodiment in the moving direction is equal between the width-direction central portion and the width-direction end portions. 
     In the fourth exemplary embodiment, as with the case of the first exemplary embodiment, the length of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  of the heat source  52  in the moving direction is shorter in the width-direction central portion than in the width-direction end portions. More specifically, the length of the area of overlap, in the moving direction, between the high-thermal-conductivity portion  53  and the downstream heat generating layer  522 C included in the plurality of heat generating layers  522  is shorter in the width-direction central portion than in the width-direction end portions. In addition, the upstream heat generating layer  522 B included in the heat generating layers  522  overlaps the high-thermal-conductivity portion  53  over the entirety thereof in the moving direction from one end to the other end in the width direction. 
     Hence, as with the case of the first exemplary embodiment, the time required for heating the fixing belt  51  to the predetermined temperature at the start of heating of the fixing belt  51  by the heat source  52  is shorter than in the case where the length of the area of overlap between the high-thermal-conductivity portion  53  and the heat generating layer  522  in the moving direction is equal between the width-direction end portions and the width-direction central portion. 
     Fifth Exemplary Embodiment 
     A fifth exemplary embodiment of the present disclosure will now be described. Elements that are the same as those described in the first exemplary embodiment are denoted by corresponding ones of the reference numerals, and detailed description of those elements is omitted herein. 
       FIGS. 9A and 9B  illustrate an arrangement of the heat source  52 , the high-thermal-conductivity portion  53 , and a low-thermal-conductivity portion  56 , to be described below, according to the fifth exemplary embodiment.  FIG. 9A  is a perspective view illustrating the heat source  52 , the high-thermal-conductivity portion  53 , and the low-thermal-conductivity portion  56 .  FIG. 9B  is a sectional view taken along line IXB-IXB illustrated in  FIG. 9A . 
     As illustrated in  FIGS. 9A and 9B , the high-thermal-conductivity portion  53  according to the fifth exemplary embodiment has the same shape as the high-thermal-conductivity portion  53  according to the first exemplary embodiment. That is, the high-thermal-conductivity portion  53  according to the fifth exemplary embodiment includes the narrow portion  53 A positioned in the width-direction central portion thereof, and the wide portions  53 B positioned at two respective width-direction ends of the narrow portion  53 A and being wider than the narrow portion  53 A in the moving direction. 
     Furthermore, although not illustrated, the heat generating layer  522  of the heat source  52  according to the fifth exemplary embodiment has the same shape as the heat generating layer  522  according to the first exemplary embodiment. That is, the length of the heat generating layer  522  in the moving direction is uniform over the entirety thereof in the width direction. 
     In the fifth exemplary embodiment, as illustrated in  FIGS. 9A and 9B , the low-thermal-conductivity portion  56  having a lower thermal conductivity than the high-thermal-conductivity portion  53  is provided between the opposite surface  52 B of the heat source  52  and the high-thermal-conductivity portion  53 . In other words, the high-thermal-conductivity portion  53  is provided on the opposite surface  52 B of the heat source  52  with the low-thermal-conductivity portion  56  interposed therebetween. 
     The low-thermal-conductivity portion  56  may have a lower thermal conductivity than a material forming the heat source  52 . The low-thermal-conductivity portion  56  is made of, for example, a heat-resisting resin material or the like, such as polyimide, and is provided in the form of a thin film. The low-thermal-conductivity portion  56  has the same shape as the high-thermal-conductivity portion  53  when seen in the direction of stacking of the low-thermal-conductivity portion  56  and the high-thermal-conductivity portion  53  on the heat source  52 . 
     In the fifth exemplary embodiment, since the low-thermal-conductivity portion  56  is provided between the heat source  52  and the high-thermal-conductivity portion  53 , the time required for heating the fixing belt  51  to the predetermined temperature at the start of heating of the fixing belt  51  by the heat source  52  is much shorter than in a case where the low-thermal-conductivity portion  56  is not provided. 
     Specifically, in the fifth exemplary embodiment, since the low-thermal-conductivity portion  56  having a low thermal conductivity is provided, the heat generated by the heat generating layer  522  of the heat source  52  is prevented from being directly conducted to the high-thermal-conductivity portion  53 . Hence, the heat generated by the heat generating layer  522  of the heat source  52  is more assuredly conducted to the fixing belt  51 . Consequently, at the start of heating of the fixing belt  51  by the heat source  52 , the temperature of the fixing belt  51  tends rise quickly with the heat generated by the heat generating layer  522 . 
     In the fifth exemplary embodiment, as described above, the low-thermal-conductivity portion  56  has the same shape as the high-thermal-conductivity portion  53 . Furthermore, the high-thermal-conductivity portion  53  has no part that is in direct contact with the heat source  52 , instead of through the low-thermal-conductivity portion  56 . Hence, the heat generated by the heat generating layer  522  of the heat source  52  is prevented from being directly conducted to the high-thermal-conductivity portion  53  and is more assuredly conducted to the fixing belt  51 . 
     When the fixing belt  51  reaches the predetermined temperature, the temperature of the low-thermal-conductivity portion  56  rises correspondingly. When the temperature of the low-thermal-conductivity portion  56  rises, the heat is gradually conducted from the low-thermal-conductivity portion  56  to the high-thermal-conductivity portion  53 . 
     For example, if the sheet P to be subjected to the fixing process has a small width and the temperature rises in the non-sheet-passing areas corresponding to the width-direction end portions of the heat source  52 , the heat is conducted from the width-direction end portions of the heat source  52  to the high-thermal-conductivity portion  53  through the low-thermal-conductivity portion  56 . The heat thus conducted from the width-direction end portions of the heat source  52  to the high-thermal-conductivity portion  53  is conducted throughout the high-thermal-conductivity portion  53  in the width direction and is supplied to the width-direction central portion of the heat source  52  that is at a low temperature. Thus, the temperature nonuniformity in the heat source  52  and in the fixing belt  51  is reduced. 
     The foregoing description of the exemplary embodiments of the present disclosure has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the disclosure and its practical applications, thereby enabling others skilled in the art to understand the disclosure for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the disclosure be defined by the following claims and their equivalents.