Patent Publication Number: US-9846397-B2

Title: Fixing device including a supplementary thermal conductor and image forming apparatus incorporating same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119(a) to Japanese Patent Application Nos. 2015-246089, filed on Dec. 17, 2015, and 2016-217937, filed on Nov. 8, 2016, in the Japan Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Embodiments of the present disclosure generally relate to a fixing device and an image forming apparatus incorporating the fixing device, and more particularly, to a fixing device for fixing a toner image onto a recording medium and an image forming apparatus for forming an image on a recording medium, incorporating the fixing device. 
     Related Art 
     Various types of electrophotographic image forming apparatuses are known, including copiers, printers, facsimile machines, and multifunction machines having two or more of copying, printing, scanning, facsimile, plotter, and other capabilities. Such image forming apparatuses usually form an image on a recording medium according to image data. Specifically, in such image forming apparatuses, for example, a charger uniformly charges a surface of a photoconductor as an image bearer. An optical writer irradiates the surface of the photoconductor thus charged with a light beam to form an electrostatic latent image on the surface of the photoconductor according to the image data. A developing device supplies toner to the electrostatic latent image thus formed to render the electrostatic latent image visible as a toner image. The toner image is then transferred onto a recording medium either directly, or indirectly via an intermediate transfer belt. Finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image onto the recording medium. Thus, the image is formed on the recording medium. 
     Such a fixing device typically includes a fixing rotator such as a roller, a belt, or a film, and an opposed rotator such as a roller or a belt pressed against the fixing rotator. The toner image is fixed onto the recording medium under heat and pressure while the recording medium is conveyed between the fixing rotator and the opposed rotator. 
     SUMMARY 
     In one embodiment of the present disclosure, a novel fixing device is described that includes a flexible endless belt formed into a loop and having an inner circumferential surface, a heater to heat the endless belt, and a nip formation assembly disposed inside the loop formed by the endless belt. The nip formation assembly includes a pressure pad and a supplementary thermal conductor to conduct heat from the heater in an axial direction of the endless belt. The supplementary thermal conductor has a belt sliding-contact face over which the inner circumferential surface of the endless belt slides. The fixing device further includes a pressure rotator to press against the nip formation assembly via the endless belt to form a fixing nip between the endless belt and the pressure rotator, through which a recording medium bearing a toner image is conveyed. The supplementary thermal conductor has an edge portion dimensioned to distance the supplementary thermal conductor from the endless belt at an end portion of the belt sliding-contact face in a longitudinal direction of the supplementary thermal conductor parallel to the axial direction of the endless belt. A distance between the edge portion of the supplementary thermal conductor and the endless belt increases toward an end portion of the endless belt in the axial direction of the endless belt. 
     Also described is a novel image forming apparatus that includes an image forming device to form a toner image and the fixing device described above, disposed downstream from the image forming device in a recording medium conveyance direction, to fix the toner image on a recording medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be more readily obtained as the same becomes better understood by reference to the following detailed description of embodiments when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic view of an image forming apparatus according to an embodiment of the present disclosure; 
         FIG. 2  is a schematic cross-sectional view of a fixing device incorporated in the image forming apparatus of  FIG. 1 , according to a first embodiment of the present disclosure; 
         FIG. 3  is an exploded perspective view of a nip formation assembly incorporated in the fixing device of  FIG. 2 , illustrating relative positions of a nip formation pad, a stay, an end heater, and a supplementary thermal conductor; 
         FIG. 4  is a cross-sectional view of the supplementary thermal conductor; 
         FIG. 5  is a perspective view of a first example of the supplementary thermal conductor having flank faces; 
         FIG. 6  is a schematic view of a fixing belt and associated components, particularly illustrating a configuration to regulate rotation of the fixing belt; 
         FIG. 7A  is a schematic view of a fixing belt and the supplementary thermal conductor having angular corners, illustrating relative positions thereof at an end position; 
         FIG. 7B  is a schematic view of the fixing belt and the supplementary thermal conductor having angular corners, illustrating relative positions thereof at a center position; 
         FIG. 8  is a schematic view of the fixing belt and the supplementary thermal conductor having flank faces, illustrating relative positions thereof; 
         FIG. 9  is a perspective view of a second example of the supplementary thermal conductor having flank faces; 
         FIG. 10  is a perspective view of a variation of the supplementary thermal conductor of  FIG. 5 ; 
         FIG. 11  is a perspective view of a variation of the supplementary thermal conductor of  FIG. 9 ; 
         FIG. 12  is a schematic view of the fixing belt and associated components, particularly illustrating another variation of the supplementary thermal conductor of  FIG. 5 ; 
         FIG. 13A  is a perspective view of a third example of the supplementary thermal conductor having flank faces; 
         FIG. 13B  is a side view of the supplementary thermal conductor of  FIG. 13A ; 
         FIG. 14  is a perspective view of a variation of the supplementary thermal conductor of  FIGS. 13A and 13B ; 
         FIG. 15A  is a perspective view of a fourth example of the supplementary thermal conductor having flank faces; 
         FIG. 15B  is a side view of the supplementary thermal conductor of  FIG. 15A ; and 
         FIG. 16  is a cross-sectional view of a fixing device according to a second embodiment of the present disclosure. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. Also, identical or similar reference numerals designate identical or similar components throughout the several views. 
     DETAILED DESCRIPTION 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have the same function, operate in a similar manner, and achieve similar results. 
     Although the embodiments are described with technical limitations with reference to the attached drawings, such description is not intended to limit the scope of the disclosure and not all of the components or elements described in the embodiments of the present disclosure are indispensable to the present disclosure. 
     In a later-described comparative example, embodiment, and exemplary variation, for the sake of simplicity like reference numerals are given to identical or corresponding constituent elements such as parts and materials having the same functions, and redundant descriptions thereof are omitted unless otherwise required. 
     As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     It is to be noted that, in the following description, suffixes Y, C, M, and K denote colors yellow, cyan, magenta, and black, respectively. To simplify the description, these suffixes are omitted unless necessary. 
     Referring now to the drawings, embodiments of the present disclosure are described below. 
     Initially with reference to  FIG. 1 , a description is given of an overall configuration of an image forming apparatus  1  according to an embodiment of the present disclosure. 
       FIG. 1  is a schematic view of the image forming apparatus  1 . 
     The image forming apparatus  1  may be a copier, a facsimile machine, a printer, a multifunction peripheral or a multifunction printer (MFP) having at least one of copying, printing, scanning, facsimile, and plotter functions, or the like. In the present embodiment, the image forming apparatus  1  a color laser printer that forms color and monochrome images on recording media by electrophotography. Alternatively, the image forming apparatus  1  may be a monochrome printer that forms a monochrome toner image on a recording medium. 
     As illustrated in  FIG. 1 , the image forming apparatus  1  includes, e.g., four image forming devices  4 Y,  4 C,  4 M, and  4 K and an intermediate transfer belt  30 . The image forming devices  4 Y,  4 C,  4 M, and  4 K are situated in the center of a housing of the image forming apparatus  1 , and arranged side by side along a direction in which the intermediate transfer belt  30  is stretched. The image forming devices  4 Y,  4 C,  4 M, and  4 K have identical configurations while containing different colors of toner as developer. Specifically, the image forming devices  4 Y,  4 C,  4 M, and  4 K contain toner of yellow (Y), cyan (C), magenta (M), and black (K), respectively. The colors yellow, cyan, magenta, and black correspond to color separation components of a color image. 
     Each of the image forming devices  4 Y,  4 C,  4 M, and  4 K is an image station that includes, e.g., a drum-shaped photoconductor  5  as a latent image bearer, a charger  6  that charges the surface of the photoconductor  5 , a developing device  7  that supplies the surface of the photoconductor  5  with toner, and a cleaner  8  that cleans the surface of the photoconductor  5 , as illustrated in the image forming device  4 K, for example. 
     Below the image forming devices  4 Y,  4 M,  4 C, and  4 K is an exposure device  9  that exposes the surface of the photoconductor  5 . The exposure device  9  includes, e.g., a light source, a polygon mirror, an f-θ lens, and a reflection mirror to irradiate the surface of the photoconductor  5  with a laser beam according to image data. 
     A transfer device  3  is disposed above the image forming devices  4 Y,  4 C,  4 M, and  4 K. The transfer device  3  includes the intermediate transfer belt  30  as a transfer body, four primary transfer rollers  31  as primary transfer devices, a secondary transfer roller  36  as a secondary transfer device, a secondary transfer backup roller  32 , a cleaning backup roller  33 , a tension roller  34 , and a belt cleaner  35 . 
     The intermediate transfer belt  30  is an endless belt entrained around the secondary transfer backup roller  32 , the cleaning backup roller  33 , and the tension roller  34 . In the present embodiment, as a driver drives and rotates the secondary transfer backup roller  32  counterclockwise, the intermediate transfer belt  30  rotates in a counter-clockwise rotational direction R 1  as illustrated in  FIG. 1  by friction therebetween. 
     The four primary transfer rollers  31  sandwich the intermediate transfer belt  30  together with the respective photoconductors  5 , thereby forming four primary transfer areas herein referred to as primary transfer nips between the intermediate transfer belt  30  and the photoconductors  5 . A power supply of the image forming apparatus  1  is connected to the primary transfer rollers  31 . The power supply applies predetermined direct current (DC) voltage and/or alternating current (AC) voltage to each of the primary transfer rollers  31 . 
     The secondary transfer roller  36  sandwiches the intermediate transfer belt  30  together with the secondary transfer backup roller  32 , thereby forming a secondary transfer area herein referred to as a secondary transfer nip between the secondary transfer roller  36  and the intermediate transfer belt  30 . Similar to the primary transfer rollers  31 , the power supply of the image forming apparatus  1  is connected to the secondary transfer roller  36 . The power supply applies predetermined DC voltage and/or AC voltage to the secondary transfer roller  36 . 
     The belt cleaner  35  includes a cleaning brush and a cleaning blade that contact an outer circumferential surface of the intermediate transfer belt  30 . 
     A bottle holder  2  is disposed in an upper portion of the housing of the image forming apparatus  1 . The bottle holder  2  accommodates removable four toner bottles  2 Y,  2 C,  2 M, and  2 K that contain fresh toner of yellow, cyan, magenta, and black, respectively. Toner supply tubes are interposed between the toner bottles  2 Y,  2 C,  2 M, and  2 K and the respective developing devices  7 . The fresh toner is supplied from the toner bottles  2 Y,  2 C,  2 M, and  2 K to the respective developing devices  7  through the toner supply tubes. 
     In a lower portion of the housing of the image forming apparatus  1  are, e.g., a sheet tray  10  and a sheet feeding roller  11 . The sheet tray  10  accommodates a plurality of sheets P as recording media. The sheet feeding roller  11  picks up and feeds the plurality of sheets P one at a time from the sheet tray  10  toward the secondary transfer nip formed between the secondary transfer roller  36  and the intermediate transfer belt  30 . The sheets P as recording media may be plain paper, thick paper, postcards, envelopes, thin paper, coated paper, art paper, tracing paper, overhead projector (OHP) transparencies, and the like. Optionally, the image forming apparatus  1  may include a bypass feeder that imports such recording media placed on a bypass tray into the image forming apparatus  1 . 
     In the housing of the image forming apparatus  1  is a conveyance passage R defined by internal components of the image forming apparatus  1 . Along the conveyance passage R, the sheet P is conveyed from the sheet tray  10  to a sheet ejection roller pair  13  via the secondary transfer nip. The sheet ejection roller pair  13  ejects the sheet P outside the housing of the image forming apparatus  1 . Along the conveyance passage R are, e.g., a registration roller pair  12 , a fixing device  20 , and the sheet ejection roller pair  13 . The registration roller pair  12  is disposed upstream from the secondary transfer roller  36  in a sheet conveyance direction A 1  as a recording medium conveyance direction. The registration roller pair  12 , as a conveyance device, conveys the sheet P to the secondary transfer nip. 
     The fixing device  20  is disposed downstream from the secondary transfer roller  36  in the sheet conveyance direction A 1 . The fixing device  20  receives the sheet P bearing a toner image and fixes the toner image on the sheet P. The sheet ejection roller pair  13  is disposed downstream from the fixing device  20  in the sheet conveyance direction A 1 . The sheet ejection roller pair  13  ejects the sheet P onto an output tray  14 . The output tray  14  is disposed atop the housing of the image forming apparatus  1 . The plurality of sheets P ejected by the sheet ejection roller pair  13  rests on the output tray  14  one by one. 
     To provide a fuller understanding of embodiments of the present disclosure, a description is now given of an image forming operation of the image forming apparatus  1  with continued reference to  FIG. 1 . 
     When a print job starts, a driver drives and rotates the photoconductor  5  of each of the image forming devices  4 Y,  4 C,  4 M, and  4 K in a clockwise rotational direction R 2  as illustrated in  FIG. 1 . The charger  6  uniformly charges the surface of the photoconductor  5  to a predetermined polarity. The exposure device  9  irradiates the surface of the photoconductor  5  thus charged, with a laser beam to form an electrostatic latent image on the surface of the photoconductor  5  according to image data. It is to be noted that the image data is single-color image data obtained by separating a desired full-color image into individual color components, that is, yellow, cyan, magenta, and black components. The developing device  7  supplies toner to the electrostatic latent image thus formed on the surface of the photoconductor  5  to render the electrostatic latent image visible as a toner image. 
     Meanwhile, when the print job starts, the driver drives and rotates the secondary transfer backup roller  32  counterclockwise in  FIG. 1  to rotate the intermediate transfer belt  30  in the rotational direction R 1 . The power supply applies a constant voltage or constant current control voltage having a polarity opposite a polarity of the toner to each of the primary transfer rollers  31 . Accordingly, a transfer electric field is generated at each of the primary transfer nips between the primary transfer rollers  31  and the respective photoconductors  5 . 
     When the toner image formed on the photoconductor  5  reaches the primary transfer nip in accordance with rotation of the photoconductor  5 , the transfer electric field thus generated transfers the toner image from the photoconductor  5  onto the intermediate transfer belt  30 . Specifically, toner images of yellow, cyan, magenta, and black are superimposed one atop another while being transferred onto the intermediate transfer belt  30 . Thus, a full-color toner image is formed on the surface of the intermediate transfer belt  30 . The cleaner  8  removes residual toner, failed to be transferred onto the intermediate transfer belt  30  and therefore remaining on the surface of the photoconductor  5 , from the photoconductor  5 . Then, a discharger discharges the surface of the photoconductor  5  to initialize the surface potential of the photoconductor  5 . 
     In the lower portion of the image forming apparatus  1 , the sheet feeding roller  11  starts rotation to feed the sheet P from the sheet tray  10  toward the registration roller pair  12  along the conveyance passage R. The registration roller pair  12  is timed to convey the sheet P to the secondary transfer nip between the secondary transfer roller  36  and the intermediate transfer belt  30  so that the sheet P meets the full-color toner image formed on the surface of the intermediate transfer belt  30  at the secondary transfer nip. The secondary transfer roller  36  is applied with a transfer voltage having a polarity opposite a polarity of the charged toner contained in the full-color toner image formed on the intermediate transfer belt  30 , thereby generating a transfer electric field at the secondary transfer nip. 
     When the full-color toner image formed on the intermediate transfer belt  30  reaches the secondary transfer nip in accordance with rotation of the intermediate transfer belt  30 , the transfer electric field thus generated transfers the toner images of yellow, cyan, magenta, and black constructing the full-color toner image from the intermediate transfer belt  30  onto the sheet P collectively. The belt cleaner  35  removes residual toner, failed to be transferred onto the sheet P and therefore remaining on the intermediate transfer belt  30 , from the intermediate transfer belt  30 . The removed toner is conveyed and collected into the waste toner container disposed in the housing of the image forming apparatus  1 . 
     The sheet P bearing the full-color toner image is conveyed to the fixing device  20  that fixes the full-color toner image onto the sheet P. Then, the sheet P bearing the fixed full-color toner image is conveyed to the sheet ejection roller pair  13  that ejects the sheet P onto the output tray  14  atop the image forming apparatus  1 . Thus, the plurality of sheets P rests on the output tray  14 . 
     As described above, the image forming apparatus  1  forms a full-color image on a recording medium. Alternatively, the image forming apparatus  1  may use one of the image forming devices  4 Y,  4 C,  4 M, and  4 K to form a monochrome image, or may use two or three of the image forming devices  4 Y,  4 C,  4 M, and  4 K to form a bicolor or tricolor image, respectively. 
     Referring now to  FIGS. 2 and 3 , a description is given of the fixing device  20  incorporated in the image forming apparatus  1  described above. 
       FIG. 2  is a schematic cross-sectional view of the fixing device  20  according to a first embodiment of the present disclosure.  FIG. 3  is an exploded perspective view of a nip formation assembly  24 U incorporated in the fixing device  20 , illustrating relative positions of a nip formation pad  24 , a stay  25 , an end heater  26 , and a supplementary thermal conductor  27 . 
     The fixing device  20  (e.g., a fuser or a fuser unit) includes a fixing belt  21  formed into a loop, a pressure roller  22 , a temperature sensor  29 , a separator  40 , and various components disposed inside the loop formed by the fixing belt  21  such as a plurality of heaters  23 A and  23 B, the nip formation pad  24 , the stay  25 , the end heater  26 , the supplementary thermal conductor  27 , and a plurality of reflectors  28 A and  28 B. The fixing belt  21  and the components disposed inside the loop formed by the fixing belt  21  constitute a belt unit  21 U detachably coupled to the pressure roller  22 . The fixing belt  21  is an endless belt formed as a thin, flexible, tubular fixing rotator rotatable in a counter-clockwise rotational direction R 3  as illustrated in  FIG. 2 . The pressure roller  22  is a pressure rotator that is rotatable in a clockwise rotational direction R 4  as illustrated in  FIG. 2  and contacts an outer circumferential surface of the fixing belt  21  at an area of contact herein referred to as a fixing nip N. The fixing belt  21  is heated by heat radiating from the heaters  23 A and  23 B disposed inside the loop formed by the fixing belt  21 . In the present embodiment, the heaters  23 A and  23 B are halogen heaters. Alternatively, the heaters  23 A and  23 B may be induction heaters, resistance heat generators, carbon heaters, or the like. 
     The nip formation pad  24  extends in an axial direction, that is, a longitudinal direction, of the fixing belt  21  inside the loop formed by the fixing belt  21 . The nip formation pad  24  faces the pressure roller  22  via the fixing belt  21 , thereby forming the fixing nip N between the fixing belt  21  and the pressure roller  22 . The stay  25  is a support that supports the nip formation pad  24  inside the loop formed by the fixing belt  21 . Specifically, the stay  25  secures and supports the nip formation pad  24  against the pressure roller  22 . Thus, the stay  25  prevents bending of the nip formation pad  24 , thereby maintaining a uniform width of the fixing nip N throughout the length of the pressure roller  22  in an axial direction thereof. The nip formation pad  24  is made of a heat-resistant material having good mechanical strength and heatproof up to about 200° C. or higher. More specifically, the nip formation pad  24  is made of a heat-resistant resin such as polyimide (PI) resin, polyether ether ketone (PEEK) resin, or one of those resins reinforced with glass fibers. Such a material prevents deformation of the nip formation pad  24  due to heat at a toner fixing temperature, thereby securing a stable fixing nip N, keeping output image quality stable. Opposed end portions of the stay  25  in a longitudinal direction thereof parallel to the axial direction of the fixing belt  21  are secured to and thus held by a side plate of the fixing device  20  or a holder mounted on the side plate of the fixing device  20 . Similarly, opposed end portions of the heaters  23 A and  23 B in a longitudinal direction thereof parallel to the axial direction of the fixing belt  21  are secured to and thus held by the side plate of the fixing device  20  or the holder mounted on the side plate of the fixing device  20 . The end heater  26 , different from main heaters or fixing heaters (i.e., heaters  23 A and  23 B), includes end heaters  26   a  and  26   b  as illustrated in  FIG. 3 . The end heaters  26   a  and  26   b  are mounted on opposed end portions of the nip formation pad  24  in a longitudinal direction thereof, parallel to the axial direction of the fixing belt  21 , as integral parts of the nip formation pad  24 . In the present embodiment, the end heater  26  is a contact, heat-transfer heater such as a ceramic heater. 
     The supplementary thermal conductor  27  (e.g., thermal equalizer) facilitates heat transfer in the axial direction of the fixing belt  21 . Inside the loop formed by the fixing belt  21 , the supplementary thermal conductor  27  covers a nip formation face  24   c  of the nip formation pad  24  and the surface of the end heater  26  (i.e., end heaters  26   a  and  26   b ), both of which face an inner circumferential surface of the fixing belt  21 . For example, when a relatively small sheet is conveyed or when the end heater  26  is activated, the supplementary thermal conductor  27  prevents heat generated by the end heater  26  from being stored locally at an end portion of the fixing belt  21  and facilitates conduction of the heat in the axial direction of the fixing belt  21 , that is, a longitudinal direction of the supplementary thermal conductor  27 , thereby equalizing the temperature of the fixing belt  21  in the axial direction thereof. The supplementary thermal conductor  27  is made of a material that conducts heat well, that is, a material having enhanced thermal conductivity. The supplementary thermal conductor  27  has a flattened belt sliding-contact face  27   a  facing and directly contacting the inner circumferential surface of the fixing belt  21 , thus serving as a flat nip formation face. Alternatively, the belt sliding-contact face  27   a  of the supplementary thermal conductor  27  may be given a concave shape or another shape. For example, a concave nip formation face directs a leading edge of the sheet P toward the pressure roller  22  as the sheet P is ejected from the fixing nip N, thereby facilitating separation of the sheet P from the fixing belt  21  and preventing a paper jam. 
     As illustrated in  FIG. 2 , the temperature sensor  29  is disposed at a predetermined position opposite an outer circumferential surface of the fixing belt  21  to detect the temperature of the fixing belt  21 . The separator  40  is disposed downstream from the fixing nip N in the sheet conveyance direction A 1  to separate the sheet P from the fixing belt  21 . A pressure device is also disposed to press the pressure roller  22  against the fixing belt  21  and to separate the pressure roller  22  from the fixing belt  21 . 
     The fixing belt  21  is an endless belt that is thin as a film and having a decreased diameter to reduce thermal capacity. The fixing belt  21  is constructed of a base layer and a release layer coating the base layer. The base layer of the fixing belt  21  is made of a metal material, such as nickel or stainless steel (e.g., steel use stainless or SUS), or a resin material such as polyimide. The release layer of the fixing belt  21  is made of, e.g., tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), or polytetrafluoroethylene (PTFE). Optionally, an elastic layer made of an elastic material such as silicon rubber, silicon rubber foam, or fluoro rubber may be interposed between the base layer and the release layer of the fixing belt  21 . As the fixing belt  21  and the pressure roller  22  sandwich and press against the toner image on the sheet P passing through the fixing nip N, slight surface asperities in the fixing belt  21  may be transferred onto the toner image on the sheet P, resulting in variation in gloss of the toner image. To address this circumstance, the elastic layer may be provided with a thickness of about 100 μm. As the elastic layer deforms, the elastic layer absorbs the slight surface asperities in the fixing belt  21 , thereby preventing such variation in gloss of the toner image. The fixing belt  21  has an overall thickness not larger than about 1 mm and a diameter of from about 20 mm to about 40 mm to reduce thermal capacity. The base layer of the fixing belt  21  has a thickness of from about 20 μm to about 50 μm. The elastic layer of the fixing belt  21  has a thickness of from about 100 μm to about 300 μm. The release layer of the fixing belt  21  has a thickness of from about 10 μm to about 50 μm. To further reduce thermal capacity, preferably, the fixing belt  21  may have an overall thickness not larger than about 0.2 mm, and more preferably, not larger than about 0.16 mm while having a diameter not larger than about 30 mm. 
     The stay  25 , having a T-shaped cross section, includes a projection  25   a  and a base  25   b.  The projection  25   a  projects from the base  25   b  away from the fixing nip N in a direction perpendicular to the longitudinal direction of the stay  25 . The projection  25   a  separates the heaters  23 A and  23 B as main heaters from each other. One of the heaters  23 A and  23 B has a heat generation range at a center portion of the longitudinal direction thereof to heat the fixing belt  21  and fix a toner image on a relatively small sheet P. The other one of the heaters  23 A and  23 B has a heat generation range at each end portion in the longitudinal direction thereof to heat the fixing belt  21  and fix a toner image on a relatively large sheet P. The heaters  23 A and  23 B generates heat under output control of the power supply disposed in the housing of the image forming apparatus  1 , based on a surface temperature of the fixing belt  21  detected by the temperature sensor  29 , thereby setting the temperature of the fixing belt  21  to a desired fixing temperature. 
     The reflectors  28 A and  28 B are interposed between the stay  25  and the heaters  23 A and  23 B, respectively, to reflect light radiated from the heaters  23 A and  23 B toward the fixing belt  21 , thereby enhancing heating efficiency of the heaters  23 A and  23 B to heat the fixing belt  21 . The reflectors  28 A and  28 B prevent light and heat radiated from the heaters  23 A and  23 B from heating the stay  25 , suppressing waste of energy. Alternatively, instead of the reflectors  28 A and  28 B, the surface of the stay  25  facing the heaters  23 A and  23 B may be insulated or given a mirror finish to reflect light or heat radiating from the heaters  23 A and  23 B toward the fixing belt  21 . 
     The pressure roller  22  is constructed of a tube (e.g., metal tube), an elastic layer coating the tube, and a release layer coating the elastic layer. The elastic layer is made of rubber such as silicone rubber form or fluororubber. The release layer is made of PFA or PTFE to facilitate separation of the sheet P from the pressure roller  22 . As a biasing mechanism (e.g., spring) presses the pressure roller  22  against the fixing belt  21 , the elastic layer of the pressure roller  22  is deformed, forming an area of contact (e.g., fixing nip N) having a predetermined width between the fixing belt  21  and the pressure roller  22 . A driver such as a motor situated inside the housing of the image forming apparatus  1  drives and rotates the pressure roller  22  in the rotational direction R 4 . As the driver generates a driving force and rotates the pressure roller  22 , the driving force is transmitted from the pressure roller  22  to the fixing belt  21  at the fixing nip N, thereby rotating the fixing belt  21  in the rotational direction R 3 . At the fixing nip N, the fixing belt  21  rotates while being sandwiched between the pressure roller  22  and the nip formation pad  24  having the nip formation face  24   c  covered by the supplementary thermal conductor  27 . On the other hand, at a circumferential span of the fixing belt  21  other than the fixing nip N, the fixing belt  21  rotates while being guided by a flange  41  situated at each end portion of the fixing belt  21  in the axial direction thereof as illustrated in  FIG. 6 . 
     In the present embodiment, the pressure roller  22  is a solid roller. Alternatively, the pressure roller  22  may be a hollow roller, i.e., a tube. If the pressure roller  22  is a hollow roller, optionally a heater such as a halogen heater may be disposed inside the pressure roller  22 . The elastic layer may be made of solid rubber. Alternatively, if no heater is situated inside the pressure roller  22 , the elastic layer may be made of sponge rubber. The sponge rubber is preferable to solid rubber because the sponge rubber has enhanced insulation that draws less heat from the fixing belt  21 . 
     As illustrated in  FIG. 3 , the nip formation assembly  24 U includes the nip formation pad  24 , the stay  25 , the supplementary thermal conductor  27 , and the end heater  26 . The nip formation pad  24  has a surface facing away from the fixing nip N and engaging a flat surface of the stay  25  facing the fixing nip N. For example, the engaged surfaces of the nip formation pad  24  and the stay  25  may have convex and concave portions such as a pin and a boss, respectively, to be coupled to each other. The supplementary thermal conductor  27  is fitted on the nip formation pad  24  given an approximately rectangular shape, covering a surface of the nip formation pad  24  facing the inner circumferential surface of the fixing belt  21 . In the present embodiment, the supplementary thermal conductor  27  engages the nip formation pad  24  with, e.g., a projection. Alternatively, the supplementary thermal conductor  27  may be attached to the nip formation pad  24  with, e.g., an adhesive. Two recesses  24   a  and  24   b  that define a difference in thickness of the nip formation pad  24  are disposed at the opposed end portions of the nip formation pad  24  in the longitudinal direction thereof. The end heaters  26   a  and  26   b  that constitute the end heater  26  illustrated in  FIG. 2  are secured to the recesses  24   a  and  24   b,  respectively. Thus, the recesses  24   a  and  24   b  accommodate the end heaters  26   a  and  26   b,  respectively. 
     Although the belt sliding-contact face  27   a  of the supplementary thermal conductor  27  faces the inner circumferential surface of the fixing belt  21 , the nip formation face  24   c  of the nip formation pad  24  facing the pressure roller  22  actually forms the fixing nip N in view of the mechanical strength that the nip formation face  24   c  of the nip formation pad  24  provides. 
     Typically, in fixing devices including an endless fixing belt formed into a loop and having a relatively low thermal capacity, the fixing belt is rotated by rotation of a pressure roller disposed opposite the fixing belt. While rotating, the fixing belt slides over a high-thermal conductor attached to a nip formation pad secured in the loop formed by the fixing belt. To reduce friction resistance between the fixing belt and the thermal conductor, a low-friction sheet is generally disposed on a belt-sliding face of the thermal conductor over which the fixing belt slides. However, heating the low-friction sheet typically having a thickness of several hundred micrometers uses extra energy, and may hamper effective energy consumption of the fixing device. Therefore, such a low-friction sheet may be excluded from the fixing device. In the fixing devices without the low-friction sheet, the fixing belt directly contacts and slides over the thermal conductor. Since the thermal conductor is typically made of metal such as copper or aluminum having a relatively high heat conductivity, the fixing belt receives a relatively high load while sliding over the thermal conductor. Specifically, an inner circumferential surface of the fixing belt contacts corners and edges at opposed end portions of the thermal conductor in a longitudinal direction thereof and receives a relatively high load from the corners and edges of the thermal conductor. As a result, the fixing belt may be scraped, have a kink or plastic recess, or the like, thus being damaged. Such damage to the fixing belt prematurely shortens the working life of the fixing belt. 
       FIG. 4  is a cross-sectional view of the supplementary thermal conductor  27 . Typically, the supplementary thermal conductor  27  has an angular, U-shaped cross section. If no treatment is given, the supplementary thermal conductor  27  may have angular corners  27   b  and  27   c  as illustrated in  FIG. 2  at each end portion in the longitudinal direction thereof. In other words, the supplementary thermal conductor  27  may have angular corners  27   b  and  27   c  on an entry side N 1  of the fixing nip N, located upstream in the sheet conveyance direction A 1 , and on an exit side N 2  of the fixing nip N, located downstream in the sheet conveyance direction A 1 , respectively. Particularly, to facilitate separation of the sheet P from the fixing belt  21 , the supplementary thermal conductor  27  is inclined toward the fixing belt  21 , forming a convex portion  27   i  on the exit side N 2  of the fixing nip N. On the exit side N 2  of the fixing nip N, the angular corner  27   c  may have strong contact with and overload the inner circumferential surface of the fixing belt  21  on the exit side N 2  of the fixing nip N, damaging the inner circumferential surface of the fixing belt  21  by, e.g., scraping or producing a kink. Similarly, on the entry side N 1  of the fixing nip N, the angular corner  27   b  may have strong contact with the inner circumferential surface of the fixing belt  21  when the fixing belt  21  enters the fixing nip N, damaging the inner circumferential surface of the fixing belt  21  by, e.g., scraping or producing a kink. 
     Hence, in the present embodiment, the angular corners  27   b  and  27   c  of the belt sliding-contact face  27   a,  which are located on the entry side N 1  and the exit side N 2  of the fixing nip N, respectively, at each end portion of the supplementary thermal conductor  27 , are recessed, forming non-contact areas corresponding the angular corners  27   b  and  27   c  between the supplementary thermal conductor  27  and the fixing belt  21 , where the supplementary thermal conductor  27  does not contact the fixing belt  21 . 
     Referring now to  FIG. 5 , a description is given of a first example of the supplementary thermal conductor  27  having flank faces. 
       FIG. 5  is a perspective view of the first example of the supplementary thermal conductor  27  having flank faces  27   d  and  27   e  on the entry side N 1  and the exit side N 2  of the fixing nip N, respectively. 
     The fixing belt  21  has a larger distance from the flank faces  27   d  and  27   e  outwards in the axial direction of the fixing belt  21 . As illustrated in  FIG. 5 , the supplementary thermal conductor  27  has the flat, triangular flank faces  27   d  and  27   e  made by removing the angular corners  27   b  and  27   c,  respectively. The flank faces  27   d  and  27   e  serve as edge portions dimensioned to distance the supplementary thermal conductor  27  from the fixing belt  21 . When the fixing belt  21  slides over the belt sliding-contact face  27   a  of the supplementary thermal conductor  27 , the flank faces  27   d  and  27   e  reduce load on the inner circumferential surface of the fixing belt  21 , thereby preventing the inner circumferential surface of the fixing belt  21  from being damaged by being scraped, having a kink, or the like. The flank faces  27   d  and  27   e  are separated from each other in the sheet conveyance direction A 1  to secure functions of the end heaters  26   a  and  26   b  as contact, heat-transfer heaters. Specifically, the end heaters  26   a  and  26   b  are located between the flank faces  27   d  and  27   e,  within an area corresponding to the belt sliding-contact face  27   a  of the supplementary thermal conductor  27 . 
     Referring now to  FIG. 6 , a description is given of a configuration to regulate rotation of the fixing belt. 
       FIG. 6  is a schematic view of the fixing belt  21  and associated components, particularly illustrating the configuration to regulate rotation of the fixing belt  21 . 
     As described above, at the fixing nip N, the fixing belt  21  rotates while being sandwiched between the pressure roller  22  and the nip formation pad  24  having the nip formation face  24   c  covered by the supplementary thermal conductor  27 . On the other hand, at a circumferential span of the fixing belt  21  other than the fixing nip N, the fixing belt  21  rotates while being guided by the flange  41  situated at each end portion of the fixing belt  21  in the axial direction thereof. That is, the flange  41  regulates a rotational trajectory of the fixing belt  21  at each end portion of the fixing belt  21  in the axial direction thereof. On the other hand, at a center portion of the fixing belt  21  in the axial direction thereof, the fixing belt  21  is not regulated and pressed at the fixing nip N. As illustrated in  FIG. 6 , the flange  41  as a belt holder is inserted into each end portion of the fixing belt  21  in the axial direction thereof to hold each end portion of the fixing belt  21  in the axial direction thereof rotatably. The flange  41  includes an inserted portion  41   a,  a regulation portion  41   b,  and a secured portion  41   c.  Specifically, the inserted portion  41   a  is inserted into each end portion of the fixing belt  21  in the axial direction thereof The regulation portion  41   b  has a larger outer diameter than an outer diameter of the inserted portion  41   a.  The secured portion  41   c  is secured to a housing of the fixing device  20 . The regulation portion  41   b  has a larger outer diameter than at least an outer diameter of the fixing belt  21  to regulate deviation of the fixing belt  21  in the axial direction thereof. The flange  41  as a belt holder has a C-shaped cross section opening on the fixing nip side. In addition, the flange  41  holds an end portion of the stay  25 , thus secures the stay  25  at a predetermined position. 
     A slip ring  42  is interposed between an end face of the fixing belt  21  and the regulation portion  41   b  of the flange  41  disposed opposite the end face of the fixing belt  21 , serving as a ring protector to protect the end face of the fixing belt  21 . 
     Accordingly, the fixing belt  21  has different trajectories in the axial direction thereof during rotation. 
     Referring now to  FIGS. 7A through 8 , a description is given of the different trajectories of the fixing belt  21 . 
       FIG. 7A  is a schematic view of the fixing belt  21  and the supplementary thermal conductor  27  having the angular corners  27   b  and  27   c,  illustrating relative positions thereof at an end position.  FIG. 7B  is a schematic view of the fixing belt  21  and the supplementary thermal conductor  27  having the angular corners  27   b  and  27   c,  illustrating relative positions thereof at a center position.  FIG. 8  is a schematic view of the fixing belt  21  and the supplementary thermal conductor  27  having the flank faces  27   d  and  27   e,  illustrating relative positions thereof. 
     The fixing belt  21  rotates in a substantially round shape at the end position as the flange  41  regulates the end portion of the fixing belt  21  in the axial direction thereof. However, as the regulation exerted by the flange  41  has a decreased influence on the fixing belt  21  toward the center portion of the fixing belt  21  in the axial direction thereof, the fixing belt  21  is substantially free from the influence from the regulation exerted by the flange  41 . Therefore, compared to the center portion of the fixing belt  21 , each end portion of the fixing belt  21  in the axial direction thereof receives a stronger contact pressure from the supplementary thermal conductor  27  on both the entry side N 1  and the exit side N 2  of the fixing nip N. In particular, on the entry side N 1  of the fixing nip N, the fixing belt  21  comes into strong contact with the supplementary thermal conductor  27  as the fixing belt  21  enters the fixing nip N. Hence, as illustrated in  FIG. 8 , the supplementary thermal conductor  27  according to the present embodiment removes the angular corners  27   b  and  27   c  therefrom to form the flank faces  27   d  and  27   e  having sufficient surface area not to contact each end portion of the fixing belt  21  that is influenced by the regulation exerted by the flange  41 . Preferably, a starting point or inner point of each of the flank faces  27   d  and  27   e  in the longitudinal direction of the supplementary thermal conductor  27  may be distanced away from the flange  41  toward a center portion of the supplementary thermal conductor  27  to decrease a contact pressure from the supplementary thermal conductor  27  to the fixing belt  21  on both the entry side N 1  and the exit side N 2  of the fixing nip N, so as not to damage the inner circumferential surface of the fixing belt  21  by, e.g., scraping or producing a kink. In other words, the flank faces  27   d  and  27   e  preferably conform to the shape of the fixing belt  21  between the end portion of the fixing belt  21  regulated by the flange  41  to the center portion of the fixing belt  21  corresponding to the fixing nip N, in the axial direction of the fixing belt  21 . 
     The surface areas of the flank faces  27   d  and  27   e  may be equal to or different from each other, provided that the flank faces  27   d  and  27   e  reduce load on the inner circumferential surface of the fixing belt  21 . Similarly, the starting points or inner points of the flank faces  27   d  and  27   e  in the longitudinal direction of the supplementary thermal conductor  27  may be equal to or different from each other, provided that the flank faces  27   d  and  27   e  reduce load on the inner circumferential surface of the fixing belt  21 . 
     Referring back to  FIG. 5 , the flank faces  27   d  and  27   e  do not enter a fixing nip span  27   f,  which is subjected to a given pressure or a higher pressure from the pressure roller  22  via the fixing belt  21 . A flank face existing in the fixing nip span  27   f  might inflect a planar surface of the fixing nip span  27   f  and deform the fixing belt  21  and the pressure roller  22  locally, resulting in damage to the fixing belt  21  and the pressure roller  22 . Hence, in the present embodiment, the flank faces  27   d  and  27   e  do not enter the fixing nip span  27   f , maintaining the planar face of the fixing nip span  27   f  and preventing damage to the fixing belt  21  and the pressure roller  22 . An entry-side span  27   g  is a span of the belt sliding-contact face  27   a  in the longitudinal direction of the supplementary thermal conductor  27  on the entry side N 1  of the fixing nip N. The entry-side span  27   g  can be shorten provided that the flank face  27   d  is not present in the fixing nip span  27   f.  That is, on the entry side N 1  of the fixing nip N, which is an upstream side of the fixing nip N in the sheet conveyance direction A 1 , the flank face  27   d  may start inside an end point of the fixing nip N in a longitudinal direction thereof parallel to the axial direction of the fixing belt  21 , in the longitudinal direction of the supplementary thermal conductor  27 . 
     By contrast, as described above and illustrated in  FIG. 4 , the supplementary thermal conductor  27  is inclined or projecting toward the fixing belt  21  on the exit side N 2  of the fixing nip N to bend the fixing belt  21  so as to facilitate separation of the sheet P from the fixing belt  21 . An exit-side span  27   h  is a span of the belt sliding-contact face  27   a  in the longitudinal direction of the supplementary thermal conductor  27  on the exit side N 2  of the fixing nip N. As an increased inclined angle of the belt sliding-contact face  27   a  facilitates separation of the sheet P from the fixing belt  21 , the exit-side span  27   h  is larger than a sheet conveyance span CS as a recording medium conveyance span in the longitudinal direction of the supplementary thermal conductor  27 . The sheet conveyance span CS corresponds to a width of a sheet P having a maximum conveyable sheet size in the axial direction of the fixing belt  21 . In addition, as described above, the flank face  27   e  is not present in the fixing nip span  27   f.  Thus, on the exit side N 2  of the fixing nip N, which is a downstream side of the fixing nip N in the sheet conveyance direction A 1 , the flank face  27   e  is outside an end point of the sheet conveyance span CS in the longitudinal direction of the supplementary thermal conductor  27 . 
     Provided that the above-described requirements are satisfied on the entry side N 1  and the exit side N 2  of the fixing nip N, the starting points of the flank faces  27   d  and  27   e  can be determined in the longitudinal direction of the supplementary thermal conductor  27 . 
     Referring now to  FIG. 9 , a description is given of a second example of the supplementary thermal conductor  27  having flank faces. 
       FIG. 9  is a perspective view of a supplementary thermal conductor  47  as the second example of the supplementary thermal conductor  27  having flank faces. 
     The supplementary thermal conductor  47  has a basic configuration identical to the configuration of the supplementary thermal conductor  27  of  FIG. 5 . For example, the supplementary thermal conductor  47  has a flattened belt sliding-contact face  47   a  facing the inner circumferential surface of the fixing belt  21 . The belt sliding-contact face  47   a  includes a fixing nip span  47   f  subjected to pressure from the pressure roller  22  via the fixing belt  21 . An entry-side span  47   g  is a span of the belt sliding-contact face  47   a  in a longitudinal direction of the supplementary thermal conductor  47  on the entry side N 1  of the fixing nip N. An exit-side span  47   h  is a span of the belt sliding-contact face  47   a  in the longitudinal direction of the supplementary thermal conductor  47  on the exit side N 2  of the fixing nip N. 
     Instead of the flank faces  27   d  and  27   e  of  FIG. 5 , however, the supplementary thermal conductor  47  has curved flank faces  47   d  and  47   e  as edge portions dimensioned to distance the supplementary thermal conductor  47  from the fixing belt  21 . The trajectory of the fixing belt  21  on the entry side N 1  and the exit side N 2  of the fixing nip N is a curved shape while the fixing belt  21  rotates. Therefore, the curved flank faces  47   d  and  47   e  further reduces load on the fixing belt  21  compared to the flat flank faces  27   d  and  27   e.    
     Referring now to  FIGS. 10 through 12 , a description is given of variation of the first and second examples of the supplementary thermal conductor  27  described above. 
       FIG. 10  is a perspective view of a supplementary thermal conductor  27 X as a variation of the supplementary thermal conductor  27  of  FIG. 5 .  FIG. 11  is a perspective view of a supplementary thermal conductor  47 X as a variation of the supplementary thermal conductor  47  of  FIG. 9 .  FIG. 12  is a schematic view of the fixing belt  21  and associated components, particularly illustrating a supplementary thermal conductor  27 Y as another variation of the supplementary thermal conductor  27  of  FIG. 5 . 
     In  FIG. 10 , the flank face  27   d  is outside the end point of the fixing nip N in the longitudinal direction thereof, in the longitudinal direction of the supplementary thermal conductor  27 , on the entry side N 1  of the fixing nip N, which is the upstream side of the fixing nip N in the sheet conveyance direction A 1 . In addition,  FIG. 10  illustrates the flank faces  27   d  and  27   e  having substantially identical starting points in a longitudinal direction of the supplementary thermal conductor  27 X. Similarly,  FIG. 11  illustrates the flank faces  47   d  and  47   e  having substantially identical starting points in a longitudinal direction of the supplementary thermal conductor  47 X. In the configurations described above, the supplementary thermal conductor  27  has flank faces on both the entry side N 1  and exit side N 2  of the fixing nip N. Alternatively, the supplementary thermal conductor  27  may have a flank face on either one of the entry side N 1  and the exit side N 2  of the fixing nip N.  FIG. 12  illustrates the supplementary thermal conductor  27 Y having the flank face  27   e  on the exit side N 2  of the fixing nip N only. 
     In the embodiment described above, the supplementary thermal conductor (e.g., supplementary thermal conductors  27  and  47 ) is an elongated, relatively thin board having a U shape to cover the nip formation pad  24 , thus being secured at a predetermined position. A drawing process is executed to press down angular corners of the board, thereby forming the board into the U shape. Thus, the supplementary thermal conductors  27  and  47  are shaped as in, e.g.,  FIGS. 5 and 9 , respectively. The supplementary thermal conductor made of a metal material having enhanced thermal conductivity such as copper or aluminum facilitates a bending process including the drawing process. As the fixing belt  21  slides over a sliding contact surface (e.g., belt sliding-contact faces  27   a  and  47   a ) of the supplementary thermal conductor, abrasion powder may be generated on the sliding contact surface and may damage the supplementary thermal conductor and the inner circumferential surface of the fixing belt  21 . To prevent such damage, the supplementary thermal conductor may be preferably made of stainless steel having enhanced abrasion resistance. 
     The supplementary thermal conductors  27  and  47  and the variations thereof are fitted on the nip formation pad  24  so as to cover the nip formation face  24   c  of the nip formation pad  24 , thereby forming a U-shaped cross section. Alternatively, the supplementary thermal conductor may form a cross section of another shape if the supplementary thermal conductor is attached to the nip formation pad  24  or if the supplementary thermal conductor serves as a nip formation pad. In short, the supplementary thermal conductor may be an elongated, relatively thick cubic board having opposed end angular corners in the longitudinal direction thereof removed. 
     Referring now to  FIGS. 13A through 15 , a description is given of examples of the supplementary thermal conductor made of an elongated, relatively thick cubic board.  FIG. 13A  is a perspective view of a supplementary thermal conductor  57  as a third example of the supplementary thermal conductor  27  having flank faces.  FIG. 13B  is a side view of the supplementary thermal conductor  57 . The supplementary thermal conductor  57  is an elongated, relatively thick cubic board having angular corners and edge portions at opposed end portions in a longitudinal direction thereof removed in, e.g., a cutting process. Thus, the supplementary thermal conductor  57  has flank faces  57   c  and  57   d.    
     In the drawing process executed on the supplementary thermal conductor (e.g., supplementary thermal conductors  27  and  47 ) to press down angular corners, metal is bent into a desired shape. Therefore, a drastic change in shape of the supplementary thermal conductor may involve processing of adjacent components. On the other hand, the cutting process determines a component shape by removal of a part of the component. Although the component needs a certain thickness, the cutting process allows an accurate drastic change in shape of the component. Accordingly, cutting not only the corners but also the edge portions of a cubic board forms the flank faces  57   c  and  57   d  of the supplementary thermal conductor  57 . Without angular corners and edges in contact with the inner circumferential surface of the fixing belt  21 , the supplementary thermal conductor  57  reduces load on the inner circumferential surface of the fixing belt  21 . 
     The cutting process also allows refined processing.  FIG. 14  is a perspective view of a supplementary thermal conductor  67  as a variation of the supplementary thermal conductor  57 . In a refined cutting process, each edge and apex of a belt sliding-contact face  67   a  is formed into an arc, curved face  67   g.  Without angular corners and edges in contact with the inner circumferential surface of the fixing belt  21 , the supplementary thermal conductor  67  reduces load on the inner circumferential surface of the fixing belt  21 . 
       FIG. 15A  is a perspective view of a supplementary thermal conductor  77  as a fourth example of the supplementary thermal conductor  27  having flank faces.  FIG. 15B  is a side view of the supplementary thermal conductor  77 . The supplementary thermal conductor  77  is an elongated, relatively thick cubic board having curved flank faces. Specifically, the supplementary thermal conductor  77  has arc, curved faces  77   c  and  77   d  at opposed end portions of a belt sliding-contact face  77   a  in a longitudinal direction of the supplementary thermal conductor  77 . In addition, the supplementary thermal conductor  77  has edge surfaces on the entry side N 1  and the exit side N 2  of the fixing nip N curved not to affect separation of the sheet P from the fixing belt  21 . Since the supplementary thermal conductor  77  has such curved corners and edges at the opposed end portions in the longitudinal direction thereof, the inner circumferential surface of the fixing belt  21  does not contact angular corners and edges that increases the load on the inner circumferential surface of the fixing belt  21 . In addition, the curved corners and edges of the supplementary thermal conductor  77  prevent an excessive bending of the fixing belt  21 . Thus, the supplementary thermal conductor  77  reduces attrition of the inner circumferential surface of the fixing belt  21 . 
     Further to reduce attrition of the inner circumferential surface of the fixing belt  21 , the belt sliding-contact face (e.g., belt sliding-contact face  77   a ) of the supplementary thermal conductor (e.g., supplementary thermal conductor  77 ) may be coated with resin to enhance smooth sliding of the inner circumferential surface of the fixing belt  21  over the belt sliding-contact face. For example, a belt sliding-contact face made of PTFE resin exhibits enhanced non-adherence, in other words, releasability, and repels water effectively, allowing the inner circumferential surface of the fixing belt  21  to slide over the belt sliding-contact face smoothly, thereby reducing attrition of the inner circumferential surface of the fixing belt  21 . As another example, a belt sliding-contact face made of polyamide imide (PAI) resin exhibits enhanced heat resistance and wear resistance while facilitating smooth sliding of the inner circumferential surface of the fixing belt  21  over the belt sliding-contact face. Accordingly, such a belt sliding-contact face made of PAI reduces attrition of both the supplementary thermal conductor and the inner circumferential surface of the fixing belt  21 . Furthermore, a lubricant may be interposed between the inner circumferential surface of the fixing belt  21  and the belt sliding-contact face of the supplementary thermal conductor to dramatically reduce attrition of both the supplementary thermal conductor and the inner circumferential surface of the fixing belt  21 . For example, a grease with enhanced viscosity, that is, a decreased fluidity, may be used as a lubricant. Such a grease may remain between the supplementary thermal conductor and the inner circumferential surface of the fixing belt  21 , thereby maintaining the lubricity therebetween for a relatively long period of time. Preferably, the grease may have a fluorine base oil that exhibits enhanced heat resistance. 
     As illustrated in  FIGS. 4, 6 and 12 , the supplementary thermal conductor  27  includes the convex portion  27   i  on the exit side N 2  of the fixing nip N through the sheet conveyance span in the longitudinal direction of the supplementary thermal conductor  27 , so as to facilitate separation of the sheet P from the fixing belt  21  when the sheet P is ejected from the fixing nip N. On the other hand, as illustrated in  FIG. 6 , the nip formation pad  24  has portions facing and conforming to the flank faces  27   d  and  27   e  of the supplementary thermal conductor  27 . In addition, the nip formation pad  24  has a portion facing and conforming to the convex portion  27   i  of the supplementary thermal conductor  27 . 
     In the embodiment described above, the supplementary thermal conductor (e.g., supplementary thermal conductor  27 ) is incorporated in the fixing device  20  that includes the nip formation pad  24  provided with the end heater  26 . Alternatively, the supplementary thermal conductor may be incorporated in a fixing device without an end heater. 
       FIG. 16  is a schematic cross-sectional view of a fixing device  80  according to a second embodiment of the present disclosure. The basic configuration of the fixing device  80  is identical to the configuration of the fixing device  20 . However, unlike the fixing device  20 , the fixing device  80  includes one heater  23  and no end heater. For example, in the fixing device  20 , the nip formation pad  24  has the recess  24   a  that accommodates the end heater  26   a  as illustrated in  FIG. 6 . By contrast, in the fixing device  80 , the nip formation pad  24  does not have the recess  24   a.  That is, the nip formation face  24   c  of the nip formation pad  24  is a uniformly flat face in the longitudinal direction of the nip formation pad  24 . 
     According to the embodiments described above, a nip formation assembly (e.g., nip formation assembly  24 U) includes a supplementary thermal conductor (e.g., supplementary thermal conductor  27 ). The supplementary thermal conductor has an edge portion dimensioned to distance the supplementary thermal conductor from an endless belt or fixing rotator (e.g., fixing belt  21 ) at an end portion of the supplementary thermal conductor in a longitudinal direction thereof parallel to an axial direction of the endless belt. A distance between the edge portion of the supplementary thermal conductor and the endless belt increases toward an end portion of the endless belt in the axial direction thereof. Thus, the end portion of the supplementary thermal conductor in the longitudinal direction thereof, more specifically, an end portion of a belt sliding-contact face over which the endless belt slides, of the supplementary thermal conductor in the longitudinal direction thereof, does not contact an inner circumferential surface of the endless belt, thereby preventing the endless belt from being scraped or having a kink. In short, damage to the endless belt can be prevented. 
     The present disclosure has been described above with reference to specific embodiments. It is to be noted that the present disclosure is not limited to the details of the embodiments described above, but various modifications and enhancements are possible without departing from the scope of the present disclosure. It is therefore to be understood that the present disclosure may be practiced otherwise than as specifically described herein. For example, elements and/or features of different embodiments may be combined with each other and/or substituted for each other within the scope of the present disclosure. The number of constituent elements and their locations, shapes, and so forth are not limited to any of the structure for performing the methodology illustrated in the drawings.