Patent Publication Number: US-9851667-B2

Title: Fixing device and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application Nos. 2015-254800, filed on Dec. 25, 2015, and 2016-220299, filed on Nov. 11, 2016, in the Japanese Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Exemplary aspects of the present disclosure relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus incorporating the fixing device. 
     Description of the Background 
     Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium 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 on the recording medium, thus forming the image on the recording medium. 
     Such fixing device may include a fixing rotator, such as a fixing roller, a fixing belt (e.g., an endless belt), and a fixing film, heated by a heater and a pressure rotator, such as a pressure roller and a pressure belt, pressed against the fixing rotator to form a fixing nip therebetween through which a recording medium bearing a toner image is conveyed. As the recording medium bearing the toner image is conveyed through the fixing nip, the fixing rotator and the pressure rotator apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium. 
     SUMMARY 
     This specification describes below an improved fixing device. In one exemplary embodiment, the fixing device includes an endless belt that is flexible and formed into a loop. The endless belt is rotatable in a rotation direction. A pressure rotator is disposed opposite an outer circumferential surface of the endless belt. A first radiant heater is disposed inside the loop formed by the endless belt. The first radiant heater includes a first heat generator to heat the endless belt. A second radiant heater is disposed inside the loop formed by the endless belt. The second radiant heater includes a second heat generator, disposed outboard from the first heat generator in an axial direction of the endless belt, to heat the endless belt. A nip formation pad, disposed inside the loop formed by the endless belt, forms a fixing nip between the endless belt and the pressure rotator. The nip formation pad includes a nip-side face disposed opposite the endless belt. A contact heater is disposed at least at one lateral end of the nip formation pad in a longitudinal direction of the nip formation pad. The contact heater includes a nip-side face disposed opposite the endless belt and a third heat generator to heat at least one lateral end of the endless belt in the axial direction of the endless belt. A thermal conduction aid, covering the nip-side face of the nip formation pad and the nip-side face of the contact heater, conducts heat applied to the endless belt in the axial direction of the endless belt. The thermal conduction aid includes a heater-side face being disposed opposite the contact heater and covering at least the third heat generator of the contact heater. 
     This specification further describes an improved image forming apparatus. In one exemplary embodiment, the image forming apparatus includes an image forming device to form a toner image and a fixing device disposed downstream from the image forming device in a recording medium conveyance direction to fix the toner image on a recording medium. The fixing device includes an endless belt that is flexible and formed into a loop. The endless belt is rotatable in a rotation direction. A pressure rotator is disposed opposite an outer circumferential surface of the endless belt. A first radiant heater is disposed inside the loop formed by the endless belt. The first radiant heater includes a first heat generator to heat the endless belt. A second radiant heater is disposed inside the loop formed by the endless belt. The second radiant heater includes a second heat generator, disposed outboard from the first heat generator in an axial direction of the endless belt, to heat the endless belt. A nip formation pad, disposed inside the loop formed by the endless belt, forms a fixing nip between the endless belt and the pressure rotator. The nip formation pad includes a nip-side face disposed opposite the endless belt. A contact heater is disposed at least at one lateral end of the nip formation pad in a longitudinal direction of the nip formation pad. The contact heater includes a nip-side face disposed opposite the endless belt and a third heat generator to heat at least one lateral end of the endless belt in the axial direction of the endless belt. A thermal conduction aid, covering the nip-side face of the nip formation pad and the nip-side face of the contact heater, conducts heat applied to the endless belt in the axial direction of the endless belt. The thermal conduction aid includes a heater-side face being disposed opposite the contact heater and covering at least the third heat generator of the contact heater. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the embodiments and many of the attendant advantages and features thereof can be readily obtained and understood from the following detailed description with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic vertical cross-sectional view of an image forming apparatus according to an exemplary embodiment of the present disclosure; 
         FIG. 2  is a vertical cross-sectional view of a fixing device incorporated in the image forming apparatus depicted in  FIG. 1 ; 
         FIG. 3  is a perspective view of a nip formation unit incorporated in the fixing device depicted in  FIG. 2 ; 
         FIG. 4  is a perspective view of the nip formation unit depicted in  FIG. 3  and halogen heaters incorporated in the fixing device depicted in  FIG. 2 ; 
         FIG. 5  is a diagram of the halogen heaters depicted in  FIG. 4  and lateral end heaters incorporated in the nip formation unit depicted in  FIG. 3 ; 
         FIG. 6  is a diagram illustrating a positional relation between a heat generator of the halogen heater and a heat generator of the lateral end heater depicted in  FIG. 5  and a heat output rate of heat output from the heat generators; 
         FIG. 7  is a graph illustrating a curve that represents a heat output rate of heat output from the halogen heater depicted in  FIG. 6  under a first pattern; 
         FIG. 8  is a graph illustrating a heat output rate of heat output from the halogen heaters depicted in  FIG. 5  under a second pattern; 
         FIG. 9  is a graph illustrating a curve that represents a combined heat output rate of heat output from the halogen heaters depicted in  FIG. 5  under the second pattern; 
         FIG. 10  is a graph illustrating a curve that represents a combined heat output rate of heat output from the halogen heaters depicted in  FIG. 5  under a third pattern; 
         FIG. 11  is a plan view of a temperature detector and a fixing belt incorporated in the fixing device depicted in  FIG. 2 ; 
         FIG. 12A  is a cross-sectional view of the lateral end heater and a thermal conduction aid incorporated in the fixing device depicted in  FIG. 2  according to a first exemplary embodiment; 
         FIG. 12B  is a front view of the lateral end heater and the thermal conduction aid depicted in  FIG. 12A ; 
         FIG. 12C  is a side view of the lateral end heater and the thermal conduction aid depicted in  FIG. 12A ; 
         FIG. 13A  is a cross-sectional view of the lateral end heater and the thermal conduction aid incorporated in the fixing device depicted in  FIG. 2  according to a variation of the first exemplary embodiment; 
         FIG. 13B  is a front view of the lateral end heater and the thermal conduction aid depicted in  FIG. 13A ; 
         FIG. 13C  is a side view of the lateral end heater and the thermal conduction aid depicted in  FIG. 13A ; 
         FIG. 14A  is a cross-sectional view of the lateral end heater and the thermal conduction aid incorporated in the fixing device depicted in  FIG. 2  according to a second exemplary embodiment; 
         FIG. 14B  is a front view of the lateral end heater and the thermal conduction aid depicted in  FIG. 14A ; 
         FIG. 14C  is a side view of the lateral end heater and the thermal conduction aid depicted in  FIG. 14A ; 
         FIG. 15A  is a cross-sectional view of the lateral end heater and the thermal conduction aid incorporated in the fixing device depicted in  FIG. 2  according to a third exemplary embodiment; 
         FIG. 15B  is a front view of the lateral end heater and the thermal conduction aid depicted in  FIG. 15A ; 
         FIG. 15C  is a side view of the lateral end heater and the thermal conduction aid depicted in  FIG. 15A ; and 
         FIG. 16  is a front view of the lateral end heater and the thermal conduction aid incorporated in the fixing device depicted in  FIG. 2 , illustrating a variation of a resistive heat generator incorporated in the lateral end heater. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. Also, identical or similar reference numerals designate identical or similar components throughout the several views. 
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this 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 a similar function, operate in a similar manner, and achieve a similar result. 
     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. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 1 , an image forming apparatus  1  according to an exemplary embodiment is explained. 
       FIG. 1  is a schematic vertical cross-sectional 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. According to this exemplary embodiment, the image forming apparatus  1  is a color printer that forms color and monochrome toner images on a recording medium by electrophotography. Alternatively, the image forming apparatus  1  may be a monochrome printer that forms a monochrome toner image on a recording medium. 
     Referring to  FIG. 1 , a description is provided of a construction of the image forming apparatus  1 . 
     As illustrated in  FIG. 1 , the image forming apparatus  1  is a color laser printer including four image forming devices  4 Y,  4 C,  4 M, and  4 K situated in a center portion of the image forming apparatus  1 . The image forming devices  4 Y,  4 C,  4 M, and  4 K are aligned in a stretch direction in which an intermediate transfer belt  30  is stretched. Although the image forming devices  4 Y,  4 C,  4 M, and  4 K contain developers in different colors, that is, yellow, cyan, magenta, and black corresponding to color separation components of a color image (e.g., yellow, cyan, magenta, and black toners), respectively, the image forming devices  4 Y,  4 C,  4 M, and  4 K have an identical structure. 
     For example, each of the image forming devices  4 Y,  4 C,  4 M, and  4 K, serving as an image forming station, includes a drum-shaped photoconductor  5  serving as a latent image bearer or an image bearer that bears an electrostatic latent image and a resultant toner image; a charger  6  that charges an outer circumferential surface of the photoconductor  5 ; a developing device  7  that supplies toner to the electrostatic latent image formed on the outer circumferential surface of the photoconductor  5 , thus visualizing the electrostatic latent image as a toner image; and a cleaner  8  that cleans the outer circumferential surface of the photoconductor  5 .  FIG. 1  illustrates reference numerals assigned to the photoconductor  5 , the charger  6 , the developing device  7 , and the cleaner  8  of the image forming device  4 K that forms a black toner image. However, reference numerals for the image forming devices  4 Y,  4 C, and  4 M that form yellow, cyan, and magenta toner images, respectively, are omitted. 
     Below the image forming devices  4 Y,  4 C,  4 M, and  4 K is an exposure device  9  that exposes the outer circumferential surface of the respective photoconductors  5  with laser beams. For example, the exposure device  9 , constructed of a light source, a polygon mirror, an f-θ lens, reflection mirrors, and the like, emits a laser beam onto the outer circumferential surface of the respective photoconductors  5  according to image data sent from an external device such as a client computer. 
     Above the image forming devices  4 Y,  4 C,  4 M, and  4 K is a transfer device  3 . For example, the transfer device  3  includes the intermediate transfer belt  30  serving as a transferred image bearer, four primary transfer rollers  31  serving as primary transferors, and a secondary transfer roller  36  serving as a secondary transferor. The transfer device  3  further includes 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 stretched taut across the secondary transfer backup roller  32 , the cleaning backup roller  33 , and the tension roller  34 . As a driver drives and rotates the secondary transfer backup roller  32  counterclockwise in  FIG. 1 , the secondary transfer backup roller  32  rotates the intermediate transfer belt  30  counterclockwise in  FIG. 1  in a rotation direction D 30  by friction therebetween. 
     The four primary transfer rollers  31  sandwich the intermediate transfer belt  30  together with the four photoconductors  5 , forming four primary transfer nips between the intermediate transfer belt  30  and the photoconductors  5 , respectively. The primary transfer rollers  31  are coupled to a power supply disposed inside the image forming apparatus  1 . The power supply applies at least one of a predetermined direct current (DC) voltage and a predetermined 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 , forming a secondary transfer nip between the secondary transfer roller  36  and the intermediate transfer belt  30 . Similar to the primary transfer rollers  31 , the secondary transfer roller  36  is coupled to the power supply disposed inside the image forming apparatus  1 . The power supply applies at least one of a predetermined direct current (DC) voltage and a predetermined alternating current (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  situated in an upper portion of the image forming apparatus  1  accommodates four toner bottles  2 Y,  2 C,  2 M, and  2 K detachably attached to the bottle holder  2 . The toner bottles  2 Y,  2 C,  2 M, and  2 K contain fresh yellow, cyan, magenta, and black toners to be supplied to the developing devices  7  of the image forming devices  4 Y,  4 C,  4 M, and  4 K, respectively. For example, the fresh yellow, cyan, magenta, and black toners are supplied from the toner bottles  2 Y,  2 C,  2 M, and  2 K to the developing devices  7  through toner supply tubes interposed between the toner bottles  2 Y,  2 C,  2 M, and  2 K and the developing devices  7 , respectively. 
     In a lower portion of the image forming apparatus  1  are a paper tray  10  that loads a plurality of sheets P serving as recording media and a feed roller  11  that picks up and feeds a sheet P from the paper tray  10  toward the secondary transfer nip formed between the secondary transfer roller  36  and the intermediate transfer belt  30 . The sheets P may be thick paper, postcards, envelopes, plain paper, thin paper, coated paper, art paper, tracing paper, overhead projector (OHP) transparencies, and the like. Optionally, a bypass tray that loads thick paper, postcards, envelopes, thin paper, coated paper, art paper, tracing paper, OHP transparencies, and the like may be attached to the image forming apparatus  1 . 
     A conveyance path R extends from the feed roller  11  to an output roller pair  13  to convey the sheet P picked up from the paper tray  10  onto an outside of the image forming apparatus  1  through the secondary transfer nip. The conveyance path R is provided with a registration roller pair  12  located below the secondary transfer nip formed between the secondary transfer roller  36  and the intermediate transfer belt  30 , that is, upstream from the secondary transfer nip in a sheet conveyance direction DP. The registration roller pair  12  serving as a conveyor conveys the sheet P conveyed from the feed roller  11  toward the secondary transfer nip. 
     The conveyance path R is further provided with a fixing device  20  located above the secondary transfer nip, that is, downstream from the secondary transfer nip in the sheet conveyance direction DP. The fixing device  20  fixes an unfixed toner image, which is transferred from the intermediate transfer belt  30  onto the sheet P, on the sheet P. The conveyance path R is further provided with the output roller pair  13  located above the fixing device  20 , that is, downstream from the fixing device  20  in the sheet conveyance direction DP. The output roller pair  13  ejects the sheet P bearing the fixed toner image onto the outside of the image forming apparatus  1 , that is, an output tray  14  disposed atop the image forming apparatus  1 . The output tray  14  stocks the sheet P ejected by the output roller pair  13 . 
     Referring to  FIG. 1 , a description is provided of an image forming operation performed by the image forming apparatus  1  having the construction described above to form a full color toner image on a sheet P. 
     As a print job starts, a driver drives and rotates the photoconductors  5  of the image forming devices  4 Y,  4 C,  4 M, and  4 K, respectively, clockwise in  FIG. 1  in a rotation direction D 5 . The chargers  6  uniformly charge the outer circumferential surface of the respective photoconductors  5  at a predetermined polarity. The exposure device  9  emits laser beams onto the charged outer circumferential surface of the respective photoconductors  5  according to yellow, cyan, magenta, and black image data constructing color image data sent from the external device, respectively, thus forming electrostatic latent images on the photoconductors  5 . The image data used to expose the respective photoconductors  5  is monochrome image data produced by decomposing a desired full color image into yellow, cyan, magenta, and black image data. The developing devices  7  supply yellow, cyan, magenta, and black toners to the electrostatic latent images formed on the photoconductors  5 , visualizing the electrostatic latent images as yellow, cyan, magenta, and black toner images, respectively. 
     Simultaneously, as the print job starts, the secondary transfer backup roller  32  is driven and rotated counterclockwise in  FIG. 1 , rotating the intermediate transfer belt  30  in the rotation direction D 30  by friction therebetween. The power supply applies a constant voltage or a constant current control voltage having a polarity opposite a polarity of the charged toner to the primary transfer rollers  31 , creating a transfer electric field at each of the primary transfer nips formed between the photoconductors  5  and the primary transfer rollers  31 , respectively. 
     When the yellow, cyan, magenta, and black toner images formed on the photoconductors  5  reach the primary transfer nips, respectively, in accordance with rotation of the photoconductors  5 , the yellow, cyan, magenta, and black toner images are primarily transferred from the photoconductors  5  onto the intermediate transfer belt  30  by the transfer electric field created at the primary transfer nips such that the yellow, cyan, magenta, and black toner images are superimposed successively on a same position on the intermediate transfer belt  30 . Thus, a full color toner image is formed on the outer circumferential surface of the intermediate transfer belt  30 . After the primary transfer of the yellow, cyan, magenta, and black toner images from the photoconductors  5  onto the intermediate transfer belt  30 , the cleaners  8  remove residual toner failed to be transferred onto the intermediate transfer belt  30  and therefore remaining on the photoconductors  5  therefrom, respectively. Thereafter, dischargers discharge the outer circumferential surface of the respective photoconductors  5 , initializing the surface potential thereof. 
     On the other hand, the feed roller  11  disposed in the lower portion of the image forming apparatus  1  is driven and rotated to feed a sheet P from the paper tray  10  toward the registration roller pair  12  through the conveyance path R. The registration roller pair  12  conveys the sheet P sent to the conveyance path R by the feed roller  11  to the secondary transfer nip formed between the secondary transfer roller  36  and the intermediate transfer belt  30  at a proper time. The secondary transfer roller  36  is applied with a transfer voltage having a polarity opposite a polarity of the charged yellow, cyan, magenta, and black toners constructing the full color toner image formed on the intermediate transfer belt  30 , thus creating a transfer electric field at the secondary transfer nip. 
     As the yellow, cyan, magenta, and black toner images constructing the full color toner image on the intermediate transfer belt  30  reach the secondary transfer nip in accordance with rotation of the intermediate transfer belt  30 , the transfer electric field created at the secondary transfer nip secondarily transfers the yellow, cyan, magenta, and black toner images from the intermediate transfer belt  30  onto the sheet P collectively. After the secondary transfer of the full color toner image from the intermediate transfer belt  30  onto the sheet P, the belt cleaner  35  removes residual toner failed to be transferred onto the sheet P and therefore remaining on the intermediate transfer belt  30  therefrom. The removed toner is conveyed and collected into a waste toner container situated inside the image forming apparatus  1 . 
     Thereafter, the sheet P bearing the full color toner image is conveyed to the fixing device  20  that fixes the full color toner image on the sheet P. The sheet P bearing the fixed full color toner image is ejected by the output roller pair  13  onto the outside of the image forming apparatus  1 , that is, the output tray  14  that stocks the sheet P. 
     The above describes the image forming operation of the image forming apparatus  1  to form the full color toner image on the sheet P. Alternatively, the image forming apparatus  1  may form a monochrome toner image by using any one of the four image forming devices  4 Y,  4 C,  4 M, and  4 K or may form a bicolor toner image or a tricolor toner image by using two or three of the image forming devices  4 Y,  4 C,  4 M, and  4 K. 
     Referring to  FIG. 2 , a description is provided of a construction of the fixing device  20  incorporated in the image forming apparatus  1  having the construction described above. 
       FIG. 2  is a schematic vertical cross-sectional view of the fixing device  20 . The fixing device  20  (e.g., a fuser or a fusing unit) includes a fixing belt  21  and a pressure roller  22 . The fixing belt  21 , serving as a fixing rotator, is an endless belt that is thin, flexible, tubular, and rotatable in a rotation direction D 21 . The pressure roller  22 , serving as a pressure rotator, contacts an outer circumferential surface of the fixing belt  21 . The pressure roller  22  is rotatable in a rotation direction D 22 . Inside a loop formed by the fixing belt  21  is a plurality of heaters or a plurality of fixing heaters, that is, a halogen heater  23 A serving as a first radiant heater and a halogen heater  23 B serving as a second radiant heater that heat the fixing belt  21  with radiant heat. Each of the halogen heaters  23 A and  23 B is a radiant heater serving as a main heater or a fixing heater. 
     Inside the loop formed by the fixing belt  21  are a nip formation pad  24 , a stay  25 , lateral end heaters  26 , a thermal conduction aid  27 , and reflectors  28 A and  28 B. The components disposed inside the loop formed by the fixing belt  21 , that is, the halogen heaters  23 A and  23 B, the nip formation pad  24 , the stay  25 , the lateral end heaters  26 , the thermal conduction aid  27 , and the reflectors  28 A and  28 B, may construct a belt unit  21 U separably coupled with the pressure roller  22 . The nip formation pad  24  presses against the pressure roller  22  via the fixing belt  21  to form a fixing nip N between the fixing belt  21  and the pressure roller  22 . The stay  25 , serving as a support, supports the nip formation pad  24 . 
     A detailed description is now given of a configuration of the nip formation pad  24 . 
     The nip formation pad  24  extending in a longitudinal direction thereof parallel to an axial direction of the fixing belt  21  is secured to and supported by the stay  25 . Accordingly, even if the nip formation pad  24  receives pressure from the pressure roller  22 , the stay  25  prevents the nip formation pad  24  from being bent by the pressure and therefore allows the nip formation pad  24  to produce a uniform nip length throughout the entire width of the pressure roller  22  in an axial direction or a longitudinal direction thereof. The nip formation pad  24  is made of a heat resistant material being resistant against temperatures up to 200 degrees centigrade and having an enhanced mechanical strength. For example, the nip formation pad  24  is made of heat resistant resin such as polyimide (PI), polyether ether ketone (PEEK), and PI or PEEK reinforced with glass fiber. Thus, the nip formation pad  24  is immune from thermal deformation at temperatures in a fixing temperature range desirable to fix a toner image on a sheet P, retaining the shape of the fixing nip N and quality of the toner image formed on the sheet P. Both lateral ends of the stay  25  and the halogen heaters  23 A and  23 B in a longitudinal direction thereof are secured to and supported by a pair of side plates of the fixing device  20  or a pair of holders provided separately from the pair of side plates, respectively. 
     A detailed description is now given of a configuration of the lateral end heaters  26 . 
     The lateral end heaters  26  are mounted on or coupled with both lateral ends of the nip formation pad  24  in the longitudinal direction thereof, respectively. The lateral end heaters  26  serve as a sub heater provided separately from the main heater or the fixing heater (e.g., the halogen heaters  23 A and  23 B). The lateral end heaters  26  heat both lateral ends of the fixing belt  21  in the axial direction thereof, respectively. The lateral end heater  26  is a contact heater that contacts the fixing belt  21  to conduct heat to the fixing belt  21 , for example, a resistive heat generator such as a ceramic heater. 
     A detailed description is now given of a configuration of the thermal conduction aid  27 . 
     The thermal conduction aid  27  also serves as a thermal equalizer that decreases a temperature gradient of the fixing belt  21  in the axial direction thereof. The thermal conduction aid  27  covers a nip-side face of each of the nip formation pad  24  and the lateral end heaters  26 , which is disposed opposite an inner circumferential surface of the fixing belt  21 . The thermal conduction aid  27  conducts and equalizes heat in a longitudinal direction of the thermal conduction aid  27  that is parallel to the axial direction of the fixing belt  21 , preventing heat from being stored at both lateral ends of the fixing belt  21  in the axial direction thereof while a plurality of small sheets P is conveyed over the fixing belt  21  or while the lateral end heaters  26  are turned on. Thus, the thermal conduction aid  27  eliminates uneven temperature of the fixing belt  21  in the axial direction thereof. Hence, the thermal conduction aid  27  is made of a material that conducts heat quickly, for example, a material having an enhanced thermal conductivity such as copper and aluminum. The thermal conduction aid  27  includes a nip-side face  27   a  being disposed opposite and in direct contact with the inner circumferential surface of the fixing belt  21 , thus serving as a nip formation face that forms the fixing nip N. 
     As illustrated in  FIG. 2 , the nip-side face  27   a  is planar. Alternatively, the nip-side face  27   a  may be curved or recessed or may have other shapes. If the nip-side face  27   a  is recessed with respect to the pressure roller  22 , the nip-side face  27   a  directs a leading edge of the sheet P toward the pressure roller  22  as the sheet P is ejected from the fixing nip N, facilitating separation of the sheet P from the fixing belt  21  and suppressing jamming of the sheet P between the fixing belt  21  and the pressure roller  22 . 
     A temperature sensor  29  is disposed opposite the outer circumferential surface of the fixing belt  21  at a proper position thereon, for example, a position upstream from the fixing nip N in the rotation direction D 21  of the fixing belt  21 . The temperature sensor  29  detects the temperature of the fixing belt  21 . A separator  41  is disposed downstream from the fixing nip N in the sheet conveyance direction DP to separate the sheet P from the fixing belt  21 . A pressurization assembly presses the pressure roller  22  against the nip formation pad  24  via the fixing belt  21  and releases pressure exerted by the pressure roller  22  to the fixing belt  21 . 
     A detailed description is now given of a construction of the fixing belt  21 . 
     In order to decrease a thermal capacity of the fixing belt  21 , the fixing belt  21 , that is, an endless belt being thin like film and having a downsized loop diameter, is constructed of a base layer serving as the inner circumferential surface of the fixing belt  21  and a release layer serving as the outer circumferential surface of the fixing belt  21 . The base layer is made of metal such as nickel and SUS stainless steel or resin such as PI. The release layer is made of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), polytetrafluoroethylene (PTFE), or the like. Optionally, an elastic layer made of rubber such as silicone rubber, silicone rubber foam, and fluoro rubber may be interposed between the base layer and the release layer. While the fixing belt  21  and the pressure roller  22  pressingly sandwich the unfixed toner image on the sheet P to fix the toner image on the sheet P, the elastic layer having a thickness of about 100 micrometers elastically deforms to absorb slight surface asperities of the fixing belt  21 , preventing variation in gloss of the toner image on the sheet P. 
     In order to decrease the thermal capacity of the fixing belt  21 , the fixing belt  21  has a total thickness not greater than 1 mm and a loop diameter in a range of from 20 mm to 40 mm. For example, the fixing belt  21  is constructed of the base layer having a thickness in a range of from 20 micrometers to 50 micrometers; the elastic layer having a thickness in a range of from 100 micrometers to 300 micrometers; and the release layer having a thickness in a range of from 10 micrometers to 50 micrometers. In order to decrease the thermal capacity of the fixing belt  21  further, the fixing belt  21  may have a total thickness not greater than 0.20 mm and preferably not greater than 0.16 mm. The loop diameter of the fixing belt  21  is not greater than 30 mm. 
     A detailed description is now given of a construction of the stay  25 . 
     The stay  25 , having a T-shape in cross-section, includes a base  25   b  disposed opposite the fixing nip N and an arm  25   a  projecting from the base  25   b  and being disposed opposite the nip formation pad  24  via the base  25   b . In other words, the arm  25   a  of the stay  25  projects from the nip formation pad  24  in a pressurization direction PR in which the pressure roller  22  presses against the nip formation pad  24  via the fixing belt  21 . The arm  25   a  is interposed between the halogen heaters  23 A and  23 B serving as the main heater to screen the halogen heater  23 A from the halogen heater  23 B. 
     A detailed description is now given of a construction of the halogen heaters  23 A and  23 B. 
     The halogen heater  23 A includes a center heat generator disposed in a center span of the halogen heater  23 A in the longitudinal direction thereof. A small sheet P is disposed opposite the center heat generator of the halogen heater  23 A. The halogen heater  23 B includes a lateral end heat generator disposed in each lateral end span of the halogen heater  23 B in the longitudinal direction thereof. A large sheet P is disposed opposite the lateral end heat generator of the halogen heater  23 B. The power supply situated inside the image forming apparatus  1  supplies power to the halogen heaters  23 A and  23 B so that the halogen heaters  23 A and  23 B generate heat. A controller operatively connected to the halogen heaters  23 A and  23 B and the temperature sensor  29  controls the halogen heaters  23 A and  23 B based on the temperature of the outer circumferential surface of the fixing belt  21 , which is detected by the temperature sensor  29  disposed opposite the outer circumferential surface of the fixing belt  21 . Thus, the temperature of the fixing belt  21  is adjusted to a desired fixing temperature. 
     A detailed description is now given of a configuration of the reflectors  28 A and  28 B. 
     The reflector  28 A is interposed between the halogen heater  23 A and the stay  25 . The reflector  28 B is interposed between the halogen heater  23 B and the stay  25 . The reflectors  28 A and  28 B reflect light and heat radiated from the halogen heaters  23 A and  23 B to the reflectors  28 A and  28 B, respectively, toward the fixing belt  21 , thus enhancing heating efficiency of the halogen heaters  23 A and  23 B to heat the fixing belt  21 . Additionally, the reflectors  28 A and  28 B prevent light and heat radiated from the halogen heaters  23 A and  23 B from heating the stay  25  with radiant heat, suppressing waste of energy. Alternatively, instead of the reflectors  28 A and  28 B, an opposed face of the stay  25  disposed opposite the halogen heaters  23 A and  23 B may be treated with insulation or mirror finish to reflect light and heat radiated from the halogen heaters  23 A and  23 B to the stay  25  toward the fixing belt  21 . 
     A detailed description is now given of a construction of the pressure roller  22 . 
     The pressure roller  22  is constructed of a cored bar; an elastic layer coating the cored bar and being made of silicone rubber foam, fluoro rubber, or the like; and a release layer coating the elastic layer and being made of PFA, PTFE, or the like. The pressurization assembly such as a spring presses the pressure roller  22  against the fixing belt  21  to form the fixing nip N. The pressure roller  22  pressingly contacting the fixing belt  21  deforms the elastic layer of the pressure roller  22  at the fixing nip N formed between the pressure roller  22  and the fixing belt  21 , thus defining the fixing nip N having a predetermined length in the sheet conveyance direction DP. 
     A driver (e.g., a motor) disposed inside the image forming apparatus  1  depicted in  FIG. 1  drives and rotates the pressure roller  22 . As the driver drives and rotates the pressure roller  22 , a driving force of the driver is transmitted from the pressure roller  22  to the fixing belt  21  at the fixing nip N, thus rotating the fixing belt  21  in accordance with rotation of the pressure roller  22  by friction between the pressure roller  22  and the fixing belt  21 . Alternatively, the driver may also be connected to the fixing belt  21  to drive and rotate the fixing belt  21 . In a nip span Na of the fixing nip N, the fixing belt  21  rotates as the fixing belt  21  is sandwiched between the pressure roller  22  and the nip formation pad  24 ; in a circumferential span of the fixing belt  21  other than the nip span Na, the fixing belt  21  rotates while the fixing belt  21  is guided by flanges secured to the pair of side plates at both lateral ends of the fixing belt  21  in the axial direction thereof, respectively. 
     According to this exemplary embodiment, the pressure roller  22  is a solid roller. Alternatively, the pressure roller  22  may be a hollow roller. In this case, a heater such as a halogen heater may be disposed inside the hollow roller. The elastic layer of the pressure roller  22  may be made of solid rubber. Alternatively, if no heater is situated inside the pressure roller  22 , the elastic layer of the pressure roller  22  may be made of sponge rubber. The sponge rubber is more preferable than the solid rubber because the sponge rubber has an increased insulation that draws less heat from the fixing belt  21 . 
     Referring to  FIG. 3 , a description is provided of a construction of a nip formation unit  200  incorporated in the fixing device  20  depicted in  FIG. 2 . 
       FIG. 3  is a perspective view of the nip formation unit  200 , illustrating a basic structure of the nip formation unit  200 . As illustrated in  FIG. 3 , the nip formation unit  200  includes the nip formation pad  24 , the stay  25 , the thermal conduction aid  27 , and lateral end heaters  26   a  and  26   b  illustrated as the lateral end heaters  26  in  FIG. 2 . The nip formation pad  24  includes a nip-side face  24   c  facing the fixing nip N and a stay-side face  24   d  being opposite the nip-side face  24   c  and facing the stay  25 . The stay  25  includes a nip-side face  25   c  being planar and facing the fixing nip N. The stay-side face  24   d  of the nip formation pad  24  contacts the nip-side face  25   c  of the stay  25 . For example, the stay-side face  24   d  of the nip formation pad  24  and the nip-side face  25   c  of the stay  25  mount a recess and a projection (e.g., a boss and a pin), respectively, so that the stay-side face  24   d  engages the nip-side face  25   c  to restrict each other with the shape of the stay-side face  24   d  and the nip-side face  25   c.    
     The thermal conduction aid  27  engages the nip formation pad  24  that is substantially rectangular such that the thermal conduction aid  27  covers the nip-side face  24   c  of the nip formation pad  24  that is disposed opposite the inner circumferential surface of the fixing belt  21 . Thus, the thermal conduction aid  27  is coupled with the nip formation pad  24 . For example, the thermal conduction aid  27  is coupled with the nip formation pad  24  with a claw, an adhesive, or the like. Two recesses  24   a  and  24   b , each of which defines a difference in thickness of the nip formation pad  24 , are disposed at both lateral ends of the nip formation pad  24  in the longitudinal direction thereof, respectively. The lateral end heaters  26   a  and  26   b  are secured to the recesses  24   a  and  24   b , thus being accommodated by the recesses  24   a  and  24   b , respectively. A description of a positional relation between the lateral end heaters  26   a  and  26   b  and the halogen heaters  23 A and  23 B is deferred. 
     The thermal conduction aid  27  includes the nip-side face  27   a  that is disposed opposite the inner circumferential surface of the fixing belt  21 . The nip-side face  27   a  serves as a slide face over which the fixing belt  21  slides. However, since the nip-side face  24   c  of the nip formation pad  24  has a mechanical strength greater than that of the nip-side face  27   a  of the thermal conduction aid  27 , the nip-side face  24   c  of the nip formation pad  24  serves as a nip formation face that faces the pressure roller  22  and forms the fixing nip N practically. 
     A description is provided of a construction of a comparative fixing device. 
     The comparative fixing device includes a thin, flexible endless belt to be heated quickly to a fixing temperature at which a toner image is fixed on a sheet and a nip formation unit located inside a loop formed by the endless belt. The nip formation unit presses against a pressure roller via the endless belt to form a fixing nip between the endless belt and the pressure roller. A plurality of halogen heaters is situated inside the loop formed by the endless belt. The halogen heaters include heat generators that have different light distributions in an axial direction of the endless belt parallel to a width direction of the sheet, respectively. 
     A plurality of lateral end heaters is disposed opposite both lateral ends of the endless belt in the axial direction thereof, respectively, and upstream from the fixing nip in a rotation direction of the endless belt so as to heat an increased heating span of the endless belt corresponding to a width of a large sheet in the axial direction of the endless belt. The lateral end heaters locally contact an inner circumferential surface or an outer circumferential surface of the endless belt. The local lateral end heaters heat the increased heating span of the endless belt corresponding to the width of the large sheet with a simple construction not incorporating an extra halogen heater used to heat the large sheet. 
     However, if the endless belt does not contact the lateral end heaters precisely, heat generated by the lateral end heaters may be conducted to the endless belt unevenly, degrading heating efficiency. 
     The lateral end heaters, disposed upstream from the fixing nip in the rotation direction of the endless belt, heat both lateral ends of the endless belt in the axial direction thereof, respectively. While the endless belt rotates, both lateral ends of the endless belt in the axial direction thereof may flap and therefore may not precisely contact the lateral end heaters, respectively. To address this circumstance, the lateral end heaters are pressed against both lateral ends of the endless belt in the axial direction thereof, respectively, with predetermined pressure. Accordingly, the endless belt is exerted with pressure at a position other than the fixing nip, resulting in faulty motion of the endless belt. 
     Additionally, the lateral end heaters may melt residual toner failed to be fixed on a previous sheet at the fixing nip and therefore remaining on the endless belt again on both lateral ends of the endless belt in the axial direction thereof, which contact the lateral end heaters, respectively. The melted toner may adhere to the endless belt and damage a toner image on a subsequent sheet, degrading quality of the toner image on the subsequent sheet. 
     To address those circumstances of the comparative fixing device, according to this exemplary embodiment illustrated in  FIG. 3 , the lateral end heaters  26   a  and  26   b  are coupled with the nip formation pad  24  to form the fixing nip N. Each of the lateral end heaters  26   a  and  26   b  includes a nip-side face  26   c  disposed opposite the inner circumferential surface of the fixing belt  21 . The nip-side face  26   c  of each of the lateral end heaters  26   a  and  26   b  is leveled with the nip-side face  24   c  of the nip formation pad  24  that is disposed opposite the inner circumferential surface of the fixing belt  21  in the pressurization direction PR depicted in  FIG. 2  of the pressure roller  22  so that the nip-side faces  26   c  and the nip-side face  24   c  define an identical plane. Accordingly, the pressure roller  22  is pressed against the lateral end heaters  26   a  and  26   b  via the fixing belt  21  and the thermal conduction aid  27  sufficiently. 
     Consequently, the fixing belt  21  rotates stably in a state in which the fixing belt  21  is pressed against the lateral end heaters  26   a  and  26   b  or adhered to the lateral end heaters  26   a  and  26   b  indirectly via the thermal conduction aid  27 . The fixing belt  21  is pressed against the lateral end heaters  26   a  and  26   b  with sufficient pressure, retaining improved heating efficiency of the lateral end heaters  26   a  and  26   b . Hence, the fixing device  20  enhances reliability. 
     Contrarily to the lateral end heaters of the comparative fixing device, the lateral end heaters  26   a  and  26   b  of the fixing device  20  depicted in  FIGS. 2 and 3  are disposed opposite the fixing nip N. Accordingly, the lateral end heaters  26   a  and  26   b  heat the fixing belt  21  in the nip span Na in the rotation direction D 21  of the fixing belt  21 . That is, the lateral end heaters  26   a  and  26   b  do not heat the fixing belt  21  in the circumferential span outboard from the nip span Na in the rotation direction D 21  of the fixing belt  21  unlike the lateral end heaters of the comparative fixing device that are disposed upstream from the fixing nip in the rotation direction of the endless belt to heat the endless belt in a circumferential span outboard from the fixing nip in the rotation direction of the endless belt. Hence, the lateral end heaters  26   a  and  26   b  of the fixing device  20  prevent residual toner failed to be fixed on a previous sheet P and therefore adhering to the fixing belt  21  from being melted again and degrading a toner image on a subsequent sheet P. 
       FIG. 4  is a perspective view of the nip formation unit  200  and the halogen heaters  23 A and  23 B. As illustrated in  FIG. 4 , the stay  25  includes a first portion  25 A and a second portion  25 B, each of which is substantially L-shaped in cross-section. Thus, the stay  25  is substantially T-shaped in cross-section. Accordingly, the stay  25  attains an enhanced rigidity that prevents the nip formation pad  24  from being bent by pressure from the pressure roller  22 . The stay  25  constructed of the first portion  25 A and the second portion  25 B extends linearly in the longitudinal direction of the nip formation pad  24 . The stay  25  is secured to the nip formation pad  24 . Accordingly, the stay  25  renders the nip-side face  24   c  depicted in  FIG. 3  of the nip formation pad  24  to form the fixing nip N precisely throughout the entire width of the fixing nip N in the longitudinal direction of the nip formation pad  24 . 
     As illustrated in  FIG. 4 , the halogen heater  23 A is disposed opposite the halogen heater  23 B via the arm  25   a  of the stay  25  in a short direction perpendicular to the longitudinal direction of the stay  25 . The arm  25   a  is interposed between the halogen heaters  23 A and  23 B to screen the halogen heater  23 A from the halogen heater  23 B. Accordingly, while the halogen heaters  23 A and  23 B are powered on, glass tubes of the halogen heaters  23 A and  23 B, respectively, do not heat each other, preventing degradation in heating efficiency of the halogen heaters  23 A and  23 B. As illustrated in  FIG. 2 , each of the halogen heaters  23 A and  23 B is not surrounded by the stay  25 . For example, a center of each of the halogen heaters  23 A and  23 B in cross-section is outside a space defined or enclosed by the stay  25 . Accordingly, the halogen heaters  23 A and  23 B attain obtuse irradiation angles α and β, respectively, of light that irradiates the fixing belt  21 , thus improving heating efficiency. 
     Alternatively, the stay  25  may have shapes other than the substantially T-shape in cross-section. The first portion  25 A and the second portion  25 B depicted in  FIG. 4  may curve and extend in the longitudinal direction of the halogen heaters  23 A and  23 B as long as the arm  25   a  interposed between the halogen heaters  23 A and  23 B screens the halogen heater  23 A from the halogen heater  23 B. The arm  25   a  of each of the first portion  25 A and the second portion  25 B may be oblique relative to the nip-side face  24   c  of the nip formation pad  24 . 
     A description is provided of arrangement of the lateral end heaters  26   a  and  26   b  to correspond to sheets P of special sizes such as an A3 extension size sheet. 
       FIG. 5  is a diagram of the halogen heaters  23 A and  23 B and the lateral end heaters  26   a  and  26   b , illustrating arrangement thereof. As illustrated in  FIG. 5 , the halogen heater  23 A includes a heat generator  40 A serving as a center heat generator having a dense light distribution in the center span of the halogen heater  23 A, which is disposed opposite a center span of the fixing belt  21  in the axial direction thereof. The halogen heater  23 B includes a heat generator  40 B serving as a lateral end heat generator having a dense light distribution in each lateral end span of the halogen heater  23 B, which is disposed opposite each lateral end span of the fixing belt  21  in the axial direction thereof. The halogen heater  23 A heats the center span of the fixing belt  21  in the axial direction thereof. The halogen heater  23 B heats each lateral end span of the fixing belt  21  in the axial direction thereof. 
     The heat generator  40 A of the halogen heater  23 A corresponds to small sheets P of small sizes such as an A4 size sheet in portrait orientation. The heat generator  40 B of the halogen heater  23 B corresponds to large sheets P of large sizes such as an A3 size sheet in portrait orientation. The heat generator  40 B is disposed outboard from the heat generator  40 A in the longitudinal direction of the halogen heater  23 A so that the heat generator  40 B heats a lateral end of the large sheet P that is outboard from the heat generator  40 A in the longitudinal direction of the halogen heater  23 B. The large sheets P include a maximum standard size sheet available in the fixing device  20 . A heat generator  40  constructed of the heat generator  40 A and the heat generators  40 B corresponds to a width of the maximum standard size sheet (e.g., the A3 size sheet in portrait orientation) and does not encompass a width of an extra-large sheet P of an extension size, which is greater than the width of the maximum standard size sheet. 
     The lateral end heaters  26   a  and  26   b  are disposed opposite both lateral ends of the halogen heater  23 B in the longitudinal direction thereof, respectively. The lateral end heaters  26   a  and  26   b  include heat generators  42   a  and  42   b  that heat both lateral ends of the extra-large sheet P greater than the maximum standard size sheet in the longitudinal direction of the halogen heater  23 B, respectively. Thus, a heat generator  42  constructed of the heat generator  40 A, the heat generators  40 B, and the heat generators  42   a  and  42   b  corresponds to the width of the extra-large sheet P of the extension size (e.g., the A3 extension size sheet and a 13-inch sheet). A part of each of the heat generators  42   a  and  42   b  overlaps the heat generator  40 B in the longitudinal direction of the halogen heater  23 B. Accordingly, the fixing belt  21  of the fixing device  20  heats both lateral ends of the extra-large sheet P greater than the maximum standard size sheet in the longitudinal direction of the halogen heater  23 B. 
     A description is provided of an amount of heat output by the halogen heaters  23 A and  23 B and the lateral end heaters  26   a  and  26   b  to heat the fixing belt  21 . 
       FIG. 6  is a diagram illustrating a positional relation between the heat generator  40 B of the halogen heater  23 B and the heat generator  42   b  of the lateral end heater  26   b  and a heat output rate of heat output by the heat generators  40 B and  42   b . An upper part of  FIG. 6  illustrates a right lateral end of the heat generator  40 B of the halogen heater  23 B. A lower part of  FIG. 6  illustrates a left lateral end of the heat generator  42   b  of the lateral end heater  26   b.    
     Generally, a heat generator, in which a filament is coiled helically, of a halogen heater suffers from decrease in heat output at a lateral end of the heat generator in a longitudinal direction of the halogen heater. The decrease in heat output varies depending on a density of the filament coiled helically. The smaller the density of the filament coiled helically is, the more the halogen heater is susceptible to the decrease in heat output. As illustrated in the upper part in  FIG. 6 , a lateral end of the heat generator  40 B in the longitudinal direction of the halogen heater  23 B, which suffers from the decrease in heat output is defined as a span from a position at which the heat generator  40 B attains a predetermined heat output rate of 100 percent to a position at which the heat generator  40 B suffers from a decreased heat output rate of 50 percent, for example. 
     As illustrated in the lower part in  FIG. 6 , the heat generator  42   b  includes a heat generation pattern  37 , that is, a pattern defined by a resistive heat generator described below. A lateral end of the lateral end heater  26   b  that is inboard from the heat generator  42   b  in a longitudinal direction of the lateral end heater  26   b  suffers from the decrease in heat output. The lateral end of the lateral end heater  26   b  in the longitudinal direction thereof fails to attain the predetermined heat output rate of 100 percent and suffers from a decreased heat output rate. 
     Accordingly, as the lateral end of the halogen heater  23 B and the lateral end heater  26   b  in the longitudinal direction thereof suffers from the decrease in heat output, a toner image formed on the lateral end of the extra-large sheet P greater than the maximum standard size sheet may not be fixed on the extra-large sheet P properly. 
     To address this circumstance, a border Bh at which heat output from the heat generator  40 B of the halogen heater  23 B starts decreasing corresponds to a border Bc at which heat output from the heat generator  42   b  of the lateral end heater  26   b  starts decreasing. Since the halogen heater  23 B is spaced apart from the lateral end heater  26   b  as illustrated in  FIG. 2 , the border Bh coincides with the border Bc in the longitudinal direction of the halogen heater  23 B on a projection. Similarly, the border Bh at which heat output from another heat generator  40 B of the halogen heater  23 B starts decreasing corresponds to the border Bc at which heat output from the heat generator  42   a  of the lateral end heater  26   a  starts decreasing. 
     Accordingly, the heat generator  42  depicted in  FIG. 5  is immune from decrease in heat output in an overlap span where the heat generator  40 B of the halogen heater  23 B overlaps the lateral end heater  26   a  and an overlap span where the heat generator  40 B of the halogen heater  23 B overlaps the lateral end heater  26   b  in the longitudinal direction of the halogen heater  23 B, thus retaining the predetermined heat output rate of 100 percent. Consequently, even when the extra-large sheet P greater than the maximum standard size sheet is conveyed over the fixing belt  21 , the toner image formed on each lateral end of the extra-large sheet P in a width direction of the extra-large sheet P is fixed on the extra-large sheet P properly. 
     As illustrated in  FIG. 6 , the border Bh at which heat output from the heat generator  40 B of the halogen heater  23 B starts decreasing coincides with the border Bc at which heat output from the heat generator  42   b  of the lateral end heater  26   b  starts decreasing. However, as illustrated in  FIG. 3 , the nip formation unit  200  incorporates the thermal conduction aid  27  having an enhanced thermal conductivity that offsets a certain amount of decrease in heat output from the heat generators  40 B and  42   b  and therefore equalizes the temperature of the fixing belt  21 . Hence, the position of the border Bc at which heat output from the heat generators  42   a  and  42   b  of the lateral end heaters  26   a  and  26   b , respectively, starts decreasing may be determined within a predetermined allowable range. 
     A description is provided of positioning of the border Bc, that is, an inboard lateral edge of the heat generator  42   b  of the lateral end heater  26   b  in the longitudinal direction of the lateral end heater  26   b , at which heat output from the heat generator  42   b  starts decreasing. 
     Referring to graphs illustrating heat output from the halogen heaters  23 A and  23 B, positioning of the border Bc is explained with three patterns. The position of the border Bc is determined within the predetermined allowable range. 
     A description is provided of a first pattern of positioning of the border Bc. 
       FIG. 7  is a graph illustrating a curve C 1  that represents a heat output rate of heat output from the halogen heater  23 B serving as a second radiant heater under the first pattern.  FIG. 7  illustrates heat output from one lateral end of the halogen heater  23 B in the longitudinal direction thereof. In the graph depicted in  FIG. 7 , a vertical axis represents a heat output rate in percentage of the halogen heater  23 B relative to a predetermined heat output rate. A horizontal axis represents the position of the halogen heater  23 B in the longitudinal direction thereof. The graph depicted in  FIG. 7  illustrates the curve C 1  with a vertex like a parabola. 
     As illustrated in  FIG. 7 , the border Bc, that is, the inboard lateral edge of the heat generator  42   b  in the longitudinal direction of the lateral end heater  26   b , at which heat output from the heat generator  42   b  of the lateral end heater  26   b  starts decreasing, is situated in a border span A. The border span A is defined from an outboard position P 1  to an inboard position P 2  in the longitudinal direction of the halogen heater  23 B. At the outboard position P 1 , heat output from the heat generator  40 B of the halogen heater  23 B attains a heat output rate of 40 percent relative to a peak heat output rate. At the inboard position P 2 , heat output from the heat generator  40 B of the halogen heater  23 B attains a heat output rate of 80 percent relative to the peak heat output rate. The border Bc situated in the border span A renders the heat output rate of heat output from an inboard lateral end of the lateral end heater  26   b  and an outboard lateral end of the halogen heater  23 B in the longitudinal direction thereof to be within the predetermined allowable range. 
     A description is provided of a second pattern of positioning of the border Bc. 
       FIG. 8  is a graph illustrating a heat output rate of heat output from the halogen heater  23 A having the heat generator  40 A situated in the center span of the halogen heater  23 A and the halogen heater  23 B having the heat generators  40 B situated in each lateral end span of the halogen heater  23 B under the second pattern. In the graph depicted in  FIG. 8 , a curve CA in a dotted line represents heat output from the halogen heater  23 A. A curve CB in a solid line represents heat output from the halogen heater  23 B. A width W 1  represents a width of an A 4  size sheet in portrait orientation in the axial direction of the fixing belt  21 . A width W 2  represents a width of an A 4  size sheet in landscape orientation in the axial direction of the fixing belt  21  as a width of the maximum standard size sheet. The halogen heaters  23 A and  23 B that have different light distributions in the longitudinal direction thereof and therefore have different heat output patterns provide different total heat output patterns, respectively. 
       FIG. 9  is a graph illustrating a curve C 2  that represents a combined heat output rate of heat output from the halogen heaters  23 A and  23 B under the second pattern. As illustrated in  FIG. 9 , the combined heat output rate of the halogen heaters  23 A and  23 B attains the predetermined heat output rate of 100 percent at a position in proximity to each lateral end of the halogen heater  23 B in the longitudinal direction thereof and a heat output rate of almost 100 percent in the center span of the halogen heater  23 A in the longitudinal direction thereof, rendering the curve C 2  to be gentle. 
     In  FIG. 9 , a span B represents a span where the combined heat output rate of the halogen heaters  23 A and  23 B attains the heat output rate of almost 100 percent constantly. A span C represents a span where the combined heat output rate of the halogen heaters  23 A and  23 B attains a heat output rate in a range of from 40 percent to almost 100 percent. The border Bc is disposed in a border span D defined from the outboard position P 1  where the halogen heater  23 B attains the heat output rate of 40 percent to an inboard position P 3  being inboard from the outboard position P 1  in the longitudinal direction of the halogen heater  23 B by a combined span of the span C and one tenth of the span B. The border Bc situated in the border span D renders the heat output rate of the inboard lateral end of the lateral end heater  26   b  and the outboard lateral end of the halogen heater  23 B in the longitudinal direction thereof to be within the predetermined allowable range. 
     A description is provided of a third pattern of positioning of the border Bc. 
       FIG. 10  is a graph illustrating a curve C 3  that represents a combined heat output rate of heat output from the halogen heaters  23 A and  23 B under the third pattern as a variation. As illustrated in  FIG. 10 , a center part C 3   c  of the curve C 3  is gentle. Both lateral end parts C 3   e  of the curve C 3  indicate a heat output rate greater than a heat output rate indicated by the center part C 3   c . The curve C 3  is obtained with the filament of each of the heat generators  40 B of the halogen heater  23 B, which is coiled more densely than the filament of the heat generator  40 A of the halogen heater  23 A. 
     In  FIG. 10 , a span B′ represents a span where the combined heat output rate of the halogen heaters  23 A and  23 B attains the heat output rate of almost 100 percent. The span B′ bridges the lateral end parts C 3   e . The span C represents the span where the combined heat output rate of the halogen heaters  23 A and  23 B attains the heat output rate in the range of from 40 percent to almost 100 percent. The border Bc is disposed in a border span D′ defined from the outboard position P 1  where the halogen heater  23 B attains the heat output rate of 40 percent to an inboard position P 3 ′ being inboard from the outboard position P 1  in the longitudinal direction of the halogen heater  23 B by a combined span of the span C and one tenth of the span B′. The border Bc situated in the border span D′ renders the heat output rate of the inboard lateral end of the lateral end heater  26   b  and the outboard lateral end of the halogen heater  23 B in the longitudinal direction thereof to be within the predetermined allowable range. 
     A description is provided of an advantageous configuration of the fixing device  20 . 
     Since the inner circumferential surface of the fixing belt  21  slides over the thermal conduction aid  27 , if the thermal conduction aid  27  is made of metal such as copper and aluminum, the thermal conduction aid  27  may increase a coefficient of friction between the fixing belt  21  and the thermal conduction aid  27 . As the coefficient of friction increases, a unit torque of the fixing device  20  may increase, shortening a life of the fixing device  20 . 
     To address this circumstance, as illustrated in  FIG. 3 , the thermal conduction aid  27  incorporates the nip-side face  27   a  being disposed opposite and in contact with the fixing belt  21  such that the fixing belt  21  slides over the nip-side face  27   a . The nip-side face  27   a  is smooth and treated with processing to reduce friction. For example, the nip-side face  27   a  is coated with a fluorine material such as PFA and PTFE or treated with other coating to reduce friction between the thermal conduction aid  27  and the inner circumferential surface of the fixing belt  21 . Alternatively, a lubricant such as fluorine grease and silicone oil is applied between the thermal conduction aid  27  and the inner circumferential surface of the fixing belt  21  to reduce friction further. 
     A description is provided of a configuration of another temperature detector separately provided from the temperature sensor  29  depicted in  FIG. 2 , which detects the temperature of the fixing belt  21  heated by the lateral end heater  26  (e.g., the lateral end heaters  26   a  and  26   b ). 
     A contact sensor (e.g., a thermistor) is employed to detect the temperature of the fixing belt  21  precisely at reduced costs. However, the contact sensor may produce slight scratches at a contact position on the fixing belt  21  where the contact sensor contacts the fixing belt  21 . The slight scratches may damage a toner image formed on a sheet P while the sheet P is conveyed over the fixing belt  21 , generating slight variation in gloss of the toner image on the sheet P or the like. To address this circumstance, in the image forming apparatus  1  that forms a color toner image on a sheet P, the contact sensor is not situated within a conveyance span in the axial direction of the fixing belt  21  where the maximum standard size sheet is conveyed over the fixing belt  21 . 
     The extra-large sheet P, that is, an extension size sheet, includes an extension portion used as an edge or a margin abutting on a toner image formed in proximity to a lateral edge of the maximum standard size sheet, a portion where a linear image called a trim mark used for alignment in printing positions is formed, or a portion where a solid patch having a small area for color adjustment is formed. Finally, the extension portion is often trimmed. Hence, even if the contact sensor produces scratches on the fixing belt  21  and the scratches damage a toner image formed on the extension portion of the extra-large sheet P with slight variation in gloss of the toner image or the like, the damaged toner image does not appear on the extra-large sheet P as a faulty toner image after the extension portion is trimmed. 
     Accordingly, as illustrated in  FIG. 11 , the fixing device  20  according to this exemplary embodiment includes a plurality of temperature detectors  45   a  and  45   b , disposed opposite both lateral ends of the fixing belt  21  in the axial direction thereof, to detect the temperature of both lateral ends of the fixing belt  21  that are heated by the lateral end heaters  26   a  and  26   b , respectively. 
     A description is provided of a configuration of the temperature detectors  45   a  and  45   b.    
       FIG. 11  is a plan view of the temperature detector  45   b  and the fixing belt  21 .  FIG. 11  omits illustration of the temperature detector  45   a  disposed symmetrical with the temperature detector  45   b.    
     Each of the temperature detectors  45   a  and  45   b  is disposed opposite the outer circumferential surface of the fixing belt  21  and disposed outboard from the conveyance span of the maximum standard size sheet in the axial direction of the fixing belt  21 . Each of the temperature detectors  45   a  and  45   b  is disposed within a span W being outboard from a lateral edge of the maximum standard size sheet and inboard from a lateral edge of the extra-large sheet P greater than the maximum standard size sheet in the axial direction of the fixing belt  21 . Accordingly, the temperature detectors  45   a  and  45   b  detect the temperature of the fixing belt  21  heated by the lateral end heaters  26   a  and  26   b , respectively, precisely at reduced costs while preventing a faulty toner image that suffers from slight variation in gloss or the like from appearing on the extra-large sheet P.  FIG. 11  illustrates the width W 2  of the A 4  size sheet in landscape orientation in the axial direction of the fixing belt  21  as the width of the maximum standard size sheet and a width W 3  of the extra-large sheet P in the axial direction of the fixing belt  21  as a width of a maximum extension size sheet. 
     The above describes the configuration of the temperature detectors  45   a  and  45   b  that detect the temperature of both lateral ends of the fixing belt  21  that are heated by the lateral end heaters  26   a  and  26   b , respectively. Alternatively, the fixing device  20  may include a sensor that detects the temperature of a part of the lateral end heaters  26   a  and  26   b  so that the controller controls the lateral end heaters  26   a  and  26   b  based on the temperature of the lateral end heaters  26   a  and  26   b  that is detected by the sensor. 
     A description is provided of three exemplary embodiments of a construction of the lateral end heaters  26   a  and  26   b  and arrangement of the lateral end heaters  26   a  and  26   b  and the thermal conduction aid  27 . 
     First, a description is provided of a construction of the lateral end heater  26   b  and arrangement of the lateral end heater  26   b  and the thermal conduction aid  27  according to a first exemplary embodiment. 
       FIG. 12A  is a cross-sectional view of the lateral end heater  26   b  and the thermal conduction aid  27 .  FIG. 12B  is a front view of the lateral end heater  26   b  and the thermal conduction aid  27 .  FIG. 12C  is a side view of the lateral end heater  26   b  and the thermal conduction aid  27 .  FIG. 12A  is a cross-sectional view of the lateral end heater  26   b  and the thermal conduction aid  27  taken on line A-A in  FIG. 12B  and seen in the sheet conveyance direction DP. Although  FIGS. 12A, 12B, and 12C  illustrate the lateral end heater  26   b , the lateral end heater  26   a  is symmetrical with the lateral end heater  26   b  and has a construction similar to a construction of the lateral end heater  26   b  described below. 
     As illustrated in  FIGS. 12A and 12B , the lateral end heater  26   b  includes a base  50 , a resistive heat generator  51 , and a plurality of electrodes  52 . The base  50  is made of ceramics or the like. The resistive heat generator  51  is mounted on the base  50  and is substantially U-shaped, for example, as illustrated in  FIG. 12B . The plurality of electrodes  52  supplies power to the resistive heat generator  51 . The electrodes  52  are attached to the resistive heat generator  51  by soldering to supply power to the resistive heat generator  51 . However, since solder does not have a sufficient heat resistance, the electrodes  52  may suffer from degradation in heat resistance. In order to increase heat resistance of the electrodes  52 , the electrodes  52  may be attached to the resistive heat generator  51  with high melting point solder or silver. 
     As illustrated in  FIG. 12B , the electrodes  52  are disposed outboard from the heat generator  42   b  of the lateral end heater  26   b  in the longitudinal direction of the lateral end heater  26   b . The electrodes  52  are coupled to lateral ends of the resistive heat generator  51  in the longitudinal direction of the lateral end heater  26   b , respectively. As the electrodes  52  are supplied with power, the resistive heat generator  51  generates heat. The temperature of the heat generator  42   b  defined by the resistive heat generator  51  increases to a high temperature. The heat generator  42   b  spans in the longitudinal direction of the thermal conduction aid  27 . The heat generator  42   b  defined by the resistive heat generator  51  has a length  43  in the rotation direction D 21  of the fixing belt  21 . The length  43  of the heat generator  42   b  is smaller than a length  44  of the thermal conduction aid  27  in the rotation direction D 21  of the fixing belt  21 . 
     As illustrated in  FIGS. 12A and 12B , the thermal conduction aid  27  includes a heater-side face  27   b  that is disposed opposite the base  50  of the lateral end heater  26   b  and covers the heat generator  42   b  of the lateral end heater  26   b .  FIG. 12C  schematically illustrates the thermal conduction aid  27 . The thermal conduction aid  27  may project from the nip formation pad  24  toward the pressure roller  22  at a position in proximity to and upstream from an exit of the fixing nip N in the rotation direction D 21  of the fixing belt  21 . Thus, the thermal conduction aid  27  facilitates separation of the sheet P from the fixing belt  21  at the exit of the fixing nip N. The above-described construction and arrangement of the lateral end heater  26   b  are also applicable to the lateral end heater  26   a . The thermal conduction aid  27  is disposed opposite the base  50  of each of the lateral end heaters  26   a  and  26   b  and covers the heat generators  42   a  and  42   b  of the lateral end heaters  26   a  and  26   b , respectively. 
     Heat generated by the lateral end heaters  26   a  and  26   b  is conducted to the fixing belt  21  through the thermal conduction aid  27 . If the thermal conduction aid  27  does not cover a part of the heat generators  42   a  and  42   b  of the lateral end heaters  26   a  and  26   b , respectively, heat may not be conducted from that part to the fixing belt  21  through the thermal conduction aid  27 , degrading heating efficiency of the lateral end heaters  26   a  and  26   b . To address this circumstance, the thermal conduction aid  27  covers at least the heat generators  42   a  and  42   b  of the lateral end heaters  26   a  and  26   b  entirely, improving heating efficiency of the lateral end heaters  26   a  and  26   b , respectively. 
     If a small sheet P that does not bridge the heat generator  42   a  of the lateral end heater  26   a  and the heat generator  42   b  of the lateral end heater  26   b  is conveyed over the fixing belt  21 , since the small sheet P is not conveyed over the lateral end span of the fixing belt  21  that is heated by the respective lateral end heaters  26   a  and  26   b , the lateral end span of the fixing belt  21  may suffer from overheating or temperature increase. To address this circumstance, the electrodes  52  that are not heat resistant sufficiently are disposed outboard from the heat generator  42   b  in the longitudinal direction of the lateral end heater  26   b , preventing overheating of the electrodes  52 . 
     A description is provided of a construction of the lateral end heater  26   b  and arrangement of the lateral end heater  26   b  and a thermal conduction aid  27 S according to a variation of the first exemplary embodiment. 
       FIG. 13A  is a cross-sectional view of the lateral end heater  26   b  and the thermal conduction aid  27 S.  FIG. 13B  is a front view of the lateral end heater  26   b  and the thermal conduction aid  27 S.  FIG. 13C  is a side view of the lateral end heater  26   b  and the thermal conduction aid  27 S.  FIG. 13A  is a cross-sectional view of the lateral end heater  26   b  and the thermal conduction aid  27 S taken on line A-A in  FIG. 13B  and seen in the sheet conveyance direction DP. A configuration of the thermal conduction aid  27 S distinguishes the variation of the first exemplary embodiment from the first exemplary embodiment. 
     As illustrated in  FIG. 13C , a thickness of the thermal conduction aid  27 S is greater than a thickness of the thermal conduction aid  27  depicted in  FIG. 12C . The thickness defines a length from a nip-side face of the thermal conduction aid  27 S that is disposed opposite the inner circumferential surface of the fixing belt  21  and a heater-side face of the thermal conduction aid  27 S that is disposed opposite the lateral end heater  26   b . Thus, the thermal conduction aid  27 S attains an enhanced rigidity compared to the thermal conduction aid  27 . The thermal conduction aid  27 S allows the pressure roller  22  to exert greater pressure to the fixing belt  21  at the fixing nip N, improving fixing performance of the fixing belt  21  to fix the toner image on the sheet P. 
     A description is provided of a construction of the lateral end heater  26   b  and arrangement of the lateral end heater  26   b  and the thermal conduction aid  27  according to a second exemplary embodiment. 
       FIG. 14A  is a cross-sectional view of the lateral end heater  26   b  and the thermal conduction aid  27 .  FIG. 14B  is a front view of the lateral end heater  26   b  and the thermal conduction aid  27 .  FIG. 14C  is a side view of the lateral end heater  26   b  and the thermal conduction aid  27 .  FIG. 14A  is a cross-sectional view of the lateral end heater  26   b  and the thermal conduction aid  27  taken on line A-A in  FIG. 14B  and seen in the sheet conveyance direction DP. Identical reference numerals are assigned to components illustrated in  FIGS. 14A, 14B, and 14C  that are identical to the components illustrated in  FIGS. 12A, 12B, and 12C  and description of the identical components is omitted. 
     As illustrated in  FIGS. 14B and 14C , the heater-side face  27   b  of the thermal conduction aid  27  that is disposed opposite the lateral end heater  26   b  contacts the heat generator  42   b  of the lateral end heater  26   b . A contact span  55  in the rotation direction D 21  of the fixing belt  21 , where the heater-side face  27   b  of the thermal conduction aid  27  contacts the heat generator  42   b  of the lateral end heater  26   b , is not smaller than the length  43  of the heat generator  42   b  of the lateral end heater  26   b  in the rotation direction D 21  of the fixing belt  21 . Similarly, the heater-side face  27   b  of the thermal conduction aid  27  contacts the heat generator  42   a  of the lateral end heater  26   a  in the contact span  55  being not smaller than the length  43  of the heat generator  42   a  of the lateral end heater  26   a  in the rotation direction D 21  of the fixing belt  21 . A part of the thermal conduction aid  27  that does not contact the heat generator  42   b  suffers from degradation in conduction of heat. To address this circumstance, the thermal conduction aid  27  contacts at least the heat generators  42   a  and  42   b  of the lateral end heaters  26   a  and  26   b  entirely, improving heating efficiency of the lateral end heaters  26   a  and  26   b , respectively. 
     A description is provided of a construction of the lateral end heater  26   b  and arrangement of the lateral end heater  26   b  and the thermal conduction aid  27  according to a third exemplary embodiment. 
       FIG. 15A  is a cross-sectional view of the lateral end heater  26   b  and the thermal conduction aid  27 .  FIG. 15B  is a front view of the lateral end heater  26   b  and the thermal conduction aid  27 .  FIG. 15C  is a side cross-sectional view of the lateral end heater  26   b  and the thermal conduction aid  27 .  FIG. 15A  is a cross-sectional view of the lateral end heater  26   b  and the thermal conduction aid  27  taken on line A-A in  FIG. 15B  and seen in the sheet conveyance direction DP. Identical reference numerals are assigned to components illustrated in  FIGS. 15A, 15B, and 15C  that are identical to the components illustrated in  FIGS. 14A, 14B, and 14C  and description of the identical components is omitted. 
     As illustrated in  FIGS. 15A and 15B , the lateral end heater  26   b  is a flat plate. Conversely, as illustrated in  FIG. 15C , the heater-side face  27   b  of the thermal conduction aid  27  that is disposed opposite the base  50  of the lateral end heater  26   b  includes a curved portion  27   c  that is curved in cross-section and a flat portion  27   d  that is flat in cross-section to facilitate separation of the sheet P from the fixing belt  21  at the exit of the fixing nip N. If the heater-side face  27   b  of the thermal conduction aid  27  that is disposed opposite the lateral end heater  26   b  is barely flat or if the flat portion  27   d  is small, the heater-side face  27   b  of the thermal conduction aid  27  may contact the heat generator  42   b  of the lateral end heater  26   b  in a decreased area. To address this circumstance, as illustrated in  FIG. 15C , an interposer  53  is interposed between the thermal conduction aid  27  and the lateral end heater  26   b.    
     A nip-side face  53   a  of the interposer  53  that is disposed opposite the thermal conduction aid  27  has a shape that corresponds to or engages the thermal conduction aid  27 . For example, the nip-side face  53   a  of the interposer  53  contacts the curved portion  27   c  and the flat portion  27   d  of the thermal conduction aid  27 . A heater-side face  53   b  of the interposer  53  that is disposed opposite the lateral end heater  26   b  has a shape that corresponds to or engages the lateral end heater  26   b . Similarly, the interposer  53  is sandwiched between the thermal conduction aid  27  and the lateral end heater  26   a . The interposer  53  sandwiched between and in contact with the thermal conduction aid  27  and the lateral end heater  26   b  increases an area where the thermal conduction aid  27  contacts the heat generator  42   b  of the lateral end heater  26   b  indirectly via the interposer  53 , facilitating conduction of heat from the heat generator  42   b  of the lateral end heater  26   b  to the thermal conduction aid  27 . 
     For example, the interposer  53  is made of copper, aluminum, or an alloy of copper and aluminum. A thermal conductivity of the interposer  53  is not smaller than a thermal conductivity of the thermal conduction aid  27 . If the thermal conductivity of the interposer  53  is smaller than the thermal conductivity of the thermal conduction aid  27 , the interposer  53  may degrade conduction of heat from the lateral end heater  26   b  to the thermal conduction aid  27  and increase waste of heat. To address this circumstance, the thermal conductivity of the interposer  53  is not smaller than the thermal conductivity of the thermal conduction aid  27 , preventing degradation in heating efficiency of the lateral end heater  26   b.    
       FIGS. 12B, 13B, 14B, and 15B  according to the first to third exemplary embodiments illustrate the resistive heat generator  51  as a heat generation pattern that is U-shaped in a front view. Alternatively, the resistive heat generator  51  may have other shapes. The heat generation pattern may be adjusted to attain a desired heat output rate and a desired temperature distribution. 
       FIG. 16  illustrates a variation of the resistive heat generator  51  as one example. As illustrated in  FIG. 16 , the resistive heat generator  51  is turned at a plurality of positions on the base  50  such that the resistive heat generator  51  is elongated. The resistive heat generator  51  depicted in  FIG. 16  defines the heat generation pattern that increases the heat output rate of the lateral end heaters  26   a  and  26   b.    
     The positional relation between the thermal conduction aid  27  and the lateral end heaters  26   a  and  26   b  serving as a contact heater may be modified properly such that the thermal conduction aid  27  covers at least the heat generators  42   a  and  42   b  of the lateral end heaters  26   a  and  26   b , respectively. As illustrated in  FIGS. 12B, 13B, 14B, and 15B , the thermal conduction aid  27  covers at least a half of the electrodes  52 . Alternatively, the thermal conduction aid  27  may cover the electrodes  52  differently. For example, the thermal conduction aid  27  may cover at least a portion of the heat generation pattern  37 , which is provided with the resistive heat generator  51 . 
     A description is provided of advantages of the fixing device  20 . 
     As illustrated in  FIG. 2 , a fixing device (e.g., the fixing device  20 ) includes an endless belt (e.g., the fixing belt  21 ) that is flexible, formed into a loop, and rotatable in a rotation direction (e.g., the rotation direction D 21 ). A pressure rotator (e.g., the pressure roller  22 ) is disposed opposite an outer circumferential surface of the endless belt. A plurality of radiant heaters (e.g., the halogen heaters  23 A and  23 B) having different light distributions in an axial direction of the endless belt, respectively, is disposed inside the loop formed by the endless belt. For example, as illustrated in  FIG. 5 , a first radiant heater (e.g., the halogen heater  23 A) includes a first heat generator (e.g., the heat generator  40 A) that heats the endless belt. A second radiant heater (e.g., the halogen heater  23 B) includes a second heat generator (e.g., the heat generator  40 B) that heats the endless belt and is disposed outboard from the first heat generator in the axial direction of the endless belt. A nip formation pad (e.g., the nip formation pad  24 ) is disposed inside the loop formed by the endless belt. The pressure rotator is pressed against the nip formation pad via the endless belt to form a fixing nip (e.g., the fixing nip N) between the endless belt and the pressure rotator. 
     As illustrated in  FIG. 3 , a contact heater (e.g., the lateral end heaters  26   a  and  26   b ) is disposed at least at one lateral end of the nip formation pad in a longitudinal direction thereof. The contact heater heats at least one lateral end of the endless belt in the axial direction thereof. The nip formation pad includes a nip-side face (e.g., the nip-side face  24   c ) disposed opposite the endless belt. The contact heater includes a nip-side face (e.g., the nip-side face  26   c ) disposed opposite the endless belt. A thermal conduction aid (e.g., the thermal conduction aid  27 ) covers the nip-side face of the nip formation pad and the nip-side face of the contact heater. The thermal conduction aid conducts heat applied to the endless belt in the axial direction of the endless belt. 
     As illustrated in  FIGS. 12A, 12B, 13A, 13B, 14A, 14B, 15A, and 15B , the contact heater includes a third heat generator (e.g., the heat generator  42   b ) that heats at least one lateral end of the endless belt in the axial direction thereof. The thermal conduction aid includes a heater-side face (e.g., the heater-side face  27   b ) being disposed opposite the contact heater and covering at least the third heat generator of the contact heater. 
     The thermal conduction aid covering the third heat generator of the contact heater facilitates conduction of heat generated by the third heat generator to the thermal conduction aid, thus improving heating efficiency of the third heat generator. 
     As illustrated in  FIG. 5 , the fixing device  20  employs a center conveyance system in which the sheet P is centered on the fixing belt  21  in the axial direction thereof. Alternatively, the fixing device  20  may employ a lateral end conveyance system in which the sheet P is conveyed in the sheet conveyance direction DP along one lateral end of the fixing belt  21  in the axial direction thereof. In this case, one of the heat generators  40 B of the halogen heater  23 B and one of the lateral end heaters  26   a  and  26   b  are eliminated. Another one of the heat generators  40 B of the halogen heater  23 B and another one of the lateral end heaters  26   a  and  26   b  are distal from the one lateral end of the fixing belt  21  in the axial direction thereof. 
     According to the exemplary embodiments described above, the fixing belt  21  serves as an endless belt. Alternatively, a fixing film, a fixing sleeve, or the like may be used as an endless belt. Further, the pressure roller  22  serves as a pressure rotator. Alternatively, a pressure belt or the like may be used as a pressure rotator. 
     The above-described embodiments are illustrative and do not limit the present disclosure. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements and features of different illustrative embodiments may be combined with each other and substituted for each other within the scope of the present invention. 
     Any one of the above-described operations may be performed in various other ways, for example, in an order different from the one described above.