Patent Publication Number: US-9851663-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. 2015-100098, filed on May 15, 2015, and 2016-065865 filed on Mar. 29, 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, 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 a fixing rotator rotatable in a predetermined direction of rotation and a pressure rotator to press against the fixing rotator to form a fixing nip between the fixing rotator and the pressure rotator. A primary heater is disposed opposite the fixing rotator to heat a circumferential span of the fixing rotator other than the fixing nip. A secondary heater is disposed outboard from the primary heater in an axial direction of the fixing rotator to heat the fixing rotator at the fixing nip. The primary heater and the secondary heater heat a bi-heating span of the fixing rotator in the axial direction of the fixing rotator. The secondary heater heats a mono-heating span of the fixing rotator in the axial direction of the fixing rotator and generates a decreased amount of heat to be conducted to the bi-heating span of the fixing rotator. The decreased amount of heat is smaller than an increased amount of heat to be conducted to the mono-heating span of the fixing rotator that is heated solely by the secondary heater of the primary heater and the secondary heater. 
     This specification further describes an improved image forming apparatus. In one exemplary embodiment, the image forming apparatus includes an image bearer to bear a toner image and a fixing rotator disposed downstream from the image bearer in a recording medium conveyance direction and rotatable in a predetermined direction of rotation. A pressure rotator presses against the fixing rotator to form a fixing nip between the fixing rotator and the pressure rotator. A primary heater is disposed opposite the fixing rotator to heat a circumferential span of the fixing rotator other than the fixing nip. A secondary heater is disposed outboard from the primary heater in an axial direction of the fixing rotator to heat the fixing rotator at the fixing nip. An electric circuit is electrically connected to the primary heater and the secondary heater to energize the primary heater and the secondary heater. The primary heater and the secondary heater heat a bi-heating span of the fixing rotator in the axial direction of the fixing rotator. The secondary heater heats a mono-heating span of the fixing rotator in the axial direction of the fixing rotator and generates a decreased amount of heat to be conducted to the bi-heating span of the fixing rotator. The decreased amount of heat is smaller than an increased amount of heat to be conducted to the mono-heating span of the fixing rotator that is heated solely by the secondary heater of the primary heater and the secondary heater. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and the many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with 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 schematic vertical cross-sectional view of a fixing device incorporated in the image forming apparatus illustrated in  FIG. 1 ; 
         FIG. 3  is a partial vertical cross-sectional view of the fixing device illustrated in  FIG. 2 ; 
         FIG. 4  is a partial perspective view of the fixing device illustrated in  FIG. 2 ; 
         FIG. 5  is a plan view of halogen heaters and lateral end heaters incorporated in the fixing device illustrated in  FIG. 2 ; 
         FIG. 6  is an exploded perspective view of a nip formation assembly incorporated in the fixing device illustrated in  FIG. 2 ; 
         FIG. 7A  is a plan view of a lateral end heater according to a first exemplary embodiment of the present disclosure that is incorporated in the nip formation assembly illustrated in  FIG. 6 ; 
         FIG. 7B  is a side view of the lateral end heater illustrated in  FIG. 7A ; 
         FIG. 8A  is a cross-sectional view of a nip formation pad and the lateral end heaters incorporated in the nip formation assembly depicted in  FIG. 6 , illustrating recesses of the nip formation pad; 
         FIG. 8B  is a cross-sectional view of the nip formation pad and the lateral end heaters depicted in  FIG. 8A , illustrating closed recesses as a first variation of the recesses illustrated in  FIG. 8A ; 
         FIG. 9  is a diagram of an electric circuit illustrating an electric connection between the halogen heaters and the lateral end heaters illustrated in  FIG. 5 ; 
         FIG. 10  is a schematic plan view of a resistive heat generator of the lateral end heater illustrated in  FIG. 7A ; 
         FIG. 11A  is a graph illustrating a relation between a position in an axial direction of a fixing belt of the fixing device illustrated in  FIG. 2  and a heat generation amount of the halogen heaters illustrated in  FIG. 5  and comparative lateral end heaters; 
         FIG. 11B  is a graph illustrating a relation between the position in the axial direction of the fixing belt and the heat generation amount of the halogen heaters and the lateral end heaters illustrated in  FIG. 5 ; and 
         FIG. 12  is a plan view of a lateral end heater according to a second exemplary embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     In describing exemplary 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 operate in a similar manner and achieve a similar result. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular to  FIG. 1 , an image forming apparatus  100  according to an exemplary embodiment of the present disclosure is explained. 
     It is to be noted that, in the drawings for explaining exemplary embodiments of this disclosure, identical reference numerals are assigned, as long as discrimination is possible, to components such as members and component parts having an identical function or shape, thus omitting description thereof once it is provided. 
       FIG. 1  is a schematic vertical cross-sectional view of the image forming apparatus  100 . The image forming apparatus  100  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  100  is a color printer that forms color and monochrome toner images on a recording medium by electrophotography. Alternatively, the image forming apparatus  100  may be a monochrome printer that forms a monochrome toner image on a recording medium. 
     A description is provided of a construction and an operation of the image forming apparatus  100 . 
     The image forming apparatus  100  is a color printer employing a tandem system in which a plurality of image forming devices for forming toner images in a plurality of colors, respectively, is aligned in a rotation direction of an intermediate transfer belt. 
     The image forming apparatus  100  includes four photoconductive drums  20 Y,  20 C,  20 M, and  20 K serving as image bearers that bear yellow, cyan, magenta, and black toner images in separation colors, respectively, that is, yellow, cyan, magenta, and black. The yellow, cyan, magenta, and black toner images formed on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K as visible images, respectively, are primarily transferred successively onto an intermediate transfer belt  11  serving as an intermediate transferor disposed opposite the photoconductive drums  20 Y,  20 C,  20 M, and  20 K as the intermediate transfer belt  11  rotates in a rotation direction Al such that the yellow, cyan, magenta, and black toner images are superimposed on a same position on the intermediate transfer belt  11  in a primary transfer process. Thereafter, the yellow, cyan, magenta, and black toner images superimposed on the intermediate transfer belt  11  are secondarily transferred onto a sheet S serving as a recording medium collectively in a secondary transfer process. Each of the photoconductive drums  20 Y,  20 C,  20 M, and  20 K is surrounded by image forming components that form the yellow, cyan, magenta, and black toner images on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K as the photoconductive drums  20 Y,  20 C,  20 M, and  20 K rotate clockwise in  FIG. 1  in a rotation direction D 20 . 
     Taking the photoconductive drum  20 K that forms the black toner image, the following describes a construction of components that form the black toner image. 
     The photoconductive drum  20 K is surrounded by a charger  30 K, a developing device  40 K, a primary transfer roller  12 K, and a cleaner  50 K in this order in the rotation direction D 20  of the photoconductive drum  20 K. Similarly, the photoconductive drums  20 Y,  20 C, and  20 M are surrounded by chargers  30 Y,  30 C, and  30 M, developing devices  40 Y,  40 C, and  40 M, primary transfer rollers  12 Y,  12 C, and  12 M, and cleaners  50 Y,  50 C, and  50 M in this order in the rotation direction D 20  of the photoconductive drums  20 Y,  20 C, and  20 M, respectively. The charger  30 K uniformly changes an outer circumferential surface of the photoconductive drum  20 K. An optical writing device  8  optically writes an electrostatic latent image on the charged outer circumferential surface of the photoconductive drum  20 K according to image data sent from an external device such as a client computer. The developing device  40 K visualizes the electrostatic latent image as a black toner image. 
     As the intermediate transfer belt  11  rotates in the rotation direction Al, the yellow, cyan, magenta, and black toner images formed on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K, respectively, are primarily transferred successively onto the intermediate transfer belt  11 , thus being superimposed on the same position on the intermediate transfer belt  11  and formed into a color toner image. In the primary transfer process, the primary transfer rollers  12 Y,  12 C,  12 M, and  12 K disposed opposite the photoconductive drums  20 Y,  20 C,  20 M, and  20 K via the intermediate transfer belt  11 , respectively, apply a primary transfer bias to the photoconductive drums  20 Y,  20 C,  20 M, and  20 K successively from the upstream photoconductive drum  20 Y to the downstream photoconductive drum  20 K in the rotation direction Al of the intermediate transfer belt  11 . The photoconductive drums  20 Y,  20 C,  20 M, and  20 K are aligned in this order in the rotation direction Al of the intermediate transfer belt  11 . The photoconductive drums  20 Y,  20 C,  20 M, and  20 K are located in four image forming stations that form the yellow, cyan, magenta, and black toner images, respectively. 
     The image forming apparatus  100  includes the four image forming stations that form the yellow, cyan, magenta, and black toner images, respectively, an intermediate transfer belt unit  10 , a secondary transfer roller  5 , an intermediate transfer belt cleaner  13 , and the optical writing device  8 . The intermediate transfer belt unit  10  is situated above and disposed opposite the photoconductive drums  20 Y,  20 C,  20 M, and  20 K. The intermediate transfer belt unit  10  incorporates the intermediate transfer belt  11  and the primary transfer rollers  12 Y,  12 C,  12 M, and  12 K. The secondary transfer roller  5  serves as a secondary transferor disposed opposite the intermediate transfer belt  11  and driven and rotated in accordance with rotation of the intermediate transfer belt  11 . The intermediate transfer belt cleaner  13  is disposed opposite the intermediate transfer belt  11  to clean the intermediate transfer belt  11 . The optical writing device  8  is situated below and disposed opposite the four image forming stations. 
     The optical writing device  8  includes a semiconductor laser serving as a light source, a coupling lens, an f 0  lens, a troidal lens, a deflection mirror, and a rotatable polygon mirror serving as a deflector. The optical writing device  8  emits light beams Lb corresponding to the yellow, cyan, magenta, and black toner images to be formed on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K thereto, forming electrostatic latent images on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K, respectively.  FIG. 1  illustrates the light beam Lb irradiating the photoconductive drum  20 K. Similarly, light beams irradiate the photoconductive drums  20 Y,  20 C, and  20 M, respectively. 
     The image forming apparatus  100  further includes a sheet feeder  61  and a registration roller pair  4 . The sheet feeder  61 , disposed in a lower portion of the image forming apparatus  100 , incorporates a paper tray that loads a plurality of sheets S to be conveyed to a secondary transfer nip formed between the intermediate transfer belt  11  and the secondary transfer roller  5 . The registration roller pair  4  serving as a conveyor conveys the sheet S conveyed from the sheet feeder  61  to the secondary transfer nip formed between the intermediate transfer belt  11  and the secondary transfer roller  5  at a predetermined time when the yellow, cyan, magenta, and black toner images superimposed on the intermediate transfer belt  11  reach the secondary transfer nip. The image forming apparatus  100  further includes a sensor for detecting that a leading edge of the sheet S reaches the registration roller pair  4 . 
     The secondary transfer roller  5  secondarily transfers the color toner image formed on the intermediate transfer belt  11  onto the sheet S as the sheet S is conveyed through the secondary transfer nip. The sheet S bearing the color toner image is conveyed to a fixing device  150  where the color toner image is fixed on the sheet S under heat and pressure. An output roller pair  7  ejects the sheet S bearing the fixed color toner image onto an output tray disposed atop the image forming apparatus  100 . In an upper portion of the image forming apparatus  100  and below the output tray are toner bottles  9 Y,  9 C,  9 M, and  9 K containing fresh yellow, cyan, magenta, and black toners, respectively. 
     The intermediate transfer belt unit  10  includes a driving roller  72  and a driven roller  73  over which the intermediate transfer belt  11  is looped, in addition to the intermediate transfer belt  11  and the primary transfer rollers  12 Y,  12 C,  12 M, and  12 K. Since the driven roller  73  also serves as a tension applicator that applies tension to the intermediate transfer belt  11 , a biasing member (e.g., a spring) biases the driven roller  73  against the intermediate transfer belt  11 . The intermediate transfer belt unit  10 , the secondary transfer roller  5 , and the intermediate transfer belt cleaner  13  constitute a transfer device  71 . The sheet feeder  61  includes a feed roller  3  that contacts an upper side of an uppermost sheet S of the plurality of sheets S loaded on the paper tray of the sheet feeder  61 . As the feed roller  3  is driven and rotated counterclockwise in  FIG. 1 , the feed roller  3  feeds the uppermost sheet S to the registration roller pair  4 . 
     The intermediate transfer belt cleaner  13  of the transfer device  71  includes a cleaning brush and a cleaning blade disposed opposite the intermediate transfer belt  11  to come into contact with the intermediate transfer belt  11 . The cleaning brush and the cleaning blade scrape a foreign substance such as residual toner particles off the intermediate transfer belt  11 , removing the foreign substance from the intermediate transfer belt  11  and thereby cleaning the intermediate transfer belt  11 . The intermediate transfer belt cleaner  13  further includes a waste toner conveyer that conveys the residual toner particles removed from the intermediate transfer belt  11 . 
     Referring to  FIG. 2 , a description is provided of a configuration of the fixing device  150  incorporated in the image forming apparatus  100  having the construction described above. 
       FIG. 2  is a schematic vertical cross-sectional view of the fixing device  150 . As illustrated in  FIG. 2 , the fixing device  150  (e.g., a fuser or a fusing unit) includes a thin, flexible, endless fixing belt  80 , serving as an endless belt or a fixing rotator, formed into a loop and rotatable in a rotation direction D 80  and a pressure roller  84  serving as a pressure rotator disposed opposite the fixing belt  80  and rotatable in a rotation direction D 84 . Inside the loop formed by the fixing belt  80  is a nip formation assembly  86  (e.g., a nip formation unit) that forms a fixing nip N between the fixing belt  80  and the pressure roller  84 , through which a sheet S serving as a recording medium is conveyed. 
     A detailed description is now given of a construction of the nip formation assembly  86 . 
     The nip formation assembly  86  includes a nip formation pad  88 , a lateral end heater  112 , and a stay  90 . The nip formation pad  88 , disposed inside the loop formed by the fixing belt  80  and disposed opposite the pressure roller  84 , presses against the pressure roller  84  via the fixing belt  80  to form the fixing nip N between the fixing belt  80  and the pressure roller  84 . The lateral end heater  112  is mounted on each lateral end of the nip formation pad  88  in a longitudinal direction thereof parallel to an axial direction of the fixing belt  80 . The stay  90  supports the nip formation pad  88  against pressure from the pressure roller  84 . 
     The stay  90  has a box shape with an opening opposite the fixing nip N. Halogen heaters  82   a  and  82   b  serving as a primary heater are disposed inside the box of the stay  90 . The halogen heaters  82   a  and  82   b  emit light that irradiates an inner circumferential surface of the fixing belt  80  directly through the opening of the stay  90 , heating the fixing belt  80  with radiant heat. A platy reflector  94  is mounted on an interior surface of the stay  90  to reflect light radiated from the halogen heaters  82   a  and  82   b  toward the fixing belt  80  so as to improve heating efficiency of the halogen heaters  82   a  and  82   b  to heat the fixing belt  80 . The reflector  94  prevents light and heat from the halogen heaters  82   a  and  82   b  from heating the stay  90 , suppressing waste of energy. Alternatively, instead of the reflector  94 , the interior surface of the stay  90  may be treated with insulation or mirror finish to reflect light radiated from the halogen heaters  82   a  and  82   b  toward the fixing belt  80 . 
     A detailed description is now given of a construction of the pressure roller  84 . 
       FIG. 3  is a partial vertical cross-sectional view of the fixing device  150 . As illustrated in  FIG. 3 , the pressure roller  84  is constructed of a hollow metal roller  84   a , an elastic layer  84   b  coating an outer circumferential surface of the metal roller  84   a  and being made of silicone rubber, and a release layer  84   c  coating an outer circumferential surface of the elastic layer  84   b . The release layer  84   c , having a layer thickness in a range of from 5 micrometers to 50 micrometers, is made of perfluoroalkoxy fluoro resin (PFA) or polytetrafluoroethylene (PTFE) to facilitate separation of the sheet S from the pressure roller  84 . As a driving force generated by a driver (e.g., a motor) situated inside the image forming apparatus  100  depicted in  FIG. 1  is transmitted to the pressure roller  84  through a gear train, the pressure roller  84  rotates in the rotation direction D 84 . Alternatively, the driver may also be connected to the fixing belt  80  to drive and rotate the fixing belt  80 . A spring or the like biases the pressure roller  84  against the fixing belt  80 . As the elastic layer  84   b  of the pressure roller  84  is pressed and deformed, the pressure roller  84  produces the fixing nip N having a predetermined length Nw in a sheet conveyance direction DS. Alternatively, the pressure roller  84  may be a solid roller. However, a hollow roller has a decreased thermal capacity. Further, a heater or a heat source such as a halogen heater may be disposed inside the pressure roller  84 . The elastic layer  84   b  may be made of solid rubber. Alternatively, if no heater is situated inside the pressure roller  84 , the elastic layer  84   b  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  80 . 
     A detailed description is now given of a construction of the fixing belt  80 . 
     The fixing belt  80  is an endless belt or film having a layer thickness in a range of from 30 micrometers to 50 micrometers and made of metal such as nickel and SUS stainless steel or resin such as polyimide. The fixing belt  80  is constructed of a base layer and a release layer. The release layer constituting an outer surface layer is made of PFA, PTFE, or the like to facilitate separation of toner of a toner image on the sheet S from the fixing belt  80 , thus preventing the toner of the toner image from adhering to the fixing belt  80 . Optionally, an elastic layer made of silicone rubber or the like may be sandwiched between the base layer and the release layer. If the fixing belt  80  does not incorporate the elastic layer, the fixing belt  80  has a decreased thermal capacity that improves fixing property of being heated quickly to a desired fixing temperature at which the toner image is fixed on the sheet S. However, as the pressure roller  84  and the fixing belt  80  sandwich and press the unfixed toner image on the sheet S passing through the fixing nip N, slight surface asperities of the fixing belt  80  may be transferred onto the toner image on the sheet S, resulting in variation in gloss of the solid toner image on the sheet S. 
     To address this circumstance, the elastic layer made of silicone rubber has a thickness not smaller than 100 micrometers. As the elastic layer deforms, the elastic layer absorbs slight surface asperities of the fixing belt  80 , suppressing variation in gloss of the toner image on the sheet S. As illustrated in  FIG. 2 , as the pressure roller  84  rotates in the rotation direction D 84 , the fixing belt  80  rotates in the rotation direction D 80  in accordance with rotation of the pressure roller  84  by friction between the pressure roller  84  and the fixing belt  80 . At the fixing nip N, the fixing belt  80  rotates as the fixing belt  80  is sandwiched between the pressure roller  84  and the nip formation pad  88 ; at a circumferential span of the fixing belt  80  other than the fixing nip N, the fixing belt  80  rotates while the fixing belt  80  is supported at each lateral end in the axial direction thereof to retain a tubular shape. Thus, the fixing belt  80  is retained circular in cross-section stably. As illustrated in  FIG. 2 , a separator  32  is disposed downstream from the fixing nip N in the sheet conveyance direction DS to separate the sheet S from the fixing belt  80 . 
     According to this exemplary embodiment, as illustrated in  FIGS. 2 and 3 , the fixing nip N is planar. Alternatively, the fixing nip N may define a curve projecting toward the fixing belt  80  to produce a recess in the fixing belt  80  in cross-section or other shapes. If the fixing nip N defines the recess in the fixing belt  80 , the recessed fixing nip N directs the leading edge of the sheet S toward the pressure roller  84  as the sheet S is ejected from the fixing nip N, facilitating separation of the sheet S from the fixing belt  80  and suppressing jamming of the sheet S. In this case, a nip formation face of the nip formation pad  88  is contoured into the recess. Similarly, a fixing nip side face of the lateral end heater  112 , serving as a secondary heater coupled with the nip formation pad  88 , may be contoured along the recessed nip formation face of the nip formation pad  88 . 
     A detailed description is now given of a configuration of the stay  90 . 
     The stay  90  supports the nip formation pad  88  against pressure from the pressure roller  84  to prevent bending of the nip formation pad  88  and produce the even length Nw of the fixing nip N in the sheet conveyance direction DS throughout the entire width of the fixing belt  80  in the axial direction thereof as illustrated in  FIG. 3 . As illustrated in  FIG. 2 , according to this exemplary embodiment, the pressure roller  84  is pressed against the fixing belt  80  to form the fixing nip N. Alternatively, the nip formation assembly  86  may be pressed against the pressure roller  84  to form the fixing nip N. The stay  90  has a mechanical strength great enough to support the nip formation pad  88  to prevent bending of the nip formation pad  88 . The stay  90  is made of metal such as stainless steel and iron, metallic oxide such as ceramics, or the like. 
     The fixing belt  80  and the components disposed inside the loop formed by the fixing belt  80 , that is, the halogen heaters  82   a  and  82   b , the nip formation pad  88 , the lateral end heater  112 , the stay  90 , and the reflector  94 , may constitute a belt unit  80 U separably coupled with the pressure roller  84 . 
       FIG. 4  is a partial perspective view of the fixing device  150 . As illustrated in  FIG. 4 , both lateral ends of the fixing belt  80  in the axial direction thereof are rotatably supported by flanges  36 , respectively. Each of the flanges  36  serves as a support projecting from a side plate  34  in the axial direction of the fixing belt  80 . Although  FIG. 4  illustrates the flange  36  and the side plate  34  situated at one lateral end of the fixing belt  80  in the axial direction thereof, the flange  36  and the side plate  34  are also situated at another lateral end of the fixing belt  80  in the axial direction thereof. The flange  36  that guides each lateral end of the fixing belt  80  in the axial direction thereof has an outer diameter substantially equivalent to an inner diameter of the fixing belt  80 . The flange  36  projects inboard from a lateral edge of the fixing belt  80  by a length in a range of from 5 mm to 10 mm in the axial direction of the fixing belt  80 . The flanges  36  guide the fixing belt  80  even when the fixing belt  80  rotates, retaining the fixing belt  80  to be circular in cross-section. The flange  36  includes a slit  36   a  disposed opposite the fixing nip N to place the nip formation assembly  86  at a predetermined position. The stay  90  depicted in  FIG. 2  has a width that spans the entire width of the fixing belt  80  in the axial direction thereof. Both lateral ends of the stay  90  in the axial direction of the fixing belt  80  are fixedly mounted on or secured to the side plates  34 , respectively, thus being supported and positioned by the side plates  34 . 
     A detailed description is now given of a configuration of the halogen heaters  82   a  and  82   b  and the lateral end heater  112 . 
       FIG. 5  is a plan view of the halogen heaters  82   a  and  82   b  and the lateral end heater  112  constructed of lateral end heaters  112   a  and  112   b , illustrating a light distribution of the halogen heaters  82   a  and  82   b  and a positional relation between the halogen heaters  82   a  and  82   b  and the lateral end heaters  112   a  and  112   b .  FIG. 5  illustrates a primary heating span S 82  in the axial direction of the fixing belt  80  where the halogen heaters  82   a  and  82   b  heat the fixing belt  80 . The primary heating span S 82  is equivalent to a width of an A3 size sheet in portrait orientation in the axial direction of the fixing belt  80 .  FIG. 5  further illustrates a combined heating span SC in the axial direction of the fixing belt  80  where the halogen heaters  82   a  and  82   b  and the lateral end heaters  112   a  and  112   b  heat the fixing belt  80 . The combined heating span SC is equivalent to a width of an A3 extension size sheet and a 13-inch sheet in portrait orientation in the axial direction of the fixing belt  80 . 
     As illustrated in  FIG. 5 , the halogen heater  82   a  serves as a primary heater having a dense light distribution in a primary heating span S 82   a  disposed opposite a center span of the fixing belt  80  in the axial direction thereof where a small sheet S having a decreased width in the axial direction of the fixing belt  80  is conveyed over the fixing belt  80 . The primary heating span S 82   a  is equivalent to a width of an A4 size sheet in portrait orientation in the axial direction of the fixing belt  80 . The halogen heater  82   b  serves as a primary heater having a dense light distribution in a primary heating span S 82   b  disposed opposite each lateral end span of the fixing belt  80  in the axial direction thereof where a medium sheet S having a medium width (e.g., an A3 size sheet) in the axial direction of the fixing belt  80  is conveyed over the fixing belt  80 . As the small sheet S is conveyed over the fixing belt  80 , the halogen heater  82   a  is powered on and the halogen heater  82   b  is not powered on, thus preventing each lateral end span, that is, a non-conveyance span, of the fixing belt  80  in the axial direction thereof where the small sheet S is not conveyed from being heated unnecessarily. 
     The width of the A3 size sheet in portrait orientation and the width of the A4 size sheet in landscape orientation are smaller than the width of the A3 extension size sheet in portrait orientation (e.g., 329 mm) and the width of the 13-inch sheet in portrait orientation (e.g., 330 mm) by a differential in a range of from 32 mm to 33 mm, respectively. Accordingly, if the fixing device  150  is configured to heat each lateral end span of the fixing belt  80  in the axial direction thereof, that is, if the fixing device  150  is configured to heat a half of the differential in the range of from 32 mm to 33 mm, that is, a span in a range of from 16.0 mm to 16.5 mm, the maximum width of sheets S available in the fixing device  150  increases from the width of the A3 size sheet equivalent to the primary heating span S 82  to the width of the A3 extension size sheet or the like equivalent to the combined heating span SC as illustrated in  FIG. 5 . In other words, if the fixing device  150  is configured to heat each outboard span of the fixing belt  80  disposed opposite each outboard span of the halogen heater  82   b  that is outboard from the primary heating span S 82   b  in the axial direction of the fixing belt  80  and does not have the dense light distribution, the large sheet S (e.g., the A3 extension size sheet) is available in the fixing device  150 . Accordingly, the fixing device  150  includes the lateral end heater  112  constructed of downsized heaters, that is, the lateral end heaters  112   a  and  112   b  serving as a secondary heater or a lateral end heater, each of which has a decreased width of about 20 mm in the axial direction of the fixing belt  80 . 
     As the large sheet S (e.g., the A3 extension size sheet and the 13-inch sheet) is conveyed through the fixing nip N, the halogen heaters  82   a  and  82   b  and the lateral end heaters  112   a  and  112   b  are energized. Conversely, as the small sheet S (e.g., a sheet not greater than the A3 size sheet) is conveyed through the fixing nip N, the halogen heaters  82   a  and  82   b  are energized or the halogen heater  82   a  is energized. Hence, the lateral end heaters  112   a  and  112   b  are not energized. If the halogen heater  82   b  is configured to define an increased heating span, that is, the combined heating span SC, to heat the large sheet S such as the A3 extension size sheet, the halogen heater  82   b  may heat the outboard span of the fixing belt  80  unnecessarily while the large sheet S is not conveyed through the fixing nip N, wasting energy. To address this circumstance, the fixing device  150  according to this exemplary embodiment incorporates a simple mechanism in addition to the halogen heaters  82   a  and  82   b , that is, the lateral end heaters  112   a  and  112   b  being disposed opposite both lateral end heating spans, that is, both secondary heating spans S 112 , in the axial direction of the fixing belt  80  or disposed in proximity to both lateral ends of the fixing belt  80  in the axial direction thereof, respectively. 
     A description is provided of a configuration of a first comparative fixing device incorporating a fixing roller. 
     The first comparative fixing device is requested to fix a toner image on sheets of various sizes. To address this request, if the first comparative fixing device employs an elongated heater to correspond to a width of a large sheet, the elongated heater may unnecessarily heat each lateral end span in an axial direction of the fixing roller, that is, a non-conveyance span, of the fixing roller where a small sheet is not conveyed, thus overheating the non-conveyance span of the fixing roller. To address this circumstance, the first comparative fixing device may convey the sheet at a decreased speed, degrading productivity. Alternatively, the first comparative fixing device may include a first halogen heater and a second halogen heater situated inside the fixing roller. The first halogen heater has a dense light distribution in a center span of the first halogen heater in the axial direction of the fixing roller. Conversely, the second halogen heater has a dense light distribution in each lateral end span of the second halogen heater in the axial direction of the fixing roller. When the small sheet is conveyed through the first comparative fixing device, the first halogen heater is energized to heat a center span of the fixing roller in the axial direction thereof where the small sheet is conveyed. 
     On the other hand, the first comparative fixing device is requested to fix a toner image on large sheets greater than the A3 size sheet such as the A3 extension size sheet and the 13-inch sheet although the large sheets are used infrequently. To address this circumstance, the first comparative fixing device may incorporate a separate halogen heater having a light distribution corresponding to those large sheets. However, it may be difficult to place the separate halogen heater inside the downsized fixing roller having a restricted diameter. 
     A description is provided of a configuration of a second comparative fixing device configured to address the above-described circumstances of the first comparative fixing device. 
     The second 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 situated 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 having different light distributions, respectively, is situated inside the loop formed by the endless belt. A plurality of lateral end heaters is disposed opposite both lateral end spans of the endless belt in an axial direction thereof, respectively, and disposed 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 the width of the large sheet in the axial direction of the endless belt. The lateral end heaters contact an inner circumferential surface or an outer circumferential surface of the endless belt. The lateral end heaters heat the increased heating span of the endless belt corresponding to the width of the large sheet in the axial direction of the endless belt with a simple construction not incorporating an extra halogen heater directed to the large sheet. 
     Additionally, the second comparative fixing device is requested to save energy. To address this request, a preheating mode of the halogen heaters in which the halogen heaters are ready to heat the endless belt before the second comparative fixing device receives a print job is barely available to reduce power consumption. In order to shorten a resuming time from an energy saver mode, a thin endless belt having a decreased thermal capacity may be employed. However, the thin endless belt may decrease an amount of heat conducted in the axial direction of the endless belt per unit time. For example, while a sheet is conveyed over a conveyance span of the endless belt, the sheet does not draw heat from a non-conveyance span of the endless belt where the sheet is not conveyed over the endless belt. Accordingly, the non-conveyance span of the endless belt may suffer from overheating. Further, the thin endless belt reduces the amount of heat conducted between the non-conveyance span and the conveyance span. Consequently, the temperature of the endless belt may vary in the axial direction of the endless belt, causing overheating of the non-conveyance span of the endless belt. To address this circumstance, the sheet may be conveyed at a decreased speed and the halogen heaters may be supplied with decreased power to prevent overheating of the endless belt, degrading satisfaction of a user. 
     To address this circumstance, a plurality of halogen heaters serving as a primary heater has a primary heating span disposed opposite a sheet not greater than the large sheet and a plurality of laminated heaters serving as a secondary heater has a secondary heating span disposed opposite each lateral end of the large sheet in the axial direction of the endless belt. The halogen heater has a heating property in which the halogen heater attains a decreased heat output at each lateral end of the halogen heater in the axial direction of the endless belt. Conversely, the laminated heater does not have such heating property. Accordingly, under combination of the halogen heaters and the laminated heaters, the halogen heater and the laminated heater produce an overlapping heating span where the primary heating span of the halogen heater overlaps the secondary heating span of the laminated heater in the axial direction of the endless belt. The endless belt may be conducted with an increased amount of heat in the overlapping heating span, varying the amount of heat conducted from the halogen heater and the laminated heater to the endless belt in the axial direction thereof. Consequently, the second comparative fixing device may suffer from fixing failure such as cold offset and hot offset and degradation in quality of a toner image fixed on a sheet such as variation in gloss of the toner image. 
     A detailed description is now given of a configuration of the plurality of halogen heaters incorporated in the second comparative fixing device. 
     The plurality of halogen heaters includes a center halogen heater having a dense light distribution in a center span of the center halogen heater in the axial direction of the endless belt and a lateral end halogen heater having a dense light distribution in each lateral end span of the lateral end halogen heater in the axial direction of the endless belt. As a small sheet is conveyed through the second comparative fixing device, the center halogen heater is powered on. As a medium sheet is conveyed through the second comparative fixing device, the lateral end halogen heater is powered on together with the center halogen heater. The center halogen heater and the lateral end halogen heater are powered on and off properly to heat sheets of various sizes. 
     Taking the sizes of the sheets and the frequency with which the sheets are conveyed, sheets up to the A3 size sheet are used frequently. The A3 size sheet is conveyed through the second comparative fixing device in portrait orientation. The A4 size sheet and a letter (LT) size sheet that are used frequently are generally conveyed in landscape orientation to enhance productivity. To address this circumstance, the center halogen heater and the lateral end halogen heater produce a heating span of about 300 mm in the axial direction of the endless belt that is great enough to heat 99 percent or more of the sizes of sheets. On the other hand, the second comparative fixing device is requested to fix a toner image on large sheets greater than the A3 size sheet in the axial direction of the endless belt such as the A3 extension size sheet and the 13-inch sheet although the large sheets are used infrequently. 
     If the plurality of halogen heaters is used as the center halogen heater and the lateral end halogen heater, respectively, the plurality of halogen heaters used to heat the small sheet is situated inside the loop formed by the endless belt or the fixing roller having a diameter of about 30 mm. Accordingly, the number of the halogen heaters is limited. To address this circumstance, the lateral end halogen heater having the dense light distribution in the lateral end span of the lateral end halogen heater may be elongated to span a width of the large sheet greater than the width of the A3 size sheet in the axial direction of the endless belt. As described above, the center halogen heater and the lateral end halogen heater heat the heating span of about 300 mm of the endless belt in the axial direction thereof frequently. However, if the elongated lateral end halogen heater is employed, the elongated lateral end halogen heater may heat an elongated heating span of about 330 mm of the endless belt in the axial direction thereof, wasting energy used to heat a differential between the heating span of about 300 mm and the elongated heating span of about 330 mm. When the A3 size sheet in portrait orientation or the A4 size sheet in landscape orientation is conveyed through the second comparative fixing device, each lateral end of the elongated heating span of the endless belt in the axial direction thereof that corresponds to the differential between the heating span of about 300 mm and the elongated heating span of about 330 mm may overheat. In order to cool the overheated lateral end of the endless belt, productivity defined by a conveyance speed of the sheets may be degraded or a fan may be installed. If a reflection plate is interposed between the lateral end halogen heater and the endless belt, each lateral end of the lateral end halogen heater in the axial direction of the endless belt may overheat. To address those circumstances, the second comparative fixing device has the configuration described above. 
     A description is provided of securing of the lateral end heaters  112   a  and  112   b  to the nip formation pad  88  and securing of the nip formation pad  88  to the stay  90 . 
       FIG. 6  is an exploded perspective view of the nip formation assembly  86 . As illustrated in  FIG. 6 , a side face  90   a  of the stay  90  that faces the pressure roller  84  mounts two ridges  90   b  and  90   c  extending in the axial direction of the fixing belt  80 . The rectangular nip formation pad  88  is sandwiched and positioned between the two ridges  90   b  and  90   c  in the sheet conveyance direction DS and is secured to the side face  90   a  with an adhesive or the like. Thus, the side face  90   a  and the two ridges  90   b  and  90   c  accommodate the nip formation pad  88 . Two recesses  88   a  and  88   b  that define a difference in thickness of the nip formation pad  88  are disposed at both lateral ends of the nip formation pad  88  in the longitudinal direction thereof. The lateral end heaters  112   a  and  112   b  are attached to the recesses  88   a  and  88   b  with an adhesive or the like or secured to the recesses  88   a  and  88   b , respectively, thus being accommodated by the recesses  88   a  and  88   b . The nip formation pad  88  includes a nip formation face  88   c  that faces the pressure roller  84 . 
     A metal plate  89  serving as a thermal conductor or a thermal conduction aid is sandwiched between the nip formation pad  88  and the fixing belt  80  at the fixing nip N. The metal plate  89  covers the nip formation pad  88 . The metal plate  89  is made of a material that conducts heat from the lateral end heaters  112   a  and  112   b  to the fixing belt  80  quickly to reduce uneven temperature of the fixing belt  80  in the axial direction thereof, for example, a material having an increased thermal conductivity such as copper, aluminum, and silver. It is preferable that the metal plate  89  is made of copper in a comprehensive view of manufacturing costs, availability, thermal conductivity, and processing. The inner circumferential surface of the fixing belt  80  slides over the metal plate  89  via a slide sheet (e.g., a low-friction sheet). The slide sheet is applied with a lubricant such as fluorine grease and silicone oil to decrease a slide torque of the fixing belt  80 . Alternatively, the metal plate  89  may contact the inner circumferential surface of the fixing belt  80  directly, not via the slide sheet. In this case, the metal plate  89  may be coated with resin or the like to decrease friction between the metal plate  89  and the fixing belt  80 . 
     As illustrated in  FIG. 6 , the nip formation assembly  86  incorporates the metal plate  89 . Alternatively, the nip formation assembly  86  may not incorporate the metal plate  89  to decrease the thermal capacity of the fixing device  150  so as to shorten a warm-up time taken to warm up the fixing belt  80  to a predetermined temperature and reduce power consumption. In this case, the fixing belt  80  slides over the nip formation pad  88 . Like in the nip formation assembly  86  incorporating the metal plate  89 , the slide sheet may be sandwiched between the fixing belt  80  and the nip formation pad  88  or the nip formation pad  88  may be treated with coating without the slide sheet sandwiched between the fixing belt  80  and the nip formation pad  88 . 
     Referring to  FIGS. 7A and 7B , a description is provided of a configuration of the lateral end heater  112  according to a first exemplary embodiment. 
       FIG. 7A  is a plan view of the lateral end heater  112 .  FIG. 7B  is a side view of the lateral end heater  112 .  FIGS. 7A and 7B  illustrate the lateral end heater  112  representing the lateral end heaters  112   a  and  112   b  depicted in  FIG. 6 . As illustrated in  FIG. 7A , the lateral end heater  112  includes a ceramic base  113 , a resistive heat generator  114  mounted on the ceramic base  113 , and an insulative layer  115  mounted on the resistive heat generator  114 . The ceramic base  113  has an outer size defined by a vertical length of about 10 mm and a horizontal length of about 20 mm in  FIG. 7A . The insulative layer  115  is a thin glass layer. Since the resistive heat generator  114  is not disposed in an outer marginal portion on the ceramic base  113 , an amount of heat generated in the outer marginal portion on the ceramic base  113  is smaller than an amount of heat generated in a center portion on the ceramic base  113 . Each of the recesses  88   a  and  88   b  depicted in  FIG. 6  contacts the outer marginal portion on the ceramic base  113  to prevent heat dissipation from the lateral end heater  112 . Each of the recesses  88   a  and  88   b  contacts a first face of the lateral end heater  112  that mounts the ceramic base  113  or a second face of the lateral end heater  112  that mounts the insulative layer  115 . If each of the recesses  88   a  and  88   b  contacts the second face of the lateral end heater  112  that mounts the insulative layer  115  constituting the thin glass layer, heat is conducted from the lateral end heater  112  to the metal plate  89  quickly, thus heating the fixing belt  80  effectively. In  FIG. 7A , a left side of the lateral end heater  112  where a circuit of the resistive heat generator  114  is turned is directed to a center of the fixing belt  80  in the axial direction. Conversely, a right side of the lateral end heater  112  where the resistive heat generator  114  is supplied with power is directed to the lateral edge of the fixing belt  80  in the axial direction thereof. 
     A description is provided of a relation between the lateral end heaters  112   a  and  112   b  and the nip formation pad  88 . 
       FIG. 8A  is a cross-sectional view of the fixing belt  80 , the nip formation pad  88 , and the lateral end heaters  112   a  and  112   b . As illustrated in  FIG. 8A , each of the lateral end heaters  112   a  and  112   b  includes a fixing belt side face  112   c  contacting the inner circumferential surface of the fixing belt  80 . The fixing belt side face  112   c  of the respective lateral end heaters  112   a  and  112   b  is leveled with the nip formation face  88   c  of the nip formation pad  88  in a pressurization direction F (e.g., a direction of a reaction force against pressure from the pressure roller  84 ) in which the nip formation pad  88  presses against the inner circumferential surface of the fixing belt  80 . In other words, the fixing belt side face  112   c  contacting the inner circumferential surface of the fixing belt  80  defines an extension of the nip formation face  88   c  in the longitudinal direction of the nip formation pad  88 . According to this exemplary embodiment, the lateral end heaters  112   a  and  112   b  are coupled with the nip formation pad  88  to form the fixing nip N. Hence, the lateral end heaters  112   a  and  112   b  are situated inside a limited space inside the loop formed by the fixing belt  80 , saving space. 
     The fixing belt side face  112   c  of the respective lateral end heaters  112   a  and  112   b  that contacts the inner circumferential surface of the fixing belt  80  is leveled with the nip formation face  88   c  of the nip formation pad  88  in the pressurization direction F to define an identical plane. Accordingly, the pressure roller  84  is pressed against the lateral end heaters  112   a  and  112   b  via the fixing belt  80  with sufficient pressure. Consequently, the fixing belt  80  rotates in a state in which the fixing belt  80  is pressed against the lateral end heaters  112   a  and  112   b , improving conduction of heat from the lateral end heaters  112   a  and  112   b  to the fixing belt  80  and thereby retaining improved heating efficiency of the lateral end heaters  112   a  and  112   b . Since the lateral end heaters  112   a  and  112   b  are situated within the fixing nip N in the axial direction of the fixing belt  80  to heat the fixing belt  80 , the lateral end heaters  112   a  and  112   b  do not heat a portion of the fixing belt  80  that is outboard from the fixing nip N in the axial direction of the fixing belt  80 , preventing residual toner failed to be fixed on the sheet S and therefore remaining on the fixing belt  80  from being melted again and transferred onto the sheet S. The pressure roller  84  also serves as a biasing member that presses the fixing belt  80  against the lateral end heaters  112   a  and  112   b  to adhere the fixing belt  80  to the lateral end heaters  112   a  and  112   b  via the metal plate  89  so as to enhance conduction of heat from the lateral end heaters  112   a  and  112   b  to the fixing belt  80 . Accordingly, a mechanism that presses the lateral end heaters  112   a  and  112   b  against the fixing belt  80  is not needed, simplifying the fixing device  150 . In other words, pressure used to form the fixing nip N is also used to press the fixing belt  80  against the lateral end heaters  112   a  and  112   b , improving conduction of heat from the lateral end heaters  112   a  and  112   b  to the fixing belt  80  without degrading rotation of the fixing belt  80 . 
     As illustrated in  FIG. 6 , each of the recesses  88   a  and  88   b  is open at each lateral edge of the nip formation pad  88  in the longitudinal direction thereof. Alternatively, each of the recesses  88   a  and  88   b  may be closed and formed in a box defined by a bottom and four walls as illustrated in  FIG. 8B .  FIG. 8B  is a cross-sectional view of the fixing belt  80 , the nip formation pad  88 , and the lateral end heaters  112   a  and  112   b  illustrating the closed recesses  88   a  and  88   b  as a variation of the recesses  88   a  and  88   b  illustrated in  FIG. 8A . Alternatively, each of the recesses  88   a  and  88   b  may be closed at both ends in the axial direction of the fixing belt  80  and open at both ends in a direction perpendicular to the axial direction of the fixing belt  80 . According to this exemplary embodiment, as illustrated in  FIG. 2 , the nip formation pad  88 , the lateral end heaters  112   a  and  112   b  illustrated as the lateral end heater  112  in  FIG. 2 , the stay  90 , and the halogen heaters  82   a  and  82   b  constitute the nip formation assembly  86 . Alternatively, the nip formation pad  88  and the lateral end heaters  112   a  and  112   b  may constitute the nip formation assembly  86 . 
     The halogen heaters  82   a  and  82   b  and the lateral end heaters  112   a  and  112   b  are energized during an initial time of a print job of conveying sheets S continuously for fixing immediately after warming up the fixing device  150 , for example, the initial time when the fixing belt  80  and the pressure roller  84  have not been heated sufficiently. Thereafter, while the sheets S are conveyed through the fixing device  150 , power supply to the halogen heaters  82   a  and  82   b  and the lateral end heaters  112   a  and  112   b  is controlled based on the temperature of the fixing belt  80 . In order to prevent decrease in heat output from the outboard span of the halogen heater  82   b , a heat generator of the halogen heater  82   b  may be elongated. However, the elongated halogen heater  82   b  may overheat the non-conveyance span of the fixing belt  80  where the small sheets or the medium sheets are not conveyed over the fixing belt  80  after the small sheets or the medium sheets are conveyed over the fixing belt  80  continuously. Additionally, the elongated halogen heater  82   b  may heat the fixing belt  80  unnecessarily, wasting energy. 
     The lateral end heaters  112   a  and  112   b  may have a positive temperature coefficient (PTC) property. If the lateral end heaters  112   a  and  112   b  have the PTC property, a resistance value increases at a preset temperature or higher and the lateral end heaters  112   a  and  112   b  do not generate heat at the preset temperature or higher. Hence, the lateral end heaters  112   a  and  112   b  do not burn or damage the fixing belt  80 , achieving the safe fixing device  150 . Additionally, each of the lateral end heaters  112   a  and  112   b  situated inside the loop formed by the fixing belt  80  emits light that irradiates the inner circumferential surface of the fixing belt  80  to heat the lateral end, secondary heating spans S 112  of the fixing belt  80  without degrading rotation of the fixing belt  80 . 
     The recesses  88   a  and  88   b  supporting the lateral end heaters  112   a  and  112   b , respectively, are coupled with the nip formation pad  88 . Hence, the lateral end heaters  112   a  and  112   b  are situated inside a limited space inside the loop formed by the fixing belt  80 , saving space. The metal plate  89  is mounted on the plane defined by the lateral end heaters  112   a  and  112   b  and the nip formation pad  88  leveled with the lateral end heaters  112   a  and  112   b . Thus, the metal plate  89  having the increased thermal conductivity retains the even temperature of the fixing belt  80 . 
     According to this exemplary embodiment, the fixing device  150  includes the two halogen heaters  82   a  and  82   b  serving as a primary heater that heats the fixing belt  80 . Alternatively, the fixing device  150  may include three or more halogen heaters to correspond to various sizes of small sheets and medium sheets. Yet alternatively, the fixing device  150  may include a single halogen heater and a lateral end heater may also heat the primary heating span S 82   b  of the fixing belt  80  on behalf of the halogen heater  82   b.    
     A description is provided of an electric connection between the halogen heaters  82   a  and  82   b  and the lateral end heaters  112   a  and  112   b.    
       FIG. 9  is a diagram of an electric circuit  91  illustrating the electric connection between the halogen heaters  82   a  and  82   b  and the lateral end heaters  112   a  and  112   b  that is employed by the fixing device  150  according to the exemplary embodiments disclosed by the present disclosure. Under a center conveyance system in which the sheet S is centered in the axial direction of the fixing belt  80  as the sheet S is conveyed over the fixing belt  80 , the lateral end heaters  112   a  and  112   b  are energized simultaneously. Accordingly, the lateral end heaters  112   a  and  112   b  are electrically connected in series to a power supply  120 . Consequently, the lateral end heaters  112   a  and  112   b  are electrically controlled more simply compared to a control in which the lateral end heater  112   a  is powered on and off separately from the lateral end heater  112   b . If one of the lateral end heaters  112   a  and  112   b  suffers from failure, the power supply  120  interrupts electric connection between the lateral end heaters  112   a  and  112   b  simultaneously, achieving safety of the fixing device  150 . The power supply  120  powers on and off the halogen heater  82   a  through a switch  120   a  , the halogen heater  82   b  through a switch  120   b , and the lateral end heaters  112   a  and  112   b  through a switch  120   c.    
     As illustrated in  FIG. 5 , according to the exemplary embodiments of the present disclosure, the single lateral end heater  112   a  is disposed opposite one lateral end of the fixing belt  80  in the axial direction thereof; the single lateral end heater  112   b  is disposed opposite another lateral end of the fixing belt  80  in the axial direction thereof. Alternatively, a plurality of lateral end heaters  112   a  may be disposed opposite one lateral end of the fixing belt  80  in the axial direction thereof and a plurality of lateral end heaters  112   b  may be disposed opposite another lateral end of the fixing belt  80  in the axial direction thereof according to various sizes of sheets S, for example. Yet alternatively, each of the lateral end heaters  112   a  and  112   b  may be disposed outboard from the halogen heater  82   b  in a longitudinal direction thereof farther than the lateral end heaters  112   a  and  112   b  illustrated in  FIG. 5 . Accordingly, the lateral end heaters  112   a  and  112   b  correspond to an increased number of sizes of sheets S and heat the fixing belt  80  precisely. 
     As illustrated in  FIG. 2 , a temperature sensor  125  is disposed opposite and in proximity to an outer circumferential surface of the fixing belt  80 . The electric circuit  91  depicted in  FIG. 9  controls the halogen heaters  82   a  and  82   b  and the lateral end heaters  112   a  and  112   b  based on the temperature of the fixing belt  80  detected by the temperature sensor  125 . As illustrated in  FIG. 2 , a safety device such as a thermostat  126  is disposed opposite and in proximity to the outer circumferential surface of the fixing belt  80  to prevent the temperature sensor  125  from being out of control when the temperature sensor  125  suffers from failure. 
     As described above, according to the first exemplary embodiment, even if a heating performance of the lateral end heaters  112   a  and  112   b  is lower than a heating performance of the halogen heaters  82   a  and  82   b , the electric circuit  91  energizes the lateral end heaters  112   a  and  112   b  separately from the halogen heaters  82   a  and  82   b . Accordingly, the electric circuit  91  controls the lateral end heaters  112   a  and  112   b  and the halogen heaters  82   a  and  82   b  to cause the lateral end heaters  112   a  and  112   b  to heat the secondary heating spans S  112  of the fixing belt  80  to a temperature identical to a temperature of the primary heating spans S 82   a  and S 82   b  of the fixing belt  80  heated by the halogen heaters  82   a  and  82   b , respectively, thus improving flexibility in selective energization of the lateral end heaters  112   a  and  112   b  and the halogen heaters  82   a  and  82   b.    
     A description is provided of a heat generation distribution of the lateral end heater  112  representing the lateral end heaters  112   a  and  112   b.    
       FIG. 10  is a schematic plan view of the resistive heat generator  114  of the lateral end heater  112  illustrating the circuit of the resistive heat generator  114  that is turned and directed to the center of the fixing belt  80  in the axial direction. As illustrated in  FIG. 10 , the resistive heat generator  114  includes a straight portion  114   a  serving as a major heat generator and a U-shaped turn portion  114   b  serving as a minor heat generator. A cross-sectional area Sb of the turn portion  114   b  is greater than a cross-sectional area Sa of the straight portion  114   a . The turn portion  114   b  is disposed inboard from the straight portion  114   a  in the axial direction of the fixing belt  80 . Since a resistance value of a conducting wire is inversely proportional to a cross-sectional area, when the resistive heat generator  114  is applied with power, a heat generation amount per unit length of the turn portion  114   b  in a longitudinal direction of the resistive heat generator  114  parallel to the axial direction of the fixing belt  80  is smaller than that of the straight portion  114   a.    
     According to this exemplary embodiment, variation in the cross-sectional area of the resistive heat generator  114  changes the resistance value to adjust an amount of power supplied to the resistive heat generator  114 . Alternatively, the conducting wire may be produced by joining materials having different electric resistivities, respectively, to change the resistance value. Yet alternatively, the circuit of the resistive heat generator  114  may be a parallel circuit or the like. As illustrated in  FIG. 10 , the turn portion  114   b  has an increased cross-sectional area. Alternatively, the straight portion  114   a  may include a plurality of sections having different cross-sectional areas, respectively, to adjust the heat generation amount of the resistive heat generator  114 . 
     A description is provided of advantages of the resistive heat generator  114  constructed of the straight portion  114   a  and the turn portion  114   b.    
       FIGS. 11A and 11B  illustrate a heat generation distribution of the halogen heaters  82   a  and  82   b  and the lateral end heaters  112   a  and  112   b  in the axial direction of the fixing belt  80 .  FIG. 11A  is a graph illustrating a relation between a position in the axial direction of the fixing belt  80  and a heat generation amount of the halogen heaters  82   a  and  82   b  and comparative lateral end heaters  112 M, each of which incorporates a comparative resistive heat generator having a uniform cross-sectional area.  FIG. 11B  is a graph illustrating a relation between the position in the axial direction of the fixing belt  80  and the heat generation amount of the halogen heaters  82   a  and  82   b  and the lateral end heaters  112   a  and  112   b , each of which incorporates the resistive heat generator  114  illustrated in  FIG. 10 . In  FIGS. 11A and 11B , reference numerals  82   a ,  82   b ,  112 M,  112   a , and  112   b  indicate the heat generation amount of the halogen heaters  82   a  and  82   b , the comparative lateral end heaters  112 M, and the lateral end heaters  112   a  and  112   b , respectively. 
     Each of the halogen heaters  82   a  and  82   b  has a heat generation property in which a heat generation amount of each lateral end of the respective halogen heaters  82   a  and  82   b  in the axial direction of the fixing belt  80  is smaller than a heat generation amount of a center of the respective halogen heaters  82   a  and  82   b  in the axial direction of the fixing belt  80 , resulting in decrease in heat generation amount at each lateral end of the respective halogen heaters  82   a  and  82   b  in the axial direction of the fixing belt  80 . Accordingly, as illustrated in  FIG. 11A , a heat generator of the halogen heater  82   b  overlaps a heat generator of the comparative lateral end heater  112 M in the axial direction of the fixing belt  80  to define a bi-heating span S 2 , that is, an overlap span, where both the halogen heater  82   b  and the comparative lateral end heater  112 M heat the fixing belt  80 . The fixing belt  80  may receive an increased amount of heat in the bi-heating span S 2  compared to other span and therefore may overheat in the bi-heating span S 2 . On the other hand, the fixing belt  80  may suffer from shortage of heat in a span not heated by the comparative lateral end heater  112 M due to decrease in heat generation amount at the lateral end of the halogen heater  82   b  in the axial direction of the fixing belt  80 . Accordingly, when the halogen heaters  82   a  and  82   b  and the comparative lateral end heater  112 M are energized to cause the fixing belt  80  to heat a large sheet S such as the A3 extension size sheet, the fixing belt  80  may suffer from uneven temperature in a span in proximity to the bi-heating span S 2 , resulting in faulty fixing, such as variation in gloss of the toner image on the large sheet S, and degradation in quality of the toner image fixed on the large sheet S, such as insufficient fixing at a low temperature lower than a desired fixing temperature. 
     Conversely, each of the lateral end heaters  112   a  and  112   b  incorporating the resistive heat generator  114  depicted in  FIG. 10  attains a resistance value in the bi-heating span S 2  that is smaller than a resistance value in a mono-heating span Si of the fixing belt  80  in the axial direction thereof where only the lateral end heaters  112   a  and  112   b  heat the fixing belt  80 . 
     Thus, a heat generation amount of the resistive heat generator  114  in the bi-heating span S 2  is smaller than a heat generation amount of the resistive heat generator  114  in the mono-heating span S  1 . Accordingly, the heat generation amount of the lateral end heaters  112   a  and  112   b  changes gently in the axial direction of the fixing belt  80 . The lateral end heaters  112   a  and  112   b  achieve the heat generation distribution that offsets decrease in heat generation amount at each lateral end of the halogen heater  82   b  in the axial direction of the fixing belt  80 . Thus, the lateral end heaters  112   a  and  112   b  even the temperature of the fixing belt  80  in the axial direction thereof Consequently, the lateral end heaters  112   a  and  112   b  reduce variation in temperature of the fixing belt  80  in the axial direction thereof, preventing fixing failure and retaining improved quality of the toner image formed on the sheet S. 
     As illustrated in  FIG. 10 , the resistance value of the turn portion  114   b  disposed opposite the bi-heating span S 2  is smaller than the resistance value of the straight portion  114   a  disposed opposite the mono-heating span S 1 , attaining the above-described simple configuration of the lateral end heaters  112   a  and  112   b . The cross-sectional area of the turn portion  114   b  disposed opposite the bi-heating span S 2  is smaller than the cross-sectional area of the straight portion  114   a  disposed opposite the mono-heating span S 1 , attaining the above-described configuration of the resistive heat generator  114  with simple modification of a circuit of the comparative resistive heat generator of the comparative lateral end heater  112 M. 
     Referring to  FIG. 12 , a description is provided of a configuration of lateral end heaters  116   a  and  116   b  according to a second exemplary embodiment. 
       FIG. 12  is a plan view of the lateral end heaters  116   a  and  116   b  serving as a secondary heater. The lateral end heaters  116   a  and  116   b  are installed in the fixing device  150  depicted in  FIG. 2  instead of the lateral end heaters  112   a  and  112   b . Unlike the rectangular lateral end heater  112  depicted in  FIG. 7A , each of the trapezoidal lateral end heaters  116   a  and  116   b  includes a ceramic base  117  instead of the ceramic base  113  and an insulative layer  119  instead of the insulative layer  115 . Unlike the ceramic base  113  and the insulative layer  115  that are rectangular, the ceramic base  117  and the insulative layer  119  are trapezoidal. For example, each of the ceramic base  117  and the insulative layer  119  is contoured in a trapezoid produced by removing one corner of a rectangle that is directed to the center of the fixing belt  80  in the axial direction thereof. Each of the lateral end heaters  116   a  and  116   b  further includes a resistive heat generator  118  instead of the resistive heat generator  114 . The resistive heat generator  118  has a wiring layout corresponding to the trapezoidal ceramic base  117  and the trapezoidal insulative layer  119 . 
     The resistive heat generator  118  includes a decreased area portion  118   a  serving as a minor heat generator and an increased area portion  118   b  serving as a major heat generator. The decreased area portion  118   a  is produced by removing the one corner of the rectangle and has a wiring density per unit length in a longitudinal direction of the resistive heat generator  118  parallel to the axial direction of the fixing belt  80  which is smaller than that of the increased area portion  118   b . Additionally, a contact area where the decreased area portion  118   a  contacts the fixing belt  80  is smaller than a contact area where the increased area portion  118   b  contacts the fixing belt  80 . Accordingly, a heat generation amount of the decreased area portion  118   a  is smaller than a heat generation amount of the increased area portion  118   b , causing the heat generation amount of the lateral end heaters  116   a  and  116   b  to change gently in the bi-heating span S 2  like the lateral end heaters  112   a  and  112   b  according to the first exemplary embodiment. Consequently, the heat generation amount of the lateral end heaters  116   a  and  116   b  changes gently in the axial direction of the fixing belt  80 . The lateral end heaters  116   a  and  116   b  achieve a heat generation distribution that offsets decrease in heat generation amount at each lateral end of the halogen heater  82   b  in the axial direction of the fixing belt  80 . Thus, the lateral end heaters  116   a  and  116   b  even the temperature of the fixing belt  80  in the axial direction thereof. 
     The decreased area portion  118   a  disposed opposite the bi-heating span S 2  has a decreased wiring density to decrease the heat generation amount of a part of the resistive heat generator  118 , attaining advantages equivalent to the advantages of the resistive heat generator  114  of the lateral end heater  112  according to the first exemplary embodiment readily. The decreased area portion  118   a  decreases the area where the resistive heat generator  118  contacts the fixing belt  80  in the bi-heating span S 2 , decreasing the heat generation amount of a part of the resistive heat generator  118  and thus attaining the advantages equivalent to the advantages of the resistive heat generator  114  of the lateral end heater  112  according to the first exemplary embodiment readily. 
     According to the exemplary embodiments described above, the halogen heaters  82   a  and  82   b  are used as a primary heater. Alternatively, a heater having a lateral end that generates a decreased amount of heat compared to a center of the heater in a longitudinal direction thereof may be used as a primary heater that attains the advantages described above. For example, an induction heating (IH) coil or the like may be used as a primary heater. Even if the IH coil isolated from the outer circumferential surface of the fixing belt  80  heats the fixing belt  80  by electromagnetic induction, a property of a magnetic field generated by electromagnetic induction may decrease the heat generation amount at each lateral end of the IH coil in the axial direction of the fixing belt  80 . Hence, the above-described configuration of the lateral end heaters  112   a ,  112   b ,  116   a , and  116   b  may be applied to the IH coil to achieve the advantages described above. According to the exemplary embodiments described above, the halogen heaters  82   a  and  82   b  heat the fixing belt  80  directly. Alternatively, a sleeve or the like may be interposed between the halogen heaters  82   a  and  82   b  and the fixing belt  80  so that the halogen heaters  82   a  and  82   b  heat the fixing belt  80  via the sleeve or the like. Yet alternatively, the fixing belt  80  may be looped over a heating roller so that the halogen heaters  82   a  and  82   b  heat the heating roller which in turn heats the fixing belt  80 . 
     The present disclosure is not limited to the details of the exemplary embodiments described above and various modifications and improvements are possible. The advantages achieved by the exemplary embodiments described above are examples and therefore are not limited to those described above. 
     A description is provided of advantages of the fixing device  150 . 
     As illustrated in  FIG. 2 , the fixing device  150  includes a fixing rotator or a flexible endless belt (e.g., the fixing belt  80 ) rotatable in a predetermined direction of rotation and a pressure rotator (e.g., the pressure roller  84 ) disposed opposite the fixing rotator to press against the fixing rotator to form the fixing nip N therebetween, through which a recording medium (e.g., a sheet S) bearing a toner image is conveyed. 
     As illustrated in  FIG. 5 , a primary heater (e.g., the halogen heaters  82   a  and  82   b ) is disposed opposite the primary heating span S 82  spanning the primary heating spans S 82   a  and S 82   b  of the fixing rotator in an axial direction thereof to heat a circumferential span of the fixing rotator other than the fixing nip N. For example, the primary heating span S 82  is a center span of the fixing rotator in the axial direction thereof. A secondary heater (e.g., the lateral end heaters  112   a ,  112   b ,  116   a , and  116   b ) is disposed opposite the secondary heating span S 112  of the fixing rotator in the axial direction of the fixing rotator to heat the fixing rotator at the fixing nip N. For example, the secondary heating span S 112  is a lateral end span of the fixing rotator in the axial direction thereof. 
     As illustrated in  FIGS. 10 and 11B , the fixing rotator has the bi-heating span S 2  and the mono-heating span  51  in the axial direction of the fixing rotator. The bi-heating span S 2  of the fixing rotator is heated by the primary heater and the secondary heater. The mono-heating span S 1  of the fixing rotator is heated by the secondary heater only. The secondary heater generates a decreased amount of heat to be conducted to the bi-heating span S 2  of the fixing rotator that is smaller than an increased amount of heat to be conducted to the mono-heating span S 1  of the fixing rotator. 
     The secondary heater achieves a heat generation distribution that offsets decrease in heat generation amount at a lateral end of the primary heater in the axial direction of the fixing rotator. Thus, the secondary heater evens the temperature of the fixing rotator in the axial direction thereof. Accordingly, the secondary heater reduces variation in temperature of the fixing rotator in the axial direction thereof, preventing fixing failure and retaining improved quality of the toner image formed on the recording medium. 
     As illustrated in  FIG. 5 , the fixing device  150  employs the center conveyance system in which the sheet S is centered on the fixing belt  80  in the axial direction thereof. Accordingly, the halogen heater  82   a  is disposed opposite the primary heating span S 82   a , that is, the center span of the fixing belt  80  in the axial direction thereof. The halogen heater  82   b  is disposed opposite the primary heating span S 82   b , that is, each lateral end span of the fixing belt  80  in the axial direction thereof. Each of the lateral end heaters  112   a  and  112   b  is disposed opposite the secondary heating span S 112 , that is, each lateral end span of the fixing belt  80  that is outboard from the primary heating span S 82   b  in the axial direction of the fixing belt  80 . Alternatively, the fixing device  150  may employ a lateral end conveyance system in which the sheet S is conveyed in the sheet conveyance direction DS along one lateral end of the fixing belt  80  in the axial direction thereof. In this case, one of heat generators of the halogen heater  82   b  and one of the lateral end heaters  112   a  and  112   b  are eliminated. Another one of the heat generators of the halogen heater  82   b  and another one of the lateral end heaters  112   a  and  112   b  are distal from the lateral end of the fixing belt  80  in the axial direction thereof 
     According to the exemplary embodiments described above, the fixing belt  80  serves as a fixing rotator. Alternatively, a fixing roller, a fixing film, a fixing sleeve, or the like may be used as a fixing rotator. Further, the pressure roller  84  serves as a pressure rotator. Alternatively, a pressure belt or the like may be used as a pressure rotator. 
     The present disclosure has been described above with reference to specific exemplary embodiments. Note 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 spirit and scope of the 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 illustrative exemplary embodiments may be combined with each other and/or substituted for each other within the scope of the present disclosure.