Patent Publication Number: US-9904220-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. 2016-034112, filed on Feb. 25, 2016, and 2016-203426, filed on Oct. 17, 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 an opposed 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 opposed 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 belt being endless and rotatable in a rotation direction. A primary heater, disposed opposite an inner circumferential surface of the fixing belt, heats the fixing belt. The primary heater includes a center heat generator disposed opposite a center span of the fixing belt in an axial direction of the fixing belt. A secondary heater, disposed opposite the inner circumferential surface of the fixing belt, heats the fixing belt. The secondary heater includes a lateral end heat generator disposed opposite a lateral end span of the fixing belt in the axial direction of the fixing belt. A nip formation pad is disposed opposite the inner circumferential surface of the fixing belt. An opposed rotator presses against the nip formation pad via the fixing belt to form a fixing nip between the fixing belt and the opposed rotator, through which a recording medium bearing a toner image is conveyed. The nip formation pad includes a base and at least one thermal conductor being interposed between the base and the fixing nip and having a thermal conductivity greater than a thermal conductivity of the base. The at least one thermal conductor includes an outboard edge disposed between an inboard edge and an outboard edge of the lateral end heat generator in the axial direction of the fixing belt and disposed outboard from a conveyance span of the fixing belt in the axial direction of the fixing belt, where the recording medium is conveyed over the fixing belt. 
     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 a fixing belt being endless and rotatable in a rotation direction. A primary heater, disposed opposite an inner circumferential surface of the fixing belt, heats the fixing belt. The primary heater includes a center heat generator disposed opposite a center span of the fixing belt in an axial direction of the fixing belt. A secondary heater, disposed opposite the inner circumferential surface of the fixing belt, heats the fixing belt. The secondary heater includes a lateral end heat generator disposed opposite a lateral end span of the fixing belt in the axial direction of the fixing belt. A nip formation pad is disposed opposite the inner circumferential surface of the fixing belt. An opposed rotator presses against the nip formation pad via the fixing belt to form a fixing nip between the fixing belt and the opposed rotator, through which the recording medium bearing the toner image is conveyed. The nip formation pad includes a base and at least one thermal conductor being interposed between the base and the fixing nip and having a thermal conductivity greater than a thermal conductivity of the base. The at least one thermal conductor includes an outboard edge disposed between an inboard edge and an outboard edge of the lateral end heat generator in the axial direction of the fixing belt and disposed outboard from a conveyance span of the fixing belt in the axial direction of the fixing belt, where the recording medium is conveyed over the fixing belt. 
    
    
     
       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 partial perspective view of the fixing device depicted in  FIG. 2 , illustrating one of belt holders incorporated therein; 
         FIG. 4  is a perspective view of the fixing device depicted in  FIG. 2 , illustrating one lateral end of the fixing device in a longitudinal direction thereof; 
         FIG. 5  is a diagram of halogen heaters incorporated in the fixing device depicted in  FIG. 2 , illustrating arrangement of the halogen heaters; 
         FIG. 6  is a graph illustrating a relation between a position on a nip formation pad incorporated in the fixing device depicted in  FIG. 2  and a temperature of a fixing belt incorporated in the fixing device depicted in  FIG. 2 ; 
         FIG. 7  is a diagram of the halogen heaters depicted in  FIG. 5  and a comparative thermal conductor that is elongated; 
         FIG. 8  is a diagram of the halogen heaters depicted in  FIG. 7  and a thermal conductor incorporated in the fixing device depicted in  FIG. 2  and provided with a slot; 
         FIG. 9  is a diagram of the halogen heaters and the thermal conductor incorporated in the fixing device depicted in  FIG. 2  and not provided with the slot; 
         FIG. 10  is a perspective view of the halogen heaters depicted in  FIG. 8 ; 
         FIG. 11  is a diagram of the halogen heaters and two thermal conductors as a first variation of the thermal conductor depicted in  FIG. 9 ; 
         FIG. 12  is a diagram of the halogen heaters and two thermal conductors as a second variation of the thermal conductor depicted in  FIG. 9 ; 
         FIG. 13  is a perspective view of the halogen heaters depicted in  FIG. 10 , illustrating a sealing portion incorporated therein; 
         FIG. 14  is a partial side view of the fixing device depicted in  FIG. 2 , illustrating one lateral end of the fixing device in the longitudinal direction thereof; and 
         FIG. 15  is a partial side view of the fixing device depicted in  FIG. 14 , illustrating a restraint incorporated therein. 
     
    
    
     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. 
     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 a description thereof once the description is provided. 
     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 M,  4 C, and  4 K situated in a center portion thereof. Although the image forming devices  4 Y,  4 M,  4 C, and  4 K contain developers (e.g., yellow, magenta, cyan, and black toners) in different colors, that is, yellow, magenta, cyan, and black corresponding to color separation components of a color image, respectively, the image forming devices  4 Y,  4 M,  4 C, and  4 K have an identical structure. 
     For example, each of the image forming devices  4 Y,  4 M,  4 C, and  4 K includes a drum-shaped photoconductor  5  serving as an image bearer or a latent 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 M, and  4 C that form yellow, magenta, and cyan toner images, respectively, are omitted. 
     Below the image forming devices  4 Y,  4 M,  4 C, 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 M,  4 C, and  4 K is a transfer device  3 . For example, the transfer device  3  includes an intermediate transfer belt  30  serving as an intermediate transferor, four primary transfer rollers  31  serving as primary transferors, a secondary transfer roller  36  serving as a secondary transferor, 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 that applies at least one of a predetermined direct current (DC) voltage and a predetermined alternating current (AC) voltage thereto. 
     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 that applies at least one of a predetermined direct current (DC) voltage and a predetermined alternating current (AC) voltage thereto. 
     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 waste toner drain tube extending from the belt cleaner  35  to an inlet of a waste toner container conveys waste toner collected from the intermediate transfer belt  30  by the belt cleaner  35  to the waste toner container. 
     A bottle holder  2  situated in an upper portion of the image forming apparatus  1  accommodates four toner bottles  2 Y,  2 M,  2 C, and  2 K detachably attached to the bottle holder  2 . The toner bottles  2 Y,  2 M,  2 C, and  2 K contain fresh yellow, magenta, cyan, and black toners to be supplied to the developing devices  7  of the image forming devices  4 Y,  4 M,  4 C, and  4 K, respectively. For example, the fresh yellow, magenta, cyan, and black toners are supplied from the toner bottles  2 Y,  2 M,  2 C, and  2 K to the developing devices  7  through toner supply tubes interposed between the toner bottles  2 Y,  2 M,  2 C, 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 timing roller pair conveys the sheet P conveyed from the feed roller  11  toward the secondary transfer nip at a proper time. 
     The conveyance path R is further provided with a fixing device  20  (e.g., a fuser or a fusing unit) 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 transferred from the intermediate transfer belt  30  onto the sheet P conveyed from the secondary transfer nip 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 M,  4 C, 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, magenta, cyan, 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, magenta, cyan, and black image data. The developing devices  7  supply yellow, magenta, cyan, and black toners to the electrostatic latent images formed on the photoconductors  5 , visualizing the electrostatic latent images as yellow, magenta, cyan, 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 the respective primary transfer nips formed between the photoconductors  5  and the primary transfer rollers  31 . 
     When the yellow, magenta, cyan, 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, magenta, cyan, 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, magenta, cyan, 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, magenta, cyan, 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  temporarily halts the sheet P conveyed through the conveyance path R. 
     Thereafter, the registration roller pair  12  resumes rotation at a predetermined time to convey the sheet P to the secondary transfer nip at a time when the full color toner image formed on intermediate transfer belt  30  reaches the secondary transfer nip. The secondary transfer roller  36  is applied with a transfer voltage having a polarity opposite a polarity of the charged yellow, magenta, cyan, and black toners of the yellow, magenta, cyan, and black toner images constructing the full color toner image formed on the intermediate transfer belt  30 , thus creating a transfer electric field at the secondary transfer nip. The transfer electric field secondarily transfers the yellow, magenta, cyan, and black toner images constructing the full color toner image formed on 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 the waste toner container. 
     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. Then, 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 M,  4 C, 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 M,  4 C, 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 . As illustrated in  FIG. 2 , the fixing device  20  (e.g., a fuser or a fusing unit) includes a fixing belt  21 , a pressure roller  22 , two halogen heaters  23   a  and  23   b , a nip formation pad  24 , a stay  25 , a reflector  26 , a stationary shield  28 , and a temperature sensor  29 . The fixing belt  21  formed into a loop serves as a fixing rotator or an endless belt rotatable in a rotation direction D 21 . The pressure roller  22  serves as an opposed rotator or a pressure rotator that is rotatable in a rotation direction D 22  and disposed opposite an outer circumferential surface of the fixing belt  21 . The two halogen heaters  23   a  and  23   b  serve as a heater or a heat source that heats the fixing belt  21 . The nip formation pad  24  is disposed opposite an inner circumferential surface of the fixing belt  21 . The stay  25  serves as a support that supports the nip formation pad  24 . The reflector  26  reflects light or heat (e.g., radiant heat) radiated from the halogen heaters  23   a  and  23   b  to the fixing belt  21 . The stationary shield  28  shields the fixing belt  21  from light or heat radiated from at least one of the halogen heaters  23   a  and  23   b  to the fixing belt  21 . According to this exemplary embodiment, the stationary shield  28  shields the fixing belt  21  from light or heat radiated from the halogen heater  23   b  having a heat generator disposed at each lateral end of the halogen heater  23   b  in a longitudinal direction thereof. The temperature sensor  29  serves as a temperature detector that detects the temperature of the outer circumferential surface of the fixing belt  21 . 
     The fixing belt  21  and 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 reflector  26 , and the stationary shield  28 , may construct a belt unit  21 U separably coupled with the pressure roller  22 . 
     A detailed description is now given of a construction of the fixing belt  21 . 
     The fixing belt  21  is a thin, flexible endless belt or film. For example, the fixing belt  21  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 polyimide (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. 
     If the fixing belt  21  does not incorporate the elastic layer, the fixing belt  21  has a decreased thermal capacity that improves fixing property of being heated quickly to a predetermined fixing temperature at which a toner image T is fixed on a sheet P. However, as the pressure roller  22  and the fixing belt  21  sandwich and press the unfixed toner image T on the sheet P passing through a fixing nip N formed between the fixing belt  21  and the pressure roller  22 , slight surface asperities of the fixing belt  21  may be transferred onto the toner image T on the sheet P, resulting in variation in gloss of the solid toner image T. To address this circumstance, the fixing belt  21  incorporates the elastic layer having a thickness not smaller than 100 micrometers. The elastic layer having the thickness not smaller than 100 micrometers elastically deforms to absorb slight surface asperities of the fixing belt  21 , preventing variation in gloss of the toner image T on the sheet P. 
     In order to decrease the thermal capacity of the fixing belt  21 , the fixing belt  21  is thin and has a decreased loop diameter. 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. Thus, the fixing belt  21  has a total thickness not greater than 1 mm. A loop diameter of the fixing belt  21  is in a range of from 20 mm to 40 mm. 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. 
     A detailed description is now given of a construction of the pressure roller  22 . 
     The pressure roller  22  is constructed of a core bar  22   a ; an elastic layer  22   b  coating the core bar  22   a  and made of rubber such as silicone rubber foam, silicone rubber, and fluoro rubber; and a release layer  22   c  coating the elastic layer  22   b  and made of PFA, PTFE, or the like. A pressurization assembly presses the pressure roller  22  against the nip formation pad  24  via the fixing belt  21 . The pressure roller  22  pressingly contacting the fixing belt  21  deforms the elastic layer  22   b  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 . 
     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  22   b  may be made of solid rubber. Alternatively, if no heater is situated inside the pressure roller  22 , the elastic layer  22   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  21 . 
     A detailed description is now given of a configuration of the halogen heaters  23   a  and  23   b.    
     The halogen heaters  23   a  and  23   b  are disposed opposite the inner circumferential surface of the fixing belt  21 . The halogen heaters  23   a  and  23   b  heat a heating span of the fixing belt  21  directly. The heating span is other than or disposed outboard from the fixing nip N in a circumferential direction, that is, the rotation direction D 21 , of the fixing belt  21 . According to this exemplary embodiment, the heating span of the fixing belt  21  is a direct heating span of the fixing belt  21  that is disposed upstream from the fixing nip N in the rotation direction D 21  of the fixing belt  21  or the sheet conveyance direction DP. The halogen heaters  23   a  and  23   b  are disposed opposite the direct heating span of the fixing belt  21  directly to heat the fixing belt  21  directly. 
     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  heat the fixing belt  21 . A controller (e.g., a processor), that is, a central processing unit (CPU) provided with a random-access memory (RAM) and a read-only memory (ROM), for example, 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  that is detected by the temperature sensor  29 . Thus, the controller adjusts the temperature of the fixing belt  21  to a desired fixing temperature. Instead of the temperature sensor  29  that detects the temperature of the fixing belt  21 , a temperature sensor that detects the temperature of the pressure roller  22  may be disposed opposite the pressure roller  22  so that the temperature of the fixing belt  21  is estimated based on a temperature of the pressure roller  22  that is detected by the temperature sensor. 
     When the fixing device  20  receives a fixing job to fix an unfixed toner image T on a sheet P, the driver drives and rotates the pressure roller  22  which in turn rotates the fixing belt  21  by friction therebetween. One or both of the halogen heaters  23   a  and  23   b  generate heat that heats the fixing belt  21 . When the temperature of the fixing belt  21  reaches the desired fixing temperature, the sheet P is conveyed through the fixing nip N. While the sheet P is conveyed through the fixing nip N, the fixing belt  21  and the pressure roller  22  fix the toner image T on the sheet P under heat and pressure. 
     According to this exemplary embodiment, the fixing device  20  incorporates the two halogen heaters  23   a  and  23   b . Alternatively, the fixing device  20  may incorporate three or more halogen heaters according to the sizes of the sheets P or the like that are available in the image forming apparatus  1 . Alternatively, instead of the halogen heaters  23   a  and  23   b , a carbon heater or the like may be employed as a heater that heats the fixing belt  21  with radiant heat. 
     A detailed description is now given of a construction of the nip formation pad  24 . 
     The nip formation pad  24  is disposed inside the loop formed by the fixing belt  21  and disposed opposite the pressure roller  22  via the fixing belt  21 . The nip formation pad  24  includes a base  241  and a thermal conductor  242 . For example, the base  241  is made of heat resistant resin such as polyether sulfone (PES), polyphenylene sulfide (PPS), liquid crystal polymer (LCP), polyether nitrile (PEN), polyamide imide (PAI), polyether ether ketone (PEEK), or the like. The thermal conductor  242  is made of a material having a thermal conductivity greater than a thermal conductivity of the base  241 . For example, the thermal conductor  242  is made of carbon nanotube having a theimal conductivity in a range of from 3,000 W/mK to 5,500 W/mK, graphite sheet having a thermal conductivity in a range of from 700 W/mK to 1,750 W/mK, silver having a thermal conductivity of 420 W/mK, copper having a thermal conductivity of 398 W/mK, aluminum having a thermal conductivity of 236 W/mK, steel electrolytic cold commercial (SECC), or the like. The thermal conductor  242  has a thermal conductivity not smaller than 236 W/mK. 
     The thermal conductor  242  is sandwiched between the base  241  and the fixing belt  21  at the fixing nip N. In other words, the base  241  is disposed opposite the fixing nip N via the thermal conductor  242 . 
     A detailed description is now given of a configuration of the stay  25 . 
     The stay  25  supports the base  241 . Accordingly, even if the nip formation pad  24  receives pressure from the pressure roller  22 , the nip formation pad  24  is not bent by the pressure and therefore produces a uniform nip length of the fixing nip N in the sheet conveyance direction DP throughout the entire width of the fixing belt  21  and the pressure roller  22  in an axial direction thereof. The stay  25  is made of metal having an increased mechanical strength, such as steel (e.g., stainless steel), to prevent bending of the nip formation pad  24 . Alternatively, the stay  25  may be made of resin having a mechanical strength great enough to prevent bending of the nip formation pad  24 . 
     A nip side face of the thermal conductor  242  is attached with a low-friction sheet  243 . As the fixing belt  21  rotates in the rotation direction D 21 , the inner circumferential surface of the fixing belt  21  slides over the low-friction sheet  243  that reduces friction between the fixing belt  21  and the nip formation pad  24 . Alternatively, the low-friction sheet  243  may be omitted. 
     A detailed description is now given of a configuration of the reflector  26 . 
     The reflector  26  is interposed between the stay  25  and the halogen heaters  23   a  and  23   b . The reflector  26  is secured to and supported by the stay  25 . The reflector  26  reflects heat or light radiated from the halogen heaters  23   a  and  23   b  toward the fixing belt  21 , suppressing conduction of heat from the halogen heaters  23   a  and  23   b  to the stay  25  and the like and thereby heating the fixing belt  21  effectively and saving energy. The reflector  26  is made of aluminum, stainless steel, or the like. If the reflector  26  is constructed of an aluminum base treated with vapor deposition of silver having a decreased emissivity and an increased reflectance, the reflector  26  enhances heating efficiency in heating the fixing belt  21 . 
     A detailed description is now given of a configuration of the stay  25 . 
     The stationary shield  28  is secured to the stay  25 . The stationary shield  28  is disposed opposite the inner circumferential surface of the fixing belt  21  at each lateral end of the fixing belt  21  in the axial direction thereof. The stationary shield  28  is disposed opposite the halogen heaters  23   a  and  23   b  to shield the fixing belt  21  from the halogen heater  23   b . Since the stationary shield  28  is requested to be heat resistant, the stationary shield  28  is made of metal such as aluminum, iron, and stainless steel or ceramics. 
     A description is provided of a configuration of a plurality of belt holders  40 . 
       FIG. 3  is a partial perspective view of the fixing device  20 , illustrating one of the plurality of belt holders  40 . The fixing device  20  further includes the plurality of belt holders  40  disposed opposite the inner circumferential surface of the fixing belt  21  at both lateral ends of the fixing belt  21  in the axial direction thereof, respectively. The belt holders  40  rotatably support the fixing belt  21  at both lateral ends of the fixing belt  21  in the axial direction thereof. Basically, no other component supports the fixing belt  21 . That is, the fixing belt  21  is not looped over or stretched taut across a roller or the like. The pair of belt holders  40 , the halogen heaters  23   a  and  23   b , and the stay  25  depicted in  FIG. 2  are secured to and supported by a pair of side plates of the fixing device  20  that is disposed at both lateral ends of the fixing device  20  in the axial direction of the fixing belt  21 , respectively. 
       FIG. 4  is a perspective view of the fixing device  20 , illustrating one lateral end of the fixing device  20  in the axial direction of the fixing belt  21 . As illustrated in  FIGS. 3 and 4 , the belt holder  40  includes a holding portion  401 , a restricting portion  402 , a mounted portion  403 , and a slit  404 . As illustrated in  FIG. 3 , the holding portion  401  is disposed inside the loop formed by the fixing belt  21  to rotatably support the fixing belt  21 . The restricting portion  402  restricts skew of the fixing belt  21  in the axial direction thereof. As illustrated in  FIG. 4 , the mounted portion  403  is mounted on and secured to a side plate  39  of the fixing device  20  with a fastener such as a screw. As illustrated in  FIG. 3 , the holding portion  401  is provided with the slit  404  at a part of the holding portion  401  in the circumferential direction of the fixing belt  21  and is partially cylindrical or tubular. As the holding portion  401  is inserted into an interior inside the loop formed by the fixing belt  21  at each lateral end of the fixing belt  21  in the axial direction thereof, the holding portion  401  rotatably supports the fixing belt  21 . Although  FIG. 3  illustrates the belt holder  40  situated at one lateral end of the fixing belt  21  in the axial direction thereof, the belt holder  40  is also situated at another lateral end of the fixing belt  21  in the axial direction thereof. 
     After the fixing device  20  is assembled, each lateral end of the nip formation pad  24  in a longitudinal direction thereof is disposed in the slit  404  of the holding portion  401 . As illustrated in  FIG. 4 , the stationary shield  28  is disposed opposite an inner circumferential surface of the holding portion  401 . The stationary shield  28  shields the belt holder  40  from the halogen heaters  23   a  and  23   b , preventing the belt holder  40  from being overheated by the halogen heaters  23   a  and  23   b  and thereby preventing the belt holder  40  from being deformed thermally and broken. 
     As illustrated in  FIG. 3 , the restricting portion  402  is greater than at least an outer loop diameter of the fixing belt  21 . The restricting portion  402  is disposed opposite a lateral edge face of the fixing belt  21  in the axial direction thereof. If the fixing belt  21  is skewed in the axial direction thereof while the fixing belt  21  rotates, the lateral edge face of the fixing belt  21  comes into contact with the restricting portion  402  which restricts skew of the fixing belt  21 . 
     A description is provided of a construction of the halogen heaters  23   a  and  23   b.    
       FIG. 5  is a diagram of the halogen heaters  23   a  and  23   b , illustrating arrangement of the halogen heaters  23   a  and  23   b . As illustrated in  FIG. 5 , the two halogen heaters  23   a  and  23   b  have different heat generation spans in a longitudinal direction of the halogen heaters  23   a  and  23   b  parallel to the axial direction of the fixing belt  21 , respectively. The halogen heater  23   a  is a center heater serving as a primary heater that includes a heat generator h 1  (e.g., an illuminator) disposed opposite a center span of the fixing belt  21  in the axial direction thereof. The halogen heater  23   b  is a lateral end heater serving as a secondary heater that includes a heat generator h 2  (e.g., an illuminator) disposed opposite each lateral end span of the fixing belt  21  in the axial direction thereof. An inboard edge h 2 in of the heat generator h 2  of the halogen heater  23   b  in the longitudinal direction thereof, which corresponds to a lateral edge of the center span of the fixing belt  21  in the axial direction thereof, is disposed opposite or corresponds to a lateral edge h 1  out of the heat generator h 1  of the halogen heater  23   a  in the longitudinal direction thereof. 
     A conveyance span A corresponds to a width of a postcard in the axial direction of the fixing belt  21 . A conveyance span A′ corresponds to a width of an A4 size sheet in portrait orientation in the axial direction of the fixing belt  21 . A conveyance span B corresponds to a width of a B4 size sheet in portrait orientation in the axial direction of the fixing belt  21 . A conveyance span C corresponds to a width of an A3 size sheet in portrait orientation in the axial direction of the fixing belt  21 . A conveyance span D corresponds to a width of an A3 extension size sheet in the axial direction of the fixing belt  21 . When the postcard is conveyed over the fixing belt  21  in the conveyance span A or the A4 size sheet in portrait orientation is conveyed over the fixing belt  21  in the conveyance span A′, the halogen heater  23   a  is energized to cause the heat generator h 1  disposed opposite the center span of the fixing belt  21  in the axial direction thereof to generate heat. Conversely, when the B4 size sheet in portrait orientation is conveyed over the fixing belt  21  in the conveyance span B, the A3 size sheet in portrait orientation is conveyed over the fixing belt  21  in the conveyance span C, or the A3 extension size sheet is conveyed over the fixing belt  21  in the conveyance span D, the halogen heaters  23   a  and  23   b  are energized to cause the heat generator h 1  disposed opposite the center span of the fixing belt  21  in the axial direction thereof and the heat generators h 2  disposed opposite both lateral end spans of the fixing belt  21  in the axial direction thereof, respectively, to generate heat. 
     A description is provided of a configuration of a comparative fixing device. 
     The comparative fixing device includes a fixing belt and a heater that heats the fixing belt. 
     As the comparative fixing device is configured to fix a toner image on sheets of an increased number of sizes, the number of conveyance spans where the sheets are conveyed over the fixing belt increases. If the number of heaters installed in the comparative fixing device increases to address this circumstance, the comparative fixing device may be manufactured at increased costs or may be upsized. To address this circumstance, the comparative fixing device decreases the number of heaters installed therein. 
     However, a plurality of heating spans of the fixing belt that is heated by the decreased number of heaters may not correspond to a plurality of sizes of sheets. Accordingly, the fixing belt may overheat in a non-conveyance span where the sheets are not conveyed over the fixing belt. The fixing belt is more susceptible to overheating than a fixing roller because a thermal capacity of the fixing belt is smaller than a thermal capacity of the fixing roller. 
     To address this circumstance, the comparative fixing device may include a thermal conductor that diffuses heat from the non-conveyance span of the fixing belt that is susceptible to overheating. 
     The thermal conductor prevents overheating of the non-conveyance span of the fixing belt, allowing the decreased number of heaters to heat the plurality of heating spans of the fixing belt that corresponds the increased number of sizes of sheets. 
     However, if the thermal conductor is excessively long, a thermal capacity of the thermal conductor may increase and the thermal conductor may diffuse heat excessively. Accordingly, when a large sheet is conveyed over the fixing belt, the fixing belt may not heat each lateral end of the large sheet sufficiently or the fixing belt may be heated slowly, degrading productivity (e.g., a fixing speed to complete a fixing job) of the comparative fixing device. Conversely, if the thermal conductor is excessively short, the thermal conductor may not diffuse heat sufficiently, causing overheating of the non-conveyance span of the fixing belt. 
     A description is provided of a configuration of the fixing device  20  to address overheating in a non-conveyance span of the fixing belt  21  where sheets P are not conveyed over the fixing belt  21  installed in the fixing device  20  having a decreased number of heaters (e.g., the halogen heaters  23   a  and  23   b ) relative to an increased number of sizes of the sheets P. 
     As illustrated in  FIG. 5 , an outboard edge h 2 out of the heat generator h 2  in the longitudinal direction of the halogen heater  23   b  is disposed outboard from the conveyance span B in the longitudinal direction of the halogen heater  23   b . Accordingly, when a sheet P is conveyed over the conveyance span B, an outboard span being outboard from the conveyance span B in the axial direction of the fixing belt  21  is the non-conveyance span where the sheet P is not conveyed over the fixing belt  21 . Although the heat generator h 2  heats the non-conveyance span of the fixing belt  21 , the sheet P conveyed in the conveyance span B of the fixing belt  21  does not draw heat from the non-conveyance span of the fixing belt  21 , resulting in overheating of the non-conveyance span of the fixing belt  21 . To address this circumstance, the fixing device  20  incorporates the thermal conductor  242  having an enhanced thermal conductivity to diffuse heat in the axial direction of the fixing belt  21 . 
     A description is provided of a referential configuration of the fixing device  20  incorporating the nip formation pad  24  that includes the thermal conductor  242  having the enhanced thermal conductivity. 
       FIG. 6  is a graph illustrating a relation between the position on the nip formation pad  24  in the longitudinal direction thereof and the temperature of the fixing belt  21 . As illustrated in  FIG. 6 , the nip formation pad  24  includes the base  241  and the thermal conductor  242  situated below the base  241  in  FIG. 6 . The base  241  is disposed opposite the fixing nip N via the thermal conductor  242 . A thermal conductivity of the thermal conductor  242  is greater than a thermal conductivity of the base  241 .  FIG. 6  illustrates a cross-section of the nip formation pad  24  in a half span of the nip formation pad  24  defined from a center to one lateral end of the nip formation pad  24  in the axial direction of the fixing belt  21 . Like the fixing device  20  depicted in  FIG. 2 , the referential configuration of the fixing device  20  involves the fixing belt  21 , the pressure roller  22 , the halogen heaters  23   a  and  23   b , and the like in addition to the nip formation pad  24 . 
     The non-conveyance span of the fixing belt  21  overheats if the width of the sheet P is shorter than a heat generation span defined by the heat generators h 1  and h 2  in the axial direction of the fixing belt  21 . With a relation between a heat generation span HS defined by the heat generators h 1  and h 2  and a conveyance span CS where the sheet P is conveyed over the fixing belt  21  as illustrated in  FIG. 6 , if the nip formation pad  24  does not incorporate the thermal conductor  242 , as indicated by a dotted line, the fixing belt  21  overheats in the non-conveyance span being outboard from the conveyance span CS in the axial direction of the fixing belt  21  at a position in proximity to the conveyance span CS. Conversely, with the relation between the heat generation span HS and the conveyance span CS as illustrated in  FIG. 6 , if the nip formation pad  24  incorporates the thermal conductor  242 , as indicated by a solid line, the thermal conductor  242  diffuses heat applied to the fixing belt  21  in the axial direction thereof, suppressing overheating of the non-conveyance span of the fixing belt  21 . 
     The thermal conductor  242  suppresses overheating of the non-conveyance span of the fixing belt  21  where the sheet P is not conveyed. The longer the thermal conductor  242  is in the axial direction of the fixing belt  21 , the more the thermal conductor  242  suppresses overheating of the fixing belt  21 . However, if the thermal conductor  242  is excessively long in the axial direction of the fixing belt  21 , each lateral end of the fixing belt  21  in the axial direction thereof may have a temperature lower than a desired temperature immediately after the fixing device  20  is powered on. 
       FIG. 7  is a diagram of the halogen heaters  23   a  and  23   b  and a comparative thermal conductor  242 C that is elongated in the axial direction of the fixing belt  21 . In order to suppress overheating of the fixing belt  21  effectively in the non-conveyance span being outboard from the conveyance span C where the A3 size sheet in portrait orientation is conveyed, the comparative thermal conductor  242 C extends to a position being outboard from the heat generator h 2  of the halogen heater  23   b  in the longitudinal direction thereof. However, in a fixing job to fix a toner image T on an A3 extension size sheet, that is, a maximum size sheet available in the fixing device  20 , conveyed in the conveyance span D, the fixing belt  21  may suffer from temperature decrease at each lateral end of the conveyance span D in the axial direction of the fixing belt  21 . For example, an outboard end of each of the heat generators h 2  of the halogen heater  23   b  in the longitudinal direction thereof generates a decreased amount of heat. 
     The comparative thermal conductor  242 C, which extends to a position being outboard from the outboard end of each of the heat generators h 2 , may diffuse an increased amount of heat outward in the axial direction of the fixing belt  21 . Accordingly, the fixing belt  21  may not store heat sufficiently. If the fixing belt  21  suffers from temperature decrease, the fixing belt  21  may degrade productivity (e.g., a fixing speed to complete a fixing job) of the fixing device  20 . In order to improve productivity, the heat generator h 2  may be elongated outward in the longitudinal direction of the halogen heater  23   b . However, the elongated heat generator h 2  may waste energy and upsize the fixing device  20 . 
     To address those circumstances, the fixing device  20  has a configuration described below with reference to  FIG. 8  to suppress overheating of the non-conveyance span of the fixing belt  21  where the sheet P is not conveyed while retaining productivity.  FIG. 8  is a diagram of the halogen heaters  23   a  and  23   b  and the thermal conductor  242 . In a description below, inboard denotes a position being closer to or situated at a center of the fixing belt  21  in the axial direction thereof; outboard denotes a position being closer to or situated at a lateral end of the fixing belt  21  in the axial direction thereof. 
     As illustrated in  FIG. 8 , a length of the thermal conductor  242  is defined such that each outboard edge  242 out of the thermal conductor  242  is interposed between the inboard edge h 2 in and the outboard edge h 2 out of each of the heat generators h 2  of the halogen heater  23   b  in the longitudinal direction thereof. The outboard edge  242 out of the thermal conductor  242  defines an inboard edge  47 in of a slot  47  abutting on each lateral end of the thermal conductor  242  in a longitudinal direction thereof. 
     Each slot  47  positions the thermal conductor  242  to the base  241  of the nip formation pad  24  depicted in  FIG. 2 . As a projection serving as a positioner projecting from the base  241  is inserted into each slot  47  disposed on the thermal conductor  242 , the thermal conductor  242  is positioned relative to the base  241  in the longitudinal direction of the thermal conductor  242 . 
     The slot  47  decreases an area where the thermal conductor  242  contacts the fixing belt  21 , thus reducing heat conduction from a portion provided with the slot  47  outward in the longitudinal direction of the thermal conductor  242 . For example, as illustrated in  FIG. 8 , a length L 2  of the slot  47  in the sheet conveyance direction DP is not smaller than a half of a length L 1  of the thermal conductor  242  in the sheet conveyance direction DP, decreasing the amount of heat conducted from the slot  47  outward in the longitudinal direction of the thermal conductor  242 . The thermal conductor  242  has a center span E spanning from one slot  47  to another slot  47  through a center of the thermal conductor  242  in the longitudinal direction thereof. The center span E serves mainly as a thermal conduction span. Conversely, the thermal conductor  242  further has an outboard span F that is disposed outboard from the outboard edge  242 out of the thermal conductor  242  in the longitudinal direction thereof or defined by the slot  47  and a portion disposed outboard from the slot  47  in the longitudinal direction of the thermal conductor  242 . Although the outboard span F conducts heat slightly, the outboard span F achieves a decreased thermal conduction compared to the center span E. Hence, the outboard span F serves mainly as a positioning span. 
     Accordingly, a lateral edge of the center span E serving as the thermal conduction span of the thermal conductor  242  to conduct heat stored in the fixing belt  21  in the axial direction thereof, which corresponds to the inboard edge  47 in of the slot  47  in the longitudinal direction of the thermal conductor  242 , defines the outboard edge  242 out of the thermal conductor  242  in the longitudinal direction thereof. Unlike the thermal conductor  242  according to this exemplary embodiment, if the length L 2  of the slot  47  in the sheet conveyance direction DP is smaller than the half of the length L 1  of the thermal conductor  242  in the sheet conveyance direction DP, the outboard span F, which is disposed outboard from the outboard edge  242 out in the longitudinal direction of the thermal conductor  242  or defined by the slot  47  and the portion disposed outboard from the slot  47  in the longitudinal direction of the thermal conductor  242 , also serves as the thermal conduction span. In this case, an outboard edge of the entire thermal conductor  242  in the longitudinal direction thereof, including the outboard span F defined by the slot  47  and the portion disposed outboard from the slot  47  in the longitudinal direction of the thermal conductor  242 , defines the outboard edge  242 out of the thermal conductor  242  in the longitudinal direction thereof. 
     An outboard edge  28 out of the stationary shield  28  in the axial direction of the fixing belt  21  is disposed outboard from the outboard edge h 2 out of the heat generator h 2  in the longitudinal direction of the halogen heater  23   b  and the center span E of the thermal conductor  242  in the longitudinal direction thereof. Since the stationary shield  28  is disposed outboard from the heat generator h 2  in the axial direction of the fixing belt  21 , the stationary shield  28  shields the fixing belt  21  from heat in an outboard span disposed outboard from the heat generator h 2  in the longitudinal direction of the halogen heater  23   b.    
     An inboard edge  28 in of the stationary shield  28  in the axial direction of the fixing belt  21  is disposed outboard from the center span E of the thermal conductor  242  in the longitudinal direction thereof. The stationary shield  28  shields the outboard span of the fixing belt  21  that is disposed outboard from the center span E in the axial direction of the fixing belt  21  from heat generated by the heat generator h 2 . In other words, the center span E where the thermal conductor  242  conducts heat is disposed inboard from the outboard span of the fixing belt  21  in the axial direction thereof. Thus, the stationary shield  28  prevents heat from conducting to the outboard span of the fixing belt  21  unnecessarily, thus preventing waste of energy. 
     Referring to  FIG. 9 , a description is provided of another configuration of the fixing device  20  to suppress overheating of the non-conveyance span of the fixing belt  21  where the sheet P is not conveyed while retaining productivity. 
       FIG. 9  is a diagram of the halogen heaters  23   a  and  23   b  and the thermal conductor  242 . As illustrated in  FIG. 9 , unlike the thermal conductor  242  depicted in  FIG. 8 , the thermal conductor  242  depicted in  FIG. 9  is not provided with the slots  47  serving as a positioner abutting on both lateral ends of the thermal conductor  242  in the longitudinal direction thereof, respectively. The thermal conductor  242  attains a uniform contact length in the sheet conveyance direction DP in which the thermal conductor  242  contacts the fixing belt  21  throughout the entire width of the thermal conductor  242  in the longitudinal direction thereof. Thus, the entire thermal conductor  242  serves as a thermal conduction span. Accordingly, as illustrated in  FIG. 9 , an outermost edge of the entire thermal conductor  242  in the longitudinal direction thereof defines the outboard edge  242 out of the thermal conductor  242  in the longitudinal direction thereof. 
     A description is provided of the inboard edge h 2 in and the outboard edge h 2 out of the heat generator h 2  of the halogen heater  23   b  in the longitudinal direction thereof. 
       FIG. 10  is a perspective view of the halogen heaters  23   a  and  23   b . As illustrated in  FIG. 10 , each of the halogen heater  23   a  serving as the center heater and the halogen heater  23   b  serving as the lateral end heater includes a glass tube  50  that is tubular or cylindrical and a filament  51  serving as a heat generator disposed inside the glass tube  50 . The filament  51  is coiled densely and continuously in the longitudinal direction of the halogen heaters  23   a  and  23   b  to define the heat generators h 1  and h 2 . According to this exemplary embodiment, since the halogen heaters  23   a  and  23   b  are used as a heater to heat the fixing belt  21 , the inboard edge h 2 in and the outboard edge h 2 out of the heat generator h 2  of the halogen heater  23   b  in the longitudinal direction thereof define an inboard edge and an outboard edge of a dense coil portion of the filament  51  in the longitudinal direction of the halogen heaters  23   a  and  23   b.    
     As described above, the length of the thermal conductor  242  in the longitudinal direction thereof is adjusted such that each outboard edge  242 out of the thermal conductor  242  is not disposed outboard from the outboard edge h 2 out of each of the heat generators h 2  of the halogen heater  23   b  in the longitudinal direction thereof, thus preventing the thermal conductor  242  from being elongated excessively in the longitudinal direction thereof. Accordingly, the thermal conductor  242  prevents heat applied to the fixing belt  21  from diffusing to a span being outboard from a maximum conveyance span of the fixing belt  21  in the axial direction thereof. Consequently, the thermal conductor  242  suppresses temperature decrease of the fixing belt  21  in a fixing job to fix the toner image T on the maximum size sheet (e.g., the A3 extension size sheet conveyed in the conveyance span D depicted in  FIG. 8 ). 
     Additionally, as described above, the length of the thermal conductor  242  in the longitudinal direction thereof is adjusted such that each outboard edge  242 out of the thermal conductor  242  is not disposed inboard from each lateral edge of the conveyance span D in the axial direction of the fixing belt  21 , thus preventing the thermal conductor  242  from being excessively short in the longitudinal direction thereof. Accordingly, the thermal conductor  242  suppresses overheating of the fixing belt  21  effectively and sufficiently. For example, when a small sheet P such as a postcard spanning the conveyance span A is conveyed over the fixing belt  21 , the thermal conductor  242  effectively suppresses overheating of the non-conveyance span of the fixing belt  21  where the small sheet P is not conveyed. 
     As illustrated in  FIGS. 8 and 9 , the outboard edge  242 out of the thermal conductor  242  is disposed outboard from the maximum conveyance span, that is, the conveyance span D, where the maximum size sheet is conveyed and disposed inboard from the outboard edge h 2 out of the heat generator h 2  of the halogen heater  23   b  in the longitudinal direction thereof. The length of the thermal conductor  242  in the longitudinal direction thereof is suppressed within a range that suppresses temperature decrease of the fixing belt  21  when the maximum size sheet is conveyed over the fixing belt  21  while the thermal conductor  242  is elongated to correspond to various widths of sheets P conveyed through the fixing device  20 . Thus, the thermal conductor  242  effectively suppresses overheating of the fixing belt  21  in the non-conveyance span of the fixing belt  21  where the sheets P are not conveyed. 
     A description is provided of a first variation of the thermal conductor  242 . 
       FIG. 11  is a diagram of the halogen heaters  23   a  and  23   b  and two thermal conductors  242   a  and  242   b  as the first variation of the thermal conductor  242 . As illustrated in  FIG. 8 , the single thermal conductor  242  extends continuously in the longitudinal direction thereof. Alternatively, as illustrated in  FIG. 11 , the two thermal conductors  242   a  and  242   b  may be aligned and abutted on each other in a longitudinal direction thereof. 
     A description is provided of a second variation of the thermal conductor  242 . 
       FIG. 12  is a diagram of the halogen heaters  23   a  and  23   b  and the two thermal conductors  242   a  and  242   b  as the second variation of the thermal conductor  242 . As illustrated in  FIG. 12 , the two thermal conductors  242   a  and  242   b  are spaced apart from each other in the longitudinal direction thereof. In order to prevent the two thermal conductors  242   a  and  242   b  from producing a step at the fixing nip N, the base  241  or the like is interposed between the thermal conductors  242   a  and  242   b  in the longitudinal direction thereof. As illustrated in  FIGS. 11 and 12 , like the thermal conductor  242  depicted in  FIGS. 8 and 9 , the outboard edge  242 out of each of the thermal conductors  242   a  and  242   b  is disposed in a span defined from the inboard edge h 2 in to the outboard edge h 2 out of the heat generator h 2  of the halogen heater  23   b  and disposed outboard from the maximum conveyance span, that is, the conveyance span D, where the maximum size sheet is conveyed, in the longitudinal direction of the halogen heater  23   b . Accordingly, the thermal conductors  242   a  and  242   b  effectively suppress overheating of the non-conveyance span of the fixing belt  21  where the small sheet P is not conveyed while the thermal conductors  242   a  and  242   b  suppress temperature decrease of each lateral end of the fixing belt  21  that is disposed opposite the maximum size sheet. 
     As illustrated in  FIG. 12 , an inboard edge  242 in of each of the thermal conductors  242   a  and  242   b  in the axial direction of the fixing belt  21  is disposed opposite or disposed inboard from each lateral edge of the conveyance span A of the postcard in the axial direction of the fixing belt  21 . In other words, each of the thermal conductors  242   a  and  242   b  is continuous outward in the axial direction of the fixing belt  21  from each lateral edge of a minimum conveyance span (e.g., the conveyance span A) where a minimum size sheet is conveyed over the fixing belt  21  or a position disposed inboard from each lateral edge of the minimum conveyance span in the axial direction of the fixing belt  21 . Accordingly, the thermal conductors  242   a  and  242   b  suppress overheating of the non-conveyance span of the fixing belt  21  where the minimum size sheet is not conveyed. 
     As described above, the thermal conductors  242 ,  242   a , and  242   b  are elongated long enough to diffuse heat stored in the non-conveyance span of the fixing belt  21 , which is disposed outboard from the conveyance span A where the small sheet P is conveyed, in the axial direction of the fixing belt  21 , thus suppressing overheating of the non-conveyance span of the fixing belt  21  effectively. On the other hand, the thermal conductors  242 ,  242   a , and  242   b  are elongated slightly beyond the conveyance span D where the maximum size sheet is conveyed. However, since the outboard edge h 2 out of the heat generator h 2  of the halogen heater  23   b  is disposed opposite substantially each lateral edge of the conveyance span D where the maximum size sheet is conveyed, heat is barely stored in the non-conveyance span of the fixing belt  21  where the maximum size sheet is not conveyed and therefore overheating of the fixing belt  21  barely occurs. 
     The fixing device  20  according to the exemplary embodiments described above is installed in the image forming apparatus  1  having a print speed (e.g., an image forming speed) defined by the number of prints per minute, that is smaller than 45 copies per minute. If the image forming apparatus  1  has a print speed that is smaller than 45 copies per minute, overheating of the non-conveyance span disposed at each lateral end of the fixing belt  21  in the axial direction thereof is limited. Therefore, the thermal conductors  242 ,  242   a , and  242   b  equalize heat stored in the fixing belt  21  in the axial direction thereof sufficiently. 
     In order to shield the non-conveyance span of the fixing belt  21  from the halogen heater  23   b , a movable shield may be interposed between the halogen heater  23   b  and the fixing belt  21 . As the movable shield moves, the movable shield changes a heating span of the fixing belt  21  that is heated by the halogen heater  23   b  according to the width of the sheet P conveyed over the fixing belt  21 . The fixing device  20  depicted in  FIG. 2  does not incorporate the movable shield. 
     Referring to  FIG. 13 , a description is provided of a configuration of a sealing portion disposed at each lateral end of the glass tube  50  in the longitudinal direction of the halogen heaters  23   a  and  23   b.    
       FIG. 13  is a perspective view of the halogen heaters  23   a  and  23   b . As illustrated in  FIG. 13 , each of the halogen heaters  23   a  and  23   b  includes a sealing portion  55  disposed at each lateral end of the glass tube  50  in the longitudinal direction of the halogen heaters  23   a  and  23   b . A loop diameter of the glass tube  50  decreases at each lateral end of the glass tube  50  in a longitudinal direction thereof. Thus, the glass tube  50  narrows at the sealing portion  55 , disposed at each lateral end of the glass tube  50  in the longitudinal direction thereof, to seal an interior of the glass tube  50 . Alternatively, the sealing portion  55  may include an outermost end of the glass tube  50  that is coupled to a lead wire. Since a loop diameter of the sealing portion  55  is small, a mechanical strength of the sealing portion  55  is smaller than a mechanical strength of other portion of the glass tube  50 . Accordingly, the sealing portion  55  is susceptible to thermal degradation and resultant breakage. 
     Referring to  FIG. 14 , a description is provided of a positional relation among the reflector  26 , the sealing portion  55 , and the belt holder  40  in the longitudinal direction of the halogen heaters  23   a  and  23   b.    
       FIG. 14  is a partial side view of the fixing device  20 , illustrating one lateral end of the fixing device  20  in a longitudinal direction thereof. As illustrated in  FIG. 14 , the belt holder  40 , the stationary shield  28 , and other components of the fixing device  20  are disposed at one lateral end of the fixing belt  21  in the axial direction thereof. In order to reflect radiant heat radiated from the halogen heaters  23   a  and  23   b  toward the fixing belt  21  to heat the fixing belt  21  effectively, the reflector  26  is disposed opposite the halogen heaters  23   a  and  23   b  and spans from each lateral end to the center of the fixing belt  21  in the axial direction thereof. 
     A connector  56  is disposed outboard from the sealing portion  55  of each of the two halogen heaters  23   a  and  23   b  in the longitudinal direction thereof. The connector  56  supports each lateral end of each of the halogen heaters  23   a  and  23   b  in the longitudinal direction thereof. A lead wire  57  is coupled to the connector  56  and extended outward from the connector  56  in the longitudinal direction of the halogen heaters  23   a  and  23   b . A diameter of the connector  56  is greater than a diameter of the halogen heaters  23   a  and  23   b . The two halogen heaters  23   a  and  23   b  are supported at an interior of the connector  56 . 
     The sealing portion  55  has an inboard edge  55 in and an outboard edge  55 out in the longitudinal direction of the halogen heaters  23   a  and  23   b . According to this exemplary embodiment, the inboard edge  55 in of the sealing portion  55  of each of the halogen heaters  23   a  and  23   b  is disposed outboard from an outboard edge  26 out of the reflector  26  in the longitudinal direction of the halogen heaters  23   a  and  23   b . Accordingly, heat or light reflected by the reflector  26  does not reach or barely reaches the sealing portion  55 . As described above, the mechanical strength of the sealing portion  55  is smaller than the mechanical strength of other portion of the glass tube  50 . As the sealing portion  55  is heated to a high temperature repeatedly by heat reflected by the reflector  26 , the sealing portion  55  may be broken over time. To address this circumstance, the sealing portion  55  is disposed outboard from the reflector  26  in the longitudinal direction of the halogen heaters  23   a  and  23   b  so that the sealing portion  55  does not overheat. Accordingly, the sealing portion  55  is protected against thermal degradation and the glass tube  50  defining the sealing portion  55  is immune from breakage such as crack. 
     An inboard edge  40 in of the belt holder  40  is disposed outboard from the outboard edge  26 out of the reflector  26  in the longitudinal direction of the halogen heaters  23   a  and  23   b . Accordingly, heat or light reflected by the reflector  26  does not reach or barely reaches the belt holder  40  or the stationary shield  28  disposed opposite the belt holder  40 . 
     Conversely, if the inboard edge  40 in of the belt holder  40  is disposed inboard from the outboard edge  26 out of the reflector  26  in the longitudinal direction of the halogen heaters  23   a  and  23   b , the belt holder  40  is requested to be made of a heat resistant material (e.g., metal) to prevent the belt holder  40  from being adversely affected by heat or light reflected by the reflector  26 . To address this circumstance, according to this exemplary embodiment, the belt holder  40  or the stationary shield  28  that protects the belt holder  40  is not susceptible to heat or light reflected by the reflector  26 . Hence, the belt holder  40  may be made of resin (e.g., resin having a reduced heat resistance). Thus, the belt holder  40  is made of a material selected from a wide variety of materials, reducing manufacturing costs. If the belt holder  40  is made of a rigid material such as metal, the belt holder  40  may cause abrasion or the like of the fixing belt  21 . To address this circumstance, the belt holder  40  is made of resin, preventing abrasion of the fixing belt  21 . 
     A description is provided of a construction of a restraint  58  incorporated in the fixing device  20 . 
       FIG. 15  is a partial side view of the fixing device  20 , illustrating the restraint  58  disposed at one lateral end of the fixing device  20  in the longitudinal direction thereof. The restraint  58  restricts motion of the halogen heaters  23   a  and  23   b  in the longitudinal direction thereof. 
     As illustrated in  FIG. 15 , the restraint  58  includes a mounted portion  581  mounted on an outer face of the side plate  39  that is opposite an inner face of the side plate  39  that mounts the mounted portion  403  of the belt holder  40  depicted in  FIG. 4 . The mounted portion  581  is secured to the side plate  39  with a fastener  59  such as a screw. The restraint  58  further includes a restricting portion  582  disposed outboard from the mounted portion  581  in the longitudinal direction of the halogen heaters  23   a  and  23   b . The restricting portion  582  includes a through hole  58   a.    
     The connector  56  is inserted into the through hole  58   a  of the restraint  58 . The connector  56  includes a rib  56   a  disposed outboard from the restricting portion  582  in the longitudinal direction of the halogen heaters  23   a  and  23   b . The rib  56   a  projects in a radial direction of the halogen heaters  23   a  and  23   b . The rib  56   a  projects beyond the through hole  58   a  in the radial direction of the halogen heaters  23   a  and  23   b . As the connector  56  moves rightward in  FIG. 15  in the axial direction of the fixing belt  21  relative to the restraint  58 , the rib  56   a  comes into contact with the restricting portion  582  of the restraint  58 . As the rib  56   a  contacts the restraint  58 , the restraint  58  restricts motion of the connector  56  rightward in  FIG. 15 , that is, inward in the longitudinal direction of the halogen heaters  23   a  and  23   b  or the axial direction of the fixing belt  21 . Thus, the restraint  58  restricts a position of the halogen heaters  23   a  and  23   b  relative to the restraint  58  and the side plate  39  in the longitudinal direction of the halogen heaters  23   a  and  23   b.    
     The restraint  58  supports the connector  56  also at another lateral end of the fixing device  20  in the longitudinal direction thereof. However, the connector  56  disposed at another lateral end of the fixing device  20  in the longitudinal direction thereof does not incorporate the rib  56   a . Hence, the restraint  58  does not restrict the position of the connector  56  disposed at another lateral end of the fixing device  20  in the longitudinal direction thereof. If the restraint  58  restricts the position of the halogen heaters  23   a  and  23   b  at each lateral end of the halogen heaters  23   a  and  23   b  in the longitudinal direction thereof, when the connector  56  and the halogen heaters  23   a  and  23   b  expand thermally due to heat generation or the like of the halogen heaters  23   a  and  23   b , thermal expansion of the connector  56  and the halogen heaters  23   a  and  23   b  is not absorbed, resulting in breakage of parts of the fixing device  20 . 
     To address this circumstance, according to this exemplary embodiment, the rib  56   a  is disposed at one of the connectors  56  that is disposed at one lateral end of the halogen heaters  23   a  and  23   b  in the longitudinal direction thereof to restrict the position of the halogen heaters  23   a  and  23   b  in the longitudinal direction thereof. Accordingly, even if the connector  56  and the halogen heaters  23   a  and  23   b  expand thermally, thermal expansion of the connector  56  and the halogen heaters  23   a  and  23   b  is absorbed at another lateral end of the halogen heaters  23   a  and  23   b  in the longitudinal direction thereof, preventing breakage of parts of the fixing device  20 . 
     The restraint  58  restricts the position of the halogen heaters  23   a  and  23   b  at one lateral end of the halogen heaters  23   a  and  23   b  in the longitudinal direction thereof and allows the halogen heaters  23   a  and  23   b  that thermally expand to elongate at another lateral end of the halogen heaters  23   a  and  23   b  in the longitudinal direction thereof. A positional relation between the halogen heaters  23   a  and  23   b  and peripheral components, that is seen from one lateral end of the halogen heaters  23   a  and  23   b  in the longitudinal direction thereof does not deviate or barely deviates due to thermal expansion of the connector  56  and the halogen heaters  23   a  and  23   b . Thus, the positional relation among the sealing portion  55 , the belt holder  40 , and the reflector  26  depicted in  FIG. 14  is retained. 
     The present disclosure is not limited to the details of the exemplary embodiments described above and various modifications and improvements are possible. For example, the exemplary embodiments of the fixing device  20  are explained with the postcard, the A4 size sheet in portrait orientation, the B4 size sheet in portrait orientation, the A3 size sheet in portrait orientation, and the A3 extension size sheet that are used as the sheets P. Alternatively, the exemplary embodiments described above are applicable to fixing devices that use a letter size sheet in portrait orientation, a double letter size sheet in portrait orientation, and the like as the sheets P. 
     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 fixing belt (e.g., the fixing belt  21 ), a primary heater (e.g., the halogen heater  23   a ), a secondary heater (e.g., the halogen heater  23   b ), a nip formation pad (e.g., the nip formation pad  24 ), and an opposed rotator (e.g., the pressure roller  22 ). The fixing belt is rotatable in a rotation direction (e.g., the rotation direction D 21 ). The primary heater and the secondary heater are disposed opposite an inner circumferential surface of the fixing belt to heat the fixing belt. 
     As illustrated in  FIG. 8 , the primary heater includes a center heat generator (e.g., the heat generator h 1 ) disposed opposite a center span of the fixing belt in an axial direction thereof. The secondary heater includes a lateral end heat generator (e.g., the heat generator h 2 ) disposed opposite a lateral end span of the fixing belt in the axial direction thereof. 
     As illustrated in  FIG. 2 , the nip formation pad is disposed opposite the inner circumferential surface of the fixing belt. The opposed rotator is disposed opposite an outer circumferential surface of the fixing belt and pressed against the nip formation pad via the fixing belt to form a fixing nip (e.g., the fixing nip N) between the fixing belt and the opposed rotator, through which a recording medium (e.g., a sheet P) bearing a toner image (e.g., a toner image T) is conveyed. The nip formation pad includes a base (e.g., the base  241 ) and a thermal conductor (e.g., the thermal conductors  242 ,  242   a , and  242   b ) being interposed between the base and the fixing nip and having a thermal conductivity greater than a thermal conductivity of the base. 
     As illustrated in  FIG. 8 , the lateral end heat generator includes an inboard edge (e.g., the inboard edge h 2 in) and an outboard edge (e.g., the outboard edge h 2 out) disposed outboard from the inboard edge in the axial direction of the fixing belt. The inboard edge of the lateral end heat generator is closer to a center of the fixing belt in the axial direction thereof than the outboard edge is. The outboard edge of the lateral end heat generator is disposed opposite a lateral end of the fixing belt in the axial direction thereof. 
     As illustrated in  FIG. 8 , the thermal conductor includes an outboard edge (e.g., the outboard edge  242 out) disposed in a span defined between the inboard edge and the outboard edge of the lateral end heat generator in a longitudinal direction of the secondary heater parallel to the axial direction of the fixing belt. The outboard edge of the thermal conductor is disposed outboard from a conveyance span (e.g., the conveyance span D) of the fixing belt in the axial direction thereof where the recording medium is conveyed over the fixing belt. The recording medium of a maximum size available in the fixing device is conveyed over the conveyance span of the fixing belt. 
     The outboard edge of the thermal conductor in the axial direction of the fixing belt is defined relative to the secondary heater and the conveyance span of the fixing belt where the recording medium is conveyed. Accordingly, the thermal conductor effectively suppresses overheating or temperature increase of a non-conveyance span of the fixing belt where the recording media of various sizes are not conveyed while the thermal conductor retains productivity defined by a fixing speed at which the fixing device performs a fixing job to fix the toner image on the recording medium. 
     According to the exemplary embodiments described above, the fixing belt  21  serves as a fixing belt. Alternatively, a fixing film, a fixing sleeve, or the like may be used as a fixing belt. Further, the pressure roller  22  serves as an opposed rotator. Alternatively, a pressure belt or the like may be used as an opposed 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.