Patent Publication Number: US-10317823-B2

Title: Fixing device and image forming apparatus having a thermal conduction aid contacting a nip formation pad

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119 to Japanese Patent Application No. 2017-063377, filed on Mar. 28, 2017, in the Japanese Patent Office, the entire disclosure 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. 
     The fixing device may further include a thermal conduction aid over which the fixing belt slides. Heat generated by the heater may diffuse to a lateral end of the thermal conduction aid in a longitudinal direction thereof, decreasing the temperature of a lateral end of the fixing belt that contacts the thermal conduction aid. Accordingly, the fixing belt may degrade fixing performance to fix the toner image on a lateral end of the sheet. 
     SUMMARY 
     This specification describes below an improved fixing device. In one embodiment, the fixing device includes a fixing rotator that is endless and rotatable in a rotation direction and a lateral end heater to heat a lateral end span of the fixing rotator in an axial direction of the fixing rotator. A thermal conduction aid contacts an inner circumferential surface of the fixing rotator. A nip formation pad contacts the thermal conduction aid. A pressure rotator presses against the nip formation pad via the fixing rotator and the thermal conduction aid to form a fixing nip between the fixing rotator and the pressure rotator, through which a recording medium bearing a toner image is conveyed. A lateral end temperature detector detects a temperature of the fixing rotator in a lateral end detection span in the axial direction of the fixing rotator. The thermal conduction aid contacts the nip formation pad in a first span including the lateral end detection span in the axial direction of the fixing rotator with a first contact area. The thermal conduction aid contacts the nip formation pad in a second span disposed outboard from the first span in the axial direction of the fixing rotator with a second contact area smaller than the first contact area. 
     This specification further describes an improved fixing device. In one embodiment, the fixing device includes a fixing rotator that is endless and rotatable in a rotation direction and a thermal conduction aid contacting an inner circumferential surface of the fixing rotator. A nip formation pad contacts the thermal conduction aid. A pressure rotator presses against the nip formation pad via the fixing rotator and the thermal conduction aid to form a fixing nip between the fixing rotator and the pressure rotator, through which a recording medium bearing a toner image is conveyed. A temperature detector detects a temperature of the fixing rotator in a detection span within a recording medium conveyance span in an axial direction of the fixing rotator, where the recording medium is conveyed. A heater heats the fixing rotator. A controller controls the heater to heat the fixing rotator to a target temperature based on the temperature of the fixing rotator detected by the temperature detector. The thermal conduction aid conducts heat to the nip formation pad in the detection span in the axial direction of the fixing rotator with a first amount. The thermal conduction aid conducts heat to the nip formation pad in an outboard span disposed outboard from the recording medium conveyance span in the axial direction of the fixing rotator with a second amount smaller than the first amount. 
     This specification further describes an improved image forming apparatus. In one embodiment, the image forming apparatus includes an image bearer to bear a toner image and a fixing device to fix the toner image on a recording medium. The fixing device includes a fixing rotator that is endless and rotatable in a rotation direction and a lateral end heater to heat a lateral end span of the fixing rotator in an axial direction of the fixing rotator. A thermal conduction aid contacts an inner circumferential surface of the fixing rotator. A nip formation pad contacts the thermal conduction aid. A pressure rotator presses against the nip formation pad via the fixing rotator and the thermal conduction aid to form a fixing nip between the fixing rotator and the pressure rotator, through which the recording medium bearing the toner image is conveyed. A lateral end temperature detector detects a temperature of the fixing rotator in a lateral end detection span in the axial direction of the fixing rotator. The thermal conduction aid contacts the nip formation pad in a first span including the lateral end detection span in the axial direction of the fixing rotator with a first contact area. The thermal conduction aid contacts the nip formation pad in a second span disposed outboard from the first span in the axial direction of the fixing rotator with a second contact area smaller than the first contact area. 
    
    
     
       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 embodiment of the present disclosure; 
         FIG. 2  is a schematic vertical cross-sectional view of a fixing device according to a first embodiment of the present disclosure, which is incorporated in the image forming apparatus depicted in  FIG. 1 ; 
         FIG. 3  is a block diagram of the image forming apparatus depicted in  FIG. 1 , illustrating a controller incorporated therein; 
         FIG. 4  is a schematic vertical cross-sectional view of a fixing device according to a second embodiment of the present disclosure; 
         FIG. 5  is a schematic perspective view of the fixing device depicted in  FIG. 2 , illustrating one lateral end of the fixing device in a longitudinal direction thereof; 
         FIG. 6  is an exploded perspective view of a nip formation assembly according to a first embodiment of the present disclosure, which is incorporated in the fixing device depicted in  FIG. 2 ; 
         FIG. 7  is an exploded perspective view of a nip formation assembly according to a second embodiment of the present disclosure, which is installable in the fixing device depicted in  FIG. 2 ; 
         FIG. 8  is an exploded perspective view of a nip formation assembly according to a third embodiment of the present disclosure, which is installable in the fixing device depicted in  FIG. 2 ; 
         FIG. 9  is a diagram of a center heater and a lateral end heater incorporated in the fixing device depicted in  FIG. 2 ; 
         FIG. 10  is a diagram of the lateral end heater and a nip formation pad of the nip formation assembly depicted in  FIG. 7 ; and 
         FIG. 11  is a graph illustrating a temperature distribution of a fixing belt incorporated in the fixing device depicted in  FIG. 2 . 
     
    
    
     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  100  according to an embodiment is explained. 
       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 two of copying, printing, scanning, facsimile, and plotter functions, or the like. According to this embodiment, the image forming apparatus  100  is a color printer that forms a color toner image 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. 
     Referring to  FIG. 1 , a description is provided of a construction of the image forming apparatus  100 . 
     As illustrated in  FIG. 1 , 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 a transfer belt. 
     The image forming apparatus  100  employs a tandem structure in which photoconductive drums  20 Y,  20 C,  20 M, and  20 Bk serving as image bearers that bear yellow, cyan, magenta, and black toner images in separation colors, respectively, are aligned. 
     Although  FIG. 1  illustrates the color printer employing the tandem system as one example of the image forming apparatus  100 , the image forming apparatus  100  may employ other systems. 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 two of copying, printing, scanning, facsimile, and plotter functions, or the like. 
     The yellow, cyan, magenta, and black toner images formed on the photoconductive drums  20 Y,  20 C,  20 M, and  20 Bk, respectively, as visible images are primarily transferred successively onto a transfer belt  11 , that is, an endless belt serving as an intermediate transferor, disposed opposite the photoconductive drums  20 Y,  20 C,  20 M, and  20 Bk as the transfer belt  11  rotates in a rotation direction A 1  in a primary transfer process. Through the primary transfer process, the yellow, cyan, magenta, and black toner images are superimposed on the transfer belt  11  and then secondarily transferred onto a recording 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 Bk is surrounded by image forming components that form a toner image on each of the photoconductive drums  20 Y,  20 C,  20 M, and  20 Bk as each of the photoconductive drums  20 Y,  20 C,  20 M, and  20 Bk rotates clockwise in  FIG. 1  in a rotation direction D 20 , thus constructing an image forming unit serving as the image forming device. Taking the image forming unit incorporating the photoconductive drum  20 Bk, the following describes an image forming operation to form the black toner image. The image forming unit includes a charger  30 Bk, a developing device  40 Bk, a primary transfer roller  12 Bk, and a cleaner  50 Bk that surround the photoconductive drum  20 Bk in this order in the rotation direction D 20  of the photoconductive drum  20 Bk. The photoconductive drums  20 Y,  20 C, and  20 M are also 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. After the charger  30 Bk charges the photoconductive drum  20 Bk, an optical writing device  8  writes an electrostatic latent image on the photoconductive drum  20 Bk with a laser beam Lb. 
     As the transfer belt  11  rotates in the rotation direction A 1 , the yellow, cyan, magenta, and black toner images formed on the photoconductive drums  20 Y,  20 C,  20 M, and  20 Bk, respectively, as visible images are primarily transferred successively onto the transfer belt  11 , thus being superimposed on a same position on the transfer belt  11 . In the primary transfer process, the primary transfer rollers  12 Y,  12 C,  12 M, and  12 Bk disposed opposite the photoconductive drums  20 Y,  20 C,  20 M, and  20 Bk via the transfer belt  11 , respectively, apply a primary transfer bias to the photoconductive drums  20 Y,  20 C,  20 M, and  20 Bk successively from the upstream photoconductive drum  20 Y to the downstream photoconductive drum  20 Bk in the rotation direction A 1  of the transfer belt  11 . 
     The photoconductive drums  20 Y,  20 C,  20 M, and  20 Bk are aligned in this order in the rotation direction A 1  of the transfer belt  11 . The photoconductive drums  20 Y,  20 C,  20 M, and  20 Bk are located in the image forming units that form the yellow, cyan, magenta, and black toner images, respectively. 
     The image forming apparatus  100  includes the four image forming units that form the yellow, cyan, magenta, and black toner images, respectively, a transfer belt unit  10 , a secondary transfer roller  5 , a transfer belt cleaner  13 , and the optical writing device  8 . The transfer belt unit  10  is situated above and disposed opposite the photoconductive drums  20 Y,  20 C,  20 M, and  20 Bk. The transfer belt unit  10  incorporates the transfer belt  11  and the primary transfer rollers  12 Y,  12 C,  12 M, and  12 Bk. The secondary transfer roller  5  is disposed opposite the transfer belt  11  and driven and rotated in accordance with rotation of the transfer belt  11 . The transfer belt cleaner  13  is disposed opposite the transfer belt  11  to clean the transfer belt  11 . The optical writing device  8  is situated below and disposed opposite the four image forming units. 
     The optical writing device  8  includes a semiconductor laser serving as a light source, a coupling lens, an fθ 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 Bk thereto, forming electrostatic latent images on the photoconductive drums  20 Y,  20 C,  20 M, and  20 Bk, respectively.  FIG. 1  illustrates the light beam Lb directed to the image forming unit that forms the black toner image. Similarly, light beams Lb are directed to the image forming units that form the yellow, cyan, and magenta toner images, respectively. 
     The image forming apparatus  100  further includes a sheet feeder  61  and a registration roller pair  4 . The sheet feeder  61  incorporates a paper tray that loads a plurality of recording sheets S to be conveyed to a secondary transfer nip formed between the transfer belt  11  and the secondary transfer roller  5 . The registration roller pair  4  conveys a recording sheet S conveyed from the sheet feeder  61  to the secondary transfer nip formed between the 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 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 recording sheet S reaches the registration roller pair  4 . 
     The image forming apparatus  100  further includes a fixing device  200 , an output roller pair  7 , an output tray  17 , and toner bottles  9 Y,  9 C,  9 M, and  9 Bk. The fixing device  200 , serving as a fusing unit employing a belt fixing system, fixes a color toner image formed by the yellow, cyan, magenta, and black toner images secondarily transferred from the transfer belt  11  onto the recording sheet S thereon. The output roller pair  7  ejects the recording sheet S bearing the fixed toner image onto an outside of the image forming apparatus  100 , that is, the output tray  17 . The output tray  17  is disposed atop the image forming apparatus  100  and stacks the recording sheet S ejected by the output roller pair  7  to the outside of the image forming apparatus  100 . The toner bottles  9 Y,  9 C,  9 M, and  9 Bk are situated below the output tray  17  and replenished with fresh yellow, cyan, magenta, and black toners, respectively. 
     The transfer belt unit  10  includes a driving roller  72  and a driven roller  73  over which the transfer belt  11  is looped, in addition to the transfer belt  11  and the primary transfer rollers  12 Y,  12 C,  12 M, and  12 Bk. 
     Since the driven roller  73  also serves as a tension applicator that applies tension to the transfer belt  11 , a biasing member (e.g., a spring) biases the driven roller  73  against the transfer belt  11 . The transfer belt unit  10 , the primary transfer rollers  12 Y,  12 C,  12 M, and  12 Bk, the secondary transfer roller  5 , and the transfer belt cleaner  13  construct a transfer device  71 . 
     The sheet feeder  61  is situated in a lower portion of the image forming apparatus  100  and includes a feed roller  3  that contacts an upper side of an uppermost recording sheet S of the plurality of recording 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 recording sheet S to the registration roller pair  4 . 
     The transfer belt cleaner  13  of the transfer device  71  includes a cleaning brush and a cleaning blade being disposed opposite and contacting the transfer belt  11 . The cleaning brush and the cleaning blade of the transfer belt cleaner  13  scrape a foreign substance such as residual toner particles off the transfer belt  11 , removing the foreign substance from the transfer belt  11  and thereby cleaning the transfer belt  11 . 
     The transfer belt cleaner  13  further includes a waste toner conveyer that conveys the residual toner particles removed from the transfer belt  11 . 
     Referring to  FIG. 2 , a description is provided of a construction of the fixing device  200  according to a first embodiment 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  200 . As illustrated in  FIG. 2 , the fixing device  200  (e.g., a fuser or a fusing unit) includes a fixing belt  201  formed into a loop as one example of a fixing rotator or a fixing member rotatable in a rotation direction D 201  and a pressure roller  203  as one example of a pressure rotator disposed opposite the fixing belt  201  and rotatable in a rotation direction D 203 . Each of the fixing belt  201  and the pressure roller  203  extends in a longitudinal direction, that is, an axial direction, which is perpendicular to a cross-section in  FIG. 2 . Each of the fixing belt  201  and the pressure roller  203  is greater than a width of the recording sheet S in the axial direction of the fixing belt  201  and the pressure roller  203 . The fixing belt  201  and the pressure roller  203  sandwich and convey the recording sheet S. The fixing device  200  further includes a heater pair  202  as one example of a heater or a heat source. The heater pair  202  includes a center heater  202 A and a lateral end heater  202 B. The center heater  202 A and the lateral end heater  202 B are disposed opposite an inner circumferential surface of the fixing belt  201  to heat the fixing belt  201  directly with radiation heat. As described below, the center heater  202 A and the lateral end heater  202 B have different heat generation spans in the axial direction of the fixing belt  201 , which heat different heating spans of the fixing belt  201  in the axial direction thereof, respectively. The heater pair  202  employs a halogen heater as one example. However, the heater pair  202  is not limited to the halogen heater. For example, the heater pair  202  is a ceramic heater that contacts and heats the fixing belt  201  or an induction heater (IH) that causes the fixing belt  201  to generate heat by electromagnetic induction. 
     The fixing device  200  further includes a temperature sensor pair  230  as one example of a temperature detector that detects the temperature of the fixing belt  201 . The temperature sensor pair  230  employs a non-contact thermopile as one example. However, the temperature sensor pair  230  is not limited to the non-contact thermopile. As described below, the temperature sensor pair  230  includes two temperature sensors, that is, a center temperature sensor  230 A and a lateral end temperature sensor  230 B which are disposed opposite an outer circumferential surface of the fixing belt  201  at different positions, respectively, in the axial direction thereof. The temperature sensor pair  230  detects the temperature of the outer circumferential surface of the fixing belt  201 . As described below, a controller  18  controls the lighting rate of the heater pair  202  according to the temperature of the fixing belt  201  detected by the temperature sensor pair  230 , thus controlling the temperature of the fixing belt  201  to a desired temperature. 
     Inside the loop formed by the fixing belt  201  is a nip formation assembly  6  including a nip formation pad  206  and a thermal conduction aid  216 . The nip formation assembly  6  is disposed opposite the pressure roller  203  via the fixing belt  201  to form a fixing nip N between the fixing belt  201  and the pressure roller  203 . For example, the nip formation assembly  6  and the pressure roller  203  sandwich the fixing belt  201  to form the fixing nip N between the fixing belt  201  and the pressure roller  203 . While a recording sheet S bearing a toner image is conveyed through the fixing nip N in a recording sheet conveyance direction DS, the toner image on the recording sheet S receives sufficient heat and pressure from the fixing belt  201  and the pressure roller  203 , thus being fixed on the recording sheet S. The nip formation pad  206  contacts the thermal conduction aid  216 . The thermal conduction aid  216  includes an inner face that contacts the nip formation pad  206  and an outer face that is opposite the inner face and contacts the inner circumferential surface of the fixing belt  201 . The fixing belt  201  rotates in the rotation direction D 201  while the fixing belt  201  contacts and slides over the thermal conduction aid  216 . 
     A description is provided of a configuration of the controller  18 . 
       FIG. 3  is a block diagram of the image forming apparatus  100 , illustrating the controller  18  for controlling the temperature of the fixing belt  201  as described above. The controller  18  (e.g., a processor) is a micro computer including a central processing unit (CPU), a read only memory (ROM), and a random access memory (RAM). The controller  18  controls power supply to the heater pair  202  based on the temperature of the fixing belt  201  detected by the temperature sensor pair  230 . For example, based on information about the temperature of the fixing belt  201  which is sent from each of the center temperature sensor  230 A and the lateral end temperature sensor  230 B, the controller  18  controls power supply to each of the center heater  202 A and the lateral end heater  202 B, that is, the halogen heater, through a triac or the like, thus performing a feedback control that adjusts the lighting rate of the center heater  202 A and the lateral end heater  202 B. The controller  18 , as one example, is disposed inside a body of the image forming apparatus  100  to control components other than the fixing device  200  that are disposed inside the image forming apparatus  100  and control communication between the image forming apparatus  100  and an external device. The controller  18  may be located in the fixing device  200  or the image forming apparatus  100 . 
     A detailed description is now given of a configuration of the thermal conduction aid  216 . 
     As illustrated in  FIG. 2 , the outer face of the thermal conduction aid  216  that is disposed opposite the pressure roller  203  is planar. Alternatively, the outer face of the thermal conduction aid  216  may not be planar. For example, the outer face of the thermal conduction aid  216  may be contoured into a recess or a curve corresponding to an outer circumferential surface of the pressure roller  203  or other shapes. If the outer face of the thermal conduction aid  216  is recessed with respect to the pressure roller  203 , the outer face of the thermal conduction aid  216  directs the leading edge of the recording sheet S toward the pressure roller  203  as the recording sheet S is ejected from the fixing nip N, facilitating separation of the recording sheet S from the fixing belt  201  and suppressing jamming of the recording sheet S between the fixing belt  201  and the pressure roller  203 . 
     Inside the loop formed by the fixing belt  201  are the nip formation pad  206 , the thermal conduction aid  216 , and a stay  207 . The nip formation pad  206  is disposed opposite the pressure roller  203 . The thermal conduction aid  216  covers an outer face of the nip formation pad  206  that is disposed opposite the inner circumferential surface of the fixing belt  201 . The stay  207  supports the nip formation pad  206  against pressure from the pressure roller  203 . Each of the nip formation pad  206 , the thermal conduction aid  216 , and the stay  207  has a length not smaller than a length of the fixing belt  201  in the axial direction thereof that is parallel to a longitudinal direction of the nip formation pad  206 , the thermal conduction aid  216 , and the stay  207 . Each of the nip formation pad  206 , the thermal conduction aid  216 , and the stay  207  has a length not smaller than a length of the fixing nip N in the recording sheet conveyance direction DS that is substantially perpendicular to the axial direction of the fixing belt  201  and parallel to a short direction of the nip formation pad  206 , the thermal conduction aid  216 , and the stay  207 . 
     The thermal conduction aid  216  prevents heat conducted to the fixing belt  201  from being stored locally and facilitates conduction of heat in the longitudinal direction of the thermal conduction aid  216 , thus reducing uneven temperature of the fixing belt  201  in the axial direction thereof. Hence, the thermal conduction aid  216  is made of a material that conducts heat quickly, for example, a metal material having an increased thermal conductivity such as copper, aluminum, and silver. It is preferable that the thermal conduction aid  216  is made of copper in a comprehensive view of manufacturing costs, availability, thermal conductivity, and processing. Thus, the thermal conduction aid  216  is a metal plate, for example. 
     According to this embodiment, at least a part of the outer face of the thermal conduction aid  216  that is disposed opposite the inner circumferential surface of the fixing belt  201  contacts the inner circumferential surface of the fixing belt  201  directly, thus serving as a nip formation face that forms the fixing nip N. 
     A detailed description is now given of a construction of the fixing belt  201 . 
     The fixing belt  201  is an endless belt or film made of metal such as nickel and SUS stainless steel or resin such as polyimide, for example. The fixing belt  201  includes a base layer and a surface layer. The surface layer is made of polytetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), polytetrafluoroethylene (PTFE), or the like to prevent toner of the toner image on the recording sheet S from adhering to the fixing belt  201 . An elastic layer may be sandwiched between the base layer and the surface layer and made of silicone rubber or the like. If the fixing belt  201  does not incorporate the elastic layer, the fixing belt  201  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 recording sheet S properly. However, as the pressure roller  203  and the fixing belt  201  sandwich and press the unfixed toner image on the recording sheet S passing through the fixing nip N, slight surface asperities of the fixing belt  201  may be transferred onto the toner image on the recording sheet S, resulting in variation in gloss of the solid toner image. To address this circumstance, the elastic layer has a thickness not smaller than 100 micrometers. As the elastic layer deforms, the elastic layer absorbs slight surface asperities of the fixing belt  201 , preventing variation in gloss of the solid toner image. 
     A detailed description is now given of a construction of the stay  207 . 
     The stay  207  includes an arm extending in a direction in which the arm separates from the fixing nip N. The center heater  202 A is disposed opposite the lateral end heater  202 B via the arm of the stay  207 . The center heater  202 A and the lateral end heater  202 B serve as a fixing heater. The center heater  202 A and the lateral end heater  202 B emit light that irradiates the inner circumferential surface of the fixing belt  201 , thus heating the fixing belt  201  directly with radiation heat. 
     The stay  207  serving as a support that supports the nip formation pad  206  to form the fixing nip N is situated inside the loop formed by the fixing belt  201 . As the nip formation pad  206  receives pressure from the pressure roller  203 , the stay  207  supports the nip formation pad  206  to prevent bending of the nip formation pad  206  and produce an even nip length in the recording sheet conveyance direction DS throughout the entire span of the fixing belt  201  in the axial direction thereof. The nip formation pad  206  includes projections  206   e  contacting the stay  207 . The projections  206   e  are extended in the longitudinal direction of the nip formation pad  206  and arranged in two rows.  FIG. 2  illustrates the two projections  206   e  disposed at a lateral end of the nip formation pad  206  in the longitudinal direction thereof. If an outer face of the stay  207  surface-contacts an inner face of the nip formation pad  206 , heat may accumulate between the outer face of the stay  207  and the inner face of the nip formation pad  206 , resulting in a failure such as deformation of the nip formation pad  206 . To address this circumstance, according to this embodiment, the outer face of the stay  207  contacts the projections  206   e  of the nip formation pad  206 , preventing heat from accumulating between the stay  207  and the nip formation pad  206 . The nip formation pad  206  further includes a boss  206   f . The stay  207  includes a boss hole through which the boss  206   f  is inserted to position the nip formation pad  206  with respect to the stay  207 . 
     A detailed description is now given of a configuration of the nip formation pad  206 . 
     The nip formation pad  206  is made of a heat resistant material being resistant against temperatures up to a range of from 200 degrees centigrade to 400 degrees centigrade, preferably a range of from 200 degrees centigrade to 350 degrees centigrade, and having an enhanced mechanical strength. For example, the nip formation pad  206  is made of heat resistant resin such as polyimide (PI), polyether ether ketone (PEEK), and PI or PEEK reinforced with glass fiber. 
     The stay  207  is mounted on and held by flanges  209  described below as a holder at both lateral ends of the stay  207  in the longitudinal direction thereof, thus being positioned inside the fixing device  200 . A reflector  208  is interposed between the center heater  202 A and the stay  207  and between the lateral end heater  202 B and the stay  207 . The reflector  208  prevents light and heat radiated from the heater pair  202  from heating the stay  207  with radiant heat, suppressing waste of energy. Alternatively, instead of the reflector  208 , an opposed face of the stay  207  disposed opposite the heater pair  202  may be treated with insulation or mirror finish to reflect light radiated from the heater pair  202  to the stay  207  toward the fixing belt  201 . The stay  207  is made of a material enhancing the mechanical strength to support the nip formation pad  206  against pressure from the pressure roller  203  and prevent bending of the nip formation pad  206 . For example, the stay  207  is made of metal such as stainless steel and iron or resin. 
     A detailed description is now given of a construction of the pressure roller  203 . 
     The pressure roller  203  is constructed of a core bar  205 , an elastic rubber layer  204  coating the core bar  205 , and a release layer coating the elastic rubber layer  204 . The elastic rubber layer  204  is made of rubber. The release layer is made of PFA or PTFE, for example, to facilitate separation of the recording sheet S from the pressure roller  203 . 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  203  through a gear train, the pressure roller  203  rotates in the rotation direction D 203 . Alternatively, the driver may also be connected to the fixing belt  201  to drive and rotate the fixing belt  201 . A spring or the like presses the pressure roller  203  against the nip formation pad  206  via the fixing belt  201 . As the spring presses and deforms the elastic rubber layer  204  of the pressure roller  203 , the pressure roller  203  produces and retains the fixing nip N having a predetermined length in the recording sheet conveyance direction DS. The pressure roller  203  may be a hollow roller or a solid roller. If the pressure roller  203  is a hollow roller, a heater such as a halogen heater may be disposed inside the hollow roller. The elastic rubber layer  204  may be made of solid rubber. Alternatively, if no heater is situated inside the pressure roller  203 , the elastic rubber layer  204  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  201 . 
     As the pressure roller  203  rotates in the rotation direction D 203 , the fixing belt  201  rotates in the rotation direction D 201  in accordance with rotation of the pressure roller  203  by friction therebetween. According to this embodiment, as the driver drives and rotates the pressure roller  203 , a driving force of the driver is transmitted from the pressure roller  203  to the fixing belt  201  at the fixing nip N, thus rotating the fixing belt  201  by friction between the pressure roller  203  and the fixing belt  201 . At the fixing nip N, the fixing belt  201  rotates as the fixing belt  201  is sandwiched between the pressure roller  203  and the nip formation pad  206 ; at a circumferential span of the fixing belt  201  other than the fixing nip N, the fixing belt  201  rotates while the fixing belt  201  is guided by the flange  209  described below at each lateral end of the fixing belt  201  in the axial direction thereof. 
     The fixing belt  201  and the components disposed inside the loop formed by the fixing belt  201 , that is, the heater pair  202 , the nip formation assembly  6 , the stay  207 , and the reflector  208 , may construct a belt unit  201 U that is coupled with the pressure roller  203 . With the construction described above, the fixing device  200  attaining quick warm-up is manufactured at reduced costs. 
     A description is provided of the length of the thermal conduction aid  216  incorporated in the fixing device  200  in the longitudinal direction of the thermal conduction aid  216 . 
     The length of the pressure roller  203  in the axial direction thereof is greater than a maximum conveyance span of 320 mm as one example where a maximum recording sheet S available in the fixing device  200  is conveyed in view of shifting of the recording sheet S when a user places the recording sheet S inside the sheet feeder  61  erroneously. The length of the thermal conduction aid  216  in the longitudinal direction thereof is greater than the length of the pressure roller  203  in the axial direction thereof. If the length of the thermal conduction aid  216  is smaller than the length of the pressure roller  203 , the fixing belt  201  may bend at a lateral end of the thermal conduction aid  216  in the longitudinal direction thereof at the fixing nip N and therefore may be damaged. To address this circumstance, the length of the thermal conduction aid  216  is greater than the length of the pressure roller  203  in view of manufacturing tolerance of each component of the fixing device  200  and play required for assembling. Accordingly, the length of the thermal conduction aid  216  is substantially greater than the maximum conveyance span of the fixing device  200 . For example, the length of the thermal conduction aid  216  is greater than the maximum conveyance span by about 20 mm at each lateral end of the thermal conduction aid  216  in the longitudinal direction thereof. 
     A description is provided of a construction of a fixing device  200 S according to a second embodiment. 
       FIG. 4  is a schematic vertical cross-sectional view of the fixing device  200 S. Identical reference numerals are assigned to components identical or equivalent to the components incorporated in the fixing device  200  illustrated in  FIG. 2 . In the fixing device  200  according to the first embodiment depicted in  FIG. 2 , the stay  207  is interposed between the center heater  202 A and the lateral end heater  202 B. Conversely, in the fixing device  200 S according to the second embodiment depicted in  FIG. 4 , the center heater  202 A and the lateral end heater  202 B are disposed upstream from the fixing nip N in the rotation direction D 201  of the fixing belt  201 . 
     As illustrated in  FIG. 2 , the stay  207  interposed between the center heater  202 A and the lateral end heater  202 B screens the center heater  202 A from the lateral end heater  202 B and screens the lateral end heater  202 B from the center heater  202 A. Accordingly, the stay  207  prevents heat radiated from one of the center heater  202 A and the lateral end heater  202 B from being absorbed by a glass tube or the like of another one of the center heater  202 A and the lateral end heater  202 B, reducing waste of energy. As illustrated in  FIG. 4 , the center heater  202 A and the lateral end heater  202 B are disposed upstream from the fixing nip N in the rotation direction D 201  of the fixing belt  201 . Accordingly, a heated portion of the fixing belt  201  heated by the center heater  202 A and the lateral end heater  202 B dissipates a reduced amount of heat before the heated portion of the fixing belt  201  reaches the fixing nip N as the fixing belt  201  rotates in the rotation direction D 201 , reducing waste of energy. Thus, the fixing device  200  is modified variously. 
       FIG. 5  is a schematic perspective view of the fixing device  200 , illustrating one lateral end of the fixing device  200  in a longitudinal direction thereof. The flange  209  is disposed at each lateral end of the fixing belt  201  in the axial direction thereof.  FIG. 5  illustrates the flange  209  disposed at one lateral end of the fixing belt  201  in the axial direction thereof. 
     The flange  209  is hollow and open at each lateral end thereof in the axial direction of the fixing belt  201 . The flange  209  includes a receiver  209   a  extending in the axial direction of the fixing belt  201  and a flange portion  209   b  projecting in a radial direction of the fixing belt  201  from the receiver  209   a  and being molded with the receiver  209   a . The receiver  209   a  includes a slit  209   c  at a part of the receiver  209   a  in a circumferential direction of the fixing belt  201  and is partially cylindrical or tubular. The nip formation pad  206  and the thermal conduction aid  216  are inserted into a space defined by the slit  209   c.    
     If the fixing belt  201  is skewed in the axial direction of the fixing belt  201 , a lateral end of the fixing belt  201  in the axial direction thereof comes into contact with the receiver  209   a  that restricts motion of the fixing belt  201  in the axial direction thereof. The flange portion  209   b  is supported by a side plate of the fixing device  200 . Optionally, a plate ring may be interposed between the receiver  209   a  and each lateral end of the fixing belt  201  in the axial direction thereof. The plate ring is made of a material that facilitates sliding of the fixing belt  201  over the plate ring. 
     A description is provided of a configuration of the nip formation pad  206  and the thermal conduction aid  216  of the nip formation assembly  6  according to a first embodiment. 
       FIG. 6  is an exploded perspective view of the nip formation assembly  6  according to the first embodiment. The center temperature sensor  230 A depicted in  FIGS. 2 and 4  is disposed opposite the fixing belt  201  at a position P 230 A indicated with alternate long and short dash lines and detects the temperature of the outer circumferential surface of the fixing belt  201  in a center detection span S 230 A in the axial direction of the fixing belt  201 . The lateral end temperature sensor  230 B depicted in  FIGS. 2 and 4  is disposed opposite the fixing belt  201  at a position P 230 B indicated with alternate long and short dash lines and detects the temperature of the outer circumferential surface of the fixing belt  201  in a lateral end detection span S 230 B in the axial direction of the fixing belt  201 . 
     For example, after the nip formation pad  206  and the thermal conduction aid  216  are installed in the fixing device  200  or  200 S and the fixing device  200  or  200 S starts operation, as illustrated in  FIGS. 2 and 4 , as the fixing belt  201  rotates in the rotation direction D 201 , the inner circumferential surface of the fixing belt  201  contacts and slides over the thermal conduction aid  216 . At the position P 230 A, the center temperature sensor  230 A detects the temperature of the outer circumferential surface of the fixing belt  201  and the inner circumferential surface of the fixing belt  201  contacts and slides over the outer face of the thermal conduction aid  216  while the fixing belt  201  rotates in the rotation direction D 201 . At the position P 230 B, the lateral end temperature sensor  230 B detects the temperature of the outer circumferential surface of the fixing belt  201  and the inner circumferential surface of the fixing belt  201  contacts and slides over the outer face of the thermal conduction aid  216  while the fixing belt  201  rotates in the rotation direction D 201 . At the positions P 230 A and P 230 B, the inner face of the thermal conduction aid  216  contacts the outer face of the nip formation pad  206 . 
     As one example, the center temperature sensor  230 A has a center detection region A indicated with a dotted circle. The lateral end temperature sensor  230 B has a lateral end detection region B indicated with a dotted circle. As the fixing belt  201  rotates, the center temperature sensor  230 A and the lateral end temperature sensor  230 B detect the temperature of the outer circumferential surface of the fixing belt  201  in the center detection span S 230 A defined by the position P 230 A as a center and the lateral end detection span S 230 B defined by the position P 230 B as a center, respectively. While the fixing belt  201  rotates, the inner circumferential surface of the fixing belt  201  contacts the thermal conduction aid  216  in the center detection span S 230 A and the lateral end detection span S 230 B. As described above, the center temperature sensor  230 A and the lateral end temperature sensor  230 B detect the temperature of the fixing belt  201  at the positions P 230 A and P 230 B, respectively. However, the center temperature sensor  230 A and the lateral end temperature sensor  230 B may not be disposed opposite the positions P 230 A and P 230 B on the fixing belt  201 , respectively. For example, if each of the center temperature sensor  230 A and the lateral end temperature sensor  230 B is a non-contact sensor isolated from the fixing belt  201 , the center temperature sensor  230 A and the lateral end temperature sensor  230 B may be shifted from the positions P 230 A and P 230 B, respectively, due to a layout of the fixing device  200 . 
     The thermal conduction aid  216  includes a nip formation portion  216   a  and a bent portion  216   b . The thermal conduction aid  216  extends in the longitudinal direction thereof and covers the outer face of the nip formation pad  206  that is disposed opposite the fixing nip N. When seen in a cross-section depicted in  FIGS. 2 and 4  that is perpendicular to the longitudinal direction of the thermal conduction aid  216 , the nip formation portion  216   a  is contoured along the outer face of the nip formation pad  206 . The nip formation portion  216   a  contacts the outer face of the nip formation pad  206  that faces the fixing nip N via the thermal conduction aid  216  and the fixing belt  201 . The nip formation portion  216   a  includes an inner face that contacts the nip formation pad  206  and an outer face that is opposite the inner face and contacts the inner circumferential surface of the fixing belt  201 . 
     The bent portion  216   b  is bent relative to the nip formation portion  216   a  at substantially a right angle. While the fixing belt  201  rotates, friction between the fixing belt  201  and the thermal conduction aid  216  may generate a force that shifts the thermal conduction aid  216  from the nip formation pad  206  in the recording sheet conveyance direction DS. To address this circumstance, the bent portion  216   b  contacts the nip formation pad  206  to prevent the thermal conduction aid  216  from shifting from the nip formation pad  206  in the recording sheet conveyance direction DS, retaining a proper positional relation between the thermal conduction aid  216  and the nip formation pad  206 . 
     The nip formation pad  206  includes grooves  206   a  and  206   b  (e.g., recesses) on the outer face that contacts the thermal conduction aid  216 , decreasing the contact area where the nip formation pad  206  contacts the thermal conduction aid  216 . The groove  206   a  originates at one lateral edge of the nip formation pad  206  and extends toward a center of the nip formation pad  206  in the longitudinal direction thereof. The groove  206   b  originates at another lateral edge of the nip formation pad  206  and extends toward the center of the nip formation pad  206  in the longitudinal direction thereof. However, the groove  206   b  does not extend to the lateral end detection span S 230 B, encompassing the position P 230 B, where the lateral end temperature sensor  230 B detects the temperature of the fixing belt  201  so as to increase the contact area where the nip formation pad  206  contacts the thermal conduction aid  216 . 
     As one example, the groove  206   b  originates at one lateral edge of the nip formation pad  206  and extends to a position outboard from the position P 230 B of the lateral end temperature sensor  230 B by 10 mm in the longitudinal direction of the nip formation pad  206 . The groove  206   a  originates at another lateral edge of the nip formation pad  206  in the longitudinal direction thereof. The groove  206   a  defines an outboard span S 4  disposed outboard from a symmetrical span S 3  that is substantially symmetrical to a first span S 1  encompassing the lateral end detection span S 230 B via a center of the fixing belt  201  in the axial direction thereof. In the symmetrical span S 3 , the nip formation pad  206  contacts the thermal conduction aid  216  in a contact area which is greater than a contact area where the nip formation pad  206  contacts the thermal conduction aid  216  in the outboard span S 4 . According to this embodiment, the groove  206   a  is substantially symmetrical to the groove  206   b  via the center of the nip formation pad  206  in the longitudinal direction thereof. Each of the grooves  206   a  and  206   b  is disposed at substantially a center of the nip formation pad  206  in the short direction thereof and has an identical width in the short direction of the nip formation pad  206 . 
     A description is provided of a configuration of a nip formation pad  206 S and the thermal conduction aid  216  of a nip formation assembly  6 S according to a second embodiment. 
       FIG. 7  is an exploded perspective view of the nip formation assembly  6 S according to the second embodiment. The nip formation pad  206 S depicted in  FIG. 7  includes a groove  206   c  disposed at a center of the nip formation pad  206 S in a longitudinal direction thereof and not disposed at both lateral end portions where the grooves  206   a  and  206   b  are not disposed on the nip formation pad  206  depicted in  FIG. 6 . As one example, the grooves  206   b  and  206   c  are not disposed in an intermediate span (e.g., the first span S 1 ) that extends inboard and outboard from the position P 230 B by 10 mm in the longitudinal direction of the nip formation pad  206 S. The grooves  206   a ,  206   b , and  206   c  are disposed outside the intermediate span, varying the contact area where the nip formation pad  206 S contacts the thermal conduction aid  216  in the longitudinal direction of the nip formation pad  206 S. The groove  206   c  decreases an amount of heat diffused from the thermal conduction aid  216  to the nip formation pad  206 S in a center span of the nip formation pad  206 S in the longitudinal direction thereof, allowing the thermal conduction aid  216  to conduct heat to the fixing belt  201  effectively to fix the toner image on the recording sheet S at the fixing nip N. 
     A description is provided of a configuration of a nip formation pad  206 T and the thermal conduction aid  216  of a nip formation assembly  6 T according to a third embodiment. 
       FIG. 8  is an exploded perspective view of the nip formation assembly  6 T according to the third embodiment. The nip formation pad  206 T depicted in  FIG. 8  includes a beam  206   d  disposed at a center of each of the grooves  206   a ,  206   b , and  206   c  in the rotation direction D 201  of the fixing belt  201  and extended in a longitudinal direction of the nip formation pad  206 T. In other words, the beam  206   d  divides each of the grooves  206   a ,  206   b , and  206   c  into a plurality of groove portions. The beam  206   d  stabilizes pressure exerted at the fixing nip N to fix the toner image on the recording sheet S. The number of the beams  206   d  may increase. In order to enhance insulation and stabilize pressure exerted at the fixing nip N, the number of the beams  206   d  may vary between the grooves  206   a ,  206   b , and  206   c . For example, if the beams  206   d  have identical lengths in the longitudinal direction and a short direction of the nip formation pad  206 T, the number of the beams  206   d  disposed on the groove  206   c  is greater than the number of the beams  206   d  disposed on each of the grooves  206   a  and  206   b , thus increasing the contact area where the nip formation pad  206 T contacts the thermal conduction aid  216 . 
     The contact area where the nip formation pad  206 T contacts the thermal conduction aid  216  is compared as below as one example. Sample regions having an identical area are extracted from an outer face of the nip formation pad  206 T that is disposed opposite the thermal conduction aid  216 . A sample region where one or more of the grooves  206   a ,  206   b , and  206   c  occupy a smaller area creates a greater contact area where the nip formation pad  206 T contacts the thermal conduction aid  216 . In order to compare the contact area in the longitudinal direction of the nip formation pad  206 T, since the outer face of the nip formation pad  206 T that is disposed opposite the thermal conduction aid  216  is substantially rectangular, sample regions having a particular unit length in the longitudinal direction of the nip formation pad  206 T are extracted to compare the rate of one or more of the grooves  206   a ,  206   b , and  206   c  that occupy in each of the sample regions. 
     Referring to  FIG. 7 , a description is provided of a first comparison example of comparing the contact area described above. 
     A first region is defined by a span inboard and outboard from the position P 230 B by 10 mm, that is, a substantially rectangular region having a span of 20 mm in the longitudinal direction of the nip formation pad  206 S. The grooves  206   b  and  206   c  are not disposed in the first region and the lateral end temperature sensor  230 B is disposed in the first region. A second region is disposed outboard from the first region in the longitudinal direction of the nip formation pad  206 S and has a span of 20 mm like the first region. Since the groove  206   b  is disposed in the second region, the first region attains a greater contact area where the nip formation pad  206 S contacts the thermal conduction aid  216  compared to the second region. 
     Referring to  FIG. 7 , a description is provided of a second comparison example of comparing the contact area described above. 
     A first region is defined by a span inboard and outboard from the position P 230 B by 5 mm, that is, a substantially rectangular region having a span of 10 mm in the longitudinal direction of the nip formation pad  206 S. The lateral end temperature sensor  230 B is disposed in the first region. A second region is disposed outboard from the first region in the longitudinal direction of the nip formation pad  206 S and has a span of 10 mm like the first region. Since the groove  206   b  is disposed in the second region, the first region attains a greater contact area where the nip formation pad  206 S contacts the thermal conduction aid  216  compared to the second region. The second region having the span of 10 mm is selected contiguously in a region outboard from the first region in the longitudinal direction of the nip formation pad  206 S. However, if the groove  206   b  is disposed in the second region, the first region attains a greater contact area where the nip formation pad  206 S contacts the thermal conduction aid  216  compared to the second region, thus achieving advantages of this embodiment. 
     Each of the nip formation pads  206 ,  206 S, and  206 T contacts the thermal conduction aid  216  such that heat conduction generates between each of the nip formation pads  206 ,  206 S, and  206 T and the thermal conduction aid  216  at least in a pressurization direction in which pressure is exerted to fix the toner image on the recording sheet S. For example, even if an intermediate component is interposed between each of the nip formation pads  206 ,  206 S, and  206 T and the thermal conduction aid  216  and the intermediate component prohibits each of the nip formation pads  206 ,  206 S, and  206 T from contacting the thermal conduction aid  216  directly, if the intermediate component has an increased thermal conductivity and a decreased thermal capacity, each of the nip formation pads  206 ,  206 S, and  206 T attains the advantages described above. The grooves  206   a ,  206   b , and  206   c  depicted in  FIGS. 6 to 8  are filled with air. Alternatively, the grooves  206   a ,  206   b , and  206   c  may be filled with a component made of a material that is different from a material of the nip formation pads  206 ,  206 S, and  206 T and has a thermal conductivity smaller than a thermal conductivity of the nip formation pads  206 ,  206 S, and  206 T. In this case also, the contact area where each of the nip formation pads  206 ,  206 S, and  206 T contacts the thermal conduction aid  216  varies depending on the position on each of the nip formation pads  206 ,  206 S, and  206 T in the longitudinal direction thereof. 
     As illustrated in  FIGS. 6 to 8 , each of the grooves  206   a ,  206   b , and  206   c  is substantially parallel to the longitudinal direction of the nip formation pads  206 ,  206 S, and  206 T. Alternatively, each of the grooves  206   a ,  206   b , and  206   c  may be angled or inclined relative to the longitudinal direction of the nip formation pads  206 ,  206 S, and  206 T. The grooves  206   a  and  206   b  originate at one lateral edge and another lateral edge of each of the nip formation pads  206 ,  206 S, and  206 T in the longitudinal direction thereof, respectively. Alternatively, the grooves  206   a  and  206   b  may be spaced apart from one lateral edge and another lateral edge of each of the nip formation pads  206 ,  206 S, and  206 T in the longitudinal direction thereof, respectively. Each of the grooves  206   a  and  206   b  does not originate at one lateral edge of each of the nip formation pads  206 ,  206 S, and  206 T in a short direction thereof. Alternatively, each of the grooves  206   a  and  206   b  may originate at and extend from one lateral edge of each of the nip formation pads  206 ,  206 S, and  206 T in the longitudinal direction thereof. 
     As illustrated in  FIGS. 6 to 8 , the nip formation pad  206  mounts the grooves  206   a  and  206   b  and each of the nip formation pads  206 S and  206 T mounts the grooves  206   a ,  206   b , and  206   c . The grooves  206   a ,  206   b , and  206   c  may be replaced with cavities having a circular shape or other shapes to vary the contact area where each of the nip formation pads  206 ,  206 S, and  206 T contacts the thermal conduction aid  216  depending on the position on each of the nip formation pads  206 ,  206 S, and  206 T in the longitudinal direction thereof. The grooves  206   a ,  206   b , and  206   c  are isolated from each other in the longitudinal direction of the nip formation pads  206 ,  206 S, and  206 T to adjust the contact area where each of the nip formation pads  206 ,  206 S, and  206 T contacts the thermal conduction aid  216 . 
     Alternatively, the grooves  206   a ,  206   b , and  206   c  or the cavities may be disposed throughout the entire span, including the lateral end detection region B, of the nip formation pads  206 ,  206 S, and  206 T in the longitudinal direction thereof. In this case, the width of each of the grooves  206   a ,  206   b , and  206   c  in the short direction of the nip formation pads  206 ,  206 S, and  206 T, the size of each of the grooves  206   a ,  206   b , and  206   c  or the cavities, and the density of the grooves  206   a ,  206   b , and  206   c  or the cavities may vary. Even if the number, the size, and the density of the grooves  206   a ,  206   b , and  206   c  or the cavities are identical, pressure with which each of the nip formation pads  206 ,  206 S, and  206 T contacts the thermal conduction aid  216  may vary. In this case also, the contact area where each of the nip formation pads  206 ,  206 S, and  206 T contacts the thermal conduction aid  216  varies depending on the position on each of the nip formation pads  206 ,  206 S, and  206 T in the longitudinal direction thereof. 
     Referring to  FIGS. 9, 10, and 11 , a description is provided of advantages of the nip formation assemblies  6 ,  6 S, and  6 T depicted in  FIGS. 6, 7, and 8 , respectively, as one example. 
       FIG. 9  is a diagram of the center heater  202 A and the lateral end heater  202 B, illustrating a positional relation between a heat generation span of each of the center heater  202 A and the lateral end heater  202 B and each of the center temperature sensor  230 A and the lateral end temperature sensor  230 B.  FIG. 9  illustrates a relation between a position of the center heater  202 A as one example of a center heater and the position P 230 A of the center temperature sensor  230 A as one example of a center temperature detector and a relation between a position of the lateral end heater  202 B as one example of a lateral end heater and the position P 230 B of the lateral end temperature sensor  230 B as one example of a lateral end temperature detector. The fixing belt  201  extends horizontally in  FIG. 9  in the longitudinal direction, that is, an axial direction A 201 , of the fixing belt  201 . Each of the center heater  202 A and the lateral end heater  202 B extends in the axial direction A 201  of the fixing belt  201 . 
     As illustrated in  FIG. 9 , the center heater  202 A includes a center heat generator G 202 A indicated with a wave. The lateral end heater  202 B includes lateral end heat generators G 202 B indicated with waves. The waves of the center heat generator G 202 A and the lateral end heat generators G 202 B indicate densely coiled portions of filaments which generate heat as the filaments are supplied with power and are coiled more densely than other portions of the filaments of the center heater  202 A and the lateral end heater  202 B. The densely coiled portions of the filaments generate a greater amount of heat compared to other portions of the filaments, thus defining heat generation spans SC and SL. In each of the center heater  202 A and the lateral end heater  202 B as a halogen heater, the filament is coiled partially densely and partially loosely and disposed inside a tubular transparent glass tube filled with halogen gas. Thus, the center heater  202 A and the lateral end heater  202 B serve as a heater that heats the fixing belt  201 .  FIG. 9  does not illustrate the fixing belt  201  to clarify the relation between the position of the center heater  202 A and the position P 230 A of the center temperature sensor  230 A and the relation between the position of the lateral end heater  202 B and the position P 230 B of the lateral end temperature sensor  230 B. 
     A description is provided of a configuration of the center heater  202 A as one example of a center heater and the lateral end heater  202 B as one example of a lateral end heater. 
     The center heater  202 A generates heat in the heat generation span SC corresponding to a width of 210 mm of an A4 size sheet in portrait orientation. The lateral end heater  202 B generates heat in the heat generation spans SL that, together with the heat generation span SC, define a heat generation span SE corresponding to a width of 320 mm of an A3 extension size sheet in portrait orientation. The heat generation spans SL as lateral end spans are combined with the heat generation span SC as a center span to define the heat generation span SE as a combined span. When the center heater  202 A and the lateral end heater  202 B generate heat, the fixing device  200  produces the heat generation span SE equivalent to the width of the A3 extension size sheet in portrait orientation as the maximum conveyance span. The center heater  202 A has the heat generation span SC in a center span of the center heater  202 A in a longitudinal direction thereof and is installed in the fixing device  200  such that the center heater  202 A heats a center span of the fixing belt  201  in the axial direction A 201 . Conversely, the lateral end heater  202 B has the heat generation spans SL at both lateral ends of the lateral end heater  202 B in a longitudinal direction thereof and is installed in the fixing device  200  such that the heat generation spans SL are substantially symmetrical with each other via the center of the fixing belt  201  in the axial direction A 201  thereof. 
     A description is provided of a configuration of the center temperature sensor  230 A as one example of a center temperature detector and the lateral end temperature sensor  230 B as one example of a lateral end temperature detector. 
     The center temperature sensor  230 A is disposed opposite the fixing belt  201  at the position P 230 A, that is, at substantially a center of the heat generation span SC corresponding to the width of the A4 size sheet in portrait orientation in the axial direction A 201  of the fixing belt  201  to detect the temperature of the outer circumferential surface of the fixing belt  201 . The lateral end temperature sensor  230 B is disposed opposite the fixing belt  201  at the position P 230 B, that is, at substantially a center of the heat generation span SL between a lateral edge of the heat generation span SC and a lateral edge of the heat generation span SL in the axial direction A 201  of the fixing belt  201  to detect the temperature of the outer circumferential surface of the fixing belt  201 . 
       FIG. 10  is a diagram of the lateral end heater  202 B and the nip formation pad  206 S of the nip formation assembly  6 S depicted in  FIG. 7  as one example, illustrating heat conduction from the lateral end heater  202 B to the thermal conduction aid  216  through the fixing belt  201  and further heat conduction from the thermal conduction aid  216  to the nip formation pad  206 S.  FIG. 10  does not illustrate the fixing belt  201  to clarify heat conduction. A length of the nip formation pad  206 S is substantially equal to a length of the thermal conduction aid  216  in the longitudinal direction thereof. Alternatively, one of the nip formation pad  206 S and the thermal conduction aid  216  may be greater than another one of the nip formation pad  206 S and the thermal conduction aid  216  in the longitudinal direction thereof. 
     Heat emitted from the lateral end heater  202 B is conducted to the thermal conduction aid  216  through the fixing belt  201 . As described above, the thermal conduction aid  216  is greater than the maximum conveyance span in the longitudinal direction of the thermal conduction aid  216 . The maximum conveyance span substantially corresponds to the heat generation span SE depicted in  FIG. 9  produced by the center heater  202 A and the lateral end heater  202 B that are powered on. As illustrated in  FIG. 10 , energy Q 1  moved to an outboard span disposed (e.g., an outboard span S 2  depicted in  FIG. 7 ) outboard from a maximum conveyance span W in the longitudinal direction of the thermal conduction aid  216  where the maximum size sheet (e.g., the A3 extension size sheet) available in the fixing device  200  is conveyed is diffused without being used to heat and fix the toner image on the recording sheet S. Accordingly, the lateral end of the fixing belt  201  suffers from temperature decrease at a lateral end of the maximum conveyance span W by the energy Q 1  diffused to an outermost end of the thermal conduction aid  216  in the longitudinal direction thereof. 
     The fixing belt  201  retains a fixing temperature sufficient to fix the toner image on the recording sheet S in the lateral end detection span S 230 B where the controller  18  depicted in  FIG. 3  performs the feedback control based on the temperature of the fixing belt  201  detected by the lateral end temperature sensor  230 B to retain a target temperature. Conversely, the lateral end temperature sensor  230 B does not detect the temperature of the fixing belt  201  in an outboard span outboard from the lateral end detection span S 230 B in the axial direction A 201  of the fixing belt  201  even if the fixing belt  201  suffers from temperature decrease. Accordingly, the controller  18  does not perform the feedback control to retain the target temperature. Consequently, even if the fixing belt  201  is heated to the fixing temperature sufficient to fix the toner image on the recording sheet S in the lateral end detection span S 230 B, the fixing belt  201  may suffer from temperature decrease at the lateral end of the maximum conveyance span W by the energy Q 1  diffused through the thermal conduction aid  216  from a lateral edge of the lateral end detection span S 230 B to a lateral edge of the thermal conduction aid  216  in the longitudinal direction thereof without being used to heat and fix the toner image on the recording sheet S. Thus, the fixing device  200  may suffer from fixing failure. 
     In the nip formation assemblies  6 ,  6 S, and  6 T depicted in  FIGS. 6, 7, and 8 , respectively, each of the nip formation pads  206 ,  206 S, and  206 T does not mount the grooves  206   b  and  206   c  in the lateral end detection span S 230 B and a periphery thereof. Accordingly, energy Q 2  moves from a contact portion of the thermal conduction aid  216  that contacts the nip formation pad  206 S and diffuses through the nip formation pad  206 S without being used to heat and fix the toner image on the recording sheet S. Consequently, the fixing belt  201  suffers from temperature decrease in the lateral end detection span S 230 B and the periphery thereof by the energy Q 2  drawn to the nip formation pad  206 S compared to a case in which heat is not drawn to the nip formation pad  206 S. The controller  18  adjusts the rate to power on the lateral end heater  202 B based on the temperature of the fixing belt  201  detected in the lateral end detection span S 230 B. Accordingly, the lateral end heater  202 B is powered on for an extended period of time to retain the target temperature compared to a case in which the energy Q 2  does not generate. Consequently, the fixing belt  201  receives an increased amount of heat from the lateral end heater  202 B also in the outboard span S 2  disposed outboard from the lateral end detection span S 230 B in the axial direction A 201  of the fixing belt  201 . 
     Hence, by adjusting the energy Q 1  and the energy Q 2  depicted in  FIG. 10 , an amount of energy drawn from the fixing belt  201  is equalized substantially in the axial direction A 201  of the fixing belt  201 , enhancing evenness of the temperature of the fixing belt  201  and preventing temperature decrease of the lateral end of the fixing belt  201  in the axial direction A 201  thereof. For example, the controller  18  adjusts the temperature of the fixing belt  201  in the axial direction A 201  thereof by adjusting an amount of the energy Q 1  diffused through the thermal conduction aid  216  and an amount of the energy Q 2  diffused to the nip formation pad  206 S in the lateral end detection span S 230 B. 
       FIG. 11  is a graph illustrating a temperature distribution of the fixing belt  201  in the axial direction A 201  thereof. In  FIG. 11 , a curve C 1  represents the temperature of the fixing belt  201  of the fixing devices  200  and  200 S according to the embodiments described above, which varies in the axial direction A 201  thereof. A curve C 2  represents the temperature of a fixing belt of a comparative fixing device that does not incorporate the nip formation assemblies  6 ,  6 S, and  6 T, which varies in an axial direction of the fixing belt. As illustrated in  FIG. 11 , when the temperature of the fixing belt  201  reaches a desired temperature under the control performed by the controller  18  based on the temperature of the fixing belt  201  detected by the lateral end temperature sensor  230 B, the temperature of the fixing belt  201  at the lateral end of the maximum conveyance span W increases compared to a general control. Consequently, according to this embodiment, the fixing belt  201  attains a fixing strength at the lateral end of the maximum conveyance span W. 
     As illustrated in  FIG. 9 , the lateral end heater  202 B has the heat generation span SL in another lateral end of the lateral end heater  202 B in the longitudinal direction thereof, in addition to the heat generation span SL in one lateral end of the lateral end heater  202 B depicted in  FIG. 10 . As the lateral end heat generators G 202 B of the lateral end heater  202 B and the center heat generator G 202 A of the center heater  202 A depicted in  FIG. 9  generate heat, while the recording sheet S is conveyed through the fixing nip N such that a substantial center of the fixing belt  201  in the axial direction A 201  thereof overlaps a substantial center of the recording sheet S, the fixing belt  201  fixes the toner image on the recording sheet S. 
     The groove  206   a  is disposed in a lateral end span of the nip formation pad  206 S where the lateral end temperature sensor  230 B is not disposed opposite the fixing belt  201  such that the groove  206   a  is substantially symmetrical with the groove  206   b  via the substantial center of the fixing belt  201  in the axial direction A 201  thereof. For example, like one lateral end span, that is, the first span S 1  encompassing the lateral end detection span S 230 B that encompasses the position P 230 B of the lateral end temperature sensor  230 B, another lateral end span, that is, the symmetrical span S 3 , which is substantially symmetrical with the first span S 1  via the substantial center of the fixing belt  201  in the axial direction A 201  thereof, also has the groove  206   a.    
     In the symmetrical span S 3 , the nip formation pad  206 S contacts the thermal conduction aid  216  in a contact area which is greater than a contact area of an outboard span S 4  outboard from the symmetrical span S 3  in the longitudinal direction of the nip formation pad  206 S. In the outboard span S 4 , the nip formation pad  206 S contacts the thermal conduction aid  216 . Accordingly, the controller  18  performs adjustment of the energy Q 1  and the energy Q 2  substantially similarly in both lateral end spans of the nip formation pad  206 S in the longitudinal direction thereof. Consequently, the fixing belt  201  attains the fixing strength at both lateral ends of the maximum conveyance span W in the axial direction A 201  of the fixing belt  201 . The shape, the depth, and the like of the grooves  206   a ,  206   b , and  206   c  may vary between both lateral end spans of the nip formation pad  206 S in the longitudinal direction thereof as long as the controller  18  adjusts the energy Q 1  and the energy Q 2  to attain the fixing strength. 
     As described above, if the thermal conduction aid  216  is configured to contact the fixing belt  201 , heat stored in the fixing belt  201  may be conducted and diffused to the thermal conduction aid  216  that is in contact with the fixing belt  201  and made of a material having a greater thermal conductivity. Heat is further conducted and diffused to the nip formation pad  206 S contacting the thermal conduction aid  216 . As heat is diffused from the lateral end of the fixing belt  201  in the axial direction A 201  thereof to the outermost end of the thermal conduction aid  216  in the longitudinal direction thereof, heat may be drawn more from the lateral end of the fixing belt  201  and therefore the fixing belt  201  may suffer from temperature decrease. 
     A length of the thermal conduction aid  216  in the longitudinal direction thereof is greater than the maximum conveyance span W where the maximum size sheet available in the fixing device  200  is conveyed in view of manufacturing error. For example, immediately after the fixing device  200  is warmed up when the entire fixing device  200  is cool and is subject to heat dissipation, energy generated by the center heater  202 A and the lateral end heater  202 B is subject to diffusion to each lateral end of the thermal conduction aid  216  in the longitudinal direction thereof. Accordingly, each lateral end of the fixing belt  201  in the axial direction A 201  thereof may suffer from heat conduction to the thermal conduction aid  216  contacting the fixing belt  201  and resultant temperature decrease. Consequently, each lateral end of the fixing belt  201  in the axial direction A 201  thereof may suffer from degradation in fixing performance, causing faulty image formation such as offset. 
     To address this circumstance, according to the embodiments described above, the thermal conduction aid  216  contacts each of the nip formation pads  206 ,  206 S, and  206 T with a decreased contact area in an outermost span of the fixing belt  201  in the axial direction A 201  thereof, thus preventing heat stored in the fixing belt  201  from being drawn to each of the nip formation pads  206 ,  206 S, and  206 T through the thermal conduction aid  216 . The thermal conduction aid  216  contacts each of the nip formation pads  206 ,  206 S, and  206 T with an increased contact area in an inboard span of the fixing belt  201  that is inboard from the outermost span in the axial direction A 201  thereof, thus facilitating conduction of heat stored in the fixing belt  201  from the fixing belt  201  to each of the nip formation pads  206 ,  206 S, and  206 T through the thermal conduction aid  216 . 
     Accordingly, an amount of energy diffused to the outermost span of the thermal conduction aid  216  in the longitudinal direction thereof is equalized to an amount of energy diffused to each of the nip formation pads  206 ,  206 S, and  206 T in the inboard span having the increased contact area. Consequently, the amount of energy stored in the fixing belt  201  is equalized in the axial direction A 201  of the fixing belt  201 . As a result, the temperature of the fixing belt  201  is even to each lateral end of the fixing belt  201  in the axial direction A 201  thereof immediately after the fixing device  200  is warmed up, thus preventing temperature decease of each lateral end of the fixing belt  201  in the axial direction A 201  thereof. 
     A description is provided of advantages of the fixing devices  200  and  200 S. 
     As illustrated in  FIGS. 2 and 4 , a fixing device (e.g., the fixing devices  200  and  200 S) includes a fixing rotator (e.g., the fixing belt  201 ), a heater (e.g., the heater pair  202 ), a nip formation assembly (e.g., the nip formation assemblies  6 ,  6 S, and  6 T), a pressure rotator (e.g., the pressure roller  203 ), and a temperature detector (e.g., the temperature sensor pair  230 ). 
     The fixing rotator is endless and rotatable in a rotation direction (e.g., the rotation direction D 201 ). The heater heats the fixing rotator. The nip formation assembly is disposed opposite an inner circumferential surface of the fixing rotator. The pressure rotator is pressed against the nip formation assembly via the fixing belt to form a fixing nip (e.g., the fixing nip N) between the fixing rotator and the pressure rotator. As a recording medium bearing a toner image is conveyed through the fixing nip, the fixing rotator and the pressure rotator fix the toner image on the recording medium. 
     The nip formation assembly includes a thermal conduction aid (e.g., the thermal conduction aid  216 ) and a nip formation pad (e.g., the nip formation pads  206 ,  206 S, and  206 T). The thermal conduction aid contacts the inner circumferential surface of the fixing rotator. The nip formation pad is disposed opposite the fixing rotator via the thermal conduction aid and contacts the thermal conduction aid. 
     The heater includes a lateral end heater (e.g., the lateral end heater  202 B) that heats a lateral end span of the fixing rotator in an axial direction (e.g., the axial direction A 201 ) thereof. 
     The temperature detector includes a lateral end temperature detector (e.g., the lateral end temperature sensor  230 B) that detects a temperature of the fixing rotator heated by the lateral end heater in a lateral end detection span (e.g., the lateral end detection span S 230 B) in the lateral end span of the fixing rotator. The lateral end detection span encompasses a detection position (e.g., the position P 230 B) where the lateral end temperature detector is disposed opposite the fixing rotator. The thermal conduction aid contacts the nip formation pad in a first span (e.g., the first span S 1 ) encompassing the lateral end detection span in the axial direction of the fixing rotator with a first contact area. The thermal conduction aid contacts the nip formation pad in a second span (e.g., the outboard span S 2 ) disposed outboard from the first span in the axial direction of the fixing rotator with a second contact area smaller than the first contact area. 
     Accordingly, the fixing device improves fixing performance even at a lateral end of the recording medium in the axial direction of the fixing rotator. 
     According to the embodiments described above, the fixing belt  201  serves as a fixing rotator. Alternatively, a fixing film or the like may be used as a fixing rotator. Further, the pressure roller  203  serves as a pressure rotator. Alternatively, a pressure belt or the like may be used as a pressure rotator. 
     The above-described embodiments are illustrative and do not limit the present disclosure. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements and features of different illustrative embodiments may be combined with each other and substituted for each other within the scope of the present invention. 
     Any one of the above-described operations may be performed in various other ways, for example, in an order different from the one described above.