Patent Publication Number: US-10782637-B1

Title: Fixing device and image forming apparatus incorporating same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application No. 2019-047036, filed on Mar. 14, 2019, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Embodiments of the present disclosure generally relate to a fixing device and an image forming apparatus incorporating the fixing device, and more particularly, to a fixing device for fixing a toner image onto a recording medium and an image forming apparatus for forming an image on a recording medium with the fixing device. 
     Related Art 
     Various types of electrophotographic image forming apparatuses are known, including copiers, printers, facsimile machines, and multifunction machines having two or more of copying, printing, scanning, facsimile, plotter, and other capabilities. Such image forming apparatuses usually form an image on a recording medium according to image data. Specifically, in such image forming apparatuses, for example, a charger uniformly charges a surface of a photoconductor as an image bearer. An optical writer irradiates the surface of the photoconductor thus charged with a light beam to form an electrostatic latent image on the surface of the photoconductor according to the image data. A developing device supplies toner to the electrostatic latent image thus formed to render the electrostatic latent image visible as a toner image. The toner image is then transferred onto a recording medium either directly, or indirectly via an intermediate transfer belt. Finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image onto the recording medium. Thus, an image is formed on the recording medium. 
     Such a fixing device often employs a fixing film system that shortens a heating startup time. In the fixing film system, a pressure roller contacts a nip formation pad via a thin fixing film to form a fixing nip between the pressure roller and the fixing film. An inner circumferential surface of the fixing film slides over a surface of the nip formation pad via a lubricant. 
     SUMMARY 
     In one embodiment of the present disclosure, a novel fixing device includes a fixing rotator, a pressure rotator, a heat source, a nip formation pad, and a support. The pressure rotator is disposed opposite the fixing rotator. The heat source is configured to heat the fixing rotator. The nip formation pad is configured to press against the pressure rotator via the fixing rotator to form a fixing nip between the fixing rotator and the pressure rotator. The support is configured to support the nip formation pad toward the fixing nip. A longitudinal end portion of the pressure rotator has an outer diameter greater than an outer diameter of a longitudinal center portion of the pressure rotator. The pressure rotator includes a grip that is configured to contact the nip formation pad via the fixing rotator outside a recording medium passing through the fixing nip and apply a frictional force to the fixing rotator. The recording medium has a maximum width conveyable in the fixing device. The nip formation pad includes a nip face having an inflection point from which a longitudinal direction of the nip face is curved toward the support within an area opposite the grip. 
     Also described is a novel image forming apparatus incorporating the fixing device. 
    
    
     
       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 view of an image forming apparatus according to an embodiment of the present disclosure; 
         FIG. 2  is a schematic view of a fixing device incorporated in the image forming apparatus illustrated in  FIG. 1 ; 
         FIG. 3  is a diagram illustrating relative lengths of components of the fixing device illustrated in  FIG. 2 , with a graph of a surface pressure distribution in a fixing nip; 
         FIG. 4  is a diagram illustrating a nip formation pad, with an enlarged view of an example of a longitudinal end portion of the nip formation pad having a shape changed; 
         FIG. 5  is a diagram illustrating the nip formation pad, with an enlarged view of another example of the longitudinal end portion of the nip formation pad curved from a middle within a grip area; 
         FIG. 6  is a diagram illustrating relative lengths of the components of the fixing device in a case in which the nip formation pad is shaped as illustrated in the enlarged view in  FIG. 4 , with a graph of a surface pressure distribution in the fixing nip; 
         FIG. 7  is a diagram illustrating a variation of the nip formation pad, with an enlarged view of an example of a longitudinal end portion of the variation of the nip formation pad; and 
         FIG. 8  is a diagram illustrating the variation of nip formation pad, with an enlarged view of another example of the longitudinal end portion of the nip formation pad curved from a middle within the grip area. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. Also, identical or similar reference numerals designate identical or similar components throughout the several views. 
     DETAILED DESCRIPTION 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of the present 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. 
     Although the embodiments are described with technical limitations with reference to the attached drawings, such description is not intended to limit the scope of the disclosure and not all of the components or elements described in the embodiments of the present disclosure are indispensable to the present disclosure. 
     In a later-described comparative example, embodiment, and exemplary variation, for the sake of simplicity, like reference numerals are given to identical or corresponding constituent elements such as parts and materials having the same functions, and redundant descriptions thereof are omitted unless otherwise required. 
     As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     It is to be noted that, in the following description, suffixes Y, C, M, and Bk denote colors of yellow, cyan, magenta, and black, respectively. To simplify the description, these suffixes are omitted unless necessary. 
     Referring to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, embodiments of the present disclosure are described below. 
     According to an embodiment of the present disclosure, a fixing device includes a fixing rotator, a pressure rotator, a heat source, a nip formation pad, and a support. The pressure rotator is disposed opposite the fixing rotator. The heat source is configured to heat the fixing rotator. The nip formation pad is configured to press against the pressure rotator via the fixing rotator to form a fixing nip between the fixing rotator and the pressure rotator. The support is configured to support the nip formation pad toward the fixing nip. A longitudinal end portion of the pressure rotator has an outer diameter greater than an outer diameter of a longitudinal center portion of the pressure rotator. The pressure rotator includes a grip that is configured to contact the nip formation pad via the fixing rotator outside a recording medium passing through the fixing nip and apply a frictional force to the fixing rotator. The recording medium has a maximum width conveyable in the fixing device. The nip formation pad includes a nip face having an inflection point from which a longitudinal direction of the nip face is curved toward the support within an area opposite the grip. 
     Initially with reference to  FIG. 1 , a description is given of an image forming apparatus  100  according to an embodiment of the present disclosure. 
       FIG. 1  is a schematic view 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 direction in which a transfer belt is stretched and rotates. 
     Alternatively, the image forming apparatus  100  may employ other structures. The image forming apparatus  100  illustrated in  FIG. 1  forms color and monochrome images on recording media by electrophotography. Alternatively, the image forming apparatus  100  may be a monochrome printer that forms monochrome images on recording media. Although  FIG. 1  illustrates the image forming apparatus  100  as a color printer, the image forming apparatus  100  may be, e.g., a copier, a facsimile machine, or a multifunction peripheral (MFP) having at least two of printing, copying, scanning, facsimile, and plotter functions. 
     As illustrated in  FIG. 1 , the image forming apparatus  100  employs a tandem structure in which four drum-shaped photoconductors  20 Y,  20 C,  20 M, and  20 Bk are arranged side by side as image bearers that bear yellow, cyan, magenta, and black toner images in separation colors, respectively. 
     The image forming apparatus  100  includes a transfer belt  11 , which is an endless belt serving as an intermediate transferor rotatable in a direction of rotation A 1  while facing the photoconductors  20 Y,  20 C,  20 M, and  20 Bk. In a primary transfer process, the yellow, cyan, magenta, and black toner images formed as visible images on the photoconductors  20 Y,  20 C,  20 M, and  20 Bk, respectively, are transferred successively onto the transfer belt  11  as the transfer belt  11  rotates in the direction of rotation A 1  in  FIG. 1 . Specifically, in the primary transfer process, the yellow, cyan, magenta, and black toner images are superimposed one atop another on the transfer belt  11 , thus being transferred onto the transfer belt  11 . Thereafter, in a secondary transfer process, the yellow, cyan, magenta, and black toner images are transferred at once onto a recording medium S, such as a recording sheet, from the transfer belt  11 . 
     Each of the photoconductors  20 Y,  20 C,  20 M, and  20 Bk is surrounded by various pieces of equipment to form a toner image in accordance with rotation of each of the photoconductors  20 Y,  20 C,  20 M, and  20 Bk. Specifically, for example, the photoconductor  20 Bk is surrounded by a charger  30 Bk, a developing device  40 Bk, a primary transfer roller  12 Bk, and a cleaner  50 Bk in this order along a direction of rotation of the photoconductor  20 Bk. A black toner image is formed on the photoconductor  20 Bk while the photoconductor  20 Bk rotates. Like the photoconductor  20 Bk, the photoconductors  20 Y,  20 C, and  20 M are surrounded by chargers  30 Y,  30 C, and  30 M, developing devices  40 Y,  40 C, and  40 M, primary transfer rollers  12 Y,  12 C, and  12 M, and cleaners  50 Y,  50 C, and  50 M in this order along a direction of rotation of the photoconductors  20 Y,  20 C, and  20 M, respectively. After the chargers  30 Y,  30 C,  30 M, and  30 Bk charge the respective photoconductors  20 Y,  20 C,  20 M, and  20 Bk, an optical writing device  8  writes electrostatic latent images on the photoconductors  20 Y,  20 C,  20 M, and  20 Bk with laser beams Lb serving as writing light, respectively. 
     As the transfer belt  11  rotates in the direction of rotation A 1 , the yellow, cyan, magenta, and black toner images formed as visible images on the photoconductors  20 Y,  20 C,  20 M, and  20 Bk, respectively, are primarily transferred onto the transfer belt  11  such that the yellow, cyan, magenta, and black toner images are superimposed one atop another 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 photoconductors  20 Y,  20 C,  20 M, and  20 Bk via the transfer belt  11 , respectively, apply a primary transfer bias to the photoconductors  20 Y,  20 C,  20 M, and  20 Bk to transfer the yellow, cyan, magenta, and black toner images onto the transfer belt  11  in this order from an upstream side to a downstream side in the direction of rotation A 1  of the transfer belt  11 . 
     That is, the photoconductors  20 Y,  20 C,  20 M, and  20 Bk are aligned in this order from the upstream side in the direction of rotation A 1  of the transfer belt  11 . The photoconductors  20 Y,  20 C,  20 M, and  20 Bk are located in four image forming stations that form the yellow, cyan, magenta, and black toner images, respectively. Hereinafter, the four image forming stations may be occasionally referred to as yellow, cyan, magenta, and black image forming stations. 
     In other words, the image forming apparatus  100  includes the yellow, cyan, magenta, and black image forming stations. In addition, the image forming apparatus  100  includes 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 disposed above and opposite the photoconductors  20 Y,  20 C,  20 M, and  20 Bk. The transfer belt unit  10  includes 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 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 disposed below and opposite the four image forming stations. 
     The optical writing device  8  includes, e.g., a semiconductor laser serving as a light source, a coupling lens, an fθ lens, a toroidal lens, a deflection mirror, and a rotatable polygon mirror serving as a deflector. According to image data of yellow, cyan, magenta, and black, the optical writing device  8  emits the laser beams Lb to the photoconductors  20 Y,  20 C,  20 M, and  20 Bk to form electrostatic latent images on the photoconductors  20 Y,  20 C,  20 M, and  20 Bk, respectively.  FIG. 1  illustrates the laser beam Lb irradiating the photoconductor  20 Bk in the black image forming station. Similarly, the laser beams Lb irradiate the other photoconductors  20 Y,  20 C, and  20 M in the yellow, cyan, and magenta image forming stations, respectively. 
     The image forming apparatus  100  further includes a sheet feeding device  61  and a registration roller pair  4 . The sheet feeding device  61  includes a sheet tray that loads a plurality of recording media S, which is conveyed one by one to an area of contact, herein referred to as a secondary transfer nip, formed between the transfer belt  11  and the secondary transfer roller  5 . Activation of the registration roller pair  4  is timed to feed a recording medium S conveyed from the sheet feeding device  61  to the secondary transfer nip formed between the transfer belt  11  and the secondary transfer roller  5  such that the recording medium S meets the yellow, cyan, magenta, and black toner images on the transfer belt  11  at the secondary transfer nip. The image forming apparatus  100  further includes a sensor to detect that a leading end of the recording medium S reaches the registration roller pair  4 . 
     In addition, the image forming apparatus  100  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  serves as a fixing or fusing unit herein employing a belt fixing system. When receiving a recording medium S bearing a toner image, the fixing device  200  fixes the toner image onto the recording medium S. The output roller pair  7  ejects the recording medium S bearing the fixed toner image outside a housing of the image forming apparatus  100 . The output tray  17  is situated atop the housing of the image forming apparatus  100 . The recording medium S is ejected onto the output tray  17  outside the housing of the image forming apparatus  100  by the output roller pair  7 . The toner bottles  9 Y,  9 C,  9 M, and  9 Bk are situated below the output tray  17 . The toner bottles  9 Y,  9 C,  9 M, and  9 Bk are replenished with fresh toner of yellow, cyan, magenta, and black, respectively. 
     In addition to the transfer belt  11  and the primary transfer rollers  12 Y,  12 C,  12 M, and  12 Bk, the transfer belt unit  10  includes a driving roller  72  and a driven roller  73 . The transfer belt  11  is entrained around the driving roller  72  and the driven roller  73 . 
     A biasing member, such as a spring, biases the driven roller  73  against the transfer belt  11 . With such a configuration, the driven roller  73  serves as a tension applicator that applies tension to the transfer belt  11 . The transfer belt unit  10 , the secondary transfer roller  5 , and the transfer belt cleaner  13  together construct a transfer device  71 . 
     The sheet feeding device  61  is disposed in a lower portion of the housing of the image forming apparatus  100 . The sheet feeding device  61  includes a sheet feeding roller  3  that contacts an upper surface of an uppermost recording medium S of the plurality of recording media S loaded on the sheet tray of the sheet feeding device  61 . As the sheet feeding roller  3  is rotated counterclockwise in  FIG. 1 , the sheet feeding roller  3  feeds the uppermost recording medium S toward the registration roller pair  4 . 
     The transfer belt cleaner  13  of the transfer device  71  includes a cleaning brush and a cleaning blade disposed so as to face and contact the transfer belt  11 . With the cleaning brush and the cleaning blade, the transfer belt cleaner  13  scrapes extraneous matter such as residual toner off the transfer belt  11 , thereby removing the extraneous matter from the transfer belt  11 . Thus, the transfer belt cleaner  13  cleans the transfer belt  11 . 
     The transfer belt cleaner  13  further includes a waste toner conveyer that conveys and discards the residual toner removed from the transfer belt  11 . 
     Referring to  FIG. 2 , a description is given of a configuration of the fixing device  200  incorporated in the image forming apparatus  100  described above. 
       FIG. 2  is a schematic view of the fixing device  200 . 
     As illustrated in  FIG. 2 , the fixing device  200  includes a fixing belt  201  as an endless belt formed into a loop, a pressure roller  203 , and various components disposed inside the loop formed by the fixing belt  201 , such as halogen heaters  202 A and  202 B, a nip formation pad  206 , a stay  207 , and reflectors  209 A and  209 B. The fixing belt  201  and the components disposed inside the loop formed by the fixing belt  201  constitute a belt unit  201 U, which is detachably coupled to the pressure roller  203 . The fixing belt  201  serves as a fixing rotator; whereas the pressure roller  203  serves as a pressure rotator disposed opposite an outer circumferential surface of the fixing belt  201 . The halogen heaters  202 A and  202 B serve as a plurality of heat sources to heat the fixing belt  201  (i.e., fixing rotator). Specifically, the fixing belt  201  is directly heated with radiation heat from the halogen heaters  202 A and  202 B, from an inside of the loop formed by the fixing belt  201 . The fixing device  200  further includes temperature sensors  230 A and  230 B to detect the temperature of the fixing belt  201 . The temperature sensors  230 A and  230 B are herein non-contact sensors that detect the temperature of fixing belt  201  without contacting the fixing belt  201 . The fixing device  200  controls the lighting rate of the halogen heaters  202 A and  202 B according to the temperature detected, thus controlling the temperature of the fixing belt  201  to a desired temperature. 
     As illustrated in  FIG. 2 , the nip formation pad  206  presses against the pressure roller  203  (i.e., pressure rotator) via the fixing belt  201  (i.e., fixing rotator) to form an area of contact, herein referred to as a fixing nip N, between the fixing belt  201  (i.e., fixing rotator) and the pressure roller  203  (i.e., pressure rotator). As the fixing belt  201  rotates, the fixing belt  201  slides over the nip formation pad  206 . Specifically, an inner circumferential surface of the fixing belt  201  slides over a thermal conduction aid  216  of the nip formation pad  206 . A toner image is fixed onto a recording medium S under heat and pressure at the fixing nip N while the recording medium S bearing the toner image is sandwiched between the fixing belt  201  and the pressure roller  203  and conveyed through the fixing nip N. Although  FIG. 2  illustrates the thermal conduction aid  216  in a flat shape, the thermal conduction aid  216  may be contoured into a recess or other shapes. In a case in which the thermal conduction aid  216  contours the fixing nip N into a recess, the recessed fixing nip N directs a leading end of the recording medium S toward the pressure roller  203  as the recording medium S is ejected from the fixing nip N. Thus, the recessed fixing nip N facilitates separation of the recording medium S from the fixing belt  201  and prevents a paper jam. 
     The thermal conduction aid  216  illustrated in  FIG. 2  has a belt-side surface (or simply referred to as a belt side) that contacts the inner circumferential surface of the fixing belt  201  (i.e., fixing rotator). The belt-side surface of the thermal conduction aid  216  is coated to reduce a frictional force generated between the thermal conduction aid  216  and the fixing belt  201 , thus restraining abrasion of the thermal conduction aid  216  and the fixing belt  201 . The belt-side surface of the thermal conduction aid  216  is coated with fluorine having low friction or a material having an increased abrasion resistance such as a diamond-like carbon (DLC). The thermal conduction aid  216  is a metal material having the belt-side surface coated. Therefore, the thermal conduction aid  216  is not deformed in a thickness direction of the thermal conduction aid  216  and affected when a nip surface pressure is increased between the thermal conduction aid  216  and a grip  301  of the pressure roller  203 . To address such a situation, it is effective to reduce a surface pressure of the grip  301  of the pressure roller  203 . 
     The nip formation pad  206  is disposed inside the loop formed by the fixing belt  201  and opposite the pressure roller  203  via the fixing belt  201 . The nip formation pad  206  includes the thermal conduction aid  216  and a base  217 . The thermal conduction aid  216  is a slide aid that covers a belt-side surface of the base  217 . The belt-side surface of the base  217  is a surface opposite the inner circumferential surface of the fixing belt  201 . The base  217  is a slide aid support that supports the thermal conduction aid  216  (i.e., slide aid). The stay  207  holds the nip formation pad  206  against pressure from the pressure roller  203 . 
     The thermal conduction aid  216  prevents heat generated by a longitudinal end heater  226  from being stored locally and facilitates conduction of heat in a longitudinal direction of the thermal conduction aid  216 . Thus, the thermal conduction aid  216  reduces uneven temperature of the fixing belt  201  in an axial direction of the fixing belt  201 . Hence, the thermal conduction aid  216  is preferably made of a material that conducts heat quickly, for example, a material having an increased thermal conductivity such as copper, aluminum, or silver. In a comprehensive view of manufacturing costs, availability, thermal conductivity, and processing, copper is a most preferable material used as the thermal conduction aid  216 . As described above, in the present embodiment, the thermal conduction aid  216  includes the belt-side surface facing the inner circumferential surface of the fixing belt  201 . The belt-side surface of the thermal conduction aid  216  directly contacts the fixing belt  201  and therefore serves as a nip formation surface. 
     The fixing belt  201  is an endless belt or film made of a metal material, such as nickel or stainless steel (e.g., steel use stainless or SUS), or a resin material such as polyimide. The fixing belt  201  is constructed of a base layer and a release layer. The release layer, as an outer surface layer of the fixing belt  201 , is made of, e.g., perfluoroalkoxy alkane (PFA) or polytetrafluoroethylene (PTFE) to facilitate separation of toner contained in a toner image on a recording medium S from the fixing belt  201 . Optionally, an elastic layer made of, e.g., silicone rubber may be interposed between the base layer and the release layer made of, e.g., PFA or PTFE of the fixing belt  201 . In a case in which the fixing belt  201  does not incorporate the elastic layer made of, e.g., silicone rubber, 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 onto the recording medium S. However, as the pressure roller  203  and the fixing belt  201  sandwich and press an unfixed toner image onto the recording medium S, slight surface asperities in the fixing belt  201  may be transferred onto the toner image on the recording medium S, resulting in variation in gloss of a solid portion of the toner image. In short, an orange peel image appears on the recording medium S. The elastic layer made of, e.g., silicone rubber having a thickness not smaller than 100 μm is preferably provided to address such an unfavorable situation. As the elastic layer made of, e.g., silicone rubber deforms, the elastic layer absorbs the slight surface asperities in the fixing belt  201 , thereby preventing formation of the faulty orange peel image. 
     The stay  207  is constructed of a first part  207 A and a second part  207 B. The first part  207 A includes an arm portion  207   c  and a base portion  207   d . The second part  207 B includes an arm portion  207   e  and a base portion  207   f . The arm portions  207   c  and  207   e  project from the base portions  207   d  and  207   f , respectively, away from the fixing nip N. The arm portions  207   c  and  207   e  are interposed between the halogen heaters  202 A and  202 B serving as fixing heat sources. The halogen heaters  202 A and  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  is a support disposed inside the loop formed by the fixing belt  201 . The stay  207  supports the nip formation pad  206  toward the fixing nip N as illustrated in  FIG. 2 . Thus, the stay  207  also supports the fixing nip N. As the nip formation pad  206  receives pressure from the pressure roller  203 , the stay  207  prevents the nip formation pad  206  from being bent by such pressure, thereby maintaining a uniform width of the fixing nip N in the axial direction of the fixing belt  201 . The stay  207  is held and secured by flanges  208  as illustrated in  FIGS. 3 and 6 . The flanges  208  serve as holders that holds opposed longitudinal end portions of the stay  207 . Thus, the stay  207  is positioned inside the fixing device  200 . Note that a longitudinal direction of the stay  207  is parallel to the axial direction of the fixing belt  201 . The reflector  209 A is interposed between the halogen heater  202 A and the stay  207 . Similarly, the reflector  209 B is interposed between the halogen heater  202 B and the stay  207 . The reflectors  209 A and  209 B thus disposed reflect the radiation heat from the halogen heaters  202 A and  202 B toward the inner circumferential surface of the fixing belt  201 . Accordingly, the reflectors  209 A and  209 B prevents the stay  207  from being heated with, e.g., the radiation heat from the halogen heaters  202 A and  202 B, thus reducing waste of energy. In a case in which the fixing device  200  excludes the reflectors  209 A and  209 B, the respective surfaces of the stay  207  facing the halogen heaters  202 A and  202 B may be insulated or given a mirror finish to reflect the radiation heat from the halogen heaters  202 A and  202 B toward the inner circumferential surface of the fixing belt  201 . As illustrated in  FIG. 2 , the reflectors  209 A and  209 B interposed between the halogen heaters  202 A and  202 B prevent the halogen heaters  202 A and  202 B from heating glass tubes of each other. Accordingly, the halogen heaters  202 A and  202 B efficiently heat the fixing belt  201 . 
     The pressure roller  203  is constructed of, e.g., a core  205 , an elastic rubber layer  204  resting on the core  205 , and a surface release layer resting on the elastic rubber layer  204 . The release layer, made of PFA or PTFE, facilitates separation of the recording medium S from the pressure roller  203 . A driver such as a motor situated inside the image forming apparatus  100  generates and transmits a driving force to the pressure roller  203  through a gear train, thus rotating the pressure roller  203 . A spring, for example, 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  forms the fixing nip N having a given width, which is a given length in a recording medium conveying direction in which the recording medium S is conveyed. The pressure roller  203  may be a hollow roller or a solid roller. In a case in which the pressure roller  203  is a hollow roller, a heat source such as a halogen heater may be disposed inside the hollow roller. The elastic rubber layer  204  may be made of solid rubber. Alternatively, in a case in which no heater is situated inside the pressure roller  203 , the elastic rubber layer  204  may be made of sponge rubber. The sponge rubber is preferable to the solid rubber because the sponge rubber has enhanced thermal insulation that draws less heat from the fixing belt  201 . 
     The fixing belt  201  rotates in accordance with rotation of the pressure roller  203 . In the example of  FIG. 2 , as a 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  through 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 while being 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 opposed axial end portions of the fixing belt  201  are guided by the flanges  208 . 
     With the configuration described above, the fixing device  200  attaining quick warm-up is manufactured at reduced costs. 
     Referring now to  FIG. 3 , a description is given of relative lengths of the components of the fixing device  200  described above. 
       FIG. 3  is a diagram illustrating the relative lengths of the components of the fixing device  200 , with a graph of a surface pressure distribution in the fixing nip N. 
     As illustrated in  FIG. 3 , the nip formation pad  206  is longer than the pressure roller  203  to prevent longitudinal edges of the nip formation pad  206  from damaging the pressure roller  203 . The pressure roller  203  is longer than a maximum recording medium conveyance width MW, which is a maximum width of a recording medium S conveyable in the image forming apparatus  100 . The pressure roller  203  includes grips  301  at opposed longitudinal end portions of the pressure roller  203  outside the maximum recording medium conveyance width MW. In other words, the pressure roller  203  (i.e., pressure rotator) includes the grips  301  that contacts the nip formation pad  206  via the fixing belt  201  (i.e., fixing rotator) outside a recording medium S with a maximum width conveyable in the fixing device  200  passing through the fixing nip N and that applies a frictional force to the fixing belt  201  (i.e., fixing rotator). Accordingly, the grips  301  prevent slippage of the fixing belt  201  that rotates in accordance with rotation of the pressure roller  203 . 
     As illustrated in  FIG. 3 , the pressure roller  203  has a so-called hourglass shape. That is, the pressure roller  203  has an outer diameter increasing from a longitudinal center portion of the pressure roller  203  toward the opposed longitudinal end portions of the pressure roller  203 . With such a configuration, the opposed longitudinal end portions of the pressure roller  203  convey a recording medium S more quickly than the longitudinal center portion of the pressure roller  203 , thereby preventing wrinkles of the recording medium S. Specifically, in the present example, the longitudinal center portion of the pressure roller  203  has an outer diameter of φ29.5 mm; whereas each of the opposed longitudinal end portions of the pressure roller  203  has an outer diameter of φ30.0 mm. In short, the longitudinal end portion of the pressure roller  203  (i.e., pressure rotator) has an outer diameter greater than an outer diameter of the longitudinal center portion of the pressure roller  203  (i.e., pressure rotator). That is, each of the opposed longitudinal end portions of the pressure roller  203  has an outer diameter greater than the longitudinal center portion of the pressure roller  203  by 0.5 mm. With such a configuration, the pressure roller  203  prevents wrinkles of the recording medium S. 
       FIG. 3  illustrates a case in which the nip formation pad  206  is flat and substantially parallel to an axis of the core  205  of the pressure roller  203 . The lower portion of  FIG. 3  illustrates the surface pressure distribution in the fixing nip N when the pressure roller  203  is pressed against the flat nip formation pad  206  to form the fixing nip N. Since the pressure roller  203  has an outer diameter increasing from the longitudinal center portion of the pressure roller  203  to the opposed longitudinal end portions of the pressure roller  203 , a deformed amount of the elastic rubber layer  204  of the pressure roller  203  pressed against the nip formation pad  206  is greater at the opposed longitudinal end portions of the pressure roller  203  than at the longitudinal center portion of the pressure roller  203 . That is, the opposed longitudinal end portions of the pressure roller  203  have a surface pressure greater than a surface pressure of the longitudinal center portion of the pressure roller  203 . In other words, as illustrated in  FIG. 3 , the grips  301  exhibit a highest surface pressure in the longitudinal direction of the pressure roller  203 . In such a situation, the inner circumferential surface of the fixing belt  201  and the belt-side surface of the nip formation pad  206  are likely to wear in a grip area  301 A, which is an area opposite the grip  301 . Abrasion of the inner circumferential surface of the fixing belt  201  or the belt-side surface of the nip formation pad  206  in the grip area  301 A increases a sliding friction at the fixing nip N. Such an increase in sliding friction may cause slippage of the fixing belt  201 , resulting in temperature abnormalities and a paper jam due to a conveyance failure of a recording medium S. 
     Referring now to  FIGS. 4 and 5 , a description is given of the nip formation pad  206  having longitudinal end portions shaped to reduce the surface pressure of the grips  301 . 
       FIG. 4  is a diagram illustrating the nip formation pad  206 , with an enlarged view of an example of the longitudinal end portion of the nip formation pad  206  having a shape changed.  FIG. 5  is a diagram illustrating the nip formation pad  206 , with an enlarged view of another example of the longitudinal end portion of the nip formation pad  206  curved from a middle within the grip area  301 A. 
     According to the present embodiment, the fixing device  200  includes the pressure roller  203  as a driving roller, the fixing belt  201  (or a fixing film) driven to rotate by the pressure roller  203 , and the nip formation pad  206 . The pressure roller  203  has an hourglass shape to prevent wrinkles of a recording medium S. The shape of the nip formation pad  206  prevents an increase in surface pressure near the opposed longitudinal end portions of the pressure roller  203  in an area of pressure between the pressure roller  203  and the nip formation pad  206 . Accordingly, the present embodiment lengthens the lifespan of the fixing device  200  while keeping stable conveyance of recording media S. 
     As illustrated in  FIG. 4 , the longitudinal position of the thermal conduction aid  216  is determined as follows. Along a longitudinal direction of the thermal conduction aid  216 , a point A corresponds to a center of the maximum recording medium conveyance width MW, which may be referred to as a maximum recording medium conveyance area. Points B and B 1  correspond to opposed ends of the maximum recording medium conveyance width MW, respectively. Points C and C 1  correspond to the respective end portions of the grips  301  in a longitudinal direction of the pressure roller  203 . Note that the respective ends of the grips  301  are located on opposed axial end sides of the fixing belt  201 , that is, on flange  208  sides. With respect to the nip formation pad  206  illustrated in  FIG. 3 , the points A, B, B 1 , C, and C 1  are aligned substantially on a straight line. By contrast, in an encircled area P in  FIG. 4 , the nip formation pad  206  is curved toward the stay  207  in the grip area  301 A as illustrated in the enlarged view in  FIG. 4 . In other words, the nip formation pad  206  includes a nip face  206   f , which has an inflection point from which a longitudinal direction of the nip face  206   f  is curved toward the stay  207  (i.e., support) within the grip area  301 A, which is an area opposite the grip  301 . 
     Specifically, in the enlarged view in  FIG. 4 , a broken line BC indicates the nip face  206   f  of the nip formation pad  206  having a planar shape; whereas a curve BD indicates the nip face  206   f  of the nip formation pad  206  having a curved shape. Specifically, the nip face  206   f  of the nip formation pad  206  contacts the fixing belt  201 . The nip formation pad  206  (more specifically, the nip face  206   f ) has an inflection point that turns at least part of the nip face  206   f  within the grip area  301 A (i.e., area opposite the grip  301 ) toward the stay  207  (i.e., support) from one of a line and a circle through the points A, B, and B 1  on the nip face  206   f . The point A is a longitudinal center point of the nip face  206   f . The points B and B 1  are opposed end points of the maximum recording medium conveyance width MW (i.e., maximum width of the recording medium S conveyable in the fixing device  200 ). Such a configuration reduces the deformed amount of the pressure roller  203  by a length of a line segment CD, resulting in reduction of the surface pressure in the fixing nip N. The length of the line segment CD is set according to the deformed amount of the pressure roller  203  and a target surface pressure of the grip  301 . In the present example, the length of the line segment CD is 0.3 mm. 
     In the enlarged view in  FIG. 4 , the nip formation pad  206  is curved from the point B. Alternatively, in consideration of variations, the nip formation pad  206  may be curved toward the stay  207  (i.e., support) starting from the middle (e.g., midpoint X of the points B and C) within the grip area  301 A (i.e., area opposite the grip  301 ) as illustrated in  FIG. 5 . 
     Referring now to  FIG. 6 , a description is given of the surface pressure distribution in the fixing nip N formed by the nip formation pad  206  shaped as illustrated in the enlarged view in  FIG. 4 . 
       FIG. 6  is a diagram illustrating relative lengths of the components of the fixing device  200  in a case in which the nip formation pad  206  is shaped as illustrated in the enlarged view in  FIG. 4 , with a graph of the surface pressure distribution in the fixing nip N. 
     As illustrated in  FIG. 6 , portions of the nip formation pad  206  opposite the grips  301  of the pressure roller  203  are curved toward the stay  207 . Such a configuration of the nip formation pad  206  reduces the deformed amount of the pressure roller  203  and the surface pressure of the grips  301  of the pressure roller  203 . 
     Referring now to  FIGS. 7 and 8 , a description is given of a variation of the nip formation pad  206 . 
       FIG. 7  is a diagram illustrating a nip formation pad  206 V as a variation of the nip formation pad  206 , with an enlarged view of an example of a longitudinal end portion of the nip formation pad  206 V.  FIG. 8  is a diagram illustrating nip formation pad  206 V, with an enlarged view of another example of the longitudinal end portion of the nip formation pad  206  curved from a middle within the grip area. 
     The pressure roller  203  is pressed against the nip formation pad  206 V to form the fixing nip N. Generally, the stay  207  supports the load applied from the pressure roller  203  via the nip formation pad  206 . However, the stay  207  may be warped depending on the strength of the stay  207 . In such a case, a longitudinal center portion of the nip formation pad  206  is separated from the pressure roller  203 . As a consequence, the fixing nip N becomes narrow at the longitudinal center portion of the nip formation pad  206 , thus reducing the pressure and causing a fixing failure. 
     To cancel the warp of the stay  207  due to the load applied from the pressure roller  203 , a longitudinal center portion of the nip formation pad  206 V projects toward the pressure roller  203  (i.e., pressure rotator) as illustrated in  FIG. 7 . In this case, the thermal conduction aid  216  of the nip formation pad  206 V becomes substantially flat when receiving the load applied from the pressure roller  203 . As a consequence, the surface pressure may be increased by an outer diameter difference of the pressure roller  203  in the grip area  301 A in the fixing nip N. To address such a situation, portions of the nip formation pad  206 V opposite the grips  301  of the pressure roller  203  are preferably curved toward the stay  207 . 
     The shape of the thermal conduction aid  216  illustrated in  FIG. 7  can be approximated by an arc because an amount of projection of the thermal conduction aid  216  toward the pressure roller  203  is smaller than a longitudinal length of the thermal conduction aid  216 . Although the points A, B, B 1 , C, and C 1  are substantially on the same arc, the portion of the nip formation pad  206 V opposite the grip  301  is preferably curved from a state indicated by a broken curve BC to a state indicated by a solid curve BD in the enlarged view in  FIG. 7 . Such a configuration reduces the deformed amount of the pressure roller  203  by a length of a line segment CD, resulting in reduction of the surface pressure in the fixing nip N. The length of the line segment CD is set according to the deformed amount of the pressure roller  203  and a target surface pressure of the grip  301 . In the present example, the length of the line segment CD is 0.3 mm. 
     In the enlarged view in  FIG. 7 , the nip formation pad  206 V is curved from the point B. Alternatively, in consideration of variations, the nip formation pad  206 V may be curved toward the stay  207  (i.e., support) starting from the middle (e.g., midpoint Y of the points B and C) within the grip area  301 A (i.e., area opposite the grip  301 ) as illustrated in  FIG. 8 . Such a configuration attains the surface pressure distribution in the fixing nip N as illustrated in  FIG. 6  and prevents abrasion of the fixing belt  201  and the nip formation pad  206 V. 
     As described above, in the present embodiment, the longitudinal end portions of the nip formation pad  206  are shaped in a direction away from the pressure roller  203 . Unlike a typical nip formation pad, each of the longitudinal end portions of the nip formation pad  206  is shaped in the direction away from the pressure roller  203  in an area of pressure between the pressure roller  203  and the nip formation pad  206 . Although the pressure roller  203  having an outer diameter increasing toward the longitudinal end portions of the pressure roller  203  prevents wrinkles of recording media S, the surface pressure between the flat nip formation pad  206  and the pressure roller  203  increases toward the longitudinal end portions of the flat nip formation pad  206  and the pressure roller  203 . To address such a situation, the nip formation pad  206  of the present embodiment is shaped in a direction away from the pressure roller  203  by an increase in the outer diameter of the pressure roller  203 . Accordingly, the pressure roller  203  is pressed against the nip formation pad  206  via the fixing belt  201  in a low surface pressure, without being separated from the nip formation pad  206 . Thus, the nip formation pad  206  of the present embodiment prevents the fixing belt  201  from slipping in the grip area  301 A while the fixing belt  201  rotates in accordance with rotation of the pressure roller  203 . In addition, the nip formation pad  206  of the present embodiment reduces abrasion of the fixing belt  201  and the nip formation pad  206  in the grip area  301 A. 
     In other words, in the present embodiment, the portion of the nip formation pad  206  opposite the grip  301  of the pressure roller  203  is shaped in the direction away from the pressure roller  203 . Such a shape of the nip formation pad  206  reduces the deformed amount of the grips  301  of the pressure roller  203  compared to other portions when the pressure roller  203  is pressed against the nip formation pad  206  via the fixing belt  201 . Accordingly, the surface pressure is reduced in the grip area  301 A. That is, the present embodiment prevents early abrasion of the nip formation pad  206  and the inner circumferential surface of the fixing belt  201  in the grip area  301 A. 
     Accordingly, the embodiments of the present disclosure lengthen the lifespan of the fixing device while keeping stable conveyance of recording media. 
     According to the embodiments described above, the fixing belt  201  serves as a fixing rotator. Alternatively, a fixing film, a fixing sleeve, 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. 
     Although the present disclosure makes reference to specific embodiments, it is to be noted that the present disclosure is not limited to the details of the embodiments described above. Thus, various modifications and enhancements are possible in light of the above teachings, without departing from the scope of the present disclosure. It is therefore to be understood that the present disclosure may be practiced otherwise than as specifically described herein. For example, elements and/or features of different embodiments may be combined with each other and/or substituted for each other within the scope of the present disclosure. The number of constituent elements and their locations, shapes, and so forth are not limited to any of the structure for performing the methodology illustrated in the drawings.