Patent Publication Number: US-8989643-B2

Title: Fixing device with endless belt 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 to Japanese Patent Application No. 2011-289278, filed on Dec. 28, 2011, in the Japanese Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Exemplary aspects of the present invention 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. 
     2. Description of the Related Art 
     Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile 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 development 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 is requested to shorten a first print time required to output the recording medium bearing the toner image onto the outside of the image forming apparatus after the image forming apparatus receives a print job. Additionally, the fixing device is requested to generate an increased amount of heat before a plurality of recording media is conveyed through the fixing device continuously at an increased speed. 
     To address these requests, the fixing device may employ an endless belt having a decreased thermal capacity and therefore heated quickly by a heater.  FIG. 1  illustrates a fixing device  20 R 1  incorporating an endless belt  100  heated by a heater  300 . As shown in  FIG. 1 , a pressing roller  400  is pressed against a tubular metal thermal conductor  200  disposed inside a loop formed by the endless belt  100  to form a fixing nip N between the pressing roller  400  and the endless belt  100 . The heater  300  disposed inside the metal thermal conductor  200  heats the entire endless belt  100  via the metal thermal conductor  200 . As the pressing roller  400  rotating clockwise and the endless belt  100  rotating counterclockwise in  FIG. 1  convey a recording medium P bearing a toner image T through the fixing nip N in a recording medium conveyance direction A 1 , the endless belt  100  and the pressing roller  400  apply heat and pressure to the recording medium P, thus fixing the toner image T on the recording medium P. 
     Since the metal thermal conductor  200  heats the endless belt  100  entirely, the endless belt  100  is heated to a predetermined fixing temperature quickly, thus meeting the above-described requests of shortening the first print time and generating the increased amount of heat for high speed printing. However, in order to shorten the first print time further and save more energy, the fixing device is requested to heat the endless belt more efficiently. To address this request, a configuration to heat the endless belt directly, not via the metal thermal conductor, is proposed as shown in  FIG. 2 . 
       FIG. 2  illustrates a fixing device  20 R 2  in which the heater  300  heats the endless belt  100  directly. Instead of the metal thermal conductor  200  depicted in  FIG. 1 , a nip formation member  500  is disposed inside the loop formed by the endless belt  100  and presses against the pressing roller  400  via the endless belt  100  to form the fixing nip N between the endless belt  100  and the pressing roller  400 . Since the nip formation member  500  does not encircle the heater  300  unlike the metal thermal conductor  200  depicted in  FIG. 1 , the heater  300  heats the endless belt  100  directly, thus improving heating efficiency for heating the endless belt  100 . 
       FIG. 3  illustrates another fixing device  20 R 3  in which the heater  300  heats the endless belt  100  directly. Instead of the nip formation member  500  depicted in  FIG. 2 , the fixing device  20 R 3  includes a nip formation assembly  503  constructed of a base pad  501  and a low-friction sheet  502  wrapped around the base pad  501 . As the endless belt  100  rotates counterclockwise in  FIG. 3 , it slides over the low-friction sheet  502  with a decreased friction therebetween, thus decreasing wear of the endless belt  100 . 
     With the configurations of the fixing devices  20 R 1  and  20 R 2  described above, as the endless belt  100  rotates in accordance with rotation of the pressing roller  400 , an upstream portion of the endless belt  100  disposed upstream from the fixing nip N in the rotation direction of the fixing belt  100  is pulled toward the fixing nip N by the rotating pressing roller  400 . For example, an upstream portion  100   a  of the endless belt  100  of the fixing device  20 R 2  shown in  FIG. 2 , as it is pulled toward the fixing nip N, may strike an upstream edge  500   a  of the nip formation member  500  and therefore may be damaged or broken. Similarly, the low-friction sheet  502  of the fixing device  20 R 3  shown in  FIG. 3  may strike an upstream edge  501   a  of the base pad  501  and therefore may wear. As the upstream portion  100   a  of the endless belt  100  strikes the upstream edge  501   a  of the base pad  501  no longer protected by the worn low-friction sheet  502 , the upstream portion  100   a  of the endless belt  100  may be damaged or broken. 
     As the thinner endless belt  100  having a decreased mechanical strength is employed to shorten the first print time further and save more energy, a technology to minimize damage and breakage of the endless belt  100  is requested. 
     SUMMARY OF THE INVENTION 
     This specification describes below an improved fixing device. In one exemplary embodiment of the present invention, the fixing device includes an endless belt rotatable in a predetermined direction of rotation; a nip formation assembly disposed opposite an inner circumferential surface of the endless belt; and an opposed rotary body pressed against a part of the nip formation assembly via the endless belt to form a fixing nip between the endless belt and the opposed rotary body through which a recording medium bearing a toner image is conveyed. The nip formation assembly includes a base pad defining the fixing nip and including a pressure portion, an extension portion, and a curved portion. The pressure portion presses against the opposed rotary body via the endless belt. The extension portion is contiguous to and disposed upstream from the pressure portion in a recording medium conveyance direction. The extension portion does not press against the opposed rotary body via the endless belt but the endless belt slides over the extension portion. The curved portion is disposed upstream from the extension portion in the recording medium conveyance direction and smoothly blends into the extension portion. The curved portion does not press against the opposed rotary body. 
     This specification further describes an improved image forming apparatus. In one exemplary embodiment of the present invention, the image forming apparatus includes the fixing device described above. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A more complete appreciation of the invention and the many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a vertical sectional view of a first related-art fixing device; 
         FIG. 2  is a vertical sectional view of a second related-art fixing device; 
         FIG. 3  is a vertical sectional view of a third related-art fixing device; 
         FIG. 4  is a schematic vertical sectional view of an image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 5  is a vertical sectional view of a fixing device installed in the image forming apparatus shown in  FIG. 4 ; 
         FIG. 6A  is a perspective view of one lateral end of a fixing belt incorporated in the fixing device shown in  FIG. 5  in an axial direction of the fixing belt; 
         FIG. 6B  is a plan view of one lateral end of the fixing belt shown in  FIG. 6A  in the axial direction thereof; 
         FIG. 6C  is a vertical sectional view of one lateral end of the fixing belt shown in  FIG. 6A  in the axial direction thereof; 
         FIG. 7  is an enlarged vertical sectional view of a fixing nip formed between the fixing belt and a pressing roller incorporated in the fixing device shown in  FIG. 5 ; 
         FIG. 8  is a vertical sectional view of a base pad and a belt holder incorporated in the fixing device shown in  FIG. 5 ; 
         FIG. 9  is a vertical sectional view of a fixing device according to another exemplary embodiment of the present invention; and 
         FIG. 10  is an enlarged vertical sectional view of the fixing nip formed between the fixing belt and the pressing roller incorporated in the fixing device shown in  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In describing exemplary embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular to  FIG. 4 , an image forming apparatus  1  according to an exemplary embodiment of the present invention is explained. 
       FIG. 4  is a schematic vertical sectional view of the image forming apparatus  1 . The image forming apparatus  1  may be a copier, a facsimile machine, a printer, a multifunction printer (MFP) having at least one of copying, printing, scanning, plotter, and facsimile functions, or the like. According to this exemplary embodiment, the image forming apparatus  1  is a color laser printer that forms a toner image on a recording medium P by electrophotography. 
     As shown in  FIG. 4 , the image forming apparatus  1  includes four image forming devices  4 Y,  4 M,  4 C, and  4 K situated at a center portion thereof. Although the image forming devices  4 Y,  4 M,  4 C, and  4 K contain yellow, magenta, cyan, and black developers (e.g., toners) that form yellow, magenta, cyan, and black toner images, respectively, resulting in a color toner image, they have an identical structure. 
     For example, the image forming devices  4 Y,  4 M,  4 C, and  4 K include drum-shaped photoconductors  5 Y,  5 M,  5 C, and  5 K serving as an image carrier that carries an electrostatic latent image and a resultant toner image; chargers  6 Y,  6 M,  6 C, and  6 K that charge an outer circumferential surface of the respective photoconductors  5 Y,  5 M,  5 C, and  5 K; development devices  7 Y,  7 M,  7 C, and  7 K that supply yellow, magenta, cyan, and black toners to the electrostatic latent images formed on the outer circumferential surface of the respective photoconductors  5 Y,  5 M,  5 C, and  5 K, thus visualizing the electrostatic latent images into yellow, magenta, cyan, and black toner images with the yellow, magenta, cyan, and black toners, respectively; and cleaners  8 Y,  8 M,  8 C, and  8 K that clean the outer circumferential surface of the respective photoconductors  5 Y,  5 M,  5 C, and  5 K. 
     Below the image forming devices  4 Y,  4 M,  4 C, and  4 K is an exposure device  9  that exposes the outer circumferential surface of the respective photoconductors  5 Y,  5 M,  5 C, and  5 K with laser beams. For example, the exposure device  9 , constructed of a light source, a polygon mirror, an f-θ lens, reflection mirrors, and the like, emits a laser beam onto the outer circumferential surface of the respective photoconductors  5 Y,  5 M,  5 C, and  5 K according to image data sent from an external device such as a client computer. 
     Above the image forming devices  4 Y,  4 M,  4 C, and  4 K is a transfer device  3 . For example, the transfer device  3  includes an intermediate transfer belt  30  serving as an intermediate transferor, four primary transfer rollers  31 Y,  31 M,  31 C, and  31 K serving as primary transferors, a secondary transfer roller  36  serving as a secondary transferor, a secondary transfer backup roller  32 , a cleaning backup roller  33 , a tension roller  34 , and a belt cleaner  35 . 
     The intermediate transfer belt  30  is an endless belt stretched over the secondary transfer backup roller  32 , the cleaning backup roller  33 , and the tension roller  34 . As a driver drives and rotates the secondary transfer backup roller  32  counterclockwise in  FIG. 4 , the secondary transfer backup roller  32  rotates the intermediate transfer belt  30  in a rotation direction R 1  by friction therebetween. 
     The four primary transfer rollers  31 Y,  31 M,  31 C, and  31 K sandwich the intermediate transfer belt  30  together with the four photoconductors  5 Y,  5 M,  5 C, and  5 K, respectively, forming four primary transfer nips between the intermediate transfer belt  30  and the photoconductors  5 Y,  5 M,  5 C, and  5 K. The primary transfer rollers  31 Y,  31 M,  31 C, and  31 K are connected to a power supply that applies a predetermined direct current voltage and/or alternating current voltage thereto. 
     The secondary transfer roller  36  sandwiches the intermediate transfer belt  30  together with the secondary transfer backup roller  32 , forming a secondary transfer nip between the secondary transfer roller  36  and the intermediate transfer belt  30 . Similar to the primary transfer rollers  31 Y,  31 M,  31 C, and  31 K, the secondary transfer roller  36  is connected to the power supply that applies a predetermined direct current voltage and/or alternating current voltage thereto. 
     The belt cleaner  35  includes a cleaning brush and a cleaning blade that contact an outer circumferential surface of the intermediate transfer belt  30 . A waste toner conveyance tube extending from the belt cleaner  35  to an inlet of a waste toner container conveys waste toner collected from the intermediate transfer belt  30  by the belt cleaner  35  to the waste toner container. 
     A bottle container  2  situated in an upper portion of the image forming apparatus  1  accommodates four toner bottles  2 Y,  2 M,  2 C, and  2 K detachably attached thereto to contain and supply fresh yellow, magenta, cyan, and black toners to the development devices  7 Y,  7 M,  7 C, and  7 K of the image forming devices  4 Y,  4 M,  4 C, and  4 K, respectively. For example, the fresh yellow, magenta, cyan, and black toners are supplied from the toner bottles  2 Y,  2 M,  2 C, and  2 K to the development devices  7 Y,  7 M,  7 C, and  7 K through toner supply tubes interposed between the toner bottles  2 Y,  2 M,  2 C, and  2 K and the development devices  7 Y,  7 M,  7 C, and  7 K, respectively. 
     In a lower portion of the image forming apparatus  1  are a paper tray  10  that loads a plurality of recording media P (e.g., sheets) and a feed roller  11  that picks up and feeds a recording medium P from the paper tray  10  toward the secondary transfer nip formed between the secondary transfer roller  36  and the intermediate transfer belt  30 . The recording media P may be thick paper, postcards, envelopes, plain paper, thin paper, coated paper, tracing paper, OHP (overhead projector) transparencies, OHP film sheets, and the like. Additionally, a bypass tray may be attached to the image forming apparatus  1  that loads postcards, envelopes, OHP transparencies, OHP film sheets, and the like. 
     A conveyance path R extends from the feed roller  11  to an output roller pair  13  to convey the recording medium P picked up from the paper tray  10  onto an outside of the image forming apparatus  1  through the secondary transfer nip. The conveyance path R is provided with a registration roller pair  12  located below the secondary transfer nip formed between the secondary transfer roller  36  and the intermediate transfer belt  30 , that is, upstream from the secondary transfer nip in a recording medium conveyance direction A 1 . The registration roller pair  12  feeds the recording medium P conveyed from the feed roller  11  toward the secondary transfer nip. 
     The conveyance path R is further provided with a fixing device  20  located above the secondary transfer nip, that is, downstream from the secondary transfer nip in the recording medium conveyance direction A 1 . The fixing device  20  fixes the color toner image transferred from the intermediate transfer belt  30  onto the recording medium P. The conveyance path R is further provided with the output roller pair  13  located above the fixing device  20 , that is, downstream from the fixing device  20  in the recording medium conveyance direction A 1 . The output roller pair  13  discharges the recording medium P bearing the fixed color toner image onto the outside of the image forming apparatus  1 , that is, an output tray  14  disposed atop the image forming apparatus  1 . The output tray  14  stocks the recording media P discharged by the output roller pair  13 . 
     With reference to  FIG. 4 , a description is provided of an image forming operation of the image forming apparatus  1  having the structure described above to form a color toner image on a recording medium P. 
     As a print job starts, a driver drives and rotates the photoconductors  5 Y,  5 M,  5 C, and  5 K of the image forming devices  4 Y,  4 M,  4 C, and  4 K, respectively, clockwise in  FIG. 4  in a rotation direction R 2 . The chargers  6 Y,  6 M,  6 C, and  6 K uniformly charge the outer circumferential surface of the respective photoconductors  5 Y,  5 M,  5 C, and  5 K at a predetermined polarity. The exposure device  9  emits laser beams onto the charged outer circumferential surface of the respective photoconductors  5 Y,  5 M,  5 C, and  5 K according to yellow, magenta, cyan, and black image data contained in image data sent from the external device, respectively, thus forming electrostatic latent images thereon. The development devices  7 Y,  7 M,  7 C, and  7 K supply yellow, magenta, cyan, and black toners to the electrostatic latent images formed on the photoconductors  5 Y,  5 M,  5 C, and  5 K, visualizing the electrostatic latent images into yellow, magenta, cyan, and black toner images, respectively. 
     Simultaneously, as the print job starts, the secondary transfer backup roller  32  is driven and rotated counterclockwise in  FIG. 4 , rotating the intermediate transfer belt  30  in the rotation direction R 1  by friction therebetween. A power supply applies a constant voltage or a constant current control voltage having a polarity opposite a polarity of the toner to the primary transfer rollers  31 Y,  31 M,  31 C, and  31 K. Thus, a transfer electric field is created at the primary transfer nips formed between the primary transfer rollers  31 Y,  31 M,  31 C, and  31 K and the photoconductors  5 Y,  5 M,  5 C, and  5 K, respectively. 
     When the yellow, magenta, cyan, and black toner images formed on the photoconductors  5 Y,  5 M,  5 C, and  5 K reach the primary transfer nips, respectively, in accordance with rotation of the photoconductors  5 Y,  5 M,  5 C, and  5 K, the yellow, magenta, cyan, and black toner images are primarily transferred from the photoconductors  5 Y,  5 M,  5 C, and  5 K onto the intermediate transfer belt  30  by the transfer electric field created at the primary transfer nips in such a manner that the yellow, magenta, cyan, and black toner images are superimposed successively on a same position on the intermediate transfer belt  30 . Thus, the color toner image is formed on the intermediate transfer belt  30 . After the primary transfer of the yellow, magenta, cyan, and black toner images from the photoconductors  5 Y,  5 M,  5 C, and  5 K onto the intermediate transfer belt  30 , the cleaners  8 Y,  8 M,  8 C, and  8 K remove residual toner not transferred onto the intermediate transfer belt  30  and therefore remaining on the photoconductors  5 Y,  5 M,  5 C, and  5 K therefrom. Thereafter, dischargers discharge the outer circumferential surface of the respective photoconductors  5 Y,  5 M,  5 C, and  5 K, initializing the surface potential thereof. 
     On the other hand, the feed roller  11  disposed in the lower portion of the image forming apparatus  1  is driven and rotated to feed a recording medium P from the paper tray  10  toward the registration roller pair  12  in the conveyance path R. The registration roller pair  12  feeds the recording medium P to the secondary transfer nip formed between the secondary transfer roller  36  and the intermediate transfer belt  30  at a time when the color toner image formed on the intermediate transfer belt  30  reaches the secondary transfer nip. The secondary transfer roller  36  is applied with a transfer voltage having a polarity opposite a polarity of the charged yellow, magenta, cyan, and black toners constituting the color toner image formed on the intermediate transfer belt  30 , thus creating a transfer electric field at the secondary transfer nip. 
     When the color toner image formed on the intermediate transfer belt  30  reaches the secondary transfer nip in accordance with rotation of the intermediate transfer belt  30 , the color toner image is secondarily transferred from the intermediate transfer belt  30  onto the recording medium P by the transfer electric field created at the secondary transfer nip. After the secondary transfer of the color toner image from the intermediate transfer belt  30  onto the recording medium P, the belt cleaner  35  removes residual toner not transferred onto the recording medium P and therefore remaining on the intermediate transfer belt  30  therefrom. The removed toner is conveyed and collected into the waste toner container. 
     Thereafter, the recording medium P bearing the color toner image is conveyed to the fixing device  20  that fixes the color toner image on the recording medium P. Then, the recording medium P bearing the fixed color toner image is discharged by the output roller pair  13  onto the output tray  14 . 
     The above describes the image forming operation of the image forming apparatus  1  to form the color toner image on the recording medium P. Alternatively, the image forming apparatus  1  may form a monochrome toner image by using any one of the four image forming devices  4 Y,  4 M,  4 C, and  4 K or may form a bicolor or tricolor toner image by using two or three of the image forming devices  4 Y,  4 M,  4 C, and  4 K. 
     With reference to  FIG. 5 , a description is provided of a construction of the fixing device  20  incorporated in the image forming apparatus  1  described above. 
       FIG. 5  is a vertical sectional view of the fixing device  20 . As shown in  FIG. 5 , the fixing device  20  (e.g., a fuser) includes a fixing belt  21  serving as a fixing rotary body or an endless belt formed into a loop and rotatable in a rotation direction R 3 ; a pressing roller  22  serving as an opposed rotary body disposed opposite an outer circumferential surface of the fixing belt  21  and rotatable in a rotation direction R 4  counter to the rotation direction R 3  of the fixing belt  21 ; a halogen heater  23  serving as a heater disposed inside the loop formed by the fixing belt  21  and heating the fixing belt  21 ; a nip formation assembly  24  disposed inside the loop formed by the fixing belt  21  and pressing against the pressing roller  22  via the fixing belt  21  to form a fixing nip N between the fixing belt  21  and the pressing roller  22 ; a stay  25  serving as a support disposed inside the loop formed by the fixing belt  21  and contacting and supporting the nip formation assembly  24 ; a reflector  26  disposed inside the loop formed by the fixing belt  21  and reflecting light radiated from the halogen heater  23  toward the fixing belt  21 ; a temperature sensor  27  serving as a temperature detector disposed opposite the outer circumferential surface of the fixing belt  21  and detecting the temperature of the fixing belt  21 ; and a separator  28  disposed opposite the outer circumferential surface of the fixing belt  21  and separating the recording medium P from the fixing belt  21 . The fixing device  20  further includes a pressurization assembly that presses the pressing roller  22  against the nip formation assembly  24  via the fixing belt  21 . 
     A detailed description is now given of a construction of the fixing belt  21 . 
     The fixing belt  21  is a thin, flexible endless belt or film. For example, the fixing belt  21  is constructed of a base layer constituting an inner circumferential surface of the fixing belt  21  and a release layer constituting the outer circumferential surface of the fixing belt  21 . The base layer is made of metal such as nickel and SUS stainless steel or resin such as polyimide (PI). The release layer is made of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), polytetrafluoroethylene (PTFE), or the like. Alternatively, an elastic layer, made of rubber such as silicone rubber, silicone rubber foam, and fluoro rubber, may be interposed between the base layer and the release layer. 
     A detailed description is now given of a construction of the pressing roller  22 . 
     The pressing roller  22  is constructed of a metal core  22   a ; an elastic layer  22   b  coating the metal core  22   a  and made of silicone rubber foam, silicone rubber, fluoro rubber, or the like; and a release layer  22   c  coating the elastic layer  22   b  and made of PFA, PTFE, or the like. The pressurization assembly presses the pressing roller  22  against the nip formation assembly  24  via the fixing belt  21 . Thus, the pressing roller  22  pressingly contacting the fixing belt  21  deforms the elastic layer  22   b  of the pressing roller  22  at the fixing nip N formed between the pressing roller  22  and the fixing belt  21 , thus creating the fixing nip N having a predetermined length in the recording medium conveyance direction A 1 . A driver (e.g., a motor) disposed inside the image forming apparatus  1  depicted in  FIG. 4  drives and rotates the pressing roller  22 . As the driver drives and rotates the pressing roller  22 , a driving force of the driver is transmitted from the pressing roller  22  to the fixing belt  21  at the fixing nip N, thus rotating the fixing belt  21  by friction between the pressing roller  22  and the fixing belt  21 . 
     According to this exemplary embodiment, the pressing roller  22  is a solid roller. Alternatively, the pressing roller  22  may be a hollow roller. In this case, a heater such as a halogen heater may be disposed inside the hollow roller. If the pressing roller  22  does not incorporate the elastic layer  22   b , the pressing roller  22  has a decreased thermal capacity that improves fixing performance of being heated to the predetermined fixing temperature quickly. However, as the pressing roller  22  and the fixing belt  21  sandwich and press a toner image T on the recording medium P passing through the fixing nip N, slight surface asperities of the fixing belt  21  may be transferred onto the toner image T on the recording medium P, resulting in variation in gloss of the solid toner image T. 
     To address this problem, it is preferable that the pressing roller  22  incorporates the elastic layer  22   b  having a thickness not smaller than about 100 micrometers. The elastic layer  22   b  having the thickness not smaller than about 100 micrometers elastically deforms to absorb slight surface asperities of the fixing belt  21 , preventing variation in gloss of the toner image T on the recording medium P. The elastic layer  22   b  is made of solid rubber. Alternatively, if no heater is disposed inside the pressing roller  22 , the elastic layer  22   b  may be made of sponge rubber. The sponge rubber is more preferable than the solid rubber because it has an increased insulation that draws less heat from the fixing belt  21 . According to this exemplary embodiment, the pressing roller  22  is pressed against the fixing belt  21 . Alternatively, the pressing roller  22  may merely contact the fixing belt  21  with no pressure therebetween. 
     A detailed description is now given of a configuration of the halogen heater  23 . 
     Both lateral ends of the halogen heater  23  in a longitudinal direction thereof parallel to an axial direction of the fixing belt  21  are mounted on side plates of the fixing device  20 , respectively. A power supply situated inside the image forming apparatus  1  supplies power to the halogen heater  23  so that the halogen heater  23  heats the fixing belt  21 . A controller  90 , that is, a central processing unit (CPU), provided with a random-access memory (RAM) and a read-only memory (ROM), for example, operatively connected to the halogen heater  23  and the temperature sensor  27  controls the halogen heater  23  based on the temperature of the fixing belt  21  detected by the temperature sensor  27  so as to adjust the temperature of the fixing belt  21  to a desired fixing temperature. Alternatively, an induction heater, a resistance heat generator, a carbon heater, or the like may be employed as a heater to heat the fixing belt  21  instead of the halogen heater  23 . 
     A detailed description is now given of a construction of the nip formation assembly  24 . 
     The nip formation assembly  24  includes a base pad  241  and a slide sheet  240  (e.g., a low-friction sheet) covering an outer surface of the base pad  241 . A longitudinal direction of the base pad  241  is parallel to an axial direction of the fixing belt  21  or the pressing roller  22 . The base pad  241  receives pressure from the pressing roller  22  to define the shape of the fixing nip N. The base pad  241  is mounted on and supported by the stay  25 . Accordingly, even if the base pad  241  receives pressure from the pressing roller  22 , the base pad  241  is not bent by the pressure and therefore produces a uniform nip width throughout the axial direction of the pressing roller  22 . The stay  25  is made of metal having an increased mechanical strength, such as stainless steel and iron, to prevent bending of the nip formation assembly  24 . The base pad  241  is also made of a rigid material having an increased mechanical strength. For example, the base pad  241  is made of resin such as liquid crystal polymer (LCP), metal, ceramic, or the like. 
     The base pad  241  is made of a heat-resistant material having a heat resistance temperature not lower than about 200 degrees centigrade. Accordingly, even if the base pad  241  is heated to a predetermined fixing temperature range, the base pad  241  is not thermally deformed, thus retaining the desired shape of the fixing nip N stably and thereby maintaining the quality of the fixed toner image T on the recording medium P. For example, the base pad  241  is made of general heat-resistant resin such as polyether sulfone (PES), polyphenylene sulfide (PPS), liquid crystal polymer (LCP), polyether nitrile (PEN), polyamide imide (PAI), polyether ether ketone (PEEK), or the like. 
     The slide sheet  240  is interposed at least between the base pad  241  and the fixing belt  21 . For example, the slide sheet  240  covers at least an opposed face  241   a  of the base pad  241  disposed opposite the fixing belt  21  at the fixing nip N. As the fixing belt  21  rotates in the rotation direction R 3 , it slides over the slide sheet  240 , decreasing a driving torque exerted on the fixing belt  21 . Accordingly, a decreased friction is imposed onto the fixing belt  21  from the nip formation assembly  24 . Alternatively, the nip formation assembly  24  may not incorporate the slide sheet  240 . 
     A detailed description is now given of a construction of the reflector  26 . 
     The reflector  26  is interposed between the stay  25  and the halogen heater  23 . According to this exemplary embodiment, the reflector  26  is mounted on the stay  25 . For example, the reflector  26  is made of aluminum, stainless steel, or the like. The reflector  26  has a reflection face  70  that reflects light radiated from the halogen heater  23  thereto toward the fixing belt  21 . Accordingly, the fixing belt  21  receives an increased amount of light from the halogen heater  23  and thereby is heated efficiently. Additionally, the reflector  26  minimizes transmission of radiation heat from the halogen heater  23  to the stay  25 , thus saving energy. 
     The fixing device  20  according to this exemplary embodiment attains various improvements to save more energy and shorten a first print time required to output a recording medium P bearing a fixed toner image T onto the outside of the image forming apparatus  1  depicted in  FIG. 4  after the image forming apparatus  1  receives a print job. As a first improvement, the fixing device  20  employs a direct heating method in which the halogen heater  23  directly heats the fixing belt  21  at a portion thereof other than a nip portion thereof facing the fixing nip N. For example, as shown in  FIG. 5 , no component is interposed between the halogen heater  23  and the fixing belt  21  at an outward portion of the fixing belt  21  disposed opposite the temperature sensor  27 . Accordingly, radiation heat from the halogen heater  23  is directly transmitted to the fixing belt  21  at the outward portion thereof. 
     As a second improvement, the fixing belt  21  is designed to be thin and have a reduced loop diameter so as to decrease the thermal capacity thereof. For example, the fixing belt  21  is constructed of the base layer having a thickness in a range of from about 20 micrometers to about 50 micrometers; the elastic layer having a thickness in a range of from about 100 micrometers to about 300 micrometers; and the release layer having a thickness in a range of from about 10 micrometers to about 50 micrometers. Thus, the fixing belt  21  has a total thickness not greater than about 1 mm. The loop diameter of the fixing belt  21  is in a range of from about 20 mm to about 40 mm. In order to decrease the thermal capacity of the fixing belt  21  further, the fixing belt  21  may have a total thickness not greater than about 0.20 mm, preferably not greater than about 0.16 mm. Additionally, the loop diameter of the fixing belt  21  may be not greater than about 30 mm. 
     According to this exemplary embodiment, the pressing roller  22  has a diameter in a range of from about 20 mm to about 40 mm so that the loop diameter of the fixing belt  21  is equivalent to the diameter of the pressing roller  22 . However, the loop diameter of the fixing belt  21  and the diameter of the pressing roller  22  are not limited to the above. For example, the loop diameter of the fixing belt  21  may be larger than the diameter of the pressing roller  22 . In this case, the curvature of the fixing belt  21  at the fixing nip N is smaller than that of the pressing roller  22 , facilitating separation of the recording medium P discharged from the fixing nip N from the fixing belt  21 . 
     Since the fixing belt  21  has a decreased loop diameter, space inside the loop formed by the fixing belt  21  is small. To address this circumstance, both ends of the stay  25  in the recording medium conveyance direction A 1  are folded into a bracket that accommodates the halogen heater  23 . Thus, the stay  25  and the halogen heater  23  are placed in the small space inside the loop formed by the fixing belt  21 . 
     In contrast to the stay  25 , the nip formation assembly  24  is compact, thus allowing the stay  25  to extend as long as possible in the small space inside the loop formed by the fixing belt  21 . For example, the length of the base pad  241  of the nip formation assembly  24  is smaller than that of the stay  25  in the recording medium conveyance direction A 1 . As shown in  FIG. 5 , the base pad  241  includes an upstream portion  24   a  disposed upstream from the fixing nip N in the recording medium conveyance direction A 1 ; a downstream portion  24   b  disposed downstream from the fixing nip N in the recording medium conveyance direction A 1 ; and a center portion  24   c  interposed between the upstream portion  24   a  and the downstream portion  24   b  in the recording medium conveyance direction A 1 . A height h 1  defines a height of the upstream portion  24   a  from the fixing nip N or its hypothetical extension E in a pressurization direction D 1  of the pressing roller  22 . A height h 2  defines a height of the downstream portion  24   b  from the fixing nip N or its hypothetical extension E in the pressurization direction D 1  of the pressing roller  22 . A height h 3 , that is, a maximum height of the base pad  241 , defines a height of the center portion  24   c  from the fixing nip N or its hypothetical extension E in the pressurization direction D 1  of the pressing roller  22 . The height h 3  is not smaller than the height h 1  and the height h 2 . 
     Hence, the upstream portion  24   a  of the base pad  241  of the nip formation assembly  24  is not interposed between the inner circumferential surface of the fixing belt  21  and an upstream curve  25   d   1  of the stay  25  in a diametrical direction of the fixing belt  21 . Similarly, the downstream portion  24   b  of the base pad  241  of the nip formation assembly  24  is not interposed between the inner circumferential surface of the fixing belt  21  and a downstream curve  25   d   2  of the stay  25  in the diametrical direction of the fixing belt  21  and the pressurization direction D 1  of the pressing roller  22 . Accordingly, the upstream curve  25   d   1  and the downstream curve  25   d   2  of the stay  25  are situated in proximity to the inner circumferential surface of the fixing belt  21 . Consequently, the stay  25  having an increased size that enhances the mechanical strength thereof is accommodated in the limited space inside the loop formed by the fixing belt  21 . As a result, the stay  25 , with its enhanced mechanical strength, supports the nip formation assembly  24  properly, preventing bending of the nip formation assembly  24  caused by pressure from the pressing roller  22  and thereby improving fixing performance. 
     As shown in  FIG. 5 , the stay  25  includes a base  25   a  contacting the nip formation assembly  24  and an upstream projection  25   b   1  and a downstream projection  25   b   2 , constituting a pair of projections, projecting from the base  25   a . The base  25   a  extends in the recording medium conveyance direction A 1 , that is, a vertical direction in  FIG. 5 . The upstream projection  25   b   1  and the downstream projection  25   b   2  project from an upstream end and a downstream end of the base  25   a , respectively, in the recording medium conveyance direction A 1  and extend in the pressurization direction D 1  of the pressing roller  22  orthogonal to the recording medium conveyance direction A 1 . The upstream projection  25   b   1  and the downstream projection  25   b   2  projecting from the base  25   a  in the pressurization direction D 1  of the pressing roller  22  elongate a cross-sectional area of the stay  25  in the pressurization direction D 1  of the pressing roller  22 , increasing the section modulus and the mechanical strength of the stay  25 . As described above, the upstream projection  25   b   1 , the base  25   a , and the downstream projection  25   b   2 , formed into a bracket in cross-section, create a recess  25   e  that houses the halogen heater  23 . 
     Additionally, as the upstream projection  25   b   1  and the downstream projection  25   b   2  elongate further in the pressurization direction D 1  of the pressing roller  22 , the mechanical strength of the stay  25  becomes greater. Accordingly, it is preferable that a front edge  25   c  of each of the upstream projection  25   b   1  and the downstream projection  25   b   2  is situated as close as possible to the inner circumferential surface of the fixing belt  21  to allow the upstream projection  25   b   1  and the downstream projection  25   b   2  to project longer from the base  25   a  in the pressurization direction D 1  of the pressing roller  22 . However, since the fixing belt  21  swings or vibrates as it rotates, if the front edge  25   c  of each of the upstream projection  25   b   1  and the downstream projection  25   b   2  is excessively close to the inner circumferential surface of the fixing belt  21 , the swinging or vibrating fixing belt  21  may come into contact with the upstream projection  25   b   1  or the downstream projection  25   b   2 . For example, if the thin fixing belt  21  is used as in this exemplary embodiment, the thin fixing belt  21  swings or vibrates substantially. Accordingly, it is necessary to position the front edge  25   c  of each of the upstream projection  25   b   1  and the downstream projection  25   b   2  with respect to the fixing belt  21  carefully. 
     Specifically, as shown in  FIG. 5 , a distance d between the front edge  25   c  of each of the upstream projection  25   b   1  and the downstream projection  25   b   2  and the inner circumferential surface of the fixing belt  21  in the pressurization direction D 1  of the pressing roller  22  is at least about 2.0 mm, preferably not smaller than about 3.0 mm. Conversely, if the fixing belt  21  is thick and therefore barely swings or vibrates, the distance d is about 0.02 mm. It is to be noted that if the reflector  26  is attached to the front edge  25   c  of each of the upstream projection  25   b   1  and the downstream projection  25   b   2  as in this exemplary embodiment, the distance d is determined by considering the thickness of the reflector  26  so that the reflector  26  does not contact the fixing belt  21 . 
     The front edge  25   c  of each of the upstream projection  25   b   1  and the downstream projection  25   b   2  situated as close as possible to the inner circumferential surface of the fixing belt  21  allows the upstream projection  25   b   1  and the downstream projection  25   b   2  to project longer from the base  25   a  in the pressurization direction D 1  of the pressing roller  22 . Accordingly, even if the fixing belt  21  has a decreased loop diameter, the stay  25  having the longer upstream projection  25   b   1  and the longer downstream projection  25   b   2  attains an enhanced mechanical strength. 
     With reference to  FIGS. 6A ,  6 B, and  6 C, a description is provided of a configuration of a lateral end of the fixing belt  21  in the axial direction thereof. 
       FIG. 6A  is a perspective view of one lateral end of the fixing belt  21  in the axial direction thereof.  FIG. 6B  is a plan view of one lateral end of the fixing belt  21  in the axial direction thereof.  FIG. 6C  is a vertical sectional view of one lateral end of the fixing belt  21  in the axial direction thereof. Although not shown, another lateral end of the fixing belt  21  in the axial direction thereof has the identical configuration shown in  FIGS. 6A to 6C . Hence, the following describes the configuration of one lateral end of the fixing belt  21  in the axial direction thereof with reference to  FIGS. 6A to 6C . 
     As shown in  FIGS. 6A and 6B , a belt holder  40  is inserted into the loop formed by the fixing belt  21  at a lateral end  21   b  of the fixing belt  21  in the axial direction thereof to rotatably support the fixing belt  21 . As shown in  FIG. 6B , the belt holder  40  contacts and rotatably supports each lateral end  21   b  of the fixing belt  21  in the axial direction thereof. Conversely, the nip formation assembly  24  contacts and supports a center  21   c  of the fixing belt  21  in the axial direction thereof. As shown in  FIG. 6C , the belt holder  40  is C-shaped in cross-section to create an opening  40   b  disposed opposite the fixing nip N where the nip formation assembly  24  is situated. As shown in  FIG. 6B , a lateral end of the stay  25  in a longitudinal direction thereof parallel to the axial direction of the fixing belt  21  is mounted on and positioned by the belt holder  40 . 
     As shown in  FIG. 6B , a slip ring  41  is interposed between a lateral edge  21   a  of the fixing belt  21  and an inward face  40   a  of the belt holder  40  disposed opposite the lateral edge  21   a  of the fixing belt  21  in the axial direction thereof. The slip ring  41  serves as a protector that protects the lateral end  21   b  of the fixing belt  21  in the axial direction thereof. For example, even if the fixing belt  21  is skewed in the axial direction thereof, the slip ring  41  prevents the lateral edge  21   a  of the fixing belt  21  from coming into contact with the inward face  40   a  of the belt holder  40  directly, thus minimizing wear and breakage of the lateral edge  21   a  of the fixing belt  21  in the axial direction thereof. Since an inner diameter of the slip ring  41  is sufficiently greater than an outer diameter of the belt holder  40 , the slip ring  41  loosely slips on the belt holder  40 . Accordingly, when the lateral edge  21   a  of the fixing belt  21  comes into contact with the slip ring  41 , the slip ring  41  is rotatable in accordance with rotation of the fixing belt  21 . Alternatively, the slip ring  41  may be stationary irrespective of rotation of the fixing belt  21 . The slip ring  41  is made of heat-resistant, super engineering plastics such as PEEK, PPS, PAI, and PTFE. 
     A shield is interposed between the halogen heater  23  and the fixing belt  21  at both lateral ends  21   b  of the fixing belt  21  in the axial direction thereof. The shield shields the fixing belt  21  against heat from the halogen heater  23 . For example, even if a plurality of small recording media P is conveyed through the fixing nip N continuously, the shield prevents heat from the halogen heater  23  from being conducted to both lateral ends  21   b  of the fixing belt  21  in the axial direction thereof where the small recording media P are not conveyed. Accordingly, both lateral ends  21   b  of the fixing belt  21  do not overheat even in the absence of large recording media P that draw heat therefrom. Consequently, the shield minimizes thermal wear and damage of the fixing belt  21 . 
     With reference to  FIG. 5 , a description is provided of a fixing operation of the fixing device  20  described above. 
     As the image forming apparatus  1  depicted in  FIG. 4  is powered on, the power supply supplies power to the halogen heater  23  and at the same time the driver drives and rotates the pressing roller  22  clockwise in  FIG. 5  in the rotation direction R 4 . Accordingly, the fixing belt  21  rotates counterclockwise in  FIG. 5  in the rotation direction R 3  in accordance with rotation of the pressing roller  22  by friction between the pressing roller  22  and the fixing belt  21 . 
     A recording medium P bearing a toner image T formed by the image forming operation of the image forming apparatus  1  described above is conveyed in the recording medium conveyance direction A 1  while guided by a guide plate and enters the fixing nip N formed between the pressing roller  22  and the fixing belt  21  pressed by the pressing roller  22 . The fixing belt  21  heated by the halogen heater  23  heats the recording medium P and at the same time the pressing roller  22  pressed against the fixing belt  21  and the fixing belt  21  together exert pressure to the recording medium P, thus fixing the toner image T on the recording medium P. 
     The recording medium P bearing the fixed toner image T is discharged from the fixing nip N in a recording medium conveyance direction A 2 . As a leading edge of the recording medium P comes into contact with a front edge of the separator  28 , the separator  28  separates the recording medium P from the fixing belt  21 . Thereafter, the recording medium P is discharged by the output roller pair  13  depicted in  FIG. 4  onto the outside of the image forming apparatus  1 , that is, the output tray  14  where the recording media P are stocked. 
     With reference to  FIG. 7 , a description is provided of a configuration of the fixing nip N formed between the pressing roller  22  and the fixing belt  21  of the fixing device  20 . 
       FIG. 7  is an enlarged vertical sectional view of the fixing nip N formed between the pressing roller  22  and the fixing belt  21 . As shown in  FIG. 7 , the base pad  241  of the nip formation assembly  24  includes an opposed face disposed opposite the pressing roller  22 , which is constructed of a pressure portion  50  (e.g., a pressure face), an extension portion  51  (e.g., an extension face), and a curved portion  52  (e.g., a curved face). The pressure portion  50  is straight in the recording medium conveyance direction A 1  and presses against the pressing roller  22  via the slide sheet  240  and the fixing belt  21 . The extension portion  51  is disposed contiguous to and upstream from the pressure portion  50  in the recording medium conveyance direction A 1 . The extension portion  51  presses against the inner circumferential surface of the fixing belt  21  via the slide sheet  240  but does not press against the pressing roller  22 . The extension portion  51  is straight in the recording medium conveyance direction A 1  on an identical hypothetical plane where the pressure portion  50  is provided. As the fixing belt  21  rotates in the rotation direction R 3 , it slides over the extension portion  51  via the slide sheet  240  and enters the fixing nip N. That is, the extension portion  51  serves as a guide that guides the fixing belt  21  to the pressure portion  50  defining the fixing nip N. 
     According to this exemplary embodiment, the pressure portion  50  and the extension portion  51  are straight in the recording medium conveyance direction A 1 . Alternatively, the pressure portion  50  and the extension portion  51  may be concave with respect to the inner circumferential surface of the fixing belt  21  or may have other shapes. After a recording medium P is conveyed through the fixing nip N formed by the concave pressure portion  50  and the concave extension portion  51 , the leading edge of the recording medium P is directed to the pressing roller  22 , facilitating separation of the recording medium P from the fixing belt  21  and thereby minimizing conveyance failure of the recording medium P such as jamming of the recording medium P. 
     The curved portion  52  is contiguous to and upstream from the extension portion  51  in the recording medium conveyance direction A 1 . The curved portion  52  is convex toward the inner circumferential surface of the fixing belt  21 . That is, the curved portion  52  projects toward the inner circumferential surface of the fixing belt  21  in the diametrical direction thereof. The curved portion  52  smoothly blends into the extension portion  51  through a border B between the curved portion  52  and the extension portion  51  so that the curved portion  52  and the extension portion  51  are not edged at the border B. 
     As described above, the base pad  241  includes the pressure portion  50  extending straight in the recording medium conveyance direction A 1  and pressing against the pressing roller  22  via the fixing belt  21 ; the extension portion  51  contiguous to and upstream from the pressure portion  50  in the recording medium conveyance direction A 1 ; and the curved portion  52  smoothly blending into the extension portion  51  and disposed upstream from the extension portion  51  in the recording medium conveyance direction A 1 . The slide sheet  240  adheres to the straight pressure portion  50 , the straight extension portion  51  and the curved portion  52 . For example, like the base pad  241 , the slide sheet  240  includes a pressure portion  60  extending straight in the recording medium conveyance direction A 1  and corresponding to the pressure portion  50  of the base pad  241 ; an extension portion  61  extending straight in the recording medium conveyance direction A 1  and corresponding to the extension portion  51  of the base pad  241 ; and a curved portion  62  corresponding to the curved portion  52  of the base pad  241 . 
     The fixing belt  21 , as it halts, is isolated from the curved portion  62  of the slide sheet  240 . Additionally, the curved portion  62  of the slide sheet  240  does not come into contact with the fixing belt  21  as the fixing belt  21  rotates on its desired rotation track without swinging or vibrating. Accordingly, even if the fixing belt  21  rotates, it does not come into contact with the curved portion  62  of the slide sheet  240 . However, since the fixing belt  21  swings or vibrates slightly as it rotates, the fixing belt  21  may come into contact with the curved portion  62  of the slide sheet  240  accidentally. To address this circumstance, the curved portion  62  smoothly blends into the contiguous extension portion  61  because the curved portion  62  and the extension portion  61  of the slide sheet  240  adhere to the curved portion  52  and the extension portion  51  of the base pad  241 . Thus, the curved portion  62  and the extension portion  61  of the slide sheet  240  minimize wear of the fixing belt  21  even if the fixing belt  21  accidentally slides over the curved portion  62  and the extension portion  61  of the slide sheet  240 . Additionally, the curved portion  52  smoothly blending into the contiguous extension portion  51  of the base pad  241  minimizes wear of the slide sheet  240  caused by contact with the base pad  241 . 
     In order to further decrease load imposed on the fixing belt  21  when the fixing belt  21  comes into contact with the curved portion  62  of the slide sheet  240 , the curved portion  52  of the base pad  241  is shaped in accordance with the desired rotation track of the fixing belt  21 . 
     As the fixing belt  21  rotates in the rotation direction R 3 , it is isolated from the curved portion  62  of the slide sheet  240  but in contact with the extension portion  61  of the slide sheet  240 . That is, the fixing belt  21  enters the fixing nip N as it slides over the extension portion  61  of the slide sheet  240 . Since the rotating fixing belt  21  is guided by the base pad  241  from the straight extension portion  51  to the straight pressure portion  50  thereof, the base pad  241  minimizes swinging or vibration of the fixing belt  21  before the fixing nip N, facilitating stable and smooth rotation of the fixing belt  21 . 
     Even if the fixing belt  21  accidentally comes into contact with the curved portion  62  of the slide sheet  240  as it swings or vibrates during rotation, the curved portion  62  of the slide sheet  240  corresponding to the curved portion  52  of the base pad  241  smoothly blends into the extension portion  61  of the slide sheet  240  corresponding to the extension portion  51  of the base pad  241 , minimizing wear of the fixing belt  21  precisely. Even if the fixing belt  21  presses the slide sheet  240  against the base pad  241  substantially, the shape of the curved portions  52  and  62  and the extension portions  51  and  61  minimizes wear of the slide sheet  240  precisely. 
     Even if the fixing belt  21  accidentally slides over the curved portion  52  and the extension portion  51  of the base pad  241 , the fixing belt  21  is isolated from the pressing roller  22 . Thus, friction between the fixing belt  21  and the pressing roller  22  that may wear the fixing belt  21  does not generate. 
     With reference to  FIG. 8 , a detailed description is now given of the position of the base pad  241 . 
       FIG. 8  is a vertical sectional view of the base pad  241  and the belt holder  40 . As shown in  FIG. 8 , the base pad  241  is situated as described below to keep the rotating fixing belt  21  away from the curved portion  62  of the slide sheet  240 . A hypothetical circle D, that is, a perfect circle or a substantially perfect circle, indicated by the dotted line overlaps an outer circumference of the C-shaped belt holder  40 . The hypothetical extension E indicated by the dotted line overlaps and extends from the fixing nip N in the recording medium conveyance direction A 1 . The curved portion  52  of the base pad  241  is situated at a position inside a region enclosed by the hypothetical circle D and the hypothetical extension E and spaced apart from the hypothetical circle D and the hypothetical extension E. Accordingly, the fixing belt  21  does not come into contact with the curved portion  62  of the slide sheet  240  adhered to the curved portion  52  of the base pad  241 , minimizing load that may be imposed on the fixing belt  21  as it accidentally slides over the curved portion  62  of the slide sheet  240  and resultant wear of the fixing belt  21  precisely. 
     With reference to  FIGS. 9 and 10 , a description is provided of a configuration of a fixing device  20 S according to another exemplary embodiment. 
       FIG. 9  is a vertical sectional view of the fixing device  20 S.  FIG. 10  is an enlarged vertical sectional view of the fixing nip N formed between the fixing belt  21  and the pressing roller  22  of the fixing device  20 S. Unlike the fixing device  20  depicted in  FIG. 5 , the fixing device  20 S includes three halogen heaters  23  serving as heaters that heat the fixing belt  21  as shown in  FIG. 9 . The three halogen heaters  23  have three different regions thereof in the axial direction of the fixing belt  21  that generate heat. Accordingly, the three halogen heaters  23  heat the fixing belt  21  in three different regions on the fixing belt  21 , respectively, in the axial direction thereof so that the fixing belt  21  heats recording media P of various widths in the axial direction of the fixing belt  21 . The fixing device  20 S further includes a metal plate  250  that partially surrounds a nip formation assembly  24 S. Thus, a stay  25 S supports the nip formation assembly  24 S via the metal plate  250 . 
     Instead of the bracket-shaped stay  25  shown in  FIG. 5 , the fixing device  20 S includes the substantially trapezoidal stay  25 S that houses the three halogen heaters  23 . For example, the stay  25 S is constructed of the base  25   a ; an upstream projection  25 Sb 1  projecting from the base  25   a  and bent downward toward the inner circumferential surface of the fixing belt  21 ; and a downstream projection  25 Sb 2  projecting from the base  25   a  and bent upward toward the inner circumferential surface of the fixing belt  21 . Instead of the reflector  26  shown in  FIG. 5 , the fixing device  20 S includes a reflector  26 S shaped in accordance with the shape of the stay  25 S and mounted on the stay  25 S. 
     As shown in  FIG. 10 , the fixing device  20 S further includes the nip formation assembly  24 S constructed of a base pad  241 S having a shape different from the shape of the base pad  241  shown in  FIG. 7  and a slide sheet  240 S wrapped around the base pad  241 S. Like the base pad  241  shown in  FIG. 7 , the base pad  241 S includes the pressure portion  50 , the extension portion  51  contiguous to the pressure portion  50 , and the curved portion  52  contiguous to the extension portion  51 , that facilitate stable and smooth rotation of the fixing belt  21  and minimize wear of the fixing belt  21 . Similar to the heights h 1 , h 2 , and h 3  shown in  FIG. 5 , the heights h 1 , h 2 , and h 3  shown in  FIG. 9  define the height of an upstream portion  24 Sa of the base pad  241 S, the height of a downstream portion  24 Sb of the base pad  241 S, and the height of a center portion  24 Sc of the base pad  241 S, respectively. In order to increase the size of the stay  25 S disposed in the limited space inside the loop formed by the fixing belt  21 , the height h 3  is not smaller than the height h 1  and the height h 2 . 
     As shown in  FIG. 10 , the extension portion  51  of the base pad  241 S guides the fixing belt  21  sliding over the extension portion  51  via the extension portion  61  of the slide sheet  240 S to the fixing nip N. Accordingly, even if only the nip formation assembly  24 S guides the fixing belt  21  at the center in the axial direction thereof, the nip formation assembly  24 S incorporating the base pad  241 S having the extension portion  51  guides and rotates the fixing belt  21  stably and smoothly. Consequently, a reduced load is imposed on the rotating fixing belt  21 . Even if the fixing belt  21  accidentally presses against the curved portion  52  of the base pad  241 S via the curved portion  62  of the slide sheet  240 S, the curved portion  52  smoothly blending into the extension portion  51  of the base pad  241 S decreases friction between the fixing belt  21  and the slide sheet  240 S wrapped around the base pad  241 S, minimizing wear of the fixing belt  21  and the slide sheet  240 S. 
     With reference to  FIGS. 5 ,  7 ,  9 , and  10 , a description is provided of advantages of the fixing devices  20  and  20 S. 
     The fixing devices  20  and  20 S for fixing a toner image T on a recording medium P include the endless belt (e.g., the fixing belt  21 ) rotatable in the predetermined direction of rotation R 3 ; the heater (e.g., the halogen heater  23 ) that heats the fixing belt  21 ; the nip formation assembly (e.g., the nip formation assemblies  24  and  24 S) disposed inside the loop formed by the fixing belt  21 ; and the opposed rotary body (e.g., the pressing roller  22 ) that presses against the nip formation assembly via the fixing belt  21  to form the fixing nip N between the pressing roller  22  and the fixing belt  21 . The nip formation assembly includes the base pad (e.g., the base pads  241  and  241 S) that defines the shape of the fixing nip N and includes the pressure portion  50 , the extension portion  51 , and the curved portion  52 . The pressure portion  50  presses against the pressing roller  22  via the fixing belt  21  so that the fixing belt  21  slides over the pressure portion  50 . The extension portion  51  is contiguous to and disposed upstream from the pressure portion  50  in the recording medium conveyance direction A 1 . The extension portion  51  does not press against the pressing roller  22  but the fixing belt  21  slides over the extension portion  51 . The curved portion  52  is disposed upstream from the extension portion  51  in the recording medium conveyance direction A 1  and smoothly blends into the extension portion  51 . The curved portion  52  does not press against the pressing roller  22 . 
     The extension portion  51  of the base pad facilitates stable and smooth rotation of the fixing belt  21  and guides the fixing belt  21  to the fixing nip N. Accordingly, a reduced load is imposed on the fixing belt  21  as it rotates in the rotation direction R 3 . Even if the fixing belt  21  accidentally presses against the curved portion  52  of the base pad, the curved portion  52  smoothly blending into the extension portion  51  minimizes load imposed on the fixing belt  21 , thus preventing wear of the fixing belt  21 . 
     As described above, the nip formation assembly minimizes load imposed on the rotating fixing belt  21  and resultant wear of the fixing belt  21 , preventing damage and breakage of the fixing belt  21  and enhancing reliability of the fixing devices  20  and  20 S. For example, it is difficult for the fixing belt  21  having a reduced thickness that decreases the thermal capacity thereof to have an increased mechanical strength. However, the nip formation assembly according to the exemplary embodiments described above has an increased mechanical strength to support and guide the fixing belt  21 , achieving the advantages described above. 
     The compact nip formation assembly guides the fixing belt  21  to the fixing nip N, facilitating stable and smooth rotation of the fixing belt  21 . Accordingly, heat is not unnecessarily consumed on a guide that guides the fixing belt  21  to the fixing nip N, decreasing the thermal capacity of the entire fixing devices  20  and  20 S. It is not necessary to provide a greater guide separately from the nip formation assembly. Hence, as shown in  FIGS. 5 and 9 , no component is interposed between the inner circumferential surface of the fixing belt  21  and the upstream curve of the stay (e.g., the upstream curve  25   d   1  of the stay  25  and an upstream curve  25 Sd 1  of the stay  25 S) in the diametrical direction of the fixing belt  21 . Similarly, no component is interposed between the inner circumferential surface of the fixing belt  21  and the downstream curve of the stay (e.g., the downstream curve  25   d   2  of the stay  25  and a downstream curve  25 Sd 2  of the stay  25 S) in the diametrical direction of the fixing belt  21  and the pressurization direction D 1  of the pressing roller  22 . That is, the upstream curve and the downstream curve of the stay are disposed opposite the inner circumferential surface of the fixing belt  21  directly. Accordingly, the upstream curve and the downstream curve of the stay are situated in proximity to the inner circumferential surface of the fixing belt  21 . Consequently, the stay having an increased size that enhances the mechanical strength thereof is accommodated in the limited space inside the loop formed by the fixing belt  21 . As a result, even if the fixing belt  21  is downsized to decrease its thermal capacity, the stay accommodated inside the downsized fixing belt  21  achieves an enhanced mechanical strength that supports the nip formation assembly properly, preventing bending of the nip formation assembly caused by pressure from the pressing roller  22  and thereby improving fixing performance. 
     According to the exemplary embodiments described above, the nip formation assemblies  24  and  24 S and the stays  25  and  25 S are employed by the fixing devices  20  and  20 S incorporating the thin fixing belt  21  having a reduced loop diameter to save more energy. Alternatively, the nip formation assemblies  24  and  24 S and the stays  25  and  25 S may be employed by other fixing devices. Additionally, as shown in  FIG. 4 , the image forming apparatus  1  incorporating the fixing device  20  or  20 S is a color laser printer. Alternatively, the image forming apparatus  1  may be a monochrome printer, a copier, a facsimile machine, a multifunction printer (MFP) having at least one of copying, printing, facsimile, and scanning functions, or the like. 
     According to the exemplary embodiments described above, the pressing roller  22  serves as an opposed rotary body disposed opposite the fixing belt  21 . Alternatively, a pressing belt or the like may serve as an opposed rotary body. Further, the halogen heater  23  disposed inside the fixing belt  21  serves as a heater that heats the fixing belt  21 . Alternatively, the halogen heater  23  may be disposed outside the fixing belt  21 . 
     The present invention has been described above with reference to specific exemplary embodiments. Note that the present invention is not limited to the details of the embodiments described above, but various modifications and enhancements are possible without departing from the spirit and scope of the invention. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative exemplary embodiments may be combined with each other and/or substituted for each other within the scope of the present invention.