Patent Publication Number: US-8971779-B2

Title: Fixing device with support 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. 2011-289277, 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  502 , disposed inside the loop formed by the endless belt  100 , 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  502  does not encircle the heater  300 , unlike the metal thermal conductor  200  depicted in  FIG. 1 , the heater  300  heals the endless belt  100  directly. However, the nip formation member  502  is subject to bending as it receives pressure from the pressing roller  400 . If the nip formation member  502  is bent, it presses against the pressing roller  400  with various levels of pressure in the axial direction of the pressing roller  400 . Accordingly, the endless belt  100  and the pressing roller  400  may not apply heat and pressure uniformly to the recording medium P conveyed through the fixing nip N, resulting in faulty fixing. 
     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; an opposed rotary body pressed against the nip formation assembly in a pressurization direction 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; and a support contacting and supporting the nip formation assembly. The support includes a base contacting the nip formation assembly; an upstream projection projecting from the base in the pressurization direction of the opposed rotary body at a position on the base corresponding to or upstream from an upstream edge of the fixing nip in a recording medium conveyance direction; and a downstream projection projecting from the base in the pressurization direction of the opposed rotary body at a position on the base corresponding to or downstream from a downstream edge of the fixing nip in the recording medium conveyance direction. The downstream projection is spaced apart from the upstream projection in the recording medium conveyance direction. 
     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 related-art fixing device; 
         FIG. 2  is a vertical sectional view of another related-art fixing device; 
         FIG. 3  is a schematic vertical sectional view of an image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 4  is a vertical sectional view of a fixing device installed in the image forming apparatus shown in  FIG. 3 ; 
         FIG. 5A  is a partial vertical sectional view of a halogen heater and a reflection face of a reflector incorporated in the fixing device shown in  FIG. 4  illustrating one example of the reflection face; 
         FIG. 5B  is a partial vertical sectional view of the halogen heater and the reflection face of the reflector illustrating another example of the reflection face; 
         FIG. 6A  is a perspective view of one lateral end of a fixing belt incorporated in the fixing device shown in  FIG. 4  in an axial direction of the fixing belt; 
         FIG. 6B  is a plan view of one lateral end of the fixing belt in the axial direction thereof shown in  FIG. 6A ; 
         FIG. 6C  is a vertical sectional view of one lateral end of the fixing belt in the axial direction thereof shown in  FIG. 6A ; 
         FIG. 7  is a partial vertical sectional view of the fixing device shown in  FIG. 4 ; 
         FIG. 8  is a partial vertical sectional view of a comparative fixing device; 
         FIG. 9  is a partial vertical sectional view of another comparative fixing device; 
         FIG. 10  is a vertical sectional view of a fixing device according to another exemplary embodiment of the present invention; and 
         FIG. 11  is a vertical sectional view of a fixing device according to yet another exemplary embodiment of the present invention. 
     
    
    
     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. 3 , an image forming apparatus  1  according to an exemplary embodiment of the present invention is explained. 
       FIG. 3  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. 3 , 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 , 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. 3 , 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. 3  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 color 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. 3 , 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. 4 , a description is provided of a construction of the fixing device  20  incorporated in the image forming apparatus  1  described above. 
       FIG. 4  is a vertical sectional view of the fixing device  20 . As shown in  FIG. 4 , 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. 3  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  may be 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 . 
     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 (PAT), polyether ether ketone (PEEK), or the like. 
     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 , the fixing belt  21  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 . Since the reflector  26  is directly heated by the halogen heater  23 , the reflector  26  is made of metal having a relatively high melting point. 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. 
     Alternatively, instead of mounting the reflector  26  on the stay  25 , an opposed face of the stay  25  disposed opposite the halogen heater  23  may be mirror finished by polishing or coating to produce a reflection face that reflects light from the halogen heater  23  toward the fixing belt  21 . The reflection face  70  of the reflector  26  or the reflection face of the stay  25  has a reflection rate not smaller than about 90 percent. 
     Since the stay  25  is required to have a predetermined mechanical strength great enough to support the nip formation assembly  24 , the shape and material of the stay  25  are limited. To address this circumstance, the reflector  26  separately provided from the stay  25  attains flexibility in the shape and material of the stay  25 . Consequently, the reflector  26  and the stay  25  are tailored to fit their specific purposes, respectively. Since the reflector  26  is interposed between the halogen heater  23  and the stay  25 , the reflector  26  is situated in proximity to the halogen heater  23 , reflecting light from the halogen heater  23  to the fixing belt  21  efficiently. 
     In order to heat the fixing belt  21  more efficiently by reflecting light from the halogen heater  23  toward the fixing belt  21 , the reflection face  70  of the reflector  26  or the reflection face of the stay  25  is directed properly. 
     With reference to  FIGS. 5A and 5B , a description is provided of the direction of the reflection face  70  of the reflector  26 . 
       FIG. 5A  is a partial vertical sectional view of the halogen heater  23  and the reflection face  70  of the reflector  26  illustrating one example of the reflection face  70 .  FIG. 5B  is a partial vertical sectional view of the halogen heater  23  and the reflection face  70  of the reflector  26  illustrating another example of the reflection face  70 . As shown in  FIG. 5A , if the reflection face  70  is concentrically shaped with respect to the halogen heater  23 , the reflection face  70  reflects light from the halogen heater  23  back to the halogen heater  23 , degrading heating efficiency for heating the fixing belt  21 . Conversely, as shown in  FIG. 5B , if the reflection face  70  is partially or entirely shaped to reflect light from the halogen heater  23  in directions other than a direction toward the halogen heater  23 , the reflection face  70  reflects a decreased amount of light toward the halogen heater  23 , improving heating efficiency for heating the fixing belt  21  by light reflection. 
     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. 3  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 of the fixing belt  21  other than a nip portion thereof facing the fixing nip N. For example, as shown in  FIG. 4 , 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 of the fixing belt  21 . 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 smaller 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 . 
     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  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  at 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 end  21   b  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. 4 , a description is provided of a fixing operation of the fixing device  20  described above. 
     As the image forming apparatus  1  depicted in  FIG. 3  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. 4  in the rotation direction R 4 . Accordingly, the fixing belt  21  rotates counterclockwise in  FIG. 4  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. 3  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 detailed description is now given of a construction of the stay  25 . 
       FIG. 7  is a partial vertical sectional view of the fixing device  20 . As shown in  FIG. 7 , 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. 7 . 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 a pressurization direction D 1  of the pressing roller  22  orthogonal to the recording medium conveyance direction A 1 . The downstream projection  25   b   2  is spaced apart from the upstream projection  25   b   1  in the recording medium conveyance direction A 1 . For example, the upstream projection  25   b   1  and the downstream projection  25   b   2  are situated outboard from the fixing nip N in the recording medium conveyance direction A 1 . In other words, the upstream projection  25   b   1  is situated upstream from an upstream edge N 1  of the fixing nip N in the recording medium conveyance direction A 1 , that is, below the fixing nip N in  FIG. 7 ; the downstream projection  25   b   2  is situated downstream from a downstream edge N 2  of the fixing nip N in the recording medium conveyance direction A 1 , that is, above the fixing nip N in  FIG. 7 . 
     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 . 
     With reference to  FIG. 8 , a description is provided of a comparative fixing device  20 C 1  incorporating a single projection  600   b.    
       FIG. 8  is a partial vertical sectional view of the comparative fixing device  20 C 1 . As shown in  FIG. 8 , the comparative fixing device  20 C 1  includes a support  600  supporting a nip formation pad  500  that receives pressure from the pressing roller  22 . The support  600  is constructed of a horizontal base  600   a  in contact with the nip formation pad  500  and the projection  600   b  projecting from the base  600   a  substantially vertically at a center of the base  600   a  in the recording medium conveyance direction A 1 . However, since the projection  600   b  is not provided at an upstream end Z 1  and a downstream end Z 2  of the base  600   a  in the recording medium conveyance direction A 1 , the base  600   a  may be bent by pressure from the pressing roller  22 . 
     With reference to  FIG. 9 , a description is provided of another comparative fixing device  20 C 2  incorporating an upstream projection  600   b   1  and a downstream projection  600   b   2  in contact with each other. 
       FIG. 9  is a partial vertical sectional view of the comparative fixing device  20 C 2 . As shown in  FIG. 9 , the comparative fixing device  20 C 2  includes two supports, that is an upstream support  600 U and a downstream support  600 D supporting a nip formation pad  501  that receives pressure from the pressing roller  22 . The upstream support  6000  is constructed of a horizontal base  600   a   1  in contact with the nip formation pad  501 ; the upstream projection  600   b   1  projecting from the base  600   a   1  substantially vertically; and a radiation adjuster  600   c   1  projecting from an upper end of the upstream projection  600   b   1  horizontally. Similarly, the downstream support  600 D is constructed of a horizontal base  600   a   2  in contact with the nip formation pad  501 ; the downstream projection  600   b   2  projecting from the base  600   a   2  substantially vertically; and a radiation adjuster  600   c   2  projecting from an upper end of the downstream projection  600   b   2  horizontally. 
     The upstream projection  600   b   1  contacts the downstream projection  600   b   2  along a vertical line extending vertically from a center of the nip formation pad  501  in the recording medium conveyance direction A 1 . Each of the radiation adjusters  600   c   1  and  600   c   2  is produced with a plurality of slits aligned in the axial direction of the fixing belt  21 , thus adjusting radiation time of light radiated from the halogen heaters  23  to the fixing belt  21  in the axial direction thereof. However, since the upstream projection  600   b   1  and the downstream projection  600   b   2  are not provided at an upstream end Z 1  and a downstream end Z 2  of the bases  600   a   1  and  600   a   2 , respectively, the bases  600   a   1  and  600   a   2  may be bent by pressure from the pressing roller  22 . 
     To address this problem, according to this exemplary embodiment shown in  FIG. 7 , the downstream projection  25   b   2  is spaced apart from the upstream projection  25   b   1  in the recording medium conveyance direction A 1 , not in contact with the upstream projection  25   b   1  unlike the configurations shown in  FIGS. 8 and 9 , thus enhancing the mechanical strength of the base  25   a  interposed between the upstream projection  25   b   1  and the downstream projection  25   b   2  in the recording medium conveyance direction A 1 . 
     Additionally, the upstream projection  25   b   1  and the downstream projection  25   b   2  are situated outboard from the upstream edge N 1  and the downstream edge N 2  of the fixing nip N, respectively, in the recording medium conveyance direction A 1 . Accordingly, the upstream projection  25   b   1  and the downstream projection  25   b   2  support the base  25   a  at both ends of the base  25   a  in the recording medium conveyance direction A 1  situated outboard from a center of the base  25   a  corresponding to the fixing nip N, where the base  25   a  receives pressure from the pressing roller  22 . According to this exemplary embodiment, the upstream projection  25   b   1  and the downstream projection  25   b   2  projecting from both ends of the base  25   a  in the recording medium conveyance direction A 1  enhance the mechanical strength of both ends of the base  25   a  in the recording medium conveyance direction A 1 , respectively. 
     According to this exemplary embodiment, unlike the configurations shown in  FIGS. 8 and 9 , even if the base  25   a  receives pressure from the pressing roller  22 , the base  25   a  is not bent at both ends thereof in the recording medium conveyance direction A 1 . Additionally, the upstream projection  25   b   1  and the downstream projection  25   b   2  enhance the mechanical strength of the base  25   a  at the center thereof interposed between the upstream projection  25   b   1  and the downstream projection  25   b   2  in the recording medium conveyance direction A 1 , thus enhancing the mechanical strength of the entire stay  25 . As a result, the stay  25  supports the nip formation assembly  24  properly, preventing bending of the nip formation assembly  24 . 
     It is to be noted that the upstream projection  25   b   1  and the downstream projection  25   b   2  project from the base  25   a  at least at portions thereof corresponding to or outboard from the upstream edge N 1  and the downstream edge N 2  of the fixing nip N, respectively. That is, the upstream projection  25   b   1  and the downstream projection  25   b   2  project from the base  25   a  at both edges of the center thereof where the base  25   a  receives pressure from the pressing roller  22  or at positions outboard from the center of the base  25   a  in the recording medium conveyance direction A 1 , thus enhancing the mechanical strength of the base  25   a  against pressure from the pressing roller  22 . Alternatively, the stay  25  may incorporate three or more projections projecting from the base  25   a  instead of the two projections, that is, the upstream projection  25   b   1  and the downstream projection  25   b   2 . 
     In order to enhance the mechanical strength of the stay  25  further, a front edge  25   c  of each of the upstream projection  25   b   1  and the downstream projection  25   b   2  is disposed as close as possible to the inner circumferential surface of the fixing belt  21 . 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. 7 , 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 may be 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. 
     In contrast to the stay  25 , the nip formation assembly  24  is compact, thus allowing the stay  25  to extend as long as possible 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. 7 , 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 the 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 . 
     Although the belt holder  40  serving as a guide that guides the rotating fixing belt  21  is interposed between the stay  25  and the fixing belt  21  at both lateral ends  21   b  of the fixing belt  21  as shown in  FIG. 6B , since no guide other than the nip formation assembly  24  is interposed between the stay  25  and the fixing belt  21 , the stay  25  is situated in proximity to the inner circumferential surface of the fixing belt  21 , thus attaining the enhanced mechanical strength of the stay  25 . 
     As shown in  FIG. 7 , the halogen heater  23  is interposed between the upstream projection  25   b   1  and the downstream projection  25   b   2  of the stay  25  or between an inner extension L of the upstream projection  25   b   1  and an inner extension L of the downstream projection  25   b   2  of the stay  25 . That is, the halogen heater  23  and the stay  25  are compacted inside the loop formed by the fixing belt  21 . Further, the halogen heater  23  is situated at a position corresponding to substantially a center of the fixing nip N in the recording medium conveyance direction A 1 . 
     Since the halogen heater  23  is partially or entirely housed by the stay  25 , the halogen heater  23  radiates light to a predetermined region on the inner circumferential surface of the fixing belt  21 . Generally, the temperature of the fixing belt  21  heated by the halogen heater  23  varies in a circumferential direction of the fixing belt  21 . For example, the temperature of a section of the fixing belt  21  situated in proximity to the halogen heater  23  is higher than the temperature of a section of the fixing belt  21  spaced apart from the halogen heater  23 . 
     To address this circumstance, according to this exemplary embodiment, the halogen heater  23  is housed by the stay  25  to concentrate light from the halogen heater  23  to the predetermined region on the inner circumferential surface of the fixing belt  21  where substantially an identical interval is provided between the halogen heater  23  and the inner circumferential surface of the fixing belt  21 . Thus, variation in temperature of the fixing belt  21  heated by the halogen heater  23  is minimized. Accordingly, the uniformly heated fixing belt  21  fixes the toner image T on the recording medium P, improving quality of the toner image T fixed on the recording medium P. 
     With reference to  FIG. 10 , a description is provided of a variation of the stay  25  depicted in  FIG. 7 . 
       FIG. 10  is a vertical sectional view of a fixing device  20 S incorporating a stay  25 S as a variation of the stay  25  depicted in  FIG. 7 . The stay  25  shown in  FIG. 7  includes the upstream projection  25   b   1  and the downstream projection  25   b   2  projecting substantially orthogonally from the base  25   a . Conversely, as shown in  FIG. 10 , the stay  25 S includes an upstream projection  25 Sb 1  disposed upstream from the base  25   a  in the recording medium conveyance direction A 1  and projecting from the base  25   a  toward the inner circumferential surface of the fixing belt  21  and a downstream projection  25 Sb 2  disposed downstream from the base  25   a  in the recording medium conveyance direction A 1  and projecting from the base  25   a  toward the inner circumferential surface of the fixing belt  21 . The upstream projection  25 Sb 1  and the downstream projection  25 Sb 2  are tilted with respect to the base  25   a . Alternatively, the stay  25 S may have other shapes. 
     With reference to  FIG. 11 , a description is provided of a configuration of a fixing device  20 T according to another exemplary embodiment. 
       FIG. 11  is a vertical sectional view of the fixing device  20 T. Unlike the fixing device  20  depicted in  FIG. 7 , the fixing device  20 T includes three halogen heaters  23  serving as heaters that heat the fixing belt  21 . 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 T further includes a metal plate  250  that partially surrounds the nip formation assembly  24 . Thus, a stay  25 T supports the nip formation assembly  24  via the metal plate  250 . 
     Instead of the bracket-shaped stay  25  shown in  FIG. 7 , the fixing device  20 T includes the substantially trapezoidal stay  25 T that houses the three halogen heaters  23 . For example, the stay  25 T is constructed of the base  25   a ; an upstream projection  25 Tb 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 Tb 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. 7 , the fixing device  20 T includes a reflector  26 T, shaped in accordance with the shape of the stay  25 T, mounted on the stay  25 T. 
     Similar to the heights h 1 , h 2 , and h 3  shown in  FIG. 7 , the heights h 1 , h 2 , and h 3  shown in  FIG. 11  define the height of the upstream portion  24   a  of the base pad  241 , the height of the downstream portion  24   b  of the base pad  241 , and the height of the center portion  24   c  of the base pad  241 , respectively. In order to increase the size of the stay  25 T, the height h 3  is not smaller than the height h 1  and the height h 2 . 
     With reference to  FIGS. 7 ,  10 , and  11 , a description is provided of advantages of the fixing devices  20 ,  20 S, and  20 T. 
     As shown in  FIGS. 7 ,  10 , and  11 , the downstream projection (e.g., the downstream projections  25   b   2 ,  25 Sb 2 , and  25 Tb 2 ) is spaced apart from the upstream projection (e.g., the upstream projections  25   b   1 ,  25 Sb 1 , and  25 Tb 1 ). The upstream projection is situated at a position on the base  25   a  corresponding to or upstream from the upstream edge N 1  of the fixing nip N in the recording medium conveyance direction A 1 . The downstream projection is situated at a position on the base  25   a  corresponding to or downstream from the downstream edge N 2  of the fixing nip N in the recording medium conveyance direction A 1 . Accordingly, the upstream projection and the downstream projection enhance the mechanical strength of the base  25   a  against pressure from the pressing roller  22  serving as an opposed rotary body. Consequently, the base  25   a  supports the nip formation assembly  24  properly, preventing the nip formation assembly  24  from being bent by pressure from the pressing roller  22 . As a result, the nip formation assembly  24  forms the uniform fixing nip N throughout the axial direction of the pressing roller  22 , which achieves uniform application of heat and pressure from the fixing belt  21  and the pressing roller  22  to the recording medium P, resulting in formation of a high quality toner image T on the recording medium P. 
     For example, it is difficult for the fixing belt  21  having a decreased loop diameter to accommodate a stay having an increased mechanical strength. However, the stay (e.g., the stays  25 ,  25 S, and  25 T) according to the exemplary embodiments described above has an increased mechanical strength that achieves the advantages described above. 
     Additionally, the front edge  25   c  of each of the upstream projection and the downstream projection of the stay is situated as close as possible to the inner circumferential surface of the fixing belt  21 , thus enhancing the mechanical strength of the stay. 
     Since the fixing belt  21  accommodates the compact nip formation assembly  24  and no guide interposed between the stay and the inner circumferential surface of the fixing belt  21 , increased space is allocated to the stay inside the loop formed by the fixing belt  21 . Accordingly, the stay has an increased size great enough to support the nip formation assembly  24  so as to prevent the nip formation assembly  24  from being bent by pressure from the pressing roller  22 . 
     The present invention is not limited to the details of the exemplary embodiments described above, and various modifications and improvements are possible. For example, as shown in  FIG. 3 , the image forming apparatus  1  incorporating the fixing device  20 ,  20 S, or  20 T 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.