Patent Publication Number: US-9405250-B2

Title: Fixing device capable of minimizing damage of endless rotary body and image forming apparatus incorporating same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a divisional of U.S. patent application Ser. No. 13/746,871 filed Jan. 22, 2013, which is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Applications No. 2012-026628 filed Feb. 9, 2012, and No. 2012-262077 filed on Nov. 30, 2012, in the Japanese Patent Office, the entire disclosures of each of which are 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 taken 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 reduce power consumption. 
     To address these requests, the fixing device may employ a thin endless belt having a decreased thermal capacity and therefore heated quickly by a heater. For example, a pressing roller is pressed against a nip formation assembly disposed inside a loop formed by the endless belt to form a fixing nip between the pressing roller and the endless belt. The heater disposed inside the loop formed by the endless belt heats the endless belt throughout the width in the axial direction thereof. As the pressing roller and the endless belt rotate and convey the recording medium bearing the toner image through the fixing nip, the endless belt and the pressing roller apply heat and pressure to the recording medium, thus fixing the toner image on the recording medium. Since the heater heats the endless belt directly, the endless belt is heated to a predetermined fixing temperature quickly, thus meeting the above-described requests of shortening the first print time and reducing power consumption. 
     As the recording medium bearing the toner image passes through the fixing nip, it travels over a center of the endless belt in the axial direction thereof. Accordingly, both lateral ends of the endless belt in the axial direction thereof where the recording medium does not travel are subject to damage, for example, thermal damage and mechanical damage. 
     For example, as the recording medium travels over the center of the endless belt in the axial direction thereof, it draws heat from the center of the endless belt. Conversely, at both lateral ends of the endless belt in the axial direction thereof where the recording medium does not travel, heat is not drawn therefrom to the recording medium. Accordingly, both lateral ends of the endless belt may overheat, resulting in thermal damage of the endless belt. 
     On the other hand, as the recording medium is discharged from the fixing nip, it may adhere to the endless belt and thereby may not be discharged from the fixing device smoothly. To address this problem, a separator may be disposed opposite the outer circumferential surface of the endless belt at each lateral end of the endless belt in the axial direction thereof. As the recording medium is discharged from the fixing nip, the separator comes into contact with the leading edge of the recording medium, separating the recording medium from the endless belt. However, if the recording medium is accidentally jammed between the endless belt and the separator, a user may pull the jammed recording medium upward to remove it from between the endless belt and the separator. Accordingly, the recording medium pulled upward lifts and spaces the separator apart from the endless belt. However, after the jammed recording medium is removed, the separator no longer lifted by the recording medium may fall and strike the endless belt by resilience of a spring anchored to the separator, thus mechanically deforming or damaging both lateral ends of the endless belt in the axial direction thereof. 
     SUMMARY OF THE INVENTION 
     This specification describes below an improved fixing device. In one exemplary embodiment of the present invention, the fixing device includes a hollow, endless rotary body rotatable in a predetermined direction of rotation and a pressing body contacting an outer circumferential surface of the endless rotary body to form a fixing nip therebetween through which a first size recording medium bearing a toner image and a second size recording medium bearing a toner image and being greater than the first size recording medium in width in an axial direction of the endless rotary body pass. The first size recording medium passes over a first passage region of the endless rotary body and the second size recording medium passes over a second passage region of the endless rotary body. At least one heater is disposed opposite an inner circumferential surface of the endless rotary body to heat the endless rotary body. A shield is interposed between the endless rotary body and the at least one heater to shield the endless rotary body from heat radiated from the at least one heater. The shield includes a notch disposed opposite a lateral end of the second passage region of the endless rotary body in the axial direction of the endless rotary body. The lateral end of the second passage region overlaps a non-passage region of the endless rotary body in the axial direction thereof where the first size recording medium does not pass. 
     This specification further describes an improved fixing device. In one exemplary embodiment of the present invention, the fixing device includes a hollow, endless rotary body rotatable in a predetermined direction of rotation and a heater disposed opposite an inner circumferential surface of the endless rotary body to heat the endless rotary body. The pressing body contacts an outer circumferential surface of the endless rotary body to form a fixing nip therebetween through which a recording medium bearing a toner image passes. A separator is disposed opposite the outer circumferential surface of the endless rotary body to contact and separate the recording medium discharged from the fixing nip from the endless rotary body. A belt holder contacts and supports each lateral end of the endless rotary body in an axial direction of the endless rotary body. The belt holder includes a base; a primary projection projecting from the base toward a center of the endless rotary body in the axial direction thereof; and a secondary projection projecting from a part of the primary projection toward the center of the endless rotary body in the axial direction thereof and disposed opposite the separator via the endless rotary body. 
     This specification further describes an improved image forming apparatus. In one exemplary embodiment of the present invention, the image forming apparatus includes an image carrier and an electrostatic latent image formation device disposed opposite the image carrier to emit light thereto to form an electrostatic latent image thereon. A development device is disposed opposite the image carrier to supply toner to the electrostatic latent image formed thereon to visualize the electrostatic latent image into a toner image. A transfer device is disposed opposite the image carrier to transfer the toner image formed thereon onto a recording medium. The image forming apparatus further includes the fixing device described above that is disposed downstream from the transfer device in a recording medium conveyance direction to fix the toner image on the recording medium. 
    
    
     
       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 schematic vertical sectional view of an image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 2  is a vertical sectional view of a fixing device according to a first exemplary embodiment of the present invention that is incorporated in the image forming apparatus shown in  FIG. 1 ; 
         FIG. 3A  is a partial perspective view of the fixing device shown in  FIG. 2  illustrating one lateral end of a fixing belt incorporated therein in an axial direction thereof; 
         FIG. 3B  is a partial plan view of the fixing device shown in  FIG. 3A ; 
         FIG. 3C  is a vertical sectional view of the fixing device shown in  FIG. 3A  illustrating one lateral end of the fixing belt in the axial direction thereof; 
         FIG. 4  is a block diagram of a controller incorporated in the image forming apparatus shown in  FIG. 1 ; 
         FIG. 5  is a partial plan view of one lateral end of the fixing belt in the axial direction thereof illustrating halogen heaters and a shield disposed opposite the fixing belt; 
         FIG. 6A  is a partial vertical sectional view of the fixing device shown in  FIG. 2  taken on the line A-A of  FIG. 3A  illustrating a heated region of the fixing belt heated by one of the halogen heaters shown in  FIG. 5 ; 
         FIG. 6B  is a partial vertical sectional view of the fixing device shown in  FIG. 2  taken on the line A-A of  FIG. 3A  illustrating another heated region of the fixing belt heated by another one of the halogen heaters shown in  FIG. 5 ; 
         FIG. 7  is a partial plan view of a fixing device according to a second exemplary embodiment of the present invention; 
         FIG. 8  is a vertical sectional view of a fixing device according to a third exemplary embodiment of the present invention; 
         FIG. 9  is an enlarged vertical sectional view of the fixing device shown in  FIG. 8  illustrating a fixing belt incorporated therein; 
         FIG. 10  is a partial perspective view of the fixing device shown in  FIG. 9  illustrating one lateral end thereof in an axial direction of the fixing belt; 
         FIG. 11A  is a perspective view of a support incorporated in the fixing device shown in  FIG. 9  seen from a heater adjacent thereto; 
         FIG. 11B  is a perspective view of the support shown in  FIG. 11A  seen from a nip formation assembly adjacent thereto; 
         FIG. 12  is a perspective view of a belt holder incorporated in the fixing device shown in  FIG. 9 ; 
         FIG. 13  is a plan view of the belt holder shown in  FIG. 12 ; 
         FIG. 14A  is a vertical sectional view of the fixing device shown in  FIG. 9  illustrating a recording medium jammed therein; 
         FIG. 14B  is a vertical sectional view of the fixing device shown in  FIG. 9  illustrating a separator incorporated therein that is spaced apart from the fixing belt; 
         FIG. 14C  is a vertical sectional view of the fixing device shown in  FIG. 9  illustrating the separator coming into contact with the fixing belt; and 
         FIG. 15  is a partial vertical sectional view of a fixing device according to a fourth 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. 1 , an image forming apparatus  1000  according to an exemplary embodiment of the present invention is explained. 
       FIG. 1  is a schematic vertical sectional view of the image forming apparatus  1000 . The image forming apparatus  1000  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  1000  is a tandem color laser printer that forms color and monochrome toner images on recording media P by electrophotography. 
     As shown in  FIG. 1 , the image forming apparatus  1000  includes a body  2  that houses an image forming station  1  situated at a center portion thereof and incorporating four image forming devices  2 Y,  2 C,  2 M, and  2 K serving as four process units that form yellow, cyan, magenta, and black toner images, respectively. The image forming devices  2 Y,  2 C,  2 M, and  2 K are aligned along a rotation direction R 1  of an endless intermediate transfer belt  11  serving as an intermediate transferor. Although the image forming devices  2 Y,  2 C,  2 M, and  2 K contain yellow, cyan, magenta, and black developers (e.g., toners) that form yellow, cyan, magenta, and black toner images, respectively, resulting in a color toner image, they have an identical structure. 
     The image forming devices  2 Y,  2 C,  2 M, and  2 K include photoconductive drums  20 Y,  20 C,  20 M, and  20 K aligned in the rotation direction R 1  of the intermediate transfer belt  11  and serving as a plurality of image carriers that carries the yellow, cyan, magenta, and black toner images, respectively. The visible yellow, cyan, magenta, and black toner images formed on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K are primarily transferred onto the intermediate transfer belt  11  that slides over the photoconductive drums  20 Y,  20 C,  20 M, and  20 K as it rotates in the rotation direction R 1  in a primary transfer process in such a manner that the yellow, cyan, magenta, and black toner images are superimposed on a same position on the intermediate transfer belt  11 . Thereafter, the yellow, cyan, magenta, and black toner images superimposed on the intermediate transfer belt  11  are secondarily transferred onto a recording medium P (e.g., a sheet) collectively in a secondary transfer process. 
     The photoconductive drums  20 Y,  20 C,  20 M, and  20 K are surrounded by various devices used to form the yellow, cyan, magenta, and black toner images on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K rotating clockwise in  FIG. 1  in a rotation direction R 2 . Taking the photoconductive drum  20 K used to form a black toner image as an example, the photoconductive drum  20 K is surrounded by a charger  30 K, a development device  40 K, a primary transfer roller  12 K serving as a primary transferor, and a cleaner  50 K incorporating a cleaning blade, which are arranged in the rotation direction R 2  of the photoconductive drum  20 K. For example, the photoconductive drum  20 K is a tube having a surface photoconductive layer including an inorganic or organic photoreceptor. The charger  30 K, disposed in close proximity to the photoconductive drum  20 K, charges the photoconductive drum  20 K by electric discharge therebetween. After the charger  30 K charges an outer circumferential surface of the photoconductive drum  20 K, an optical writer  8 , serving as an exposure device or an electrostatic latent image formation device, exposes the charged outer circumferential surface of the photoconductive drum  20 K, writing an electrostatic latent image thereon. 
     For example, the optical writer  8  is constructed of a semiconductor laser serving as a light source, a coupling lens, an f-θ lens, a troidal lens, reflection mirrors, and a rotatable polygon mirror serving as an optical deflector. The optical writer  8  emits laser beams Lb onto the outer circumferential surface of the respective photoconductive drums  20 Y,  20 C,  20 M, and  20 K according to image data sent from an external device such as a client computer, thus forming electrostatic latent images on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K, respectively. 
     Each of the development devices  40 Y,  40 C,  40 M, and  40 K, detachably attached to the image forming devices  2 Y,  2 C,  2 M, and  2 K, is constructed of a toner supply portion and a development portion. The toner supply portion supplies toner to the development portion that supplies the toner to the electrostatic latent image formed on the respective photoconductive drums  20 Y,  20 C,  20 M, and  20 K. 
     As the intermediate transfer belt  11  rotates in the rotation direction R 1 , the yellow, cyan, magenta, and black toner images formed on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K are primarily transferred onto the intermediate transfer belt  11  in such a manner that the yellow, cyan, magenta, and black toner images are superimposed on the same position on the intermediate transfer belt  11 . For example, the photoconductive drums  20 Y,  20 C,  20 M, and  20 K are disposed opposite primary transfer rollers  12 Y,  12 C,  12 M, and  12 K, serving as primary transferors, respectively, via the intermediate transfer belt  11 . As a primary transfer bias is applied to the primary transfer rollers  12 Y,  12 C,  12 M, and  12 K, the yellow, cyan, magenta, and black toner images formed on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K are primarily transferred onto the intermediate transfer belt  11  successively at different times from the upstream photoconductive drum  20 Y to the downstream photoconductive drum  20 K in the rotation direction R 1  of the intermediate transfer belt  11 . 
     The primary transfer rollers  12 Y,  12 C,  12 M, and  12 K sandwich the intermediate transfer belt  11  together with the photoconductive drums  20 Y,  20 C,  20 M, and  20 K, forming primary transfer nips between the intermediate transfer belt  11  and the photoconductive drums  20 Y,  20 C,  20 M, and  20 K. A power supply connected to the primary transfer rollers  12 Y,  12 C,  12 M, and  12 K applies a primary transfer bias, that is, a predetermined direct current voltage and/or an alternating current voltage, to the primary transfer rollers  12 Y,  12 C,  12 M, and  12 K. 
     After the primary transfer of the yellow, cyan, magenta, and black toner images from the photoconductive drums  20 Y,  20 C,  20 M, and  20 K, the cleaners  50 Y,  50 C,  50 M, and  50 K, each of which is constructed of an elastic rubber band and a toner removal brush, remove residual toner failed to be transferred onto the intermediate transfer belt  11  therefrom. 
     The photoconductive drums  20 Y,  20 C,  20 M, and  20 K are aligned in this order in the rotation direction R 1  of the intermediate transfer belt  11 . As described above, the photoconductive drums  20 Y,  20 C,  20 M, and  20 K are incorporated in the four image forming devices  2 Y,  2 C,  2 M, and  2 K that form yellow, cyan, magenta, and black toner images, respectively. 
     Above the photoconductive drums  20 Y,  20 C,  20 M, and  20 K are a transfer belt unit  10 , a secondary transfer roller  5  serving as a secondary transferor, and a transfer belt cleaner  13 . Below the photoconductive drums  20 Y,  20 C,  20 M, and  20 K is the optical writer  8  described above. 
     In addition to the endless intermediate transfer belt  11  and the plurality of primary transfer rollers  12 Y,  12 C,  12 M, and  12 K, the transfer belt unit  10  further includes a driving roller  72  and a driven roller  73  that support the intermediate transfer belt  11  looped thereover. As a driver drives and rotates the driving roller  72  counterclockwise in  FIG. 1 , the driving roller  72  rotates the intermediate transfer belt  11  in the rotation direction R 1  by friction therebetween. The driving roller  72  also serves as a secondary transfer backup roller disposed opposite the secondary transfer roller  5  via the intermediate transfer belt  11 . Similarly, the driven roller  73  also serves as a cleaning backup roller disposed opposite the belt cleaner  13  via the intermediate transfer belt  11 . The driven roller  73  is attached with a biasing member such as a spring that presses the driven roller  73  against the belt cleaner  13  via the intermediate transfer belt  11 . Thus, the driven roller  73  also stretches the intermediate transfer belt  11 . The transfer belt unit  10 , the primary transfer rollers  12 Y,  12 C,  12 M, and  12 K, the secondary transfer roller  5 , and the belt cleaner  13  constitute a transfer device  71 . 
     The secondary transfer roller  5  contacting the intermediate transfer belt  11  rotates in accordance with rotation of the intermediate transfer belt  11  by friction therebetween. The secondary transfer roller  5  sandwiches the intermediate transfer belt  11  together with the driving roller  72  to form a secondary transfer nip between the secondary transfer roller  5  and the intermediate transfer belt  11 . Similar to the primary transfer rollers  12 Y,  12 C,  12 M, and  12 K, the secondary transfer roller  5  is connected to the power supply that applies a secondary transfer bias, that is, a predetermined direct current voltage and/or alternating current voltage thereto. 
     The belt cleaner  13 , interposed between the secondary transfer nip and the image forming device  2 Y in the rotation direction R 1  of the intermediate transfer belt  11 , is disposed opposite the driven roller  73  via the intermediate transfer belt  11  and cleans an outer circumferential surface of the intermediate transfer belt  11 . The belt cleaner  13  includes a cleaning brush and a cleaning blade that contact the outer circumferential surface of the intermediate transfer belt  11  to remove residual toner from the intermediate transfer belt  11 . A waste toner conveyance tube extending from the belt cleaner  13  to an inlet of a waste toner container conveys waste toner collected from the intermediate transfer belt  11  by the belt cleaner  13  to the waste toner container. 
     Below the transfer device  71  are a paper tray  61 , a registration roller pair  4 , and a recording medium sensor. The paper tray  61  loads a plurality of recording media P. The registration roller pair  4  feeds a recording medium P sent from the paper tray  61  to the secondary transfer nip. The recording medium sensor detects a leading edge of the recording medium P. For example, the paper tray  61  is situated in a lower portion of the image forming apparatus  1000  and is attached with a feed roller  3  that picks up and feeds an uppermost recording medium P of the plurality of recording media P loaded in the paper tray  61 . As the feed roller  3  is driven and rotated counterclockwise in  FIG. 1 , the feed roller  3  feeds the uppermost recording medium P toward the registration roller pair  4 . 
     A conveyance path R extends from the feed roller  3  to an output roller pair  7  to convey the recording medium P picked up from the paper tray  61  onto an outside of the image forming apparatus  1000  through the secondary transfer nip. The conveyance path R is provided with the registration roller pair  4  situated upstream from the secondary transfer nip formed between the secondary transfer roller  5  and the intermediate transfer belt  11  in a recording medium conveyance direction A 1  to feed the recording medium P to the secondary transfer nip. For example, the registration roller pair  4  feeds the recording medium P conveyed from the paper tray  61  to the secondary transfer nip at a proper time when the color toner image formed on the intermediate transfer belt  11  by the image forming station  1  as described above reaches the secondary transfer nip. Specifically, when a predetermined time elapses after the recording medium sensor, interposed between the feed roller  3  and the registration roller pair  4 , detects the leading edge of the recording medium P conveyed from the feed roller  3 , the recording medium P is temporarily halted by the registration roller pair  4  as it strikes the registration roller pair  4 . Then, the registration roller pair  4  resumes its rotation at a predetermined time to feed the recording medium P to the secondary transfer nip, for example, at a time when the color toner image formed on the intermediate transfer belt  11  reaches the secondary transfer nip. 
     The recording media P may be thick paper, postcards, envelopes, plain paper, thin paper, coated paper, art paper, tracing paper, OHP (overhead projector) transparencies, recording sheets, and the like. In addition to the paper tray  61 , the image forming apparatus  1000  may be equipped with a bypass tray that loads thick paper, postcards, envelopes, thin paper, tracing paper, OHP transparencies, and the like. 
     Downstream from the secondary transfer nip in the recording medium conveyance direction A 1  are a fixing device  100 , the output roller pair  7 , and an output tray  17 . The fixing device  100  fixes the color toner image transferred from the intermediate transfer belt  11  onto the recording medium P thereon. The output roller pair  7  discharges the recording medium P bearing the fixed color toner image onto the outside of the image forming apparatus  1000 , that is, the output tray  17 . The output tray  17 , disposed atop the image forming apparatus  1000 , stocks the recording medium P discharged by the output roller pair  7 . 
     A plurality of toner bottles  9 Y,  9 C,  9 M, and  9 K containing yellow, cyan, magenta, and black toners is detachably attached to a plurality of toner bottle holders, respectively, disposed in an upper portion of the image forming apparatus  1000  situated below the output tray  17 . A toner supply tube is interposed between the toner bottles  9 Y,  9 C,  9 M, and  9 K and the development devices  40 Y,  40 C,  40 M, and  40 K, respectively, thus supplying the yellow, cyan, magenta, and black toners from the toner bottles  9 Y,  9 C,  9 M, and  9 K to the development devices  40 Y,  40 C,  40 M, and  40 K. 
     As described above, the belt cleaner  13  of the transfer device  71  includes the cleaning brush and the cleaning blade that contact the outer circumferential surface of the intermediate transfer belt  11 . The cleaning brush and the cleaning blade scrape and remove a foreign substance such as residual toner off the intermediate transfer belt  11 , thus cleaning the intermediate transfer belt  11 . The belt cleaner  13  includes a waste toner discharger that discharges the residual toner collected from the intermediate transfer belt  11  into the waste toner conveyance tube described above. 
     With reference to  FIG. 1 , a description is provided of an image forming operation of the image forming apparatus  1000  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 photoconductive drums  20 Y,  20 C,  20 M, and  20 K of the image forming devices  2 Y,  2 C,  2 M, and  2 K, respectively, clockwise in  FIG. 1  in the rotation direction R 2 . The chargers  30 Y,  30 C,  30 M, and  30 K uniformly charge the outer circumferential surface of the respective photoconductive drums  20 Y,  20 C,  20 M, and  20 K at a predetermined polarity. The optical writer  8  emits laser beams Lb onto the charged outer circumferential surface of the respective photoconductive drums  20 Y,  20 C,  20 M, and  20 K according to yellow, cyan, magenta, and black image data contained in image data sent from the external device, respectively, thus forming electrostatic latent images thereon. The development devices  40 Y,  40 C,  40 M, and  40 K supply yellow, cyan, magenta, and black toners to the electrostatic latent images formed on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K, visualizing the electrostatic latent images into yellow, cyan, magenta, and black toner images, respectively. 
     Simultaneously, as the print job starts, the driving roller  72  is driven and rotated counterclockwise in  FIG. 1 , rotating the intermediate transfer belt  11  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  12 Y,  12 C,  12 M, and  12 K. Thus, a predetermined transfer electric field is created at the primary transfer nips formed between the primary transfer rollers  12 Y,  12 C,  12 M, and  12 K and the photoconductive drums  20 Y,  20 C,  20 M, and  20 K, respectively. 
     When the yellow, cyan, magenta, and black toner images formed on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K reach the primary transfer nips, respectively, in accordance with rotation of the photoconductive drums  20 Y,  20 C,  20 M, and  20 K, the yellow, cyan, magenta, and black toner images are primarily transferred from the photoconductive drums  20 Y,  20 C,  20 M, and  20 K onto the intermediate transfer belt  11  by the transfer electric field created at the primary transfer nips in such a manner that the yellow, cyan, magenta, and black toner images are superimposed successively on a same position on the intermediate transfer belt  11 . Thus, a color toner image is formed on the intermediate transfer belt  11 . After the primary transfer of the yellow, cyan, magenta, and black toner images from the photoconductive drums  20 Y,  20 C,  20 M, and  20 K onto the intermediate transfer belt  11 , the cleaners  50 Y,  50 C,  50 M, and  50 K remove residual toner failed to be transferred onto the intermediate transfer belt  11  and therefore remaining on the photoconductive drums  20 Y,  20 C,  20 M, and  20 K therefrom. Thereafter, dischargers discharge the outer circumferential surface of the respective photoconductive drums  20 Y,  20 C,  20 M, and  20 K, initializing the surface potential thereof for a next image forming operation. 
     On the other hand, the feed roller  3  disposed in the lower portion of the image forming apparatus  1000  is driven and rotated to feed a recording medium P from the paper tray  61  toward the registration roller pair  4  in the conveyance path R. The registration roller pair  4  feeds the recording medium P to the secondary transfer nip formed between the secondary transfer roller  5  and the intermediate transfer belt  11  at a time when the color toner image formed on the intermediate transfer belt  11  reaches the secondary transfer nip. The secondary transfer roller  5  is applied with a transfer voltage having a polarity opposite a polarity of the charged yellow, cyan, magenta, and black toners constituting the color toner image formed on the intermediate transfer belt  11 , thus creating a predetermined transfer electric field at the secondary transfer nip. 
     When the color toner image formed on the intermediate transfer belt  11  reaches the secondary transfer nip in accordance with rotation of the intermediate transfer belt  11 , the color toner image is secondarily transferred from the intermediate transfer belt  11  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  11  onto the recording medium P, the belt cleaner  13  removes residual toner failed to be transferred onto the recording medium P and therefore remaining on the intermediate transfer belt  11  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  100  where the color toner image is fixed on the recording medium P. Then, the recording medium P bearing the fixed color toner image is discharged by the output roller pair  7  onto the output tray  17 . 
     The above describes the image forming operation of the image forming apparatus  1000  to form the color toner image on the recording medium P. Alternatively, the image forming apparatus  1000  may form a monochrome toner image by using any one of the four image forming devices  2 Y,  2 C,  2 M, and  2 K or may form a bicolor or tricolor toner image by using two or three of the image forming devices  2 Y,  2 C,  2 M, and  2 K. 
     With reference to  FIG. 2 , a description is provided of a construction of the fixing device  100  incorporated in the image forming apparatus  1000  described above. 
       FIG. 2  is a vertical sectional view of the fixing device  100  according to a first exemplary embodiment. As shown in  FIG. 2 , the fixing device  100  (e.g., a fuser) includes a fixing belt  121  serving as an endless rotary body, a heating rotary body, or a fixing rotary body, that is, an endless belt formed into a loop and rotatable in a rotation direction R 3 ; a pressing roller  122  serving as a pressing body or an opposed rotary body disposed opposite an outer circumferential surface of the fixing belt  121  to form a fixing nip N therebetween and rotatable in a rotation direction R 4  counter to the rotation direction R 3  of the fixing belt  121 ; and a halogen heater set  123  serving as a heater disposed inside the loop formed by the fixing belt  121  and heating the fixing belt  121 . 
     A detailed description is now given of a construction of the halogen heater set  123 . 
     The halogen heater set  123  radiates light, that is, radiation heat, to the fixing belt  121 , thus heating the fixing belt  121  directly. The halogen heater set  123  includes three halogen heaters  123 A,  123 B, and  123 C disposed inside the loop formed by the fixing belt  121  such that they are disposed opposite an inner circumferential surface of the fixing belt  121 . The halogen heaters  123 A,  123 B, and  123 C serve as heaters or heat sources that have three different heating regions thereof in an axial direction of the fixing belt  121  that generate heat, respectively. Accordingly, the three halogen heaters  123 A,  123 B, and  123 C heat the fixing belt  121  in three different regions on the fixing belt  121 , respectively, in the axial direction thereof so that the fixing belt  121  heats recording media P of various widths in the axial direction of the fixing belt  121 . 
     For example, the halogen heater  123 A serves as a third heater or a center heater that heats a center of the fixing belt  121  in the axial direction thereof where a small recording medium P is conveyed. The center of the fixing belt  121  has a width in the axial direction thereof that is equivalent to a width of a letter size recording medium P in portrait orientation. The halogen heater  123 B serves as a first heater or a first lateral end heater that heats each lateral end of the fixing belt  121  in the axial direction thereof where each lateral end of a medium recording medium P in the axial direction of the fixing belt  121  is conveyed. The medium recording medium P is a double letter size recording medium P having a width in portrait orientation greater than that of the letter size recording medium P in the axial direction of the fixing belt  121 . The halogen heater  123 C serves as a second heater or a second lateral end heater that heats each lateral end of the fixing belt  121  in the axial direction thereof where each lateral end of a large recording medium P in the axial direction of the fixing belt  121  is conveyed. The large recording medium P is an A3 size recording medium P having a width in portrait orientation greater than that of the double letter size recording medium P. 
     While a small recording medium P having a width in portrait orientation equivalent to or smaller than that of a letter size recording medium P, that is, a letter size recording medium P or smaller, is conveyed through the fixing nip N formed between the fixing belt  121  and the pressing roller  122 , the halogen heater  123 A is turned on but the halogen heaters  123 B and  123 C are turned off. While a medium recording medium P in portrait orientation, that is, a double letter size recording medium P, is conveyed through the fixing nip N, the halogen heaters  123 A and  123 B are turned on. While a large recording medium P in portrait orientation, that is, an A3 size recording medium P, is conveyed through the fixing nip N, the halogen heaters  123 A and  123 C are turned on. 
     As shown in  FIG. 2 , the halogen heaters  123 A,  123 B, and  123 C are situated inside the loop formed by the fixing belt  121  in such a manner that three axes  123 Ax,  123 Bx, and  123 Cx of the three halogen heaters  123 A,  123 B, and  123 C constitute three vertices of a triangle Ta in cross-section, respectively. The halogen heater  123 C is situated closer to a nip formation assembly  124  producing the fixing nip N than the halogen heaters  123 A and  123 B are. That is, the halogen heater  123 C is interposed between the halogen heaters  123 A and  123 B and the nip formation assembly  124  in a diametrical direction of the fixing belt  121 . This is because the fixing device  100  is designed for letter size recording media P and double letter size recording media P rather than for A3 size recording media P. Since letter size recording media P and double letter size recording media P are used more frequently than A3 size recording media P, the halogen heaters  123 A and  123 B configured to heat the letter size recording media P and the double letter size recording media P are disposed closer to the inner circumferential surface of the fixing belt  121  than the halogen heater  123 C configured to heat the A3 size recording media P, thus heating the letter size recording media P and the double letter size recording media P through the fixing belt  121  efficiently. 
     It is to be noted that since the width of a double letter size recording medium P in portrait orientation is equivalent to the width of a letter size recording medium P in landscape orientation in the axial direction of the fixing belt  121  orthogonal to the recording medium conveyance direction A 1 , the halogen heaters  123 A and  123 B are turned on to heat the letter size recording medium P in landscape orientation. Similarly, since the width of an A3 size recording medium P in portrait orientation is equivalent to the width of an A4 size recording medium P in landscape orientation in the axial direction of the fixing belt  121 , the halogen heaters  123 A and  123 C are turned on to heat the A4 size recording medium P in landscape orientation. 
     The portrait orientation defines an orientation in which the long side of the recording medium P is parallel to the recording medium conveyance direction A 1 . Conversely, the landscape orientation defines an orientation in which the short side of the recording medium P is parallel to the recording medium conveyance direction A 1 . 
     As shown in  FIG. 2 , the fixing device  100  further includes the nip formation assembly  124  pressing against the pressing roller  122  via the fixing belt  121  to form the fixing nip N between the fixing belt  121  and the pressing roller  122 ; a metal plate  132  partially surrounding the nip formation assembly  124 ; a stay  125  serving as a support that supports the nip formation assembly  124  via the metal plate  132 ; and a reflector  126  that reflects light radiated from the halogen heater set  123  thereto toward the fixing belt  121 . 
     The fixing device  100  further includes a temperature sensor  127  serving as a temperature detector disposed opposite the outer circumferential surface of the fixing belt  121  and detecting the temperature of the fixing belt  121 ; a separator  128  disposed opposite the outer circumferential surface of the fixing belt  121  and separating the recording medium P from the fixing belt  121 ; and a pressurization assembly that presses the pressing roller  122  against the nip formation assembly  124  via the fixing belt  121 . 
     The fixing belt  121  is heated directly by light radiated from the halogen heater set  123  disposed opposite the inner circumferential surface of the fixing belt  121 . The nip formation assembly  124  is disposed opposite the inner circumferential surface of the fixing belt  121 . As the fixing belt  121  rotates in the rotation direction R 3 , the inner circumferential surface of the fixing belt  121  slides over the nip formation assembly  124 . 
     As shown in  FIG. 2 , the nip formation assembly  124  has an opposed face  124   a  disposed opposite the fixing belt  121  at the fixing nip N and linearly extending in the recording medium conveyance direction A 1  to produce the planar fixing nip N. Alternatively, the opposed face  124   a  of the nip formation assembly  124  may be concave with respect to the fixing belt  121  or have other shapes. If the concave opposed face  124   a  of the nip formation assembly  124  produces the concave fixing nip N, the concave fixing nip N directs a leading edge of a recording medium P toward the pressing roller  122  as the recording medium P is discharged from the fixing nip N, thus facilitating separation of the recording medium P from the fixing belt  121  and thereby minimizing jamming of the recording medium P. 
     A detailed description is now given of a construction of the fixing belt  121 . 
     The fixing belt  121  is a thin, flexible endless belt or film. For example, the fixing belt  121  is constructed of a base layer constituting the inner circumferential surface of the fixing belt  121  and a release layer constituting the outer circumferential surface of the fixing belt  121 . 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. The release layer prevents adhesion of toner from the recording medium P to the fixing belt  121 . 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. As the fixing belt  121  and the pressing roller  122  exert pressure to a toner image T on a recording medium P, the elastic layer of the pressing roller  122  prevents slight surface asperities of the fixing belt  121  from being transferred onto the toner image T on the recording medium P, thus minimizing variation in gloss of the solid toner image T, that is, minimizing formation of an orange peel image. It is preferable that the elastic layer of the pressing roller  122  has a thickness not smaller than about 100 micrometers, for example, to prevent formation of an orange peel image effectively. As the elastic layer of the pressing roller  122  is deformed by pressure between the pressing roller  122  and the fixing belt  121 , the elastic layer absorbs slight surface asperities of the fixing belt  121 , preventing formation of an orange peel image. 
     A detailed description is now given of a construction of the pressing roller  122 . 
     The pressing roller  122  is constructed of a metal core  122   a ; an elastic layer  122   b  coating the metal core  122   a  and made of silicone rubber foam, silicone rubber, fluoro rubber, or the like; and a release layer  122   c  coating the elastic layer  122   b  and made of PFA, PTFE, or the like. The pressurization assembly including a spring presses the pressing roller  122  against the nip formation assembly  124  via the fixing belt  121 . Thus, the pressing roller  122  pressingly contacting the fixing belt  121  deforms the elastic layer  122   b  of the pressing roller  122  at the fixing nip N formed between the pressing roller  122  and the fixing belt  121 , 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  1000  depicted in  FIG. 1  drives and rotates the pressing roller  122  through a gear train. As the driver drives and rotates the pressing roller  122 , a driving force of the driver is transmitted from the pressing roller  122  to the fixing belt  121  at the fixing nip N, thus rotating the fixing belt  121  by friction between the pressing roller  122  and the fixing belt  121 . 
     The fixing belt  121  rotates in accordance with rotation of the pressing roller  122 . For example, as described above, as the driver such as the motor drives and rotates the pressing roller  122  in the rotation direction R 4 , a driving force of the driver is transmitted from the pressing roller  122  to the fixing belt  121  at the fixing nip N, thus rotating the fixing belt  121  by friction between the pressing roller  122  and the fixing belt  121 . At the fixing nip N, the fixing belt  121  is nipped between the pressing roller  122  and the nip formation assembly  124  and is rotated by friction with the pressing roller  122 . Conversely, at a position other than the fixing nip N, the fixing belt  121  is rotated while guided by a belt holder  140  described below at both lateral ends of the fixing belt  121  in the axial direction thereof. 
     According to this exemplary embodiment, the pressing roller  122  is a solid roller. Alternatively, the pressing roller  122  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  122  does not incorporate the elastic layer  122   b , the pressing roller  122  has a decreased thermal capacity that improves fixing performance of being heated to a predetermined fixing temperature quickly. However, as the pressing roller  122  and the fixing belt  121  sandwich and press the toner image T on the recording medium P passing through the fixing nip N, slight surface asperities of the fixing belt  121  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  122  incorporates the elastic layer  122   b  having a thickness not smaller than about 100 micrometers. The elastic layer  122   b  having the thickness not smaller than about 100 micrometers elastically deforms to absorb slight surface asperities of the fixing belt  121 , preventing variation in gloss of the toner image T on the recording medium P. 
     The elastic layer  122   b  of the pressing roller  122  is made of solid rubber. Alternatively, if no heater is disposed inside the pressing roller  122 , the elastic layer  122   b  may be made of insulative rubber, such as sponge rubber. The insulative rubber such as sponge rubber is more preferable than the solid rubber because it has an increased insulation that draws less heat from the fixing belt  121 . According to this exemplary embodiment, the pressing roller  122  is pressed against the fixing belt  121 . Alternatively, the pressing roller  122  may merely contact the fixing belt  121  with no pressure therebetween. 
     A detailed description is now given of a configuration of the halogen heater set  123 . 
     Both lateral ends of the halogen heater set  123  in a longitudinal direction thereof parallel to the axial direction of the fixing belt  121  are mounted on side plates of the fixing device  100 , respectively. A power supply situated inside the image forming apparatus  1000  supplies power to the halogen heater set  123  so that the halogen heater set  123  heats the fixing belt  121 . A controller  200 , 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 set  123  and the temperature sensor  127  controls the halogen heater set  123 , that is, turns on and off the halogen heater set  123  or adjusts an amount of power supplied to the halogen heater set  123  based on the temperature of the fixing belt  121  detected by the temperature sensor  127  so as to adjust the temperature of the fixing belt  121  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  121  instead of the halogen heater set  123 . 
     A detailed description is now given of a construction of the nip formation assembly  124 . 
     The nip formation assembly  124  includes a base pad  131  and a slide sheet  130  (e.g., a low friction sheet) covering an outer surface of the base pad  131 . A longitudinal direction of the base pad  131  in which it extends is parallel to the axial direction of the fixing belt  121  or the pressing roller  122 . The base pad  131  receives pressure from the pressing roller  122  to define the shape of the fixing nip N. 
     The base pad  131  of the nip formation assembly  124  is mounted on and supported by the stay  125 . Accordingly, even if the base pad  131  receives pressure from the pressing roller  122 , the base pad  131  is not bent by the pressure and therefore produces a uniform nip width throughout the entire width of the pressing roller  122  in the axial direction thereof. The base pad  131  is made of a heat-resistant material having heat resistance against temperatures up to about 200 degrees centigrade. Accordingly, even if the base pad  131  is heated to a predetermined fixing temperature range, the base pad  131  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  131  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  130  is interposed at least between the base pad  131  and the fixing belt  121 . For example, the slide sheet  130  covers at least the opposed face  124   a  of the base pad  131  disposed opposite the fixing belt  121  at the fixing nip N. As the fixing belt  121  rotates in the rotation direction R 3 , it slides over the slide sheet  130 , decreasing a driving torque exerted on the fixing belt  121 . Accordingly, a decreased friction is imposed onto the fixing belt  121  from the nip formation assembly  124 . According to this exemplary embodiment, the fixing belt  121  slides over the base pad  131  indirectly via the slide sheet  130 . Alternatively, the nip formation assembly  124  may not incorporate the slide sheet  130  so that the fixing belt  121  slides over the base pad  131  directly. 
     The stay  125  is made of metal having an increased mechanical strength, such as stainless steel and iron, to support the nip formation assembly  124  against pressure from the pressing roller  122 , preventing bending of the nip formation assembly  124 . The base pad  131  is also made of a rigid material having an increased mechanical strength. For example, the base pad  131  is made of resin such as LCP, metal, ceramic, or the like. 
     A detailed description is now given of a configuration of the reflector  126 . 
     The reflector  126  is interposed between the stay  125  and the halogen heater set  123 . According to this exemplary embodiment, the reflector  126  is mounted on the stay  125 . For example, the reflector  126  is made of aluminum, stainless steel, or the like. The reflector  126  has a reflection face  126   a  that reflects light, that is, radiation heat, radiated from the halogen heater set  123  thereto toward the fixing belt  121 . Accordingly, the fixing belt  121  receives an increased amount of light from the halogen heater set  123  and thereby is heated efficiently. Additionally, the reflector  126  minimizes transmission of light from the halogen heater set  123  to the stay  125 , thus minimizing energy wasted in unnecessarily heating the stay  125  by light from the halogen heater set  123  and thereby saving energy. Instead of mounting the reflector  126 , a surface of the stay  125  may be treated with insulation or mirror finished to attain the advantages described above. 
     The fixing device  100  according to this exemplary embodiment attains various improvements to save more energy and shorten a first print time taken to output a recording medium P bearing a fixed toner image T onto the outside of the image forming apparatus  1000  depicted in  FIG. 1  after the image forming apparatus  1000  receives a print job. As a first improvement, the fixing device  100  employs a direct heating method in which the halogen heater set  123  directly heats the fixing belt  121  at a portion thereof other than a nip portion thereof facing the fixing nip N. For example, as shown in  FIG. 2 , no component is interposed between the halogen heater set  123  and the fixing belt  121  at an outward portion of the fixing belt  121  disposed opposite the temperature sensor  127 . Accordingly, light from the halogen heater set  123  is directly transmitted to the fixing belt  121  at the outward portion thereof. 
     As a second improvement, the fixing belt  121  is designed to be thin and have a reduced loop diameter so as to decrease the thermal capacity thereof. For example, the fixing belt  121  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  121  has a total thickness not greater than about 1 mm. The loop diameter of the fixing belt  121  is in a range of from about 20 mm to about 40 mm. In order to decrease the thermal capacity of the fixing belt  121  further, the fixing belt  121  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  121  may be not greater than about 30 mm. 
     According to this exemplary embodiment, the pressing roller  122  has a diameter in a range of from about 20 mm to about 40 mm so that the loop diameter of the fixing belt  121  is equivalent to the diameter of the pressing roller  122 . However, the loop diameter of the fixing belt  121  and the diameter of the pressing roller  122  are not limited to the above. For example, the loop diameter of the fixing belt  121  may be smaller than the diameter of the pressing roller  122 . In this case, a curvature of the fixing belt  121  at the fixing nip N is greater than that of the pressing roller  122 , facilitating separation of the recording medium P discharged from the fixing nip N from the fixing belt  121 . 
     Since the fixing belt  121  has a reduced loop diameter, space inside the loop formed by the fixing belt  121  is small. To address this circumstance, both ends of the stay  125  in the recording medium conveyance direction A 1  are folded into a square bracket that accommodates the halogen heater set  123 . Thus, the stay  125  and the halogen heater set  123  are placed in the small space inside the loop formed by the fixing belt  121 . 
     With reference to  FIGS. 3A, 3B, and 3C , a description is provided of a configuration of a lateral end of the fixing belt  121  in the axial direction thereof. 
       FIG. 3A  is a partial perspective view of one lateral end of the fixing belt  121  in the axial direction thereof.  FIG. 3B  is a partial plan view of one lateral end of the fixing belt  121  in the axial direction thereof parallel to a width direction of a recording medium P.  FIG. 3C  is a vertical sectional view of one lateral end of the fixing belt  121  in the axial direction thereof. Although not shown, another lateral end of the fixing belt  121  in the axial direction thereof has the identical configuration shown in  FIGS. 3A to 3C . Hence, the following describes the configuration of one lateral end of the fixing belt  121  in the axial direction thereof with reference to  FIGS. 3A to 3C . 
     As shown in  FIGS. 3A and 3B , the belt holder  140  is inserted into the loop formed by the fixing belt  121  at each lateral end of the fixing belt  121  in the axial direction thereof orthogonal to a circumferential direction thereof to rotatably support the fixing belt  121 . As shown in  FIG. 3C , the belt holder  140  is a flange that is C-shaped in cross-section to create an opening disposed opposite the fixing nip N where the nip formation assembly  124  is situated. As shown in  FIG. 3A , the belt holder  140  is mounted on a side plate  142 . Each lateral end of the stay  125  in a longitudinal direction thereof is also mounted on and positioned by the side plate  142 . Like the stay  125 , the side plate  142  is made of metal such as stainless steel and iron. Since the side plate  142  and the stay  125  are made of the common material, the stay  125  is mounted on the side plate  142  precisely. 
     As shown in  FIG. 3B , a shield  133  (e.g., a shield plate) is situated at each lateral end of the fixing belt  121  in the axial direction thereof in such a manner that the shield  133  projects from the belt holder  140  to the halogen heaters  123 B and  123 C in the axial direction of the fixing belt  121 . For example, the shield  133  overlaps an outboard lateral end  123 Ca of the halogen heater  123 C and an outboard lateral end  123 Ba of the halogen heater  123 B in the axial direction of the fixing belt  121 . The shield  133  is interposed between the halogen heaters  123 B and  123 C and the fixing belt  121  and the belt holder  140 , thus shielding the fixing belt  121  and the belt holder  140  from light, that is, radiation heat, emitted by the halogen heaters  123 B and  123 C. A detailed description of the shield  133  is deferred. 
     A slip ring is interposed between a lateral edge of the fixing belt  121  and an inward face of the belt holder  140  disposed opposite the lateral edge of the fixing belt  121  in the axial direction thereof. The slip ring serves as a protector that protects the lateral edge of the fixing belt  121  in the axial direction thereof. For example, even if the fixing belt  121  is skewed in the axial direction thereof, the slip ring prevents the lateral edge of the fixing belt  121  from coming into direct contact with the belt holder  140 , thus minimizing abrasion and breakage of the lateral edge of the fixing belt  121  in the axial direction thereof. Since an inner diameter of the slip ring is sufficiently greater than an outer diameter of the belt holder  140 , the slip ring loosely slips on the belt holder  140 . Accordingly, when the lateral edge of the fixing belt  121  comes into contact with the slip ring, the slip ring is rotatable in accordance with rotation of the fixing belt  121  by friction therebetween. Alternatively, the slip ring may remain at rest irrespective of rotation of the fixing belt  121 . The slip ring is made of heat-resistant, super engineering plastics such as PEEK, PPS, PAI, and PTFE. 
     With reference to  FIG. 2 , a detailed description is now given of a construction of the stay  125 . 
     As shown in  FIG. 2 , in contrast to the stay  125 , the nip formation assembly  124  is compact, thus allowing the stay  125  to extend as long as possible in the small space inside the loop formed by the fixing belt  121 . For example, the length of the base pad  131  of the nip formation assembly  124  is smaller than that of the stay  125  in the recording medium conveyance direction A 1 . As shown in  FIG. 2 , the base pad  131  includes an upstream portion  131   a  disposed upstream from the fixing nip N in the recording medium conveyance direction A 1 ; a downstream portion  131   b  disposed downstream from the fixing nip N in the recording medium conveyance direction A 1 ; and a center portion  131   c  interposed between the upstream portion  131   a  and the downstream portion  131   b  in the recording medium conveyance direction A 1 . A height h 1  defines a height of the upstream portion  131   a  from the fixing nip N or its hypothetical extension E in a pressurization direction D 1  of the pressing roller  122  in which the pressing roller  122  is pressed against the nip formation assembly  124 . A height h 2  defines a height of the downstream portion  131   b  from the fixing nip N or its hypothetical extension E in the pressurization direction D 1  of the pressing roller  122 . A height h 3 , that is, a maximum height of the base pad  131 , defines a height of the center portion  131   c  from the fixing nip N or its hypothetical extension E in the pressurization direction D 1  of the pressing roller  122 . The height h 3  is not smaller than the height h 1  and the height h 2 . 
     Hence, the upstream portion  131   a  of the base pad  131  of the nip formation assembly  124  is not interposed between the inner circumferential surface of the fixing belt  121  and an upstream curve  125   d   1  of the stay  125  in the diametrical direction of the fixing belt  121 . Similarly, the downstream portion  131   b  of the base pad  131  of the nip formation assembly  124  is not interposed between the inner circumferential surface of the fixing belt  121  and a downstream curve  125   d   2  of the stay  125  in the diametrical direction of the fixing belt  121 . Accordingly, the upstream curve  125   d   1  and the downstream curve  125   d   2  of the stay  125  are situated in proximity to the inner circumferential surface of the fixing belt  121 . Consequently, the stay  125  having an increased size that enhances the mechanical strength thereof is accommodated in the limited space inside the loop formed by the fixing belt  121 . As a result, the stay  125 , with its enhanced mechanical strength, supports the nip formation assembly  124  properly, preventing bending of the nip formation assembly  124  caused by pressure from the pressing roller  122  and thereby improving fixing performance. 
     As shown in  FIG. 2 , the stay  125  includes a base  125   a  contacting the nip formation assembly  124  and an upstream arm  125   b   1  and a downstream arm  125   b   2 , constituting a pair of projections, projecting from the base  125   a . The base  125   a  extends in the recording medium conveyance direction A 1 , that is, a vertical direction in  FIG. 2 . The upstream arm  125   b   1  and the downstream arm  125   b   2  project from an upstream end and a downstream end of the base  125   a , respectively, in the recording medium conveyance direction A 1  and extend in the pressurization direction D 1  of the pressing roller  122  orthogonal to the recording medium conveyance direction A 1 . The upstream arm  125   b   1  and the downstream arm  125   b   2  projecting from the base  125   a  in the pressurization direction D 1  of the pressing roller  122  elongate a cross-sectional area of the stay  125  in the pressurization direction D 1  of the pressing roller  122 , increasing the section modulus and the mechanical strength of the stay  125 . 
     Additionally, as the upstream arm  125   b   1  and the downstream arm  125   b   2  elongate further in the pressurization direction D 1  of the pressing roller  122 , the mechanical strength of the stay  125  becomes greater. Accordingly, it is preferable that a front edge  125   c  of each of the upstream arm  125   b   1  and the downstream arm  125   b   2  is situated as close as possible to the inner circumferential surface of the fixing belt  121  to allow the upstream arm  125   b   1  and the downstream arm  125   b   2  to project longer from the base  125   a  in the pressurization direction D 1  of the pressing roller  122 . However, since the fixing belt  121  swings or vibrates as it rotates, if the front edge  125   c  of each of the upstream arm  125   b   1  and the downstream arm  125   b   2  is excessively close to the inner circumferential surface of the fixing belt  121 , the swinging or vibrating fixing belt  121  may come into contact with the upstream arm  125   b   1  or the downstream arm  125   b   2 . For example, if the thin fixing belt  121  is used as in this exemplary embodiment, the thin fixing belt  121  swings or vibrates substantially. Accordingly, it is necessary to position the front edge  125   c  of each of the upstream arm  125   b   1  and the downstream arm  125   b   2  with respect to the fixing belt  121  carefully. 
     Specifically, as shown in  FIG. 2 , a distance d between the front edge  125   c  of each of the upstream arm  125   b   1  and the downstream arm  125   b   2  and the inner circumferential surface of the fixing belt  121  in the pressurization direction D 1  of the pressing roller  122  is at least about 2.0 mm, preferably not smaller than about 3.0 mm. Conversely, if the fixing belt  121  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  126  is attached to the front edge  125   c  of each of the upstream arm  125   b   1  and the downstream arm  125   b   2  as in this exemplary embodiment, the distance d is determined by considering the thickness of the reflector  126  so that the reflector  126  does not contact the fixing belt  121 . 
     The front edge  125   c  of each of the upstream arm  125   b   1  and the downstream arm  125   b   2  situated as close as possible to the inner circumferential surface of the fixing belt  121  allows the upstream arm  125   b   1  and the downstream arm  125   b   2  to project longer from the base  125   a  in the pressurization direction D 1  of the pressing roller  122 . Accordingly, even if the fixing belt  121  has a decreased loop diameter, the stay  125  having the longer upstream arm  125   b   1  and the longer downstream arm  125   b   2  attains an enhanced mechanical strength. 
     With reference to  FIG. 2 , a description is provided of a fixing operation of the fixing device  100  described above. 
     As the image forming apparatus  1000  depicted in  FIG. 1  is powered on, the power supply supplies power to the halogen heater set  123  and at the same time the driver drives and rotates the pressing roller  122  clockwise in  FIG. 2  in the rotation direction R 4 . Accordingly, the fixing belt  121  rotates counterclockwise in  FIG. 2  in the rotation direction R 3  in accordance with rotation of the pressing roller  122  by friction between the pressing roller  122  and the fixing belt  121 . 
     A recording medium P bearing a toner image T formed by the image forming operation of the image forming apparatus  1000  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  122  and the fixing belt  121  pressed by the pressing roller  122 . The fixing belt  121  heated by the halogen heater set  123  heats the recording medium P and at the same time the pressing roller  122  pressed against the fixing belt  121  and the fixing belt  121  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  128 , the separator  128  separates the recording medium P from the fixing belt  121 . Thereafter, the separated recording medium P is discharged by the output roller pair  7  depicted in  FIG. 1  onto the outside of the image forming apparatus  1000 , that is, the output tray  17  where the recording media P are stocked. 
     With reference to  FIG. 2 , a description is provided of advantages of the fixing device  100  having the configuration described above. 
     The nip formation assembly  124  guides the fixing belt  121  to the fixing nip N, minimizing vibration or swinging of the fixing belt  121  before the fixing belt  121  enters the fixing nip N and thereby facilitating stable and smooth entry of the fixing belt  121  into the fixing nip N. Accordingly, even if no guide other than the nip formation assembly  124  is configured to guide a center interposed between both lateral ends of the fixing belt  121  in the axial direction thereof to the fixing nip N, the nip formation assembly  124  guides and rotates the fixing belt  121  stably and smoothly. Consequently, the nip formation assembly  124  minimizes load imposed on the rotating fixing belt  121  and resultant wear of the fixing belt  121 , preventing damage and breakage of the fixing belt  121  and enhancing reliability of the fixing device  100 . For example, it is difficult for the fixing belt  121  having a reduced thickness that decreases the thermal capacity thereof to have an increased mechanical strength. However, the nip formation assembly  124  supports and guides the thin fixing belt  121 , preventing damage and breakage of the fixing belt  121 . 
     The nip formation assembly  124  incorporated in the fixing device  100  depicted in  FIG. 2  guides the fixing belt  121  to the fixing nip N, resulting in the simple, compact fixing device  100  manufactured at reduced costs. Accordingly, the compact fixing device  100  has a reduced thermal capacity that shortens a warm-up time thereof, thus saving more energy and shortening a first print time taken to output a recording medium P bearing a toner image T onto the outside of the image forming apparatus  1000  after the image forming apparatus  1000  receives a print job. 
     Since the nip formation assembly  124  serves as a guide that guides the fixing belt  121  to the fixing nip N, it is not necessary to provide a guide separately from the nip formation assembly  124 . Hence, no component is interposed between the inner circumferential surface of the fixing belt  121  and the upstream curve  125   d   1  of the stay  125  in the diametrical direction of the fixing belt  121 . Similarly, no component is interposed between the inner circumferential surface of the fixing belt  121  and the downstream curve  125   d   2  of the stay  125  in the diametrical direction of the fixing belt  121 . That is, the upstream curve  125   d   1  and the downstream curve  125   d   2  of the stay  125  are disposed opposite the inner circumferential surface of the fixing belt  121  directly. Accordingly, the upstream curve  125   d   1  and the downstream curve  125   d   2  of the stay  125  are situated in proximity to the inner circumferential surface of the fixing belt  121 . Consequently, the stay  125  having an increased size that enhances the mechanical strength thereof is accommodated in the limited space inside the loop formed by the fixing belt  121 . As a result, even if the fixing belt  121  is downsized to decrease its thermal capacity, the stay  125  accommodated inside the downsized fixing belt  121  achieves an enhanced mechanical strength that supports the nip formation assembly  124  properly, preventing bending of the nip formation assembly  124  caused by pressure from the pressing roller  122  and thereby improving fixing performance. 
     While the pressing roller  122  is isolated from the fixing belt  121 , the nip formation assembly  124  is spaced apart from the inner circumferential surface of the fixing belt  121  so that the upstream portion  131   a  and the downstream portion  131   b  of the base pad  131  of the nip formation assembly  124  do not pressingly contact the fixing belt  121 . Accordingly, the fixing belt  121  does not slide over the nip formation assembly  124 , minimizing load imposed on the fixing belt  121  and resultant abrasion of the fixing belt  121 . Additionally, the fixing belt  121  contacts the nip formation assembly  124  with a reduced friction therebetween, producing a desired path through which the fixing belt  21  enters the fixing nip N. 
     With reference to  FIG. 4 , a description is provided of a control method for controlling the fixing device  100  incorporated in the image forming apparatus  1000  depicted in  FIG. 1 . 
       FIG. 4  is a block diagram of the controller  200  for controlling the fixing device  100 . As shown in  FIG. 4 , the controller  200  includes a controller unit  200   a  and an engine control unit  200   b . The controller unit  200   a  including the CPU, the ROM, and the RAM is operatively connected to the engine control unit  200   b , a control panel  151 , and an external communication interface  152 . The controller unit  200   a , by executing a preloaded control program, controls operation of the entire image forming apparatus  1000  and input from the external communication interface  152  and the control panel  151 . For example, the controller unit  200   a  receives an instruction from a user input by using the control panel  151  disposed atop the image forming apparatus  1000  and performs various processes according to the instruction. Additionally, the controller unit  200   a  receives a print job, that is, an image forming job, and image data from an external client computer through the external communication interface  152  and controls the engine control unit  200   b , thus controlling an image forming operation to form a toner image T, that is, a monochrome toner image T and a color toner image T, on a recording medium P and output the recording medium P bearing the toner image T. 
     The engine control unit  200   b  is operatively connected to the temperature sensor  127 , the halogen heater set  123 , and a pressing roller driver  129  incorporated in the fixing device  100 . The engine control unit  200   b  including the CPU, the ROM, and the RAM, by executing a preloaded control program, controls a printer engine including the plurality of image forming devices  2 Y,  2 C,  2 M, and  2 K, the optical writer  8 , and the fixing device  100  depicted in  FIG. 1  that performs the image forming processes described above according to an instruction from the controller unit  200   a . For example, the engine control unit  200   b , in an image forming mode to form a toner image T on a recording medium P, controls power supply to the halogen heater set  123  to heat the fixing belt  121  to a predetermined target temperature based on the temperature of the fixing belt  121  detected by the temperature sensor  127  and controls the pressing roller driver  129  that drives and rotates the pressing roller  122 . 
     The image forming apparatus  1000  has three modes: the image forming mode to perform the image forming processes described above; a standby mode to wait for an instruction to start the image forming processes; and a sleep mode to consume less power than the standby mode. For example, in the image forming mode, the fixing belt  121  of the fixing device  100  is warmed up to a predetermined fixing temperature in a range of from about 158 degrees centigrade to about 170 degrees centigrade, and then the fixing device  100  performs the fixing process for fixing the toner image T on the recording medium P. In the standby mode, the fixing belt  121  of the fixing device  100  is maintained at a predetermined lower temperature of about 90 degrees centigrade lower than the predetermined fixing temperature set in the image forming mode. In the sleep mode, power is not supplied to the engine control unit  200   b  and the printer engine including the fixing device  100 , and thus the halogen heater set  123  and the pressing roller  122  are turned off. 
     With reference to  FIG. 5 , a description is provided of a relation between the position of the halogen heaters  123 A,  123 B, and  123 C and the shield  133  and a passage region and a non-passage region of the fixing belt  121  for recording media P of various sizes. 
       FIG. 5  is a partial plan view of one lateral end of the fixing belt  121  in the axial direction thereof illustrating the halogen heaters  123 A,  123 B, and  123 C and the shield  133 . As shown in  FIG. 5 , the halogen heater  123 A is disposed opposite a passage region P 3  of the fixing belt  121  where a letter size recording medium in portrait orientation passes. The halogen heaters  123 A and  123 B are disposed opposite a passage region P 1  of the fixing belt  121  where a double letter size recording medium in portrait orientation passes. Specifically, the halogen heater  123 B is disposed opposite a lateral end P 1   e  of the passage region P 1  of the fixing belt  121  where the double letter size recording medium passes. The halogen heaters  123 A and  123 C are disposed opposite a passage region P 2  of the fixing belt  121  where an A3 size recording medium in portrait orientation passes. 
     As shown in  FIG. 5 , the outboard lateral end  123 Ba of the halogen heater  123 B in the axial direction of the fixing belt  121  parallel to the width direction of the recording medium P is disposed opposite a non-passage region NP 1  of the fixing belt  121  where a double letter size recording medium in portrait orientation (hereinafter referred to as a double letter size recording medium DLT) does not pass. Accordingly, after a plurality of double letter size recording media DLT passes over the fixing belt  121  continuously while the halogen heater  123 B is turned on, the non-passage region NP 1  of the fixing belt  121  may overheat because the plurality of double letter size recording media DLT does not pass over the non-passage region NP 1  of the fixing belt  121  and therefore does not draw heat therefrom. To address this problem, the shield  133  shields the non-passage region NP 1  of the fixing belt  121  from light radiated from the halogen heater  123 B, thus decreasing an amount of light radiated from the halogen heater  123 B that reaches the non-passage region NP 1  of the fixing belt  121 . 
     Similarly, the outboard lateral end  123 Ca of the halogen heater  123 C in the axial direction of the fixing belt  121  is disposed opposite a non-passage region NP 2  of the fixing belt  121  where an A3 size recording medium in portrait orientation (hereinafter referred to as an A3 size recording medium A3T) does not pass. Accordingly, after a plurality of A3 size recording media A3T passes over the fixing belt  121  continuously while the halogen heater  123 C is turned on, the non-passage region NP 2  of the fixing belt  121  may overheat because the plurality of A3 size recording media A3T does not pass over the non-passage region NP 2  of the fixing belt  121  and therefore does not draw heat therefrom. To address this problem, the shield  133  shields the non-passage region NP 2  of the fixing belt  121  from light radiated from the halogen heater  123 C, thus decreasing an amount of light radiated from the halogen heater  123 C that reaches the non-passage region NP 2  of the fixing belt  121 . 
     Hence, the shield  133  shields the non-passage regions NP 1  and NP 2  of the fixing belt  121  from light radiated from the halogen heaters  123 B and  123 C, minimizing overheating of the non-passage regions NP 1  and NP 2  of the fixing belt  121  after the plurality of double letter size recording media DLT and the plurality of A3 size recording media A3T continuously pass over the fixing belt  121 , respectively, and thereby preventing wear and damage of the fixing belt  121  caused by heat from the halogen heaters  123 B and  123 C. 
     The shield  133  is made of a heat-resistant material having resistance against temperatures up to about 400 degrees centigrade. According to this exemplary embodiment, the shield  133  is a metal sheet made of SUS stainless steel and having a thickness of about 0.5 mm. Thus, even if the shield  133  is heated by light from the halogen heaters  123 B and  123 C, the heat-resistant shield  133  minimizes its wear that may arise due to overheating. 
     An opposed face  133   c  depicted in  FIG. 3A  of the shield  133  disposed opposite the halogen heaters  123 B and  123 C has an overall reflectance not greater than about 80 percent, preventing light reflected by the opposed face  133   c  of the shield  133  from heating components located in proximity to the shield  133  and thereby minimizing thermal wear of these components. 
     As shown in  FIG. 3A , the shield  133  is in contact with the stay  125 . Accordingly, heat received from the halogen heaters  123 B and  123 C is conducted from the shield  133  to the stay  125 , minimizing temperature increase of the shield  133  and thereby preventing overheating and resultant thermal wear of the shield  133 . Additionally, the shield  133  shields the belt holder  140  from light radiated from the halogen heaters  123 B and  123 C, minimizing thermal wear of the belt holder  140 . 
     If the shield  133  shields the entire outboard lateral end  123 Ba of the halogen heater  123 B disposed opposite the non-passage region NP 1  of the fixing belt  121  depicted in  FIG. 5 , the shield  133  prevents almost all of light radiated from the halogen heater  123 B from reaching the non-passage region NP 1  of the fixing belt  121  while the double letter size recording medium DLT passes over the fixing belt  121 . However, if the shield  133  is configured to shield the entire outboard lateral end  123 Ba of the halogen heater  123 B, during passage of the double letter size recording medium DLT, the shield  133  may also prevent light radiated from the halogen heater  123 C from reaching the fixing belt  121  unnecessarily. Accordingly, such shield  133  may unnecessarily restrict heating of an area on the fixing belt  121  that need to be heated by the halogen heater  123 C. For example, a lateral end P 2   e  of the passage region P 2  of the fixing belt  121  in the axial direction thereof where the A3 size recording medium A3T passes may not be heated by the halogen heater  123 C to the predetermined fixing temperature, resulting in fixing failure. 
     To address this problem, the shield  133  has a shape that reduces overheating of the non-passage region NP 1  of the double letter size recording medium DLT and the non-passage region NP 2  of the A3 size recording medium A3T and at the same time minimizes fixing failure at the lateral end P 2   e  of the passage region P 2  where the A3 size recording medium A3T passes that may arise due to insufficient heating. For example, as shown in  FIG. 5 , the shield  133  is produced with a rectangular notch  133   a  disposed opposite the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  where the A3 size recording medium A3T passes. Specifically, at the lateral end P 2   e  of the passage region P 2  of the fixing belt  121 , the non-passage region NP 1  of the fixing belt  121  where the double letter size recording medium DLT does not pass overlaps the passage region P 2  of the fixing belt  121  where the A3 size recording medium A3T passes in the axial direction of the fixing belt  121 . 
     With reference to  FIGS. 6A and 6B , a description is provided of a heated region of the fixing belt  121  heated by light radiated from the halogen heaters  123 B and  123 C. 
       FIG. 6A  is a partial vertical sectional view of the fixing device  100  taken on the line A-A of  FIG. 3A  illustrating the heated region of the fixing belt  121  heated by light radiated from the halogen heater  123 B.  FIG. 6B  is a partial vertical sectional view of the fixing device  100  taken on the line A-A of  FIG. 3A  illustrating the heated region of the fixing belt  121  heated by light radiated from the halogen heater  123 C. The line A-A of  FIG. 3A  is in the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  shown in  FIG. 5  where the non-passage region NP 1  of the fixing belt  121  where the double letter size recording medium DLT does not pass overlaps the passage region P 2  of the fixing belt  121  where the A3 size recording medium A3T passes in the axial direction of the fixing belt  121 . 
     As shown in  FIG. 6A , a heated region HrB of the fixing belt  121  in the rotation direction R 3  of the fixing belt  121  is disposed opposite the halogen heater  123 B through the notch  133   a  of the shield  133 . Hence, the heated region HrB of the fixing belt  121  is heated by light radiated from the halogen heater  123 B and irradiated thereto through the notch  133   a  of the shield  133 . Conversely, a non-heated region NHrB of the fixing belt  121  in the rotation direction R 3  of the fixing belt  121  is disposed opposite the halogen heater  123 B via the shield  133 . Hence, the non-heated region NHrB of the fixing belt  121  is shielded from light radiated from the halogen heater  123 B by the shield  133  and therefore is not heated by the halogen heater  123 B. Accordingly, at the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  where the A3 size recording medium A3T passes which overlaps the non-passage region NP 1  of the fixing belt  121  where the double letter size recording medium DLT does not pass in the axial direction of the fixing belt  121 , the shield  133  shields the fixing belt  121  from light radiated from the halogen heater  123 B at a predetermined rate. Consequently, the shield  133 , compared to a configuration without the shield  133 , reduces overheating of the non-passage region NP 1  of the fixing belt  121  where the double letter size recording medium DLT does not pass during printing on the double letter size recording medium DLT. 
     As shown in  FIG. 6B , a non-heated region NHrC of the fixing belt  121  in the rotation direction R 3  thereof is disposed opposite the halogen heater  123 C via the shield  133 . Hence, the non-heated region NHrC of the fixing belt  121  is shielded from light radiated from the halogen heater  123 C by the shield  133  and therefore is not heated by the halogen heater  123 C. Conversely, a heated region HrC of the fixing belt  121  in the rotation direction R 3  thereof is disposed opposite the halogen heater  123 C through the notch  133   a  of the shield  133 . Hence, the heated region HrC of the fixing belt  121  is heated by light radiated from the halogen heater  123 C and irradiated thereto through the notch  133   a  of the shield  133 . Accordingly, as shown in  FIG. 5 , the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  where the A3 size recording medium A3T passes is heated by light radiated from the halogen heater  123 C and irradiated thereto through the notch  133   a  of the shield  133 . The shield  133 , compared to a configuration without the notch  133   a , allows the halogen heater  123 C to heat the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  where the A3 size recording medium A3T passes with an increased area, thus minimizing fixing failure that may arise due to a decreased temperature lower than the predetermined fixing temperature at the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  during printing on the A3 size recording medium A3T. 
     With the configuration described above, the fixing device  100  reduces overheating of the non-passage region NP 1  of the fixing belt  121  where the double letter size recording medium DLT does not pass and the non-passage region NP 2  of the fixing belt  121  where the A3 size recording medium A3T does not pass. Simultaneously, the fixing device  100  minimizes fixing failure that may arise due to decreased temperature at the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  where the A3 size recording medium A3T passes, which is disposed at both lateral ends of the fixing belt  121  in the axial direction thereof. 
     With reference to  FIGS. 1, 2, 5, 6A, and 6B , a description is provided of advantages of the fixing device  100  and the image forming apparatus  1000  incorporating the fixing device  100  according to the exemplary embodiments described above. 
     As shown in  FIGS. 2 and 5 , the fixing device  100  includes the fixing belt  121  serving as a hollow, endless rotary body; the pressing roller  122  serving as a pressing body that contacts the outer circumferential surface of the fixing belt  121 ; the nip formation assembly  124  disposed opposite the inner circumferential surface of the fixing belt  121  and pressing against the pressing roller  122  via the fixing belt  121  to form the fixing nip N between the fixing belt  121  and the pressing roller  122 ; and the halogen heater set  123  serving as a heater set disposed opposite the inner circumferential surface of the fixing belt  121  and irradiating the fixing belt  121  with light, that is, radiation heat. The fixing device  100  allows recording media P of at least two sizes to pass between the fixing roller  121  and the pressing roller  122 , that is, a first size recording medium (e.g., a double letter size recording medium DLT in portrait orientation) and a second size recording medium (e.g., an A3 size recording medium A3T in portrait orientation) greater that the first size recording medium in width in the axial direction of the fixing belt  121 . The halogen heater set  123  includes a plurality of heaters disposed opposite different regions on the fixing belt  121  in the axial direction thereof, respectively. The plurality of heaters includes at least the halogen heater  123 B serving as a first heater disposed opposite each lateral end P 1   e  of the passage region P 1 , that is, a first passage region, of the fixing belt  121  in the axial direction thereof where the first size recording medium passes and the halogen heater  123 C serving as a second heater disposed opposite each lateral end P 1   e  of the passage region P 1  and each lateral end P 2   e  of the passage region P 2 , that is, a second passage region, of the fixing belt  121  in the axial direction thereof where the second size recording medium passes. The fixing device  100  further includes the shield  133  interposed between the fixing belt  121  and the halogen heaters  123 B and  123 C to shield the fixing belt  121  from heat radiated from the halogen heaters  123 B and  123 C. The shield  133  includes the notch  133   a  disposed opposite each lateral end P 2   e  of the passage region P 2  of the fixing belt  121  which overlaps the non-passage region NP 1  of the fixing belt  121  in the axial direction thereof where the first size recording medium does not pass. 
     Accordingly, the shield  133  reduces overheating of the non-passage region NP 1  of the fixing belt  121  where the first size recording medium does not pass and the non-passage region NP 2  of the fixing belt  121  where the second size recording medium does not pass. Simultaneously, the shield  133  prevents temperature decrease in the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  where the second size recording medium passes, thus minimizing fixing failure that may arise due to the decreased temperature of the fixing belt  121 . 
     As shown in  FIG. 6A , the halogen heater  123 C is disposed downstream from the halogen heater  123 B in the rotation direction R 3  of the fixing belt  121 . 
     As shown in  FIG. 5 , the shield  133  further includes a body  133   d  and an upstream arm  133   b  projecting from the body  133   d  toward a center of the fixing belt  121  in the axial direction thereof and disposed upstream from the notch  133   a  in the rotation direction R 3  of the fixing belt  121 . The notch  133   a  is formed into a rectangle extending in the axial direction of the fixing belt  121  by eliminating a downstream portion of the shield  133  in the rotation direction R 3  of the fixing belt  121  such that the rectangular notch  133   a  extends in the axial direction of the fixing belt  121  along the adjacent upstream arm  133   b . Accordingly, the notch  133   a  allows heat radiated from the halogen heater  123 C to be conducted to the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  where the second size recording medium passes without being blocked by the shield  133 , thus facilitating efficient heating of the fixing belt  121  by the halogen heater  123 C. 
     As shown in  FIG. 3B , the fixing device  100  further includes the belt holder  140  disposed opposite the inner circumferential surface of the fixing belt  121  at each lateral end of the fixing belt  121  in the axial direction thereof and serving as a guide that guides the fixing belt  121  rotating in a predetermined rotation locus. The shield  133  is interposed between the belt holder  140  and the halogen heaters  123 B and  123 C to shield the belt holder  140  from heat radiated from the halogen heaters  123 B and  123 C. Accordingly, the shield  133  minimizes thermal wear of the belt holder  140 . 
     As shown in  FIG. 3A , the fixing device  100  further includes the stay  125  contacting the shield  133  and serving as a dissipator that dissipates heat conducted from the shield  133 . Accordingly, the stay  125  prevents overheating of the shield  133 , minimizing thermal wear of the shield  133 . 
     As shown in  FIG. 3A , the opposed face  133   c  of the shield  133  disposed opposite the halogen heaters  123 B and  123 C has an overall reflectance not greater than about 80 percent. Accordingly, the shield  133  minimizes thermal wear of the components surrounding the shield  133  due to temperature increase. 
     The shield  133  has resistance against temperatures up to about 400 degrees centigrade. Accordingly, the shield  133  minimizes thermal wear of itself due to temperature increase. 
     As shown in  FIG. 5 , the fixing device  100  further includes the halogen heater  123 A serving as a third heater disposed opposite and heating the center passage region P 3  of the fixing belt  121  in the axial direction thereof where a third size recording medium (e.g., the letter size recording medium in portrait orientation) passes. As shown in  FIG. 2 , the three axes  123 Ax,  123 Bx, and  123 Cx of the three halogen heaters  123 A,  123 B, and  123 C constitute the three vertices of the triangle Ta in cross-section. The halogen heater  123 C is interposed between the nip formation assembly  124  and the halogen heaters  123 A and  123 B in the diametrical direction of the fixing belt  121 . Accordingly, before a recording medium of frequently used size, that is, the first size recording medium or the third size recording medium, is conveyed through the fixing nip N, the halogen heaters  123 A and  123 B disposed opposite the passage regions P 3  and P 1  of the fixing belt  121  where the third and first size recording media pass, respectively, and situated closer to the inner circumferential surface of the fixing belt  121  than the halogen heater  123 C heat the fixing belt  121  efficiently. 
     As shown in  FIG. 1 , the image forming apparatus  1000  includes an image carrier (e.g., the photoconductive drums  20 Y,  20 C,  20 M, and  20 K); an electrostatic latent image formation device (e.g, the optical writer  8 ) that forms an electrostatic latent image on the image carrier; a development device (e.g., the development devices  40 Y,  40 C,  40 M, and  40 K) that visualizes the electrostatic latent image into a toner image with toner; a transfer device (e.g., the transfer device  71 ) that transfers the toner image formed on the image carrier onto a recording medium; and the fixing device described above (e.g., the fixing device  100 ) that fixes the toner image on the recording medium. Accordingly, the fixing device  100  incorporated in the image forming apparatus  1000 , with the above-described configuration of the shield  133 , reduces overheating of the fixing belt  121  in the non-passage region NP 1  of the fixing belt  121  where the first size recording medium does not pass and the non-passage region NP 2  of the fixing belt  121  where the second size recording medium does not pass. Simultaneously, the fixing device  100 , with the above-described configuration of the notch  133   a  of the shield  133 , minimizes fixing failure that may arise due to decreased temperature at the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  where the second size recording medium passes. 
     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. 1 , the image forming apparatus  1000  incorporating the fixing device  100  is a color laser printer. Alternatively, the image forming apparatus  1000  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. 
     As shown in  FIGS. 5 and 6B , the shield  133  shields the non-passage region NP 2  of the fixing belt  121  where the second size recording medium does not pass from heat radiated from the halogen heater  123 C. Similarly, the shield  133 , with the upstream arm  133   b , shields the non-passage region NP 1  of the fixing belt  121  where the first size recording medium does not pass while the shield  133 , with the notch  133   a , allows heat radiated from the halogen heater  123 C to be conducted to the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  where the second size recording medium passes without being blocked by the shield  133 . Accordingly, compared to a configuration without the shield  133 , the fixing device  100  reduces overheating of the non-passage region NP 2  of the fixing belt  121  where the second size recording medium does not pass that may be caused by heat from the halogen heater  123 C. Similarly, the fixing device  100  reduces overheating of the non-passage region NP 1  of the fixing belt  121  where the first size recording medium does not pass that may be caused by heat from the halogen heater  123 B. 
     For example, the notch  133   a  of the shield  133  disposed opposite the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  where the second size recording medium passes, which overlaps the non-passage region NP 1  of the fixing belt  121  where the first size recording medium does not pass in the axial direction thereof, allows heat from the halogen heaters  123 B and  123 C to reach the fixing belt  121 . Accordingly, as shown in  FIG. 6B , the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  where the second size recording medium passes is heated by heat radiated from the halogen heater  123 C and irradiating thereto through the notch  133   a . Consequently, the shield  133 , compared to a configuration without the notch  133   a , allows the halogen heater  123 C to heat the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  where the second size recording medium passes with an increased area, thus minimizing fixing failure that may arise due to a decreased temperature lower than the predetermined fixing temperature at each lateral end P 2   e  of the passage region P 2  during printing on the second size recording medium. 
     According to the exemplary embodiments described above, the shield  133  having the notch  133   a  is employed in the fixing device  100  incorporating the plurality of halogen heaters  123 A,  123 B, and  123 C. Alternatively, the shield  133  may be employed in a fixing device  100 S incorporating a single halogen heater  123 H as shown in  FIG. 7 . 
     With reference to  FIG. 7 , a description is provided of a configuration of the fixing device  100 S incorporating the single halogen heater  123 H and the shield  133 . 
       FIG. 7  is a partial plan view of the fixing device  100 S according to a second exemplary embodiment illustrating one lateral end of the fixing belt  121  in the axial direction thereof. As shown in  FIG. 7 , the fixing device  100 S includes the halogen heater  123 H instead of the halogen heaters  123 A,  123 B, and  123 C depicted in  FIG. 5 . The halogen heater  123 H extends throughout substantially the entire width of the fixing belt  121  in the axial direction thereof, thus heating both the double letter size recording medium DLT and the A3 size recording medium A3T. For example, the halogen heater  123 H is disposed opposite the non-passage region NP 1  of the fixing belt  121  where the double letter size recording medium DLT in portrait orientation does not pass. Accordingly, after the plurality of double letter size recording media DLT passes over the fixing belt  121  continuously while the halogen heater  123 H is turned on, the non-passage region NP 1  of the fixing belt  121  may overheat because the plurality of double letter size recording media DLT does not pass over the non-passage region NP 1  of the fixing belt  121  and therefore does not draw heat therefrom. To address this problem, the shield  133  shields a part of the non-passage region NP 1  of the fixing belt  121  from light radiated from the halogen heater  123 H, thus decreasing an amount of light radiated from the halogen heater  123 H that reaches the non-passage region NP 1  of the fixing belt  121 . 
     Similarly, an outboard lateral end  123 Ha of the halogen heater  123 H in the axial direction of the fixing belt  121  is disposed opposite the non-passage region NP 2  of the fixing belt  121  where the A3 size recording medium A3T in portrait orientation does not pass. Accordingly, after the plurality of A3 size recording media A3T passes over the fixing belt  121  continuously while the halogen heater  123 H is turned on, the non-passage region NP 2  of the fixing belt  121  may overheat because the plurality of A3 size recording media A3T does not pass over the non-passage region NP 2  of the fixing belt  121  and therefore does not draw heat therefrom. To address this problem, the shield  133  shields the non-passage region NP 2  of the fixing belt  121  from light radiated from the halogen heater  123 H, thus decreasing an amount of light radiated from the halogen heater  123 H that reaches the non-passage region NP 2  of the fixing belt  121 . 
     Hence, the shield  133  shields the non-passage regions NP 1  and NP 2  of the fixing belt  121  from light radiated from the halogen heater  123 H, minimizing overheating of the non-passage regions NP 1  and NP 2  of the fixing belt  121  after the plurality of double letter size recording media DLT and the plurality of A3 size recording media A3T continuously pass over the fixing belt  121 , respectively, and thereby preventing wear and damage of the fixing belt  121  caused by heat from the halogen heater  123 H. 
     However, if the shield  133  is configured to shield the entire non-passage region NP 1  of the fixing belt  121  where the double letter size recording medium DLT does not pass, during passage of the double letter size recording medium DLT, the shield  133  may also prevent light radiated from the halogen heater  123 H from reaching the fixing belt  121  unnecessarily. Accordingly, such shield  133  may unnecessarily restrict heating of an area on the fixing belt  121  that need to be heated by the halogen heater  123 H. For example, the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  in the axial direction thereof where the A3 size recording medium A3T passes may not be heated by the halogen heater  123 H to the predetermined fixing temperature, resulting in fixing failure. 
     To address this problem, the shield  133  has the shape that reduces overheating of the non-passage region NP 1  of the double letter size recording medium DLT and the non-passage region NP 2  of the A3 size recording medium A3T and at the same time minimizes fixing failure at the lateral end P 2   e  of the passage region P 2  of the A3 size recording medium A3T that may arise due to insufficient heating. For example, as shown in  FIG. 7 , the shield  133  is produced with the rectangular notch  133   a  disposed opposite the lateral end P 2   e  of the passage region P 2  of the fixing belt  121  where the A3 size recording medium A3T passes. Specifically, at the lateral end P 2   e  of the passage region P 2  of the fixing belt  121 , the non-passage region NP 1  of the fixing belt  121  where the double letter size recording medium DLT does not pass overlaps the passage region P 2  of the fixing belt  121  where the A3 size recording medium A3T passes in the axial direction of the fixing belt  121 . 
     According to the exemplary embodiments described above, the heaters (e.g., the halogen heaters  123 A,  123 B, and  123 C depicted in  FIG. 5  and the halogen heater  123 H depicted in  FIG. 7 ) are situated symmetrically via a center of the fixing belt  121  in the axial direction thereof. Alternatively, the heaters may be aligned along one lateral edge of the fixing belt  121  in the axial direction thereof such that the non-passage regions NP 1  and NP 2  are produced only at one lateral end of the fixing belt  121  in the axial direction thereof. In this case, the single shield  133  may be disposed opposite the non-passage regions NP 1  and NP 2  situated only at one lateral end of the fixing belt  121  in the axial direction thereof. 
     With reference to  FIG. 8 , a description is provided of a configuration of a fixing device  100 T according to a third exemplary embodiment. 
       FIG. 8  is a vertical sectional view of the fixing device  100 T. As shown in  FIG. 8 , the fixing device  100 T includes a fixing belt  21  formed into a loop; a pressing roller  22  disposed opposite an outer circumferential surface of the fixing belt  21 ; a heater  23  disposed inside the loop formed by the fixing belt  21 ; a reflector  24  disposed opposite the heater  23 ; a nip formation assembly  25  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 support  26  contacting and supporting the nip formation assembly  25 ; a separator  27  disposed opposite the outer circumferential surface of the fixing belt  21 ; a pair of belt holders  28  contacting and supporting the fixing belt  21  at both lateral ends in an axial direction thereof; and a pair of protectors  29  contactably disposed opposite the fixing belt  21  at both lateral ends in the axial direction thereof. 
     The fixing device  100 T further includes a cabinet  31  housing the components of the fixing device  100 T described above; a plurality of bolts  32  that bolts the belt holder  28  to the cabinet  31 ; the temperature sensor  127 ; and the controller  200  operatively connected to the temperature sensor  127  and the heater  23  to control the heater  23  based on the temperature of the fixing belt  21  detected by the temperature sensor  127 . The fixing device  100 T is detachably installed inside the body  2  of the image forming apparatus  1000  depicted in  FIG. 1 . 
     As a recording medium P bearing a toner image T is conveyed through the fixing nip N formed between the fixing belt  21  and the pressing roller  22 , the fixing belt  21  heated by the heater  23  and the pressing roller  22  apply heat and pressure to the recording medium P, thus fixing the toner image T on the recording medium P. As the recording medium P bearing the fixed toner image T is discharged from the fixing nip N, the separator  27  separates the recording medium P from the fixing belt  21 . Thereafter, the recording medium P is conveyed through the conveyance path R to the output roller pair  7  depicted in  FIG. 1 . 
     With reference to  FIG. 9 , a detailed description is now given of a construction of the fixing belt  21 . 
       FIG. 9  is an enlarged vertical sectional view of the fixing device  100 T illustrating the fixing belt  21  and the components situated inside the loop formed by the fixing belt  21 . As shown in  FIG. 9 , the fixing belt  21  is constructed of a base layer  21   a ; an elastic layer  21   b  coating the base layer  21   a ; and a release layer  21   c  coating the elastic layer  21   b . The flexible fixing belt  21  has a thickness of about 1 mm. The fixing belt  21  has a long width corresponding to a width of the recording medium P in the axial direction of the fixing belt  21 . The fixing belt  21  has a loop diameter of about 25 mm in cross-section orthogonal to the axial direction of the fixing belt  21 . 
     Alternatively, the fixing belt  21  may not incorporate the elastic layer  21   b . In this case, the fixing belt  21  has a reduced thermal capacity that facilitates heating of the fixing belt  21  by the heater  23  and thereby saving energy. Further, the loop diameter of the fixing belt  21  may be in a range of from about 15 mm to about 120 mm according to settings of the fixing device  100 T. As shown in  FIG. 8 , as the pressing roller  22  rotates in the rotation direction R 4 , the fixing belt  21  rotates in the rotation direction R 3  in accordance with rotation of the pressing roller  22 . That is, the fixing belt  21  is driven and rotated by the pressing roller  22 . As the fixing belt  21  and the pressing roller  22  rotate in the rotation directions R 3  and R 4 , respectively, the recording medium P is conveyed through the fixing nip N in the recording medium conveyance direction A 1  and discharged from the fixing nip N. 
     As shown in  FIG. 9 , the base layer  21   a  of the fixing belt  21  is made of a material having a desired mechanical strength, for example, metal such as nickel (Ni) and SUS stainless steel or resin such as polyimide and has a thickness in a range of from about 20 micrometers to about 100 micrometers. For example, the base layer  21   a  may be thin, metal or resin film. 
     The elastic layer  21   b  of the fixing belt  21  is made of rubber such as silicone rubber (Q) and fluoro rubber (FKM) and has a thickness in a range of from about 20 micrometers to about 900 micrometers. The elastic layer  21   b  absorbs surface asperities of the fixing belt  21  and the recording medium P. Accordingly, as the fixing belt  21  and the pressing roller  22  apply heat and pressure to the recording medium P conveyed through the fixing nip N, the elastic layer  21   b , by absorbing surface asperities of the fixing belt  21  and the recording medium P, facilitates uniform application of heat and pressure to the recording medium P. As the fixing belt  21  and the pressing roller  22  exert pressure to the toner image T on the recording medium P to fix the toner image T on the recording medium P, slight surface asperities of the fixing belt  21  may be transferred onto the toner image T on the recording medium P, producing variation in gloss on the solid toner image T that results in formation of an orange peel image. To address this problem, the elastic layer  21   b  of the fixing belt  21  having a thickness not smaller than about 100 micrometers deforms and absorbs slight surface asperities of the fixing belt  21 , thus minimizing variation in gloss of the solid toner image T, that is, minimizing formation of an orange peel image. 
     The release layer  21   c  of the fixing belt  21  is made of a material that facilitates separation of the recording medium P and the toner image T formed thereon from the fixing belt  21 , that is, a material that prevents adhesion and sticking of toner of the toner image T to the fixing belt  21  and is used on a surface of a die, for example. For example, the release layer  21   c  is made of resin such as PFA, PTFE, polyether imide (PEI), and PES and has a thickness in a range of from about 1 micrometer to about 200 micrometers. 
     With reference to  FIG. 8 , a detailed description is now given of a construction of the pressing roller  22 . 
     As shown in  FIG. 8 , the pressing roller  22  is constructed of a roller-shaped metal core  22   a , an elastic layer  22   b  coating the metal core  22   a , and a release layer  22   c  coating the elastic layer  22   b . A driving mechanism disposed inside the image forming apparatus  1000  depicted in  FIG. 1  generates a driving force that drives and rotates the pressing roller  22 . For example, the driving mechanism is constructed of a driver (e.g., a motor) and a reduction gearing (e.g., reduction gears). As a pressurization assembly presses the pressing roller  22  against the nip formation assembly  25  via the fixing belt  21 , the elastic layer  22   b  of the pressing roller  22  is elastically deformed by pressure from the pressurization assembly, thus forming the fixing nip N. 
     The metal core  22   a , that is, a solid tube having a desired mechanical strength, is made of thermally conductive metal such as carbon steel (e.g., SC and STKM) and aluminum (Al). Alternatively, the metal core  22   a  may be a hollow tube accommodating a heater such as a halogen heater that heats the recording medium P conveyed through the fixing nip N via the metal core  22   a , the elastic layer  22   b , and the release layer  22   c.    
     Similar to the elastic layer  21   b  of the fixing belt  21  described above, the elastic layer  22   b  of the pressing roller  22  is made of synthetic rubber such as silicone rubber (Q) and fluoro rubber (FKM). The synthetic rubber is relatively rigid, non-foaming solid rubber. If no heater is situated inside the metal core  22   a , the elastic layer  22   b  may be made of foaming synthetic rubber such as sponge rubber. The sponge rubber, as it contains foam, provides an increased insulation that insulates the pressing roller  22  from the fixing belt  21  heated by the heater  23 . Hence, heat is not drawn from the fixing belt  21  to the pressing roller  22 , saving energy. 
     Like the release layer  21   c  of the fixing belt  21 , the release layer  22   c  of the pressing roller  22  is made of a thermally conductive, durable material that facilitates separation of the recording medium P from the pressing roller  22  and enhances durability of the elastic layer  22   b . For example, the release layer  22   c  is produced by coating of the elastic layer  22   b  with PFA or fluoroplastic coating made of PFA or PTFE. Alternatively, the release layer  22   c  may be a silicone rubber layer or a fluoro rubber layer. 
     With reference to  FIG. 8 , a detailed description is now given of a construction of the heater  23 . 
     The heater  23  mounted on the cabinet  31  is situated inside the loop formed by the fixing belt  21  and spaced apart from an inner circumferential surface of the fixing belt  21 . The heater  23  has a single light emission region that generates radiation heat to heat the fixing belt  21  directly. The heater  23  is a radiant heater such as a halogen heater incorporating a halogen lamp that generates radiation heat, a carbon heater incorporating a quartz tube filled with carbon fiber in inert gas, and a ceramic heater including resistance wiring embedded inside ceramic. The controller  200  controls powering on and off of the heater  23 . 
     With reference to  FIG. 9 , a detailed description is now given of a construction of the reflector  24 . 
     As shown in  FIG. 9 , the reflector  24  is constructed of a mount  24   a  mounted on the cabinet  31 ; a reflection face  24   b  that reflects light emitted from the heater  23  toward the inner circumferential surface of the fixing belt  21 ; and a cover  24   c  that covers the support  26 . The mount  24   a  is situated at each lateral end of the reflector  24  in the axial direction of the fixing belt  21  and mounted on the cabinet  31  through the belt holder  28 . The reflection face  24   b  is interposed between the support  26  and the heater  23  in a diametrical direction of the fixing belt  21 . The reflection face  24   b , disposed opposite the heater  23 , is bent at a center thereof in the recording medium conveyance direction A 1  to house the heater  23 . 
     With reference to  FIG. 9 , a detailed description is now given of a construction of the nip formation assembly  25 . 
     As shown in  FIG. 9 , the nip formation assembly  25  has a long width in a width direction of the recording medium P parallel to the axial direction of the fixing belt  21 . A cross-section of the nip formation assembly  25  perpendicular to the width direction of the recording medium P is substantially rectangular. As the fixing belt  21  rotates in the rotation direction R 3 , it slides over the nip formation assembly  25 . For example, the nip formation assembly  25  is constructed of a contact face portion  25   a  over which the fixing belt  21  slides and a coupling portion  25   b  coupled with the support  26 . The nip formation assembly  25  is disposed opposite the inner circumferential surface of the fixing belt  21  and is mounted on the cabinet  31 . 
     The contact face portion  25   a  has a plane disposed opposite the pressing roller  22  via the fixing belt  21 . As the pressing roller  22  presses the fixing belt  21  against the nip formation assembly  25 , the fixing belt  21  comes into contact with the plane of the contact face portion  25   a  of the nip formation assembly  25 . Simultaneously, as the pressing roller  22  presses the fixing belt  21  against the nip formation assembly  25 , the elastic layer  22   b  depicted in  FIG. 8  of the pressing roller  22  is pressed and deformed into a plane corresponding to the plane of the contact face portion  25   a  of the nip formation assembly  25 . The elastic layer  22   b  deformed into the plane produces the fixing nip N having a predetermined length in the recording medium conveyance direction A 1 . 
     According to this exemplary embodiment, the contact face portion  25   a  of the nip formation assembly  25  has the plane as described above. Alternatively, the contact face portion  25   a  may have other shapes. For example, the contact face portion  25   a  may have a concave curve with respect to the fixing belt  21  that corresponds to a circumference of the pressing roller  22 . The concave curve of the contact face portion  25   a  directs a leading edge of the recording medium P discharged from the fixing nip N toward the pressing roller  22 , thus facilitating separation of the recording medium P from the fixing belt  21  and thereby preventing jamming of the recording medium P conveyed through the fixing device  100 T. 
     With reference to  FIGS. 10, 11A, and 11B , a detailed description is now given of a construction of the support  26 . 
       FIG. 10  is a partial perspective view of the fixing device  100 T illustrating one lateral end thereof in the axial direction of the fixing belt  21 .  FIG. 11A  is a perspective view of the support  26  seen from the heater  23  depicted in  FIG. 9 .  FIG. 11B  is a perspective view of the support  26  seen from the nip formation assembly  25  depicted in  FIG. 9 . As shown in  FIGS. 10 and 11A , like the nip formation assembly  25  depicted in  FIG. 10 , the support  26  has a long width in the width direction of the recording medium P parallel to the axial direction of the fixing belt  21 . As shown in  FIG. 9 , a cross-section of the support  26  perpendicular to the width direction of the recording medium P is formed into a square bracket producing an opening  26   d  that houses the heater  23 . 
     As shown in  FIGS. 9 and 11A , the support  26  is constructed of a support portion  26   a  that contacts and supports the nip formation assembly  25 ; a housing portion  26   b  producing the opening  26   d  that houses the heater  23  and the reflector  24 ; and an engagement portion  26   c  disposed at each lateral end of the support  26  in a longitudinal direction thereof parallel to the axial direction of the fixing belt  21  and engaged with the cabinet  31 . The support portion  26   a  of the support  26  is coupled with the coupling portion  25   b  of the nip formation assembly  25  to support the nip formation assembly  25  against pressure from the pressing roller  22 , thus preventing bending of the nip formation assembly  25  in the axial direction of the fixing belt  21 . Accordingly, the support  26  helps the nip formation assembly  25  produce the fixing nip N evenly throughout the entire width of the recording medium P in the axial direction of the fixing belt  21 . Like the nip formation assembly  25 , the support  26 , disposed opposite the inner circumferential surface of the fixing belt  21 , is mounted on the cabinet  31  with the engagement portions  26   c  that are fastened to the cabinet  31  with a fastener. 
     As shown in  FIG. 9 , the cover  24   c  of the reflector  24  covers substantially the entire opening  26   d  of the support  26  in the axial direction of the fixing belt  21 . Thus, the reflector  24  protects the support  26  against heat radiated from the heater  23 , minimizing waste of energy. Alternatively, instead of mounting the reflector  24 , an inner circumferential surface of the housing portion  26   b  depicted in  FIG. 11A  of the support  26  may be mirror finished to attain the advantages described above. Further, the inner circumferential surface of the housing portion  26   b  of the support  26  may be formed of an insulator that insulates the support  26  from heat conducted from the heater  23 . 
     With reference to  FIGS. 8 and 10 , a detailed description is now given of a construction of the separator  27 . 
     It is to be noted that the fixing belt  21  is not illustrated in  FIG. 10 . As shown in  FIG. 10 , the separator  27  is constructed of a separation plate  41  disposed opposite the outer circumferential surface of the fixing belt  21 ; a pair of support shafts  42  in contact with both lateral ends of the separation plate  41  in the axial direction of the fixing belt  21 , respectively, to rotatably support the separation plate  41 ; and a spring that biases the separation plate  41  against the fixing belt  21 . The separation plate  41  is constructed of a pair of supported portions  41   a , a separation portion  41   b , and a pair of positioning portions  41   c . The pair of supported portions  41   a  is disposed at both lateral ends of the separator  27 , respectively, in the axial direction of the fixing belt  21 . The supported portion  41   a  is contacted and supported by the support shaft  42 . The planar separation portion  41   b  contacts the leading edge of the recording medium P discharged from the fixing nip N, thus separating the recording medium P from the fixing belt  21 . The positioning portion  41   c  is contiguous to the separation portion  41   b  at each lateral end of the separation portion  41   b  in the axial direction of the fixing belt  21  and includes a bent front  41   ca  that contacts the outer circumferential surface of the fixing belt  21 . As the bent front  41   ca  of the positioning portion  41   c  of the separation plate  41  comes into contact with the fixing belt  21 , the separation plate  41  is positioned with respect to the fixing belt  21 . 
     With reference to  FIGS. 12 and 13 , a detailed description is now given of a construction of the belt holder  28 . 
       FIG. 12  is a perspective view of the belt holder  28 .  FIG. 13  is a plan view of the belt holder  28 . As shown in  FIGS. 12 and 13 , the belt holder  28  is constructed of a flange  51 , a base  52 , a primary projection  53 , and a secondary projection  54 , which are integrally molded. The belt holder  28  is made of super engineering plastics having enhanced mechanical strength and heat resistance, for example, resin such as PPS, PAI, and PEEK. 
     The flange  51  is constructed of a planar plate  51   d  and through-holes  51   a  and  52   b  produced through the plate  51   d  and used to attach the flange  51  to the cabinet  31  depicted in  FIG. 8 . For example, the bolts  32  depicted in  FIG. 8  are inserted into the through-holes  51   a  and  52   b , respectively, to fasten the flange  51  to the cabinet  31 . As shown in  FIG. 13 , the flange  51  further includes a protrusion  51   c  protruding from the plate  51   d  in a direction opposite a direction in which the primary projection  53  projects from the base  52 . The protrusion  51   c  positions the belt holder  28  with respect to the cabinet  31  as the belt holder  28  is attached to the cabinet  31 . 
     The base  52  is formed into a ring or a tube projecting from the flange  51  toward a center of the fixing belt  21  in the axial direction thereof. As shown in  FIG. 10 , the protector  29  (e.g., a slip ring) is rotatably attached to or hung on the base  52 . Thus, the base  52  and the protector  29  restrict movement of the fixing belt  21  in the axial direction thereof if the fixing belt  21  is skewed accidentally. 
     The primary projection  53  is formed into a ring or a tube projecting from the base  52  toward the center of the fixing belt  21  in the axial direction thereof. Since the primary projection  53 , disposed opposite the inner circumferential surface of the fixing belt  21 , contacts and supports the fixing belt  21  at each lateral end in the axial direction thereof, the primary projection  53  serves as an endless rotary body guide that guides the fixing belt  21  as it rotates in the rotation direction R 3 . As shown in  FIG. 12 , a recess  52   a  is produced across the primary projection  53  and the base  52  at the fixing nip N, that is, at a position opposite the through-holes  51   a  and  51   b  via the secondary projection  54  in the diametrical direction of the fixing belt  21 . As shown in  FIG. 10 , each lateral end of the nip formation assembly  25  and the support  26  in the axial direction of the fixing belt  21  is situated inward from the recess  52   a . Thus, the nip formation assembly  25  and the support  26 , held by the belt holder  28  at each lateral end of the nip formation assembly  25  and the support  26  in the axial direction of the fixing belt  21 , are supported by the cabinet  31  mounting the belt holder  28 . 
     As shown in  FIG. 13 , the secondary projection  54  projects from a part of the primary projection  53  toward the center of the fixing belt  21  in the axial direction thereof. As shown in  FIG. 10 , the secondary projection  54  is disposed opposite the positioning portion  41   c  of the separation plate  41  of the separator  27  via the fixing belt  21 . For example, the positioning portion  41   c  presses against the secondary projection  54  via the fixing belt  21 . As shown in  FIGS. 9 and 10 , the belt holder  28  is mounted on the cabinet  31  such that the secondary projection  54  is disposed downstream from the nip formation assembly  25  in the rotation direction R 3  of the fixing belt  21  or the recording medium conveyance direction A 1 . 
     As shown in  FIG. 9 , the secondary projection  54  is disposed opposite a back face  24   d  of the reflector  24  opposite the reflection face  24   b  that reflects light radiated from the heater  23  toward the fixing belt  21 . Further, the secondary projection  54  is disposed opposite a back face  26   e  of the support  26  opposite the housing portion  26   b  depicted in  FIG. 11A  housing the heater  23 . 
     The secondary projection  54  has a friction coefficient different from that of the primary projection  53 . For example, a coefficient of static friction and a coefficient of kinetic friction of the secondary projection  54  are smaller than those of the primary projection  53 , respectively. The friction coefficient of the secondary projection  54  may be smaller than that of the primary projection  53  by coating an outer circumferential face  54   a  of the secondary projection  54 , disposed opposite the positioning portion  41   c  of the separation plate  41  of the separator  27  via the fixing belt  21 , with fluoroplastic (e.g., fluorocarbon polymers). 
     Alternatively, the outer circumferential face  54   a  of the secondary projection  54  may be made of a material having a friction coefficient smaller than that of the primary projection  53 , thus rendering the friction coefficient of the secondary projection  54  to be smaller than that of the primary projection  53 . Yet alternatively, a piece made of a material having a friction coefficient smaller than that of the primary projection  53  may be embedded in or attached to the outer circumferential face  54   a  of the secondary projection  54 . 
     With reference to  FIG. 10 , a detailed description is now given of a configuration of the protector  29 . 
     As shown in  FIG. 10 , the protector  29  is a ring produced with a center through-hole  29   a  into which the primary projection  53  and the secondary projection  54  of the belt holder  28  are inserted. The protector  29  rotatably attached to or hung on the base  52  of the belt holder  28 , together with the base  52 , restricts movement of the fixing belt  21  in the axial direction thereof as the fixing belt  21  is skewed accidentally. As a lateral edge of the fixing belt  21  in the axial direction thereof comes into contact with a planar face of the protector  29  disposed opposite the lateral edge of the fixing belt  21 , the protector  29  rotates in accordance with rotation of the fixing belt  21  by friction therebetween while the protector  29  remains in contact with the fixing belt  21 . To address this circumstance, the protector  29  is made of a relatively elastic material that makes the planar face of the protector  29  smooth and relatively small in friction coefficient. 
     With reference to  FIG. 10 , a detailed description is now given of a configuration of the cabinet  31 . 
     As shown in  FIG. 10  illustrating one lateral end, that is, a right end, of the fixing device  100 T in the axial direction of the fixing belt  21 , the cabinet  31  includes a right side plate  31   a  mounting the belt holder  28  that supports the nip formation assembly  25  and the support  26  at a right end thereof. Although not shown, a left side plate is situated at another lateral end, that is, a left end, of the fixing device  100 T in the axial direction of the fixing belt  21 . Like the right side plate  31   a , the left side plate mounts another belt holder  28  that supports the nip formation assembly  25  and the support  26  at a left end thereof. The cabinet  31  further includes a coupling plate that couples the right side plate  31   a  with the left side plate. Thus, the right side plate  31   a , the left side plate, and the coupling plate are combined. The cabinet  31  mounts a grip gripped by a user to attach and detach the fixing device  100 T to and from the body  2  of the image forming apparatus  1000  depicted in  FIG. 1 . 
     With reference to  FIGS. 14A, 14B, and 14C , a description is provided of operations of the separator  27  described above. 
       FIG. 14A  is a vertical sectional view of the fixing device  100 T illustrating a recording medium P jammed therein.  FIG. 14B  is a vertical sectional view of the fixing device  100 T illustrating the separator  27  spaced apart from the fixing belt  21 .  FIG. 14C  is a vertical sectional view of the fixing device  100 T illustrating the separator  27  coming into contact with the fixing belt  21 . 
     As shown in  FIG. 14A , as a recording medium P is discharged from the fixing nip N, the separator  27  may fail to separate the recording medium P from the fixing belt  21  and thereby the recording medium P may be jammed between the fixing belt  21  and the separator  27  at a position downstream from the fixing nip N in the rotation direction R 3  of the fixing belt  21 . To address this circumstance, the user removes the jammed recording medium P from the fixing device  100 T. Since the recording medium P is jammed between the fixing belt  21  and the separator  27 , as the user pulls the jammed recording medium P, the separator  27  is rotated and lifted by the recording medium P in a rotation direction R 5  and therefore the separator  27  is spaced apart from the fixing belt  21  as shown in  FIG. 14B . After the jammed recording medium P is removed from the fixing device  100 T, that is, after the jammed recording medium P separates from the separator  27  and thereby no longer lifts the separator  27 , resilience F of a spring anchored to the separator  27  causes the positioning portion  41   c  of the separation plate  41  of the separator  27  to strike the fixing belt  21 . 
     To address this problem, the fixing device  100 T includes the secondary projection  54  of the belt holder  28  that is disposed opposite the positioning portion  41   c  of the separator  27  via the fixing belt  21  as shown in  FIG. 10 . Accordingly, even if the positioning portion  41   c  of the separator  27  strikes the fixing belt  21 , the secondary projection  54  of the belt holder  28  supports the fixing belt  21  against impact exerted from the separator  27  onto the fixing belt  21 . Consequently, the secondary projection  54  of the belt holder  28  absorbs impact exerted from the positioning portion  41   c  of the separator  27  to the fixing belt  21 . 
     With reference to  FIGS. 8, 9, 10, 12, and 13 , a description is provided of advantages of the fixing device  100 T described above. 
     As shown in  FIG. 8 , the fixing device  100 T includes the fixing belt  21  serving as an endless rotary body rotatable in the rotation direction R 3 ; the pressing roller  22  serving as a pressing body pressing against the outer circumferential surface of the fixing belt  21 ; the heater  23  disposed opposite the fixing belt  21  to heat the fixing belt  21 ; the nip formation assembly  25  pressing against the pressing roller  22  via the fixing belt  21  to form the fixing nip N through which a recording medium P bearing a toner image T passes; the support  26  contacting and supporting the nip formation assembly  25 ; the separator  27  disposed opposite the outer circumferential surface of the fixing belt  21  to separate the recording medium P discharged from the fixing nip N from the fixing belt  21 ; and the pair of belt holders  28  contacting and supporting the fixing belt  21  at both lateral ends in the axial direction thereof perpendicular to the recording medium conveyance direction A 1 . 
     As shown in  FIG. 10 , the belt holder  28  includes the base  52 ; the primary projection  53  projecting from the base  52  toward the center of the fixing belt  21  in the axial direction thereof; and the secondary projection  54  projecting from a part of the primary projection  53  toward the center of the fixing belt  21  in the axial direction thereof. The secondary projection  54  is disposed opposite the positioning portion  41   c  of the separator  27  via the fixing belt  21 . 
     The secondary projection  54  of the belt holder  28  and the positioning portion  41   c  of the separator  27  prevent buckling and plastic deformation of the fixing belt  21 . For example, as shown in  FIG. 14B , as the user pulls and removes the jammed recording medium P from between the fixing belt  21  and the separator  27 , the recording medium P rotates and lifts the separator  27  in the rotation direction R 5 . After the jammed recording medium P is removed from between the fixing belt  21  and the separator  27 , resilience F of the spring anchored to the separator  27  may cause the separator  27  to strike the fixing belt  21 , thus generating buckling and plastic deformation of the fixing belt  21 . 
     To address this problem, the secondary projection  54  of the belt holder  28  is disposed opposite the positioning portion  41   c  of the separator  27 . Accordingly, even if the separator  27  strikes the fixing belt  21 , the secondary projection  54  supporting the fixing belt  21  absorbs impact exerted from the separator  27  onto the fixing belt  21 . Consequently, the secondary projection  54  of the belt holder  28  prevents damages, that is, buckling and plastic deformation, of the fixing belt  21 . 
     As shown in  FIG. 9 , the secondary projection  54  of the belt holder  28  is disposed downstream from the nip formation assembly  25  in the rotation direction R 3  of the fixing belt  21 . Hence, as the fixing belt  21  is driven and rotated in the rotation direction R 3 , the secondary projection  54  does not come into contact with the fixing belt  21 . For example, at a position downstream from the nip formation assembly  25  in the rotation direction R 3  of the fixing belt  21 , the pressing roller  22  rotating in the rotation direction R 4  pushes the fixing belt  21  away from the nip formation assembly  25 , slackening the fixing belt  21  with decreased tension. Since the fixing belt  21  is slackened as it rotates, the fixing belt  21  does not strike the secondary projection  54 . Accordingly, the fixing belt  21  contacts the secondary projection  54  with reduced friction therebetween, decreasing resistance between the rotating fixing belt  21  and the secondary projection  54  and thereby minimizing rotation torque of the fixing belt  21 . 
     As shown in  FIG. 9 , the secondary projection  54  of the belt holder  28  is disposed opposite the back face  24   d  of the reflector  24  opposite the reflection face  24   b  of the reflector  24  that is disposed opposite the heater  23  to reflect light radiated from the heater  23 . Accordingly, heat radiated from the heater  23  is not conducted to the secondary projection  54  directly. Consequently, it is not necessary to select a heat-resistant material for the belt holder  28 , increasing flexibility in design and selection of moldable materials at reduced costs. Additionally, since heat radiated from the heater  23  is not conducted to the secondary projection  54  of the belt holder  28  directly, durability of the belt holder  28  improves. 
     As shown in  FIG. 9 , the secondary projection  54  of the belt holder  28  is disposed opposite the back face  26   e  of the support  26  opposite the housing portion  26   b  depicted in  FIG. 11A  housing the heater  23 . Accordingly, like the reflector  24  described above, the support  26  prohibits heat radiated from the heater  23  from being conducted to the secondary projection  54  directly. Consequently, it is not necessary to select a heat-resistant material for the belt holder  28 , increasing flexibility in design and selection of moldable materials at reduced costs. 
     Even if a front face  26   f  of the support  26  disposed opposite the heater  23  is configured to be mirror finished by coating or attaching of a reflection material, instead of attaching the reflector  24  to the support  26 , the support  26  prohibits heat radiated from the heater  23  from being conducted to the secondary projection  54  directly. Hence, durability of the belt holder  28  improves. 
     For example, the coefficient of static friction and the coefficient of kinetic friction of the secondary projection  54  are smaller than those of the primary projection  53 , respectively, by coating the secondary projection  54  with fluoroplastic or using a material for the secondary projection  54  that is different from a material used for other components. Accordingly, even if a fixing belt that differs from the fixing belt  21  in design specification is installed in the fixing device  100 T and the fixing belt  21  receives a force that may twist or warp the fixing belt  21  as the fixing belt  21  slides over the secondary projection  54 , the coefficient of static friction and the coefficient of kinetic friction of the secondary projection  54  that are smaller than those of the components other than the secondary projection  54 , for example, the primary projection  53 , prevent the fixing belt  21  from being twisted and warped. Consequently, the fixing belt  21  rotates smoothly, improving its durability. 
     Further, even if the fixing belt  21  comes into contact with the secondary projection  54 , the coefficient of static friction and the coefficient of kinetic friction of the secondary projection  54  that are smaller than those of the components other than the secondary projection  54  decrease resistance between the rotating fixing belt  21  and the secondary projection  54 , minimizing torque required to rotate the fixing belt  21 . 
     The fixing device  100 T is installable in the image forming apparatus  1000  depicted in  FIG. 1 . Accordingly, even if the separator  27  strikes the fixing belt  21  upon removal of the jammed recording medium P from between the fixing belt  21  and the separator  27 , the secondary projection  54  of the belt holder  28  that supports the fixing belt  21  absorbs impact exerted from the separator  27  onto the fixing belt  21 , thus preventing damages, that is, buckling and plastic deformation, of the fixing belt  21 . Consequently, durability of the image forming apparatus  1000  improves. Additionally, heat radiated from the heater  23  is not conducted to the secondary projection  54  directly. Consequently, it is not necessary to select a heat-resistant material for the belt holder  28 , increasing flexibility in design and selection of moldable materials at reduced costs. As a result, the image forming apparatus  1000  provides flexibility in design at reduced costs. Since the belt holder  28  has the decreased coefficient of static friction and the decreased coefficient of kinetic friction, it minimizes damage and abrasion of the fixing belt  21 , enhancing durability of the fixing belt  21 . Thus, the image forming apparatus  1000  incorporating the durable fixing belt  21  enhances its durability. 
     The fixing device  100 T depicted in  FIG. 9  that includes the separator  27  and the belt holder  28  may incorporate the shield  133  depicted in  FIGS. 5 and 7 . 
     With reference to  FIG. 15 , a description is provided of a configuration of a fixing device  100 T′ according to a fourth exemplary embodiment that incorporates the shield  133  having the notch  133   a.    
       FIG. 15  is a partial vertical sectional view of the fixing device  100 T′. The fixing device  100 T′ has a configuration equivalent to the configuration of the fixing device  100 T described above. 
     As shown in  FIG. 15 , the fixing device  100 T′ includes the shield  133  having the notch  133   a  shown in  FIGS. 5 and 7 . Alternatively, the heater  23  may be replaced by the halogen heaters  123 A,  123 B, and  123 C depicted in  FIG. 5  or the halogen heater  123 H depicted in  FIG. 7 . Further, the fixing devices  100  and  100 S depicted in  FIGS. 2 and 7 , respectively, may incorporate the separator  27  and the belt holder  28  shown in  FIG. 10 . 
     According to the exemplary embodiments described above, the pressing rollers  122  and  22  serve as a pressing body disposed opposite the fixing belts  121  and  21 , respectively. Alternatively, a pressing belt, a pressing plate, a pressing pad, or the like may serve as a pressing body. 
     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.