Patent Publication Number: US-9405239-B2

Title: Fixing device, image forming apparatus, and fixing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application Nos. 2012-203268, filed on Sep. 14, 2012, and 2013-092560, filed on Apr. 25, 2013, in the Japanese Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     Exemplary aspects of the present invention relate to a fixing device, an image forming apparatus, and a fixing method, and more particularly, to a fixing device for fixing an image on a recording medium, an image forming apparatus incorporating the fixing device, and a fixing method for fixing a toner image on a recording medium. 
     2. Description of the Background 
     Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a 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 may include a fixing rotary body heated by a heater and an opposed body contacting the fixing rotary body to form a nip therebetween through which a recording medium bearing a toner image is conveyed. As the fixing rotary body and the opposed body rotate and convey the recording medium bearing the toner image through the nip, the fixing rotary body heated to a predetermined fixing temperature and the opposed body together heat and melt toner of the toner image, thus fixing the toner image on the recording medium. 
     Since the recording medium passing through the nip draws heat from the fixing rotary body, a temperature sensor detects the temperature of the fixing rotary body to maintain the fixing rotary body at a desired temperature. However, at each lateral end of the fixing rotary body in an axial direction thereof, the recording medium is not conveyed over the fixing rotary body and therefore does not draw heat from the fixing rotary body. Accordingly, after a plurality of recording media is conveyed through the nip continuously, a non-conveyance span situated at each lateral end of the fixing rotary body may overheat. 
     To address this circumstance, the fixing device may incorporate a heat shield to shield the non-conveyance span of the fixing rotary body from the heater, thus preventing overheating of the fixing rotary body. For example, the heat shield may be movable between a plurality of positions to correspond to a plurality of non-conveyance spans varying depending on the size of recording media. However, the heat shield is retained at an identical position during a print job for forming a toner image on a plurality of recording media of an identical size. Accordingly, if the temperature of the non-conveyance span of the fixing rotary body increases accidentally during the print job, the heat shield may not be able to prevent the fixing rotary body from overheating. 
     SUMMARY 
     This specification describes below an improved fixing device. In one exemplary embodiment, the fixing device includes a fixing rotary body rotatable in a predetermined direction of rotation, a heater disposed opposite and heating the fixing rotary body, an opposed body contacting the fixing rotary body to form a nip therebetween through which a recording medium is conveyed, and a heat shield movably disposed opposite the fixing rotary body. The heat shield includes a noncircular shield portion disposed opposite a lateral end of the fixing rotary body in an axial direction thereof to shield the fixing rotary body from the heater and a recess defined by the shield portion in the axial direction of the fixing rotary body. A temperature detector is disposed opposite at least one of the fixing rotary body and the opposed body to detect a temperature of the at least one of the fixing rotary body and the opposed body. A controller is operatively connected to the heat shield and the temperature detector to determine a rotation angled position to which the heat shield is moved based on a size of the recording medium and the temperature of the at least one of the fixing rotary body and the opposed body detected by the temperature detector. 
     This specification further describes an improved image forming apparatus. In one exemplary embodiment, the image forming apparatus includes the fixing device described above. 
     This specification further describes an improved fixing method. In one exemplary embodiment, the fixing method includes placing a heat shield at a retracted position where the heat shield is not interposed between a heater and a fixing rotary body, conveying a recording medium over the fixing rotary body, determining that a temperature of the fixing rotary body is not lower than a predetermined first temperature, moving the heat shield to a first rotation angled position where the heat shield is interposed between the heater and the fixing rotary body to shield the fixing rotary body from the heater, determining that the temperature of the fixing rotary body is not lower than a predetermined second temperature higher than the first temperature, and moving the heat shield to a second rotation angled position where the heat shield is interposed between the heater and the fixing rotary body to shield the fixing rotary body more fully from the heater than at the first rotation angled position. 
    
    
     
       BRIEF DESCRIPTION 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 incorporated in the image forming apparatus shown in  FIG. 1  illustrating a heat shield incorporated therein situated at a shield position; 
         FIG. 3  is a block diagram of the fixing device shown in  FIG. 2 ; 
         FIG. 4  is a vertical sectional view of the fixing device shown in  FIG. 2  illustrating the heat shield situated at a retracted position; 
         FIG. 5  is a partial perspective view of the fixing device shown in  FIG. 4 ; 
         FIG. 6  is a partial perspective view of the fixing device shown in  FIG. 2  illustrating one lateral end of the heat shield in an axial direction thereof; 
         FIG. 7  is a partial perspective view of the fixing device shown in  FIG. 2  illustrating a driver incorporated therein; 
         FIG. 8  is a schematic diagram of the fixing device shown in  FIG. 4  illustrating a halogen heater pair incorporated therein, the heat shield, and the sizes of recording media; 
         FIG. 9  is a schematic diagram of the fixing device shown in  FIG. 2  illustrating the heat shield at the shield position; 
         FIG. 10  is a partially enlarged plan view of the heat shield shown in  FIG. 8 ; 
         FIG. 11  is a flowchart illustrating control processes for controlling the rotation angle of the heat shield shown in  FIG. 10 ; 
         FIG. 12  is a schematic diagram of a fixing device according to another exemplary embodiment of the present invention; and 
         FIG. 13  is a partial schematic diagram of the fixing device shown in  FIG. 12 . 
     
    
    
     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  1  according to an exemplary embodiment of the present invention is explained. 
       FIG. 1  is a schematic vertical sectional view of the image forming apparatus  1 . The image forming apparatus  1  may be a copier, a facsimile machine, a printer, a multifunction peripheral or a multifunction printer (MFP) having at least one of copying, printing, scanning, facsimile, and plotter functions, or the like. According to this exemplary embodiment, the image forming apparatus  1  is a color laser printer that forms color and monochrome toner images on recording media by electrophotography. 
     As shown in  FIG. 1 , the image forming apparatus  1  includes four image forming devices  4 Y,  4 M,  4 C, and  4 K situated at a center portion thereof. Although the image forming devices  4 Y,  4 M,  4 C, and  4 K contain yellow, magenta, cyan, and black developers (e.g., toners) that form yellow, magenta, cyan, and black toner images, respectively, resulting in a color toner image, they have an identical structure. 
     For example, each of the image forming devices  4 Y,  4 M,  4 C, and  4 K includes a drum-shaped photoconductor  5  serving as an image carrier that carries an electrostatic latent image and a resultant toner image; a charger  6  that charges an outer circumferential surface of the photoconductor  5 ; a development device  7  that supplies toner to the electrostatic latent image formed on the outer circumferential surface of the photoconductor  5 , thus visualizing the electrostatic latent image as a toner image; and a cleaner  8  that cleans the outer circumferential surface of the photoconductor  5 . It is to be noted that, in  FIG. 1 , reference numerals are assigned to the photoconductor  5 , the charger  6 , the development device  7 , and the cleaner  8  of the image forming device  4 K that forms a black toner image. However, reference numerals for the image forming devices  4 Y,  4 M, and  4 C that form yellow, magenta, and cyan toner images, respectively, are omitted. 
     Below the image forming devices  4 Y,  4 M,  4 C, and  4 K is an exposure device  9  that exposes the outer circumferential surface of the respective photoconductors  5  with laser beams. For example, the exposure device  9 , constructed of a light source, a polygon mirror, an f-θ lens, reflection mirrors, and the like, emits a laser beam onto the outer circumferential surface of the respective photoconductors  5  according to image data sent from an external device such as a client computer. 
     Above the image forming devices  4 Y,  4 M,  4 C, and  4 K is a transfer device  3 . For example, the transfer device  3  includes an intermediate transfer belt  30  serving as an intermediate transferor, four primary transfer rollers  31  serving as primary transferors, a secondary transfer roller  36  serving as a secondary transferor, a secondary transfer backup roller  32 , a cleaning backup roller  33 , a tension roller  34 , and a belt cleaner  35 . 
     The intermediate transfer belt  30  is an endless belt stretched across the secondary transfer backup roller  32 , the cleaning backup roller  33 , and the tension roller  34 . As a driver drives and rotates the secondary transfer backup roller  32  counterclockwise in  FIG. 1 , the secondary transfer backup roller  32  rotates the intermediate transfer belt  30  in a rotation direction R 1  by friction therebetween. 
     The four primary transfer rollers  31  sandwich the intermediate transfer belt  30  together with the four photoconductors  5 , respectively, forming four primary transfer nips between the intermediate transfer belt  30  and the photoconductors  5 . The primary transfer rollers  31  are connected to a power supply that applies a predetermined direct current voltage and/or alternating current voltage thereto. 
     The secondary transfer roller  36  sandwiches the intermediate transfer belt  30  together with the secondary transfer backup roller  32 , forming a secondary transfer nip between the secondary transfer roller  36  and the intermediate transfer belt  30 . Similar to the primary transfer rollers  31 , the secondary transfer roller  36  is connected to the power supply that applies a predetermined direct current voltage and/or alternating current voltage thereto. 
     The belt cleaner  35  includes a cleaning brush and a cleaning blade that contact an outer circumferential surface of the intermediate transfer belt  30 . A waste toner conveyance tube extending from the belt cleaner  35  to an inlet of a waste toner container conveys waste toner collected from the intermediate transfer belt  30  by the belt cleaner  35  to the waste toner container. 
     A bottle holder  2  situated in an upper portion of the image forming apparatus  1  accommodates four toner bottles  2 Y,  2 M,  2 C, and  2 K detachably attached thereto to contain and supply fresh yellow, magenta, cyan, and black toners to the development devices  7  of the image forming devices  4 Y,  4 M,  4 C, and  4 K, respectively. For example, the fresh yellow, magenta, cyan, and black toners are supplied from the toner bottles  2 Y,  2 M,  2 C, and  2 K to the development devices  7  through toner supply tubes interposed between the toner bottles  2 Y,  2 M,  2 C, and  2 K and the development devices  7 , respectively. 
     In a lower portion of the image forming apparatus  1  are a paper tray  10  that loads a plurality of recording media P (e.g., sheets) and a feed roller  11  that picks up and feeds a recording medium P from the paper tray  10  toward the secondary transfer nip formed between the secondary transfer roller  36  and the intermediate transfer belt  30 . The recording media P may be thick paper, postcards, envelopes, plain paper, thin paper, coated paper, art paper, tracing paper, OHP (overhead projector) transparencies, OHP film sheets, and the like. Additionally, a bypass tray that loads postcards, envelopes, OHP transparencies, OHP film sheets, and the like may be attached to the image forming apparatus  1 . 
     A conveyance path R extends from the feed roller  11  to an output roller pair  13  to convey the recording medium P picked up from the paper tray  10  onto an outside of the image forming apparatus  1  through the secondary transfer nip. The conveyance path R is provided with a registration roller pair  12  located below the secondary transfer nip formed between the secondary transfer roller  36  and the intermediate transfer belt  30 , that is, upstream from the secondary transfer nip in a recording medium conveyance direction A 1 . The registration roller pair  12  serving as a timing roller pair feeds the recording medium P conveyed from the feed roller  11  toward the secondary transfer nip. 
     The conveyance path R is further provided with a fixing device  20  located above the secondary transfer nip, that is, downstream from the secondary transfer nip in the recording medium conveyance direction A 1 . The fixing device  20  fixes a toner image transferred from the intermediate transfer belt  30  onto the recording medium P conveyed from the secondary transfer nip. The conveyance path R is further provided with the output roller pair  13  located above the fixing device  20 , that is, downstream from the fixing device  20  in the recording medium conveyance direction A 1 . The output roller pair  13  discharges the recording medium P bearing the fixed toner image onto the outside of the image forming apparatus  1 , that is, an output tray  14  disposed atop the image forming apparatus  1 . The output tray  14  stocks the recording medium P discharged by the output roller pair  13 . 
     With reference to  FIG. 1 , a description is provided of an image forming operation of the image forming apparatus  1  having the structure described above to form a color toner image on a recording medium P. 
     As a print job starts, a driver drives and rotates the photoconductors  5  of the image forming devices  4 Y,  4 M,  4 C, and  4 K, respectively, clockwise in  FIG. 1  in a rotation direction R 2 . The chargers  6  uniformly charge the outer circumferential surface of the respective photoconductors  5  at a predetermined polarity. The exposure device  9  emits laser beams onto the charged outer circumferential surface of the respective photoconductors  5  according to yellow, magenta, cyan, and black image data contained in image data sent from the external device, respectively, thus forming electrostatic latent images thereon. The development devices  7  supply yellow, magenta, cyan, and black toners to the electrostatic latent images formed on the photoconductors  5 , visualizing the electrostatic latent images into yellow, magenta, cyan, and black toner images, respectively. 
     Simultaneously, as the print job starts, the secondary transfer backup roller  32  is driven and rotated counterclockwise in  FIG. 1 , rotating the intermediate transfer belt  30  in the rotation direction R 1  by friction therebetween. The power supply applies a constant voltage or a constant current control voltage having a polarity opposite a polarity of the toner to the primary transfer rollers  31 , creating a transfer electric field at each primary transfer nip formed between the photoconductor  5  and the primary transfer roller  31 . 
     When the yellow, magenta, cyan, and black toner images formed on the photoconductors  5  reach the primary transfer nips, respectively, in accordance with rotation of the photoconductors  5 , the yellow, magenta, cyan, and black toner images are primarily transferred from the photoconductors  5  onto the intermediate transfer belt  30  by the transfer electric field created at the primary transfer nips such that the yellow, magenta, cyan, and black toner images are superimposed successively on a same position on the intermediate transfer belt  30 . Thus, a color toner image is formed on the intermediate transfer belt  30 . After the primary transfer of the yellow, magenta, cyan, and black toner images from the photoconductors  5  onto the intermediate transfer belt  30 , the cleaners  8  remove residual toner failed to be transferred onto the intermediate transfer belt  30  and therefore remaining on the photoconductors  5  therefrom. Thereafter, dischargers discharge the outer circumferential surface of the respective photoconductors  5 , initializing the surface potential thereof. 
     On the other hand, the feed roller  11  disposed in the lower portion of the image forming apparatus  1  is driven and rotated to feed a recording medium P from the paper tray  10  toward the registration roller pair  12  in the conveyance path R. As the recording medium P comes into contact with the registration roller pair  12 , the registration roller pair  12  that interrupts its rotation temporarily halts the recording medium P. 
     Thereafter, the registration roller pair  12  resumes its rotation and conveys the recording medium P to the secondary transfer nip at a time when the color toner image formed on the intermediate transfer belt  30  reaches the secondary transfer nip. The secondary transfer roller  36  is applied with a transfer voltage having a polarity opposite a polarity of the charged yellow, magenta, cyan, and black toners constituting the color toner image formed on the intermediate transfer belt  30 , thus creating a transfer electric field at the secondary transfer nip. The transfer electric field secondarily transfers the yellow, magenta, cyan, and black toner images constituting the color toner image formed on the intermediate transfer belt  30  onto the recording medium P collectively. After the secondary transfer of the color toner image from the intermediate transfer belt  30  onto the recording medium P, the belt cleaner  35  removes residual toner failed to be transferred onto the recording medium P and therefore remaining on the intermediate transfer belt  30  therefrom. The removed toner is conveyed and collected into the waste toner container. 
     Thereafter, the recording medium P bearing the color toner image is conveyed to the fixing device  20  that fixes the color toner image on the recording medium P. Then, the recording medium P bearing the fixed color toner image is discharged by the output roller pair  13  onto the output tray  14 . 
     The above describes the image forming operation of the image forming apparatus  1  to form the color toner image on the recording medium P. Alternatively, the image forming apparatus  1  may form a monochrome toner image by using any one of the four image forming devices  4 Y,  4 M,  4 C, and  4 K or may form a bicolor or tricolor toner image by using two or three of the image forming devices  4 Y,  4 M,  4 C, and  4 K. 
     With reference to  FIGS. 2 and 3 , a description is provided of a construction of the fixing device  20  incorporated in the image forming apparatus  1  described above. 
       FIG. 2  is a vertical sectional view of the fixing device  20 .  FIG. 3  is a block diagram of the fixing device  20 . As shown in  FIG. 2 , the fixing device  20  (e.g., a fuser) includes a fixing belt  21  serving as a fixing rotary body or an endless belt formed into a loop and rotatable in a rotation direction R 3 ; a pressing roller  22  serving as an opposed body disposed opposite an outer circumferential surface of the fixing belt  21  and rotatable in a rotation direction R 4  counter to the rotation direction R 3  of the fixing belt  21 ; a halogen heater pair  23  serving as a heater disposed inside the loop formed by the fixing belt  21  and heating the fixing belt  21 ; a nip formation assembly  24  disposed inside the loop formed by the fixing belt  21  and pressing against the pressing roller  22  via the fixing belt  21  to form a fixing nip N between the fixing belt  21  and the pressing roller  22 ; a stay  25  serving as a support disposed inside the loop formed by the fixing belt  21  and contacting and supporting the nip formation assembly  24 ; a reflector  26  disposed inside the loop formed by the fixing belt  21  and reflecting light radiated from the halogen heater pair  23  thereto toward the fixing belt  21 ; a heat shield  27  interposed between the halogen heater pair  23  and the fixing belt  21  to shield the fixing belt  21  from light radiated from the halogen heater pair  23 ; a temperature sensor  28   a  serving as a first temperature detector disposed opposite the outer circumferential surface of the fixing belt  21  and detecting the temperature of the fixing belt  21 ; a temperature sensor  28   b  serving as a second temperature detector disposed opposite an outer circumferential surface of the pressing roller  22  and detecting the temperature of the pressing roller  22 ; and a controller  90  depicted in  FIG. 3  operatively connected to the temperature sensors  28   a  and  28   b  and the heat shield  27  to control the rotation angle of the heat shield  27 . 
     A detailed description is now given of a construction of the fixing belt  21 . 
     The fixing belt  21  is a thin, flexible endless belt or film. For example, the fixing belt  21  is constructed of a base layer constituting an inner circumferential surface of the fixing belt  21  and a release layer constituting the outer circumferential surface of the fixing belt  21 . The base layer is made of metal such as nickel and SUS stainless steel or resin such as polyimide (PI). The release layer is made of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), polytetrafluoroethylene (PTFE), or the like. Alternatively, an elastic layer made of rubber such as silicone rubber, silicone rubber foam, and fluoro rubber may be interposed between the base layer and the release layer. 
     If the fixing belt  21  does not incorporate the elastic layer, the fixing belt  21  has a decreased thermal capacity that improves fixing performance of being heated to a predetermined fixing temperature quickly. However, as the pressing roller  22  and the fixing belt  21  sandwich and press a toner image T on a recording medium P passing through the fixing nip N, slight surface asperities of the fixing belt  21  may be transferred onto the toner image T on the recording medium P, resulting in variation in gloss of the solid toner image T. To address this problem, it is preferable that the fixing belt  21  incorporates the elastic layer having a thickness not smaller than about 80 micrometers. The elastic layer having the thickness not smaller than about 80 micrometers elastically deforms to absorb slight surface asperities of the fixing belt  21 , preventing variation in gloss of the toner image T on the recording medium P. 
     According to this exemplary embodiment, the fixing belt  21  is designed to be thin and have a reduced loop diameter so as to decrease the thermal capacity thereof. For example, the fixing belt  21  is constructed of the base layer having a thickness in a range of from about 20 micrometers to about 50 micrometers; the elastic layer having a thickness in a range of from about 80 micrometers to about 300 micrometers; and the release layer having a thickness in a range of from about 3 micrometers to about 50 micrometers. Thus, the fixing belt  21  has a total thickness not greater than about 1 mm. A loop diameter of the fixing belt  21  is in a range of from about 20 mm to about 40 mm. In order to decrease the thermal capacity of the fixing belt  21  further, the fixing belt  21  may have a total thickness not greater than about 0.20 mm and preferably not greater than about 0.16 mm. Additionally, the loop diameter of the fixing belt  21  may not be greater than about 30 mm. 
     A detailed description is now given of a construction of the pressing roller  22 . 
     The pressing roller  22  is constructed of a metal core  22   a ; an elastic layer  22   b  coating the metal core  22   a  and made of silicone rubber foam, silicone rubber, fluoro rubber, or the like; and a release layer  22   c  coating the elastic layer  22   b  and made of PFA, PTFE, or the like. A pressurization assembly presses the pressing roller  22  against the nip formation assembly  24  via the fixing belt  21 . Thus, the pressing roller  22  pressingly contacting the fixing belt  21  deforms the elastic layer  22   b  of the pressing roller  22  at the fixing nip N formed between the pressing roller  22  and the fixing belt  21 , thus creating the fixing nip N having a predetermined length in the recording medium conveyance direction A 1 . According to this exemplary embodiment, the pressing roller  22  is pressed against the fixing belt  21 . Alternatively, the pressing roller  22  may merely contact the fixing belt  21  with no pressure therebetween. 
     A driver (e.g., a motor) disposed inside the image forming apparatus  1  depicted in  FIG. 1  drives and rotates the pressing roller  22 . As the driver drives and rotates the pressing roller  22 , a driving force of the driver is transmitted from the pressing roller  22  to the fixing belt  21  at the fixing nip N, thus rotating the fixing belt  21  by friction between the pressing roller  22  and the fixing belt  21 . 
     According to this exemplary embodiment, the pressing roller  22  is a solid roller. Alternatively, the pressing roller  22  may be a hollow roller. In this case, a heater such as a halogen heater may be disposed inside the hollow roller. Further, the elastic layer  22   b  may be made of solid rubber. Alternatively, if no heater is disposed inside the pressing roller  22 , the elastic layer  22   b  may be made of sponge rubber. The sponge rubber is more preferable than the solid rubber because it has an increased insulation that draws less heat from the fixing belt  21 . 
     The halogen heater pair  23  is situated inside the loop formed by the fixing belt  21  and upstream from the fixing nip N in the recording medium conveyance direction A 1 . For example, the halogen heater pair  23  is situated lower than and upstream from a hypothetical line L passing through a center Q of the fixing nip N in the recording medium conveyance direction A 1  and an axis O of the pressing roller  22  in  FIG. 2 . The power supply situated inside the image forming apparatus  1  supplies power to the halogen heater pair  23  so that the halogen heater pair  23  heats the fixing belt  21 . 
     As shown in  FIG. 3 , the controller  90  (e.g., a processor), 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 pair  23  and the temperature sensor  28   a  controls the halogen heater pair  23  based on the temperature of the fixing belt  21  detected by the temperature sensor  28   a  so as to adjust the temperature of the fixing belt  21  to a desired fixing temperature. The controller  90  is also operatively connected to the temperature sensor  28   b  to control the halogen heater pair  23  based on the temperature of the pressing roller  22  detected by the temperature sensor  28   b.    
     As shown in  FIG. 2 , according to this exemplary embodiment, two halogen heaters constituting the halogen heater pair  23  are situated inside the loop formed by the fixing belt  21 . Alternatively, one halogen heater or three or more halogen heaters may be situated inside the loop formed by the fixing belt  21  according to the sizes of recording media P available in the image forming apparatus  1 . However, it is preferable that one or two halogen heaters are situated inside the loop formed by the fixing belt  21  in view of manufacturing costs and limited space inside the loop formed by the fixing belt  21 . Alternatively, instead of the halogen heater pair  23 , a resistance heat generator, a carbon heater, or the like may be employed as a heater that heats the fixing belt  21  by radiation heat. 
     A detailed description is now given of a construction of the nip formation assembly  24 . 
     The nip formation assembly  24  includes a base pad  241  and a slide sheet  240  (e.g., a low-friction sheet) covering an outer surface of the base pad  241 . For example, the slide sheet  240  covers an opposed face of the base pad  241  disposed opposite the fixing belt  21 . A longitudinal direction of the base pad  241  is parallel to an axial direction of the fixing belt  21  or the pressing roller  22 . The base pad  241  receives pressure from the pressing roller  22  to define the shape of the fixing nip N. According to this exemplary embodiment, the fixing nip N is planar in cross-section as shown in  FIG. 2 . Alternatively, the fixing nip N may be concave with respect to the pressing roller  22  or have other shapes. The slide sheet  240  reduces friction between the base pad  241  and the fixing belt  21  sliding over the base pad  241 . Alternatively, the base pad  241  may be made of a low friction material. In this case, the slide sheet  240  is not interposed between the base pad  241  and the fixing belt  21 . 
     The base pad  241  is made of a heat resistant material resistant against temperatures of 200 degrees centigrade or more to prevent thermal deformation of the nip formation assembly  24  by temperatures in a fixing temperature range desirable to fix the toner image T on the recording medium P, thus retaining the shape of the fixing nip N and quality of the toner image T formed on the recording medium P. For example, the base pad  241  is made of general heat resistant resin such as polyether sulfone (PES), polyphenylene sulfide (PPS), liquid crystal polymer (LCP), polyether nitrile (PEN), polyamide imide (PAI), polyether ether ketone (PEEK), or the like. 
     The base pad  241  is mounted on and supported by the stay  25 . Accordingly, even if the base pad  241  receives pressure from the pressing roller  22 , the base pad  241  is not bent by the pressure and therefore produces a uniform nip width throughout the entire width of the pressing roller  22  in the axial direction thereof. The stay  25  is made of metal having an increased mechanical strength, such as stainless steel and iron, to prevent bending of the nip formation assembly  24 . The base pad  241  is also made of a rigid material having an increased mechanical strength. For example, the base pad  241  is made of resin such as LCP, metal, ceramic, or the like. 
     A detailed description is now given of a construction of the reflector  26 . 
     The reflector  26  is mounted on and supported by the stay  25  and disposed opposite the halogen heater pair  23 . The reflector  26  reflects light or heat radiated from the halogen heater pair  23  thereto onto the fixing belt  21 , suppressing conduction of heat from the halogen heater pair  23  to the stay  25 . Thus, the reflector  26  facilitates efficient heating of the fixing belt  21 , saving energy. For example, the reflector  26  is made of aluminum, stainless steel, or the like. If the reflector  26  includes an aluminum base treated with silver-vapor-deposition to decrease radiation and increase reflectance of light, the reflector  26  heats the fixing belt  21  effectively. An opposed face of the reflector  26  disposed opposite the halogen heater pair  23  spans in a circumferential direction of the fixing belt  21  over the inner circumferential surface of the fixing belt  21 . The reflector  26  includes lateral end portions  26   a  disposed opposite a lower face of the halogen heater pair  23  in  FIG. 2  and in proximity to the inner circumferential surface of the fixing belt  21 . The lateral end portions  26   a  are curved along the inner circumferential surface of the fixing belt  21  in the circumferential direction thereof. The lateral end portions  26   a  are disposed opposite lateral ends of the halogen heater pair  23  in a longitudinal direction thereof parallel to the axial direction of the fixing belt  21  to shield the fixing belt  21  from light radiated from the halogen heater pair  23 . That is, the lateral end portions  26   a  do not extend throughout the entire width of the reflector  26  in a longitudinal direction thereof parallel to the axial direction of the fixing belt  21 . 
     With reference to  FIGS. 2 and 4 , a detailed description is now given of a configuration of the heat shield  27 . 
       FIG. 4  is a vertical sectional view of the fixing device  20 . The heat shield  27  is a metal plate, having a thickness in a range of from about 0.1 mm to about 1.0 mm, curved in the circumferential direction of the fixing belt  21  along the inner circumferential surface thereof. As shown in  FIG. 2 , the heat shield  27  is not circular in the circumferential direction of the fixing belt  21 . For example, the heat shield  27  is an arc in cross-section arched along the inner circumferential surface of the fixing belt  21 . The heat shield  27  is rotatable clockwise and counterclockwise in  FIGS. 2 and 4  in the circumferential direction of the fixing belt  21  on a track interposed between the halogen heater pair  23  and the fixing belt  21 . As shown in  FIG. 2 , a circumference of the fixing belt  21  is divided into two sections: a circumferential, direct heating span DH where the halogen heater pair  23  is disposed opposite and heats the fixing belt  21  directly and a circumferential, indirect heating span IH where the halogen heater pair  23  is disposed opposite the fixing belt  21  indirectly via the components other than the heat shield  27 , that is, the reflector  26 , the stay  25 , the nip formation assembly  24 , and the like. The heat shield  27  moves to a shield position shown in  FIG. 2  where the heat shield  27  is interposed between the halogen heater pair  23  and the fixing belt  21  in the direct heating span DH to shield the fixing belt  21  from light radiated from the halogen heater pair  23 . Conversely, the heat shield  27  moves to a retracted position shown in  FIG. 4  where the heat shield  27  retracts from the direct heating span DH to the indirect heating span IH and therefore is not interposed between the halogen heater pair  23  and the fixing belt  21 . That is, the heat shield  27  is behind the reflector  26  and the stay  25  and therefore disposed opposite the halogen heater pair  23  via the reflector  26  and the stay  25 . The heat shield  27  is made of a heat resistant material, for example, metal such as aluminum, iron, and stainless steel or ceramic. 
     With reference to  FIG. 5 , a description is provided of a configuration of flanges  40  incorporated in the fixing device  20 . 
       FIG. 5  is a partial perspective view of the fixing device  20 . As shown in  FIG. 5 , the flanges  40  serving as a belt holder are inserted into both lateral ends of the fixing belt  21  in the axial direction thereof, respectively, to rotatably support the fixing belt  21 . Both lateral ends of the flanges  40 , the halogen heater pair  23 , and the stay  25  in the axial direction of the fixing belt  21  are mounted on and supported by a pair of side plates of the fixing device  20 , respectively. 
     With reference to  FIG. 6 , a description is provided of a support mechanism that supports the heat shield  27 . 
       FIG. 6  is a partial perspective view of the fixing device  20  illustrating one lateral end of the heat shield  27  in the axial direction of the fixing belt  21 . As shown in  FIG. 6 , the heat shield  27  is supported by an arcuate slider  41  rotatably or slidably attached to the flange  40 . For example, a projection  27   a  disposed at each lateral end of the heat shield  27  in the axial direction of the fixing belt  21  is inserted into a hole  41   a  produced in the slider  41 . Thus, the heat shield  27  is attached to the slider  41 . The slider  41  includes a tab  41   b  projecting inboard in the axial direction of the fixing belt  21  toward the heat shield  27 . As the tab  41   b  of the slider  41  is inserted into an arcuate groove  40   a  produced in the flange  40 , the slider  41  is slidably movable in the groove  40   a . Accordingly, the heat shield  27 , together with the slider  41 , is rotatable or movable in a circumferential direction of the flange  40 . The flange  40  and the slider  41  are made of resin. 
     Although  FIG. 6  illustrates the support mechanism that supports the heat shield  27  at one lateral end thereof in the axial direction of the fixing belt  21 , another lateral end of the heat shield  27  in the axial direction of the fixing belt  21  is also supported by the support mechanism shown in  FIG. 6 . Thus, another lateral end of the heat shield  27  is also rotatably or movably supported by the slider  41  slidable in the groove  40   a  of the flange  40 . 
     With reference to  FIG. 7 , a description is provided of a construction of a driver  91  that drives and rotates the heat shield  27 . 
       FIG. 7  is a partial perspective view of the fixing device  20  illustrating the driver  91 . As shown in  FIG. 7 , the driver  91  includes a motor  42  serving as a driving source and a plurality of gears  43 ,  44 , and  45  constituting a gear train. The gear  43  serving as one end of the gear train is connected to the motor  42 . The gear  45  serving as another end of the gear train is connected to a gear  41   c  produced on the slider  41  along a circumferential direction thereof. Accordingly, as the motor  42  is driven, a driving force is transmitted from the motor  42  to the gear  41   c  of the slider  41  through the gear train, that is, the gears  43  to  45 , thus rotating the heat shield  27  supported by the slider  41 . 
     With reference to  FIG. 8 , a description is provided of a relation between the shape of the heat shield  27 , heat generators of the halogen heater pair  23 , and the sizes of recording media. 
       FIG. 8  is a schematic diagram of the fixing device  20  illustrating the halogen heater pair  23 , the heat shield  27 , and the sizes of recording media. 
     First, a detailed description is given of the shape of the heat shield  27 . 
     It is to be noted that an axial direction of the heat shield  27  defines a direction in which an axis of the heat shield  27  extends in the axial direction of the fixing belt  21 . A circumferential direction of the heat shield  27  defines a direction in which the heat shield  27  rotates in the circumferential direction of the fixing belt  21 . 
     As shown in  FIG. 8 , the heat shield  27  includes a pair of shield portions  48  constituting both lateral ends of the heat shield  27  in the axial direction thereof, respectively; a bridge  49  bridging the shield portions  48  in the axial direction of the heat shield  27 ; and a recess  50  defined by the shield portions  48  and the bridge  49 , and in turn itself defining an inboard edge of each shield portion  48 . The recess  50  between the pair of shield portions  48  in the axial direction of the heat shield  27  is defined and enclosed by the inboard edge of each shield portion  48  in the axial direction of the heat shield  27  and an inner edge  54  of the bridge  49 , that is, one end of the bridge  49  in the circumferential direction of the heat shield  27 , constituting a bottom of the recess  50 . The shield portions  48  are disposed opposite both lateral ends of the halogen heater pair  23  in the axial direction of the fixing belt  21 , respectively, to shield both lateral ends of the fixing belt  21  in the axial direction thereof from the halogen heater pair  23 . In the present embodiment, the pair of shield portions  48  and the bridge  49  constituting the heat shield  27  are in a single metal plate. The recess  50  between the pair of shield portions  48  in the axial direction of the heat shield  27  does not shield the fixing belt  21  from the halogen heater pair  23  and therefore allows light radiated from the halogen heater pair  23  to irradiate the fixing belt  21 . 
     Each shield portion  48  includes an axially straight edge  53  constituting one end of the shield portion  48  in the circumferential direction of the heat shield  27  and extending in the axial direction thereof. The axially straight edge  53  extends substantially throughout the entire width of the shield portion  48  in the axial direction of the heat shield  27  except for a sloped edge  52 , a detailed description of which is deferred. The axially straight edge  53  of the shield portion  48  is disposed downstream from the inner edge  54  of the bridge  49  in the rotation direction R 3  of the fixing belt  21  depicted in  FIG. 2 . For example, the shield portions  48  are disposed downstream from the bridge  49  in a shield direction Y, equivalent to the rotation direction R 3  of the fixing belt  21 , in which the heat shield  27  rotates and moves to the shield position shown in  FIG. 2 . The inner edge  54  of the bridge  49  is connected to the axially straight edge  53  of one shield portion  48  through the inboard edge of the shield portion  48  that is disposed opposite the inboard edge of another shield portion  48 . The inboard edge of the shield portion  48  includes a circumferentially straight edge  51  extending parallel to the circumferential direction of the heat shield  27  in which the heat shield  27  rotates and the sloped edge  52  angled relative to the circumferentially straight edge  51 . 
     As shown in  FIG. 8 , the sloped edge  52  is contiguous to the circumferentially straight edge  51  substantially in the shield direction Y. The sloped edge  52  is angled outboard from the circumferentially straight edge  51  substantially in the shield direction Y such that an interval between the sloped edge  52  and another sloped edge  52  increases. Accordingly, the recess  50  has a uniform, decreased width defined by the circumferentially straight edges  51  in the axial direction of the heat shield  27  and an increased width defined by the sloped edges  52  in the axial direction of the heat shield  27  that increases gradually in the shield direction Y. An outer edge  55  of the heat shield  27  situated at another end of the heat shield  27  in the circumferential direction thereof and defining an outer edge of the bridge  49  and the shield portions  48  extends straight in the axial direction of the heat shield  27 . 
     Next, a detailed description is given of a relation between the heat generators of the halogen heater pair  23  and the sizes of recording media. 
     As shown in  FIG. 8 , the halogen heater pair  23  has a plurality of heat generators having different lengths in the axial direction of the fixing belt  21  and being situated at different positions in the axial direction of the fixing belt  21  to heat different axial spans on the fixing belt  21  according to the size of the recording medium P. For example, the halogen heater pair  23  is constructed of the lower halogen heater  23  having a center heat generator  23   a  disposed opposite a center of the fixing belt  21  in the axial direction thereof and the upper halogen heater  23  having lateral end heat generators  23   b  disposed opposite both lateral ends of the fixing belt  21  in the axial direction thereof, respectively. The center heat generator  23   a  spans a conveyance span S 2  corresponding to a width W 2  of a medium recording medium P 2  in the axial direction of the fixing belt  21 . Conversely, the lateral end heat generators  23   b , together with the center heat generator  23   a , span a conveyance span S 3  corresponding to a width W 3  of a large recording medium P 3  greater than the width W 2  of the medium recording medium P 2  and a conveyance span S 4  corresponding to a width W 4  of an extra-large recording medium P 4  greater than the width W 3  of the large recording medium P 3 . 
     A detailed description is now given of a relation between the shape of the heat shield  27  and the sizes of the recording media P 2 , P 3 , and P 4 . 
     Each circumferentially straight edge  51  is situated inboard from and in proximity to an edge of the conveyance span S 3  corresponding to the width W 3  of the large recording medium P 3  in the axial direction of the fixing belt  21 . Each sloped edge  52  overlaps a side edge of a standard size recording medium in the axial direction of the fixing belt  21 . According to this exemplary embodiment, each sloped edge  52  overlaps the edge of the conveyance span S 3  corresponding to the width W 3  of the large recording medium P 3  as the standard size recording medium in the axial direction of the fixing belt  21 . 
     For example, the medium recording medium P 2  is a letter size recording medium having a width W 2  of 215.9 mm or an A4 size recording medium having a width W 2  of 210 mm. The large recording medium P 3  is a double letter size recording medium having a width W 3  of 279.4 mm or an A3 size recording medium having a width W 3  of 297 mm. The extra-large recording medium P 4  is an A3 extension size recording medium having a width W 4  of 329 mm. However, examples of the sizes of recording media are not limited to the above. Additionally, the medium, large, and extra-large sizes mentioned herein are relative terms. Hence, instead of the medium, large, and extra-large sizes, small, medium, and large sizes may be used. 
     With reference to  FIG. 2 , a description is provided of a fixing operation of the fixing device  20  described above. 
     As the image forming apparatus  1  depicted in  FIG. 1  is powered on, the power supply supplies power to the halogen heater pair  23  and at the same time the driver drives and rotates the pressing roller  22  clockwise in  FIG. 2  in the rotation direction R 4 . Accordingly, the fixing belt  21  rotates counterclockwise in  FIG. 2  in the rotation direction R 3  in accordance with rotation of the pressing roller  22  by friction between the pressing roller  22  and the fixing belt  21 . 
     A recording medium P bearing a toner image T formed by the image forming operation of the image forming apparatus  1  described above is conveyed in the recording medium conveyance direction A 1  while guided by a guide plate and enters the fixing nip N formed between the fixing belt  21  and the pressing roller  22  pressed against the fixing belt  21 . The fixing belt  21  heated by the halogen heater pair  23  heats the recording medium P and at the same time the pressing roller  22  pressed against the fixing belt  21 , together with the fixing belt  21 , exerts pressure on 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 a separator, the separator separates the recording medium P from the fixing belt  21 . Thereafter, the separated recording medium P is discharged by the output roller pair  13  depicted in  FIG. 1  onto the outside of the image forming apparatus  1 , that is, the output tray  14  where the recording medium P is stocked. 
     With reference to  FIG. 8 , a description is provided of control of the halogen heater pair  23  and the heat shield  27  according to the sizes of recording media. 
     As the medium recording medium P 2  is conveyed over the fixing belt  21  depicted in  FIG. 2 , the controller  90  depicted in  FIG. 3  turns on the center heat generator  23   a  to heat the conveyance span S 2  of the fixing belt  21  corresponding to the width W 2  of the medium recording medium P 2 . As the extra-large recording medium P 4  is conveyed over the fixing belt  21 , the controller  90  turns on the lateral end heat generators  23   b  as well as the center heat generator  28   a  to heat the conveyance span S 4  of the fixing belt  21  corresponding to the width W 4  of the extra-large recording medium P 4 . 
     However, as described above, the halogen heater pair  23  is configured to heat the conveyance span S 2  corresponding to the width W 2  of the medium recording medium P 2  and the conveyance span S 4  corresponding to the width W 4  of the extra-large recording medium P 4 . Accordingly, if the center heat generator  23   a  is turned on as the large recording medium P 3  is conveyed over the fixing belt  21 , the center heat generator  23   a  does not heat each outboard span S 2   a  outboard from the conveyance span S 2  in the axial direction of the fixing belt  21 . Consequently, the large recording medium P 3  is not heated throughout the entire width W 3  thereof. Conversely, if the lateral end heat generators  23   b  are turned on in addition to the center heat generator  23   a , the lateral end heat generators  23   b  and the center heat generator  23   a  heat the conveyance span S 4  greater than the conveyance span S 3  corresponding to the width W 3  of the large recording medium P 3 . If the large recording medium P 3  is conveyed over the fixing belt  21  while the lateral end heat generators  23   b  and the center heat generator  23   a  are turned on, the lateral end heat generators  23   b  may heat both outboard spans S 3   a  outboard from the conveyance span S 3  corresponding to the width W 3  of the large recording medium P 3 , resulting in overheating of the fixing belt  21  in the outboard spans S 3   a.    
     To address this circumstance, as the large recording medium P 3  is conveyed over the fixing belt  21 , the heat shield  27  moves to the shield position as shown in  FIG. 9 .  FIG. 9  is a schematic diagram of the fixing device  20 . At the shield position shown in  FIG. 9 , the shield portions  48  of the heat shield  27  shield the fixing belt  21  in a region in proximity to both side edges of the large recording medium P 3  and the outboard spans S 3   a , thus suppressing overheating of the fixing belt  21  in the outboard spans S 3   a  where the large recording medium P 3  is not conveyed. 
     Since the shield portions  48  are not endless in the circumferential direction of the fixing belt  21 , as the heat shield  27  rotates, the shield portions  48  shield the fixing belt  21  from the halogen heater pair  23  in a variable area on the fixing belt  21 . For example, as the heat shield  27  rotates in the shield direction Y toward the shield position shown in  FIG. 2 , the shield portions  48  shield the fixing belt  21  in an increased area. Conversely, as the heat shield  27  rotates in a retract direction counter to the shield direction Y toward the retracted position shown in  FIG. 4 , the shield portions  48  shield the fixing belt  21  in a decreased area. 
     Since each shield portion  48  includes the sloped edge  52 , as the rotation angle of the heat shield  27  changes, the shield portions  48  shield the fixing belt  21  in a variable area changed by stepless adjustment, especially at a smallest interval between the lateral end heat generators  23   b  and the fixing belt  21 . 
     With reference to  FIG. 10 , a description is provided of rotation angled positions of the heat shield  27 . 
       FIG. 10  is a partially enlarged plan view of the heat shield  27 .  FIG. 10  illustrates the heat shield  27  at three rotation angled positions, that is, a first rotation angled position AP 1 , a second rotation angled position AP 2 , and a third rotation angled position AP 3  selectable according to the size of the recording medium P. Alternatively, the heat shield  27  may be rotatable to two rotation angled positions or four or more rotation angled positions. 
     As shown in  FIG. 10 , a plurality of rotation angled positions is available to correspond to the plurality of sizes of the recording media. For example, the sloped edge  52  overlapping the side edge of the large recording medium P 3  in the axial direction of the heat shield  27  as shown in  FIG. 9  overlaps the lateral end heat generator  23   b  partially illustrated in  FIG. 10  in the circumferential direction of the heat shield  27  at the plurality of rotation angled positions of the heat shield  27 . 
     With reference to  FIG. 9 , a description is provided of the slope of the shield portion  48  of the heat shield  27 . 
     As shown in  FIG. 9 , the shield portion  48  may include a sloped edge  53 ′, indicated by the alternate long and short dashed line in  FIG. 9 , which forms the shield portion  48  into a triangle, instead of the sloped edge  52  and the axially straight edge  53 . The sloped edge  53 ′ is contiguous to and angled relative to the inner edge  54  of the bridge  49  extending in the axial direction of the heat shield  27 , increasing the slope of the shield portion  48  that changes the variable area on the fixing belt  21  shielded by the shield portion  48 . However, since the sloped edge  53 ′ decreases the area of the shield portion  48  compared to the sloped edge  52 , the sloped edge  53 ′ decreases an amount of light from the halogen heater pair  23  that is shielded by the shield portion  48 , overheating the fixing belt  21 . To address this circumstance, it is preferable that the shield portion  48  includes the axially straight edge  53  indicated by the solid line in  FIG. 9  that extends in the axial direction of the heat shield  27  at one end of the heat shield  27  in the circumferential direction thereof. 
     Alternatively, the shield portion  48  may include a sloped edge  52 ′ indicated by the alternate long and two short dashed line in  FIG. 9  that forms the shield portion  48  into a trapezoid, instead of the sloped edge  52 . The sloped edge  52 ′ is contiguous to the axially straight edge  53  and the inner edge  54  of the bridge  49  and angled relative to the inner edge  54  of the bridge  49 . Since the sloped edge  52 ′ decreases the area of the recess  50 , the sloped edge  52 ′ may allow the halogen heater pair  23  to heat the fixing belt  21  in a decreased area, resulting in insufficient heating of the fixing belt  21  in the conveyance span S 3  corresponding to the width W 3  of the large recording medium P 3 , for example. To address this circumstance, it is preferable that the shield portion  48  includes the circumferentially straight edge  51  abutting the recess  50  to secure the desired area of the recess  50 . 
     When a fixing job is finished or the temperature of the outboard span S 3   a  of the fixing belt  21  where the large recording medium P 3  is not conveyed decreases to a predetermined threshold and therefore the heat shield  27  is no longer requested to shield the fixing belt  21 , the controller  90  moves the heat shield  27  to the retracted position shown in  FIG. 4 . Thus, the fixing device  20  performs the fixing job precisely by moving the heat shield  27  to the shield position shown in  FIG. 2  at a proper time without decreasing the rotation speed of the fixing belt  21  and the pressing roller  22  to convey the large recording medium P 3 . Whether the heat shield  27  is at the shield position shown in  FIG. 2  or at the retracted position shown in  FIG. 4 , the bridge  49  of the heat shield  27  is disposed opposite the indirect heating span IH shown in  FIGS. 2 and 4 . Accordingly, the bridge  49  does not receive light from the halogen heater pair  23  directly. 
     As shown in  FIGS. 2 and 4 , a rotation axis of the heat shield  27  is situated in proximity to a center of the fixing belt  21  in cross-section, that is, a rotation axis of the fixing belt  21 ; a center of the halogen heater pair  23 , that is, a center of a filament of each of the center heat generator  23   a  and the lateral end heat generators  23   b  is situated closer to the inner circumferential surface of the fixing belt  21  than the rotation axis of the heat shield  27  is. Accordingly, at the shield position shown in  FIG. 2 , the heat shield  27  is disposed opposite the halogen heater pair  23  with a decreased interval therebetween. Conversely, at the retracted position shown in  FIG. 4 , the heat shield  27  is disposed opposite the halogen heater pair  23  with an increased interval therebetween. Consequently, at the retracted position, the heat shield  27  is less exposed to light radiated from the halogen heater pair  23  and therefore is less susceptible to overheating. 
     As shown in  FIG. 4 , since the nip formation assembly  24  is situated inside the loop formed by the fixing belt  21 , the nip formation assembly  24  prohibits the heat shield  27  from moving to the fixing nip N. To address this circumstance, the halogen heater pair  23  is situated upstream from the fixing nip N in the rotation direction R 3  of the fixing belt  21  so that the heat shield  27  is movable between the shield position shown in  FIG. 2  where the heat shield  27  is situated at an upstream position upstream from the fixing nip N in the rotation direction R 3  of the fixing belt  21  and the retracted position shown in  FIG. 4  where the heat shield  27  is situated at a downstream position downstream from the fixing nip N in the rotation direction R 3  of the fixing belt  21 . Accordingly, the heat shield  27  retracts to the downstream, retracted position shown in  FIG. 4  where the nip formation assembly  24  does not interfere with movement of the heat shield  27  while increasing a circumferential moving span of the heat shield  27  that moves in the circumferential direction of the fixing belt  21 . Such configuration to increase the circumferential moving span of the heat shield  27  is advantageous for the fixing device  20  incorporating the fixing belt  21  having a smaller diameter to reduce its thermal capacity because the smaller fixing belt  21  creates a smaller loop, and thus, a smaller enclosed, interior space. 
     The temperature sensor  28   a  for detecting the temperature of the fixing belt  21  is disposed opposite an axial span on the fixing belt  21  where the fixing belt  21  is subject to overheating. According to this exemplary embodiment, as shown in  FIG. 8 , the temperature sensor  28   a  is disposed opposite each outboard span S 3   a  outboard from the conveyance span S 3  corresponding to the width W 3  of the large recording medium P 3  because the fixing belt  21  is subject to overheating in the outboard span S 3   a . Since the fixing belt  21  is subject to overheating by the lateral end heat generators  23   b , the temperature sensors  28   a  are disposed opposite the lateral end heat generators  23   b , respectively. 
     With reference to  FIGS. 2 to 4 and 11 , a description is provided of one example of a control method for controlling the rotation angle of the heat shield  27 . 
       FIG. 11  is a flowchart illustrating control processes of the control method. It is to be noted that the heat shield  27  is at the retracted position shown in  FIG. 4  by default. 
     In step S 1 , upon receipt of a print job, the controller  90  receives information about the size of a recording medium, that is, a large recording medium P 3  of A3 size according to this example, and the number of prints, that is, the number of the recording media P 3  conveyed through the fixing nip N. In step S 2 , immediately after receiving the print job, the controller  90  determines storage of heat of the fixing device  20  based on the temperature of the outer circumferential surface of the pressing roller  22  detected by the temperature sensor  28   b . For example, the controller  90  determines whether or not the temperature of the pressing roller  22  is a predetermined temperature of 80 degrees centigrade or smaller. If the temperature of the pressing roller  22  is 80 degrees centigrade or lower (YES in step S 2 ), the controller  90  determines that the fixing device  20  stores an insufficient amount of heat, retaining the heat shield  27  at the default retracted position shown in  FIG. 4  without moving the heat shield  27  in step S 3 . Conversely, if the temperature of the pressing roller  22  is higher than 80 degrees centigrade (NO in step S 2 ), the controller  90  determines that the fixing device  20  stores a sufficient amount of heat and controls the driver  91  to move the heat shield  27  to the shield position shown in  FIG. 2  in step S 4 . For example, the heat shield  27  halts at the first rotation angled position AP 1  selected from among the first rotation angled position AP 1 , the second rotation angled position AP 2 , and the third rotation angled position AP 3  shown in  FIG. 10  that are available for the width W 3  of the large recording medium P 3 . 
     It is to be noted that the controller  90  determines storage of heat of the fixing device  20  based on the temperature of the pressing roller  22  immediately after receipt of the print job. Conversely, the controller  90  moves the heat shield  27  based on such determination after the temperature of the outer circumferential surface of the fixing belt  21  reaches a predetermined fixing temperature and before the large recording medium P 3  enters the fixing nip N. 
     Thus, before the large recording medium P 3  is conveyed through the fixing nip N, the controller  90  moves the heat shield  27  based on storage of heat of the fixing device  20 , that is, the temperature of the pressing roller  22 , and halts the heat shield  27  at the rotation angled position determined based on the size of the large recording medium P 3  and storage of heat of the fixing device  20 , that is, the temperature of the pressing roller  22 . 
     In step S 5 , the large recording medium P 3  enters the fixing nip N. While the large recording medium P 3  is conveyed through the fixing nip N, the controller  90  monitors the temperature of the fixing belt  21  detected by the temperature sensor  28   a  constantly. For example, the controller  90  determines whether or not the temperature of the fixing belt  21  is a predetermined first temperature of 200 degrees centigrade or higher in step S 6 . If the controller  90  determines that the temperature of the fixing belt  21  is 200 degrees centigrade or higher (YES in step S 6 ), the controller  90  controls the driver  91  to move the heat shield  27  to the first rotation angled position AP 1  in step S 8 . Conversely, if the controller  90  determines that the temperature of the fixing belt  21  is lower than 200 degrees centigrade (NO in step S 6 ), the controller  90  does not move the heat shield  27  and therefore retains the heat shield  27  at the default retracted position in step S 9 . It is to be noted that if the heat shield  27  is already at the first rotation angled position AP 1  based on storage of heat of the fixing device  20  or a conveyance time elapsed from starting of conveyance of the large recording medium P, a detailed description of which is deferred, even if the temperature of the fixing belt  21  is 200 degrees centigrade or higher, the controller  90  does not move the heat shield  27  and therefore retains the heat shield  27  at the first rotation angled position AP 1 . 
     According to this exemplary embodiment, in addition to monitoring the temperature of the fixing belt  21 , the controller  90  monitors the conveyance time elapsed from starting of conveyance of the large recording medium P 3  through the fixing nip N. For example, the controller  90  determines whether or not a predetermined first conveyance time of 10 seconds has elapsed after the large recording medium P 3  enters the fixing nip N in step S 7 . If the controller  90  determines that the first conveyance time has elapsed (YES in step S 7 ), the controller  90  controls the driver  91  to move the heat shield  27  to the first rotation angled position AP 1  selected from among the first rotation angled position AP 1 , the second rotation angled position AP 2 , and the third rotation angled position AP 3  that are available for the large recording medium P 3  in step S 8 . Conversely, if the controller  90  determines that the first conveyance time has not elapsed (NO in step S 7 ), the controller  90  retains the heat shield  27  at the retracted position in step S 9 . It is to be noted that if the heat shield  27  is already at the first rotation angled position AP 1  based on storage of heat of the fixing device  20  or the temperature of the fixing belt  21 , even if the first conveyance time of 10 seconds has elapsed, the controller  90  does not move the heat shield  27  and therefore retains the heat shield  27  at the first rotation angled position AP 1 . 
     Thereafter, as the print job continues, the controller  90  determines whether or not the temperature of the fixing belt  21  is a predetermined second temperature of 210 degrees centigrade or higher in step S 10 . Simultaneously, the controller  90  determines whether or not a predetermined second conveyance time of 20 seconds has elapsed in step S 11 . If the controller  90  determines that the temperature of the fixing belt  21  is the predetermined second temperature or higher (YES in step S 10 ) or the predetermined second conveyance time has elapsed (YES in step S 11 ), the controller  90  controls the driver  91  to move the heat shield  27  to the second rotation angled position AP 2  in step S 12 . Conversely, if the controller  90  determines that the temperature of the fixing belt  21  is lower than the predetermined second temperature (NO in step S 10 ) and the predetermined second conveyance time of 20 seconds has not elapsed (NO in step S 11 ), the controller  90  does not move the heat shield  27  and therefore retains the heat shield  27  at the first rotation angled position AP 1  in step S 13 . 
     Thereafter, as the print job continues, the controller  90  determines whether or not the temperature of the fixing belt  21  is a predetermined third temperature of 220 degrees centigrade or higher in step S 14 . Simultaneously, the controller  90  determines whether or not a predetermined third conveyance time of 30 seconds has elapsed in step S 15 . If the controller  90  determines that the temperature of the fixing belt  21  is the predetermined third temperature or higher (YES in step S 14 ) or the predetermined third conveyance time has elapsed (YES in step S 15 ), the controller  90  controls the driver  91  to move the heat shield  27  to the third rotation angled position AP 3  in step S 16 . Conversely, if the controller  90  determines that the temperature of the fixing belt  21  is lower than the predetermined third temperature (NO in step S 14 ) and the predetermined third conveyance time of 30 seconds has not elapsed (NO in step S 15 ), the controller  90  does not move the heat shield  27  and therefore retains the heat shield  27  at the second rotation angled position AP 2  in step S 17 . 
     Thus, during the print job, that is, from starting of conveyance of the large recording medium P 3  through the fixing nip N until the print job is finished, the controller  90  moves the heat shield  27  based on the temperature of the fixing belt  21  predicted from the temperature of the pressing roller  22  and selects the rotation angled position from among the first rotation angled position AP 1 , the second rotation angled position AP 2 , and the third rotation angled position AP 3  based on the size of the large recording medium P 3  and the temperature of the fixing belt  21 . 
     Finally, in step S 18 , the print job is finished. 
     It is to be noted that when the received print job is finished, printing stops even during the processes described above. For example, if the controller  90  controls the rotation angle of the heat shield  27  based on the size of the recording medium P, once the controller  90  receives information about the size of the recording medium P, the controller  90  moves the heat shield  27  to the rotation angled position corresponding to the size of the recording medium P irrespective of the temperature of the fixing belt  21  and the pressing roller  22 . In this case, when the fixing device  20  is maintained substantially at an ambient temperature upon starting a print job after the fixing device  20  is turned off for a substantial time, the heat shield  27  may move to the shield position shown in  FIG. 2  even when the fixing belt  21  is heated insufficiently, thus shielding the fixing belt  21  from the halogen heater pair  23  and thereby increasing a warm-up time to warm up the fixing belt  21 . Further, once the heat shield  27  moves to the rotation angled position corresponding to the size of the recording medium P, the heat shield  27  is retained at the rotation angled position until the print job is finished. Accordingly, even if the temperature of the fixing belt  21  increases accidentally, the heat shield  27  may not be moved. 
     To address this circumstance, the fixing device  20  moves the heat shield  27  based on storage of heat of the fixing device  20 , that is, the temperature of the pressing roller  22 , before the recording medium P enters the fixing nip N. Conversely, the fixing device  20  moves the heat shield  27  based on the temperature of the fixing belt  21  during the print job. Accordingly, the controller  90  moves the heat shield  27  at the proper time based on the temperature of the fixing belt  21  or the pressing roller  22 , thus shielding the fixing belt  21  from the halogen heater pair  23 . 
     As shown in  FIG. 8 , since the temperature sensor  28   a  is disposed opposite a part of the fixing belt  21  in the axial direction thereof, that is, the outboard span S 3   a , the temperature sensor  28   a  does not detect the temperature of other part of the fixing belt  21  in the axial direction thereof, that is, a center of the fixing belt  21  in the axial direction thereof, for example. Further, the temperature of the fixing belt  21  may increase sharply during the print job. In this case, if the controller  90  moves the heat shield  27  after the temperature sensor  28   a  detects the temperature of the fixing belt  21 , it may be too late to prevent overheating of the fixing belt  21 . Hence, if the controller  90  moves the heat shield  27  based on the temperature of the fixing belt  21 , the heat shield  27  may not move at the proper time, resulting in overheating of the fixing belt  21  in both lateral ends in the axial direction thereof. 
     To address this circumstance, the controller  90  of the fixing device  20  moves the heat shield  27  based on the conveyance time elapsed after the recording medium P enters the fixing nip N in addition to the temperature of the fixing belt  21 . For example, the controller  90  obtains in advance data about a relation between the conveyance time for conveying the recording media P of sizes available in the fixing device  20  and the temperature of the fixing belt  21  from past print data and experimental results. Thus, the controller  90  presets the conveyance time based on which the heat shield  27  is moved according to the relation between the conveyance time and the temperature of the fixing belt  21 . Hence, by moving the heat shield  27  based on the conveyance time and the temperature of the fixing belt  21 , even if it is difficult to prevent overheating of the fixing belt  21  by moving the heat shield  27  solely based on the temperature of the fixing belt  21 , the controller  90  moves the heat shield  27  at the proper time to shield the fixing belt  21  from the halogen heater pair  23 . 
     The controller  90  incorporated in the fixing device  20  determines the rotation angled position of the heat shield  27  based on the size of the recording medium P and the temperature of the pressing roller  22  or the fixing belt  21 . Accordingly, the controller  90  determines a shielded span on the fixing belt  21  that is shielded by the heat shield  27  from the halogen heater pair  23  so that the temperature of the fixing belt  21  is in a proper range. Consequently, overheating of the fixing belt  21  at both lateral ends in the axial direction thereof is prevented. For example, the sloped edge  52  of the heat shield  27  allows fine adjustment of the shielded span on the fixing belt  21 . Accordingly, the heat shield  27  is moved to a desired rotation angled position based on the temperature of the pressing roller  22  or the fixing belt  21 . Further, the controller  90  selects a single rotation angled position from among the plurality of rotation angled positions, that is, the first rotation angled position AP 1 , the second rotation angled position AP 2 , and the third rotation angled position AP 3  available to the plurality of sizes of the recording media P, based on the temperature of the pressing roller  22  or the fixing belt  21 . Accordingly, the controller  90  determines a desired rotation angled position using an uncomplicated control process. 
     Additionally, the controller  90  moves the heat shield  27  based on the size of the recording medium P and the conveyance time. Accordingly, even if it is difficult for the controller  90  to determine the rotation angled position of the heat shield  27  based on the temperature of the fixing belt  21  during the print job, the controller  90  predicts the temperature of the fixing belt  21  from the conveyance time, thus determining the rotation angled position of the heat shield  27  precisely. 
     A description is provided of alternative configurations of the fixing device  20 . 
     With reference to  FIGS. 2 and 11 , a detailed description is now given of a first alternative configuration of the fixing device  20 . 
     According to the exemplary embodiments described above with reference to  FIG. 11 , the controller  90  of the fixing device  20  determines the rotation angled position of the heat shield  27  based on the temperature of the fixing belt  21  during the print job. Alternatively, the controller  90  may determine the rotation angled position of the heat shield  27  based on the temperature of the pressing roller  22  as mentioned in steps S 6 , S 10 , and S 14  in  FIG. 11 . Since the pressing roller  22  rotates in the rotation direction R 4  while contacting the fixing belt  21 , the controller  90  predicts the temperature of the fixing belt  21  from the temperature of the pressing roller  22 . Accordingly, even if it is difficult to add the temperature sensor  28   a  for detecting the temperature of the fixing belt  21  due to limited space inside the fixing device  20  or the image forming apparatus  1 , the controller  90  determines a time to move the heat shield  27  and the rotation angled position of the heat shield  27  based on the temperature of the pressing roller  22  detected by the temperature sensors  28   b . In this case, similarly to the temperature sensors  28   a , the temperature sensors  28   b  for detecting the temperature of the pressing roller  22  are disposed opposite the outboard spans S 3   a  on the pressing roller  22  outboard from the conveyance span S 3  corresponding to the width W 3  of the large recording medium P 3  in the axial direction of the pressing roller  22  as shown in  FIG. 8 . 
     With reference to  FIGS. 2 and 11 , a detailed description is now given of a second alternative configuration of the fixing device  20 . 
     According to the exemplary embodiments described above with reference to  FIG. 11 , the controller  90  of the fixing device  20  determines the rotation angled position of the heat shield  27  during the print job based on the size of the recording medium P and the conveyance time as mentioned in steps S 7 , S 11 , and S 15  in  FIG. 11 . Additionally or alternatively, the controller  90  may determine the rotation angled position of the heat shield  27  during the print job based on the size of the recording medium P and the number of prints, that is, the number of the recording media P conveyed through the fixing nip N. For example, the controller  90  obtains in advance data about a relation between the number of prints, that is, the number of the recording media P conveyed through the fixing nip N, for printing on the recording media P of sizes available in the fixing device  20  and the temperature of the fixing belt  21  from past print data and from experimental results. Thus, the controller  90  presets the number of prints based on which the heat shield  27  is moved according to the relation between the number of prints and the temperature of the fixing belt  21 . When the actual number of prints exceeds the preset number of prints based on which the heat shield  27  is moved, the controller  90  moves the heat shield  27  to the rotation angled position corresponding to the actual number of prints. 
     With reference to  FIG. 8 , a detailed description is now given of a third alternative configuration of the fixing device  20 . 
     According to the exemplary embodiments described above with reference to  FIG. 8 , the controller  90  of the fixing device  20  moves the heat shield  27  in the shield direction Y. Alternatively, the controller  90  may move the heat shield  27  in a retract direction counter to the shield direction Y based on the temperature of the fixing belt  21  or the pressing roller  22 . For example, when the temperature of the fixing belt  21  or the pressing roller  22  is a predetermined temperature or lower during the print job, the controller  90  moves the heat shield  27  in the retract direction counter to the shield direction Y toward the retracted position shown in  FIG. 4 . Accordingly, an increased amount of light from the halogen heater pair  23  irradiates the fixing belt  21 , heating the fixing belt  21  to a desired fixing temperature quickly. 
     With reference to  FIGS. 12 and 13 , a description is provided of a configuration of a fixing device  20 S incorporating a heat shield  27 S according to another exemplary embodiment. 
       FIG. 12  is a schematic diagram of the fixing device  20 S.  FIG. 13  is a partial schematic diagram of the fixing device  20 S. As shown in  FIG. 12 , the heat shield  27 S includes a pair of shield portions  48 S disposed at both lateral ends of the heat shield  27 S in an axial direction thereof, respectively. Each of the shield portions  48 S has two steps. Each shield portion  48 S includes a first shield section  48   b  having an increased length in a longitudinal direction of the heat shield  27 S parallel to the axial direction thereof and a second shield section  48   a  having a decreased length in the longitudinal direction of the heat shield  27 S. The bridge  49  bridges the first shield section  48   b  of one shield portion  48 S situated at one lateral end of the heat shield  27 S and the first shield section  48   b  of another shield portion  48 S situated at another lateral end of the heat shield  27 S in the axial direction thereof. The second shield section  48   a  is contiguous to and outboard from the first shield section  48   b  in the axial direction of the heat shield  27 S. An axially straight edge  53   a  situated at one end of the second shield section  48   a  in a circumferential direction of the heat shield  27 S, that is, the rotation direction R 3  of the fixing belt  21 , is disposed downstream from an axially straight edge  53   b  situated at one end of the first shield section  48   b  in the circumferential direction of the heat shield  27 S in the shield direction Y. The axially straight edge  53   b  is disposed downstream from the inner edge  54  of the bridge  49  in the shield direction Y. A sloped edge  52   a , that is, an inboard edge of one second shield section  48   a  in the axial direction of the heat shield  27 S is disposed opposite another sloped edge  52   a , that is, an inboard edge of another second shield section  48   a  in the axial direction of the heat shield  27 S. Similarly, a sloped edge  52   b , that is, an inboard edge of one first shield section  48   b  in the axial direction of the heat shield  27 S is disposed opposite another sloped edge  52   b , that is, an inboard edge of another first shield section  48   b  in the axial direction of the heat shield  27 S. That is, the sloped edges  52   a  and  52   b  constitute an inboard edge of the shield portion  48 S in the axial direction of the heat shield  27 S. The recess  50  between the pair of shield portions  48 S in the axial direction of the heat shield  27 S is defined and enclosed by the sloped edge  52   a  of each second shield section  48   a , the axially straight edge  53   b  and the sloped edge  52   b  of each first shield section  48   b , and the inner edge  54  of the bridge  49 . 
     At least four sizes of recording media P including a small recording medium P 1 , a medium recording medium P 2 , a large recording medium P 3 , and an extra-large recording medium P 4  are available in the fixing device  20 S. For example, the small recording medium P 1  includes a postcard having a width of 100 mm. The medium recording medium P 2  includes an A4 size recording medium having a width of 210 mm. The large recording medium P 3  includes an A3 size recording medium having a width of 297 mm. The extra-large recording medium P 4  includes an A3 extension size recording medium having a width of 329 mm. However, the small recording medium P 1 , the medium recording medium P 2 , the large recording medium P 3 , and the extra-large recording medium P 4  may include recording media of other sizes. 
     A width W 1  of the small recording medium P 1  is smaller than the length of the center heat generator  23   a  in the longitudinal direction of the halogen heater pair  23  parallel to the axial direction of the heat shield  27 S. The sloped edge  52   b  of the first shield section  48   b  overlaps a side edge of the small recording medium P 1 . The sloped edge  52   a  of the second shield section  48   a  overlaps a side edge of the large recording medium P 3 . It is to be noted that a description of the relation between the position of recording media other than the small recording medium P 1 , that is, the medium recording medium P 2 , the large recording medium P 3 , and the extra-large recording medium P 4 , and the position of the center heat generator  23   a  and the lateral end heat generators  23   b  of the fixing device  20 S is omitted because it is similar to that of the fixing device  20  described above. 
     As the small recording medium P 1  is conveyed through the fixing nip N, the center heat generator  23   a  is turned on. However, since the center heat generator  23   a  heats the conveyance span S 2  on the fixing belt  21  corresponding to the width W 2  of the medium recording medium P 2  that is greater than the width W 1  of the small recording medium P 1 , the controller  90  moves the heat shield  27 S to the shield position shown in  FIG. 13 . At the shield position, each first shield section  48   b  of the heat shield  27 S shields the fixing belt  21  from the center heat generator  23   a  in an outboard span S 1   a  outboard from a conveyance span S 1  corresponding to the width W 1  of the small recording medium P 1  in the axial direction of the fixing belt  21 . Accordingly, the fixing belt  21  does not overheat in each outboard span S 1   a  where the small recording medium P 1  is not conveyed over the fixing belt  21 . 
     As the medium recording medium P 2 , the large recording medium P 3 , and the extra-large recording medium P 4  are conveyed through the fixing nip N, the controller  90  performs a control for controlling the halogen heater pair  23  and the heat shield  27 S that is similar to the control for controlling the halogen heater pair  23  and the heat shield  27  described above. In this case, each second shield section  48   a  of the heat shield  27 S shields the fixing belt  21  from the halogen heater pair  23  as each shield portion  48  of the fixing device  20  does. 
     Like the shield portion  48  of the fixing device  20  that has the sloped edge  52 , the second shield section  48   a  and the first shield section  48   b  have the sloped edges  52   a  and  52   b , respectively. Accordingly, by changing the rotation angled position of the heat shield  27 S, the controller  90  changes the span on the fixing belt  21  shielded from the center heat generator  23   a  and the lateral end heat generators  23   b  of the halogen heater pair  23  by the second shield section  48   a  and the first shield section  48   b  of each shield portion  48 S. 
     The present invention is not limited to the details of the exemplary embodiments described above, and various modifications and improvements are possible. For example, the fixing belt  21  is used as a fixing rotary body. Alternatively, a hollow, tubular fixing roller, a solid fixing roller, a fixing film, or the like may be used as a fixing rotary body. The pressing roller  22  is used as an opposed body. Alternatively, a pressing belt, a pressing plate, a pressing pad, or the like may be used as an opposed body. Further, the shape of the heat shield is not limited to those of the heat shields  27  and  27 S. For example, the heat shield may have three or more steps corresponding to the sizes of recording media available in the fixing device. 
     Further, when the heat shield  27  is at the retracted position shown in  FIG. 4 , a part of the heat shield  27  is disposed opposite the direct heating span DH on the fixing belt  21  and therefore heated by the halogen heater pair  23  directly. Alternatively, the entire heat shield  27  may be configured to be disposed opposite the indirect heating span IH on the fixing belt  21  by modifying the shape and the circumferential moving span of the heat shield  27  or the shape of the stay  25  and the reflector  26 . In this case, the heat shield  27  at the retracted position is not heated by the halogen heater pair  23  and thereby is not subject to thermal deformation and wear. 
     With reference to  FIGS. 2 to 4, 8, and 12 , a description is provided of advantages of the fixing devices  20  and  20 S described above. 
     The fixing devices  20  and  20 S include a fixing rotary body (e.g., the fixing belt  21 ); a heater (e.g., the halogen heater pair  23 ) to heat the fixing rotary body; an opposed body (e.g., the pressing roller  22 ) contacting an outer circumferential surface of the fixing rotary body to form a nip (e.g., the fixing nip N) therebetween through which a recording medium is conveyed; a heat shield (e.g., the heat shields  27  and  27 S) movably disposed opposite the heater to shield the fixing rotary body from the heater; a temperature detector (e.g., the temperature sensors  28   a  and  28   b ) to detect the temperature of the fixing rotary body or the opposed body; and a controller (e.g., the controller  90 ) to move the heat shield between a plurality of rotation angled positions. The heat shield includes a non-circular shield portion (e.g., the shield portions  48  and  48 S) disposed opposite a lateral end of the fixing rotary body in an axial direction thereof to shield the fixing rotary body from the heater and a recess (e.g., the recess  50 ) contiguous to the shield portion. The shield portion is not circular in a circumferential direction of the fixing rotary body. The controller determines the rotation angled position of the heat shield based on the size of the recording medium and the temperature of the fixing rotary body or the opposed body detected by the temperature detector. 
     The controller determines the rotation angled position of the heat shield based on the size of the recording medium and the temperature of the fixing rotary body or the opposed body. Accordingly, even if the temperature of an outboard span (e.g., the outboard spans S 1   a , S 2   a , and S 3   a ) of the fixing rotary body where the recording medium is not conveyed increases accidentally, the controller moves the heat shield to the rotation angled position where the heat shield shields the fixing rotary body from the heater in an increased span in the axial direction of the fixing rotary body based on the temperature of the fixing rotary body or the opposed body contacting the fixing rotary body. Consequently, the heat shield prevents overheating of the fixing rotary body in the outboard span where the recording medium is not conveyed. 
     According to the exemplary embodiments described above, the recording medium conveyed over the fixing belt  21  is centered in the axial direction thereof. Alternatively, the recording medium may be conveyed along one edge of the fixing belt  21  in the axial direction thereof. In this case, the heat shields  27  and  27 S may include a single shield portion equivalent to the shield portion  48  or  48 S that is disposed opposite one lateral end of the fixing belt  21  in the axial direction thereof. 
     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.