Patent Publication Number: US-9429891-B2

Title: Fixing device and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. application Ser. No. 14/040,866, filed Sep. 30, 2013, and is based upon and claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2012-233990, filed on Oct. 23, 2012, in the Japanese Patent Office, and the entire contents of each of the above are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing an image on a recording medium and an image forming apparatus incorporating the fixing device. 
     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 an endless belt heated by a heater and a pressing roller pressed against the endless belt to form a fixing nip therebetween through which a recording medium bearing a toner image is conveyed. As the recording medium is conveyed through the fixing nip, the endless belt and the pressing roller apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium. 
     Since the endless belt has a decreased heat capacity, it is heated by the heater quickly, shortening a warm-up time taken to heat the endless belt to a predetermined fixing temperature at which the toner image is fixed on the recording medium. For example, a metal heat conductor may be disposed opposite an inner circumferential surface of the endless belt. As the heater situated inside the substantially tubular, heat conductor heats the heat conductor, the heat conductor in turn heats the endless belt. A nip formation pad disposed opposite the inner circumferential surface of the endless belt presses the endless belt against the pressing roller to form the fixing nip between the endless belt and the pressing roller. A heat insulator is interposed between the heater and the nip formation pad to shield the nip formation pad from the heater. Thus, the heat insulator facilitates heating of the heat conductor and enhances durability of the nip formation pad. 
     However, the heat insulator, if it has an increased heat capacity, may decrease an amount of heat conducted to the heat conductor, degrading heating of the heat conductor. As a result, it may take longer to warm up the endless belt to the predetermined fixing temperature, consuming an increased amount of energy. 
     SUMMARY 
     This specification describes below an improved fixing device. In one exemplary embodiment, the fixing device includes a flexible endless belt rotatable in a predetermined direction of rotation and a heat conductor disposed opposite an inner circumferential surface of the endless belt to heat the endless belt. A heater is disposed opposite an inner circumferential surface of the heat conductor to heat the heat conductor. A pressing rotary body is disposed opposite the endless belt. A nip formation pad is disposed opposite the inner circumferential surface of the endless belt and presses the endless belt against the pressing rotary body to form a fixing nip between the endless belt and the pressing rotary body through which a recording medium bearing a toner image is conveyed. The nip formation pad includes an abutment face. A support is disposed opposite the inner circumferential surface of the heat conductor and contacts the abutment face of the nip formation pad to support the nip formation pad against pressure from the pressing rotary body. A heat insulator is interposed between the heater and the nip formation pad and the support to shield the nip formation pad and the support from the heater. 
     This specification further describes an improved image forming apparatus. In one exemplary embodiment, the image forming apparatus includes the fixing device described above. 
    
    
     
       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 ; 
         FIG. 3  is a partial vertical sectional view of a fixing belt incorporated in the fixing device shown in  FIG. 2 ; 
         FIG. 4  is a side view of the fixing device shown in  FIG. 2 ; 
         FIG. 5  is a perspective view of a heat insulator incorporated in the fixing device shown in  FIG. 2 ; 
         FIG. 6  is a perspective view of a heat conductor incorporated in the fixing device shown in  FIG. 2 ; 
         FIG. 7A  is a perspective view of a nip formation pad incorporated in the fixing device shown in  FIG. 2  before being attached to the heat conductor; 
         FIG. 7B  is a perspective view of the nip formation pad shown in  FIG. 7A  attached to the heat conductor; 
         FIG. 8  is a vertical sectional view of the nip formation pad and the heat conductor shown in  FIG. 7B ; 
         FIG. 9  is a schematic side view of the fixing belt and the heat conductor incorporated in the fixing device shown in  FIG. 4 ; 
         FIG. 10  is a schematic side view of the heat conductor shown in  FIG. 9 ; 
         FIG. 11  is a graph showing a relation between the Vickers hardness of the heat conductor shown in  FIG. 10  and the temperature of the fixing belt shown in  FIG. 9  at which the heat conductor is crimped; and 
         FIG. 12  is a vertical sectional view of a fixing device incorporating a heat insulator as a variation of the heat insulator shown in  FIG. 5 . 
     
    
    
     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 tandem color printer that forms color and monochrome toner images on recording media by electrophotography. 
     As shown in  FIG. 1 , the image forming apparatus  1  includes image forming devices  4 Y,  4 M,  4 C, and  4 K that form yellow, magenta, cyan, and black toner images, respectively, a paper tray  12 , a fixing device  20 , an intermediate transfer unit  85 , and a bottle holder  101 . 
     The bottle holder  101  situated in an upper portion of the image forming apparatus  1  holds four toner bottles  102 Y,  102 M,  102 C, and  102 K detachably attached thereto and containing fresh yellow, magenta, cyan, and black toners, respectively. 
     Below the bottle holder  101  is the intermediate transfer unit  85  that includes an intermediate transfer belt  78 , four primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K, an intermediate transfer belt cleaner  80 , a secondary transfer backup roller  82 , a cleaning backup roller  83 , and a tension roller  84 . 
     The intermediate transfer belt  78  of the intermediate transfer unit  85  is disposed opposite the image forming devices  4 Y,  4 M,  4 C, and  4 K aligned along a rotation direction R 1  of the intermediate transfer belt  78 . The image forming devices  4 Y,  4 M,  4 C, and  4 K include photoconductive drums  5 Y,  5 M,  5 C, and  5 K, chargers  75 Y,  75 M,  75 C, and  75 K, development devices  76 Y,  76 M,  76 C, and  76 K, cleaners  77 Y,  77 M,  77 C, and  77 K, and dischargers, respectively. 
     A description is provided of image forming processes performed on the photoconductive drums  5 Y,  5 M,  5 C, and  5 K. 
     A driver (e.g., a motor) drives and rotates the photoconductive drums  5 Y,  5 M,  5 C, and  5 K clockwise in  FIG. 1  in a rotation direction R 2 . The image forming processes include a charging process, an exposure process, a development process, a primary transfer process, and a cleaning process. 
     In the charging process, the chargers  75 Y,  75 M,  75 C, and  75 K disposed opposite the photoconductive drums  5 Y,  5 M,  5 C, and  5 K uniformly charge an outer circumferential surface of the respective photoconductive drums  5 Y,  5 M,  5 C, and  5 K. 
     In the exposure process, an exposure device  3  situated below the photoconductive drums  5 Y,  5 M,  5 C, and  5 K emits laser beams Ly, Lm, Lc, and Lk onto the charged outer circumferential surface of the respective photoconductive drums  5 Y,  5 M,  5 C, and  5 K that scan and expose the outer circumferential surface of the respective photoconductive drums  5 Y,  5 M,  5 C, and  5 K according to yellow, magenta, cyan, and black image data sent from an external device such as a client computer, thus forming electrostatic latent images thereon. 
     In the development process, the development devices  76 Y,  76 M,  76 C, and  76 K disposed opposite the photoconductive drums  5 Y,  5 M,  5 C, and  5 K develop the electrostatic latent images formed on the photoconductive drums  5 Y,  5 M,  5 C, and  5 K with yellow, magenta, cyan, and black toners supplied from the toner bottles  102 Y,  102 M,  102 C, and  102 K into yellow, magenta, cyan, and black toner images, respectively. 
     The photoconductive drums  5 Y,  5 M,  5 C, and  5 K are disposed opposite the primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K via the intermediate transfer belt  78  to form primary transfer nips between the intermediate transfer belt  78  and the photoconductive drums  5 Y,  5 M,  5 C, and  5 K, respectively. In the primary transfer process, the primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K primarily transfer the yellow, magenta, cyan, and black toner images formed on the photoconductive drums  5 Y,  5 M,  5 C, and  5 K, respectively, onto the intermediate transfer belt  78 . After the primary transfer process, a slight amount of residual toner failed to be transferred onto the intermediate transfer belt  78  remains on the photoconductive drums  5 Y,  5 M,  5 C, and  5 K. 
     To address this circumstance, in the cleaning process, a cleaning blade of the respective cleaners  77 Y,  77 M,  77 C, and  77 K disposed opposite the photoconductive drums  5 Y,  5 M,  5 C, and  5 K mechanically collects the residual toner from the photoconductive drums  5 Y,  5 M,  5 C, and  5 K. Finally, the discharger disposed opposite the respective photoconductive drums  5 Y,  5 M,  5 C, and  5 K eliminates residual potential from the photoconductive drums  5 Y,  5 M,  5 C, and  5 K. 
     A description is provided of the primary transfer process and a secondary transfer process performed on the intermediate transfer belt  78  after the image forming processes described above. 
     First, a description is given of the primary transfer process. 
     The intermediate transfer belt  78  is stretched taut across the secondary transfer backup roller  82 , the cleaning backup roller  83 , and the tension roller  84 . The four primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K and the photoconductive drums  5 Y,  5 M,  5 C, and  5 K sandwich the intermediate transfer belt  78  to form the primary transfer nips between the photoconductive drums  5 Y,  5 M,  5 C, and  5 K and the intermediate transfer belt  78 . A transfer bias having a polarity opposite a polarity of toner is applied to the primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K. 
     As the secondary transfer backup roller  82  drives and rotates the intermediate transfer belt  78  in the rotation direction R 1 , the yellow, magenta, cyan, and black toner images formed on the photoconductive drums  5 Y,  5 M,  5 C, and  5 K are primarily transferred successively onto the intermediate transfer belt  78  passing through the primary transfer nips formed between the intermediate transfer belt  78  and the primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K. Thus, the yellow, magenta, cyan, and black toner images are superimposed on the same position on the intermediate transfer belt  78 , forming a color toner image on the intermediate transfer belt  78 . Next, a description is given of the secondary transfer process performed on the intermediate transfer belt  78 . 
     A secondary transfer roller  89  is disposed opposite the secondary transfer backup roller  82  via the intermediate transfer belt  78  to form a secondary transfer nip between the secondary transfer roller  89  and the intermediate transfer belt  78 . As the color toner image formed on the intermediate transfer belt  78  reaches the secondary transfer nip, the color toner image is secondarily transferred onto a recording medium P conveyed through the secondary transfer nip. After the secondary transfer, the intermediate transfer belt cleaner  80  disposed opposite the intermediate transfer belt  78  collects residual toner failed to be transferred onto the recording medium P and therefore remaining on the intermediate transfer belt  78  therefrom. 
     The paper tray  12  situated in a lower portion of the image forming apparatus  1  loads a plurality of recording media P (e.g., transfer sheets). 
     A description is provided of conveyance of the recording medium P from the paper tray  12  to the secondary transfer nip. 
     As a feed roller  97  is driven and rotated counterclockwise in  FIG. 1 , an uppermost recording medium P of the plurality of recording media P placed on the paper tray  12  is conveyed to a roller nip formed between two registration rollers  98   a  and  98   b . As the recording medium P comes into contact with the registration rollers  98   a  and  98   b , the registration rollers  98   a  and  98   b  that interrupt their rotation halt the recording medium P at the roller nip formed between the registration rollers  98   a  and  98   b  temporarily. At a time when the color toner image formed on the intermediate transfer belt  78  reaches the secondary transfer nip, the registration rollers  98   a  and  98   b  resume their rotation to feed the recording medium P to the secondary transfer nip. As the recording medium P is conveyed through the secondary transfer nip, the color toner image formed on the intermediate transfer belt  78  is secondarily transferred onto the recording medium P. 
     Thereafter, the recording medium P bearing the color toner image is conveyed to the fixing device  20 . As the recording medium P bearing the color toner image is conveyed between a fixing belt  21  and a pressing roller  31 , the fixing belt  21  and the pressing roller  31  apply heat and pressure to the recording medium P, fixing the color toner image on the recording medium P. Thereafter, the recording medium P bearing the fixed color toner image is discharged by output rollers  99   a  and  99   b  and stacked on an outside of the image forming apparatus  1 , that is, an output tray  100  disposed atop the image forming apparatus  1 . Thus, a series of image forming processes performed by the image forming apparatus  1  is completed. 
     With reference to  FIGS. 2 to 4 , a description is provided of a configuration 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 . As shown in  FIG. 2 , the fixing device  20  (e.g., a fuser) includes the fixing belt  21  serving as an endless belt, a heat conductor  22 , a support  23 , a heater  25 , a nip formation pad  26 , a heat insulator  27 , a low-friction sheet  28 , the pressing roller  31  serving as a pressing rotary body, a temperature sensor  40 , and a pressurization assembly  50 . 
     A detailed description is now given of a construction of the fixing belt  21 . 
     The fixing belt  21  is a thin, flexible endless belt rotatable counterclockwise in  FIG. 2  in a rotation direction R 3 . For example, the endless, fixing belt  21  is formed in a seamless belt manufactured by combining both ends of a band.  FIG. 3  is a partial vertical sectional view of the fixing belt  21 . As shown in  FIG. 3 , the fixing belt  21 , having a thickness of about 1 mm or smaller, is constructed of a base layer  21   b  constituting an inner circumferential surface  21   a ; an elastic layer  21   c  coating the base layer  21   b ; and a surface release layer  21   d  coating the elastic layer  21   c . The base layer  21   b , having a thickness in a range of from about 30 micrometers to about 100 micrometers, is made of metal such as nickel and stainless steel or resin such as polyimide. However, the configuration of the base layer  21   b  of the fixing belt  21  is not limited to the above. It is to be noted that the base layer  21   b  is made of a basic material. Since the metal heat conductor  22  is interposed between the heater  25  and the fixing belt  21 , light emitted from the heater  25  does not irradiate the fixing belt  21  directly. Accordingly, the base layer  21   b  of the fixing belt  21  is not requested to be made of a material having relatively great heat resistance. Hence, the base layer  21   b  of the fixing belt  21  is made of resin manufactured at reduced costs. 
     The elastic layer  21   c , having a thickness in a range of from about 100 micrometers to about 300 micrometers, is made of rubber such as silicone rubber, silicone rubber foam, and fluoro rubber. However, the configuration of the elastic layer  21   c  of the fixing belt  21  is not limited to the above. The elastic layer  21   c  absorbs slight surface asperities of the fixing belt  21  at a fixing nip N formed between the fixing belt  21  and the pressing roller  31  when the pressing roller  31  is pressed against the nip formation pad  26  via the fixing belt  21 , facilitating even conduction of heat from the fixing belt  21  to a toner image T on a recording medium P passing through the fixing nip N. Accordingly, the elastic layer  21   c  suppresses formation of an orange peel image on the recording medium P. The orange peel image defines a faulty toner image having lots of slight surface asperities on a surface thereof. 
     The release layer  21   d , having a thickness in a range of from about 10 micrometers to about 50 micrometers, is made of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), polytetrafluoroethylene (PTFE), polyimide, polyether imide, polyether sulfone (PES), or the like. However, the configuration of the release layer  21   d  of the fixing belt  21  is not limited to the above. The release layer  21   d  facilitates separation of the toner image T on the recording medium P from the fixing belt  21 . A loop diameter of the fixing belt  21  is in a range of from about 15 mm to about 120 mm. According to this exemplary embodiment, the loop diameter of the fixing belt  21  is about 30 mm. However, the configuration of the fixing belt  21  is not limited to the above. 
     A detailed description is now given of a configuration of the nip formation pad  26 . 
     The nip formation pad  26  is made of heat resistant resin such as liquid crystal polymer. As shown in  FIG. 2 , the nip formation pad  26  has an opposed face  26   d  disposed opposite the pressing roller  31  via the fixing belt  21  and is curved or concave with respect to the pressing roller  31  in accordance with the curvature of the pressing roller  31 , that is, a curve of the pressing roller  31  at the fixing nip N. Accordingly, the curved opposed face  26   d  of the nip formation pad  26  directs the recording medium P discharged from the fixing nip N along the curve of the pressing roller  31 , facilitating separation of the recording medium P bearing the fixed toner image T from the fixing belt  21  and preventing the recording medium P from adhering to the fixing belt  21 . 
       FIG. 4  is a side view of the fixing device  20 . As shown in  FIG. 4 , both lateral ends of the nip formation pad  26  in a longitudinal direction thereof parallel to an axial direction of the fixing belt  21  are mounted on and supported by side plates  43  of the fixing device  20 , respectively. Since the nip formation pad  26  is mounted on the side plates  43 , the nip formation pad  26  is immovable at least in a recording medium conveyance direction Y 10 . The low-friction sheet  28  interposed between the nip formation pad  26  and the fixing belt  21  reduces frictional resistance between the nip formation pad  26  and the fixing belt  21  sliding thereover. The low-friction sheet  28  is made of a material having a decreased friction coefficient and resistance against abrasion and heat such as porous fluoroplastic. As shown in  FIG. 2 , the low-friction sheet  28  is substantially U-shaped in cross-section. 
     A detailed description is now given of a configuration of the heat conductor  22 . 
     As shown in  FIG. 4 , both lateral ends of the heat conductor  22  in a longitudinal direction thereof parallel to the axial direction of the fixing belt  21  are mounted on and supported by the side plates  43  of the fixing device  20 , respectively. The heat conductor  22  is a pipe or a tube having a thickness of about 0.2 mm or less. However, the configuration of the heat conductor  22  is not limited to the above. For example, the heat conductor  22  is made of conductive metal such as aluminum, iron, and stainless steel. 
     The heat conductor  22  having the thickness of about 0.2 mm or less, as it is heated by the heater  25 , heats the fixing belt  21  effectively. According to this exemplary embodiment, the heat conductor  22  has a thickness of about 0.1 mm and made of stainless steel. However, the configuration of the heat conductor  22  is not limited to the above. As shown in  FIG. 2 , the heat conductor  22  is in proximity to or in contact with the inner circumferential surface  21   a  of the fixing belt  21  at a position other than the fixing nip N. At the fixing nip N, the heat conductor  22  is bent to produce a recess  22   d  defining an opening  22   a.    
     At ambient temperature, a clearance A greater than 0 mm and not greater than about 1 mm is provided between the fixing belt  21  and the heat conductor  22  at the position other than the fixing nip N. However, the size of the clearance A is not limited to the above. The clearance A decreases the area on the fixing belt  21  where the fixing belt  21  slides over the heat conductor  22  and thereby suppresses abrasion of the fixing belt  21 . Simultaneously, since the heat conductor  22  is not isolated from the fixing belt  21  with an excessively great clearance therebetween, the heat conductor  22  heats the fixing belt  21  effectively. Additionally, since the heat conductor  22  is in proximity to the fixing belt  21 , even if the flexible fixing belt  21  deforms, the heat conductor  22  supports the fixing belt  21 , retaining the circular loop shape of the fixing belt  21  and thereby reducing deformation and resultant wear of the fixing belt  21 . A lubricant, such as fluorine grease, is applied between the heat conductor  22  and the fixing belt  21  sliding thereover to reduce frictional resistance therebetween. 
     The heat conductor  22  is a thin metal plate. As the heat conductor  22  is heated by radiation heat from the heater  25  mounted on the side plates  43  of the fixing device  20 , the heat conductor  22  in turn heats the fixing belt  21 . That is, the heat conductor  22  is heated by the heater  25  directly. The fixing belt  21  is heated by the heater  25  indirectly through the heat conductor  22 . The fixing belt  21  heats the toner image T on the recording medium P conveyed over the outer circumferential surface of the fixing belt  21 . 
     The heater  25  is a halogen heater, a carbon heater, or the like. The temperature sensor  40  (e.g., a thermistor) disposed opposite the outer circumferential surface of the fixing belt  21  detects the temperature of the outer circumferential surface of the fixing belt  21 . A controller (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 heater  25  and the temperature sensor  40  controls the heater  25  based on the temperature of the fixing belt  21  detected by the temperature sensor  40  so as to adjust the temperature of the fixing belt  21  to a desired fixing temperature to fix the toner image T on the recording medium P. 
     The heat conductor  22  having the configuration described above heats the fixing belt  21  over substantially the entire span of the fixing belt  21  in a circumferential direction thereof, not over a partial span of the fixing belt  21 . Accordingly, even when the recording medium P is conveyed through the fixing nip N at high speed, the heat conductor  22  heats the fixing belt  21  sufficiently, minimizing faulty fixing that may arise due to a decreased temperature of the fixing belt  21  lower than the desired fixing temperature. 
     A detailed description is now given of a configuration of the support  23 . 
     As shown in  FIG. 2 , the support  23  is stationarily situated inside the loop formed by the fixing belt  21  to support the nip formation pad  26  against pressure from the pressing roller  31 . As shown in  FIG. 4 , both lateral ends of the support  23  in a longitudinal direction thereof parallel to the axial direction of the fixing belt  21  are mounted on and supported by the side plates  43  of the fixing device  20 , respectively. The support  23  presses against the pressing roller  31  via the nip formation pad  26  and the fixing belt  21 , supporting the nip formation pad  26  against pressure from the pressing roller  31  at the fixing nip N and thereby protecting the nip formation pad  26  from substantial deformation by pressure from the pressing roller  31 . The support  23  is made of metal having a relatively great mechanical strength such as stainless steel and ferro-alloy that achieves the advantages of the support  23  described above to support the nip formation pad  26 . 
     Conventionally, no heat insulator is interposed between the heater  25  and the nip formation pad  26 . For example, if the heat insulator  27  is not provided inside the loop formed by the fixing belt  21 , the heater  25  may heat the support  23  and the nip formation pad  26  as well as the heat conductor  22  and therefore may not heat the fixing belt  21  efficiently. Since the support  23  is mounted on and supported by the side plates  43  of the fixing device  20 , the side plates  43  may draw heat from the support  23 , resulting in inefficient heating of the fixing belt  21 . Further, since the nonmetallic nip formation pad  26  includes a plurality of projections  26   a  projecting beyond the heat conductor  22  toward the heater  25 , the nip formation pad  26  may be heated by the heater  25  directly, degrading its durability. To address this circumstance, the heat insulator  27  is disposed opposite the inner circumferential surface  21   a  of the fixing belt  21 . 
     A detailed description is now given of a configuration of the heat insulator  27 . 
     As shown in  FIG. 2 , the heat insulator  27  made of a single structural component is interposed between the heater  25  and the nip formation pad  26  and mounted on the support  23 . The heat insulator  27  is made of a material having an infrared reflectance not smaller than about 90 percent to prevent the heater  25  from heating the support  23  and the nip formation pad  26 . 
       FIG. 5  is a perspective view of the heat insulator  27 . As shown in  FIG. 5 , the heat insulator  27  includes a first reflection face  27   a , a second reflection face  27   b , and a curved, third reflection face  27   c , which are disposed opposite the heater  25 . The curved, third reflection face  27   c  bridges the first reflection face  27   a  and the second reflection face  27   b . The first reflection face  27   a , the second reflection face  27   b , and the curved, third reflection face  27   c  of the heat insulator  27  reflect light radiated from the heater  25  thereto toward the heat conductor  22 , allowing the light to irradiate and heat the heat conductor  22  efficiently. The heat insulator  27  is made of high intensity aluminum having a thickness of about 0.5 mm. However, the configuration of the heat insulator  27  is not limited to the above. 
     A detailed description is now given of a construction of the pressing roller  31 . 
     As shown in  FIG. 2 , the pressing roller  31  serves as a pressing rotary body contacting an outer circumferential surface of the fixing belt  21  at the fixing nip N. The pressing roller  31  having a diameter in a range of from about 30 mm to about 40 mm is constructed of a hollow metal core  32  and an elastic layer  33  coating the metal core  32 . However, the construction of the pressing roller  31  is not limited to the above. The pressing roller  31  is pressed against the nip formation pad  26  via the fixing belt  21  to form the desired fixing nip N between the pressing roller  31  and the fixing belt  21 . 
     The pressing roller  31  mounts a gear engaging a driving gear of a driver that drives and rotates the pressing roller  31  clockwise in  FIG. 2  in a rotation direction R 4 . As shown in  FIG. 4 , both lateral ends of the pressing roller  31  in an axial direction thereof are rotatably mounted on the side plates  43  of the fixing device  20  through bearings  42 , respectively. As shown in  FIG. 2 , a pressurization direction D 1  in which the pressing roller  31  is pressed against the nip formation pad  26  is disposed opposite the support  23 . 
     The elastic layer  33  is made of silicone rubber foam, silicone rubber, fluoro rubber, or the like. Optionally, a thin, surface release layer made of PFA, PTFE, or the like may coat the elastic layer  33 . If the elastic layer  33  of the pressing roller  31  is made of sponge such as silicone rubber foam, the pressing roller  31  exerts reduced pressure to the nip formation pad  26  at the fixing nip N, reducing bending of the nip formation pad  26 . The elastic layer  33  suppresses heat conduction from the fixing belt  21  to the pressing roller  31 , improving heating efficiency of the fixing belt  21 . 
     With reference to  FIG. 2 , a detailed description is now given of a construction of the pressurization assembly  50 . 
     The pressurization assembly  50  brings the pressing roller  31  into contact with and isolation from the fixing belt  21 . The pressurization assembly  50  is constructed of a pressing lever  51 , an eccentric cam  52 , a spring  53 , and a spring support plate  54 . 
     The pressing lever  51  is pivotable about a shaft  51   a  attached to one end of the pressing lever  51  in a longitudinal direction thereof and mounted on the side plate  43  of the fixing device  20 . A center of the pressing lever  51  in the longitudinal direction thereof contacts the bearing  42  depicted in  FIG. 4  that bears the pressing roller  31  and is movably supported by an elongate hole produced in the side plate  43 . 
     The spring  53  is anchored to another end of the pressing lever  51  in the longitudinal direction thereof and the spring support plate  54 . The spring support plate  54  contacts the eccentric cam  52 . The eccentric cam  52  is rotatable by a driving motor. 
     During a fixing job, as the driving motor rotates the eccentric cam  52 , the pressing lever  51  pivots about the shaft  51   a . When the eccentric cam  52  is at a pressurization position shown in  FIG. 2 , the pressing lever  51  presses the pressing roller  31  against the fixing belt  21 , forming the desired fixing nip N therebetween. Conversely, while a fixing job is not performed, for example, while the recording medium P is jammed between the pressing roller  31  and the fixing belt  21 , the eccentric cam  52  rotates a half-turn from the pressurization position shown in  FIG. 2 , causing the pressing lever  51  to isolate the pressing roller  31  from the fixing belt  21  or to press the pressing roller  31  against the fixing belt  21  with decreased pressure therebetween. 
     With reference to  FIG. 6 , a description is provided of manufacturing and installation of the heat conductor  22 . 
       FIG. 6  is a perspective view of the heat conductor  22 . The heat conductor  22  is formed into a pipe or a tube by bending a tractable, stainless steel plate having a thickness of about 0.1 mm. However, manufacturing of the heat conductor  22  is not limited to the above. As the stainless steel plate is bent into a substantial pipe or tube to create the opening  22   a  as shown in  FIG. 6 , the stainless steel plate may widen the opening  22   a  in directions D 2   a  and D 2   b  by its springback. To address this circumstance, the heat conductor  22  includes the recess  22   d  defining the opening  22   a  and produced with a plurality of through-holes  22   b . As the projections  26   a  of the nip formation pad  26  depicted in  FIG. 2  are inserted into the through-holes  22   b  of the heat conductor  22 , the nip formation pad  26  is attached to the heat conductor  22 , restricting springback of the heat conductor  22  and forming the heat conductor  22  into a desired shape. 
     With reference to  FIGS. 7A, 7B, and 8 , a description is provided of assembly of the heat conductor  22  and the nip formation pad  26 . 
       FIG. 7A  is a perspective view of the nip formation pad  26  before being attached to the heat conductor  22 .  FIG. 7B  is a perspective view of the nip formation pad  26  attached to the heat conductor  22 .  FIG. 8  is a vertical sectional view of the nip formation pad  26  attached to the heat conductor  22 . 
     As shown in  FIG. 7A , the plurality of through-holes  22   b  is aligned in the recess  22   d  of the heat conductor  22  in the longitudinal direction of the nip formation pad  26 . Similarly, the plurality of projections  26   a  of the nip formation pad  26  is aligned in the longitudinal direction of the nip formation pad  26  such that the plurality of projections  26   a  corresponds to the plurality of through-holes  22   b . As the nip formation pad  26  is embedded in the recess  22   d  of the heat conductor  22 , the projections  26   a  of the nip formation pad  26  are inserted into the through-holes  22   b  of the heat conductor  22 . Thus, the nip formation pad  26  is attached to the heat conductor  22  as shown in  FIG. 7B . 
     As shown in  FIG. 8 , a restriction face  26   b  of the projection  26   a  of the nip formation pad  26  contacts a restriction face  22   c  of the recess  22   d  of the heat conductor  22  that defines the through-hole  22   b , preventing the opening  22   a  from being widened by springback of the heat conductor  22 . The restriction face  26   b  of the respective projections  26   a  of the nip formation pad  26  contacts the restriction face  22   c  defining the respective through-holes  22   b  of the heat conductor  22 . That is, the restriction face  26   b  of the nip formation pad  26  contacts the restriction face  22   c  of the heat conductor  22  at a plurality of positions in the longitudinal direction of the nip formation pad  26  and the heat conductor  22 , preventing partial deformation and widening of the heat conductor  22  throughout the longitudinal direction thereof. Since the heat conductor  22  neither deforms nor widens partially in the longitudinal direction thereof, the heat conductor  22  does not come in contact with the fixing belt  21  in an increased area, suppressing abrasion of the fixing belt  21 . 
     As shown in  FIGS. 2 and 8 , the nip formation pad  26  includes an abutment face  26   c  abutting the support  23  to receive pressure from the pressing roller  31  throughout the long width of the nip formation pad  26  in the longitudinal direction thereof. If the abutment face  26   c  of the nip formation pad  26  is configured to abut the support  23  at a part of the long width of the nip formation pad  26 , another part not abutting the support  23  may not receive pressure from the pressing roller  31  precisely, decreasing pressure exerted between the pressing roller  31  and the fixing belt  21  at the fixing nip N and resulting in formation of a faulty toner image T. To address this circumstance, according to this exemplary embodiment, the abutment face  26   c  of the nip formation pad  26  abuts the support  23  throughout the entire width of the abutment face  26   c  in the longitudinal direction of the nip formation pad  26  that is greater than at least the width of the maximum size recording medium P available in the image forming apparatus  1 . 
     As the pressing roller  31  rotates in the rotation direction R 4 , the nip formation pad  26  receives friction from the pressing roller  31  through the fixing belt  21 . However, the support  23  mounted on and supported by the side plates  43  abuts the abutment face  26   c  of the nip formation pad  26  throughout the entire width of the nip formation pad  26  in the longitudinal direction thereof to support the nip formation pad  26 , thus preventing the nip formation pad  26  from being deformed by friction from the pressing roller  31 . 
     As described above, the heat conductor  22  is manufactured by bending a metal plate (e.g., a stainless steel plate) into a substantial pipe or tube. The thin heat conductor  22  is heated by the heater  25  quickly, shortening the warm-up time of the fixing device  20 . However, since the thin heat conductor  22  has a decreased rigidity, as it receives pressure from the pressing roller  31 , it may not resist the pressure and may be deformed or bent. If the heat conductor  22  is deformed or bent, the fixing nip N may not have a desired length in the recording medium conveyance direction Y 10 , degrading fixing quality to fix the toner image T on the recording medium P. To address this circumstance, as shown in  FIG. 8 , a predetermined clearance is secured between the opening  22   a  of the heat conductor  22  and the nip formation pad  26 . Accordingly, the heat conductor  22  does not receive pressure from the pressing roller  31  and therefore is not deformed or bent by pressure from the pressing roller  31 . 
     With reference to  FIGS. 1 and 2 , a description is provided of a fixing operation of the fixing device  20  having the configuration described above to fix a toner image T on a recording medium P. 
     As a power switch of the image forming apparatus  1  is turned on, a power supply supplies power to the heater  25 . Simultaneously, the pressing roller  31  rotates in the rotation direction R 4 . Accordingly, the fixing belt  21  rotates in the rotation direction R 3  in accordance with rotation of the pressing roller  31  by friction therebetween at the fixing nip N. Thereafter, as a recording medium P conveyed from the paper tray  12  reaches the secondary transfer nip, the secondary transfer roller  89  secondarily transfers a toner image T formed on the intermediate transfer belt  78  onto the recording medium P. 
     The recording medium P bearing the toner image T is conveyed in the recording medium conveyance direction Y 10  while guided by a guide plate and enters the fixing nip N formed between the fixing belt  21  and the pressing roller  31  pressed against the fixing belt  21 . As the recording medium P is conveyed through the fixing nip N, the recording medium P receives heat from the fixing belt  21  heated by the heater  25  through the heat conductor  22  and pressure from the pressing roller  31  and the fixing belt  21  pressed against the pressing roller  31  by the nip formation pad  26  supported by the support  23 . Thus, the toner image T is fixed on the recording medium P by the heat and pressure. Thereafter, the recording medium P bearing the fixed toner image T is discharged from the fixing nip N and conveyed in a recording medium conveyance direction Y 11 . 
     With reference to  FIG. 9 , a description is provided of thermal deformation of the heat conductor  22 . 
       FIG. 9  is a schematic side view of the fixing belt  21  and the heat conductor  22 . Diagram (a) of  FIG. 9  illustrates the fixing belt  21  and the heat conductor  22  at ambient temperature. As the heat conductor  22  is heated by the heater  25  depicted in  FIG. 2 , the heat conductor  22  is thermally deformed and bent as shown in diagram (b) of  FIG. 9 , producing a bending B in a diametrical direction of the heat conductor  22 . The clearance A created between the fixing belt  21  and the heat conductor  22  at ambient temperature decreases as the bending B of the heat conductor  22  increases. Under a condition in which the heat conductor  22  is heated and cooled and vice versa, when the heat conductor  22  is cooled to ambient temperature, the clearance A is retrieved as shown in diagram (a) of  FIG. 9 . 
     A detailed description is now given of change in the bending B of the heat conductor  22  as the heat conductor  22  is heated from ambient temperature. 
     During warm-up of the fixing device  20 , the heat conductor  22  at ambient temperature or a temperature close to ambient temperature is heated by the heater  25  relatively quickly to a target fixing temperature of the fixing belt  21  at which the toner image T is fixed on the recording medium P. Immediately after the heater  25  starts heating the heat conductor  22 , an outer circumferential surface of the heat conductor  22  that is situated farther from the heater  25  than an inner circumferential surface of the heat conductor  22  has a temperature lower than a temperature of the inner circumferential surface of the heat conductor  22 . Further, the relatively sharp temperature gradient is created in the diametrical direction of the heat conductor  22 . Thus, the temperature distribution of the heat conductor  22  is uneven throughout the entire heat conductor  22 . Accordingly, thermal expansion of the heat conductor  22  varies partially, bending the heat conductor  22  by thermal deformation. The maximum bending of the heat conductor  22  is defined as a maximum bending Bmax. 
     As the fixing device  20  is ready to fix the toner image T on the recording medium P and the temperature of the fixing belt  21  is maintained at or near the target fixing temperature, the temperature of the heat conductor  22  is even throughout the entire heat conductor  22  with a decreased temperature gradient in the diametrical direction of the heat conductor  22 . Accordingly, the bending B of the heat conductor  22  decreases compared to that immediately after the heater  25  starts heating the heat conductor  22 . Thus, a stable bending Bave of the heat conductor  22  is retained. 
     The clearance A between the fixing belt  21  and the heat conductor  22  is defined by a formula (1) below.
 
 B max≧ A&gt;B ave  (1)
 
     The inner diameter of the fixing belt  21 , the outer diameter of the heat conductor  22 , the material, thickness, and type of the heat conductor  22 , and fixing conditions of the fixing device  20  such as the target fixing temperature are determined to satisfy the formula (1). 
     According to this exemplary embodiment, the inner diameter of the fixing belt  21  is about 30 mm; the outer diameter of the heat conductor  22  is about 29.5 mm. Hence, the clearance A is about 0.5 mm. However, the clearance A is not limited to the above. The heat conductor  22  is made of SUS 430 stainless steel having a thickness of about 0.1 mm and heated by the heater  25 . The target fixing temperature is about 180 degrees centigrade. However, the configuration of the heat conductor  22  is not limited to the above. Accordingly, the maximum bending Bmax of the heat conductor  22  is about 1.3 mm. The stable bending Bave of the heat conductor  22  is about 0.4 mm. Thus, the clearance A, the maximum bending Bmax, and the stable bending Bave satisfy the formula (1). However, the maximum Bmax and the stable bending Bave of the heat conductor  22  are not limited to the above. 
     Since the maximum bending Bmax of the heat conductor  22  is not smaller than the clearance A, during warm-up of the fixing device  20  while the fixing belt  21  halts, the inner circumferential surface  21   a  of the fixing belt  21  comes into contact with the heat conductor  22  precisely. That is, an air layer is not interposed between the heat conductor  22  and the fixing belt  21  and thus heat is conducted from the heat conductor  22  to the fixing belt  21  effectively, improving heating efficiency of the heat conductor  22  to heat the fixing belt  21 . 
     Since the stable bending Bave of the heat conductor  22  is smaller than the clearance A, during fixing, the inner circumferential surface  21   a  of the fixing belt  21  is disposed opposite the heat conductor  22  with a slight clearance therebetween. Even if the fixing belt  21  comes into contact with the heat conductor  22 , it contacts the heat conductor  22  with slight pressure therebetween. Accordingly, the heat conductor  22  heats the fixing belt  21  effectively while reducing abrasion of the fixing belt  21  and the heat conductor  22 . 
     With reference to  FIG. 10 , a description is provided of thermal deformation of the heat conductor  22 . 
       FIG. 10  is a schematic side view of the heat conductor  22 . The straight heat conductor  22  shown in diagram (a) of  FIG. 10 , as it is heated by the heater  25 , is bent by thermal deformation as shown in diagram (b) of  FIG. 10 . As the heat conductor  22  is cooled to ambient temperature, the bent heat conductor  22  is subject to reversible deformation and recovers its original straight shape. However, as the heat conductor  22  is heated in an increased amount, the bent heat conductor  22  is subject to irreversible deformation and does not recover its original straight shape. 
     When the bent heat conductor  22  is subject to irreversible deformation and does not recover its original shape even at ambient temperature, the heat conductor  22  is crimped by plastic deformation. Once the heat conductor  22  is crimped by plastic deformation, as the recording medium P is conveyed through the fixing nip N, a part of the heat conductor  22  may come into contact with the inner circumferential surface  21   a  of the fixing belt  21  with increased pressure therebetween. Accordingly, the heat conductor  22  may scratch the inner circumferential surface  21   a  of the heat conductor  22  or cause variation in the temperature of the fixing belt  21 , resulting faulty fixing or variation in gloss of the toner image T on the recording medium P. 
     Crimping of the heat conductor  22  is prevented by optimizing the hardness of the heat conductor  22 . Generally, if the hardness of the heat conductor  22  is excessively great, the heat conductor  22  does not recover from thermal deformation and therefore is crimped. Conversely, if the hardness of the heat conductor  22  is relatively small, even if the heat conductor  22  is thermally deformed, it is flexible enough to recover from thermal deformation to its original shape. That is, the heat conductor  22  having the relatively small hardness is susceptible to reversible thermal deformation. 
     With reference to  FIG. 11 , a description is provided of an experiment for examining occurrence of crimping of the heat conductor  22 . 
       FIG. 11  is a graph showing a relation between the Vickers hardness of the heat conductor  22  and the temperature of the fixing belt  21  at which the heat conductor  22  is crimped. A plurality of experimental pieces is prepared by adhering a fixing belt to a surface of a plurality of metal heat conductors having various Vickers hardnesses. The metal heat conductors have a thickness of 0.1 mm. The fixing belt is constructed of a nickel layer contacting the metal heat conductor and having a thickness of 35 micrometers; a silicone rubber layer coating the nickel layer and having a thickness of 200 micrometers; and a PFA layer coating the silicone rubber layer and having a thickness of 15 micrometers. As the metal heat conductor is heated to a predetermined temperature quickly, whether or not the metal heat conductor is crimped is examined as shown in  FIG. 11 . 
     In  FIG. 11 , the horizontal axis represents the Vickers hardness of the metal heat conductor. The vertical axis represents the surface temperature of the fixing belt, that is, the temperature of the PFA layer of the fixing belt. “●” indicates no crimping of the metal heat conductor. Conversely, “x” indicates crimping of the metal heat conductor. For example, as shown in  FIG. 11 , the metal heat conductor having a Vickers hardness of about 300 HV, as the fixing belt is heated to about 190 degrees centigrade quickly, is not crimped. Conversely, the metal heat conductor having a Vickers hardness of about 300 HV, as the fixing belt is heated to about 210 degrees centigrade quickly, is crimped. The metal heat conductor having a Vickers hardness not greater than about 280 HV, regardless of the target fixing temperature, is not crimped. Even the metal heat conductor having a Vickers hardness not greater than about 340 HV, if the target fixing temperature is not greater than 180 degrees centigrade, is not crimped. 
     According to this exemplary embodiment, the metal heat conductor  22  has a thickness not greater than about 0.1 mm and a Vickers hardness not greater than about 280 HV. However, the thickness and the Vickers hardness of the heat conductor  22  are not limited to the above. 
     The heat conductor  22  is made of ferrite stainless steel such as SUS 430 stainless steel having a relatively small heat capacity ratio per unit volume. For example, SUS 430 stainless steel has a density of 7.73×10 −3  kg/m 3 , a specific heat of 0.46 kJ/kg° C., a Young&#39;s modulus of 206 Gpa, a Vickers hardness of 250 HV, and a heat capacity ratio per unit volume of 3.56. However, property of stainless steel SUS 430 of the heat conductor  22  is not limited to the above. Accordingly, the heat conductor  22  is heated effectively and is not crimped. 
     Nickel has a density of 8.9×10 −3  kg/m 3 , a specific heat of 0.439 kJ/kg° C., a Young&#39;s modulus of 210 Gpa, a Vickers hardness of 96 HV, and a heat capacity ratio per unit volume of 3.91. 
     SUS 304—½H stainless steel has a density of 7.93×10 −3  kg/m 3 , a specific heat of 0.502 kJ/kg° C., a Young&#39;s modulus of 197 Gpa, a Vickers hardness of 250 HV, and a heat capacity ratio per unit volume of 3.98. 
     During warm-up of the fixing belt  21 , the heat conductor  22  disposed opposite the inner circumferential surface  21   a  of the fixing belt  21  deforms in the maximum bending Bmax. While the recording medium P is conveyed through the fixing nip N, the heat conductor  22  retains the relatively small, stable bending Bave. Utilizing such deformation of the heat conductor  22 , the clearance A between the fixing belt  21  and the heat conductor  22  is optimized. Accordingly, even if the fixing device  20  is configured to be warmed up quickly, achieve a shortened first print time taken to output the recording medium P bearing the fixed toner image T after receiving a print job, and convey the recording medium P at high speed, the heat conductor  22  heats the fixing belt  21  efficiently, fixing the toner image T on the recording medium P precisely. Further, the fixing belt  21  does not come into contact with the heat conductor  22  as it rotates in the rotation direction R 3 , reducing abrasion of the fixing belt  21  by friction between the fixing belt  21  and the heat conductor  22 . 
     As shown in  FIG. 2 , the fixing device  20  includes the fixing belt  21  serving as a flexible endless belt formed into a loop and rotatable in the rotation direction R 3  and the nip formation pad  26  disposed opposite the inner circumferential surface  21   a  of the fixing belt  21  and pressing against the pressing roller  31  via the fixing belt  21  to form the fixing nip N between the fixing belt  21  and the pressing roller  31  through which a recording medium P bearing a toner image T is conveyed. 
     The heat conductor  22  is disposed opposite the inner circumferential surface  21   a  of the fixing belt  21  to heat the fixing belt  21 . The heater  25  is disposed opposite the inner circumferential surface of the heat conductor  22  to heat the heat conductor  22 . As the pressing roller  31  is pressed against the nip formation pad  26  via the fixing belt  21 , the support  23 , disposed opposite the inner circumferential surface of the heat conductor  22  and contacting the abutment face  26   c  of the nip formation pad  26 , supports the nip formation pad  26 . The heat insulator  27  is interposed between the heater  25  and the nip formation pad  26  and the support  23  to shield the nip formation pad  26  and the support  23  from the heater  25 . The heat insulator  27  is made of a single component. Thus, the heat insulator  27  prohibits the heater  25  from heating the nonmetallic nip formation pad  26  directly, preventing degradation in durability of the nip formation pad  26  by heat radiated from the heater  25 . 
     Since the heat insulator  27  is made of a single component, the heat insulator  27  is assembled with a reduced number of processes. Further, the heat insulator  27  has a decreased heat capacity that shortens the warm-up time to heat the fixing belt  21  to the desired fixing temperature and saves energy. 
     The heater  25  is an infrared heater. Hence, the heater  25  is versatile, simple, and manufactured at low-cost. 
     The heat insulator  27  includes an infrared reflection plate to reflect light, that is, heat, radiated from the heater  25 . The heat insulator  27  is made of high intensity aluminum having an infrared reflectance not smaller than about 90 percent. Accordingly, the heat insulator  27  reflects light emitted from the heater  25  toward the support  23  and the nip formation pad  26  to the heat conductor  22 , heating the heat conductor  22  and therefore improving heating efficiency of the heat conductor  22  to heat the fixing belt  21 . Consequently, the heat insulator  27  shortens the warm-up time to warm up the fixing belt  21 , saving energy. 
     The heat insulator  27  mounted on and supported by the support  23  insulates the nip formation pad  26  from heat radiated from the heater  25  toward the nip formation pad  26 . The nip formation pad  26  is made of heat resistant resin. Accordingly, the heat insulator  27  insulates the nip formation pad  26  from heat radiated from the heater  25  toward the nip formation pad  26 , preventing degradation in durability of the nip formation pad  26  made of nonmetallic, heat resistant resin. 
     The base layer  21   b  of the fixing belt  21  is made of heat resistant resin. As shown in  FIG. 2 , the opening  22   a  of the heat conductor  22  is disposed opposite the pressing roller  31  via the nip formation pad  26  and the fixing belt  21 . The heat conductor  22  having a decreased heat capacity and retaining a predetermined shape is disposed opposite the inner circumferential surface  21   a  of the fixing belt  21 . Accordingly, the heat conductor  22  prohibits the heater  25  from heating the fixing belt  21  directly. Further, the heat conductor  22  heated by the heater  25  heats the entire fixing belt  21  evenly and effectively. 
     Since the fixing belt  21  is not heated by the heater  25  directly, the base layer  21   b  of the fixing belt  21  is made of low-cost, heat resistant resin. Hence, the fixing belt  21  is manufactured at reduced costs. 
     A description is provided of variations of the components incorporated in the fixing device  20 . 
     According to the exemplary embodiments described above, the heat insulator  27  is bent and curved as shown in  FIG. 5  such that the curved, third reflection face  27   c  bridges the first reflection face  27   a  and the second reflection face  27   b . Alternatively, the heat insulator  27  may not include the curved, third reflection face  27   c  and therefore the first reflection face  27   a  may be coupled with the second reflection face  27   b  as shown in  FIG. 12 . 
       FIG. 12  is a vertical sectional view of a fixing device  20 S. As shown in  FIG. 12 , the fixing device  20 S includes a heat insulator  27 S constructed of the first reflection face  27   a  and the second reflection face  27   b  coupled with the first reflection face  27   a . The first reflection face  27   a  adjoins the second reflection face  27   b  at a right angle. 
     Yet alternatively, instead of the curved, third reflection face  27   c  shown in  FIG. 5 , a planar face may bridge the first reflection face  27   a  and the second reflection face  27   b . Accordingly, the heat insulator  27  is manufactured by pressing, reducing manufacturing costs. 
     According to the exemplary embodiments described above, the heat insulator  27  is made of a material having an increased surface reflectance. Alternatively, a surface of the heat insulator  27  may be coated with a material having an increased reflectance or treated with vacuum deposition to improve surface reflectance. 
     According to the exemplary embodiments described above, the heat insulator  27  is mounted on the support  23 . Alternatively, the heat insulator  27  may be mounted on and supported by the side plates  43  of the fixing device  20 . 
     As shown in  FIG. 2 , the opposed face  26   d  of the nip formation pad  26  disposed opposite the pressing roller  31  at the fixing nip N is concave with respect to the pressing roller  31  in cross-section. Alternatively, the opposed face  26   d  of the nip formation pad  26  may be planar. Accordingly, the nip formation pad  26  prevents the recording medium P conveyed through the fixing nip N from creasing. Additionally, the nip formation pad  26  increases the curvature of the fixing belt  21  at an exit of the fixing nip N, facilitating separation of the recording medium P discharged from the fixing nip N from the fixing belt  21 . 
     According to the exemplary embodiments described above, a lubricant, such as fluorine grease, is applied between the heat conductor  22  and the fixing belt  21  sliding thereover to reduce frictional resistance therebetween. Alternatively, the outer circumferential surface of the heat conductor  22  that contacts the fixing belt  21  may be made of a material having a decreased friction coefficient. Yet alternatively, the inner circumferential surface  21   a  of the fixing belt  21  may be made of fluoroplastic. 
     According to the exemplary embodiments described above, the heat conductor  22  is substantially circular in cross-section. Alternatively, the heat conductor  22  may be polygonal in cross-section. 
     As shown in  FIG. 2 , no heater is situated inside the pressing roller  31 . Alternatively, a heater such as a halogen heater may be situated inside the pressing roller  31 . 
     According to the exemplary embodiments described above, the loop diameter of the fixing belt  21  is equivalent to the diameter of the pressing roller  31 . Alternatively, the loop diameter of the fixing belt  21  may be smaller than the diameter of the pressing roller  31 . In this case, the curvature of the fixing belt  21  at the fixing nip N is greater than that of the pressing roller  31 , facilitating separation of the recording medium P discharged from the fixing nip N from the fixing belt  21 . Alternatively, the loop diameter of the fixing belt  21  may be greater than the diameter of the pressing roller  31 . According to the exemplary embodiments described above, regardless of a relation between the loop diameter of the fixing belt  21  and the diameter of the pressing roller  31 , the heat conductor  22  does not receive pressure from the pressing roller  31 . 
     With reference to  FIGS. 2 and 8 , a description is provided of advantages of the fixing device  20 . 
     The fixing device  20  includes a flexible endless belt (e.g., the fixing belt  21 ) formed into a loop and rotatable in the rotation direction R 3 ; a pressing rotary body (e.g., the pressing roller  31 ) disposed opposite the endless belt; and the nip formation pad  26  disposed opposite the inner circumferential surface  21   a  of the endless belt and pressing the endless belt against the pressing rotary body to form the fixing nip N between the endless belt and the pressing rotary body through which a recording medium P bearing a toner image T is conveyed. The heat conductor  22  is disposed opposite the inner circumferential surface  21   a  of the endless belt to heat the endless belt. The heater  25  is disposed opposite the inner circumferential surface of the heat conductor  22  to heat the heat conductor  22 . The support  23  is disposed opposite the inner circumferential surface of the heat conductor  22 . As the pressing rotary body is pressed against the nip formation pad  26  via the endless belt, the support  23  contacting the abutment face  26   c  of the nip formation pad  26  supports the nip formation pad  26  against pressure from the pressing rotary body. The heat insulator  27  is interposed between the heater  25  and the nip formation pad  26  and the support  23  to shield the nip formation pad  26  and the support  23  from the heater  25 . The heat insulator  27  is constructed of a single component. 
     The fixing device  20  incorporating the heat insulator  27  reduces the number of the components incorporated therein and the number of assembly processes, thus shortening the warm-up time to warm up the endless belt and saving energy. Additionally, the heat insulator  27  prevents degradation in durability of the nip formation pad  26 . 
     According to the exemplary embodiments described above, the pressing roller  31  is used as a pressing rotary body. Alternatively, a pressing belt or the like may be used as a pressing rotary body. Further, the fixing belt  21  is used as an endless belt. As used herein, the term “endless belt” is not to be limited to a belt as commonly known but is to be understood to include an endless film and the like. 
     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.