Patent Publication Number: US-8526870-B2

Title: Fixing device and image forming apparatus incorporating same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based on and claims priority to Japanese Patent Application No. 2010-029961, filed on Feb. 15, 2010, in the Japan Patent Office, which is hereby incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium, and an image forming apparatus including the fixing device. 
     2. Description of the Related Art 
     Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; 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. 
     The fixing device used in such image forming apparatuses may include a fixing belt or a fixing film to apply heat to the recording medium bearing the toner image.  FIG. 1  is a sectional view of a fixing device  140 R 1  including a fixing belt  5 . The fixing belt  5  is looped around a heating roller  2  and a fixing roller  3  in a state in which a tension roller  4  biases the fixing belt  5 . A pressing roller  6 R presses against the fixing roller  3  via the fixing belt  5  to form a fixing nip between the pressing roller  6 R and the fixing belt  5 . The fixing belt  5  is heated by a heater  1  provided inside the heating roller  2 . As a recording medium  7  bearing a toner image passes between the fixing roller  3  and the pressing roller  6 R on the fixing belt  5 , the fixing belt  5  and the pressing roller  6 R apply heat and pressure to the recording medium  7  bearing the toner image to fix the toner image on the recording medium  7 . 
     One problem with such an arrangement, however, is that the heating roller  2  has a relatively large heat capacity, resulting in a longer warm-up time for the fixing device  140 R 1 . To address this problem, instead of the fixing belt  5  the fixing device may include a fixing film having a relatively small heat capacity.  FIG. 2  is a sectional view of a fixing device  140 R 2  including a fixing film  14 . A ceramic heater  13  is provided inside a loop formed by the fixing film  14  and supported by a stay  12  via a holder  11 . The pressing roller  6 R presses against the ceramic heater  13  via the fixing film  14  to form a fixing nip between the pressing roller  6 R and the fixing film  14 . As a recording medium bearing a toner image passes between the pressing roller  6 R and the fixing film  14 , the fixing film  14  heated by the ceramic heater  13  and the pressing roller  6 R apply heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium. 
     However, the fixing film  14  also has a drawback in that, over time, friction between the ceramic heater  13  and the fixing film  14  sliding over the ceramic heater  13  increases, resulting eventually in unstable movement of the fixing film  14  and increasing the required driving torque of the fixing device  140 R 2 . 
     To address the above-described problems, instead of the ceramic heater  13  the fixing device may include a hollow cylindrical heating member stationarily provided inside the loop formed by a fixing member (e.g., the fixing belt or the fixing film) across a clearance therebetween. A heater provided inside the hollow cylinder of the heating member heats the heating member, which in turn heats the fixing member, to maintain the fixing member at the proper temperature. The pressing roller is pressed against a nip formation member provided inside the loop formed by the fixing member via the fixing member to form the fixing nip between the fixing member and the pressing roller. 
     However, the heating member described above also has a drawback in that pressure applied by the pressing roller bends the nip formation member and the heating member supporting the nip formation member in such a manner that the nip formation member is sandwiched between the heating member and the fixing member. Specifically, the pressure applied by the pressing roller bends the center portion of the nip formation member in the longitudinal direction of the nip formation member parallel to the axial direction of the fixing member substantially. Accordingly, the heating member supporting the nip formation member is also bent. To address this problem, the center portion of the nip formation member in the longitudinal direction of the nip formation member may be given a convex shape protruding inward toward the heating member. However, when the pressing roller applies substantial pressure to the nip formation member having the convex center portion, the heating member contacted by the nip formation member is deformed like a bow along the convex shape of the nip formation member. Specifically, the center portion of the heating member in the axial direction of the fixing member, which is pressed by the convex center portion of the nip formation member, moves closer to the inner circumferential surface of the fixing member at the position diametrically opposite the fixing nip. By contrast, the lateral end portions of the heating member in the axial direction of the fixing member move away from the inner circumferential surface of the fixing member. In other words, the clearance provided between the fixing member and the heating member becomes excessively large at the lateral ends of the fixing member in the axial direction of the fixing member. Accordingly, heat is not transmitted from the heating member to the fixing member effectively at the lateral ends of the fixing member. As a result, the fixing member is not heated quickly, increasing the warm-up time of the fixing device. 
     BRIEF SUMMARY OF THE INVENTION 
     This specification describes below an improved fixing device. In one exemplary embodiment of the present invention, the fixing device, which fixes a toner image on a recording medium, includes a flexible endless belt-shaped fixing member, a heating member, a nip formation member, and a pressing member. The fixing member rotates in a predetermined direction of rotation and is formed into a loop. The heating member is provided inside the loop formed by the fixing member and faces an inner circumferential surface of the fixing member to heat the fixing member. The nip formation member is provided inside the loop formed by the fixing member and is supported by the heating member. The pressing member is pressed against the nip formation member via the fixing member to form a fixing nip between the fixing member and the pressing member through which the recording medium bearing the toner image passes. The nip formation member includes a central convex portion of increased thickness provided at a center of the nip formation member in an axial direction of the fixing member and protruding toward the heating member. The heating member has a tapered shape corresponding to the convex portion of the nip formation member, in which the heating member is tapered from lateral ends of increased diameter toward a center of reduced diameter of the heating member in the axial direction of the fixing member. 
     This specification further describes below an 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 sectional view of one related-art fixing device; 
         FIG. 2  is a sectional view of another related-art fixing device; 
         FIG. 3  is a schematic view of an image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 4  is a vertical sectional view of a fixing device included in the image forming apparatus shown in  FIG. 3 ; 
         FIG. 5  is a perspective view of the fixing device shown in  FIG. 4 ; 
         FIG. 6  is a side view of the fixing device shown in  FIG. 5 ; 
         FIG. 7A  is a sectional view of a nip formation member included in the fixing device shown in  FIG. 4 ; 
         FIG. 7B  is a side view of the nip formation member shown in  FIG. 7A ; 
         FIG. 8A  is a sectional view of a comparative fixing device; 
         FIG. 8B  is a plan view of the comparative fixing device shown in  FIG. 8A ; 
         FIG. 9A  is a sectional view of the comparative fixing device shown in  FIG. 8A  when a pressing roller is pressed against a nip formation member; 
         FIG. 9B  is a plan view of the comparative fixing device shown in  FIG. 9A ; 
         FIG. 10  is a partially enlarged sectional view of the comparative fixing device shown in  FIG. 9A  at a lateral end of a fixing belt included in the comparative fixing device in an axial direction of the fixing belt; 
         FIG. 11A  is a sectional view of the comparative fixing device shown in  FIG. 10  at the lateral end of the fixing belt in the axial direction of the fixing belt; 
         FIG. 11B  is a plan view of the comparative fixing device shown in  FIG. 11A ; 
         FIG. 12A  is a sectional view of the fixing device shown in  FIG. 5  when a pressing roller is not pressed against a nip formation member; 
         FIG. 12B  is a plan view of the fixing device shown in  FIG. 12A ; 
         FIG. 13A  is a sectional view of the fixing device shown in  FIG. 12A  when the pressing roller is pressed against the nip formation member; 
         FIG. 13B  is a plan view of the fixing device shown in  FIG. 13A ; 
         FIG. 14A  is a perspective view of a metal pipe and a support member included in the fixing device shown in  FIG. 12A  before being assembled; 
         FIG. 14B  is a sectional view of the support member shown in  FIG. 14A ; 
         FIG. 14C  is a plan view of the support member shown in FIG.  14 B; 
         FIG. 14D  is a perspective view of the metal pipe and the support member shown in  FIG. 14A  after being assembled; and 
         FIG. 15  is a side view of the metal pipe shown in  FIG. 14A  and a flange attached to the metal pipe. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In describing exemplary embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular to  FIG. 3 , an image forming apparatus  900  according to an exemplary embodiment of the present invention is explained. 
       FIG. 3  is a schematic view of the image forming apparatus  900 . As illustrated in  FIG. 3 , the image forming apparatus  900  may be a copier, a facsimile machine, a printer, a multifunction printer having at least one of copying, printing, scanning, plotter, and facsimile functions, or the like. According to this exemplary embodiment of the present invention, the image forming apparatus  900  is a tandem color printer for forming a color image on a recording medium by electrophotography or electrostatic recording. 
     As illustrated in  FIG. 3 , the image forming apparatus  900  includes an image forming device  170  provided in a center portion of the image forming apparatus  900 , toner bottles  160   a ,  160   b ,  160   c , and  160   d  provided above the image forming device  170 , a writing unit  190  provided below the image forming device  170 , a paper tray unit  180  provided below the writing unit  190  in a lower portion of the image forming apparatus  900 , and a fixing device  140  provided above the image foaming device  170  in an upper portion of the image forming apparatus  900 . 
     The image forming device  170  includes an intermediate transfer belt  100  formed into a loop, photoconductors  110   a ,  110   b ,  110   c , and  110   d  disposed opposite the intermediate transfer belt  100 , transfer rollers  130   a ,  130   b ,  130   c , and  130   d  provided inside the loop formed by the intermediate transfer belt  100  and facing an inner circumferential surface of the intermediate transfer belt  100 , a second transfer roller  150  provided outside the loop formed by the intermediate transfer belt  100  and facing an outer circumferential surface of the intermediate transfer belt  100 , and a belt cleaning unit  120  provided outside the loop formed by the intermediate transfer belt  100  and facing the outer circumferential surface of the intermediate transfer belt  100 . 
     The image forming apparatus  900  can have the known overall structure. For example, components provided in the image forming device  170  and used to form a toner image, such as chargers, an exposure device (e.g., the writing unit  190 ), development devices, and cleaners, surround the photoconductors  110   a ,  110   b ,  110   c , and  110   d , respectively. 
     Specifically, as illustrated in  FIG. 3 , the intermediate transfer belt  100  looped over a plurality of rollers is provided in substantially the center portion of the image forming apparatus  900  in such a manner that the intermediate transfer belt  100  is disposed diagonally down from left to right and is rotatable counterclockwise in  FIG. 3 . 
     The photoconductors  110   a ,  110   b ,  110   c , and  110   d , the chargers, the development devices, and the cleaners are integrated into process cartridges, respectively, which are arranged below the intermediate transfer belt  100  along the outer circumferential surface of the intermediate transfer belt  100  in such a manner that the photoconductors  110   a ,  110   b ,  110   c , and  110   d  contact the lower outer circumferential surface of the intermediate transfer belt  100 . The four process cartridges have an identical structure except that the process cartridges use toners in colors (e.g., yellow, cyan, magenta, and black) different from each other. Specifically, charging rollers serving as the chargers, development units serving as the development devices, and cleaning units serving as the cleaners surround the photoconductors  110   a ,  110   b ,  110   c , and  110   d , respectively. The four development units use the toners in colors different from each other. The toner bottles  160   a ,  160   b ,  160   c , and  160   d  are provided above the intermediate transfer belt  100 , and contain yellow, cyan, magenta, and black toners to be supplied to the development units through conveyance paths, respectively, in a predetermined amount. 
     The writing unit  190  is provided below the process cartridges, and is constructed of four light sources including laser diodes (LD) corresponding to yellow, cyan, magenta, and black image data sent from a client computer, for example; a polygon scanner including a hexagonal polygon mirror and a polygon motor; fθ lenses provided in optical paths of the respective light sources; lenses (e.g., long cylindrical lenses); mirrors; and the like. Laser beams emitted by the laser diodes according to the yellow, cyan, magenta, and black image data are deflected by the polygon scanner, and irradiate and scan the photoconductors  110   a ,  110   b ,  110   c , and  110   d  to form electrostatic latent images on the photoconductors  110   a ,  110   b ,  110   c , and  110   d , respectively. 
     The transfer rollers  130   a ,  130   b ,  130   c , and  130   d  are disposed opposite the photoconductors  110   a ,  110   b ,  110   c , and  110   d  via the intermediate transfer belt  100 , respectively, and are connected to a power source that applies a predetermined voltage to the transfer rollers  130   a ,  130   b ,  130   c , and  130   d . The transfer rollers  130   a ,  130   b ,  130   c , and  130   d  primarily transfer yellow, cyan, magenta, and black toner images formed on the photoconductors  110   a ,  110   b ,  110   c , and  110   d  by the development units that visualize the yellow, cyan, magenta, and black electrostatic latent images with the yellow, cyan, magenta, and black toners contained in the development units, respectively, onto the intermediate transfer belt  100  so that the yellow, cyan, magenta, and black toner images are superimposed on a same position on the intermediate transfer belt  100  to form a color toner image on the intermediate transfer belt  100 . 
     The second transfer roller  150  is pressed against one of the plurality rollers supporting the intermediate transfer belt  100 , that is, the rightmost roller in  FIG. 3 , via the intermediate transfer belt  100 , and is connected to a power source that applies a predetermined voltage to the second transfer roller  150 . The second transfer roller  150  pressed against the rightmost roller inside the loop formed by the intermediate transfer belt  100  contacts the intermediate transfer belt  100  to form a second transfer nip between the second transfer roller  150  and the intermediate transfer belt  100 . Thus, when the second transfer roller  150  receives the predetermined voltage from the power source, the second transfer roller  150  secondarily transfers the color toner image formed on the intermediate transfer belt  100  onto a sheet serving as a recording medium conveyed from the paper tray unit  180  which is provided in the lower portion of the image forming apparatus  900  and contains sheets to be conveyed to the second transfer nip formed between the second transfer roller  150  and the intermediate transfer belt  100 . 
     On the other hand, the belt cleaning unit  120  faces another one of the plurality of rollers supporting the intermediate transfer belt  100 , that is, the leftmost roller in  FIG. 3 , via the intermediate transfer belt  100 . The belt cleaning unit  120  cleans the outer circumferential surface of the intermediate transfer belt  100  after the color toner image formed on the intermediate transfer belt  100  is transferred onto the sheet. 
     The fixing device  140  is provided above the second transfer nip formed between the second transfer roller  150  and the intermediate transfer belt  100 , and fixes the color toner image on the sheet sent from the second transfer nip semi-permanently. 
     Thereafter, the sheet bearing the fixed toner image is conveyed from the fixing device  140 , and then discharged onto an output tray provided on top of the image forming apparatus  900 . 
     Referring to  FIGS. 4 to 6 , the following describes the fixing device  140 .  FIG. 4  is a vertical sectional view of the fixing device  140 .  FIG. 5  is a perspective view of the fixing device  140 .  FIG. 6  is a side view of the fixing device  140 . 
     As illustrated in  FIG. 4 , the fixing device  140  includes a fixing belt  31  formed into a loop, a nip formation member  32 , a support member  33 , a metal pipe  34 , a halogen heater  35 , a heat insulator  36 , and a lubrication sheet  37 , which are provided inside the loop formed by the fixing belt  31 , a thermistor  38  provided outside the loop formed by the fixing belt  31 , and a pressing roller  6  disposed opposite the fixing belt  31 . 
     The pressing roller  6  serving as a rotary pressing member or a pressing member is pressed against the nip formation member  32  via the fixing belt  31  serving as a fixing member. The metal pipe  34  formed into a substantially hollow cylinder is stationarily disposed close to an inner circumferential surface of the fixing belt  31  so that the inner circumferential surface of the fixing belt  31  slides over an outer circumferential surface of the metal pipe  34 . For example, a gap not greater than 1 mm is provided between the inner circumferential surface of the fixing belt  31  and the outer circumferential surface of the metal pipe  34 . The metal pipe  34  supports the nip formation member  32  via the heat insulator  36  that prevents heat transmission to the nip formation member  32 , improving heating efficiency of the metal pipe  34  for heating the fixing belt  31 . The support member  33  is stationarily disposed inside the hollow cylinder of the metal pipe  34 , and supports a concave portion of the metal pipe  34 , which houses and supports the nip formation member  32 . A lubricant (e.g., silicon oil and fluorine grease) may be applied between the metal pipe  34  and the fixing belt  31  to decrease wear of the fixing belt  31  due to friction generated between the metal pipe  34  and the fixing belt  31  that slides over the metal pipe  34 . 
     The metal pipe  34  is manufactured by bending a thin sheet of metal (hereinafter “sheet metal”) such as aluminum, iron, or stainless steel into a generally cylindrical shape. According to this exemplary embodiment, the metal pipe  34  is made of SUS stainless steel. As illustrated in  FIG. 6 , the fixing device  140  further includes side plates  40  and flanges  41  attached to the side plates  40 . Lateral ends of the metal pipe  34  in an axial direction of the metal pipe  34  are fixedly supported by the side plates  40  of the fixing device  140  via the flanges  41 , respectively. As illustrated in  FIG. 4 , the metal pipe  34  has substantially a circular shape in cross-section. Alternatively, the metal pipe  34  may have an oval shape, a polygonal shape, or other shape in cross-section. It is to be noted that even if the metal pipe  34  has any of the circular, oval, polygonal, and other shapes in cross-section, the metal pipe  34  can have a cylindrical shape. Yet alternatively, a slit may be provided on a circumferential surface of the metal pipe  34 . The mesh-like lubrication sheet  37  is provided between the nip formation member  32  and the fixing belt  31 , but is not essential. The fixing belt  31  is heated directly by the metal pipe  34  and indirectly by the halogen heater  35  serving as a heat source or a heater provided inside the metal pipe  34  via the metal pipe  34 . The thermistor  38  is used to adjust a temperature of the fixing belt  31 . The heat source for heating the metal pipe  34  is not limited to the halogen heater  35 , and therefore may be an induction heater, a resistant heat generator, a carbon heater, or the like. 
     As illustrated in  FIGS. 4 and 5 , the pressing roller  6  is pressed against the nip formation member  32  via the fixing belt  31  to form a concave fixing nip N between the pressing roller  6  and the fixing belt  31 . Alternatively, the fixing nip N may have a planar shape or other shape. However, when the fixing nip N has the concave shape, a leading edge of a sheet discharged from the fixing nip N is directed to the pressing roller  6 , thus facilitating separation of the sheet from the fixing belt  31  and thereby preventing jamming of the sheet at the fixing nip N. 
     The fixing belt  31  is a thin, flexible endless belt, and is constructed of a base layer, an elastic layer provided on the base layer, and a release layer provided on the elastic layer. The base layer is made of a metal material such as nickel and SUS stainless steel or a resin material such as polyimide. The elastic layer is made of silicon rubber. The release layer is made of tetrafluoroethylene perfluoroalkylvinylether copolymer (PFA) and/or polytetrafluoroethylene (PTFE). The fixing belt  31  has a thickness not greater than about 1 mm. Alternatively, the elastic layer may be omitted because the fixing belt  31  without the elastic layer has a smaller heat capacity that improves fixing property. However, when the fixing belt  31  and the pressing roller  6  apply pressure to a sheet bearing an unfixed toner image to fix the toner image on the sheet, slight surface asperities on a surface of the fixing belt  31  are transferred onto the toner image, roughening the solid toner image into an orange-peel image. To address this problem, the fixing belt  31  can preferably include the elastic layer made of silicon rubber which has a thickness not smaller than about 100 μm. 
     The pressing roller  6  is constructed of a hollow metal roll, a silicon rubber layer provided on the metal roll, and a release layer provided on the silicon rubber layer as a surface layer. Like the fixing belt  31 , the pressing roller  6  includes the silicon rubber layer serving as an elastic layer. However, a thickness of the silicon rubber layer of the pressing roller  6  is different from the thickness of the elastic layer of the fixing belt  31 . The release layer made of PFA or PTFE provides separation property for separating the sheet from the pressing roller  6 . The pressing roller  6  receives a driving force transmitted from a driver (e.g., a motor) provided in the image forming apparatus  900  via a gear train, and is rotated by the driving force in a rotation direction R 2 . Consequently, the fixing belt  31  pressed by the pressing roller  6  at the fixing nip N rotates in a rotation direction R 1  in accordance with rotation of the pressing roller  6 . A spring presses the pressing roller  6  against the nip formation member  32  via the fixing belt  31  and deforms the silicon rubber layer of the pressing roller  6  to provide a predetermined nip length of the fixing nip N in a sheet conveyance direction. 
     Alternatively, the pressing roller  6  may be a solid roller. However, the hollow pressing roller  6  has a desired smaller heat capacity. Optionally, a heat source (e.g., a halogen heater) may be provided inside the hollow pressing roller  6 . The silicon rubber layer of the pressing roller  6  can be made of solid rubber. Alternatively, when no heat source is provided inside the pressing roller  6 , the pressing roller  6  may be made of sponge rubber. The pressing roller  6  made of sponge rubber can improve heat insulation to suppress heat transmission from the fixing belt  31  to the pressing roller  6 . 
     A width of the support member  33 , which is fixedly provided inside the metal pipe  34  to support the nip formation member  32 , in a width direction, that is, a longitudinal direction, of the support member  33  parallel to an axial direction of the fixing belt  31  is equivalent to at least a width of the nip formation member  32  and the metal pipe  34  in a width direction, that is, a longitudinal direction, of the nip formation member  32  and the metal pipe  34  parallel to the axial direction of the fixing belt  31 . Lateral ends of the support member  33  in the width direction of the support member  33  are fixedly supported by the side plates  40  of the fixing device  140 , respectively. A surface of the support member  33  is insulated or mirror-finished so that the support member  33  is not heated by the halogen heater  35 , thus reducing wasteful energy consumption and utilizing radiation heat generated by the halogen heater  35  to heat the metal pipe  34 . 
     As illustrated in  FIG. 5 , when the pressing roller  6  presses the fixing belt  31  against the nip formation member  32 , pressure applied by the pressing roller  6  bends the support member  33  and the nip formation member  32  supported by the support member  33 . Specifically, the nip formation member  32  and the support member  33  are bent substantially at a center of the fixing belt  31  in the axial direction of the fixing belt  31 . Accordingly, a predetermined nip width of the fixing nip N formed between the fixing belt  31  and the pressing roller  6  is not provided. 
     To address this problem, the nip formation member  32  includes a central convex portion  32 A at a center of the nip formation member  32  in the longitudinal direction of the nip formation member  32  parallel to the axial direction of the fixing belt  31  as illustrated in  FIG. 7B . The central convex portion  32 A protrudes toward the metal pipe  34  depicted in  FIG. 4  with respect to lateral end portions  32 B of the nip formation member  32  provided at lateral ends of the nip formation member  32  in the longitudinal direction of the nip formation member  32  so as to provide the predetermined nip width of the fixing nip N also at the center of the fixing belt  31 . 
     Referring to  FIGS. 7A ,  7 B,  8 A,  8 B,  9 A, and  9 B, the following describes the nip formation member  32  having the central convex portion  32 A.  FIG. 7A  is a sectional view of the nip formation member  32 .  FIG. 7B  is a side view of the nip formation member  32 . As illustrated in  FIG. 7B , the central convex portion  32 A of the nip formation member  32  has a height sufficient to offset bending of the nip formation member  32  due to pressure applied by the pressing roller  6 . Accordingly, even when the support member  33  is bent by pressure applied by the pressing roller  6 , a surface of the nip formation member  32  facing the fixing nip N is flattened, providing the predetermined nip width of the fixing nip N. 
       FIG. 8A  is a sectional view of a comparative fixing device  140 R when the pressing roller  6  is not pressed against the nip formation member  32 .  FIG. 8B  is a plan view of the pressing roller  6 , the nip formation member  32 , and a comparative metal pipe  34 R when the pressing roller  6  is not pressed against the nip formation member  32 .  FIG. 9A  is a sectional view of the comparative fixing device  140 R when the pressing roller  6  is pressed against the nip formation member  32 .  FIG. 9B  is a plan view of the pressing roller  6 , the nip formation member  32 , and the comparative metal pipe  34 R when the pressing roller  6  is pressed against the nip formation member  32 . 
     As illustrated in  FIGS. 8B and 9B , the nip formation member  32  includes the rib-shaped central convex portion  32 A at the center of the nip formation member  32  in the longitudinal direction of the nip formation member  32 . When the pressing roller  6  is not pressed against the nip formation member  32  as illustrated in  FIG. 8A , the lateral end portions  32 B of the nip formation member  32  provided at the lateral ends of the nip formation member  32  in the longitudinal direction of the nip formation member  32  are slightly separated from the comparative metal pipe  34 R serving as a heating member as illustrated in  FIG. 8B . By contrast, when the pressing roller  6  is pressed against the nip formation member  32  as illustrated in  FIG. 9A , the nip formation member  32  and the support member  33  are bent by pressure applied by the pressing roller  6  in a state in which the surface of the nip formation member  32  facing the fixing nip N is planar as illustrated in  FIG. 9B . Accordingly, the predetermined nip width is provided both at the center and the lateral ends of the nip formation member  32  in the longitudinal direction of the nip formation member  32  parallel to the axial direction of the fixing belt  31 . Consequently, the comparative fixing device  140 R provides proper fixing property at a predetermined temperature. 
     However, a substantial clearance may arise between an outer circumferential surface of the comparative metal pipe  34 R and the inner circumferential surface of the fixing belt  31 , which adversely increases toward lateral ends of the fixing belt  31  in the axial direction of the fixing belt  31 .  FIG. 10  is a partially enlarged sectional view of the comparative fixing device  140 R at the lateral end of the fixing belt  31  in the axial direction of the fixing belt  31 . The halogen heater  35  provided inside the comparative metal pipe  34 R heats the comparative metal pipe  34 R, and then heat is transmitted from the comparative metal pipe  34 R to the fixing belt  31  to heat the fixing belt  31 . With this configuration, if the clearance between the outer circumferential surface of the comparative metal pipe  34 R and the inner circumferential surface of the fixing belt  31  is too large, heat is not transmitted from the comparative metal pipe  34 R to the fixing belt  31  effectively. Accordingly, the fixing belt  31  is not heated to a proper fixing temperature quickly, resulting in faulty fixing at the lateral ends of the fixing belt  31  in the axial direction of the fixing belt  31 . 
     Referring to  FIGS. 11A and 11B , the following describes the mechanism that generates the substantial clearance between the fixing belt  31  and the comparative metal pipe  34 R at the lateral ends of the fixing belt  31  in the axial direction of the fixing belt  31 .  FIG. 11A  is a sectional view of the comparative fixing device  140 R at the lateral end of the fixing belt  31  in the axial direction of the fixing belt  31 .  FIG. 11B  is a plan view of the pressing roller  6 , the fixing belt  31 , the nip formation member  32 , and the comparative metal pipe  34 R. 
     The nip formation member  32  has the central convex portion  32 A in the longitudinal direction of the nip formation member  32 , so that the surface of the nip formation member  32  facing the fixing nip N is flattened when the pressing roller  6  is pressed against the nip formation member  32  via the fixing belt  31 . When the pressing roller  6  applies pressure to the nip formation member  32 , the support member  33  and the nip formation member  32  supported by the support member  33  are deformed or bent. Consequently, the comparative metal pipe  34 R provided between the support member  33  and the nip formation member  32  is also bent in accordance with bending of the nip formation member  32  and the support member  33 .  FIG. 11B  illustrates the bent comparative metal pipe  34 R in the axial direction of the fixing belt  31 . 
     On the other hand, the fixing belt  31  has an identical circumferential length both at a center portion and lateral end portions of the fixing belt  31  in the axial direction of the fixing belt  31 . Accordingly, the clearance between the comparative metal pipe  34 R and the fixing belt  31  is appropriate at the center of the fixing belt  31  in the axial direction of the fixing belt  31  at which the comparative metal pipe  34 R is bent. By contrast, the clearance between the comparative metal pipe  34 R and the fixing belt  31  is too large at the lateral ends of the fixing belt  31  in the axial direction of the fixing belt  31  at a position farthest from the fixing nip N in the rotation direction R 1  of the fixing belt  31  as illustrated in  FIG. 10 . Consequently, heat is not transmitted from the comparative metal pipe  34 R to the fixing belt  31  quickly. 
     A lubricant (e.g., grease) is applied between the outer circumferential surface of the comparative metal pipe  34 R and the inner circumferential surface of the fixing belt  31  to cause the fixing belt  31  to rotate and slide over the outer circumferential surface of the comparative metal pipe  34 R smoothly. When the comparative metal pipe  34 R is disposed close to the fixing belt  31  only at the center portion of the fixing belt  31  in the axial direction of the fixing belt  31 , the lubricant is accumulated at the lateral end portions of the fixing belt  31  in the axial direction of the fixing belt  31 . To address this problem, in the comparative fixing device  140 R, the clearance between the comparative metal pipe  34 R and the fixing belt  31  is small, and the lubricant applied between the comparative metal pipe  34 R and the fixing belt  31  is circulated through the clearance in a circumferential direction of the fixing belt  31  to prevent wear of the nip formation member  32  due to friction generated between the fixing belt  31  and the nip formation member  32  and decrease load applied to the rotating fixing belt  31 . 
     However, when the substantial clearance is provided between the comparative metal pipe  34 R and the fixing belt  31  as illustrated in  FIG. 10 , the lubricant is not circulated through the clearance by rotation of the fixing belt  31  due to viscosity of the lubricant. Accordingly, the lubricant is accumulated in the clearance between the comparative metal pipe  34 R and the fixing belt  31 . Thus, the blocked circulation of the lubricant accelerates wear of the nip formation member  32 , degrading durability of the comparative fixing device  140 R. Moreover, the lubricant applied in the clearance between the comparative metal pipe  34 R and the fixing belt  31  at the center portion of the fixing belt  31  in the axial direction of the fixing belt  31  is pushed toward the lateral end portions of the fixing belt  31  in the axial direction of the fixing belt  31  at which the larger clearance is provided between the comparative metal pipe  34 R and the fixing belt  31 . Accordingly, the lubricant leaks from the lateral end portions of the fixing belt  31  in the axial direction of the fixing belt  31 . 
     To address the above-described problems, the fixing device  140  according to this exemplary embodiment has the configuration described below.  FIG. 12A  is a sectional view of the fixing device  140  when the pressing roller  6  is not pressed against the nip formation member  32 .  FIG. 12B  is a plan view of the pressing roller  6 , the nip formation member  32 , and the metal pipe  34  when the pressing roller  6  is not pressed against the nip formation member  32 .  FIG. 13A  is a sectional view of the fixing device  140  when the pressing roller  6  is pressed against the nip formation member  32 .  FIG. 13B  is a plan view of the pressing roller  6 , the fixing belt  31 , the nip formation member  32 , and the metal pipe  34  when the pressing roller  6  is pressed against the nip formation member  32 . In  FIGS. 12A and 12B , the fixing belt  31 , the halogen heater  35 , the heat insulator  36 , the lubrication sheet  37 , and the thermistor  38  depicted in  FIG. 4  are omitted. In  FIGS. 13A and 13B , the halogen heater  35 , the heat insulator  36 , the lubrication sheet  37 , and the thermistor  38  depicted in  FIG. 4  are omitted. 
     The metal pipe  34  has a thickness in a range of from about 0.1 mm to about 0.2 mm. The clearance between the outer circumferential surface of the metal pipe  34  and the inner circumferential surface of the fixing belt  31  is about 0.5 mm at maximum. According to this exemplary embodiment, an inner diameter of the fixing belt  31  is about 30 mm. The nip formation member  32  has a convex shape in which the central concave portion  32 A of the rib of the nip formation member  32  in the longitudinal direction of the nip formation member  32  parallel to the axial direction of the fixing belt  31  protrudes toward the metal pipe  34  by about 0.6 mm compared to the lateral end portions  32 B of the rib of the nip formation member  32  in the longitudinal direction of the nip formation member  32 . Thus, when the pressing roller  6  presses the fixing belt  31  against the nip formation member  32  as illustrated in  FIG. 13A , the nip formation member  32  forms the planar fixing nip N between the pressing roller  6  and the fixing belt  31 , as illustrated in  FIG. 13B . 
     When the pressing roller  6  applies pressure to the nip formation member  32  having the above-described structure, the clearance of about 0.5 mm is provided between the outer circumferential surface of the metal pipe  34  and the inner circumferential surface of the fixing belt  31  at the center portion of the fixing belt  31  in the axial direction of the fixing belt  31 . With a conventional, straight metal pipe having a uniform diameter, the clearance between the straight metal pipe and the fixing belt  31  is enlarged to about 1.1 mm at the lateral end portions of the fixing belt  31  in the axial direction of the fixing belt  31 . Accordingly, the fixing belt  31  is not heated by the straight metal pipe sufficiently. To address this problem, in the fixing device  140  according to this exemplary embodiment, lateral end portions  34 B of the metal pipe  34  in cross-section in the longitudinal direction of the metal pipe  34  parallel to the axial direction of the fixing belt  31  are greater than a center portion  34 A of the metal pipe  34  in cross-section in the longitudinal direction of the metal pipe  34 , as illustrated in  FIG. 12B . In other words, a diameter of the metal pipe  34  increases from the center portion  34 A toward the lateral end portions  34 B of the metal pipe  34  in the longitudinal direction of the metal pipe  34 . For example, a diameter of the lateral end portions  34 B of the metal pipe  34  in the longitudinal direction of the metal pipe  34  is greater than a diameter of the center portion  34 A of the metal pipe  34  in the longitudinal direction of the metal pipe  34  by about 0.6 mm. In other words, the difference between the diameter of the lateral end portions  34 B and the diameter of the center portion  34 A in the longitudinal direction of the metal pipe  34  corresponds to the height of the convex portion  32 A of the nip formation member  32 , which is sufficient to offset bending of the nip formation member  32  caused by pressure applied by the pressing roller  6  to the nip formation member  32  so as to provide substantially uniform clearance between the inner circumferential surface of the fixing belt  31  and the outer circumferential surface of the metal pipe  34  in the axial direction of the fixing belt  31 . Accordingly, when the pressing roller  6  presses the fixing belt  31  against the nip formation member  32 , the clearance between the metal pipe  34  and the fixing belt  31  at the lateral end portions  34 B of the metal pipe  34  in the axial direction of the fixing belt  31  is decreased by about 0.6 mm. Consequently, the clearance between the metal pipe  34  and the fixing belt  31  at the center portion  34 A of the metal pipe  34  in the axial direction of the fixing belt  31  is equivalent to the clearance between the metal pipe  34  and the fixing belt  31  at the lateral end portions  34 B of the metal pipe  34  in the axial direction of the fixing belt  31 . 
     In order to provide the increased outer diameter of the lateral end portions  34 B of the metal pipe  34  in the longitudinal direction of the metal pipe  34  in a state in which the metal pipe  34  is assembled into the fixing device  140 , the metal pipe  34  can be tapered. However, the metal pipe  34  serving as a heating member is formed of sheet metal having a thickness of about 0.1 mm. Accordingly, spring-back generated by the sheet metal processed into the metal pipe  34  may prevent the metal pipe  34  from being tapered precisely. Further, an amount of spring-back generated by the metal pipe  34  may fluctuate. Accordingly, increased precision in processing the sheet metal into the tapered metal pipe  34  may decrease production yields of the metal pipe  34 , resulting in increased manufacturing costs. 
     To address this problem, another method for manufacturing the metal pipe  34  uses a conventional, straight cylindrical metal pipe without a tapered shape, which is manufactured by bending a flat thin plate into a cylinder. Referring to  FIGS. 14A ,  14 B,  14 C, and  14 D, the following describes the manufacturing method using the conventional, straight cylindrical metal pipe.  FIG. 14A  is a perspective view of the metal pipe  34  and the support member  33  before being assembled.  FIG. 14B  is a sectional view of the support member  33 .  FIG. 14C  is a plan view of the support member  33 .  FIG. 14D  is a perspective view of the metal pipe  34  and the support member  33  after being assembled. 
     As illustrated in  FIG. 14A , the metal pipe  34  includes a concave portion  34 H. The metal pipe  34  is elastically deformed in such a manner that the concave portion  34 H of the metal pipe  34 , which houses the nip formation member  32  depicted in  FIG. 5 , is widened toward the lateral end portions  34 B (depicted in  FIG. 12B ) of the metal pipe  34  in the longitudinal direction of the metal pipe  34  parallel to the axial direction of the fixing belt  31 . Accordingly, the metal pipe  34  is tapered. Specifically, when the metal pipe  34  is bent into a cylinder, the metal pipe  34  generates an elastic force in a direction in which the bent metal pipe  34  returns to its original flat shape. The metal pipe  34  is tapered by using such elastic force. 
     For example, as illustrated in  FIG. 14C , the support member  33  includes a groove  33   r  provided in the support member  33  in such a manner that the groove  33   r  is tapered in the longitudinal direction of the support member  33  parallel to the axial direction of the fixing belt  31  depicted in  FIG. 5 . Specifically, the groove  33   r  includes a center portion  33   r A provided at a center of the groove  33   r  in a longitudinal direction of the groove  33   r  parallel to the axial direction of the fixing belt  31  of reduced diameter, and lateral end portions  33   r B provided at lateral ends of the groove  33   r  in the longitudinal direction of the groove  33   r  of increased diameter compared to the diameter of the center portion  33   r A. In other words, the diameter of the groove  33   r  becomes greater from the center portion  33   r A toward the lateral end portions  33   r B thereof. The concave portion  34 H of the metal pipe  34  is placed in the groove  33   r  of the support member  33 , and is adhered to interior walls of the groove  33   r  with screws or the like. Thus, the metal pipe  34  is tapered. 
     Referring to  FIG. 15 , the following describes yet another method for manufacturing the metal pipe  34 .  FIG. 15  is a side view of the metal pipe  34  and a flange  45  attached to the metal pipe  34  and including a tapered portion  45 A. 
     The tapered portion  45 A of the flange  45  is inserted into the metal pipe  34  at the lateral end portion  34 B of the metal pipe  34  in the longitudinal direction of the metal pipe  34  parallel to the axial direction of the fixing belt  31 , so as to arbitrarily adjust an amount of tapering of the metal pipe  34 . Specifically, the amount of tapering of the metal pipe  34  is changed by adjusting a depth of insertion of the tapered portion  45 A of the flange  45  into the metal pipe  34 . In other words, the outer diameter of the lateral end portion  34 B of the metal pipe  34  in the longitudinal direction of the metal pipe  34  is enlarged by adjusting the depth of insertion of the tapered portion  45 A of the flange  45  into the metal pipe  34 . In  FIG. 15 , the flange  45  is shrunk to simplify the drawing. However, the flange  45  has a proper length in the longitudinal direction of the metal pipe  34  with respect to a length of the metal pipe  34  in the longitudinal direction of the metal pipe  34  to provide a predetermined width of the fixing nip N formed between the fixing belt  31  and the pressing roller  6  when the pressing roller  6  presses the fixing belt  31  against the nip formation member  32  as illustrated in  FIG. 4 . 
     Referring to  FIG. 5 , the following describes effects provided by the fixing device  140 . With the configuration described above, particularly, with the tapered metal pipe  34  corresponding to the nip formation member  32  having the convex portion  32 A, the clearance between the inner circumferential surface of the fixing belt  31 , that is, an endless belt serving as a fixing member, and the outer circumferential surface of the metal pipe  34  serving as a cylindrical heating member does not increase toward the lateral end portions of the fixing member in the axial direction of the fixing member, preventing faulty fixing due to inefficient heat transmission from the heating member to the fixing member. The clearance between the fixing member and the heating member provides effect of resistance against heat transmission from the heating member to the fixing member. Further, the pressing roller  6  serving as a pressing member is pressed against the nip formation member  32  serving as a nip formation member to form the fixing nip N between the pressing member and the nip formation member via the fixing member. Even when pressure applied by the pressing member deforms the nip formation member, the clearance between the fixing member and the heating member at the lateral end portions of the fixing member in the axial direction of the fixing member does not become excessively greater than the clearance between the fixing member and the heating member at the center portion of the fixing member in the axial direction of the fixing member in accordance with the deformation of the nip formation member. Accordingly, heat is transmitted from the heating member to the fixing member effectively, providing desired fixing property. 
     Further, heat transmission efficiency at the center portion of the fixing member in the axial direction of the fixing member is equivalent to heat transmission efficiency at the lateral end portions of the fixing member in the axial direction of the fixing member, preventing variation of heat distribution of the fixing member. 
     When a lubricant is applied to the clearance between the fixing member and heating member, the lubricant is spread and circulated through the clearance. Accordingly, rotation of the fixing member constantly supplies the lubricant to the nip formation member, improving durability of the nip formation member and suppressing leakage of the lubricant from the clearance between the fixing member and the heating member at the lateral end portions of the fixing member in the axial direction of the fixing member. 
     In the fixing device  140  according to the above-described exemplary embodiments, the pressing roller  6  is used as a pressing member. Alternatively, a pressing belt, a pressing pad, or a pressing plate may be used as a pressing member to provide effects equivalent to the effects provided by the pressing roller  6 . 
     Further, the fixing belt  31  is used as a fixing member. Alternatively, an endless fixing film may be used as a fixing member. 
     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.