Patent Publication Number: US-8532553-B2

Title: Fixing device and image forming apparatus including same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present patent application claims priority pursuant to 35 U.S.C. §119 from Japanese Patent Application No. 2010-055651, filed on Mar. 12, 2010 in the Japan Patent Office, which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field 
     Exemplary embodiments of the present disclosure relate to an image forming apparatus and a fixing device employed in the image forming apparatus, and more specifically, an electrophotographic image forming apparatus capable of forming images at high speed, and a fixing device employed in the image forming apparatus. 
     2. Description of the Background Art 
     Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction apparatuses having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. In such an image forming apparatus, 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. 
     In recent years, image forming apparatuses have advanced in terms of color processing, high image quality, and high throughput. In order to fix a color image at a high image quality, it is preferable that the time (hereinafter “nip time”) during which a recording sheet contacts a fixing nip is, for example, 40 milliseconds to 60 milliseconds, and the pressure at the nip is equal to or more than, for example, 7 N/cm 2 . In particular, for high-speed image forming apparatuses having a throughput of 50 ppm or more for A4 size recording media, recording sheets are transported at a relatively high speed. Accordingly, in order to obtain the above-described nip time, it is preferable that the width of the fixing nip is equal to or more than, e.g., 10 mm, and the surface pressure thereof is equal to or more than, e.g., 7 N/cm 2 . Further, coated sheets (e.g., paper sheets coated with resin), which are generally used with high-speed machines, tends to produce defective toner images due to water vapor when passing through the nip. Accordingly, it is preferable to apply a higher surface pressure of, e.g., 12 N/cm 2  or more at the nip. 
     Further, in order to enhance the image quality, it is preferable to separate a recording sheet from the nip exit naturally, without excessive force or an unduly complicated structure. Therefore, various proposals have been made regarding the shape of the nip, the roller curvature at the nip exit, and the guide member. Also, there is a method for bringing a separation claw or the like into contact with the roller for forcibly separating recording sheets therefrom, but this method may cause wear flaws in the roller to be transferred to images, thereby degrading images. 
     Such high-speed image forming apparatuses often use belt fixing devices capable of easily obtaining the preferable nip width, like that described in JP-H04-050883-A. However, in order to obtain a large nip width of, e.g., 10 mm or more, and a high surface pressure, it is preferable to provide a large pressing force between the fixing roller and the pressing roller. Additionally, in order to obtain a nip width which is kept uniform in the axial direction without being bent by the pressing force, it is preferable to use a large-diameter roller of a diameter of, e.g., 50 mm or more, and having excellent strength. 
     Such a belt fixing device having a large-diameter roller inevitably results in a large size of the device itself, thus increasing the cost of components. Further, a reduced roller curvature at the nip exit tends to hinder natural separation of recording sheets, thus resulting in failures, such as curling of the recording sheet around the fixing belt due to the adhesion force of fused toner. Further, in cases of forcible separation using separation claws, images may be degraded. 
     Further, for a fixing device including a stationary member as a fixing member like those described in the above-described JP-2004-252354-A and JP-2004-198556-A, using the stationary member as the fixing member causes the fixing belt to slide over the stationary member, resulting in a reduced running performance of the fixing belt due to the sliding resistance, wear degradation of the respective members, and the like. 
     To cope with such a challenge, for example, conventional techniques like those described in JP-2004-252354-A and JP-2004-198556-A propose to use a roller in the stationary member to reduce the sliding resistance or modify the materials of components and grease. However, the stationary member is smaller than the large-diameter roller, resulting in insufficient strength. As a result, a high surface pressure cannot be obtained, thus preventing the enhancement of image quality. 
     For the fixing device like that described in JP-2007-334205-A, it is conceivable to obtain a high surface pressure by increasing the diameter of the heating member and using a larger reinforcement member, and to obtain a wider nip by adjusting the shape of the nip. However, if an increased diameter of the heating member is used, the roller curvature at the nip exit is reduced, preventing natural separation of a recording sheet. As a result, the recording sheet may be curled around the fixing belt by the adhesion force of fused toner, as with the belt fixing device described in JP-H04-050883-A. Further, it is hard to optimize both the heat transmission performance and the running performance of the fixing belt simultaneously, thus making it hard to employ the above-described belt fixing device in a high-speed image forming apparatus. 
     SUMMARY 
     In an aspect of this disclosure, there is provided an improved fixing device including a rotary heating member, a heat source, a stationary member, a flexible fixing belt, and a rotary pressure member. The heat source is disposed near the rotary heating member to heat the rotary heating member. The stationary member is disposed in sliding contact with a portion of an outer circumferential surface of the rotary heating member. The fixing belt is looped around the rotary heating member and the stationary member. The rotary pressure member is disposed in pressure contact with the stationary member via the fixing belt to form a nip between the fixing belt and the rotary pressure member. A pressing force acting between the rotary heating member and the rotary pressure member via the fixing belt and the stationary member creates pressure at the nip. 
     In an aspect of this disclosure, there is provided an improved image forming apparatus including the fixing device described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Additional aspects, features, and advantages of the present disclosure will be readily ascertained 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 configuration view of an image forming apparatus according to an exemplary embodiment of the present disclosure; 
         FIG. 2  is a schematic configuration view of a fixing device according to an exemplary embodiment of the present disclosure; 
         FIG. 3  is a side view of a fixing device according to an exemplary embodiment of the present disclosure; 
         FIG. 4  is a side view of a fixing device according to an exemplary embodiment of the present disclosure; 
         FIG. 5  is a schematic view of a rotation mechanism of a heating roller and a pressure roller; 
         FIG. 6  is a schematic view of a fixing device including an induction heating device as a heat source; 
         FIG. 7  is a schematic view of a stationary member including rollers; 
         FIG. 8  is a schematic view of a stationary member having protrusions; 
         FIG. 9  is a schematic view of a conventional roller-fixing device; and 
         FIG. 10  is a schematic view of a conventional belt-fixing device. 
     
    
    
     The accompanying drawings are intended to depict exemplary embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent 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 similar results. 
     Although the exemplary embodiments are described with technical limitations with reference to the attached drawings, such description is not intended to limit the scope of the invention and all of the components or elements described in the exemplary embodiments of this disclosure are not necessarily indispensable to the present invention. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, exemplary embodiments of the present disclosure are described below with reference to  FIGS. 1 to 8 . 
       FIG. 1  is a schematic view of an image forming apparatus including a fixing device according to an exemplary embodiment of the present disclosure. 
     In  FIG. 1 , the image forming apparatus  100  is a color printer. It is to be noted that the image forming apparatus is not limited to the color printer as illustrated in  FIG. 1  and may be any other suitable type of image forming apparatus. The image forming apparatus  100  includes an intermediate transfer member  10  of, e.g., an endless-belt type at the center thereof. 
     The intermediate transfer member  10  is looped around a driving roller  14 , a first driven roller  15  and a second driven roller  16  to circulate in the clockwise direction in  FIG. 1 . The intermediate transfer member  10  may be looped around four or more rollers including, for example, a roller for correcting the positional deviation thereof. In  FIG. 1 , the intermediate transfer member  10  is extended substantially horizontally. Alternatively, the intermediate transfer member  10  may be extended obliquely. 
     The image forming apparatus  100  further includes a belt cleaning device near the second driven roller  16  to remove residual toner remaining on the intermediate transfer member  10  after image transfer. 
     Further, above the intermediate transfer member  10  extended between the driving roller  14  and the second driven roller  16 , four single-color image forming units  18  for, e.g., black, yellow, magenta and cyan, are arranged along a running direction of the intermediate transfer member  10 . The single-color image forming units  18  collectively form a tandem image forming device  20 . An exposure device  21  is further provided above the tandem image forming device  20 . 
     On the other hand, under an area where the intermediate transfer member  10  is extended is provided a secondary transfer device  22  including a roller. The secondary transfer device  22  is pressed against the first driven roller  15  to transfer single-color images from the intermediate transfer member  10  onto a recording medium P. In  FIG. 1 , beside the secondary transfer device  22 , a conveyance belt  24  of an endless shape is extended between two rollers  23 . Beside the conveyance belt  24  is provided a fixing device  25  to fix the transferred images on the recording medium P. 
     The secondary transfer device  22  also feeds the recording medium P having the images transferred from the intermediate transfer member  10  to the conveyance belt  24 . It is to be noted that a non-contact type charger may be used as the secondary transfer device  22 . In such a case, a sheet feed mechanism may be provided separately from the non-contact type charger of the secondary transfer device  22 . 
     Further, below the secondary transfer device  22 , the conveyance belt  24  and the fixing device  25  is provided a medium housing cassette  28  that store recording media P, such as paper sheets or OHP films. 
     When copying is performed using the image forming apparatus, a document G is set on a document table  30  of a scanner  200  and pressed by a document retainer. Further, when a start button is pressed, the scanner  200  is activated to optically read the document. Thus, when the document G is illuminated by a light source  31 , e.g., a halogen lamp, reflected light from the document G is further reflected by a mirror  32 . The reflected light passes through a lens  33  to be condensed onto a CCD  34  and converted at the CCD  34  to electric signals. Thus, an image on the document G is converted to electrical signals. 
     Further, when the start button is pressed, a driving motor starts to rotate the driving roller  14 . In accordance with rotation of the driving roller  14 , the driven rollers  15  and  16  rotate, thus circulating the intermediate transfer member  10 . At the same time, in the single-color image forming units  18 , image bearing members  40  are rotated to form single-color images of black, yellow, magenta, and cyan thereon. Further, as the intermediate transfer member  10  circulates, the single-color images are transferred and superimposed onto the intermediate transfer member  10  to form a composite color image on the intermediate transfer member  10 . 
     Meanwhile, when the start button is pressed, feed rollers  35  rotates to feed a recording medium P from the sheet tray  28  into a feed path  36 . The recording medium P fed by the feed rollers  35  impinges on registration rollers  37  and stops there. 
     Further, in synch with conveyance of the composite color on the intermediate transfer member  10 , the registration rollers  37  are rotated to feed the recording medium P to a secondary transfer nip between the intermediate transfer member  10  and the secondary transfer device  22 . Thus, the composite color image on the intermediate transfer member  10  is collectively transferred onto the recording medium P, thus obtaining a desired color image on the recording medium P. 
     After the image transfer at the secondary transfer device  22 , the recording medium P is conveyed by the conveyance belt  24  to the fixing device  25 . In the fixing device  25 , heat and pressure are applied to the recording medium P to fix the transferred images on the recording medium P. Then, the recording medium P is stacked on an output tray  38 . 
     Meanwhile, after the image transfer at the secondary transfer device  22 , the belt cleaning device removes residual toner remaining on the intermediate transfer member  10  to get ready for next image formation by the tandem image forming device  20 . 
     In each of the single-color image forming units  18  of the tandem image forming device  20 , the image bearing member  40  of a drum shape is surrounded by, for example, a charging device  41 , a development device  42 , a primary transfer device  43 , a cleaning device  44 , and a discharging device, which is not illustrated, and the like. 
     All or some of the components of each single-color image forming unit  18  may be integrally formed as a process cartridge removably mountable to the image forming apparatus  100 , thus enhancing easiness of servicing operation. 
     The charging devices  41  (e.g., charging rollers in  FIG. 1 ) apply voltage to the image bearing members  40  in contact therewith, thus electrically charging the image bearing members  40 . 
     Each of the development devices  42  uses two-component developer containing magnetic carrier particles and nonmagnetic toner particles. 
     The primary transfer devices  43  of a roller type are pressed against the corresponding image bearing members  40  with the intermediate transfer member  10  interposed therebetween. It is to be noted that the primary transfer devices  43  are not limited to such a roller type and may be brush-type or non-contact-type chargers. 
     The cleaning devices  44  include cleaning members, such as cleaning blades or cleaning brushes, in contact with the image bearing members  40 , and remove residual toner on the image bearing members  40  by using the cleaning members. 
     The discharging devices are for example, lamps to initialize the surface potential of the image bearing members  40  by directing light thereto. 
     Further, as each of the image bearing members  40  rotate, the surface of the corresponding image bearing member  40  is uniformly charged by the corresponding charging device  41 . In the exposure device  21 , writing light L is emitted from a light source, such as a laser or an LED in accordance with image data optically read by the scanner  200 . The writing light L is reflected by a polygon mirror  47 , reflected by mirrors  48 , and directed to the image bearing members  40  to form electrostatic latent images on the image bearing members  40 . 
     The development devices  42  supplies toners to the image bearing members  40  to convert the electrostatic latent images into visible images, and the visible images are transferred onto the intermediate transfer member  10  by the primary transfer device  43 . After the image transfer, the cleaning devices  44  remove the residual toners from the surfaces of the image bearing members  40 , and the discharging devices discharge the intermediate transfer member  10  to get ready for next image formation. 
     Next, the fixing device according to an exemplary embodiment of the present disclosure is described in more detail. The fixing device includes, for example, a rotary heating member (a heating roller  78 ) to rotate while being heated by a heat source (a heat source  84 ), a stationary member (a stationary member  74 ) in sliding contact with a portion of the outer circumferential surface of the rotary heating member, a flexible fixing belt (a fixing belt  77 ) looped around the rotary heating member and the stationary member, and a rotary pressure member (a pressure roller  72 ) in pressure contact with the stationary member with the fixing belt interposed therebetween to form a nip between the fixing belt and the rotary pressure member. The pressure at the nip is created by the pressing force acting between the rotary heating member and the rotary pressure member with the fixing belt and the stationary member interposed therebetween. Further, the term “peripheral direction” used herein refers to a rotation direction of the rotary heating member, and the term “axial direction” used herein refers to a direction parallel to the rotation axis of the rotary heating member. 
       FIG. 2  is a schematic configuration view (a front view) of the fixing device  25  according to this exemplary embodiment. Further,  FIG. 3  is a right-side view of the fixing device illustrated in  FIG. 2 . 
     The fixing device  25  includes, for example, the heating roller  78 , the stationary member  74 , the flexible fixing belt  77 , and the pressure roller  72 . The heating roller  78  is rotatably supported by a frame. The stationary member  74  is disposed in sliding contact with a portion of the outer circumferential surface of the heating roller  78  and held so as to be movable only in the vertical direction in  FIG. 2 . The flexible fixing belt  77  is looped in, e.g., an elliptical shape around the outside of the heating roller  78  and the stationary member  74 . The pressure roller  72  is pressed against the stationary member  74  with the fixing belt  77  interposed therebetween, thereby forming a fixing nip (which refers to a portion at which the fixing belt  77  and the pressure roller  72  contact with each other). 
     The fixing belt  77  is closely contacted with the stationary member  74  by the pressure roller  72 . In addition, the fixing belt  77  induces a force (tension) for restoring its original circular shape and, thus, comes into close contact with the heating roller  78 . 
     A pressing force acts on the pressure roller  72  in a substantially-vertically upward direction in  FIG. 2 , and is transmitted to the fixing belt  77 , the stationary member  74  and the heating roller  78 . Consequently, pressure is applied to the fixing nip, a portion at which the stationary member  74  and the fixing belt  77  contact with each other, and a portion at which the stationary member  74  and the heating roller  78  contact with each other. 
     Near the exit of the fixing nip for the fixing belt  77 , a separation plate  83  for assisting separation of a recording sheet P from the fixing belt  77  is disposed at a position at which a front edge of the recording medium P does not contact the fixing belt  77 . The separation plate  83  has a rotational fulcrum at a portion downstream thereof in the conveyance direction of the recording sheet P and has a positioning portion near one end thereof (outside the width of the recording sheet P in the axial direction). The positioning portion is biased toward the fixing belt  77  by a spring member or other biasing member, thus creating a minute gap between the end of the separation plate  83  and the fixing belt  77 . Accordingly, the recording sheet P is naturally (smoothly) separated from the fixing belt  77  near the exit of the fixing nip and guided by the separation plate  83 , thus preventing the recording sheet P from curling around the fixing belt  77 . 
     The heating roller  78  includes the heat source  84  inside thereof. The heat source may be, for example, a halogen heater, an infrared heater, an induction heater, or a thermal resistance. 
     A thermopile  85 - 1  detects the surface temperature of the fixing belt  77  looped around the heating roller  78  at a position at which the fixing belt  77  passes immediately after separating from the heating roller  78 . Further, the thermopile  85 - 1  is disposed away from the fixing belt  77  within a range of the widths of compatible recording media P in the axial direction, i.e., within a sheet-pass area indicated by a double arrow X in  FIG. 3 . 
     Further, a thermistor  85 - 2  detects the surface temperature of the fixing belt  77  at a portion at which the fixing belt  77  is looped around the heating roller  78 . The thermistor  85 - 2  is disposed in contact with the fixing belt  77  outside the range of the widths of compatible recording media P in the axial direction, i.e., outside the sheet-pass area indicated by the double arrow X in  FIG. 3 . 
     When the fixing belt  77  is stopped, a controller control the turning on/off of the heat source  84  in accordance with the temperature detected by the thermistor  85 - 2  to maintain the heating roller  78  at a predetermined temperature. By contrast, when the fixing belt  77  is rotating, the controller controls the turning on/off of the heat source  84  in accordance with the temperature detected by the thermistor  85 - 1  to maintain the heating roller  78  at a predetermined temperature. 
     The pressure roller  72  also includes a heat source  86  such as a halogen heater inside thereof, and a thermistor  87  is pressed against the pressure roller  72 . A controller controls the turning on/off of the heat source  86  to maintain the pressure roller  72  at a predetermined temperature in accordance with the temperature detected by the thermistor  87 . 
     An entry guide  88  is provided at the entry-side of the fixing device  25  to guide a recording sheet P toward the fixing nip. 
     The thermistor  85 - 2  and the butting portion of the separation plate  83  contacting the surface of the fixing belt  77  are disposed outside the sheet width. Such a configuration prevents wear scars in the sheet width area of the fixing belt  77 , thus preventing image degradation due to the scars transferred onto a toner image on a recording sheet P. 
     For the present exemplary embodiment, the pressure roller  72  includes a metal pipe made of, e.g., steel and a silicon rubber layer with a thickness of 2 mm on the metal pipe, has a diameter of 50 mm, and further includes journal portions  60  with a reduced diameter of 20 mm at its opposite ends. Bearings  61  are provided on the journal portions  60  at the opposite ends and pressed toward the heating roller  78  by a pressing unit including springs  62  and pressing levers  82  which are pivotably provided in a frame  81 . A gear  63  on an end portion of the pressure roller is driven by a driving unit to rotate in the counterclockwise direction in  FIG. 2 . Thus, the pressure roller  72  having the above-described structure is rotated, thereby rotating the fixing belt  77  at the fixing nip. 
     The heating roller  78  includes, for example, an aluminum pipe (of a hollow cylindrical shape) having a high heat conductivity, a thickness of approximately 0.5 mm to approximately 3 mm, and a diameter of approximately 50 mm. The outer surface of the pressure roller  78  is processed by, e.g., alumite processing or fluorocarbon-resin processing to prevent wear due to contact thereof with the fixing belt  77  and slidable contact thereof with the stationary member  74 . The inner surface of the pressure roller  78  is processed by heat-resistant black coating to facilitate absorption of heat from the heat source  84 . 
     The heating roller  78  is rotatably fixed and supported, at its opposite end portions, to and on the frame  81  with bearings  64  interposed therebetween. The heating roller  78  having the above-described structure heats the fixing belt  77  while being rotated by the rotation of the fixing belt  77 . The heating roller  78  has rigidity enough to be hardly bent even when receiving pressure from the pressure roller  72 . 
     The stationary member  74  slidingly contacts the heating roller  78  at its upper surface and the fixing belt  77  at its lower surface. In  FIG. 3 , the length of the stationary member  74  in the horizontal direction is set smaller than the outer diameter of the heating roller  78  and larger than the width of the fixing nip. Further, the size of the stationary member  74  in the vertical direction is set to a size so that the fixing belt  77  is loosely extended by both the heating roller  78  and the stationary member  74 . 
     It is preferable that the stationary member  74  includes a heat-resistant resin that has high heat-resistant characteristics and slidability and is less prone to induce heat migration from the heating roller  78 . For example, polyphenylene sulfide (PPS), polyamide-imide (PAI), polyimide (PI), and the like may be used. 
     Further, an opposing surface of the stationary member  74  opposing the fixing belt  77  may partially include a heat-resistant elastic member  74 - 2 , e.g., a silicon rubber, thus enhancing the close contact between the fixing belt  77  and a recording sheet P and increasing image quality. Further, it is preferable that the opposing surface of the stationary member  74  is coated with fluorocarbon-resin or provided with a fluorocarbon-resin sheet to reduce the sliding resistance. 
     Further, an opposing surface of the stationary member  74  opposing the heating roller  78  slidingly contacts the heating roller  78  at, e.g., two contact portions  74 - 1  in the circumferential direction in  FIG. 2 . It is preferable that the opposing surface of the stationary member  74  opposing the heating roller  78  has a low sliding resistance to the heating roller  78  and is less prone to induce heat migration. For example, the contact portions  74 - 1  may be rotary members, e.g., rollers as illustrated in  FIG. 7 , or a plurality of semispherical protrusions  74 - 5  on the opposing surface of the stationary member  74  opposing the heating roller  78 , as illustrated in  FIG. 8 , to reduce the contact area. Such configurations can reduce the sliding resistance and heat migration between the stationary member  74  and the heating roller  78 . 
     The opposing surface of the stationary member  74  opposing the fixing belt  77  has a surface curved in the same direction as that of the outer surface of the pressure roller  78 , thus forming the fixing nip. For the convex shape of the fixing nip protruding toward the stationary member  74 , a recording sheet P is ejected along a direction closer to the pressure roller  72  than the fixing belt  77 , thus preventing the recording sheet P from curling around the fixing belt  77 . It is to be noted that the shape of the fixing nip is not limited to the convex shape. For example, the surface of the stationary member  74  opposing the fixing belt  77  may be flattened to form a flat shape of fixing nip. Such a flat shape of the fixing nip obtains a good conveyance performance of a recording sheet P, thus suppressing occurrence of sheet feeding failure, such as cockling. 
     Further, a fixing-belt introducing portion (entry)  74 - 3  and/or a fixing-belt exit  74 - 4  at left and right portions in the opposing surface of the stationary member  74  opposing the fixing belt  77  have shapes of smaller R values in the same direction as that of the outer surface of the heating roller. The fixing belt  77  is extended to contact the two R portions. If a fixing nip is formed by a pair of rollers of a diameter of 50 mm, an exit of the fixing nip has a smaller curvature (R=25 mm), thus preventing natural separation of a recording sheet therefrom. By contrast, for the above-described configuration, the exit of the fixing nip has a larger curvature (e.g., R=8 mm), thus facilitating natural separation of recording sheets therefrom. 
     Further, it is preferable that the fixing-belt introducing portion  74 - 3  and the fixing-belt exit  74 - 4  have smaller sliding resistances to the fixing belt  77  which the fixing-belt introducing portion  74 - 3  and the fixing-belt exit  74 - 4  slidingly contact. For example, as illustrated in  FIG. 7 , the contact portions of the fixing-belt introducing portion  74 - 3  and the fixing-belt exit  74 - 4  with the fixing belt  77  may be rotational members, e.g., rollers, thus reducing the sliding resistance. 
     The fixing belt  77  is a belt of a diameter of, for example, 58 mm and including a front layer and a back layer coated by a front release layer and a back release layer. Each of the front layer and the back layer is made of, e.g., highly heat-resistant polyimide resin of a thickness of from 0.05 mm to 0.2 mm. The front release layer may be formed with, for example, a silicon rubber, a fluorocarbon resin, a two-layer structure of a silicon rubber and a fluorocarbon resin, a mixed material of a silicon rubber and a fluorocarbon resin, or other materials, and has elasticity to conform to the concavity and convexity of toners on recording sheets. Further, the back release layer is processed with fluorocarbon resin to reduce the sliding resistance to the stationary member  74 . Further, the back release layer may be preferably coated with a lubrication material such as a fluorine grease. Further, as the material of the fixing belt  77 , for example, metal, such as stainless steel, nickel, and copper or rubber as well as resins may be used. 
     The fixing belt  77  having the above-described structure is heated by being wound around the heating roller  78  having a larger diameter and applies heat and pressure to the recording sheet at the fixing nip to fix a toner image on the recording sheet P. 
     As described above, for the fixing device according to the present exemplary embodiment, not only the stationary member but also the rotary heating member receives the pressure applied to the nip. Such a configuration can form a large width and high surface pressure of fixing nip without the size (and strength) of the stationary member, thus obtaining high-quality images at high speed. Further, since the stationary member has a relatively small size and contacts the rotary heating member, the perimeter of the fixing belt can be shortened, thus allowing downsizing of the fixing device. Further, since the shape of the stationary member can be adjusted to enlarge the curvature of the fixing belt, the above-described configuration can obtain an excellent separation performance, thus allowing natural separation of a recording sheet. Further, since the rotary heating member contacts the fixing belt while rotating, the above-described configuration can obtain an excellent efficiency in heat transmission from the rotary heating member to the fixing belt. 
     (Second Exemplary Embodiment) 
     In the above-described exemplary embodiment, as illustrated in  FIG. 3 , the pressing unit is provided in the pressure-roller side of the fixing device  25 . Alternatively, as illustrated in  FIG. 4 , the pressing unit may be provided in the heating-roller side of the fixing device  25 . In this case, the pressure roller  72  is rotatably fixed at the frame  81  and pressed by the pressing unit including the springs  62  and the pressing levers  82  provided in the heating-roller side of the fixing device  25 . Alternatively, the pressing unit may be provided in the heating-roller side and the pressure-roller side of the fixing device  25 . In this case, the stationary member  74  is fixed to be pressed from both the heating-roller side and the pressure-roller side. 
     (Third Exemplary Embodiment) 
     As described above, the heating roller  78  is rotated by the rotation of the fixing belt  77 . Such a configuration can prevent a speed difference between the heating roller  78  and the fixing belt  77 , thus suppressing wear of the heating roller  78  and the fixing belt  77 . 
     However, since the heating roller  78  is rotated while sliding over the stationary member  74 , if the sliding resistance of the heating roller  78  to the stationary member  74  becomes larger than the force for driving the heating roller  78  transmitted from the fixing belt  77 , the heating roller  78  cannot be rotated. If the heating roller  78  cannot be rotated, the efficiency of heating the fixing belt  77  may decrease and, furthermore, may act as a resistance to the rotation of the fixing belt  77 , thus preventing the rotation of the fixing belt  77 . 
     Hence, as illustrated in  FIG. 5 , it is preferable to provide a rotary unit including gears  65  and  66  through which the pressure roller  72  and the heating roller  78  are linked and an one-way clutch  67  between the heating roller  78  and the gear  65 . In this case, the heating roller  78  is rotated by the rotation of the fixing belt  77 , and the gear  65  rotates at a speed lower by several % (e.g., approximately 1% to 5 approximately %) than that of the heating roller  78 . 
     Accordingly, the one-way clutch  67  is in a free state when the rotation speed of the heating roller  78  is higher than the rotation speed of the heating-roller gear  65 . By contrast, when the rotation speed of the heating roller  78  decreases due to, for example, slip, the one-way clutch  67  is locked at a state where the rotation speed of the heating roller  78  is equal to the rotation speed of the heating-roller gear  65 , thus causing the heating roller  78  to rotate at a rotation speed reduced by several %. As described above, even if slip occurs between the fixing belt  77  and the heating roller  78 , the heating roller  78  is rotated to rotate the fixing belt  77 , thus preventing the rotation of the fixing belt  77  from being stopped due to the slip. 
     (Fourth Exemplary Embodiment) 
     As described above, the fixing belt  77  is rotated by the rotation of the pressure roller  72 . However, since the fixing belt  77  is rotated while sliding over the stationary member  74 , if the sliding resistance of the fixing belt  77  to the stationary member  74  becomes larger than the force for driving the fixing belt transmitted from the pressure roller  72 , the fixing belt  77  cannot be rotated. In particular, when a recording sheet is passing through the fixing nip, the recording sheet P exists between the pressure roller  72  and the fixing belt  77 , thus reducing the driving force for driving the fixing belt  77  transmitted from the pressure roller  72 . Namely, if the fixing belt  77  cannot be rotated, the recording sheet P cannot be conveyed through the nip. 
     Hence, as described above ( FIG. 5 ), it is preferable to provide gear coupling between the pressure roller  72  and the heating roller  78  through gears  65  and  66  and to drive the heating roller  78  using a driving unit. 
     In this case, the speed of the heating roller  78  is set to be substantially equal (±1%) to or higher (by the range of approximately 0% to approximately 10%) than the speed at which the fixing belt  77  is driven by the pressure roller  72 . 
     As described above, the inner surface of the fixing belt  77  is processed with fluorocarbon resin and coated with a lubricating material. Accordingly, the surface of the fixing belt  77  contacting the heating roller  78  tends to slip. As a result, even when the heating roller  78  is rotated at a speed differing from that of the fixing belt  77 , the fixing belt  77  slips over the heating roller  78  in a stable manner. Accordingly, the fixing belt  77  is normally traveled, while the fixing belt  77  can be driven. However, there is a need for paying attentions to wear degradation due to the speed difference between the heating roller  78  and the fixing belt  77 . 
     As described above, when a recording sheet is being conveyed through the fixing nip, the driving force for driving the fixing belt  77  transmitted from the pressure roller  72  may decrease. However, for this configuration, the heating roller  78  creates a driving force for driving the fixing belt  77 , preventing the rotation of the fixing belt  77  from being stopped. Further, since the heating roller  78  is rotated by a driving unit, this configuration can prevent the rotation of the heating roller  78  from stopping due to slip between the fixing belt  77  and the heating roller  78 . 
     The heating roller  78  may be rotated at a speed lower than the speed of the fixing belt  77 . In this case, however, the heating roller  78  may act as a resistance to the rotation of the fixing belt, and as a result, the fixing belt may be loosened at the nip exit, thus resulting in unstable sheet separation performance. Hence, it is preferable to rotate the heating roller  78  at the same speed (±1%) as that of the fixing belt  77  or at a higher speed than that of the fixing belt  77  (by the range of approximately 0% to approximately 10%). 
     (Fifth Exemplary Embodiment) 
     As illustrated in  FIG. 6 , the fixing device  25  may include an induction heating device  89 . The induction heating device  89  is disposed near and away from the outer surface of the fixing belt  77  at a certain distance to heat a heat-generating layer provided in the layers of the fixing belt  77 . The induction heating device  89  may be disposed near the inner surface of the heating roller to heat the heating roller  78  and indirectly heat the fixing belt  77 . 
     By employing the induction heating device  89  near the outer surface of the fixing belt  77 , the heating roller  78  need not necessarily include a heat source inside it. Accordingly, as illustrated in  FIG. 6 , a rib  68  may be provided on the inner surface of the heating roller  78  to increase the strength thereof, thus allowing the heating roller  78  to receive higher surface pressure. 
     The fixing device  25  having the above-described structure is employed in the image forming apparatus  100  illustrated in  FIG. 1 , thus allowing the image forming apparatus to perform the above-described functions. 
     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present disclosure may be practiced otherwise than as specifically described herein. With some embodiments having thus been described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the present disclosure and appended claims, and all such modifications are intended to be included within the scope of the present disclosure and appended claims.