Patent Publication Number: US-8977156-B2

Title: Fixing device and image forming apparatus including same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority pursuant to 35 U.S.C. §119 from Japanese patent application number 2013-021735, filed on Feb. 6, 2013, the entire disclosure of which is incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a fixing device for use in an image forming apparatus such as a printer, a facsimile machine, a copier, and the like, and to an image forming apparatus incorporating such a fixing device. 
     2. Related Art 
     Conventionally, an image forming apparatus is configured such that a latent image formed on an image carrier based on image data is developed by toner supplied from a developing device so that a visible toner image is formed on the image carrier. The toner image on the image carrier is transferred onto a recording medium by a transfer device, and is fixed on the recording medium by the fixing device. 
     JP-H06-138793-A discloses a fixing device in which a rotatably-mounted pressure roller and a flexible endless fixing belt are pressed against each other to form an area of contact herein referred to as a nip or nip portion. The thus-formed fixing device includes a heating member connected to a power source and disposed at an interior side of the fixing belt that forms the nip portion. The heating member contacts the interior surface of the fixing belt and heats the fixing belt electrically, and includes a plurality of heat sources arranged along the width of the sheet perpendicular to a sheet conveyance direction. The heating sources are selectively controlled so that the area heated by each heat source is changed based on the image data and corresponding to an unfixed image on the sheet. As a result, the fixing belt is heated by the heating member so that only that portion of the fixing belt bearing the unfixed toner image is heated. The unfixed image on the sheet that has been conveyed to the nip portion is then fixed onto the sheet with heat and pressure. Compared to a case in which an entire fixing belt is heated up to the fixing temperature, such an arrangement saves energy. 
     However, heat transfer from a rear side to a surface side of the fixing belt heated by the heating member requires time. Thus, there is a delay between the time the heating member is activated and the time at which the fixing belt attains the desired fixing temperature, thereby possibly causing a fixing error due to insufficient heat. Accordingly, based on the image data of the unfixed image existing in the nip portion after one rotation of the fixing belt, the fixing belt is again heated by the heating member by previously changing the area to be heated by each heat source one rotation of the fixing belt before the unfixed image exists in the nip portion. Accordingly, a predetermined amount of heat can be transferred to the surface of the fixing belt by the time the fixing belt rotates one cycle and the heated portion of the fixing belt again arrives at the nip portion, thereby preventing a fixing error from occurring due to insufficient heat. 
     However, because the temperature of the fixing belt decreases due to dissipation of the heat from the surface of the fixing belt during rotation of the belt, a large amount of heat needs to be supplied from the heating member to the fixing belt which in turn necessitates heavy energy consumption. 
     SUMMARY 
     The present invention provides an optimal fixing device capable of reducing the amount of power required for operation. Such an energy-saving fixing device includes a rotatable, endless fixing belt; a rotatable contact member configured to contact a circumferential surface of the fixing belt to form a nip in association with the fixing belt; and a heating member disposed at an interior surface of the fixing belt and configured to heat the fixing belt. The heating member includes a plurality of heat sources arranged along the width of a sheet of recording media in a direction perpendicular to a sheet conveyance direction. An area to be heated by each heat source is changed based on image data and corresponding to an unfixed image on the sheet. The fixing belt is heated by the heating member so that the unfixed image on the sheet that has been conveyed to the nip is fixed onto the sheet at least with heat. The heating member is disposed at a portion other than the fixing nip. 
     These and other objects, features, and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic configuration of an image forming apparatus according to a first embodiment of the present invention; 
         FIG. 2  illustrates a schematic configuration of a fixing device according to the first embodiment of the present invention; 
         FIG. 3A  is an external view of the fixing device and  FIG. 3B  is a schematic view of a heating member; 
         FIGS. 4A and 4B  are plan views each illustrating an image pattern; 
         FIG. 5  is a graph depicting a relation between image and blank areas and first and second target temperatures; 
         FIGS. 6A and 6B  are plan views each illustrating an image pattern; 
         FIG. 7  is a schematic configuration of a fixing device according to a second embodiment; 
         FIG. 8  is a schematic configuration of a fixing device according to a third embodiment; 
         FIG. 9  is a schematic configuration of a fixing device according to a fourth embodiment; 
         FIG. 10  is a schematic configuration of a fixing device according to a fifth embodiment; 
         FIG. 11  is a schematic configuration of a fixing device according to a sixth embodiment; 
         FIG. 12  is a schematic configuration of a fixing device according to a seventh embodiment; 
         FIG. 13  is a schematic configuration of a fixing device according to an eighth embodiment; 
         FIG. 14  is a schematic configuration of a fixing device according to a ninth embodiment; and 
         FIG. 15  is a schematic configuration of a fixing device according to a tenth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, preferred embodiments of the present invention will be described with reference to accompanying drawings. 
       FIG. 1  illustrates a schematic configuration of a printer as an image forming apparatus according to a first embodiment of the present invention. 
     As illustrated in  FIG. 1 , the image forming apparatus includes a sheet feeder  4 , a registration roller pair  6 , a photoreceptor drum  8  as an image carrier, a transfer device  10 , and a fixing device  12 . 
     The sheet feeder  4  includes a paper tray  14  and a sheet feed roller  16 . The paper tray  14  contains multiple sheets S each as a recording medium stacked thereon. The sheet feed roller  16  separates and sends each sheet one by one from the top of the stacked sheets. 
     The sheet S sent out by the sheet feed roller  16  is once stopped by the registration roller pair  6 , which corrects an alignment error of the sheet S. Then, the sheet S is sent to a transfer portion N by the registration roller pair  6  at a timing synchronizing with a rotation of the photoreceptor drum  8 , that is, when a leading end of the toner image formed on the photoreceptor drum  8  is matched with a predetermined position of a leading end of the sheet S in the conveyance direction. 
     Around the photoreceptor drum  8 , a charging roller  18 , a mirror  20 , a part of exposure means, a developing device  22  including a developing roller  22   a , a transfer device  10 , and a cleaning device  24  including a cleaning blade  24   a  are sequentially disposed along the rotation direction of the photoreceptor drum  8 . Exposure light Lb is emitted to an exposure portion  26  on the photoreceptor drum  8  via the mirror  20  at a position between the charging roller  18  and the developing device  22  and scanning is performed. 
     Image formation in the printer is performed in a manner similar to the conventional technology. Specifically, when the photoreceptor drum  8  starts to rotate, the surface of the photoreceptor drum  8  is charged uniformly by the charging roller  18  and irradiated and scanned by the exposure light Lb based on the image data is, so that a latent image corresponding to the image to be formed is created on the photoreceptor drum  8 . The rotation of the photoreceptor drum  8  moves the latent image to a position opposite the developing device  22 , where toner is supplied to the latent image from the developing device  22 , so that the latent image is rendered visible as a toner image. The toner image formed on the photoreceptor drum  8  is transferred by the transfer device  10  onto the sheet S that has entered into a transfer portion N at a predetermined timing. The sheet S, on which the toner image has been transferred, is conveyed to the fixing device  12  and the toner image on the sheet S is fixed onto the sheet S by the fixing device  12 . The sheet S is then discharged onto a paper discharge tray, not shown, and is stacked thereon. 
     Residual toner remaining on the photoreceptor drum  8  without being transferred from the photoreceptor drum  8  to the sheet S in the transfer portion N is conveyed along with the rotation of the photoreceptor drum  8  to the cleaning device  24 , and is scraped off from the photoreceptor drum  8  by the cleaning blade  24   a , so that the surface of the photoreceptor drum is cleaned. Thereafter, the residual electric charge on the photoreceptor drum  8  is removed by a discharger, not shown, and the photoreceptor drum  8  is prepared for a next image formation process. 
       FIG. 2  is a schematic configuration of a fixing device  12  according to a first embodiment of the present invention.  FIG. 3A  is an external view of the fixing device  12 ; and  FIG. 3B  is a schematic view of a heating member  56 . 
     As illustrated in  FIG. 2 , the fixing device  12  includes a fixing belt  38 , a pressure roller  30 , and a heating member  56 . The heating member  56  is a sheet-shaped heat generator such as a thermal heater. The pressure roller  30  contacts an external surface of the rotatably disposed fixing belt  38  so that a fixing nip portion SN is formed between the fixing belt  38  and the pressure roller  30 . In the present embodiment, the pressure roller  30  is pressed against the fixing belt  38  via a biasing member, not shown. 
     In addition, an elastic roller  40  to which a biasing force is applied from a biasing member, not shown, and which presses against the fixing belt  38 , is disposed at a position opposite the heating member  56  via the fixing belt  38  and away from the pressure roller  30 . With this configuration, even while the fixing belt  38  is rotating, good contact between the heating member  56 , having a substantially flat contact portion with the fixing belt  36 , and the fixing belt  38  can be maintained. 
     The heating member  56  is mounted on a stay-like member  57  and is so disposed as to contact an interior surface of the fixing belt  38 . Because the heating member  56  is disposed contacting the interior surface of the fixing belt  38 , the heating member  56  does not damage an outer circumferential surface of the fixing belt  38  contacting the toner image on the sheet S, thereby lengthening a lifetime of the fixing belt  38 . 
     The heating member  56  includes a plurality of heaters  55   a ,  55   b ,  55   c ,  55   d ,  55   e ,  55   f , and  55   g  disposed to cover an entire image forming area over the width of the sheet in a direction perpendicular to the sheet conveyance direction of the sheet S. As illustrated in  FIG. 3A , the heating member  56  includes seven heaters arranged along the width of the sheet, each of which can heat the fixing belt  38  independently. 
     According to the present embodiment, there are seven heaters  55  of the heating member  56 . However, the number of heaters is not limited thereto, and may, for example, be fewer or more. 
     A thermistor  34 , a temperature sensor to detect a surface temperature of the fixing belt  38  is disposed downstream in the rotation direction of the fixing belt than the fixing nip portion SN and upstream in the fixing belt rotation direction than the heating member  56 . In addition, another thermistor  36  as a temperature sensor to detect a temperature of the heating member  56  is disposed on an opposite side surface at which the heating member  56  contacts the interior surface of the fixing belt  38 . 
     The heating member  56  connects to a power supply  39  which supplies electric power to the heating member  56 . When the electric power is supplied to the heater  55  of the heating member  56  from the power supply  39 , the heater  55  of the heating member  56  generates heat. In addition, based on the temperature detected by the thermistors  34  and  36 , a controller  37  controls the power supply  39  to cause the power supply  39  to supply electricity to the heater  55  of the heating member  56 . The controller  37  allows the power supply  39  to independently supply power to each of the divided heaters  55  of the heating member  56 . Specifically, the controller  37  is configured as a microcomputer including a CPU, a ROM, a RAM, an I/O interface, and the like. 
     The fixing belt  38  includes a base member  38   a  formed of a stainless steel having an external diameter of 40 mm and a thickness of 40 μm, and an elastic layer  38   b  coated on a surface of the base member  38   a . The elastic layer  38   b  is formed of a silicon rubber and has a thickness of 100 μm. Further, a release layer  38   c  formed of fluorine resins such as tetrafluoroethylene-perfluoroalkyl vinylether copolymer (PFA) or polytetrafluoroethylene (PTFE) having a thickness of from 5 μm to 50 μm is formed on an external surface of the elastic layer  38   b  to improve durability and releaseability of the fixing belt  38 . The base member  38   a  of the fixing belt  38  may employ polyimide as a material. When polyimide is employed for the base member  38   a , thermal capacity of the fixing belt  38  can be reduced and a highly responsive base member  38   a  is formed even with the same thickness compared to the case in which the metal material is used. Because of flexibility greater than that of the metal belt, a pressing load is reduced and thus the torque can be reduced. 
     Other than the heating member  56 , the fixing device  12  further includes, in an interior of the fixing belt  38 , a belt support member  61  and a nip forming member  60 . The belt support member  61  supports the fixing belt  38  and the nip forming member  60  forms the fixing nip portion SN in association with the pressure roller  30  with the fixing belt  38  in between. These members are connected to a side plate of the apparatus, not shown, and are supported thereby. 
     The belt support member  61  is inserted at both lateral ends of the fixing belt  38  in the axial direction perpendicular to the rotation direction of the fixing belt  38 . Each end of the fixing belt  38  is rotatably supported by the belt support member  61 . 
     The pressure roller  30  includes a metal core  30   a  formed of iron having an external diameter of 40 mm and a thickness of 2 mm, and an elastic layer  30   b  coated on a surface of the metal core  30   a . The elastic layer  30   b  is formed of a silicon rubber and has a thickness of 5 mm. Further, it is preferred that a fluorine resin layer having a thickness of approximately 40 μm be provided on a surface of the elastic layer  30   b  to increase releaseability. 
     In addition, in the present embodiment, a contact surface between the heating member  56  and the fixing belt  38  is configured to be substantially flat. Conceivably, the heating member  56  could be formed in a semicircular column shape to conform to the shape of an interior surface of the fixing belt  38  so that the heating member  56  can be optimally contacted against the interior surface of the fixing belt  38 ; however, assembling/mounting heater members and circuitry to a curved surface requires complicated processes and the semicircular column shape is not adequate for the higher precision and production performance required compared to assembling heaters and circuitry on a flat surface. As a result, in the present embodiment, a flat-type heat generator that excels in the precision and productivity is used as the heating member  56 , so that heat generation efficiency is improved due to the high precision mounting. 
     On the other hand, when the fixing belt  38  is employed as a fixing member for the purposes of low thermal capacity and compact size, securing a flat portion of the fixing belt  38  is difficult to achieve simply by disposing the flat-shaped heating member  56  alone. Accordingly, the interior surface of the rotating fixing belt  38  and the heating member  56  need to be contacted sufficiently. 
     If the heating member  56  is disposed at a portion of the fixing belt forming the fixing nip portion SN along with the pressure roller  30 , even when the fixing belt  38  is rotating, contact between the fixing belt  38  and the heating member  56  can be maintained. However, heat transfer from a rear side to a surface side of the fixing belt  38  heated by the heating member  56  requires time. Accordingly, even though the fixing belt  38  is heated by the heating member  56  at the portion of the fixing belt forming the fixing nip portion SN, the heat is not transferred to the toner on the sheet S at the fixing nip portion SN immediately after the heating. 
     The heat of the fixing belt  38  heated by the heating member  56  is transferred to the surface of the fixing belt while the fixing belt  38  is rotating. However, until the fixing belt  38  rotates one cycle and the heated portion of the fixing belt  38  again arrives at the fixing nip portion SN, the heat transferred to the surface of the fixing belt continues to dissipate from the surface thereof. Considering the fact that the temperature of the fixing belt  38  decreases due to the dissipation of the heat, a large amount of heat needs to be supplied from the heating member  56  to the fixing belt  38 , which results in heavy energy consumption. 
     In the fixing device  12  according to the present embodiment as illustrated in  FIG. 2 , the heating member  56  is disposed at a portion other than the fixing nip portion SN of the fixing belt  38 . As a result, compared to a case in which the heating member  56  is disposed at the fixing nip portion SN, the discharged heat amount from the surface of the fixing belt from when the portion of the fixing belt  38  heated by the heating member  56  rotates one cycle and until the heated portion of the fixing belt  38  again arrives at the fixing nip portion SN, can be reduced. As a result, the heat amount to be supplied to the fixing member  38  from the heating member  56  so that the fixing temperature at the fixing nip portion SN can be secured can be reduced, thereby reducing the amount of electricity to be supplied to the heating member  56  from the power supply  39  and saving energy. 
     The elastic roller  40  has an external diameter φ of from 15 to 30 mm and is comprised of a metal core  40   a  formed of iron having an external diameter φ of 8 mm and an elastic layer  40   b  coated on a surface of the metal core  40   a . The elastic layer  40   b  is formed of a silicon rubber having a thickness ranging from 3.5 to 11 mm. It is preferred that a fluorine resin layer having a thickness of approximately 40 μm be provided on a surface of the elastic layer  40   b  to increase releaseability. 
     In the fixing device  12  according to the present embodiment, the controller  37  controls a supply of electricity from the power supply  39  to each heater  55  of the heating member  56  based on the image data to form an image on the sheet S to achieve energy saving. Hereinafter, an example of controlling operation will be described. 
       FIG. 4A  shows image formation patterns on the sheet S sequentially from a leading end of the sheet in the sheet conveyance direction, including an image area a, a blank area b, and an image area a′. Fixing of images onto the sheet is required in the image areas a and a′; however, because there is no image in the blank area b and the toner as a target for fixation does not exist, fixing operation is not required. 
     Image data of the above image formation patterns is input into the controller  37  from an image processor, not shown. Then, the controller  37  controls the heating member  56  such that the temperature of the portion corresponding to the blank area b of the fixing belt  38  becomes lower than that of the portions corresponding to the image area a and the image area a′ of the fixing belt  38 . Specifically, the controller  37  controls the power supply  39  such that the heating member  56  is given enough power to obtain the fixing temperature at the portions corresponding to the image area a and the image area a′ and is given less power that does not obtain than the fixing temperature at the blank area b. “Portions corresponding to the image area or the blank area” means the positions at which the fixing belt  38  contacts. 
       FIG. 4B  shows image formation patterns on the sheet S sequentially from a leading end of the sheet in the sheet conveyance direction, including an image area a and a blank area b. Similarly to the case illustrated in  FIG. 4A , the controller  37  controls the heating member  56  such that the temperature of the portion corresponding to the blank area b of the fixing belt  38  becomes lower than that of the portions corresponding to the image area a of the fixing belt  38 . Specifically, the controller  37  controls the power supply  39  such that the heating member  56  is given enough power to obtain the fixing temperature at the portions corresponding to the image area a and is given less power at the blank area b. 
     Conceivably, the controller  38  could control the power supply to the heating member  56  to shut off completely at the portion corresponding to the blank area b of the fixing belt  38 . However, lowering the temperature of the fixing belt  38  excessively simply delays a rise of the temperature up to the fixing temperature for a next image area. Accordingly, as illustrated in  FIG. 5 , it is preferred that the temperature of the fixing belt  38  be kept at a second temperature which is lower than the fixing temperature and higher than room temperature. 
     With such a configuration, the heating member  56  is supplied with electricity and heated at the portion corresponding to the blank area b of the fixing belt  38 , but the power consumption is reduced compared to a case in which the temperature of the portion corresponding to the blank area b of the fixing belt  38  is set to the fixing temperature. Specifically, because the supplied electricity in areas P′ is lower than that in areas P, energy saving is achieved. 
       FIG. 6A  shows an image formation pattern on the sheet S mixing an image area ‘c’ and a blank area  FIG. 6B  shows an image formation pattern on the sheet S, in which sequentially from a leading end of the sheet in the sheet conveyance direction, the image area a, and a mixed area ‘h’ including an image area ‘c’ and a blank area ‘d’ in the width of the sheet exist. 
     Similarly to the case described above, the controller  37  controls the heating member  56  such that the temperature of the portion corresponding to the blank area ‘d’ of the fixing belt  38  becomes lower than that of the portions corresponding to the image area a and the image area of the fixing belt  38 . Specifically, the controller  37  controls the power supply  39  such that the heating member  56  is given enough power to obtain the fixing temperature at the portions corresponding to the image area a and the image area ‘c’ and is given less power at the blank area ‘d’. 
     Specifically, when the heating member  56  heats the portion corresponding to the image area a of the fixing belt  38 , all heaters  55   a ,  55   b ,  55   c ,  55   d ,  55   e ,  55   f , and  55   g  included in the heating member  56  are supplied electricity (see  FIG. 3A ). On the other hand, when the heating member  56  heats the portion corresponding to the blank area c of the fixing belt  38 , all heaters  55   a ,  55   b ,  55   c , and  55   d  included in a heating area e are supplied electricity (see  FIG. 3A ). Specifically, the heaters  55   e ,  55   f , and  55   g  included in a heating area f are not supplied with electricity. 
     In addition, in the present embodiment, when electricity is supplied to the heating member  56  from the power supply  39  such that a preliminary heating area g, which is a portion in the sheet conveyance direction before the image area enters into the fixing nip portion SN, is preliminarily heated, as illustrated by shaded areas in  FIGS. 4 and 6 . The preliminary heating area g is an area that becomes necessary due to a length required for heat generation in the circumferential direction of the heating member  56  or because the heating member  56  itself needs a time for temperature rise. The preliminary heating area g is preferably as small as possible from the viewpoint of energy saving. 
       FIG. 7  illustrates a second embodiment of the present invention. As illustrated in  FIG. 7 , the fixing device  12  includes a pressure pad  41  formed of an elastic material and disposed opposite the heating member  56  via the fixing belt  38 . 
     The pressure pad  41  is supported by a stay  41   a , to which a load is given from a biasing member, not shown, and the pressure pad  41  is pressed against the fixing belt  38  and the heating member  56 . With this configuration, even while the fixing belt  38  is rotating, good contact between the fixing belt  38  and the heating member  56  can be maintained. In addition, by using a pad member as a pressurizing member, a contact area becomes wider than the case of using the elastic roller  40  as described in the first embodiment and the contact between the heating member  56  and the fixing belt  38  can be stably secured. 
     On the surface of the pressure pad  41 , a felt or sheet material formed of silicon rubber, or heat-resistance fiber such as aramid fiber is employed. If the fiber material is used for the surface of the pressure pad  41 , because the fiber material soaks up silicon oils, it reduces friction resistance relative to the surface of the fixing belt  38  and improves releaseability. 
     On the other hand, a fluorine resin layer having a thickness of from 50 to 100 μm can be formed on the surface of the pressure pad  41 , thereby reducing friction resistance relative to the surface of the fixing belt  38  and improving releaseability. 
       FIG. 8  illustrates a third embodiment of the present invention. As illustrated in  FIG. 8 , the fixing device  12  includes a pressure brush  42  disposed opposite the heating member  56  via the fixing belt  38 . 
     The pressure brush  42  is supported by a stay  42   a , to which a load is given from a biasing member, not shown, and the pressure brush  42  is pressed against the fixing belt  38  and the heating member  56 . With this configuration, even while the fixing belt  38  is rotating, good contact between the fixing belt  38  and the heating member  56  can be maintained. In addition, using a brush member as a pressurizing member allows a contact area to be wider than using the elastic roller  40  and the contact between the heating member  56  and the fixing belt  38  can be stably maintained. 
     A pile member in which fibers having heat-resistant property such as polyimide are mixed in a base formed of aramid fibers, may preferably be used for the pressure brush  42 . In addition, an entire pile length is from 1 to 8 mm and the pressure is applied such that the pile bites the surface of the fixing belt  38  in the depth of 1 to 2 mm. 
     In addition, when the pressure member is the pressure brush  42 , a contact resistance between the fixing belt  38  and the pressure brush  42  is small and the torque load of the fixing belt  38  can be reduced. 
       FIG. 9  illustrates a fourth embodiment of the present invention. As illustrated in  FIG. 9 , the fixing device  12  includes a pressure brush roller  43  disposed opposite the heating member  56  via the fixing belt  38  and rotatable about a rotary axis thereof. 
     The pressure brush roller  43  having an external diameter φ of from 15 to 30 mm includes a metal core  43   a  made of iron having an external diameter φ of 8 mm and a brush portion  43   b  formed of fibers with heat resistant property such as polyimide. A pile member in which heat resistant fibers are mixed in a base formed of aramid fibers may preferably be used for the brush portion  43   b . In addition, an entire pile length is from 1 to 8 mm and the pressure is applied such that the pile bites the surface of the fixing belt  38  in the depth of 1 to 2 mm. 
     Because the pressure brush roller  43  is pressed against the fixing belt  38 , the pressure brush roller  43  can be driven by the rotation of the fixing belt  38  without disposing a driving means to rotatably drive the pressure brush roller  43 , or may be configured to be driven by any driving means. A structure in which the pressure brush roller  43  is driven by a rotation of the fixing belt  38  without the driving means is more cost effective and can save space. However, driving the pressure brush roller  43  to rotate by the driving means can provide more stable contact between the pressure brush roller  43  and the fixing belt  38  than when being driven solely by the rotation of the fixing belt  38 . 
     When a surface speed of the pressure brush roller  43  is equal to the surface speed of the fixing belt  38 , friction between the two is reduced and a longer lifetime is possible. Conversely, when the surface speeds of the both are different, including both cases in which the surface speed of the pressure brush roller  43  relative to that of the fixing belt  38  is positive or negative, the greater difference the more stable the contact becomes. In addition, a cleaning effect of the surface of the fixing belt due to the pressure brush roller  43  is improved. 
     In the present embodiment, the pressure brush roller  43  and the fixing belt  38  rotate in a same direction in the portion where the pressure brush roller  43  and the fixing belt  38  are disposed oppositely; however, the rotation direction of the pressure brush roller  43  can be set opposite that of the fixing belt  38 . 
     In addition, although not illustrated in the present fourth embodiment, the pressure brush roller  43  may be provided with a cleaning device and can be cleaned by the cleaning device. 
       FIG. 10  illustrates a fifth embodiment of the present invention. As illustrated in  FIG. 10 , the fixing device  12  includes the nip forming member  60  disposed on an interior surface of the fixing belt  38  opposite the fixing nip portion SN so as to form the fixing nip portion SN. Then, the heating member  56  and the elastic roller  40  opposite the heating member  56  are disposed on an extended line connecting substantially the center of the fixing nip portion SN and substantially the center line of the pressure roller  30 . 
     In addition, the elastic roller  40  which is applied a biasing force from a biasing member, not shown, and presses against the fixing belt  38  is disposed at a position opposite the heating member  56  via the fixing belt  38 . With this configuration, even while the fixing belt  38  is rotating, good contact between the fixing belt  38  and the heating member  56  can be maintained. 
     Herein, the pressure member to press against the fixing belt  38  at a position opposite the heating member  56  via the fixing belt  38  is not limited to the elastic roller  40 . For example, the pressure pad  41  or the pressure brush  42 , and the pressure brush roller  43  may be used as the pressure member as long as the structure can maintain good contact between the fixing belt  38  and the heating member  56 . 
     In an interior of the fixing belt  38 , the belt support member  61  and the nip forming member  60  both supporting the fixing belt  38  are disposed. They are connected to and supported by a side plate of the apparatus, not shown. 
     In the present embodiment as illustrated in  FIG. 10 , a load of the elastic roller  40  pressing against the heating member  56  is applied downwardly and a load of the pressure roller  30  pressing against the nip forming member  60  is applied upwardly. As a result, both loads are canceled and the load of the side plate of the apparatus supporting the belt support member  61  and the nip forming member  60  is reduced. Thus, the side plate is not bent so much, thereby suppressing biasing of the belt due to the bending of the side plate. Further, the side plate may be simply constructed. 
       FIG. 11  illustrates a sixth embodiment of the present invention. As illustrated in  FIG. 11 , the nip forming member  60  to form the fixing nip portion SN is disposed on an interior surface of the fixing belt  38  corresponding to the fixing nip portion SN. Then, the heating member  56  and the elastic roller  40  opposite the heating member  56  are disposed on an extended line connecting substantially the center of the fixing nip portion SN and substantially the center line of the pressure roller  30 . 
     The pressure member to press against the fixing belt  38  at a position opposite the heating member  56  via the fixing belt  38  is not limited to the elastic roller  40 . For example, the pressure pad  41  or the pressure brush  42 , and the pressure brush roller  43  may be used as the pressure member as long as the structure can maintain good contact between the fixing belt  38  and the heating member  56 . 
     On an interior side of the fixing belt  38 , the belt support member  61  and the nip forming member  60  both supporting the fixing belt  38  are disposed. They are connected to and supported by the side plate of the apparatus, not shown. 
     The load of the elastic roller  40  pressing against the heating member  56  and the load of the pressure roller  30  pressing against the nip forming member  60  are applied to the side plate of the apparatus supporting the belt support member  61  and the nip forming member  60 . The load is finally applied to a stay  62  supporting the heating member  56  or the nip forming member  60 . 
     If the stay  62  is not constructed rigidly enough to support the load and is easily bent by each load of the elastic roller  40  and the pressure roller  30 , good contact between the fixing belt  38  and the heating member  56  or an even nip width at the fixing nip portion SN cannot be obtained. As a result, enough rigidity is required for the stay  62  so as not to be bent due to each load. 
     However, even though a rigidity of the stay  62  is secured by enlarging the size of the stay  62  or by using multiple stays, the entire size of the apparatus may also be enlarged as well. As a result, because it is necessary to both downsize the fixing belt  38  and strengthen the stay  62 , the nip forming member  60  and the heating member  56  are supported by the stay  62  with enough rigidity. Accordingly, the nip forming member  60 , the heating member  56 , and the stay  62  can be disposed on the interior surface of the fixing belt  38  with a smaller diameter. 
     In general, the load applied to the nip forming member  60  is greater than that applied to the heating member  56  because the upward load applied to the nip forming member  60  is greater, and therefore, the rigidity of the stay  62  should be strong enough to endure the load applied to the nip forming member  60  and not to cause bending of the stay  62  itself. 
     Next, seventh to tenth embodiments according to the present invention will be described. A redundant description concerning the same structure as in the first to sixth embodiments will be omitted. 
     The seventh embodiment referring to  FIG. 12  will be described. As illustrated in  FIG. 12 , the fixing device  12  includes an elastic roller  140  configured to press against the fixing belt  38  by being applied with a biasing force by a biasing member, not shown. The elastic roller  140  is disposed opposite the heating member  56  via the fixing belt  38 , so as to press-contact the fixing belt  38 . With this configuration, even while the fixing belt  38  is rotating, good contact between the fixing belt  38  and the heating member  56  can be maintained. 
     The elastic roller  140  functions also as an oil applicator to coat the oil on the surface of the fixing belt  38  so that adhesion of the toner from the sheet S to the fixing belt  38  is prevented. In addition, the elastic roller  140  includes a metal core  140   a , sponge-like foam  140   b , and a semi-transparent film  140   c . The foam  140   b  is disposed on a circumference of the metal core  140   a  and includes silicon oils in the sponge-like body. The semi-transparent film  140  including minute holes is wound around the circumference of the foam  140   b  once or twice. The silicon oil included in the foam  140   b  is leaked through the semi-transparent film  140 , so that a slight amount of oil is coated on a surface of the fixing belt  38 . 
     If occasionally a sheet jamming occurs in the conveyance path of the sheet S inside the image forming apparatus, the toner adhered from the sheet S to the fixing belt  38  may be adhered from the fixing belt  38  to the surface of the elastic roller  140 . To prevent this, the semi-transparent film  140  to form the surface layer of the elastic roller  140  employs materials with good releaseability capable of preventing the toner adhered on the surface of the elastic roller  140  from setting. This is because if the toner is set on the surface of the elastic roller  140 , the minute holes on the semi-transparent film  140  through which oils can be leaked are clogged by the toner, so that coating the oil on the elastic roller  140  becomes impossible. Preferable materials for the semi-transparent film  140  include Gore-Tex (a registered trademark of W. L. Gore and Associates) films because of its releasable property. 
     In the present seventh embodiment, a conductive layer is coated on the elastic layer  30   b  of the pressure roller  30 . A discharging brush  44  is disposed near the pressure roller  30  so as to discharge an electrical charge from the pressure roller  30 . As a result, the charged charge of the pressure roller  30  is reduced and electrical charge difference between the pressure roller  30  and the fixing belt  38  is reduced, so that an electrostatic offset in which the toner is adhered from the sheet S to the fixing belt  38  by an electrostatic force, can be reduced. 
     Preferred materials for the conductive layer coated on the elastic layer  30   b  of the pressure roller  30  include for example PFA mixed with carbon. When the conductive layer is formed on the elastic layer  30   b  of the pressure roller  30 , a circumferential surface of the pressure roller  30  becomes conductive. If the surface resistivity of the pressure roller  30  is in a range from 1*10 8 Ω/m 2  to 1*10 6 Ω/m 2 , it is conceived that the pressure roller  30  has conductivity. 
     The discharging brush  44  includes an electrode  44   a  and a holder  44   b . The electrode  44   a  is fixed to the holder  44   b  formed of a conductive material. Preferred materials for the electrode  44   a  include, for example, (1) multiple fiber-like stainless steel bound together; (2) acrylic fiber dispersed with carbon particles in a streak or dyed with copper ions; (3) carbon fiber alone; and (4) conductive unwoven cloth. 
     The discharging brush  44  is disposed on the left in  FIG. 12  so that the electrode  44   a  can good contact or be away at a certain distance relative to the circumferential surface of the pressure roller  30 . The discharging brush  44  can be disposed in the bottom of the pressure roller  30 . 
     As described above, when the pressure roller  30  is electrically discharged by the discharging brush  44 , the electrical charge difference between the pressure roller  30  and the fixing belt  38  is reduced and the amount of toner electrostatically attracted from the sheet S to the fixing belt  38  is reduced, so that the electrostatic offset is reduced. Accordingly, the amount of offset toner to be adhered from the fixing belt  38  to the elastic roller  140  can be reduced. As a result, durability of the elastic roller  140  is improved, and because the offset toner is not developed to agglomerated particles, a quality image is formed on the sheet S. 
     An eighth embodiment will be described referring to  FIG. 13 . As illustrated in  FIG. 13 , the fixing device  12  includes the elastic roller  140  configured to press against the fixing belt  38  by being applied with a biasing force by a biasing member, not shown. The elastic roller  140  is disposed opposite the heating member  56  via the fixing belt  38 , so as to press-contact the fixing belt  38 . With this configuration, even while the fixing belt  38  is rotating, good contact between the fixing belt  38  and the heating member  56  can be maintained. This elastic roller  140  functions as an oil applicator to coat the oil on the surface of the fixing belt  38  even in the present eighth embodiment. 
     In the present eighth embodiment, a cleaning roller  141  formed of a metallic roller to clean the surface of the elastic roller  140  is pressed by a biasing member, not shown, toward the elastic roller  140 . 
     The surface of the cleaning roller  141  may be formed of the metallic material alone as is, but may be coated with fluorine rubber having less releaseability than that of the surface layer (i.e., the semi-transparent film  140 ) of the elastic roller  140  on its metal surface thereof with a thickness of from 10 to 100 μm. In addition, resins that can be bound with the toner can be coated on the surface of the metallic roller. 
     With this structure, smears due to toner or paper dust adhered to the elastic roller  140  can be removed by the cleaning roller  141  so that the surface of the elastic roller  140  is cleaned, thereby maintaining the surface of the elastic roller  140  without contamination. 
     A ninth embodiment will be described referring to  FIG. 14 . As illustrated in  FIG. 14 , the fixing device  12  includes the elastic roller  140  configured to press against the fixing belt  38  by being applied with a biasing force by a biasing member, not shown. The elastic roller  140  is disposed opposite the heating member  56  via the fixing belt  38 , so as to press-contact the fixing belt  38 . With this configuration, even while the fixing belt  38  is rotating, good contact between the fixing belt  38  and the heating member  56  can be maintained. The elastic roller  140  functions as an oil applicator to coat the oil on the surface of the fixing belt  38  even in the present structure. 
     In the present ninth embodiment, as illustrated in  FIG. 14 , a cleaning roller  142  configured to clean the surface of the fixing belt  38  is disposed in contact with the fixing belt  38  at a side opposite the elastic roller  140  via the fixing belt  38 . The cleaning roller  142  is formed of foamed silicon rubber and preferably includes an uppermost layer formed of a material having less releaseability than that of the surface layer (i.e., the semi-transparent film  140   c ) of the fixing belt  38 , for example, conductive PFA. 
     In the ninth embodiment as illustrated in  FIG. 14 , the cleaning roller  142  is disposed on the left in the figure; however, the cleaning roller  142  may be disposed above the fixing belt  38 . In addition, the cleaning roller  142  being biased by a biasing member, not shown, contacts the fixing belt  38  with pressure. 
     When the toner image on the sheet S is melted while being crushed on the fixing belt  38 , the toner adhered/offset from the sheet S to the fixing belt  38  is attracted to the cleaning roller  142  and is removed from the fixing belt  38 . In addition, the adhered/offset toner to the fixing belt  38  is conveyed to the elastic roller  140 , and, before adhering to the elastic roller  140 , the toner is attracted by the cleaning roller  142  and is removed from the fixing belt  38 . Thus, the amount of the offset toner to be adhered to the elastic roller  140  can be reduced. As a result, durability of the elastic roller  140  is improved, and because the offset toner is not developed to agglomerated particles, a quality image is formed on the sheet S. 
     A tenth embodiment according to the present invention will be described referring to  FIG. 15 . As illustrated in  FIG. 15 , the fixing device  12  includes a cleaning web unit  45  configured to clean the surface of the fixing belt  38  and coat a release agent thereon. 
     The cleaning web unit  45  includes a cleaning web  45   d , a supply roll  45   c , a wind-up roll  45   b , and a web pressing roll  45   a . The supply roll  45   c  supplies the cleaning web  45   d  and the wind-up roll  45   b  winds up the cleaning web  45   d . The supply roll  45   a  presses the cleaning web  45   d  against the surface of the fixing belt  38 . Then, the cleaning web  45   d  moves little by little responsive to the fixing operation while being pressed against the surface of the fixing belt  38  by the web pressing roll  45   a.    
     In the present configuration, the web pressing roll  45   a  is positioned opposite the heating member  56  via the fixing belt  38  and the cleaning web  45   d . Because the web pressing roll  45   a  is biased by a biasing member, not shown, and serves as a pressing member to press against the fixing belt  38 . With this configuration, even while the fixing belt  38  is rotating, good contact between the fixing belt  38  and the heating member  56  can be maintained. 
     Preferred materials for the cleaning web  45   d  include in general aromatic polyamide resins and polyester unwoven fibers. The cleaning web  45   d  includes a release agent such as silicon oil, which is soaked in the cleaning web  45   d . With this structure, the cleaning web  45   d  applies the release agent to the surface of the fixing belt  38  so that the releaseability of the surface of the fixing belt  38  may be improved, thereby preventing the toner from adhering on the surface of the fixing belt  38  from the sheet S. In addition, when the cleaning web  45   d  slidably contacts the fixing belt  38 , the adhered/offset toner on the fixing belt  38  is collected to the cleaning web  45   d , thereby cleaning the surface of the fixing belt  38 . 
     In addition, in the present tenth embodiment, a conductive layer is coated on the elastic layer  30   b  of the pressure roller  30 . The discharging brush  44  is disposed near the pressure roller  30  so as to discharge an electrical charge from the pressure roller  30 . As a result, the charged charge of the pressure roller  30  is reduced and the electrical charge difference between the pressure roller  30  and the fixing belt  38  is reduced, so that an electrostatic offset in which the toner is adhered from the sheet S to the fixing belt  38  by an electrostatic force, can be reduced. 
     Preferred materials for the conductive layer coated on the elastic layer  30   b  of the pressure roller  30  include for example tetrafluoroethylene-perfluoroalkyl vinylether copolymer (PFA) mixed with carbon. By coating the PFA mixed with carbon on the elastic layer  30   b  of the pressure roller  30 , the circumferential surface of the pressure roller  30  exerts conductivity. If the surface resistivity of the pressure roller  30  is in a range from 1*10 8 Ω/m 2  to 1*10 6 Ω/m 2 , it is assumed that the pressure roller  30  has conductivity. 
     The discharging brush  44  includes an electrode  44   a  and a holder  44   b . The electrode  44   a  is fixed to the holder  44   b  formed of a conductive material. Preferred materials for the electrode  44   a  include, for example, (1) multiple fiber-like stainless steel bound together; (2) acrylic fiber dispersed with carbon particles in a streak or dyed with copper ions; (3) carbon fiber alone; and (4) conductive unwoven cloth. 
     The discharging brush  44  is disposed on the left in  FIG. 15  so that the electrode  44   a  can contact or be away at a certain distance relative to the circumferential surface of the pressure roller  30 . Alternatively, the discharging brush  44  can be disposed in the bottom of the pressure roller  30 . 
     As described above, when the pressure roller  30  is electrically discharged by the discharging brush  44 , the electrical charge difference between the pressure roller  30  and the fixing belt  38  is reduced and the amount of toner electrostatically attracted from the sheet S to the fixing belt  38  is reduced, so that the electrostatic offset is reduced. Accordingly, the amount of offset toner to be adhered from the fixing belt  38  to the cleaning web  45   d  can be reduced. As a result, durability of the cleaning web  45   d  is improved, and because the offset toner is not developed to agglomerated particles, a quality image is formed on the sheet S. 
     Additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.