Patent Publication Number: US-8116670-B2

Title: Fixing device and image forming apparatus using the same

Description:
PRIORITY STATEMENT 
     The present patent application claims priority under 35 U.S.C. §119 upon Japanese Patent Applications No. JP2006-307954 filed on Nov. 14, 2006 and No. JP 2007-125208 filed on May 10, 2007 in the Japan Patent Office, the entire contents of each of which are hereby incorporated herein by reference. 
     BACKGROUND 
     An image forming apparatus used as a printer, facsimile machine, copier, or multi-functional device thereof may use a fixing member to fix a toner image on a recording medium. Such a fixing member may be a fixing belt, film, or roller, for example. 
     For example, a conventional image forming apparatus may include a fixing belt having an endless shape, a plurality of roller members for extending and supporting the fixing belt, a heater provided inside one of the plurality of roller members, and a pressure roller serving as a pressure member. When the fixing belt is heated by the heater via the one roller member, a recording medium having a toner image thereon is conveyed to a nip formed between the fixing belt and the pressure roller. When heat and pressure are applied to the recording medium at the nip, the toner image is fixed on the recording medium. 
     Another conventional image forming apparatus uses an on-demand fixing device operable with a relatively short warm-up time. Such an on-demand fixing device may include an endless fixing film serving as a fixing member, a pressure roller serving as a pressure member, and a heater, for example, a ceramic heater. The heater is disposed at an inner surface side of the fixing film to heat the fixing film. The heater also forms a nip by abutting the pressure roller via the fixing film. When a recording medium is conveyed to the nip, heat and pressure are applied to a toner image on the recording medium at the nip and thus the toner image is fixed on the recording medium. 
     The belt-type fixing device as described above is relatively advantageous in increasing its operation speed compared to a roller-type fixing device. However, such belt-type fixing device may not be so effective to shorten so called “warm-up time,” which is the time required to reach a temperature suitable for operation, and “first print time,” which is the time required for completing a print operation starting from receipt of a print request, printing, and ejection of a printed medium. 
     On the other hand, for the on-demand fixing device using a fixing film, the warm-up time and first print time may be shortened by reducing a heat capacity thereof, thereby reducing the size thereof. However, in the on-demand fixing device, only the nip of the fixing film is locally heated while the other portions may be not sufficiently heated. As a result, during rotation, the fixing film may have a relatively low temperature at the sheet entry side of the nip, thereby causing defective fixing. Further, when the fixing device is operated at higher speed, the fixing film is also rotated at higher speed, thereby further facilitating heat dissipation at the portions other than the nip. 
     In order to prevent such defective fixing, a tubular metal heat conductor may be provided so as to face an inner surface of an endless fixing member. By directly or indirectly heating the metal heat conductor, the entire portion of the fixing member may be sufficiently and uniformly heated. 
     However, in such a configuration, the tubular metal heat conductor might be undesirably deformed by pressing force, which is applied at the nip from the pressure member to the metal heat conductor. Because the tubular metal heat conductor has been preferably formed thinner in order to enhance the heat efficiency of the fixing member, such a configuration may more easily cause defective fixing. 
     Deformation of the metal heat conductor may cause severe rubbing against the inner surface of the fixing member at the deformed portion. As a result, the inner surface of the fixing member may be unevenly worn out, thereby deteriorating the durability thereof. Alternatively, a driving torque of the fixing member may be increased, thereby causing slip of the fixing member and jam of a recording medium at the nip. 
     The deformation of the metal heat conductor may also reduce its contact area with the pressure member on a central portion in a width direction thereof. As a result, the nip width of the deformed portion may become smaller than the nip width of each end portion of the metal heat conductor. In such state, the amount of heat which the metal heat conductor applies to the recording medium is different between the central portion and each end portion. Consequently, cold offset may be caused at a central portion of an output image, while hot offset may be caused at end portions thereof. Thus, a relatively large difference in glossiness may occur between the central portion and the respective end portions. Further, an extremely large deviation in nip width may occur between the central portion and each end portion. Such deviation may significantly affect the output image and, in some cases, might cause cockling in the recording medium. 
     SUMMARY 
     The present disclosure provides an image forming apparatus and a fixing device operable while suppressing defective fixing and/or other failures even when the image forming apparatus and/or the fixing device operates at a relatively high speed with a shortened warm-up time and first print time. 
     In at least one example embodiment of the present specification, a fixing device for use in an image forming apparatus includes a flexible fixing member, a pressure member, an abutting member, and an interior member. The flexible fixing member is formed in an endless shape and is configured to heat and melt a toner image. The pressure member is disposed so as to be in contact with the fixing member with pressure. The abutting member is fixedly disposed on an inner surface of the fixing member so as to form a nip portion by abutting the pressure member via the fixing member. The interior member is fixedly disposed so as to face the inner surface of the fixing member except the nip portion, and is configured to heat the fixing member. The abutting member and the interior member are provided independent of each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the subject matter of this disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic view illustrating an overall configuration of an image forming apparatus according to an example embodiment; 
         FIG. 2  is a schematic view illustrating a fixing device according to an example embodiment; 
         FIG. 3  is a schematic view of the fixing device of  FIG. 2  viewed from a width direction side; 
         FIG. 4  is a bent state of an abutting member; 
         FIG. 5  is a schematic view illustrating a configuration of a fixing device according to an example embodiment; 
         FIG. 6  is a schematic view illustrating a configuration of a fixing device according to an example embodiment; 
         FIG. 7  is a schematic view illustrating a configuration of a fixing device according to an example embodiment; 
         FIG. 8  is a schematic view illustrating a configuration of a fixing device according to an example embodiment; and 
         FIG. 9  is a schematic view illustrating an overall configuration of an image forming apparatus according to an example embodiment. 
     
    
    
     The accompanying drawings are intended to depict example 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 EXAMPLE EMBODIMENTS 
     It will be understood that if an element or layer is referred to as being “on”, “against”, “connected to” or “coupled to” another element or layer, then it can be directly on, against, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, if an element is referred to as being “directly on”, “directly connected to” or “directly coupled to” another element or layer, then there are no intervening elements or layers present. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper” and the like may be used herein to facilitate description of one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, a term such as “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors herein interpreted accordingly. 
     Although the terms first, second, etc., may be used herein to describe various elements, components, regions, layers, and/or sections, it should be understood that these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are used only to distinguish one element, component, region, layer, or section from another region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of the present disclosure. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present disclosure. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes” and/or “including”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     In describing example 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. For the sake of simplicity, the same reference numerals are used in the drawings and the descriptions for the same materials and constituent parts having the same functions, and descriptions thereof are omitted unless otherwise stated. 
     Example embodiments of the present disclosure are now described below with reference to the accompanying drawings. It should be noted that, in a later-described comparative example, example embodiment, and alternative example, the same reference numerals are used for the same constituent elements such as parts and materials having the same functions, and descriptions thereof omitted. 
     First, an image forming apparatus according to an example embodiment is described with reference to  FIG. 1 . 
       FIG. 1  is a schematic view of a configuration of an image forming apparatus  1 . In this example embodiment, the image forming apparatus  1  is configured as a monochrome multifunctional copier. However, an image forming apparatus according to an example embodiment is not limited to the multifunctional copier, but may be a copier, a printer, a facsimile, or any other suitable type of multifunctional device having functions thereof. 
     As illustrated in  FIG. 1 , the image forming apparatus  1  includes a document reading unit  2 , an exposing unit  3 , an imaging unit  4  including a photoconductor drum  5 , a transfer unit  7 , a document conveying unit  10 , sheet feeding units  12  to  15 , and a fixing device  20 , for example. 
     The document reading unit  2  optically reads image information of an original document D. The exposing unit  3  irradiates exposure light L onto the photoconductor drum  5  according to the image information read in the document reading unit  2 . The imaging unit  4  forms a toner image on the photoconductor drum  5 . The transfer unit  7  transfers the toner image onto a recording medium P. The document conveying unit  10  conveys the document D to the document reading unit  2 . Each of the sheet feeding units  12  to  14  accommodates a stack of recording media P, for example, transfer sheets. The fixing device  20  fixes an unfixed image transferred on the recording medium P. In this example embodiment, the fixing device  20  is provided with a fixing belt  21  serving as a fixing member and a pressure roller  31  serving as a pressure member. 
     Next, a normal image-forming operation of the image forming apparatus  1  is described with reference to  FIG. 1 . An original document D on a document table is conveyed using conveyance rollers of the document conveying unit  10  in a direction indicated by an arrow A in  FIG. 1 , and is passed over the document reading unit  2 . Meanwhile, the document reading unit  2  optically reads image information of the original document D. 
     The image information optically read by the document reading unit  2  is converted into electric signals and is transmitted to the exposing unit  3  or an optical writing unit. Based on the electric signals of image information, the exposing unit  3  irradiates exposure light L, for example, a laser beam onto the photoconductor drum  5  of the imaging unit  4 . 
     Meanwhile, in the imaging unit  4 , the photoconductor drum  5  is rotated in the clockwise direction in  FIG. 1 . Through imaging processes including charging, exposing, and developing processes, a toner image corresponding to the image information is formed on the photoconductor drum  5 . The transfer unit  7  transfers the toner image from the photoconductor drum  5  onto a recording medium P conveyed by registration rollers. 
     In this regard, the recording medium P is conveyed to the transfer unit  7  as follows. One of the sheet feeding units  12  to  14  of the image forming apparatus  1  is automatically or manually selected. Here, for example, assume that the topmost sheet feeding unit  12  is selected. A topmost sheet of a stack of recording media P accommodated in the sheet feeding unit  12  is conveyed to an entrance of a conveyance path K. 
     When the recording medium P is conveyed to the registration rollers through the conveyance path K, the registration rollers feed the recording medium P toward the transfer unit  7 . The registration rollers feed the recording medium P in such a timing that the toner image formed on the photoconductor drum  5  is properly transferred on the recording medium P. 
     The recording medium P is passed over the transfer unit  7  and is further conveyed along the conveyance path K to the fixing device  20 . When the recording medium P is sent into a nip between the fixing belt  21  and the pressure roller  31 , the toner image is fixed on the recording medium P by heat of the fixing belt  21  and pressure of the fixing belt  21  and the pressure roller  31 . The recording medium P having the toner image fixed thereon is sent out from the nip between the fixing belt  21  and the pressure roller  31 , and ejected from the image forming apparatus  1 . Thus, a series of image forming processes is performed. 
     Next, a configuration and a fixing operation of the fixing device  20  are described in further detail with reference to  FIGS. 2 to 4 .  FIG. 2  is a schematic view illustrating a configuration of the fixing device  20 .  FIG. 3  is a schematic view in the width direction of the fixing device  20 . 
     As illustrated in  FIG. 2 , the fixing device  20  includes a fixing belt  21  serving as a fixing member, an abutting member  22 , an interior member  23 , a heater  25  serving as a heat source, a pressure roller  31  serving as a pressure member, a temperature sensor  40 , a guide plate  35 , and a guide plate  37 , for example. 
     Here, the fixing belt  21  is a thin, flexible endless belt and is rotated in the clockwise direction indicated by an arrow B in  FIG. 2 . The fixing belt  21  may include a substrate and additionally an elastic layer and a releasing layer, which are laminated in order on the substrate. The fixing belt  21  may be formed in a total thickness of 1 mm or less, for example. 
     The substrate of the fixing belt  21  has a thickness of approximately 30 to 50 μm, for example and is made of nickel, stainless steel, or any other suitable metal and/or polyimide or any other suitable resin. 
     The elastic layer of the fixing belt  21  has a thickness of approximately 100 to 300 μm, for example, and is made of silicone rubber, silicone rubber foam, fluorocarbon rubber, or any other suitable rubber material. By providing the fixing belt  21  with the elastic layer, minute irregularities may be prevented from being formed on the surface of the fixing belt  21  at the nip. As a result, heat of the fixing belt  21  can be uniformly applied over a toner image T, which may prevent formation of a defective image, for example, a so-called orange peel image. 
     The releasing layer of the fixing belt  21  has a thickness of approximately 10 to 50 μm, for example, and is made of tetrafluoroethylene-perfluoroalkylvinylether copolymer resin (PFA), polyimide, polyetherimide, polyethersulfide (PES), or any other suitable material. By providing the fixing belt  21  with the releasing layer, the fixing belt  21  may be securely released or separated from the toner image T. 
     The fixing belt  21  is configured to have a diameter in a range from 15 to 120 mm, for example. In this example embodiment, the diameter of the fixing belt  21  is assumed to be 30 mm. 
     The heater  25 , the abutting member  22 , and the interior member  23  are fixedly disposed at the inner surface side of the fixing belt  21 . The fixing belt  21  is in contact with the abutting member  22  with pressure to form the nip between the fixing belt  21  and the pressure roller  31 . 
     The abutting member  22  is fixedly disposed at the inner surface side of the fixing belt  21  so as to form the nip by abutting the pressure roller  31  via the fixing belt  21 . As illustrated in  FIG. 3 , the abutting member  22  is fixedly supported at end portions in a width direction thereof by side plates  43  of the fixing device  20  via support members  41 . Further, as illustrated in  FIG. 2 , the abutting member  22  has a face opposite to the pressure roller  31 , which is formed to have a curvature similar to, if not same as, that of the pressure roller  31 . Thus, the recording medium P is sent out from the nip along a curved surface of the pressure roller  31 . Such a configuration may prevent the recording medium P from remaining unseparated from and attracted on the fixing belt  21  after the fixing process. 
     The abutting member  22  is preferably made of material having such a rigidity as not to be severely bent by a pressing force of the pressure roller  31 . The abutting member  22  has a sliding contact face to slidingly contact the fixing belt  21 . The sliding contact face is preferably made of material having a relatively low friction coefficient so as to suppress abrasion of the fixing belt  21  during sliding contact. 
     The heater  25  serving as a heat source is a halogen heater, a carbon heater, or any other suitable heater. As illustrated in  FIG. 3 , the heater  25  has end portions fixed on the side plates  43  of the fixing device  20  via the support members  41 . 
     The interior member  23  is heated by radiation heat radiated from the heater  25 , the output of which is controlled by a power supply of the image forming apparatus  1 . The fixing belt  21  is entirely heated except the nip by the interior member  23 . From the outer surface of the fixing belt  21  thus heated, heat is transmitted onto the toner image T carried on the recording medium P. The output control of the heater  25  is performed based on detection results of the surface temperature of the fixing belt  21  detected by the temperature sensor  40 , for example, a thermistor. As illustrated in  FIG. 2 , the temperature sensor  40  is disposed at a position opposite to the outer surface of the fixing belt  21 . By performing the output control of the heater  25  as described above, the surface temperature of the fixing belt  21 , i.e., fixing temperature is set to a desired temperature. 
     As illustrated in  FIG. 2 , the interior member  23  is fixedly provided so as to face the inner surface of the fixing belt  21  except the nip. The interior member  23 , heated by the radiation heat of the heater  25 , heats or transmits heat to the fixing belt  21 . The interior member  23  may be made of heat-conductive metal, for example, aluminum, iron, or stainless steel. As illustrated in  FIG. 3 , the interior member  23  is fixedly supported at both end portions thereof by the side plates  43  of the fixing device  20  via the support members  41 . 
     The gap δ between the fixing belt  21  and the interior member  23  is preferably larger than 0 mm and not larger than 1 mm. Such a configuration may suppress abrasion of the fixing belt  21  that may be caused by the sliding contact between the interior member  23  and the fixing belt  21 . Such a configuration may also suppress reduction in the heating efficiency of the fixing belt  21  that may be caused by an undesirably large gap between the interior member  23  and the fixing belt  21 . Further, by disposing the interior member  23  close to the fixing belt  21 , a circular shape of the fixing belt  21  having a certain degree of flexibility is appropriately maintained, thereby suppressing degradation or damage of the fixing belt  21  that may be caused by deformation thereof. 
     As described above, the fixing device  20  according to this example embodiment is capable of effectively heating the fixing belt  21  with a relatively simple configuration. Thus, the warm-up time of the fixing device  20  and/or the first print time of the image forming apparatus  1  may be shortened while achieving the downsizing of the fixing device  20 . 
     In this example embodiment, the abutting member  22  forming the nip and the interior member  23  heating the fixing belt  21  are provided independent of each other. In other words, each of the abutting member  22  and the interior member  23  is formed as an independent member. 
     As a result, without locally heating only a part of the fixing belt  21 , the inner surface of the fixing belt  21  is substantially entirely heated by the interior member  23 . Thus, even when the image forming apparatus  1  operates at a relatively high speed, the fixing belt  21  may be sufficiently heated, thereby suppressing defective fixing. 
     Alternatively, when the interior member  23  is provided independent of the abutting member  22  receiving a pressing force from the pressure roller  31 , the interior member  23  may be formed in a thinner shape in order to increase the heating efficiency of the fixing belt  21 , as described above. Such a configuration may suppress severe rubbing of the interior member  23  against the inner surface of the fixing belt  21  or an increased driving torque of the fixing belt  21  that might be caused by the deformation of the interior member  23 . For example, as illustrated in  FIG. 4 , the central portion in the width direction of the abutting member  22  may be bent by pressing forces of the pressure roller  31  indicated by arrows E in  FIG. 4 . Even in such a case, the deformation of the abutting member  22  does not cause deformation of the interior member  23 . 
     Further, as illustrated in  FIG. 2 , the abutting member  22  and the interior member  23  are disposed apart from each other. In other words, a sufficient gap is provided between the abutting member  22  and the interior member  23  so that the two members  22  and  23  do not contact each other. Such a configuration provides a space between the abutting member  22  and the interior member  23  to serve as a heat insulator, thereby suppressing reduction in the heating efficiency of the fixing belt  21  that might be caused by heat transmission from the interior member  23  to the abutting member  22 . In other words, the heat, which would be otherwise transmitted to the abutting member  22 , is efficiently transmitted to the fixing belt  21 , thereby entirely heating the fixing belt  21  except the nip in an efficient manner. 
     As described above, in this example embodiment, the interior member  23  actively heats the fixing belt  21  while the abutting member  22  does not actively heat the fixing belt  21 . As a result, the fixing belt  21  is sufficiently heated by the interior member  23  up to a temperature suitable for fixing before a relevant portion thereof reaches the nip. On the other hand, the nip is not actively heated and therefore a certain amount of heat of the fixing belt  21  is transmitted to the unfixed toner image T on the recording medium P, that is, consumed as an energy for melting and fixing the toner image T, thereby decreasing the temperature of the fixing belt  21 . At this time, in a state where the temperature of an interface between the fixing belt  21  and an image forming surface of the recording medium P is lower than the fixing temperature, the recording medium P is sent out from the nip. As a result, while the adhesive force of toner is being reduced, the recording medium P is separated from the fixing belt  21 . Thus, such a configuration that the abutting member  22  does not actively heat the fixing belt  21  may facilitate separation of the recording medium P from the fixing belt  21 . 
     As illustrated in  FIG. 2 , the pressure roller  31  serving as a pressure member has a diameter of approximately 30 mm, for example. The pressure roller  31  has an elastic layer  33  formed on a hollow core metal  32 . The elastic layer  33  of the pressure roller  31  is made of material, for example, fluorocarbon rubber, silicone rubber, or silicone rubber foam. On a surface of the elastic layer  33  may be provided a thin releasing layer made of PFA, polytetrafluoroethylene (PTFE), or any other suitable material. The pressure roller  31  is in contact with the fixing belt  21  with pressure to form the nip having a desired distance between the pressure roller  31  and the fixing belt  21 . Further, as illustrated in  FIG. 3 , the pressure roller  31  may be provided with a gear  45  for engaging a driving gear of a driving mechanism, not illustrated, so as to be rotationally driven in the counterclockwise direction indicated by an arrow C in  FIG. 2 . The pressure roller  31  is rotatably supported at end portions in a width direction thereof by the side plates  43  via bearings  42 . On the interior of the pressure roller  31  may be provided a halogen heater or any other heat source. 
     In this example embodiment, the fixing belt  21  is formed to have a diameter similar to, if not same as, the diameter of the pressure roller  31 . However, the diameter of the fixing belt  21  may be smaller than the diameter of the processing roller  31 . In such a case, the curvature of the fixing belt  21  is also smaller than the curvature of the pressure roller  31  at the nip. As a result, the recording medium P sent out form the nip may be more securely separated from the fixing belt  21 . 
     At the entry side of the abutting portion, i.e., the nip between the fixing belt  21  and the pressure roller  31  is provided a guide plate  35 , or an entry guide plate, for guiding the recording medium P to be conveyed into the nip. On the other hand, at the exit side of the nip is provided a guide plate  37 , or an exit guide plate, for guiding the recording medium P sent out from the nip. The guide plates  35  and  37  are fixedly provided on the side plates  43  of the fixing device  20 . 
     Next, an operation of the fixing device  20  configured as described above is briefly described. 
     When the image forming apparatus  1  is powered on, an electric power is supplied to the heater  25  and the pressure roller  31  is rotationally driven in the direction indicated by the arrow C in  FIG. 2 . By a frictional force generated between the pressure roller  31  and the fixing belt  21 , the fixing belt  21  is dependently driven or rotated in the direction indicated by the arrow B in  FIG. 2 . 
     When a recording medium P is fed from one of the sheet feeding units  12  to  14 , an unfixed toner image T is carried on the recording medium P at the imaging unit  4 . While being guided by the guide plate  35 , the recording medium P having the unfixed toner image T thereon is conveyed in a direction indicated by an arrow Y 10  in  FIG. 2  and sent into the nip between the fixing belt  21  and the pressure roller  31 , which are pressingly contacted with each other. 
     By applying heat of the fixing belt  21  heated by the interior member  23 , or the heater  25 , and pressing forces of the pressure roller  31  and the abutting member  22  onto the unfixed toner image T, the toner image T is fixed on the surface of the recording medium P. The recording medium P is sent out from the nip and is conveyed in a direction indicated by an arrow Y 11  in  FIG. 2 . 
     As described above, in this example embodiment, the abutting member  22  forming the nip by abutting the pressure roller  31  via the fixing belt  21  serving as a fixing member is provided independent of the interior member  23  disposed so as to face the inner surface of the fixing belt  21  to entirely heat the fixing belt  21  except the nip. Such a configuration may suppress a failure, for example, defective fixing even when the image forming apparatus  1  operates at a relatively high speed with a shortened warm-up or first print time. 
     As described above, in this example embodiment, the fixing device  20  employs the pressure roller  31  as a pressure member. However, the pressure member may be a pressing belt or pad. In such cases, similar effects to those of this example embodiment may be obtained. 
     Further, in this example embodiment, the fixing belt  21  having a plurality of layers is employed as the fixing member. However, the fixing member is not limited to the fixing belt  21  but may be an endless fixing film made of polyimide, polyamide, fluorocarbon resin, or metal. In such cases, similar effects to that of this example embodiment may be obtained. 
     In this example embodiment, the face of the abutting member  22  opposing the heater  25  may be subjected to mirror finishing. Alternatively, a heat insulator may be disposed on the opposing face of the abutting member  22 . In such a case, the heat irradiated from the heater  25  toward the abutting member  22  serves to heat the interior member  23 . Thus, the heating efficiency of the fixing belt  21  or the interior member  23  may be further enhanced. 
     Next, another example embodiment is described in detail with reference to  FIG. 5 .  FIG. 5  is a schematic view illustrating a configuration of a fixing device  20  according to another example embodiment.  FIG. 5  corresponds to  FIG. 2  regarding the above-described example embodiment. The fixing device  20  of  FIG. 5  is different from the fixing device  20  of  FIG. 2  in that an abutting member  22  of  FIG. 5  has a substantially cuboid shape and that a heat insulator  27  is disposed between the abutting member  22  and a fixing belt  21 . 
     As illustrated in  FIG. 5 , the fixing device  20  includes the fixing belt  21  serving as a fixing member, the abutting member  22 , an interior member  23 , a heater  25 , and a pressure roller  31  serving as a pressure member, for example. 
     In this example embodiment, the abutting member  22  is formed in a substantially cuboid shape unlike the abutting member  22  illustrated in  FIG. 2 . The abutting member  22  has a flat face opposing the pressure roller  31 . Such a configuration may allow a nip between the fixing belt  21  and the pressure roller  31  to be formed in substantially parallel with an image forming surface of a recording medium P. Thus, the coherence between the fixing belt  21  and the recording medium P may be increased, thereby enhancing the fixing of a toner image on the recording medium P. In this configuration, the fixing belt  21  has a relatively large curvature at the exit side of the nip, further facilitating separation of the recording medium P from the fixing belt  21 . 
     In a direction in which the abutting member  22  faces the pressure roller  31  or the heater  25 , the abutting member  22  has a larger thickness than that of the interior member  23 . Such a configuration allows the abutting member  22 , which preferably has a higher strength and/or a larger thickness in order to securely form the nip, and the interior member  23 , which preferably has a lower heat capacity or a smaller thickness in order to increase heating efficiency, to be formed independent of each other. 
     Further, in order to obtain similar effects to the above-described effects, the abutting member  22  may be made of material having relatively high rigidity compared to material of the interior member  23 . In other words, the abutting member  22 , preferably having a strength enough to securely form the nip, may be made of material having relatively high rigidity, and the interior member  23 , preferably having a lower heat capacity in order to increase its heating efficiency, may be made of material having a higher heat conductivity. For example, the interior member  23  may be made of aluminum and the abutting member  22  may be made of stainless steel or iron. In such cases, even when the abutting member  22  and the interior member  23  are formed to have a substantially identical thickness, the fixing device  20  may obtain similar effects to the above-described effects. 
     In this example embodiment, as illustrated in  FIG. 5 , the heat insulator  27  is disposed between the abutting member  22  and the fixing belt  21 . The heat insulator  27  may be made of silicone foam, heat-resistant felt, or any other suitable material. Such a configuration may suppress reduction in the temperature of the fixing belt  21  and resultant defective fixing that might be caused by the heat transmission from the fixing belt  21  to the abutting member  22  at the nip. 
     As described above, the abutting member  22  forming the nip by abutting the pressure roller  31  via the fixing belt  21  is provided independent of the interior member  23  disposed so as to face the inner surface of the fixing belt  21  except the nip to heat the fixing belt  21 . Such a configuration may suppress a failure, for example, defective fixing even when an image forming apparatus including the fixing device  20  operates at a relatively high speed with a shortened warm-up or first print time. 
     Next, another example embodiment is described with reference to  FIG. 6 . 
       FIG. 6  is a schematic view illustrating a configuration of a fixing device  20   c  according to another example embodiment.  FIG. 6  corresponds to  FIG. 5  of the above-described embodiment. The fixing device  20  of  FIG. 6  is different from the fixing device  20  of  FIG. 5  in that an interior member  23  of  FIG. 6  is heated by electromagnetic induction and that heat insulators  28  are provided between an abutting member  22  and the interior member  23 . 
     As illustrated in  FIG. 6 , the fixing device  20   c  includes a fixing belt  21  serving as a fixing member, the abutting member  22 , the interior member  23 , and a pressure roller  31  serving as a pressure member, for example. 
     The fixing device  20  further includes an induction heater  50 . The interior member  23  is heated by electromagnetic induction of the induction heater  50  instead of radiation heat of the heat source  25  illustrated in  FIG. 5 . 
     The induction heater  50  may include exciting coils, cores, and a coil guide. As illustrated in  FIG. 6 , the exciting coils may further include litz wires, each having a bundle of fine wires, extending in a width direction of the fixing belt  21  to partially cover the fixing belt  21 . The coil guide is made of resin material having relatively high heat resistivity and supports the exciting coils and the cores. The cores are semi-cylindrical members made of ferromagnet, for example, ferrite having a relative permeability of approximately 1000 to 3000, for example. The cores include a central core or side cores in order to efficiently form magnetic flux toward the interior member  23 . The cores are disposed so as to face the exciting coils extending in the width direction of the fixing belt  21 . 
     The fixing device  20   c  thus configured operates as follows. When the fixing belt  21  is rotationally driven in the clockwise direction in  FIG. 6 , the fixing belt  21  is heated at positions facing the induction heater  50 . For example, by applying a high-frequency alternating current to the exciting coils, magnetic lines having alternating directions are formed around the interior member  23 . 
     At this time, an eddy current is generated at the surface of the interior member  23  and thus Joule heat is generated by the electrical resistance of the interior member  23 . Such Joule heat heats the interior member  23 , and the heated interior member  23  heats the fixing belt  21 . 
     Alternatively, in order to efficiently heat the interior member  23  by electromagnetic induction, the induction heater  50  may be disposed so as to face the outer surface of the interior member  23 . The interior member  23  may be made of nickel, stainless steel, iron, copper, cobalt, chrome, aluminum, gold, platinum, silver, tin, palladium, or an alloy of at least two of these metals. 
     In this example embodiment, as illustrated in  FIG. 6 , the heat insulators  28  are disposed between the abutting member  22  and the interior member  23 . The heat insulators  28  may be made of silicone foam, heat-resistant felt, or any other suitable material. Such a configuration may suppress reduction in the heating efficiency of the fixing belt  21  that might be caused by the heat transmission from the interior member  23  to the abutting member  22 . In other words, the heat, which would be otherwise transmitted to the abutting member  22 , is intensively transmitted to the fixing belt  21 . Thus, the fixing belt  21  may be entirely heated except the nip by the interior member  23 . 
     As described above, in this example embodiment, the abutting member  22  forming the nip by abutting the pressure roller  31  via the fixing belt  21  is formed independent of the interior member  23  disposed so as to face the inner surface of the fixing belt  21  except the nip to heat the fixing belt  21 . Thus, such a configuration may suppress a failure, for example, defect fixing even when an image forming apparatus including the fixing device  20  operates at a relatively high speed with a shortened warm-up or first print time. 
     Next, another example embodiment is described with reference to  FIG. 7 .  FIG. 7  is a schematic view illustrating a fixing device  20  according to another example embodiment.  FIG. 7  corresponds to  FIG. 5  of the above-described example embodiment. The fixing device  20  of  FIG. 7  is different from the fixing device  20  of  FIG. 5  in that an interior member  23  of  FIG. 7  is heated by heat of a resistance heater  60 . 
     As illustrated in  FIG. 7 , the fixing device  20  includes a fixing belt  21  serving as a fixing member, an abutting member  22 , the interior member  23 , a pressure roller  31  serving as a pressure member, and a temperature sensor  40 , for example. 
     In this embodiment, the fixing device  20  further includes the resistance heater  60  provided to the inner surface side of the fixing belt  21 . In this example embodiment, the interior member  23  is heated by heat of the resistance heater  60  instead of radiation heat of the heat source  25  as illustrated in  FIG. 5 . 
     The resistance heater  60  may be a sheet heating element, for example, a ceramic heater and connected to a power supply, not illustrated, at both end portions thereof. When an electric current is applied to the resistance heater  60 , the temperature of the resistance heater  60  is increased by electric resistance thereof, thereby heating the interior member  23  in contact therewith. The interior member  23  thus heated heats the fixing belt  21 . Thus, by using the resistance heater  60  as a heat source for heating the interior member  23 , the fixing belt  21  may be effectively heated at relatively low cost. 
     As illustrated in  FIG. 7 , in this example embodiment, the resistance heater  60  is disposed near the entry of the nip rather than the exit of the nip. Such a configuration may allow the fixing belt  21  to be effectively heated before a relevant portion of the fixing belt  21  reaches the nip. 
     In this regard, a plurality of resistance heaters  60  may be provided so as to be able to change a heat distribution in the width direction of the fixing belt  21 . For example, three resistance heaters  60  may be provided at one central portion and end portions in the width direction of the fixing belt  21 . During a normal operation, all the resistance heaters  60  are activated so as to entirely heat the fixing belt  21  or the interior member  23  except the nip. 
     Alternatively, when a recording medium P having a relatively small size in the width direction of the fixing belt  21  is passed through the nip, only the resistance heater  60  disposed at the central portion of the fixing device  21  is activated to heat the central portion thereof. Thus, even when a plurality of recording media P having a relatively small size in the width direction of the fixing belt  21  are sequentially passed through the nip, the fixing device  20  may suppress excess increase in temperature of each end portion in the width direction of the fixing belt  21 . 
     Further, the resistance heater  60  may preferably have a Curie point temperature, i.e., a temperature at which the resistance value thereof drastically changes so as to stop increase in temperature thereof. In a normal state where the resistance heater  60  does not reach the Curie point temperature, when an electric current is applied to the resistance heater  60 , the temperature of the resistance heater  60  is increased, thereby heating the fixing belt  21  or the interior member  23  to a given temperature. On the other hand, when the temperature of the resistance heater  60  reaches the Curie point temperature, the resistance value of the resistance heater  60  rapidly increases, thereby suppressing the flow of the electric current therethrough. When the heat generation of the resistance heater  60  is stopped, an excess increase in temperature of the resistance heater  60  may be prevented. 
     With this configuration, even when media sheets of a small size are sequentially passed through the nip, the temperature of the fixing belt  21  may be prevented from excessively increasing at the end portions in its width direction. 
     The Curie point temperature of the resistance heater  60  is preferably set to a given temperature which may not cause an offset on a recording medium. For example, the Curie point temperature of the resistance heater  60  may be set to 180 degrees C. 
     As described above, in this example embodiment, the abutting member  22  forming the nip by abutting the pressure roller  31  via the fixing belt  21  is provided independent of the interior member  23  disposed so as to face the inner surface of the fixing belt  21  except the nip. Such a configuration may suppress a failure, for example, defective fixing even when an image forming apparatus including the fixing device  20  operates at a relatively high speed with a shortened warm-up time or first print time. 
     Next, another example embodiment is described with reference to  FIG. 8 .  FIG. 8  is a schematic view illustrating a fixing device  20  according to another example embodiment.  FIG. 8  corresponds to  FIG. 7  of the above-described example embodiment. The fixing device  20  of  FIG. 8  is different from the fixing device  20  of  FIG. 7  in that an inner surface of an interior member  23  of  FIG. 8  is entirely, not partially, in contact with a resistance heater  60 . 
     As illustrated in  FIG. 8 , the fixing device  20  includes a fixing belt  21  serving as a fixing member, an abutting member  22 , the interior member  23 , a pressure roller  31  serving as a pressure member, and the resistance heater  60 , for example. 
     In this example embodiment, the resistance heater  60  is disposed so as to be in contact with all areas of the inner surface of the interior member  23 . Such a configuration allows the interior member  23  to be uniformly heated over all the areas of the inner surface thereof, thereby enhancing the heating efficiency of the fixing belt  21 . 
     In this example embodiment, the resistance heater  60  is employed as a heat source for heating the interior member  23 . However, the interior member  23  may be configured as a heat source. Such a configuration may obtain a similar effect to that of this example embodiment. 
     As described above, in this example embodiment, the abutting member  22  forming the nip by abutting the pressure roller  31  via the fixing belt  21  is provided independent of the interior member  23  disposed so as to face the inner surface of the fixing belt  21  except the nip. Such a configuration may suppress a failure, for example, defective fixing even when an image forming apparatus including the fixing device  20  operates at a relatively high speed with a shortened warm-up time or first print time. 
     Next, another example embodiment is described with reference to  FIG. 9 . 
       FIG. 9  is a schematic view illustrating an overall configuration of an image forming apparatus  1  including a fixing device  20  according to another example embodiment. The image forming apparatus  1  of  FIG. 9  is configured as a color image forming apparatus unlike the image forming apparatus  1  of  FIG. 1 , which is configured as a monochrome image forming apparatus. 
     As illustrated in  FIG. 9 , the image forming apparatus  1  may be a tandem color image forming apparatus. The image forming apparatus  1  includes a bottle casing  101  at an upper portion thereof. In the bottle casing  101  are detachably and replaceably mounted four toner bottles  102 Y,  102 M,  102 C, and  102 K containing toner of yellow, magenta, cyan, and black colors, respectively. 
     Below the bottle casing  101  is provided an intermediate transfer unit  85  including an intermediate transfer belt  78 . Imaging units  4 Y,  4 M,  4 C, and  4 K for forming images of yellow, magenta, cyan, and black colors are disposed at positions opposing the intermediate transfer belt  78 . 
     The imaging units  4 Y,  4 M,  4 C, and  4 K include photoconductor drums  5 Y,  5 M,  5 C, and  5 K, respectively. For example, the photoconductor drum  5 Y is surrounded by a charger  75 , a developing unit  76 , a cleaning unit  77 , and a discharger, not illustrated. The imaging unit  4 Y performs imaging processes including a charging process, an exposing process, a developing process, primary and secondary transfer processes, and a cleaning process, for example. Through the imaging processes, a yellow color image is formed on the photoconductor drums  5 Y. 
     The photoconductor drum  5 Y is rotationally driven by a driving motor, not illustrated, in the clockwise direction in  FIG. 9 . In the charging process, the surface of the photoconductor drum  5 Y is uniformly charged by the corresponding charger  75 . In the exposing process, the exposing unit  3  emits laser light L onto the surface of the photoconductor drum  5 Y to form an electrostatic latent images of yellow color thereon. 
     In the developing process, when a relevant portion of the surface of the photoconductor drum  5 Y reaches a position opposing the developing unit  76 , the developing unit  76  develops the electric latent image to form a toner image of yellow color. 
     In the primary transfer process, when the relevant portion of the photoconductor drum  5 Y reaches a position opposing a transfer bias roller  79 Y, the yellow toner image on the photoconductor drum  5 Y is transferred onto the intermediate transfer belt  78 . At this time, a slight amount of untransferred toner may remain on the photoconductor drum  5 Y. 
     In the cleaning process, when the relevant portion of the photoconductor drum  5 Y reaches a position opposing the corresponding cleaning unit  77 , the untransferred toner is mechanically cleaned by a cleaning blade or any other suitable cleaning member of the cleaning unit  77 . 
     In the discharging process, when the relevant portion of the photoconductor drum  5 Y reaches a position opposing the corresponding discharger, the discharger discharges residual charges remaining on the photoconductor drum  5 Y. Thus, a series of imaging processes to be performed on the photoconductor drum  5 Y is finished. 
     Similarly, a series of imaging processes is performed on each of the other imaging unit  4 M,  4 C, and  4 K is performed to form toner images of magenta, cyan, and black on the photoconductor drums  5 M,  5 C, and  5 K. 
     The toner images of respective colors formed on the photoconductor drums  5 Y,  5 M,  5 C, and  5 K are transferred in a superimposing manner to form a composite color toner image on the intermediate transfer belt  78 . 
     As illustrated in  FIG. 9 , the intermediate transfer unit  85  may include the intermediate transfer belt  78 , the primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K, a secondary transfer backup roller  82 , a cleaning backup roller  83 , a tension roller  84 , and an intermediate transfer cleaning unit  80 . The intermediate transfer belt  78  is extended and supported by the three rollers  82  to  84  and is endlessly moved by rotational driving of the secondary transfer backup roller  82  in a direction indicated by an arrow F in  FIG. 9 . 
     The primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K sandwich the intermediate transfer belt  78  with the photoconductor drums  5 Y,  5 M,  5 C, and  5 K to form primary transfer nips. Each of the primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K is applied with a transfer bias having a polarity opposite to a polarity of each color toner. 
     The intermediate transfer belt  78  is moved in the direction indicated by the arrow F so as to sequentially pass through the respective primary transfer nips between the photoconductor drums  5 Y,  5 M,  5 C, and  5 K and the primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K. Thus, the respective color toner images on the photoconductor drums  5 Y,  5 M,  5 C, and  5 K are transferred on the intermediate transfer belt  78  in a superimposing manner. 
     Then, the composite color toner image superimposed on the intermediate transfer belt  78  reaches a position facing the secondary transfer roller  89 . Meanwhile, the secondary transfer backup roller  82  sandwiches the intermediate transfer belt  78  with the secondary transfer roller  89  to form a secondary transfer nip. The composite color toner image is transferred on a recording medium P conveyed to the secondary transfer nip. At this time, untransferred toner, which has not been transferred onto the recording medium P, may remain on the intermediate transfer belt  78 . Such untransferred toner on the intermediate transfer belt  78  is cleaned by the intermediate transfer cleaning unit  80 . 
     Thus, a series of transfer processes to be performed on the intermediate transfer belt  78  is finished. 
     In this regard, the recording medium P is conveyed from a sheet feeding unit  12 , which is disposed at a lower portion of the image forming apparatus  1 , via a sheet feeding roller  97 , a pair of registration rollers  98 , and other rollers. 
     The sheet feeding unit  12  accommodates a stack of recording media P, for example, transfer sheets. When the sheet feeding roller  97  is rotationally driven in the counterclockwise direction in  FIG. 9 , a topmost one of the recording media P is fed between the pair of registration rollers  98 . 
     The recording medium P conveyed to the pair of registration rollers  98  is temporarily stopped at a roller nip between the pair of registration rollers  98 . The pair of registration rollers  98  are rotationally driven to convey the recording medium P to the secondary transfer nip at such a timing that the color image on the intermediate transfer belt  78  is transferred onto the recording medium P. Thus, the desired color image is appropriately transferred on the recording medium P. 
     When the recording medium P having the color image transferred is conveyed to the fixing device  20 , the fixing device  20  fixes the color image on the recording medium P. In this example embodiment, the fixing device  20  of  FIG. 9  is similar in configuration and operation to the fixing device  20  of  FIG. 2 . In other words, in the fixing device  20  of  FIG. 9 , the abutting member  22  forming the nip by abutting the pressure roller  31  via the fixing belt  21  is provided independent of the interior member  23  disposed so as to face the inner surface of the fixing belt  21  except the nip to heat the fixing belt  21 . 
     The recording medium P is passed through a nip of a pair of ejection rollers  99  and is ejected to the outside of the image forming apparatus  1 . The ejected recording medium P having the desired color image is stacked on a stack portion  100 . Thus, a series of image forming processes of the image forming apparatus  1  is completed. 
     As described above, in this example embodiment, the abutting member  22  forming the nip by abutting the pressure roller  31  via the fixing belt  21  is provided independent of the interior member  23  disposed so as to face the inner surface of the fixing belt  21  except the nip to heat the fixing belt  21 . Such a configuration may suppress a failure, for example, defective fixing even when an image forming apparatus including the fixing device  20  operates at a relatively high speed with a shortened warm-up or first print time. 
     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 disclosure of this application may be practiced otherwise than as specifically described herein. 
     Further, elements and/or features of different example embodiments and/or examples may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. 
     Example embodiments and/or examples being thus described, it should be apparent to one skilled in the art after reading this disclosure that the example embodiments and examples may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present disclosure, and such modifications are not excluded from the scope of the following claims.