Patent Publication Number: US-8112023-B2

Title: Image forming apparatus and fixing device

Description:
PRIORITY STATEMENT 
     This application is a continuation application of and claims priority under 35 U.S.C. §§120/121 to U.S. patent application Ser. No. 11/785,271, filed on Apr. 17, 2007 now U.S. Pat. No. 7,885,590, which claims priority under 35 U.S.C. §119 upon Japanese Patent Application No. 2006-112952 filed on Apr. 17, 2006 and Japanese Patent Application No. 2007-009483 filed on Jan. 18, 2007 in the Japan Patent Office, the entire contents of each of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Technical Field 
     Some example embodiments of the present invention generally relate to an image forming apparatus and/or a fixing device, for example, for fixing a toner image on a recording medium, e.g., by induction heating. 
     2. Description of Background Art 
     A background image forming apparatus, for example, a copying machine, a facsimile machine, a printer, or a multifunction printer having copying, printing, scanning, and facsimile functions, forms a toner image on a recording medium (e.g., a sheet) according to image data by an electrophotographic method. For example, a charger charges a surface of a photoconductor. An optical writer emits a light beam on the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to image data. The electrostatic latent image is developed with a developer (e.g., toner) to form a toner image on the photoconductor. The toner image is transferred from the photoconductor onto a sheet. A fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image on the sheet. Thus, the toner image is formed on the sheet. 
     One example of a background fixing device uses induction heating to shorten a time period needed for the fixing device to be heated up to a proper fixing temperature after being powered on, so as to save energy. The fixing device includes a magnetic flux generator including a coil, a fixing roller including a heat generating layer, and/or a pressing roller. The magnetic flux generator opposes a part of an outer circumferential surface of the fixing roller. The pressing roller pressingly contacts another part of the outer circumferential surface of the fixing roller to form a fixing nip. At the fixing nip, the fixing roller and the pressing roller apply heat and pressure to a sheet bearing a toner image conveyed to the fixing nip to fix the toner image on the sheet. The coil extends in a width direction (i.e., a direction perpendicular to a sheet conveyance direction) of the magnetic flux generator. 
     For example, a power source applies a high-frequency alternating current to the coil to form an alternating magnetic field around the coil. An eddy current generates in the heat generating layer. An electric resistance of the heat generating layer generates Joule heat. The Joule heat increases the temperature of the whole fixing roller. Induction heating may heat the fixing roller up to a desired temperature in a shortened time period by consuming less energy compared to heating with a heating lamp, for example. 
     Another example of a background fixing device includes a magnetic flux generator, a pressing roller, and/or a fixing roller. The magnetic flux generator is disposed inside the pressing roller. The fixing roller contacts the pressing roller, and includes a temperature-sensitive, magnetic metal pipe. A member including a non-magnetic material (e.g., aluminum) having a low electric resistivity is disposed inside the temperature-sensitive, magnetic metal pipe. The temperature-sensitive, magnetic metal pipe includes a magnetic shunt alloy providing self-control of temperature. Thus, in this example fixing device, induction heating may effectively heat the fixing roller. 
     Yet another example of a background fixing device includes a fixing roller including a heat generating layer having various layer thicknesses in a width direction of the heat generating layer (i.e., a width direction of the fixing roller). For example, a layer thickness of a center portion of the heat generating layer in the width direction of the heat generating layer is greater than a layer thickness of both end portions of the heat generating layer in the width direction of the heat generating layer. Thus, the fixing device may provide a proper width of the fixing nip which may prevent faulty fixing. 
     The above-described background fixing devices may perform faulty fixing due to a varied temperature distribution in the width direction of the fixing roller. For example, both end portions of the fixing roller in the width direction of the fixing roller dissipate heat in a greater amount than a center portion of the fixing roller in the width direction of the fixing roller. Especially during a warm-up period of the fixing device when the fixing device is powered on after a long time period has elapsed since the fixing device was powered off, the fixing device is heated from a relatively low temperature up to a proper fixing temperature. Accordingly, the amount of dissipated heat substantially differs between the both end portions and the center portion of the fixing roller in the width direction of the fixing roller. Namely, the temperature of the both end portions of the fixing roller is lower than the temperature of the center portion of the fixing roller in the width direction of the fixing roller. 
     SUMMARY 
     At least one embodiment of the present invention provides a fixing device for fixing a toner image on a recording medium by applying heat to the recording medium. The fixing device includes a magnetic flux generator and a heat generating member. The magnetic flux generator generates a magnetic flux. The heat generating member opposes the magnetic flux generator and includes a heat generating layer. The heat generating layer generates heat by the magnetic flux generated by the magnetic flux generator and has an eddy current load, obtained by dividing a volume resistivity by a layer thickness, varying depending on a position in a width direction of the heat generating layer. The heat generating layer includes a magnetic layer having a Curie point in a range from about 100 degrees centigrade to about 300 degrees centigrade. 
     At least one embodiment of the present invention provides an image forming apparatus that includes an image carrier to carry a toner image and a fixing device (such as mentioned above regarding another embodiment of the present invention) to fix the toner image transferred from the image carrier onto a recording medium by applying at least heat to at least one of the toner image and the recording medium. 
     Additional features and advantages of example embodiments will be more fully apparent from the following detailed description, the accompanying drawings, and the associated claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of example embodiments and the many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic view of an image forming apparatus according to an example embodiment of the present invention; 
         FIG. 2  is a sectional view (according to an example embodiment of the present invention) of a fixing device of the image forming apparatus shown in  FIG. 1 ; 
         FIG. 3  is an enlarged sectional view (according to an example embodiment of the present invention) of a part of a fixing roller of the fixing device shown in  FIG. 2 ; 
         FIG. 4A  is a sectional view (according to an example embodiment of the present invention) of the fixing roller shown in  FIG. 3  for illustrating a flow of a magnetic flux; 
         FIG. 4B  is a sectional view (according to an example embodiment of the present invention) of the fixing roller shown in  FIG. 3  for illustrating another flow of a magnetic flux; 
         FIG. 5  is a sectional view (according to an example embodiment of the present invention) of a heat generating layer of the fixing roller shown in  FIG. 3  corresponding to a width direction of the fixing roller; 
         FIG. 6  is a graph (according to an example embodiment of the present invention) illustrating a relationship between an eddy current load and an amount of generated heat of the heat generating layer shown in  FIG. 5 ; 
         FIG. 7  is a graph (according to an example embodiment of the present invention) illustrating a relationship between a position in a width direction of the fixing roller shown in  FIG. 3  and a fixing temperature; 
         FIG. 8  is a sectional view of a heat generating layer of a fixing roller corresponding to a width direction of the fixing roller according to another example embodiment of the present invention; 
         FIG. 9  is a sectional view of a heat generating layer of a fixing roller corresponding to a width direction of the fixing roller according to yet another example embodiment of the present invention; 
         FIG. 10  is a sectional view of a heat generating layer of a fixing roller corresponding to a width direction of the fixing roller according to yet another example embodiment of the present invention; 
         FIG. 11  is a sectional view of a heat generating layer of a fixing roller corresponding to a width direction of the fixing roller according to yet another example embodiment of the present invention; 
         FIG. 12  is a sectional view of a heat generating layer of a fixing roller corresponding to a width direction of the fixing roller according to yet another example embodiment of the present invention; 
         FIG. 13  is a sectional view of a heat generating layer of a fixing roller corresponding to a width direction of the fixing roller according to yet another example embodiment of the present invention; 
         FIG. 14  is a sectional view of a fixing device according to yet another example embodiment of the present invention; and 
         FIG. 15  is an enlarged sectional view (according to an example embodiment of the present invention) of a part of a fixing belt of the fixing device shown in  FIG. 14 . 
     
    
    
     The accompanying drawings are intended to depict example embodiments of the present invention 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 for ease of description to describe 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, 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 used herein are 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 invention. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. 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 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. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 1 , an image forming apparatus  1  according to an example embodiment of the present invention is explained. 
     As illustrated in  FIG. 1 , the image forming apparatus  1  includes a document feeder  3 , a reader  4 , a writer  2 , photoconductors  11 Y,  11 M,  11 C, and  11 BK, chargers  12 Y,  12 M,  12 C, and  12 BK, development devices  13 Y,  13 M,  13 C, and  13 BK, a paper tray  7 , a feeding roller  8 , a registration roller pair  9 , a transfer belt  17 , transfer bias rollers  14 Y,  14 M,  14 C, and  14 BK, cleaners  15 Y,  15 M,  15 C, and  15 BK, a separating charger  18 , a belt cleaner  16 , and/or a fixing device  19 . The reader  4  includes an exposure glass  5 . 
     The image forming apparatus  1 , e.g., may be a copying machine, a facsimile machine, a printer, a multifunction printer having copying, printing, scanning, and facsimile functions, or the like. As a more particular example, the image forming apparatus  1  may be a tandem type color copying machine for forming a color image on a recording medium by an electrophotographic method. 
     Referring to  FIG. 1 , the following describes operations of the image forming apparatus  1  for forming a color toner image on a recording medium. 
     A user places an original D on an original tray (not shown) of the document feeder  3 . A feeding roller (not shown) of the document feeder  3  feeds the original D placed on the original tray in a direction A to the exposure glass  5  of the reader  4 . When the original D reaches the exposure glass  5  and is thereby placed on the exposure glass  5 , the reader  4  optically reads an image on the original D and sends image data created according to the read image to the writer  2 . 
     For example, the reader  4  scans an image on the original D while a lamp (not shown) of the reader  4  emits a light beam onto the original D. The light beam reflected by the original D travels through mirrors (not shown) and a lens (not shown) of the reader  4  and forms an image in a color sensor (not shown) of the reader  4 . The color sensor reads color image data in the light beam into RGB (red, green, blue) image data and converts the RGB image data into electric, RGB image signals. An image processor (not shown) of the reader  4  performs color conversion processing, color correction processing, space frequency correction processing, and/or the like based on the RGB image signals to create color image data for yellow, magenta, cyan, and black colors. 
     The reader  4  sends the yellow, magenta, cyan, and black image data to the writer  2 . The writer  2  emits laser beams corresponding to the yellow, magenta, cyan, and black image data onto the photoconductors  11 Y,  11 M,  11 C, and  11 BK, respectively. 
     The four photoconductors  11 Y,  11 M,  11 C, and  11 BK, serving as image carriers, have a drum shape and rotate in a rotating direction B. In a charging process, the chargers  12 Y,  12 M,  12 C, and  12 BK uniformly charge surfaces of the photoconductors  11 Y,  11 M,  11 C, and  11 BK at positions at which the chargers  12 Y,  12 M,  12 C, and  12 BK oppose the photoconductors  11 Y,  11 M,  11 C, and  11 BK, respectively. Thus, a charging potential is formed on each of the photoconductors  11 Y,  11 M,  11 C, and  11 BK. 
     In an exposing process, four light sources (not shown) of the writer  2  emit laser beams corresponding to the yellow, magenta, cyan, and black image data onto the photoconductors  11 Y,  11 M,  11 C, and  11 BK, respectively. The laser beams corresponding to the yellow, magenta, cyan, and black image data travel on optical paths different from each other. 
     The laser beam corresponding to the yellow image data irradiates the surface of the photoconductor  11 Y (i.e., a first photoconductor from the left in  FIG. 1 ). For example, a polygon mirror (not shown) rotating at a high speed causes the laser beam corresponding to the yellow image data to scan in an axial direction of the photoconductor  11 Y (i.e., a main scanning direction). Thus, an electrostatic latent image corresponding to the yellow image data is formed on the surface of the photoconductor  11 Y charged by the charger  12 Y. 
     Similarly, the laser beam corresponding to the magenta image data irradiates the surface of the photoconductor  11 M (i.e., a second photoconductor from the left in  FIG. 1 ) to form an electrostatic latent image corresponding to the magenta image data. The laser beam corresponding to the cyan image data irradiates the surface of the photoconductor  11 C (i.e., a third photoconductor from the left in  FIG. 1 ) to form an electrostatic latent image corresponding to the cyan image data. The laser beam corresponding to the black image data irradiates the surface of the photoconductor  11 BK (i.e., a fourth photoconductor from the left in  FIG. 1 ) to form an electrostatic latent image corresponding to the black image data. 
     When the electrostatic latent images formed on the surfaces of the photoconductors  11 Y,  11 M,  11 C, and  11 BK reach positions at which the development devices  13 Y,  13 M,  13 C, and  13 BK oppose the photoconductors  11 Y,  11 M,  11 C, and  11 BK, respectively, the development devices  13 Y,  13 M,  13 C, and  13 BK supply yellow, magenta, cyan, and black toners onto the surfaces of the photoconductors  11 Y,  11 M,  11 C, and  11 BK to develop the electrostatic latent images formed on the photoconductors  11 Y,  11 M,  11 C, and  11 BK to form yellow, magenta, cyan, and black toner images, respectively, in a developing process. 
     The paper tray  7  loads a recording medium (e.g., sheets P). The feeding roller  8  feeds the sheets P one by one toward the registration roller pair  9 . When the sheet P passes a guide (not shown) and reaches the registration roller pair  9 , the registration roller pair  9  feeds the sheet P to the transfer belt  17  at a proper time. 
     The transfer belt  17  rotates in a rotating direction C. The transfer bias rollers  14 Y,  14 M,  14 C, and  14 BK are disposed to contact an inner circumferential surface of the transfer belt  17  at positions at which the photoconductors  11 Y,  11 M,  11 C, and  11 BK oppose an outer circumferential surface of the transfer belt  17 . When the yellow, magenta, cyan, and black toner images formed on the surfaces of the photoconductors  11 Y,  11 M,  11 C, and  11 BK reach positions at which the outer circumferential surface of the transfer belt  17  opposes the photoconductors  11 Y,  11 M,  11 C, and  11 BK, respectively, the transfer bias rollers  14 Y,  14 M,  14 C, and  14 BK transfer and superimpose the yellow, magenta, cyan, and black toner images formed on the surfaces of the photoconductors  11 Y,  11 M,  11 C, and  11 BK onto the sheet P conveyed on the outer circumferential surface of the transfer belt  17 , respectively, in a transfer process. Thus, a color toner image is formed on the sheet P. 
     When portions on the surfaces of the photoconductors  11 Y,  11 M,  11 C, and  11 BK from which the yellow, magenta, cyan, and black toner images are transferred onto the sheet P reach positions at which the cleaners  15 Y,  15 M,  15 C, and  15 BK oppose the photoconductors  11 Y,  11 M,  11 C, and  11 BK, respectively, the cleaners  15 Y,  15 M,  15 C, and  15 BK remove toners not transferred and remaining on the surfaces of the photoconductors  11 Y,  11 M,  11 C, and  11 BK, respectively, in a cleaning process. 
     The portions on the surfaces of the photoconductors  11 Y,  11 M,  11 C, and  11 BK cleaned by the cleaners  15 Y,  15 M,  15 C, and  15 BK pass dischargers (not shown), respectively. Thus, a series of image forming processes performed on the photoconductors  11 Y,  11 M,  11 C, and  11 BK is completed. 
     The sheet P bearing the color toner image is conveyed on the transfer belt  17  toward the separating charger  18 . When the sheet P reaches a position at which the separating charger  18  opposes the transfer belt  17 , the separating charger  18  neutralizes electric charge stored on the sheet P so as to separate the sheet P from the transfer belt  17  without dispersing toner particles from the color toner image formed on the sheet P. 
     When a portion on the outer circumferential surface of the transfer belt  17  on which the sheet P has been carried reaches a position at which the belt cleaner  16  opposes the transfer belt  17 , the belt cleaner  16  removes substances adhered to the outer circumferential surface of the transfer belt  17 . 
     The sheet P separated from the transfer belt  17  is conveyed toward the fixing device  19 . In the fixing device  19 , a fixing roller (not shown) and a pressing roller (not shown) opposing each other nip the sheet P to fix the color toner image on the sheet P. An output roller (not shown) feeds the sheet P bearing the fixed color toner image to the outside of the image forming apparatus  1 . Thus, a series of image forming processes performed by the image forming apparatus  1  is completed. 
     Referring to  FIGS. 2 and 3 , the following describes a structure and operations of the fixing device  19 .  FIG. 2  is a sectional view of the fixing device  19 . As illustrated in  FIG. 2 , the fixing device  19  includes a pressing roller  30 , an induction heater  24 , and/or a fixing roller  20 . The pressing roller  30  includes a cylinder  32  and/or an elastic layer  31 . The induction heater  24  includes a coil guide  27 , a coil  25 , and/or a core  26 . The core  26  includes a center core  26   a  and/or a side core  26   b . The fixing roller  20  includes a core  205 , an elastic layer  204 , a heat generating layer  203 , another elastic (e.g., silicon rubber) layer  202 , and/or a releasing layer  201 . 
     The pressing roller  30  serves as a pressing member for pressing the fixing roller  20  via a sheet P bearing a toner image T. For example, the pressing roller  30  pressingly contacts the fixing roller  20  to form a fixing nip between the pressing roller  30  and the fixing roller  20 . A sheet P bearing a toner image T conveyed in a direction Y 1  enters the fixing nip. The induction heater  24  heats the fixing roller  20  by induction heating. The fixing roller  20  and the pressing roller  30  apply heat and pressure to the sheet P to fix the toner image T on the sheet P at the fixing nip. 
     The cylinder  32 , e.g., includes aluminum and/or copper. The elastic layer  31 , e.g., includes a fluorocarbon rubber and/or a silicon rubber, and is formed on the cylinder  32 . The elastic layer  31  has a layer thickness, e.g., from about 0.5 mm to about 2.0 mm and an Asker hardness, e.g., from about 60 degrees to about 90 degrees. 
     The induction heater  24  serves as a magnetic flux generator for generating a magnetic flux. At least a portion of the fixing roller  20  is disposed in the magnetic flux. The induction heater  24  is disposed adjacent to and, e.g., is obversely shaped with respect to, an outer circumferential surface of the fixing roller  20 . The coil guide  27  includes a heat-resistant resin. The coil guide  27  covers a part of the outer circumferential surface of the fixing roller  20  and supports the coil  25 . The coil  25  may be an exciting coil, e.g., including a litz wire, e.g., formed by bundling thin wires. The litz wire is coiled and extends in a width direction (i.e., a longitudinal direction) of the fixing roller  20 . The core  26  is disposed adjacent to and, e.g., is obversely shaped with respect to, the coil  25  and thus extends similarly in the width direction of the fixing roller  20 . The core  26  may be an exciting coil core and includes ferromagnet (e.g., ferrite) having a relative permeability, e.g., from about 1,000 to about 3,000. The center core  26   a  and the side core  26   b  are provided in a center and a side of the core  26  in a direction perpendicular to the width direction of the fixing roller  20 , respectively, so as to effectively generate a magnetic flux toward the fixing roller  20 . 
     A thermistor (not shown) contacts the surface of the fixing roller  20 . The thermistor includes a temperature-sensitive element having an increased thermal response, and detects the temperature (e.g., fixing temperature) of the fixing roller  20 . The heating level of the induction heater  24  is adjusted based on a detection result provided by the thermistor. 
     The fixing roller  20  serves as a heat generating member for generating heat by induction heating performed by the induction heater  24 . The fixing roller  20  also serves as a fixing member for melting a toner image T on a sheet P by applying heat to the sheet P. The fixing roller  20  has a multilayered structure. For example, the core  205 , serving as an auxiliary layer, e.g., includes aluminum and has, e.g., a hollow, cylindrical shape. The elastic layer  204  is formed on the core  205 . The heat generating layer  203  is formed on the elastic layer  204 . The silicon rubber layer  202  is formed on the heat generating layer  203 . The releasing layer  201  (e.g., a PFA (perfluoroalkoxy) layer) is formed on the silicon rubber layer  202 . 
       FIG. 3  is a sectional view of a part of the fixing roller  20 . As illustrated in  FIG. 3 , the heat generating layer  203  of the fixing roller  20  includes a magnetic layer  203   a  and/or a low resistance layer  203   b.    
     In addition to a function for maintaining a strength of the whole fixing roller  20 , the core  205  provides a function for serving as an auxiliary layer (e.g., a demagnetizing layer in sense of exhibiting at least reduced ferromagnetic properties relative to the magnetic layer  203   a , if not exhibiting paramagnetic properties or non-magnetic properties) for supporting an effective action of self-control of the temperature of the magnetic layer  203   a . For example, the core  205  is provided at a position in the fixing roller  20 , that is, on an inner circumferential side relative to the heat generating layer  203 . The core  205  has a volume resistivity lower than a volume resistivity of the magnetic layer  203   a  (e.g., a magnetic shunt alloy layer). For example, the core  205  has a volume resistivity, e.g., not greater than about 1.0×10 −7  Ω·m and more particularly, e.g., has a volume resistivity not greater than about 5.0×10 −8  Ω·m. To satisfy the above-described conditions, the core  205  can, e.g., include aluminum. 
     When the core  205  is configured as described above, the magnetic layer  203   a  including the magnetic shunt alloy provides an improved self-control of the temperature. For example, when the temperature of the magnetic layer  203   a  does not reach a Curie point, a magnetic flux generated by the induction heater  24  is concentrated in the heat generating layer  203 , as illustrated by arrows in  FIG. 4A . Thus, the heat generating layer  203  is sufficiently heated by induction heating. When the temperature of the magnetic layer  203   a  reaches a Curie point (i.e., the temperature at which the magnetic layer  203   a  loses its magnetism, or in other words, exhibits paramagnetic properties instead of ferromagnetic properties), a magnetic flux generated by the induction heater  24  penetrates the heat generating layer  203  and reaches the core  205 , as illustrated by arrows in  FIG. 4B . Thus, the heat generating layer  203  is not sufficiently heated by induction heating. Namely, when the temperature of the magnetic layer  203   a  reaches a Curie point, the core  205  functions as a demagnetizing layer. 
     As illustrated in  FIG. 3 , according to this example embodiment, the core  205  including aluminum is used as an auxiliary layer. Alternatively, an auxiliary layer may be provided on an outer circumferential side relative to a core, e.g., stainless steel. Namely, the auxiliary layer is sandwiched between the core and a heat generating layer. In this case, the auxiliary layer may also provide the above-described effects provided by the core  205  serving as an auxiliary layer. 
     The elastic layer  204  is sandwiched between the heat generating layer  203  and the core  205 . According to this example embodiment, the elastic layer  204  includes an elastic material (e.g., a silicon rubber), and has a layer thickness, e.g., not greater than about 5 mm. Thus, the elastic layer  204  is deformable to provide a fixing nip formed between the fixing roller  20  and the pressing roller  30  (depicted in  FIG. 2 ) opposing each other. As a result, a sheet P is properly separated from the fixing roller  20  and the pressing roller  30  after the fixing roller  20  and the pressing roller  30  fix a toner image T on the sheet P. The heat generating layer  203  and the core  205  are not positioned far from each other, resulting in the above-described effects provided by the core  205 . Namely, the layer thickness of the elastic layer  204  can be determined, e.g., to satisfy both a proper separation of a sheet P from the fixing roller  20  and the pressing roller  30  and a proper self-control of the temperature of the fixing roller  20 . 
     The heat generating layer  203  includes the magnetic layer  203   a  and/or the low resistance layer  203   b . The magnetic layer  203   a  has a Curie point in a range, e.g., from about 100 degrees centigrade to about 300 degrees centigrade, for example, a temperature a bit higher than an upper limit of a target fixing temperature. The magnetic layer  203   a  includes magnetic shunt alloys (e.g., an iron-nickel alloy, a copper-nickel alloy, a nickel-iron-chrome alloy, and/or the like). As described above, when the heat generating layer  203  includes the magnetic layer  203   a  having a reference Curie point, the fixing roller  20  is properly heated by induction heating without being excessively heated. The magnetic layer  203   a  may have a desired Curie point when an amount of materials and processing conditions are adjusted. 
     The low resistance layer  203   b  provided on an outer circumferential side (e.g., a side facing the induction heater  24  depicted in  FIG. 2 ) from the magnetic layer  203   a  has a volume resistivity, e.g., not greater than about 1.0×10 −7  Ω·m and more particularly, e.g., has a volume resistivity not greater than about 5.0×10 −8  Ω·m. According to this example embodiment, the low resistance layer  203   b  has a volume resistivity, e.g., of about 1.7×10 −8  Ω·m and includes a non-magnetic material (e.g., copper). The heat generating layer  203  is heated by induction heating caused by a magnetic flux generated by the induction heater  24 , when the magnetic layer  203   a  does not reach a Curie point. 
     According to this example embodiment, in the heat generating layer  203 , an eddy current load obtained by dividing a volume resistivity by a layer thickness varies depending on a position in the width direction (again, along the longitudinal axis) of the fixing roller  20  (i.e., a width direction of the heat generating layer  203 ). As illustrated in  FIG. 5 , the magnetic layer  203   a  has a uniform layer thickness in the width direction (i.e., a thrust direction or an axial direction) of the fixing roller  20 . The low resistance layer  203   b  has a layer thickness varying depending on a position in the width direction of the fixing roller  20 . The heat generating layer  203  has a uniform volume resistivity in the width direction of the fixing roller  20 . 
     As illustrated in  FIG. 3 , the silicon rubber layer  202  has a layer thickness, e.g., not greater than about 500 μm. The silicon rubber layer  202  prevents oxidation of the low resistance layer  203   b  (which can include, e.g., copper), and provides elasticity near the outer circumferential surface of the fixing roller  20 . 
     The releasing layer  201  includes, e.g., a fluorochemical (e.g., PFA) and has a layer thickness, e.g., of about 30 μm. The releasing layer  201  increases a toner releasing property on the outer circumferential surface of the fixing roller  20  directly touching a toner image T on a sheet P (depicted in  FIG. 2 ). 
     As described above, the fixing roller  20  has a multilayered structure including a plurality of layers (e.g., the core  205 , the elastic layer  204 , the heat generating layer  203 , the silicon rubber layer  202 , and/or the releasing layer  201 ). The layer thickness of the plurality of layers of the fixing roller  20  is substantially uniform in the width direction of the fixing roller  20  (i.e., a direction perpendicular to a conveyance direction of a sheet P). Accordingly, the fixing roller  20  has a flat surface, providing proper fixing of a toner image T on a sheet P and a proper conveyance of a sheet P. 
     Referring to  FIG. 2 , the following describes operations of the fixing device  19 . When a driving motor (not shown) rotates the fixing roller  20  in a rotating direction D, the pressing roller  30  rotates in a rotating direction E. A magnetic flux generated by the induction heater  24  heats the fixing roller  20  at an opposing position at which the induction heater  24  opposes the fixing roller  20 . 
     For example, a power source (not shown) applies a current, e.g., a high-frequency alternating current, in a range, e.g., from about 10 kHz to about 1 MHz (more particularly, e.g., in a range from about 20 kHz to about 800 kHz) to the coil  25 . Magnetic lines of force are formed toward the heat generating layer  203 . Directions of the magnetic lines of force alternately switch in opposite directions to form an alternating magnetic field. When the magnetic layer  203   a  (depicted in  FIG. 3 ) has a temperature not greater than a Curie point, an eddy current generates in the heat generating layer  203 . An electric resistance of the heat generating layer  203  generates Joule heat. Thus, the fixing roller  20  is heated by the Joule heat generated by the heat generating layer  203 . 
     A portion on the outer circumferential surface of the fixing roller  20  heated by the induction heater  24  rotates to a contact position (e.g., the fixing nip) at which the fixing roller  20  contacts the pressing roller  30 . At the contact position, the fixing roller  20  applies heat to a sheet P conveyed in the direction Y 1  to melt a toner image T on the sheet P. 
     For example, a guide (not shown) guides a sheet P bearing a toner image T formed in the above-described image forming processes to the fixing nip formed between the fixing roller  20  and the pressing roller  30 . Thus, the sheet P is conveyed in the direction Y 1  and enters the fixing nip. At the fixing nip, the fixing roller  20  and the pressing roller  30  apply heat and pressure to the sheet P to fix the toner image T on the sheet P. The sheet P bearing the fixed toner image T moves out of the fixing nip. 
     The portion on the outer circumferential surface of the fixing roller  20  heated by the induction heater  24  reaches the opposing position at which the induction heater  24  opposes the fixing roller  20  again after moving out of the fixing nip. The above-described operations of the fixing device  19  are repeated to complete a fixing process in an image forming process. 
     In the fixing process, when the magnetic layer  203   a  has a temperature greater than a Curie point, a heat generating level of the heat generating layer  203  is restricted. For example, the temperature of the magnetic layer  203   a  heated by the induction heater  24  exceeds a Curie point, the magnetic layer  203   a  loses its magnetism, and thereby generation of an eddy current is restricted near a surface of the heat generating layer  203 . Thus, Joule heat in a decreased amount generates in the heat generating layer  203 , preventing the heat generating layer  203  from being excessively heated. 
     In the fixing device  19  according to this example embodiment, an eddy current load in the heat generating layer  203  varies depending on a position in the width direction of the fixing roller  20  (i.e., the width direction of the heat generating layer  203 ). 
     Referring to  FIGS. 5 and 6 , the following describes the eddy current load in the heat generating layer  203 .  FIG. 5  illustrates a front view of the fixing roller  20  taken along the width direction (i.e., the longitudinal direction) of the fixing roller  20 .  FIG. 5  further illustrates a sectional view of the heat generating layer  203  corresponding to the width direction of the fixing roller  20 .  FIG. 5  further illustrates a graph showing an eddy current load of the heat generating layer  203  corresponding to the width direction of the fixing roller  20 .  FIG. 6  is a graph illustrating a relationship between an eddy current load and an amount of generated heat of the heat generating layer  203  when the power source applies a current, e.g., a high-frequency alternating current, e.g., of about 30 kHz, to the coil  25  (depicted in  FIG. 2 ). 
     The eddy current load is a factor determining a heat generating property of the heat generating layer  203  and is calculated according to an Equation 1 below. In the Equation 1, “d” represents an eddy current load of the heat generating layer  203 . “ρ” represents a volume resistivity of the heat generating layer  203 . “t” represents a layer thickness of the heat generating layer  203 .
 
 d=ρ/t   Equation 1
 
     However, when the layer thickness t of the heat generating layer  203  is greater than a skin thickness (e.g., a permeance depth) of the heat generating layer  203 , a magnetic flux does not penetrate the heat generating layer  203  and the eddy current load d is calculated according to an Equation 2 below. In the Equation 2, “δ” represents a skin thickness of the heat generating layer  203 .
 
 d=ρ/δ   Equation 2
 
     The skin thickness δ is calculated according to an Equation 3 below. In the Equation 3, “ρ′” represents a volume resistivity of a material. “μ” represents a relative permeability of a material. “f” represents a frequency of an alternating current for exciting a material. 
     
       
         
           
             
               
                 
                   δ 
                   = 
                   
                     5.03 
                     ⁢ 
                     
                       ( 
                       
                         10 
                         3 
                       
                       ) 
                     
                     * 
                     
                       
                         
                           r 
                           ′ 
                         
                         mf 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
               
             
           
         
       
     
     As illustrated in  FIG. 6 , the amount of heat generated by the heat generating layer  203  (depicted in  FIG. 5 ) does not proportionally increase as the eddy current load increases. For example, when the eddy current load is not greater than a reference value (e.g., when the eddy current load is in a range illustrated in an area F), the amount of generated heat of the heat generating layer  203  increases as the eddy current load increases. When the eddy current load is not smaller than a reference value (e.g., when the eddy current load is in a range illustrated in an area G), the amount of generated heat of the heat generating layer  203  decreases as the eddy current load increases. 
     According to this example embodiment, the eddy current load of the heat generating layer  203  is set in the range illustrated in the area G. As illustrated in  FIG. 5 , a center portion of the heat generating layer  203  in the width direction of the fixing roller  20  has an eddy current load greater than an eddy current load of both end portions of the heat generating layer  203  in the width direction of the fixing roller  20 . Namely, according to this example embodiment, the heat generating layer  203  has an eddy current load of three levels. For example, the low resistance layer  203   b  has a layer thickness varying in the width direction of the fixing roller  20 . Thus, the eddy current load of the center portion of the heat generating layer  203  is greater than the eddy current load of the both end portions of the heat generating layer  203  in the width direction of the fixing roller  20 . 
     The both end portions of the heat generating layer  203  in the width direction of the fixing roller  20  may have a decreased temperature. To address this problem, the both end portions have a decreased eddy current load. Thus, the heat generating layer  203  may have a uniform temperature distribution (i.e., a uniform amount of generated heat) in the width direction of the fixing roller  20 . 
       FIG. 7  illustrates a result of an experiment for examining effects of this example embodiment. In  FIG. 7 , a horizontal axis represents a position in the width direction of the fixing roller  20  (depicted in  FIG. 5 ). A line H represents a center position in the width direction of the fixing roller  20 . Lines I and J represent both end positions of an image forming area in the width direction of the fixing roller  20 . A vertical axis represents a surface temperature (e.g., a fixing temperature) of the fixing roller  20 . A graph R 1  illustrates a fixing temperature distribution when the fixing roller  20  of the fixing device  19  (depicted in  FIG. 2 ) according to this example embodiment is used. A graph R 2  illustrates a fixing temperature distribution when the magnetic layer  203   a  (depicted in  FIG. 5 ) having a uniform layer thickness in the width direction of the fixing roller  20  is used. The graphs R 1  and R 2  show that the fixing roller  20  has a uniform temperature distribution in the width direction of the fixing roller  20  when the eddy current load of the heat generating layer  203  (depicted in  FIG. 5 ) may be optimized according to a position in the width direction of the fixing roller  20 . 
     According to this example embodiment, when an eddy current load obtained by dividing a volume resistivity by a layer thickness of the heat generating layer  203  is optimized according to a position in the width direction of the fixing roller  20 , the layer thickness of the low resistance layer  203   b  (depicted in  FIG. 5 ) is a variable, and the volume resistivity of the heat generating layer  203  and the layer thickness of the magnetic layer  203   a  are constants. However, at least one of the layer thickness of the magnetic layer  203   a , the volume resistivity of the magnetic layer  203   a , the layer thickness of the low resistance layer  203   b , and the volume resistivity of the low resistance layer  203   b  may be a variable, so as to optimize the eddy current load of the whole heat generating layer  203  according to a position in the width direction of the fixing roller  20 . 
     As illustrated in  FIG. 2 , the fixing device  19  according to this example embodiment uses an induction heating method and includes the fixing roller  20  including the heat generating layer  203  including the magnetic layer  203   a  (depicted in  FIG. 3 ) having a reference Curie point. Thus, the eddy current load of the heat generating layer  203  varies depending on a position in the width direction of the fixing roller  20 . Thus, the fixing roller  20  may provide an improved heating efficiency with a relatively simple structure, a uniform temperature distribution in the width direction of the fixing roller  20  when heated by the induction heater  24 , proper fixing of a toner image T on a sheet P, and proper prevention of an excessively increased temperature of the fixing roller  20 . 
     According to this example embodiment, the fixing roller  20  is used as the heat generating member. However, the pressing roller  30 , in addition to the fixing roller  20 , may be used as the heat generating member so as to improve a fixing property of the fixing device  19 . In this case, the pressing roller  30  includes a heat generating layer including a magnetic layer having a reference Curie point. A magnetic flux generator is provided at a position opposing the pressing roller  30 . The pressing roller  30  may provide the effects provided by the fixing roller  20  according to this example embodiment, when the eddy current load of the heat generating layer of the pressing roller  30  varies depending on a position in a width direction (i.e., a longitudinal direction) of the pressing roller  30  or the heat generating layer. 
     Referring to  FIG. 8 , the following describes a fixing roller  20   b  including a heat generating layer  203   e   2  according to another example embodiment of the present invention.  FIG. 8  illustrates a front view of the fixing roller  20   b  taken along a longitudinal direction (i.e., a width direction) of the fixing roller  20   b .  FIG. 8  further illustrates a sectional view of the heat generating layer  203   e   2  corresponding to the width direction of the fixing roller  20   b .  FIG. 8  further illustrates a graph showing an eddy current load of the heat generating layer  203   e   2  corresponding to the width direction of the fixing roller  20   b.    
     Like the fixing roller  20  (depicted in  FIG. 3 ), the fixing roller  20   b , serving as the heat generating member and the fixing member, includes the core  205  serving as the auxiliary layer, the elastic layer  204 , the heat generating layer  203   e   2 , the silicon rubber layer  202 , and/or the releasing layer  201  layered in this order. However, the heat generating layer  203   e   2  has a structure different from the structure of the heat generating layer  203  (depicted in  FIG. 5 ). For example, the heat generating layer  203   e   2  includes a magnetic layer  203   a   2 , a low resistance layer  203   b   2 , a second low resistance layer  203   c , and/or a third low resistance layer  203   d . The magnetic layer  203   a   2  and the low resistance layer  203   b   2  have structures common to the magnetic layer  203   a  and the low resistance layer  203   b  (depicted in  FIG. 5 ), respectively, except shapes of the magnetic layer  203   a   2  and the low resistance layer  203   b   2 . Like the low resistance layer  203   b , the second low resistance layer  203   c  and the third low resistance layer  203   d  have a volume resistivity, e.g., not greater than about 5.0×10 −8  Ω·m. Namely, the heat generating layer  203   e   2  includes the low resistance layer  203   b   2 , the second low resistance layer  203   c , and the third low resistance layer  203   d  including three different materials, respectively. 
     Like the heat generating layer  203  (depicted in  FIG. 5 ), according to this example embodiment, an eddy current load of the heat generating layer  203   e   2  is set in the range illustrated in the area G in  FIG. 6 . As illustrated in  FIG. 8 , a center portion of the heat generating layer  203   e   2  in the width direction of the fixing roller  20   b  (i.e., a width direction of the heat generating layer  203   e   2 ) has an eddy current load greater than an eddy current load of both end portions of the heat generating layer  203   e   2  in the width direction of the fixing roller  20   b . Namely, according to this example embodiment, the heat generating layer  203   e   2  has eddy current loads of three levels. For example, the magnetic layer  203   a   2 , the low resistance layer  203   b   2 , the second low resistance layer  203   c , and the third low resistance layer  203   d  have volume resistivities different from each other. Thus, the eddy current load of the center portion of the heat generating layer  203   e   2  is greater than the eddy current load of the both end portions of the heat generating layer  203   e   2  in the width direction of the fixing roller  20   b . The layer thickness of the magnetic layer  203   a   2  varies depending on a position in the width direction of the fixing roller  20   b . The low resistance layer  203   b   2  has a uniform layer thickness. The second low resistance layer  203   c  and the third low resistance layer  203   d  are formed at reference positions in the width direction of the fixing roller  20   b , respectively. 
     The both end portions of the heat generating layer  203   e   2  in the width direction of the fixing roller  20   b  may have a decreased temperature. To address this problem, the both end portions have a decreased eddy current load. Thus, the heat generating layer  203   e   2  may have a uniform temperature distribution (i.e., a uniform amount of generated heat) in the width direction of the fixing roller  20   b , as illustrated in the area G in  FIG. 6 . 
     As described above, the fixing roller  20   b  according to this example embodiment illustrated in  FIG. 8 , like the fixing roller  20  depicted in  FIG. 5 , includes the heat generating layer  203   e   2  including the magnetic layer  203   a   2  having a reference Curie point. The eddy current load of the heat generating layer  203   e   2  varies depending on a position in the width direction of the fixing roller  20   b . Thus, the fixing roller  20   b  may provide an improved heating efficiency with a relatively simple structure, a uniform temperature distribution in the width direction of the fixing roller  20   b  when heated by the induction heater  24  (depicted in  FIG. 2 ) serving as the magnetic flux generator, proper fixing of a toner image T on a sheet P, and proper prevention of an excessively increased temperature of the fixing roller  20   b.    
     Referring to  FIG. 9 , the following describes a fixing roller  20   c  including a heat generating layer  203   e   3  according to yet another example embodiment of the present invention.  FIG. 9  illustrates a front view of the fixing roller  20   c  taken along a longitudinal direction (i.e., a width direction) of the fixing roller  20   c .  FIG. 9  further illustrates a sectional view of the heat generating layer  203   e   3  corresponding to the width direction of the fixing roller  20   c .  FIG. 9  further illustrates a graph showing an eddy current load of the heat generating layer  203   e   3  corresponding to the width direction of the fixing roller  20   c.    
     Like the fixing roller  20  (depicted in  FIG. 3 ), the fixing roller  20   c , serving as the heat generating member and the fixing member, includes the core  205  serving as the auxiliary layer, the elastic layer  204 , the heat generating layer  203   e   3 , the silicon rubber layer  202 , and/or the releasing layer  201  layered in this order. However, the heat generating layer  203   e   3  has a structure different from the structure of the heat generating layer  203  (depicted in  FIG. 5 ). For example, the heat generating layer  203   e   3  includes the magnetic layer  203   a , a low resistance layer  203   b   3 , a second low resistance layer  203   c   3 , and/or a third low resistance layer  203   d   3 . The low resistance layer  203   b   3 , the second low resistance layer  203   c   3 , and the third low resistance layer  203   d   3  have structures common to the structures of the low resistance layer  203   b  (depicted in  FIG. 5 ), the second low resistance layer  203   c  (depicted in  FIG. 8 ), and the third low resistance layer  203   d  (depicted in  FIG. 8 ), respectively, except shapes of the low resistance layer  203   b   3 , the second low resistance layer  203   c   3 , and the third low resistance layer  203   d   3 . Like the low resistance layer  203   b , the second low resistance layer  203   c   3  and the third low resistance layer  203   d   3  have a volume resistivity, e.g., not greater than about 5.0×10 −8  Ω·m. Namely, the heat generating layer  203   e   3  includes the low resistance layer  203   b   3 , the second low resistance layer  203   c   3 , and the third low resistance layer  203   d   3  including three different materials, respectively. 
     Like the heat generating layer  203  (depicted in  FIG. 5 ), according to this example embodiment, an eddy current load of the heat generating layer  203   e   3  is set in the range illustrated in the area G in  FIG. 6 . As illustrated in  FIG. 9 , a center portion of the heat generating layer  203   e   3  in the width direction of the fixing roller  20   c  (i.e., a width direction of the heat generating layer  203   e   3 ) has an eddy current load greater than an eddy current load of both end portions of the heat generating layer  203   e   3  in the width direction of the fixing roller  20   c . Namely, according to this example embodiment, the heat generating layer  203   e   3  has eddy current loads of three levels. For example, the magnetic layer  203   a , the low resistance layer  203   b   3 , the second low resistance layer  203   c   3 , and the third low resistance layer  203   d   3  have volume resistivities different from each other. Thus, the eddy current load of the center portion of the heat generating layer  203   e   3  in the width direction of the fixing roller  20   c  is greater than the eddy current load of the both end portions of the heat generating layer  203   e   3  in the width direction of the fixing roller  20   c . The magnetic layer  203   a  has a uniform layer thickness. The low resistance layer  203   b   3 , the second low resistance layer  203   c   3 , and the third low resistance layer  203   d   3  are formed at reference positions in the width direction of the fixing roller  20   c , respectively. 
     The both end portions of the heat generating layer  203   e   3  in the width direction of the fixing roller  20   c  may have a decreased temperature. To address this problem, the both end portions have a decreased eddy current load. Thus, the heat generating layer  203   e   3  may have a uniform temperature distribution (i.e., a uniform amount of generated heat) in the width direction of the fixing roller  20   c , as illustrated in the area G in  FIG. 6 . 
     As described above, the fixing roller  20   c  according to this example embodiment illustrated in  FIG. 9 , like the fixing roller  20  depicted in  FIG. 5 , includes the heat generating layer  203   e   3  including the magnetic layer  203   a  having a reference Curie point. The eddy current load of the heat generating layer  203   e   3  varies depending on a position in the width direction of the fixing roller  20   c . Thus, the fixing roller  20   c  may provide an improved heating efficiency with a relatively simple structure, a uniform temperature distribution in the width direction of the fixing roller  20   c  when heated by the induction heater  24  (depicted in  FIG. 2 ) serving as the magnetic flux generator, proper fixing of a toner image T on a sheet P, and proper prevention of an excessively increased temperature of the fixing roller  20   c.    
     Referring to  FIG. 10 , the following describes a fixing roller  20   d  including a heat generating layer  203   e   4  according to yet another example embodiment of the present invention.  FIG. 10  illustrates a front view of the fixing roller  20   d  taken along a longitudinal direction (i.e., a width direction) of the fixing roller  20   d .  FIG. 10  further illustrates a sectional view of the heat generating layer  203   e   4  corresponding to the width direction of the fixing roller  20   d .  FIG. 10  further illustrates a graph showing an eddy current load of the heat generating layer  203   e   4  corresponding to the width direction of the fixing roller  20   d.    
     Like the fixing roller  20  (depicted in  FIG. 3 ), the fixing roller  20   d , serving as the heat generating member and the fixing member, includes the core  205  serving as the auxiliary layer, the elastic layer  204 , the heat generating layer  203   e   4 , the silicon rubber layer  202 , and/or the releasing layer  201  layered in this order. However, the heat generating layer  203   e   4  has a structure different from the structure of the heat generating layer  203  (depicted in  FIG. 5 ). For example, the heat generating layer  203   e   4  includes the magnetic layer  203   a  and/or a low resistance layer  203   b   4 . The low resistance layer  203   b   4  has a structure common to the structure of the low resistance layer  203   b  (depicted in  FIG. 5 ), except a shape of the low resistance layer  203   b   4 . For example, the low resistance layer  203   b   4  has a layer thickness that gradually varies. Namely, the low resistance layer  203   b   4  includes a thick portion having a thick layer thickness, a thin portion having a thin layer thickness, and/or a tapered portion. The tapered portion is provided between the thick portion and the thin portion. In the tapered portion, the layer thickness of the low resistance layer  203   b   4  gradually decreases from the layer thickness of the thick portion to the layer thickness of the thin portion. 
     Like the heat generating layer  203  (depicted in  FIG. 5 ), according to this example embodiment, an eddy current load of the heat generating layer  203   e   4  is set in the range illustrated in the area G in  FIG. 6 . As illustrated in  FIG. 10 , a center portion of the heat generating layer  203   e   4  in the width direction of the fixing roller  20   d  (i.e., a width direction of the heat generating layer  203   e   4 ) has an eddy current load greater than an eddy current load of both end portions of the heat generating layer  203   e   4  in the width direction of the fixing roller  20   d . Namely, according to this example embodiment, the heat generating layer  203   e   4  has an eddy current load that gradually varies. 
     The both end portions of the heat generating layer  203   e   4  in the width direction of the fixing roller  20   d  may have a decreased temperature. To address this problem, the both end portions have a decreased eddy current load. Thus, the heat generating layer  203   e   4  may have a uniform temperature distribution (i.e., a uniform amount of generated heat) in the width direction of the fixing roller  20   d , as illustrated in the area G in  FIG. 6 . 
     As described above, the fixing roller  20   d  according to this example embodiment illustrated in  FIG. 10 , like the fixing roller  20  depicted in  FIG. 5 , includes the heat generating layer  203   e   4  including the magnetic layer  203   a  having a reference Curie point. The eddy current load of the heat generating layer  203   e   4  varies depending on a position in the width direction of the fixing roller  20   d . Thus, the fixing roller  20   d  may provide an improved heating efficiency with a relatively simple structure, a uniform temperature distribution in the width direction of the fixing roller  20   d  when heated by the induction heater  24  (depicted in  FIG. 2 ) serving as the magnetic flux generator, proper fixing of a toner image T on a sheet P, and proper prevention of an excessively increased temperature of the fixing roller  20   d.    
     Referring to  FIG. 11 , the following describes a fixing roller  20   e  including a heat generating layer  203   e   5  according to yet another example embodiment of the present invention.  FIG. 11  illustrates a front view of the fixing roller  20   e  taken along a longitudinal direction (i.e., a width direction) of the fixing roller  20   e .  FIG. 11  further illustrates a sectional view of the heat generating layer  203   e   5  corresponding to the width direction of the fixing roller  20   e .  FIG. 11  further illustrates a graph showing a volume resistivity and an eddy current load of the heat generating layer  203   e   5  corresponding to the width direction of the fixing roller  20   e.    
     Like the fixing roller  20  (depicted in  FIG. 3 ), the fixing roller  20   e , serving as the heat generating member and the fixing member, includes the core  205  serving as the auxiliary layer, the elastic layer  204 , the heat generating layer  203   e   5 , the silicon rubber layer  202 , and/or the releasing layer  201  layered in this order. However, the heat generating layer  203   e   5  has a structure different from the structure of the heat generating layer  203  (depicted in  FIG. 5 ). For example, the heat generating layer  203   e   5  includes the magnetic layer  203   a  and/or low resistance layers  203   b   51 ,  203   b   52 , and  203   b   53 . The low resistance layers  203   b   51 ,  203   b   52 , and  203   b   53  have volume resistivities different from each other by varying an amount of filler added to a material of the low resistance layers  203   b   51 ,  203   b   52 , and  203   b   53 . The three low resistance layers  203   b   51 ,  203   b   52 , and  203   b   53  have volume resistivities, e.g., not greater than about 5.0×10 −8  Ω·m, respectively. 
     Unlike the heat generating layer  203  (depicted in  FIG. 5 ), according to this example embodiment, an eddy current load of the heat generating layer  203   e   5  is set in the range illustrated in the area F in  FIG. 6 . As illustrated in  FIG. 11 , a center portion of the heat generating layer  203   e   5  in the width direction of the fixing roller  20   e  (i.e., a width direction of the heat generating layer  203   e   5 ) has a volume resistivity smaller than a volume resistivity of both end portions of the heat generating layer  203   e   5  in the width direction of the fixing roller  20   e . Accordingly, the center portion of the heat generating layer  203   e   5  in the width direction of the fixing roller  20   e  has an eddy current load smaller than an eddy current load of the both end portions of the heat generating layer  203   e   5  in the width direction of the fixing roller  20   e . For example, the magnetic layer  203   a  and the low resistance layers  203   b   51 ,  203   b   52 , and  203   b   53  have volume resistivities different from each other to cause the eddy current load of the center portion of the heat generating layer  203   e   5  in the width direction of the fixing roller  20   e  to be smaller than the eddy current load of the both end portions of the heat generating layer  203   e   5  in the width direction of the fixing roller  20   e . Namely, the magnetic layer  203   a  has a uniform layer thickness. The low resistance layers  203   b   51 ,  203   b   52 , and  203   b   53  also have a uniform layer thickness and are arranged at reference positions in the width direction of the fixing roller  20   e , respectively. 
     The both end portions of the heat generating layer  203   e   5  in the width direction of the fixing roller  20   e  may have a decreased temperature. To address this problem, the both end portions have an increased eddy current load. Thus, the heat generating layer  203   e   5  may have a uniform temperature distribution (i.e., a uniform amount of generated heat) in the width direction of the fixing roller  20   e , as illustrated in the area F in  FIG. 6 . 
     As described above, the fixing roller  20   e  according to this example embodiment illustrated in  FIG. 11 , like the fixing roller  20  depicted in  FIG. 5 , includes the heat generating layer  203   e   5  including the magnetic layer  203   a  having a reference Curie point. The eddy current load of the heat generating layer  203   e   5  varies depending on a position in the width direction of the fixing roller  20   e . Thus, the fixing roller  20   e  may provide an improved heating efficiency with a relatively simple structure, a uniform temperature distribution in the width direction of the fixing roller  20   e  when heated by the induction heater  24  (depicted in  FIG. 2 ) serving as the magnetic flux generator, proper fixing of a toner image T on a sheet P, and proper prevention of an excessively increased temperature of the fixing roller  20   e.    
     Referring to  FIG. 12 , the following describes a fixing roller  20   f  including a heat generating layer  203   e   6  according to yet another example embodiment of the present invention.  FIG. 12  illustrates a front view of the fixing roller  20   f  taken along a longitudinal direction (i.e., a width direction) of the fixing roller  20   f    FIG. 12  further illustrates a sectional view of the heat generating layer  203   e   6  corresponding to the width direction of the fixing roller  20   f .  FIG. 12  further illustrates a graph showing a volume resistivity and an eddy current load of the heat generating layer  203   e   6  corresponding to the width direction of the fixing roller  20   f.    
     Like the fixing roller  20  (depicted in  FIG. 3 ), the fixing roller  20   f , serving as the heat generating member and the fixing member, includes the core  205  serving as the auxiliary layer, the elastic layer  204 , the heat generating layer  203   e   6 , the silicon rubber layer  202 , and/or the releasing layer  201  layered in this order. However, the heat generating layer  203   e   6  has a structure different from the structure of the heat generating layer  203  (depicted in  FIG. 5 ). For example, the heat generating layer  203   e   6  includes the magnetic layer  203   a , a low resistance layer  203   b   6 , a second low resistance layer  203   c   6 , and/or a third low resistance layer  203   d   6 . The low resistance layer  203   b   6 , the second low resistance layer  203   c   6 , and the third low resistance layer  203   d   6  have structures common to the low resistance layer  203   b  (depicted in  FIG. 5 ), the second low resistance layer  203   c  (depicted in  FIG. 8 ), and the third low resistance layer  203   d  (depicted in  FIG. 8 ), respectively, except shapes of the low resistance layer  203   b   6 , the second low resistance layer  203   c   6 , and the third low resistance layer  203   d   6 . Like the low resistance layer  203   b , the second low resistance layer  203   c   6  and the third low resistance layer  203   d   6  have a volume resistivity, e.g., not greater than about 5.0×10 −8  Ω·m. Namely, the heat generating layer  203   e   6  includes the low resistance layer  203   b   6 , the second low resistance layer  203   c   6 , and the third low resistance layer  203   d   6  including three different materials, respectively. 
     Like the heat generating layer  203   e   5  (depicted in  FIG. 11 ), according to this example embodiment, an eddy current load of the heat generating layer  203   e   6  is set in the range illustrated in the area F in  FIG. 6 . As illustrated in  FIG. 12 , a center portion of the heat generating layer  203   e   6  in the width direction of the fixing roller  20   f  (i.e., a width direction of the heat generating layer  203   e   6 ) has a volume resistivity smaller than a volume resistivity of both end portions of the heat generating layer  203   e   6  in the width direction of the fixing roller  20   f . Accordingly, the center portion of the heat generating layer  203   e   6  in the width direction of the fixing roller  20   f  has an eddy current load smaller than an eddy current load of the both end portions of the heat generating layer  203   e   6  in the width direction of the fixing roller  20   f . For example, the magnetic layer  203   a , the low resistance layer  203   b   6 , the second low resistance layer  203   c   6 , and the third low resistance layer  203   d   6  cause the eddy current load of the center portion of the heat generating layer  203   e   6  in the width direction of the fixing roller  20   f  to be smaller than the eddy current load of the both end portions of the heat generating layer  203   e   6  in the width direction of the fixing roller  20   f . Namely, the magnetic layer  203   a  has a uniform layer thickness. The low resistance layer  203   b   6 , the second low resistance layer  203   c   6 , and the third low resistance layer  203   d   6  also have a uniform layer thickness and are arranged at reference positions in the width direction of the fixing roller  20   f , respectively. 
     The both end portions of the heat generating layer  203   e   6  in the width direction of the fixing roller  20   f  may have a decreased temperature. To address this problem, the both end portions have an increased eddy current load. Thus, the heat generating layer  203   e   6  may have a uniform temperature distribution (i.e., a uniform amount of generated heat) in the width direction of the fixing roller  20   f , as illustrated in the area F in  FIG. 6 . 
     As described above, the fixing roller  20   f  according to this example embodiment illustrated in  FIG. 12 , like the fixing roller  20  depicted in  FIG. 5 , includes the heat generating layer  203   e   6  including the magnetic layer  203   a  having a reference Curie point. The eddy current load of the heat generating layer  203   e   6  varies depending on a position in the width direction of the fixing roller  20   f . Thus, the fixing roller  20   f  may provide an improved heating efficiency with a relatively simple structure, a uniform temperature distribution in the width direction of the fixing roller  20   f  when heated by the induction heater  24  (depicted in  FIG. 2 ) serving as the magnetic flux generator, proper fixing of a toner image T on a sheet P, and proper prevention of an excessively increased temperature of the fixing roller  20   f.    
     Referring to  FIG. 13 , the following describes a fixing roller  20   g  including a heat generating layer  203   e   7  according to yet another example embodiment of the present invention.  FIG. 13  illustrates a front view of the fixing roller  20   g  taken along a longitudinal direction (i.e., a width direction) of the fixing roller  20   g .  FIG. 13  further illustrates a sectional view of the heat generating layer  203   e   7  corresponding to the width direction of the fixing roller  20   g .  FIG. 13  further illustrates a graph showing a volume resistivity of the heat generating layer  203   e   7  corresponding to the width direction of the fixing roller  20   g .  FIG. 13  further illustrates a graph showing an eddy current load of the heat generating layer  203   e   7  corresponding to the width direction of the fixing roller  20   g.    
     Like the fixing roller  20  (depicted in  FIG. 3 ), the fixing roller  20   g , serving as the heat generating member and the fixing member, includes the core  205  serving as the auxiliary layer, the elastic layer  204 , the heat generating layer  203   e   7 , the silicon rubber layer  202 , and/or the releasing layer  201  layered in this order. However, the heat generating layer  203   e   7  has a structure different from the structure of the heat generating layer  203  (depicted in  FIG. 5 ). For example, the heat generating layer  203   e   7  includes the magnetic layer  203   a , a low resistance layer  203   b   7 , and/or a second low resistance layer  203   c   7 . The low resistance layer  203   b   7  and the second low resistance layer  203   c   7  have structures common to the low resistance layer  203   b  (depicted in  FIG. 5 ) and the second low resistance layer  203   c  (depicted in  FIG. 8 ), respectively, except shapes of the low resistance layer  203   b   7  and the second low resistance layer  203   c   7 . The low resistance layer  203   b   7  and the second low resistance layer  203   c   7  have a volume resistivity, e.g., not greater than about 5.0×10 −8  Ω·m. Namely, the heat generating layer  203   e   7  includes the low resistance layer  203   b   7  and the second low resistance layer  203   c   7  including two different materials, respectively. 
     Like the heat generating layer  203  (depicted in  FIG. 5 ), according to this example embodiment, an eddy current load of the heat generating layer  203   e   7  is set in the range illustrated in the area G in  FIG. 6 . As illustrated in  FIG. 13 , a center portion of the heat generating layer  203   e   7  in the width direction of the fixing roller  20   g  (i.e., a width direction of the heat generating layer  203   e   7 ) has a volume resistivity greater than a volume resistivity of both end portions of the heat generating layer  203   e   7  in the width direction of the fixing roller  20   g . As illustrated in  FIG. 13 , the center portion of the heat generating layer  203   e   7  in the width direction of the fixing roller  20   g  has an eddy current load greater than an eddy current load of the both end portions of the heat generating layer  203   e   7  in the width direction of the fixing roller  20   g . For example, the magnetic layer  203   a , the low resistance layer  203   b   7 , and the second low resistance layer  203   c   7  cause the center portion of the heat generating layer  203   e   7  in the width direction of the fixing roller  20   g  to have the eddy current load greater than the eddy current load of the both end portions of the heat generating layer  203   e   7  in the width direction of the fixing roller  20   g . The magnetic layer  203   a  has a uniform layer thickness. The layer thickness of each of the low resistance layer  203   b   7  and the second low resistance layer  203   c   7  varies depending on a position in the width direction of the fixing roller  20   g.    
     The both end portions of the heat generating layer  203   e   7  in the width direction of the fixing roller  20   g  may have a decreased temperature. To address this problem, the both end portions have a decreased eddy current load. Thus, the heat generating layer  203   e   7  may have a uniform temperature distribution (i.e., a uniform amount of generated heat) in the width direction of the fixing roller  20   g , as illustrated in the area G in  FIG. 6 . 
     As described above, the fixing roller  20   g  according to this example embodiment illustrated in  FIG. 13 , like the fixing roller  20  depicted in  FIG. 5 , includes the heat generating layer  203   e   7  including the magnetic layer  203   a  having a reference Curie point. The eddy current load of the heat generating layer  203   e   7  varies depending on a position in the width direction of the fixing roller  20   g . Thus, the fixing roller  20   g  may provide an improved heating efficiency with a relatively simple structure, a uniform temperature distribution in the width direction of the fixing roller  20   g  when heated by the induction heater  24  (depicted in  FIG. 2 ) serving as the magnetic flux generator, proper fixing of a toner image T on a sheet P, and proper prevention of an excessively increased temperature of the fixing roller  20   g.    
     Referring to  FIGS. 14 and 15 , the following describes a fixing device  19   h  according to another example embodiment of the present invention.  FIG. 14  is a sectional view of the fixing device  19   h . As illustrated in  FIG. 14 , the fixing device  19   h  includes the induction heater  24  and/or the pressing roller  30  which are common to the fixing device  19  depicted in  FIG. 2 , but further includes an auxiliary fixing roller  50 , a support roller  41 , and/or a fixing belt  60 . Namely, the fixing device  19   h  includes the fixing belt  60  instead of the fixing roller  20  (depicted in  FIG. 2 ) serving as a fixing member for melting a toner image T on a sheet P by applying heat to the sheet P. 
     The fixing device  19   h  fixes a toner image T on a sheet P conveyed in the direction Y 1 . The auxiliary fixing roller  50  includes a core (not shown) and/or an elastic layer (not shown). The core includes stainless steel. The elastic layer includes a silicon rubber and is formed on the core. The elastic layer has a layer thickness, e.g., from about 1 mm to about 5 mm and an Asker hardness, e.g., from about 30 degrees to about 60 degrees. The support roller  41  may include stainless steel and rotates in a rotating direction K. 
     The fixing belt  60  is looped over the auxiliary fixing roller  50  and the support roller  41 . Namely, the auxiliary fixing roller  50  and the support roller  41  serve as rollers for supporting the fixing belt  60 . The fixing belt  60  serves as a heat generating member for generating heat by induction heating performed by the induction heater  24 . The fixing belt  60  also serves as a fixing member for melting a toner image T on a sheet P by applying heat to the sheet P. 
       FIG. 15  is a sectional view of a part of the fixing belt  60 . As illustrated in  FIG. 15 , the fixing belt  60  includes an auxiliary layer  605 , an elastic layer  604 , a heat generating layer  603 , a silicon rubber layer  602 , and/or a releasing layer  601 . The heat generating layer  603  includes a magnetic layer  603   a  and/or a low resistance layer  603   b . The auxiliary layer  605 , the elastic layer  604 , the heat generating layer  603 , the silicon rubber layer  602 , and the releasing layer  601  are layered in this order from an inner circumferential side to an outer circumferential side of the fixing belt  60 , and have structures similar to the structures of the core  205 , the elastic layer  204 , the heat generating layer  203 , the silicon rubber layer  202 , and the releasing layer  201  depicted in  FIG. 3 , respectively. The heat generating layer  603  has an eddy current load varying depending on a position in a width direction of the fixing belt  60  (i.e., a width direction of the heat generating layer  603 ). 
     The fixing belt  60  rotates in a rotating direction L (depicted in  FIG. 14 ). When the temperature of the magnetic layer  603   a  does not reach a Curie point, the induction heater  24  (depicted in  FIG. 14 ) heats the heat generating layer  603  by generating a magnetic flux. 
     Referring to  FIGS. 14 and 15 , the following describes operations of the fixing device  19   h . The auxiliary fixing roller  50  is driven to rotate the fixing belt  60  in the rotating direction L. The rotating fixing belt  60  rotates the support roller  41  in the rotating direction K. Accordingly, the pressing roller  30  rotates in a rotating direction M. The induction heater  24  opposes the fixing belt  60  at an opposing position at which the induction heater  24  heats the fixing belt  60 . 
     For example, a power source (not shown) applies a high-frequency alternating current in a range, e.g., from about 10 kHz to about 1 MHz (more particularly, e.g., in a range from about 20 kHz to about 800 kHz) to the coil  25 . Magnetic lines of force are formed toward the heat generating layer  603 . Directions of the magnetic lines of force alternately switch in opposite directions to form an alternating magnetic field. An eddy current generates in the heat generating layer  603 . An electric resistance of the heat generating layer  603  generates Joule heat. Thus, the fixing belt  60  is heated by the Joule heat generated by the heat generating layer  603 . 
     A portion on an outer circumferential surface of the fixing belt  60  heated by the induction heater  24  moves to a contact position (e.g., a fixing nip) at which the fixing belt  60  contacts the pressing roller  30 . At the contact position, the fixing belt  60  applies heat to a sheet P conveyed in the direction Y 1  to fix a toner image T on the sheet P. 
     The portion on the outer circumferential surface of the fixing belt  60  heated by the induction heater  24  reaches the opposing position at which the induction heater  24  opposes the fixing belt  60  again after moving out of the fixing nip. The above-described operations of the fixing device  19  are repeated to complete a fixing process in an image forming process. 
     As described above, the fixing belt  60  according to this example embodiment includes the heat generating layer  603  including the magnetic layer  603   a  having a reference Curie point. An eddy current load of the heat generating layer  603  varies depending on a position in the width direction of the fixing belt  60 . Thus, the fixing belt  60  may provide an improved heating efficiency with a relatively simple structure, a uniform temperature distribution in the width direction of the fixing belt  60  when heated by the induction heater  24  serving as a magnetic flux generator for generating a magnetic flux, proper fixing of a toner image T on a sheet P, and proper prevention of an excessively increased temperature of the fixing belt  60 . 
     According to this example embodiment, the fixing belt  60  is used as the heat generating member. However, both the support roller  41  and the fixing belt  60  may be used as the heat generating members. In this case, the support roller  41  and the fixing belt  60  may provide the effects provided by the fixing belt  60  according to this example embodiment. 
     According to this example embodiment, the fixing belt  60  includes the auxiliary layer  605  including aluminum. However, the support roller  41  may include aluminum to serve as an auxiliary layer. In this case, the fixing belt  60  may not include the auxiliary layer  605 . Thus, the support roller  41  and/or the fixing belt  60  may provide the effects provided by the fixing belt  60  according to this example embodiment. 
     The present invention has been described above with reference to specific example embodiments. Nonetheless, the present invention is not limited to the details of example embodiments described above, but various modifications and improvements are possible without departing from the spirit and scope of the present invention. It is therefore to be understood that within the scope of the associated claims, the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative example embodiments may be combined with each other and/or substituted for each other within the scope of the present invention.