Patent Publication Number: US-9411280-B2

Title: Fixing device and image forming apparatus incorporating same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application Nos. 2013-088663, filed on Apr. 19, 2013, and 2014-072640, filed on Mar. 31, 2014, in the Japan Patent Office, the entire disclosures of which are hereby incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     Embodiments of this disclosure generally relate to a fixing device and to an electrophotographic image forming apparatus incorporating the fixing device. 
     2. Related Art 
     Some typical heating devices include a heat-resistant film, a heat generator and a pressure member. The heat generator is fixedly supported at the center in a lateral direction on one side of a heater substrate and divided into six sub-heat generators in a direction perpendicular to a direction in which a recording medium is conveyed. The pressure member is disposed facing the heat generator via the heat-resistant film. The pressure member and the heat generator are pressed against each other via the heat-resistant film to form an area of contact referred to herein as a fixing nip therebetween. The recording medium is conveyed to the fixing nip, between the heat-resistant film and the pressure member, thereby passing through the fixing nip together with the heat-resistant film. Thus, heat energy is transmitted from the heat generator to the recording medium via the heat-resistant film. 
     Such heating devices are capable of significantly reducing energy consumption by selectively supplying power to the sub-heat generators for individually heating their respective images, thereby reaching a fixing temperature. 
     In addition, the heating devices include electrical conduction paths to supply power to the heat generator. The electrical conduction paths are provided, e.g., in a substantially symmetrical manner, on both sides of the heater substrate in the lateral direction thereof with the six sub-heat generators interposed therebetween. 
     Some other typical heating devices, specifically image heating devices, include a heating rotator to heat an image carried by a recording medium, and a heating member to contact and heat the surface of the heating rotator. The heating member includes a heater having a heat generator on a heat-conductive substrate, and a heat-conductive member that contacts the substrate. The heat-conductive member is made of a material having a higher heat conductivity than a material of the substrate. Thus, the typical image heating devices have a heating area allowing both of the heater and the heat-conductive member to heat the surface of the heating rotator. 
     With such a configuration, the typical image heating devices can shorten warm-up time and lower the temperature of the heater. 
     Some of them also include substantially one heat generator disposed at a position downstream in a rotational direction of a fixing roller in the heating area. 
     However, with such a typical wiring pattern, in which electrical conduction paths are disposed on both sides of the heater substrate in the lateral direction thereof, with the heat generator interposed therebetween in a substantially symmetrical manner, a space, that is, a line width of wiring is insufficient to dispose the heat generator having a plurality of heat-generating areas, at a position closer to the fixing nip. 
     SUMMARY 
     In one embodiment of this disclosure, an improved fixing device for fixing an unfixed toner image formed on a recording medium onto the recording medium under heat and pressure in a fixing nip includes a first rotator, a second rotator pressed against the first rotator to form the fixing nip, and a heater to heat the first rotator. The heater includes an elongated substrate extending in an axial direction of the first rotator, a heat generator disposed on a surface of the substrate facing the first rotator, at a position downstream in a rotational direction of the first rotator, a plurality of first conduction paths connected to the heat generator and grounded downstream of the heat generator in the rotational direction of the first rotator, and a second conduction path on which the plurality of first conduction paths are grounded. The heat generator has a heat-generating area divided into a plurality of sub-heat-generating areas in the axial direction of the first rotator. The plurality of sub-heat-generating areas are grounded on the second conduction path via the plurality of first conduction paths. 
     In another embodiment of this disclosure, an improved fixing device for fixing an unfixed toner image formed on a recording medium onto the recording medium under heat and pressure in a fixing nip includes a first rotator, a second rotator pressed against the first rotator to form a fixing nip, and a heater to heat the first rotator. The heater includes an elongated substrate extending in an axial direction of the first rotator, a heat generator disposed on a surface of the substrate facing the first rotator, at a position downstream in a rotational direction of the first rotator, and a plurality of conduction paths connected to the heat generator and grounded downstream of the heat generator in the rotational direction of the first rotator. The elongated substrate has a plurality of through-holes. The heat generator has a heat-generating area divided into a plurality of sub-heat-generating areas in the axial direction of the first rotator. The plurality of conduction paths are wired on a backside of the elongated substrate via the through-holes and grounded. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be more readily obtained as the same becomes better understood by reference to the following detailed description of embodiments when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic overall view of an image forming apparatus according to embodiments of this disclosure; 
         FIG. 2  is a schematic sectional view of a fixing device according to a first embodiment incorporated in the image forming apparatus of  FIG. 1 ; 
         FIG. 3  is a sectional view of a heater incorporated in the fixing device of  FIG. 2 ; 
         FIG. 4  is a plan view of the heater of  FIG. 3 , with a holder removed therefrom; 
         FIG. 5A  is a plan view of a recording medium, illustrating an image formation pattern, with sub-heat-generating areas of a heat generator; 
         FIG. 5B  is a graph of a relationship between the respective fixing temperatures of the sub-heat-generating areas and positions of the recording medium in a longitudinal direction thereof; 
         FIG. 6  is a sectional view of a heater; 
         FIG. 7  is a plan view of the heater of  FIG. 6 , with a holder removed therefrom; 
         FIG. 8  is a schematic sectional view of a fixing device according to a second embodiment; 
         FIG. 9  is a schematic sectional view of a fixing device according to a third embodiment; and 
         FIG. 10  is a schematic sectional view of a fixing device according to a fourth embodiment. 
     
    
    
     The accompanying drawings are intended to depict embodiments of this 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 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have the same function, operate in a similar manner, and achieve similar results. 
     Although the embodiments are described with technical limitations with reference to the attached drawings, such description is not intended to limit the scope of the invention and all of the components or elements described in the embodiments of this disclosure are not necessarily indispensable to the present invention. 
     In a later-described comparative example, embodiment, and exemplary variation, for the sake of simplicity like reference numerals are given to identical or corresponding constituent elements such as parts and materials having the same functions, and redundant descriptions thereof omitted unless otherwise required. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, embodiments of this disclosure are described below. 
     Initially with reference to  FIG. 1 , a description is given of an overall configuration and operation of an image forming apparatus  100  according to some embodiments of this disclosure. 
       FIG. 1  is a schematic overall view of the image forming apparatus  1  according to some embodiments of this disclosure. 
     It is to be noted that, in the following description, suffixes Y, M, C, and K denote colors yellow, magenta, cyan, and black, respectively, and may be omitted where unnecessary. 
     As illustrated in  FIG. 1 , the image forming apparatus  1  of this embodiment is a tandem-type color printer. A bottle container  101  is disposed in an upper portion of the image forming apparatus  1 . The bottle container  101  accommodates four removable toner bottles  102 Y,  102 M,  102 C, and  102 K. The toner bottles  102 Y,  102 M,  102 C, and  102 K accommodates toner of yellow, magenta, cyan, and black, respectively. Thus, the four toner bottles  102 Y,  102 M,  102 C, and  102 K are replaceable. 
     An intermediate transfer unit  85  is disposed below the bottle container  101 . The intermediate transfer unit  85  includes, e.g., an intermediate transfer belt  78 , four 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 cleaner  80 . 
     Four imaging units  4 Y,  4 M,  4 C, and  4 K are arranged side by side, facing the intermediate transfer belt  78  to form toner images of yellow, magenta, cyan, and black, respectively. 
     The four imaging units  4 Y,  4 M,  4 C, and  4 K include photoconductive drums  5 Y,  5 M,  5 C, and  5 K, respectively. The photoconductive drums  5 Y,  5 M,  5 C, and  5 K serve as image carriers. Each of the photoconductive drums  5 Y,  5 M,  5 C, and  5 K is surrounded by various pieces of imaging equipment, such as a charging device  75 , a developing device  76 , a cleaning device  77 , and a charge neutralizing device. Imaging processes, namely, charging, exposure, development, transfer, and cleaning processes are performed on each of the photoconductive drums  5 Y,  5 M,  5 C, and  5 K. Accordingly, the toner images of yellow, magenta, cyan, and black are formed on the photoconductive drums  5 Y,  5 M,  5 C, and  5 K, respectively. 
     The photoconductive drums  5 Y,  5 M,  5 C, and  5 K are rotated in a counterclockwise direction in  FIG. 1  by a driving motor. Surfaces of the photoconductive drums  5 Y,  5 M,  5 C, and  5 K are uniformly charged where the surfaces of the photoconductive drums  5 Y,  5 M,  5 C, and  5 K face the respective charging devices  75  (charging process). Then, as the photoconductive drums  5 Y,  5 M,  5 C, and  5 K rotate and reach a position opposite an exposure device  3 , the surfaces of the photoconductive drums  5 Y,  5 M,  5 C, and  5 K are scanned with and exposed by laser light L emitted from the exposure device to form electrostatic latent images of yellow, magenta, cyan and black on the photoconductive drums  5 Y,  5 M,  5 C, and  5 K, respectively (exposure process). 
     Then, the photoconductive drums  5 Y,  5 M,  5 C, and  5 K rotate further and reach a position at which the surfaces of the photoconductive drums  5 Y,  5 M,  5 C, and  5 K face the respective development devices  76 , where the electrostatic latent images are developed with toner of yellow, magenta, cyan and black into visible images, also known as toner images of yellow, magenta, cyan and black, respectively (development process). Then, the photoconductive drums  5 Y,  5 M,  5 C, and  5 K rotate further and reach a position at which the surfaces of the photoconductive drums  5 Y,  5 M,  5 C, and  5 K face primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K, respectively, via the intermediate transfer belt  78 , where the toner images are transferred from the photoconductive drums  5 Y,  5 M,  5 C, and  5 K onto the intermediate transfer belt  78  (primary-transfer process). At this time, a small amount of toner may remain untransferred on the surfaces of the photoconductive drums  5 Y,  5 M,  5 C, and  5 K as residual toner. 
     Then, the photoconductive drums  5 Y,  5 M,  5 C, and  5 K rotate further and reach a position at which the surfaces of the photoconductive drums  5 Y,  5 M,  5 C, and  5 K face the respective cleaning devices  77 , where the cleaning devices  77  mechanically collect the residual toner on the surfaces of the photoconductive drums  5 Y,  5 M,  5 C, and  5 K with cleaning blades incorporated in the cleaning devices  77 , respectively (cleaning process). 
     Finally, the photoconductive drums  5 Y,  5 M,  5 C, and  5 K rotate and reach a position at which the surfaces of the photoconductive drums  5 Y,  5 M,  5 C, and  5 K face the respective charge neutralizing devices, where residual potential is removed from the surfaces of the photoconductive drums  5 Y,  5 M,  5 C, and  5 K. 
     Thus, a series of image forming processes performed on the surfaces of the photoconductive drums  5 Y,  5 M,  5 C, and  5 K is completed. 
     After the series of image forming processes, the toner images formed on the surfaces of the photoconductive drums  5 Y,  5 M,  5 C, and  5 K through the development process are transferred onto the intermediate transfer belt  78  while being superimposed one atop another to form a multicolor toner image on the intermediate transfer belt  78 . 
     The intermediate transfer belt  78  is stretched over the secondary-transfer backup roller  82 , the cleaning backup roller  83 , and the tension roller  84 , and rotated in a direction indicated by arrow A in  FIG. 1  by rotation of the secondary-transfer backup roller  82 . 
     The four primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K and the photoconductive drums  5 Y,  5 M,  5 C, and  5 K press against each other to form areas of contact via the intermediate transfer belt  78  herein called primary transfer nips, respectively. 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 a polarity of toner. 
     The intermediate transfer belt  78  travels in the direction indicated by arrow A and successively passes through the primary transfer nips formed between the primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K, on the one hand, and the photoconductive drums  5 Y,  5 M,  5 C, and  5 K, respectively, on the other. Thus, the toner images formed on the respective photoconductive drums  5 Y,  5 M,  5 C, and  5 K are primarily transferred onto the intermediate transfer belt  78  while being superimposed one atop another. 
     Then, the intermediate transfer belt  78  carrying the multicolor toner image reaches a position at which the intermediate transfer belt  78  faces the secondary transfer roller  89 , where the secondary transfer backup roller  82  and the secondary transfer roller  89  press against each other to form an area of contact via the intermediate transfer belt  78 , herein called a secondary transfer nip. 
     The multicolor color toner image formed on the intermediate transfer belt  78  is transferred onto a recording medium R at the secondary transfer nip. At this time, a small amount of toner may remain untransferred on the intermediate transfer belt  78  as residual toner. Then, the intermediate transfer belt  78  reaches a position at which the intermediate transfer belt  78  faces the intermediate transfer cleaner  80 . At the position, the residual toner on the intermediate transfer belt  78  is collected. 
     Thus, a series of transfer processes performed on the intermediate transfer belt  78  is completed. 
     Now, a detailed description is given of movement of the recording medium R. The recording medium R is fed by a sheet tray  12  disposed in a lower portion of the image forming apparatus  1 , and conveyed to the secondary transfer nip via a feed roller  97  and a pair of registration rollers  98  pressed against each other. Specifically, the sheet tray  12  accommodates a stack of recording media R, such as transfer sheets, one atop another. 
     When the feed roller  97  is rotated in a counterclockwise direction in  FIG. 1 , an uppermost recording medium R of the plurality of recording media R is fed toward an area of contact of rollers of the pair of registration rollers  98 . 
     The recording medium R conveyed to the pair of registration rollers  98  temporarily stops at a predetermined position as the pair of registration rollers  98  stops rotating. 
     The pair of registration rollers  98  is rotated again to convey the recording medium R to the secondary transfer nip in synchronization with the movement of the intermediate transfer belt  78  carrying the multicolor toner image. Thus, the multicolor toner image is transferred onto the recording medium R. 
     Thereafter, the recording medium R carrying the multicolor toner image is conveyed to a fixing device  20 , described later. In the fixing device  20 , the multicolor toner image is fixed onto the recording medium R under heat and pressure applied by an endless fixing belt  21  and a pressing roller  25 . 
     Then, the recording medium R passes through rollers of a pair of output rollers  99  pressed against each other, and is discharged onto an output tray  100  outside the image forming apparatus  1 . 
     Thus, the plurality of recording media R carrying output images rest one atop another on the output tray  100 . Accordingly, a series of image forming processes performed in the image forming apparatus  1  is completed. 
     Referring now to  FIGS. 2 to 5 , a detailed description is given of the fixing device  20  according to a first embodiment. Initially with reference  FIG. 2 , a description is given of an overall configuration of the fixing device  20 . 
       FIG. 2  is a schematic sectional view of the fixing device  20  according to the first embodiment, incorporated in the image forming apparatus  1  described above. As illustrated in  FIG. 2 , the fixing device  20  according to the first embodiment includes the fixing belt  21  serving as a first rotator, a stationary member  22 , a base  22   a , a heater  23  that directly contacts and heats the fixing belt  21 , a pressure roller  24 , and the pressing roller  25  serving as a second rotator. The fixing belt  21  and the components disposed inside a loop defined by the fixing belt  21 , that is, the stationary member  22 , the base  22   a , and the heater  23  may constitute a belt unit  121  separably coupled with the pressure roller  24  and the pressing roller  25 . 
     The fixing belt  21  is a thin, flexible, endless belt having a predetermined width, constructed of a base layer, an elastic layer, and a release layer resting in this order from an inner circumferential surface side thereof. The fixing belt  21  has a total thickness not greater than 1 mm. The base layer of the fixing belt  21  has a thickness of, e.g., about 30 μm to about 100 μm, and is made of a metal material, such as nickel or stainless steel, or a resin material such as polyimide. 
     The elastic layer of the fixing belt  21  has a thickness of, e.g., about 100 μm to about 300 μm, and is made of a rubber material such as silicon rubber, silicon rubber foam, or fluoro rubber. The elastic layer eliminates slight surface asperities of the fixing belt  21  in an area of contact between the pressing roller  25  and the fixing belt  21 , herein called a fixing nip FN. Accordingly, heat is uniformly transmitted to a toner image T on the recording medium R, thereby suppressing formation of a rough image such as an orange peel image. 
     The release layer of the fixing belt  21  has a thickness of, e.g., about 10 μm to about 50 μm. The release layer is made of, e.g., tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), polytetrafluoroethylene (PTFE), polyimide, polyetherimide, or polyether sulfide (PES). The release layer ensures releasability of the toner image T. In other words, the release layer reliably separates the toner image T from the fixing belt  21 . 
     The stationary member  22  is a member having a substantially square bar shape. The stationary member  22  has a flat surface to form the fixing nip FN in concert with the pressing roller  25 , described later. The stationary member  22  is positioned inside the loop defined by the fixing belt  21 , and both ends of the stationary member  22  are supported by a frame of the fixing device  20 . The stationary member  22  has another flat surface opposite the flat surface for forming the fixing nip FN, on which the base  22   a  is mounted to support the heater  23 , described later. 
     Referring now to  FIG. 3 , a detailed description is given of the heater  23 . 
       FIG. 3  is a sectional view of the heater  23  incorporated in the fixing device  20  described above. The heater  23  is constructed of at least a substrate  231 , a heat generator  232 , and an overcoat layer  234 . In this embodiment, the heater  23  also includes a holder  235 . As illustrated in  FIG. 3 , the substrate  231  rests on the holder  235 . The heat generator  232  rests on the substrate  231 . The overcoat layer  234  coats the substrate  231  and the heat generator  232 . 
     The substrate  231  has an elongated shape extending in an axial direction, that is, a width direction of the fixing belt  21 . The substrate  231  is a glass substrate having print wiring on a surface facing the fixing belt  21 . Wiring patterns of the print wiring are described later. 
     The heat generator  232  is a ceramic heater having an elongated planar shape. As illustrated in  FIG. 3  and also in  FIG. 4  referred to later, the heat generator  232  is disposed on the surface of the substrate  231  facing the fixing belt  21 , at a position downstream in a rotational direction of the fixing belt  21 . The heat generator  232  has a heat-generating area divided into a plurality of sub-heat-generating areas in the axial direction of the fixing belt  21  according to the wiring patterns of print wiring, described later. 
     Referring now to  FIG. 4 , a detailed description is given of a first example of the wiring patterns of print wiring. 
       FIG. 4  is a plan view of the heater  23 , with a holder removed therefrom, illustrating the first example of a wiring pattern. The first example of a wiring pattern for supplying power to the heat generator  232  includes a plurality of first conduction paths (connection terminals) W 1 , a second conduction path W 2 , and a plurality of third conduction paths W 3 . 
     The plurality of first conduction paths W 1  are connected to both sides of the heat generator  232  in a lateral direction thereof, that is, the horizontal direction in  FIG. 4 , and aligned at a predetermined interval in a longitudinal direction of the heat generator  232 , that is, the vertical direction in  FIG. 4 . 
     A description is now given of power supply to the heat generator  232  and grounding of the heat generator  232 . 
     The substrate  231  has a planar shape elongated in the axial direction of the fixing belt  21 . The fixing belt  21  moves along the overcoat layer  234  that covers the surface of the substrate  231  facing the fixing belt  21  out of two surfaces of the substrate  231 , namely, front and back surfaces. The heat generator  232  is disposed in such a manner that the longitudinal direction thereof is along a longitudinal direction of the substrate  231 . The respective longitudinal directions of the substrate  231  and the heat generator  232  correspond to the axial direction of the fixing belt  21 . The respective lateral directions of the substrate  231  and the heat generator  232  correspond to the rotational direction of the fixing belt  21 . 
     As illustrated in  FIG. 4 , the plurality of first conduction paths W 1  are disposed on both sides of the heat generator  232  in the lateral direction thereof at a predetermined interval along the longitudinal direction of the heat generator  232 . 
     The plurality of first conduction paths W 1  include a plurality of first conduction paths W 1 A on one side of the heat generator  232  in the lateral direction thereof, and the plurality of first conduction paths W 1 B on the other side of the heat generator  232  in the lateral direction thereof. More specifically, the plurality of first conduction paths W 1 A are disposed downstream of the heat generator  232  in the rotational direction of the fixing belt  21  while the plurality of first conduction paths W 1 B are disposed upstream of the heat generator  232  in the rotational direction of the fixing belt  21 . Power is supplied to the plurality of first conduction paths W 1 B, and the plurality of first conduction paths W 1 A are grounded. Thus, a circuit is configured to supply power to the heat generator  232 . With such a configuration, the heat-generating area of the heat generator  232  generates heat. 
     In this embodiment, power is supplied to each unit of the plurality of first conduction paths W 1  to heat the corresponding sub-heat-generating area. Each of the plurality of third conduction paths W 3  is disposed to supply power to the corresponding unit of the plurality of first conduction paths W 1 . Thus, the plurality of third conduction paths W 3  are disposed as a wiring pattern for power supply. The plurality of third conduction paths W 3  are disposed on the surface of the substrate  231  facing the fixing belt  21 , on one side of the heat generator  232  in the lateral direction thereof, and more specifically, upstream of the heat generator  232  in the rotational direction of the fixing belt  21 . 
     The plurality of third conduction paths W 3  are bidimentionally arranged side by side. Accordingly, the wiring pattern for power supply occupies a considerable area on the substrate  231 , and more specifically, upstream of the heat generator  232  in the rotational direction of the fixing belt  21 . 
     Thus, according to this embodiment, the plurality of third conduction paths W 3  are disposed upstream of the heat generator  232  in the rotational direction of the fixing belt  21  to ensure a sufficient space, that is, a sufficient line width of wiring to dispose the heat generator  232  on the substrate  231 , at a position downstream in the rotational direction of the fixing belt  21 , that is, closer to the fixing nip FN. 
     As described above, the plurality of first conduction paths W 1 A are disposed on the surface of the substrate  231  facing the fixing belt  21 , on one side of the heat generator  232  in the lateral direction thereof, and more specifically, downstream of the heat generator  232  in the rotational direction of the fixing belt  21 . The plurality of first conduction paths W 1 A are grounded on a single elongated conduction path, that is, the second conduction path W 2 , and thus connected thereto. The second conduction path W 2  extends in the longitudinal direction of the heat generator  232 . The second conduction path W 2  is disposed on the substrate  231 , downstream of the heat generator  232  in the rotational direction of the fixing belt  21 , as a grounding wiring pattern. 
     As described above, the plurality of first conduction paths W 1 A are commonly grounded on a single conduction path, that is, the second conduction path W 2 . Accordingly, the plurality of first conduction paths W 1 A are not necessarily controlled individually. 
     In this embodiment, all the plurality of first conduction paths W 1 A connected to the heat generator  232  and disposed downstream of the heat generator  232  in the rotational direction of the fixing belt  21  are grounded on the second conduction path W 2 . 
     Each of the plurality of first conduction paths W 1 A belongs to any one of the sub-heat-generating areas of the heat generator  232 . In other words, the sub-heat-generating areas of the heat generator  232  are grounded via the plurality of first conduction paths W 1 A. Accordingly, the sub-heat-generating areas of the heat generator  232  are commonly grounded via the plurality of first conduction paths W 1 A on the single second conduction path W 2 . 
     The above-description is given of power supply to the heat generator  232  and grounding of the heat generator  232 . 
     The first conduction paths W 1 B are connected to the plurality of third conduction paths W 3 . Specifically, each of the plurality of third conduction paths W 3  is connected to a predetermined number of the first conduction paths W 1 B, which, in the example shown in  FIG. 4 , is four first conduction paths W 1 B. The plurality of third conduction paths W 3  extend in the longitudinal direction of the heat generator  232 . In the example shown in  FIG. 4 , three third conduction paths W 3  are disposed upward from a center in the longitudinal direction of the heat generator  232 . Similarly, three other third conduction paths W 3  are disposed downward from the center in the longitudinal direction of the heat generator  232 . 
     When a given third conduction path W 3  of the plurality of third conduction paths W 3  is supplied with power, an area of the heat generator  232  corresponding to the given third conduction paths W 3  generates heat. Thus, the heat-generating area of the heat generator  232  is divided into a plurality of sub-heat-generating areas in the axial direction of the fixing belt  21 . 
     According to the above-described first embodiment, one heat generator  232  is illustrated. Alternatively, a plurality of heat generators  232  may be linearly aligned and connected to the plurality of first conduction paths W 1 A, respectively. In such a case, the plurality of first conduction paths W 1 A may be commonly grounded on the conduction path W 2 . Alternatively, a thermal head may be used. 
     Referring back to  FIG. 3 , the overcoat layer  234  covers the substrate  231 , the heat generator  232 , the plurality of first conduction paths W 1 , the second conduction path W 2  and the plurality of third conduction paths W 3 . Preferably, the overcoat layer  234  is made of a material having good heat conductivity. 
     As illustrated in  FIG. 3 , the holder  235  is a bar-like member having a substantially H-shaped cross-section to hold the substrate  231 . The holder  235  is made of, e.g., a resin material having good heat resistance. As illustrated in  FIG. 2 , the base  22   a  engages a bottom of the holder  235 . Accordingly, the heater  23  is coupled to the stationary member  22 . 
     The pressure roller  24  is constructed of a metal core  241  and an elastic layer  242  covering the metal core  241 . The elastic layer  242  is made of, e.g., silicon rubber foam, silicon rubber, or fluoro rubber. 
     The pressure roller  24  is rotatable, and configured to be pressed against the heater  23  to form an area of contact via the fixing belt  21 , herein called a nip to transmit heat from the heat generator  232  to the fixing belt  21 . Preferably, the elastic layer  242  is covered by a heat insulation layer made of a flexible, high-heat insulation material. 
     The heater  23  has the overcoat layer  234 , as an outmost layer of the heater  23 , slidably pressing an inner surface of the fixing belt  21 . The heater  23  and associated components are disposed inside the loop defined by the fixing belt  21 , thereby enhancing effective use of space. The overcoat layer  234  directly contacts the inner surface of the fixing belt  21 . The contact area therebetween is a surface-contact area, which is larger than a point-contact area or a line-contact area. Accordingly, heat is transmitted from the heater  23  to the fixing belt  21  with relatively high heat conductivity. 
     The pressing roller  25  is constructed of a metal core  251 , and an elastic layer  252  covering the metal core  251 . The pressure roller  24  is rotatable, and configured to be pressed against the stationary member  22  via the fixing belt  21 . Thus, a desired fixing nip FN is formed between the pressing roller  25  and the fixing belt  21 . 
     The elastic layer  252  of the pressing roller  25  is made of, e.g., silicon rubber foam, silicon rubber, or fluoro rubber. Preferably, the elastic layer  252  is covered by a thin release layer made of, e.g., PFA or PTFE. 
     A description is now given of operation and effects of the fixing device  20  according to the first embodiment. 
     As described above, the toner image T formed on the recording medium R is fixed onto the recording medium R under heat and pressure applied by the fixing belt  21  and the pressing roller  25 . Thus, a multicolor image is formed on the recording medium R. In the image forming apparatus  1  incorporating the fixing device  20  according to the first embodiment, the heat generator  232  has a plurality of sub-heat-generating areas for selectively generating heat. In other words, the image forming apparatus  1  incorporating the fixing device  20  according to the first embodiment can divide the recording medium R into an imaged area and a blank area according to image data, and only the imaged area is heated at a fixing temperature when the unfixed toner image T formed on the recording medium R is fixed onto the recording medium R. 
     A specific example is illustrated in  FIGS. 5A and 5B . 
       FIG. 5A  is a plan view of a recording medium R, illustrating an image formation pattern, with a plurality of sub-heat-generating areas A through F of the heat generator  232 .  FIG. 5B  is a graph of a relationship between the respective fixing temperatures of the sub-heat-generating areas A through F and positions of the recording medium R in a longitudinal direction thereof. 
     In this example, the unfixed toner image T of an image formation pattern including a blank area carried by the recording medium R illustrated in  FIG. 5A  is fixed under heat generated by the heat generator  232  having the plurality of sub-heat-generating areas A through F. Initially, the image forming apparatus  1  divides the recording medium R into an imaged area and a blank area according to the image data, and controls the plurality of sub-heat-generating areas A through F to achieve the respective fixing temperatures illustrated in  FIG. 5B . 
     Specifically, an image is formed in an area between a position P1 and a position P2 in the longitudinal direction of the recording medium R across the width of the recording medium R. That is, the area between the position P1 and the position P2 is an imaged area. The image forming apparatus  1  supplies power to all the sub-heat-generating areas A through F, thereby controlling the respective fixing temperatures to reach a fixing temperature T2, which is a first target temperature. At this time, the power is supplied to the sub-heat-generating areas A through F so that the respective temperatures reach the fixing temperature T2 right before the position P1 of the recording medium R reaches the fixing nip FN. 
     An image is formed in an area between the position P2 and a position P3 in the longitudinal direction of the recording medium R across the width of the recording medium R, except for a blank area formed at a position corresponding to the sub-heat-generating area D. Accordingly, the image forming apparatus  1  supplies less power to the sub-heat-generating area D right after the position P2 of the recording medium R passes through the fixing nip FN, thereby lowering the fixing temperature of the sub-heat-generating area D to a preliminary heating temperature T1, which is a second target temperature. Accordingly, the startup time is shortened when the temperature is raised to a predetermined fixing temperature again. 
     Similarly, an image is formed in an area between the position P3 and a position P4 in the longitudinal direction of the recording medium R across the width of the recording medium R, except for a blank area formed at a position corresponding to the sub-heat-generating areas C through F. Accordingly, the image forming apparatus  1  supplies less power to the sub-heat-generating areas C, E, and F right after the position P3 of the recording medium R passes through the fixing nip FN while continuously supplying the less power to the sub-heat-generating area D, thereby lowering the fixing temperature of the sub-heat-generating areas C, E, and F to the second target temperature, that is, the preliminary heating temperature T1. 
     At a position P4, an image is formed in an area backward from the position P4 in a direction of conveyance, that is, a longitudinal direction of the recording medium R across the width of the recording medium R. In short, the area backward from the position P4 is an imaged area. The image forming apparatus  1  supplies power to all the sub-heat-generating areas A through F so that the respective fixing temperatures reach or maintain the first temperature, that is, the fixing temperature T2. At this time, the power is supplied to the sub-heat-generating areas A through F so that the respective temperatures reach or maintain the fixing temperature T2 right before the position P4 of the recording medium R reaches the fixing nip FN. 
     As described above, the image forming apparatus  1  incorporating the fixing device  20  according to the first embodiment supplies power to ensure that a sub-heat-generating area positioned corresponding to an imaged area is maintained at the fixing temperature T2, and that another sub-heat-generating area positioned corresponding to a blank area is maintained at the preliminary heating temperature T1, which is lower than the fixing temperature T2. 
     In the fixing device  20  according to the first embodiment, the heat generator  232  is disposed on the surface of the substrate  231  facing the fixing belt  21 , and more specifically, at a position downstream in the rotational direction of the fixing belt  21 . The elongated second conduction path W 2  is a single conduction path on which the plurality of first conduction paths W 1 A are grounded. Such a simple grounding wiring pattern obviates a space necessary for a wiring pattern in which a plurality of conduction paths, such as the plurality of third conduction paths W 3 , are disposed in the rotational direction of the fixing belt  21 . Accordingly, the wiring pattern of print wiring for supplying power to the heat generator  232  is grounded on the second conduction path W 2  via the plurality of first conduction paths W 1 A disposed closer to the fixing nip FN. 
     With this wiring pattern, the distance between an edge of the heat generator  232  closer to the fixing nip FN and the edge of the substrate  231  closer to the fixing nip FN is shorter compared to a typical wiring pattern. Accordingly, an amount of heat absorbed from the fixing belt  21  is decreased during warm-up time. 
     For example, as illustrated in  FIG. 5B , the power supply starts to heat the heat generator  232  so that the respective temperatures of the sub-heat-generating areas A through F reach the fixing temperature T1 right before the position P1 of the recording medium R reaches the fixing nip FN. Thus, the power supply in the fixing device  20  starts a time Δt later than the power supply in a fixing device according to a comparative example. 
     Thus, in the image forming apparatus  1  incorporating the fixing device  20  according to the first embodiment, the heat-generating area of the heat generator  232  is divided into a plurality of sub-heat-generating areas (e.g., sub-heat-generating areas A through F), and the heat generator  232  is located at a position downstream in the rotational direction of the fixing belt  21 , that is, closer to the fixing nip FN, in an area of contact between the fixing belt  21  and the heater  23  incorporating the heat generator  232 . Accordingly, the power consumption can be reduced and the warm-up time can be shortened. 
     Referring now to  FIGS. 6 and 7 , a description is given of a second example of the wiring patterns of print wiring. 
       FIG. 6  is a sectional view of a heater  23 ′.  FIG. 7  is a plan view of the heater  23 ′, with the holder  235  removed therefrom, illustrating a second example of a wiring pattern. 
     According to the second example of a wiring pattern, a plurality of first conduction paths W 1 A are connected to the heat generator  232  and provided closer to the fixing nip FN to be grounded. Each of the plurality of first conduction paths W 1 A is wired on a backside of the substrate  231  via a through-hole  236 . The plurality of first conduction paths W 1 A thus wired on the backside of the substrate  231  may be grounded on a single conduction path such as the second conduction path W 2 , or may be grounded on a plurality of conduction paths such as the plurality of third conduction paths W 3 . 
     The above-description is given of the fixing device  20  and the image forming apparatus  1  incorporating the fixing device  20 . The image forming apparatus  1  incorporates the fixing device  20 , but is not limited thereto. Alternatively, the image forming apparatus  1  may incorporate a fixing device  20 S according to a second embodiment. 
     Referring now to  FIG. 8 , a description is given of the fixing device  20 S according to the second embodiment. 
       FIG. 8  is a schematic sectional view of the fixing device  20 S according to the second embodiment. The fixing device  20 S includes a pair of rotatable rollers  27  and  28 , an endless fixing belt  21  stretched over the pair of rotatable rollers  27  and  28 , a pressing roller  25  pressed against the roller  27  to form an area of contact via the fixing belt  21 , herein called a fixing nip FN. The fixing device  20 S also includes a heater  23 S incorporating a heat generator  232 . As illustrated in  FIG. 8 , the heater  23 S directly contacts and heats the fixing belt  21 . The fixing belt  21  and the components disposed inside a loop defined by the fixing belt  21 , that is, the pair of rotatable rollers  27  and  28 , and the heater  23 S, may constitute a belt unit  121 S separably coupled with the pressing roller  25 . 
     The above-described wiring patterns of print wiring are also applicable to the heater  23 S incorporated in the fixing device  20 S. 
     Alternatively, the image forming apparatus  1  may incorporate a fixing device  20 T according to a third embodiment, or a fixing device  20 U according to a fourth embodiment. 
     Referring now to  FIG. 9 , a description is given of the fixing device  20 T according to the third embodiment. 
       FIG. 9  is a schematic view of the fixing device  20 T according to the third embodiment. The fixing device  20 T has a configuration similar to that of the fixing device  20  and that of the fixing device  20 S as below. Differently from the fixing devices  20  and  20 S, the fixing device  20 T includes a curved-surface member  40  and an elastic member  42 . The curved-surface member  40  is interposed between an endless fixing belt  21  and a heater  23 T, thereby serving as an intermediate member. In short, the heater  23  indirectly contacts the fixing belt  21  via the curved-surface member  40 . 
     The fixing device  20 T includes the fixing belt  21  serving as a first rotator, the heater  23 T to heat the fixing belt  21 , and a pressing roller  25  serving as a second rotator pressed against the fixing belt  21  to form an area of contact herein called a fixing nip FN. The fixing device  20 T also includes a stationary member  22 A, a base  22 B, the curved-surface member  40  and the elastic member  42 . The fixing belt  21  and the components disposed inside a loop defined by the fixing belt  21 , that is, the stationary member  22 A, the base  22 B, the heater  23 T, the curved-surface member  40 , and the elastic member  42 , may constitute a belt unit  121 T separably coupled with the pressing roller  25 . 
     While a recording medium R carrying an unfixed toner image T thereon passes through the fixing nip FN, the toner image T is fixed onto the recording medium R under heat and pressure. 
     The heater  23 T includes, e.g., an elongated substrate  231  extending in an axial direction of the fixing belt  21 , and a heat generator  232 . 
     The heat generator  232  is disposed on a surface of the substrate  231  facing the fixing belt  21 , and more specifically, at a position downstream in a rotational direction of the fixing belt  21 . The heat generator  232  has a heat-generating area divided into a plurality of sub-heat-generating areas in the axial direction of the fixing belt  21 . 
     The heater  23 T also includes electrical conduction paths connected to the individual sub-heat-generating areas of the heat generator  232  and disposed on the surface of the substrate facing the fixing belt  21 . The sub-heat-generating areas of the heat generator  232  are commonly grounded on a single electrical conduction path via a plurality of electrical conduction paths disposed downstream of the heat generator  232  in the rotational direction of the fixing belt  21 . 
     The pressing roller  25  is constructed of a metal core  251 , and an elastic layer  252  covering the metal core  251 . 
     In this embodiment, a pressure roller such as the pressure roller  24  incorporated in the fixing device  20  according to the first embodiment may be omitted. Instead of using the pressure roller  24 , the curved-surface member  40  and the elastic member  42  are used in this embodiment. The curved-surface member  40  is disposed inside the loop defined by the fixing belt  21 . The elastic member  42  presses the curved-surface member  40  so that the curved-surface member  40  contacts the inner surface of the fixing belt  21 . 
     The curved-surface member  40 , serving as an intermediate member, has a pillar shape and a length equal to, or substantially equal to the length of the heater  23 T, extending in the rotational direction of the fixing belt  21 . The curved-surface member  40  has a curved surface that conforms to the shape of the inner surface of the fixing belt  21 , thereby evenly adhering to a curved surface of a cylindrical shape of the fixing belt  21 . 
     The curved-surface member  40  has a contact surface opposite the curved surface. The heater  23 T is fixed to the contact surface of the curved-surface member  40 . 
     The heater  23 T includes an overcoat layer  234  in addition to the substrate  231  and the heat generator  232 . The overcoat layer  234  covers one side of the heat generator  232 . An opposite side of the heat generator  232  is coupled to the substrate  231 , as illustrated in  FIG. 3  or  FIG. 6 . Thus, the heater  23 T and the curved-surface member  40  are coupled to each other via the overcoat layer  234 . 
     In this embodiment, the heater  23 T does not include a holder, such as the holder  235 . Accordingly, the elastic member  42  contacts and presses the substrate  231  as illustrated in  FIG. 9 . Alternatively, the heater  23 T may include the holder. In such a case, the substrate  232  is coupled to the holder, and the elastic member  42  contacts and presses the holder. 
     Thus, the curved-surface member  40  and the heater  23 T are coupled to each other with the heat generator  232  enclosed therein at the contact portion of the curved-surface member  40  and the heater  23 T. The curved-surface member  40  is made of metal. Preferably, the curved-surface member  40  is made of a metal material having good heat conductivity and easy to be heated, such as aluminum having low specific heat. 
     The stationary member  22 A is pressed by the pressing roller  25  via the fixing belt  21 , thereby forming the fixing nip FN. The stationary member  22 A has a shape substantially the same as the stationary member  22  illustrated in  FIG. 2 . The stationary member  22 A is positioned inside the loop defined by the fixing belt  21 , extending in the axial direction of the fixing belt  21 . Both ends of the stationary member  22 A are supported by a frame of the fixing device  20 T. 
     The base  22 B is coupled to a surface of the stationary member  22 A opposite a surface facing the fixing nip FN. The base  22 B has a rectangular, square-tube shape. The base  22 B faces the heater  23 T. The elastic member  42  is interposed between the base  22 B and the heater  23 T to separate the heater  23 T from the base  22 B. In this embodiment, an extensible spring is used as the elastic member  42 . 
     The elastic member  42  has an end that presses an outermost part of the heater  23 T that is, the substrate  231  in this case. The elastic member  42  presses the heater  23 T with its elastic force so that the curved-surface member  40  closely contacts the fixing belt  21 . Thus, the overcoat layer  234  of the heater  23 T indirectly contacts the fixing belt  21  via the curved-surface member  40 . 
     The fixing belt  21  is made of a material described above, and has elasticity, rigidity, and tension. The elastic member  42  applies its force in a direction in which the fixing belt  21  is extended. 
     The overcoat layer  234  is a thin layer that protects the heat generator  232 . The curved-surface member  40  interposed between the heater  23 T and the fixing belt  21  has a curved-surface that conforms to the curvature of the fixing belt  21 . 
     A rotational force of the fixing belt  21  is given by a torque of the pressing roller  25 , using a frictional force between an elastic layer  252  of the pressing roller  25  and the fixing belt  21 . 
     For example, the fixing belt  21  having a perfect round shape without receiving any external force is slightly deformed into an elliptical shape by the elastic member  42 , thereby keeping its balance with the force of the elastic member  42 . A pressing force of the curved-surface member  40  against the fixing belt  21  can be adjusted to prevent an excessive torque of the pressing roller  25  from stopping rotation of the fixing belt  21 . For example, the spring used as the elastic member  42  may be replaced with another spring having a different elastic modulus. 
     The curved-surface member  40  and the heater  23  are together displaced by an action of the elastic member  42 . A guide member may be provided to determine a direction of the displacement for a smooth movement. 
     The curved-surface member  40  closely contacts and slides on the cylindrical inner surface of the fixing belt  21  to transmit heat from the heater  23  to the fixing belt  21 . The rotatable fixing belt  21  and the pressing roller  25  convey the recording medium R through the fixing nip FN, in which the toner image T formed on the recording medium R is fixed onto the recording medium R. 
     The above-described wiring patterns of print wiring are also applicable to the heater  23 T incorporated in the fixing device  20 T. 
     Referring now to  FIG. 10 , a description is given of the fixing device  20 U according to the fourth embodiment. 
       FIG. 10  is a schematic view of the fixing device  20 U according to the fourth embodiment. In the fixing devices  20 ,  20 S and  20 T described above, the heater  23 ,  23 S and  23 T are disposed away from the fixing nip FN, respectively. In the fixing device  20 U, a heater  23 U is disposed facing a fixing nip FN. The fixing device  20 U has a configuration similar to those of the fixing devices  20 ,  20 S and  20 T described above. 
     The fixing device  20 U includes an endless fixing belt  21  serving as a first rotator, a heater  23 U that directly contacts and heats the fixing belt  21 , and a pressing roller  25  serving as a second rotator pressed against the fixing belt  21  to form an area of contact herein called a fixing nip FN. The fixing device  20 U also includes a stationary member  22 C and a holder  240  that holds the heater  23 U. The fixing belt  21  and the components disposed inside a loop defined by the fixing belt  21 , that is, the stationary member  22 C, the heater  23 U, and the holder  240 , may constitute a belt unit  121 U separably coupled with the pressing roller  25 . 
     While a recording medium R carrying an unfixed toner image T thereon passes through the fixing nip FN, the toner image T is fixed onto the recording medium R under heat and pressure. 
     The heater  23 U has an elongated planner shape and includes, e.g., an elongated substrate  231  extending in an axial direction of the fixing belt  21 , a heat generator  232 , and an overcoat layer  234 . 
     The heat generator  232  is disposed on a surface of the substrate  231  facing the fixing belt  21 , and more specifically, at a position downstream in a rotational direction of the fixing belt  21 . The heat generator  232  has a heat-generating area divided into a plurality of sub-heat-generating areas in the axial direction of the fixing belt  21 . 
     The overcoat layer  234  covers the substrate  231 , the heat generator  232 , and the electrical conduction paths. 
     The heater  23 U also includes electrical conduction paths connected to the individual sub-heat-generating areas of the heat generator  232  and disposed on the surface of the substrate facing the fixing belt  21 . The sub-heat-generating areas of the heat generator  232  are commonly grounded on a single electrical conduction path via a plurality of electrical conduction paths disposed downstream of the heat generator  232  in the rotational direction of the fixing belt  21 . 
     The pressing roller  25  is constructed of a metal core  251 , and an elastic layer  252  covering the metal core  251 . 
     A stationary member  22 C is a rigid member having a grooved shape, and disposed inside a loop defined by the fixing belt  21 , extending in the axial direction of the fixing belt  21 . Both ends of the stationary member  22 C are supported by a frame of the fixing device  20 U. End surfaces of the stationary member  22 C facing the pressing roller  25  are coupled to the holder  240 . The holder  240  supports the stationary member  22 C at both ends in the rotational direction of the fixing belt  21 . 
     The holder  240  is coupled to the stationary member  22 C with a back surface thereof facing the pressing roller  25 . The back surface of the holder  240  has a recessed portion  50  extending in the axial direction of the fixing belt  21 . The heater  23 U is disposed in the recessed portion  50  of the holder  240 . The heater  23 U has an elongated planar shape and includes, e.g., the substrate  231 , the heat generator  232 , the overcoat layer  234 , and electric conduction paths as described above. 
     The pressing roller  25  is pressed against the overcoat layer  234  via the fixing belt  21  to form the fixing nip FN. The stationary member  22 C supports a pressing force applied by the pressing roller  25  in the fixing nip FN. 
     The heater  23  faces the fixing nip FN and is supported in the recessed portion  50  of the holder  240 . Alternatively, the recessed portion  50  may directly hold the substrate  231 . The holder  240  has shoulders  51   a  and  51   b  formed on both sides of the recessed portion  50  and chamfered along the rotational direction of the fixing belt  21  to support and smoothly rotate the fixing belt  21 . 
     A rotational force of the fixing belt  21  is given by a torque of the pressing roller  25 , using a frictional force between the elastic layer  252  of the pressing roller  25  and the fixing belt  21 . An unsupported portion of the fixing belt  21  may be supported by a support member. 
     The fixing belt  21  is heated by heat applied by the heater  23  in the fixing nip FN. The rotatable fixing belt  21  and the pressing roller  25  convey the recording medium R through the fixing nip FN, in which the toner image T formed on the recording medium R is fixed onto the recording medium R. 
     The above-described wiring patterns of print wiring are also applicable to the heater  23 U incorporated in the fixing device  20 U. 
     The present invention, although it has been described above with reference to specific exemplary embodiments, is not limited to the details of the embodiments described above, and various modifications and enhancements are possible without departing from the scope of the invention. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative exemplary embodiments may be combined with each other and/or substituted for each other within the scope of this invention. The number of constituent elements and their locations, shapes, and so forth are not limited to any of the structure for performing the methodology illustrated in the drawings.