Patent Publication Number: US-11378904-B2

Title: Image forming apparatus and heating device comprising plural pressing devices configured to generate different pressing forces

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119 to Japanese Patent Application No. 2019-216897 filed on Nov. 29, 2019 in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Embodiments of the present disclosure generally relate to a heating device and an image forming apparatus. 
     Background Art 
     The image forming apparatuses often include a heating device. One example of the heating device is a fixing device that fixes toner onto a recording medium under heat. Another example of the heating device is a drying device that dries ink on a recording medium. 
     SUMMARY 
     This specification describes a heating device that includes a rotator, an opposed rotator configured to contact the rotator to form a nip, a heater configured to heat the rotator, and a plurality of pressing devices. The heater includes a first portion and a second portion. The second portion of the heater generates a smaller amount of heat than the first portion of the heater. The pressing devices are arranged in a longitudinal direction of the heater and each configured to press at least one of the rotator and the opposed rotator and cause the rotator and the opposed rotator to press each other. The pressing devices include a first pressing device corresponding to the first portion of the heater and a second pressing device corresponding to the second portion of the heater, and the first pressing device generates a smaller pressing force than the second pressing device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aforementioned and other aspects, features, and advantages of the present disclosure would be better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic cross-sectional view of an image forming apparatus according to an embodiment of the present disclosure; 
         FIG. 2  is a schematic cross-sectional view of a fixing device incorporated in the image forming apparatus depicted in  FIG. 1 ; 
         FIG. 3  is a perspective view of the fixing device depicted in  FIG. 2 ; 
         FIG. 4  is an exploded perspective view of the fixing device depicted in  FIG. 2 ; 
         FIG. 5  is a perspective view of a heating unit incorporated in the fixing device depicted in  FIG. 2 ; 
         FIG. 6  is an exploded perspective view of the heating unit depicted in  FIG. 5 ; 
         FIG. 7  is a plan view of a heater incorporated in the heating unit depicted in 
         FIG. 5 ; 
         FIG. 8  is an exploded perspective view of the heater depicted in  FIG. 7 ; 
         FIG. 9  is a perspective view illustrating the connector connected to the heater, according to the embodiment of the present disclosure; 
         FIG. 10  is a schematic diagram illustrating a circuit to supply power to the heater according to the embodiment of the present disclosure; 
         FIG. 11  is an explanatory view illustrating typical current paths in the heater depicted in  FIG. 7 ; 
         FIG. 12  is an explanatory view illustrating current paths in the heater depicted in  FIG. 7  in which an unintended shunt occurs; 
         FIG. 13  is an explanatory view illustrating heat generation amounts generated by power supply lines in each block of the heater depicted in  FIG. 7  in which the unintended shunt occurs; 
         FIG. 14  is a graph illustrating the total heat generation amount generated by the power supply lines in each block of the heater illustrated in  FIG. 13 ; 
         FIG. 15  is an explanatory view illustrating heat generation amounts generated by power supply lines in each block of the heater depicted in  FIG. 7  when all heat generator groups are energized; 
         FIG. 16  is a graph illustrating the total heat generation amount generated by the power supply lines in each block of the heater illustrated in  FIG. 15 ; 
         FIG. 17  is an explanatory view illustrating pressing forces applied by pressing devices under an equal pressure condition; 
         FIG. 18A  is an explanatory view illustrating the heater and a longitudinal positional relationship of parts in the fixing device depicted in  FIG. 2  and a small sheet when the small sheet passes through the fixing device, an upper part of  FIG. 18A  illustrates the heater, and a lower part of  FIG. 18A  illustrates the longitudinal positional relationship; 
         FIG. 18B  is a graph illustrating a temperature distribution in the longitudinal direction of a fixing belt in the fixing device depicted in  FIG. 18A  when the small sheet passes through the fixing device; 
         FIG. 19A  is an explanatory view illustrating the heater and a longitudinal positional relationship of parts in the fixing device depicted in  FIG. 2  and a large sheet when the large sheet passes through the fixing device, an upper part of  FIG. 19A  illustrates the heater, and a lower part of  FIG. 19A  illustrates the longitudinal positional relationship; 
         FIG. 19B  is a graph illustrating a temperature distribution in the longitudinal direction of the fixing belt in the fixing device depicted in  FIG. 19A  when the large sheet passes through the fixing device; 
         FIG. 20  is a flowchart illustrating a pressure condition control according to an embodiment of the present disclosure; 
         FIG. 21  is a flowchart illustrating a pressure condition control according to another embodiment of the present disclosure, which is different from the pressure control in  FIG. 20 ; 
         FIG. 22A  is an explanatory view illustrating the heater and a longitudinal positional relationship of a plurality of temperature detectors disposed in the longitudinal direction and other parts in the fixing device according to an embodiment of the present disclosure, an upper part of  FIG. 22A  illustrates the heater, and a lower part of  FIG. 22A  illustrates the longitudinal positional relationship; 
         FIG. 22B  is a graph illustrating a temperature distribution in the longitudinal direction of the fixing belt in the fixing device including the plurality of temperature detectors and depicted in  FIG. 22A ; 
         FIG. 23  is a flowchart illustrating a pressure condition control based on results detected by the plurality of temperature detectors according to an embodiment of the present disclosure; 
         FIG. 24A  is an explanatory view illustrating the heater and a longitudinal positional relationship of a plurality of temperature detectors disposed in the longitudinal direction and other parts in the fixing device according to an embodiment of the present disclosure, an upper part of  FIG. 24A  illustrates the heater, and a lower part of  FIG. 24A  illustrates the longitudinal positional relationship; 
         FIG. 24B  is a graph illustrating a temperature distribution in the longitudinal direction of the fixing belt in the fixing device including the plurality of temperature detectors and depicted in  FIG. 24A ; 
         FIG. 25  is a flowchart illustrating a pressure condition control based on results detected by the plurality of temperature detectors of  FIG. 24A  according to an embodiment of the present disclosure; 
         FIGS. 26A and 26B  are explanatory views illustrating pressing devices under the equal pressure condition, according to an embodiment of the present disclosure; 
         FIGS. 27A and 27B  are explanatory views illustrating the pressing devices under a first pressure condition, according to an embodiment of the present disclosure; 
         FIGS. 28A and 28B  are explanatory views illustrating the pressing devices under a second pressure condition, according to an embodiment of the present disclosure; 
         FIGS. 29A and 29B  are explanatory views illustrating pressing devices according to another embodiment; 
         FIG. 30  is an explanatory view illustrating a pressing device according to still another embodiment; 
         FIG. 31  is an explanatory view illustrating a fixing device including pressing devices that presses a pressure roller; 
         FIG. 32  is a plan view of the heater, illustrating a short-side dimension of the heater and a short-side dimension of the resistive heat generators; 
         FIGS. 33A and 33B  are plan views of heaters according to variations of the present disclosure; 
         FIG. 34  is a schematic cross-sectional view illustrating a configuration of another fixing device according to an embodiment of the present disclosure; 
         FIG. 35  is a schematic cross-sectional view illustrating a configuration of still another fixing device according to an embodiment of the present disclosure; 
         FIG. 36  is a schematic cross-sectional view illustrating a configuration of still another fixing device according to an embodiment of the present disclosure; 
         FIG. 37  is a schematic diagram illustrating a circuit to supply power to the heater according to another embodiment of the present disclosure; 
         FIG. 38  is an explanatory view illustrating heat generation amounts generated by power supply lines in each block of the heater depicted in  FIG. 37  in which the unintended shunt occurs; 
         FIG. 39  is a graph illustrating the total heat generation amount generated by the power supply lines in each block of the heater illustrated in  FIG. 38 ; 
         FIG. 40  is an explanatory view illustrating heat generation amounts generated by power supply lines in each block of the heater depicted in  FIG. 37  when all heat generator groups are energized; and 
         FIG. 41  is a graph illustrating the total heat generation amount generated by the power supply lines in each block of the heater illustrated in  FIG. 40 . 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION 
     In describing 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 have a similar function, operate in a similar manner, and achieve a similar result. 
     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 disclosure, and all of the components or elements described in the embodiments of this disclosure are not necessarily indispensable. 
     Referring to the drawings, embodiments of the present disclosure are described below. Identical reference numerals are assigned to identical components or equivalents and a description of those components is simplified or omitted. In the following description of each embodiment, a fixing device that fixes a toner image onto a sheet by heat is described as an example of a heating device. 
     A monochrome image forming apparatus  1  illustrated in  FIG. 1  includes a photoconductor drum  10 . The photoconductor drum  10  is a drum-shaped rotator that bears toner as a developer of a toner image on an outer circumferential surface of the photoconductor drum  10  and rotates in a direction indicated by arrow in  FIG. 1 . Around the photoconductor drum  10 , the image forming apparatus  1  includes a charging roller  11 , a developing device  12 , and a cleaning blade  13 . The charging roller  11  uniformly charges the surface of the photoconductor drum  10 . The developing device  12  includes a developing roller  7  that supplies toner to the surface of the photoconductor drum  10 . The cleaning blade  13  cleans the surface of the photoconductor drum  10 . 
     In addition, the image forming apparatus  1  includes an exposure device  3 . The exposure device  3  irradiates the surface of the photoconductor drum  10  with a laser light Lb based on image data via a mirror  14 . 
     The image forming apparatus  1  includes a transfer device  15  including a transfer charger opposite the photoconductor drum  10 . The transfer device  15  transfers the toner image on the surface of the photoconductor drum  10  to a sheet P. 
     A sheet feeder  4  is disposed in a lower portion of the image forming apparatus  1 . The sheet feeder  4  includes a sheet tray  16  and a sheet feeding roller  17 . Downstream from the sheet feeding roller  17  in a sheet conveyance direction, registration rollers  18  are disposed. The sheet tray  16  accommodates sheets P as recording media. The sheet feeding roller  17  conveys the sheet P from the sheet tray  16  to a conveyance path  5 . 
     The fixing device  9  includes a fixing belt  20 , a pressure roller  21 , and a heater described below. The heater heats the fixing belt  20 . The pressure roller  21  presses the fixing belt  20 . 
     Next, a description is given of a basic operation of the image forming apparatus  1  with reference to  FIG. 1 . 
     At the beginning of a print operation (i.e. an image forming operation), the photoconductor drum  10  rotates, and the charging roller  11  charges the surface of the photoconductor drum  10 . Subsequently, the exposure device  3  irradiates the photoconductor drum  10  with the laser light Lb based on the image data. An electric potential at the position of the photoconductor drum  10  that receives the laser light Lb decreases, and an electrostatic latent image is formed on the surface of the photoconductor drum  10 . The developing device  12  supplies toner to the surface of the photoconductor drum  10  on which the electrostatic latent image is formed. As a result, the electrostatic latent image is visualized as a toner image (i.e. a developer image). The transfer device  15  transfers the toner image onto the sheet P, and the cleaning blade  13  removes the toner remaining on the photoconductor drum  10 . 
     On the other hand, as the printing operation starts, the sheet feeding roller  17  of the sheet feeder  4  disposed in the lower portion of the image forming apparatus  1  is driven and rotated to feed the sheet P from the sheet tray  16  toward the registration rollers  18  through the conveyance path  5 . 
     The registration rollers  18  convey the sheet P sent to the conveyance path  5  to a transfer portion, timed to coincide with the toner image on the photoconductor drum  10 . The transfer portion is a portion at which the transfer device  15  faces the photoconductor drum  10 . A transfer bias is applied between the transfer device  15  and the photoconductor drum  10 , and the transfer device  15  transfers the toner image onto the sheet P. 
     The sheet P bearing the toner image is conveyed to the fixing device  9 . The heated fixing belt  20  and the pressure roller  21  heat and press the sheet P. As a result, the toner image is fixed on the sheet P. The sheet P bearing the fixed toner image thereon is separated from the fixing belt  20 , conveyed by a conveyance roller pair disposed downstream from the fixing device  9 , and ejected to an output tray disposed outside the image forming apparatus  1 . 
     Next, a configuration of the fixing device  9  is described. 
     As illustrated in  FIG. 2 , the fixing device  9  according to the present embodiment includes a fixing belt  20  as a fixing rotator, a pressure roller  21  as an opposed rotator or a pressure rotator, and a heating unit  19 . The pressure roller  21  contacts the outer circumferential surface of the fixing belt  20  to form a nip N. The heating unit  19  heats the fixing belt  20 . The heating unit  19  includes a laminated heater  22  as a heater, a heater holder  23  as a holder to hold the heater  22 , and a stay  24  as a supporter to support the heater holder  23 . 
     The fixing belt  20  is an endless belt. The fixing belt  20  has a tubular base layer and a release layer. The tubular base layer is made of, for example, polyimide (PI) and has an outer diameter of 25 mm and a thickness of 40 to 120 μm. The release layer is formed as the outermost surface layer of the fixing belt. The release layer is formed of a fluorine-based resin such as PFA or PTFE and has a thickness of 5 to 50 μm to enhance durability of the fixing belt  20  and facilitate separation of the sheet P and a foreign substance from the fixing belt  20 . Optionally, an elastic layer that is made of rubber or the like and has a thickness in a range of from 50 micrometers to 500 micrometers may be interposed between the base layer and the release layer. The base layer of the fixing belt  20  may be made of heat resistant resin such as polyetheretherketone (PEEK) or metal such as nickel (Ni) or stainless steel (SUS), instead of polyimide. An inner circumferential surface of the fixing belt  20  may be coated with polyimide, PTFE, or the like to produce a slide layer. 
     The pressure roller  21  has an outer diameter of 25 mm, for example. The pressure roller  21  includes a core  21   a , an elastic layer  21   b , and a release layer  21   c . The core  21   a  is a solid core made of iron. The elastic layer  21   b  coats the circumferential surface of the core  21   a . The release layer  21   c  coats an outer circumferential surface of the elastic layer  21   b . The elastic layer  21   b  is made of silicone rubber and has a thickness of 3.5 mm, for example. In order to facilitate separation of the sheet P and the foreign substance from the pressure roller  21 , the release layer  21   c  that is made of fluororesin and has a thickness of about 40 micrometers, for example, is preferably disposed on the outer surface of the elastic layer  21   b.    
     The pressing device described below presses the fixing belt  20  against the pressure roller  21 , and the fixing belt  20  contacts and presses the pressure roller  21 . Thus, the fixing nip N is formed between the fixing belt  20  and the pressure roller  21 . In addition, the pressure roller  21  functions as a drive roller. That is, the pressure roller  21  receives a driving force from a motor disposed in the main body of the image forming apparatus  1  and rotates. The fixing belt  20  is driven and rotated by the pressure roller  21  as the pressure roller  21  rotates. When the fixing belt  20  rotates, the fixing belt  20  slides on the heater  22 . In order to facilitate sliding performance of the fixing belt  20 , a lubricant such as oil or grease may be interposed between the heater  22  and the fixing belt  20 . 
     The heater  22  extends in a longitudinal direction thereof throughout an entire width of the fixing belt  20  in a rotation axis direction of the fixing belt  20 , referred to as a longitudinal direction of the fixing belt  20  below. The heater  22  contacts the inner circumferential surface of the fixing belt  20  at a position corresponding to the pressure roller  21 . The heater  22  heats the fixing belt  20  as a heated member to a predetermined fixing temperature. 
     The heater  22  has a base  50  and a heat generator  60 . In the present embodiment, the heat generator  60  is disposed on the base  50  to face the fixing belt  20 . Alternatively, the heat generator  60  may be disposed on a surface of the base  50  facing the heater holder  23 , that is, the surface opposite to a surface of the base  50  facing the fixing belt  20 . In that case, since the heat of the heat generator  60  is transmitted to the fixing belt  20  through the base  50 , it is preferable that the base  50  be made of a material with high thermal conductivity such as aluminum nitride. In the heater  22  according to the present embodiment, another insulation layer may be further disposed on a surface of the base  50  facing the heater holder  23 , that is, the surface opposite to the surface of the base  50  facing the fixing belt  20 . 
     The heater  22  may not contact the fixing belt  20  or may be disposed opposite the fixing belt  20  indirectly via a low-friction sheet or the like. However, the heater  22  that contacts the fixing belt  20  directly as in the present embodiment enhances conduction of heat from the heater  22  to the fixing belt  20 . The heater  22  may contact the outer circumferential surface of the fixing belt  20 . However, if the outer circumferential surface of the fixing belt  20  is brought into contact with the heater  22  and damaged, the fixing belt  20  may degrade quality of fixing the toner image on the sheet P. Hence, preferably, the heater  22  contacts the inner circumferential surface of the fixing belt  20 . 
     The heater holder  23  and the stay  24  are disposed inside the inner circumferential surface of the fixing belt  20 . The stay  24  is configured by a channeled metallic member, and both side plates of the fixing device  9  support both end portions of the stay  24 . The stay  24  supports a stay side face of the heater holder  23 , that faces the stay  24  and is opposite a heater side face of the heater holder  23 . Accordingly, the stay  24  retains the heater  22  and the heater holder  23  to be immune from being bent substantially by pressure from the pressure roller  21 , forming the fixing nip N between the fixing belt  20  and the pressure roller  21 . 
     Since the heater holder  23  is subject to temperature increase by heat from the heater  22 , the heater holder  23  is preferably made of a heat-resistant material. For example, the heater holder  23  made of heat-resistant resin having low thermal conduction, such as a liquid crystal polymer (LCP), reduces heat transfer from the heater  22  to the heater holder  23  and provides efficient heating of the fixing belt  20 . 
     As a print job starts, the heater  22  supplied with power causes the heat generator  60  to generate heat, thus heating the fixing belt  20 . The motor drives and rotates the pressure roller  21 , and the fixing belt  20  starts rotating with the rotation of the pressure roller  21 . When the temperature of the fixing belt  20  reaches a predetermined target temperature called a fixing temperature, as illustrated in  FIG. 2 , the sheet P bearing an unfixed toner image is conveyed to the nip N between the fixing belt  20  and the pressure roller  21  in a direction indicated by arrow A in  FIG. 2 , and the unfixed toner image is heated and pressed onto the sheet P and fixed thereon. 
       FIG. 3  is a perspective view of the fixing device  9 .  FIG. 4  is an exploded perspective view of the fixing device  9 . 
     As illustrated in  FIGS. 3 and 4 , the fixing device  9  includes a device frame  40  that includes a first device frame  25  and a second device frame  26 . The first device frame  25  includes a pair of side walls  28  and a front wall  27 . The second device frame  26  includes a rear wall  29 . The pair of side walls  28  are disposed at one side and another side of the fixing belt  20 , respectively, in the longitudinal direction of the fixing belt  20 . The side walls  28  support both sides of each of the pressure roller  21  and the heating unit  19 , respectively. Each of the side walls  28  includes a plurality of engaging projections  28   a . As the engaging projections  28   a  engage corresponding coupling holes  29   a  in the rear wall  29 , the first device frame  25  is coupled to the second device frame  26 . 
     Each of the side walls  28  includes a slot  28   b  through which a rotation shaft and the like of the pressure roller  21  are inserted. The slot  28   b  opens toward the rear wall  29  and closes at a portion opposite the rear wall  29 , and the portion of the slot  28   b  opposite the rear wall  29  serves as a contact portion. A bearing  30  that supports the rotation shaft of the pressure roller  21  is disposed at an end of the contact portion. As both sides of the rotation shaft of the pressure roller  21  are attached to the corresponding bearings  30 , the side walls  28  rotatably support the pressure roller  21 . 
     A driving force transmission gear  31  serving as a driving force transmitter is disposed at an axial end side of the rotation shaft of the pressure roller  21 . In a state in which the side walls  28  support the pressure roller  21 , the driving force transmission gear  31  is exposed outside the side wall  28 . Accordingly, when the fixing device  9  is installed in the body of the image forming apparatus  1 , the driving force transmission gear  31  is coupled to a gear disposed inside the body of the image forming apparatus  1  so that the driving force transmission gear  31  transmits the driving force from the driver to the pressure roller  21 . Alternatively, a driving force transmitter to transmit the driving force to the pressure roller  21  may be pulleys over which a driving force transmission belt is stretched taut, a coupler, and the like instead of the driving force transmission gear  31 . 
     A pair of flanges  32  that support the fixing belt  20 , the heater holder  23 , the stay  24 , and the like is disposed at both sides of the heating unit  19  in a longitudinal direction thereof, respectively. Each flange  32  has a guide groove  32   a . As edges of the slot  28   b  of the side wall  28  enter the guide grooves  32   a , respectively, the flange  32  is attached to the side wall  28 . 
     A pair of springs  33  serving as a pair of biasing members contact flanges  32 , respectively. As the springs  33  bias the flanges  32  and the stay  24  toward the pressure roller  21 , respectively, the fixing belt  20  is pressed against the pressure roller  21  to form the fixing nip between the fixing belt  20  and the pressure roller  21 . A pressure lever described below presses the end of the spring  33  on the side opposite to the side in contact with the flange  32 . 
     As illustrated in  FIG. 4 , a hole  29   b  is disposed near one end of the rear wall  29  of the second device frame  26  in a longitudinal direction of the second device frame  26 . The hole  29   b  serves as a positioner of the fixing device  9  that positions the body of the fixing device  9  with respect to the body of the image forming apparatus  100 . Similarly, the body of the image forming apparatus  1  includes a projection  101  as a positioner fixed on the image forming apparatus  1 . The projection  101  is inserted into the hole  29   b  of the fixing device  9 . Accordingly, the projection  101  engages the hole  29   b , positioning the body of the fixing device  9  with respect to the body of the image forming apparatus  1  in the longitudinal direction of the fixing belt  20 . Note that although the hole  29   b  serving as the positioner is disposed at one side of the rear wall  29  in the longitudinal direction of the second device frame  26 , a positioner is not disposed at another side of the rear wall  29 . Thus, the second device frame  26  does not restrict thermal expansion and shrinkage of the body of the fixing device  9  in the longitudinal direction of the fixing belt  20  due to temperature change. 
       FIG. 5  is a perspective view of the heating unit  19 .  FIG. 6  is an exploded perspective view of the heating unit  19 . 
     As illustrated in  FIGS. 5 and 6 , the heater holder  23  includes an accommodating recess  23   a  disposed on a fixing belt side face of the heater holder  23 , that is a face in front side of  FIGS. 5 and 6 . The accommodating recess  23   a  is rectangular and accommodates the heater  22 . The accommodating recess  23   a  has a similar shape and size of the heater  22 , but a length L 2  of the accommodating recess  23   a  in the longitudinal direction of the heater holder  23  is set slightly longer than a length L 1  of the heater  22  in the longitudinal direction of the heater  22 . The accommodating recess  23   a  formed slightly longer than the heater  22  does not interfere the heater  22  even when the heater  22  expands in the longitudinal direction due to thermal expansion. The accommodating recess  23   a  accommodates the heater  22 , and a connector as power supplying member described below sandwiches the heater  22  and the heater holder  23 , thus the heater  22  is held by the connector. 
     In addition to the guide grooves  32   a  described above, each of the pair of flanges  32  includes a belt support  32   b , a belt restrictor  32   c , and a supporting recess  32   d . The belt support  32   b  is C-shaped and inserted into the loop of the fixing belt  20 , thus contacting the inner circumferential surface of the fixing belt  20  to support the fixing belt  20 . The belt restrictor  32   c  contacts an edge face of the fixing belt  20  to restrict motion (e.g., skew) of the fixing belt  20  in the longitudinal direction of the fixing belt  20 . The supporting recess  32   d  is inserted with a lateral end of each of the heater holder  23  and the stay  24  in the longitudinal direction thereof, thus the flanges  32  support the heater holder  23  and the stay  24 . The belt supports  32   b  inserted into the inner periphery of the fixing belt  20  in both ends support the fixing belt  20  in a state in which the fixing belt  20  is not tensioned in a circumferential direction thereof while the fixing belt  20  does not rotate, that is, by a free belt system. 
     As illustrated in  FIGS. 5 and 6 , the heater holder  23  includes a positioning recess  23   e  as a positioner disposed at one side of the heater holder  23  in the longitudinal direction thereof. The flange  32  further includes an engagement  32   e  illustrated in a left part in  FIGS. 5 and 6 . The engagement  32   e  engages the positioning recess  23   e , positioning the heater holder  23  with respect to the flange  32  in the longitudinal direction of the fixing belt  20 . The flange  32  illustrated in right parts in  FIGS. 5 and 6  does not include the engagement  32   e  and therefore the heater holder  23  is not positioned with respect to the flange  32  in the longitudinal direction of the fixing belt  20 . Positioning the heater holder  23  with respect to the flange  32  at one side of the heater holder  23  in the longitudinal direction of the fixing belt  20  does not restrict an expansion and contraction of the heater holder  23  in the longitudinal direction of the fixing belt  20  due to a temperature change. 
     As illustrated in  FIG. 6 , the stay  24  includes step portions  24   a  at both ends in the longitudinal direction of the stay  24  to set the stay  24  in the flanges  32 . Each step portion  24   a  abuts the flange  32  to restrict movement of the stay  24  in the longitudinal direction with respect to the flange  32 . However, at least one of the step portions  24   a  is arranged to have a gap, that is, loose fit with play between the step portion  24   a  and the flange  32 . The above-described arrangement of the gap between the flange  32  and at least one of the step portions  24   a  enables an expansion and contraction of the stay  24  in the longitudinal direction of the fixing belt  20  due to the temperature change. 
       FIG. 7  is a plan view of the heater  22 .  FIG. 8  is an exploded perspective view of the heater  22 . 
     As illustrated in  FIG. 8 , the heater  22  includes the base  50 , a first insulation layer  51  disposed on the base  50 , a conductor layer  52  disposed on the first insulation layer  51 , and a second insulation layer  53  that covers the conductor layer  52 . The conductor layer  52  includes the heat generator  60 . In the present embodiment, the base  50 , the first insulation layer  51 , the conductor layer  52  including the heat generator  60 , and the second insulation layer  53  are layered in this order toward the fixing belt  20 , that is, the nip N. Heat generated from the heat generator  60  is transmitted to the fixing belt  20  via the second insulation layer  53  (see  FIG. 2 ). 
     The base  50  is a long plate made of a metal such as stainless steel (SUS), iron, or aluminum. The base  50  may be made of ceramic, glass, etc. instead of metal. If the base  50  is made of an insulating material such as ceramic, the first insulation layer  51  sandwiched between the base  50  and the conductor layer  52  may be omitted. Since metal has an excellent durability when it is rapidly heated and is processed readily, using metal to make the base  50  reduces the manufacturing cost of the base  50 . Among metals, aluminum and copper are preferable for the material of the base  50  because aluminum and copper have high thermal conductivity and are less likely to cause uneven temperature. Stainless steel is advantageous because stainless steel is manufactured at reduced costs compared to aluminum and copper. 
     The first insulation layer  51  and the second insulation layer  53  are made of material having electrical insulation, such as heat-resistant glass. Alternatively, each of the first insulation layer  51  and the second insulation layer  53  may be made of ceramic, polyimide (PI), or the like. 
     The conductor layer  52  includes the heat generator  60 , a plurality of electrodes  61 , and a plurality of power supply lines  62  as conductors. The heat generator  60  includes a plurality of resistive heat generators  59 . The power supply line  62  electrically connects the heat generator  60  and the electrodes  61 . Each of the resistive heat generators  59  is electrically connected to any two of the three electrodes  61  in parallel to each other via the plurality of power supply lines  62  disposed on the base  50 . Thus, the resistive heat generators  59  are electrically connected in parallel to each other. 
     The heat generator  60  is produced by mixing silver-palladium (AgPd), glass powder, and the like into a paste. The paste is coated on the base  50  by screen printing or the like. Thereafter, the base  50  is fired to form the heat generator  60 . Alternatively, the heat generator  60  may be made of a resistive material such as a silver alloy (AgPt) and ruthenium oxide (RuO2). 
     The power supply lines  62  are made of a conductor having an electrical resistance lower than that of the heat generator  60 . The power supply lines  62  and the electrodes  61  may be made of a material prepared with silver (Ag), silver-palladium (AgPd), or the like. Screen-printing such a material forms the power supply lines  62  and the electrodes  61 . 
       FIG. 9  is a perspective view illustrating a connector  70  connected to the heater  22 . 
     As illustrated in  FIG. 9 , the connector  70  includes a housing  71  made of resin and a plurality of contact terminals  72  fixed to the housing  71 . Each contact terminal  72  is configured by a flat spring and connected to a power supply harness  73 . 
     As illustrated in  FIG. 9 , the connector  70  is attached to the heater  22  and the heater holder  23  such that a front side of the connector  70  sandwiches the heater  22  and the heater holder  23  together with a back side of the connector  70 . Thus, the contact portions  72   a  disposed at ends of the contact terminals  72  elastically contact and press against the electrodes  61  each corresponding to the contact terminals  72 , and the heat generator  60  is electrically connected to the power supply provided in the image forming apparatus via the connector  70 . The above-described configuration allows the power supply to supply power to the heat generator  60 . Note that, as illustrated in  FIG. 7 , at least part of each of the electrodes  61  is not coated by the second insulation layer  53  and therefore exposed to secure connection with the corresponding connector  70 . 
     As illustrated in  FIG. 10 , in the present embodiment, the heat generator  60  includes a first heat generator group  60 A serving as a heat generation part and a second heat generator group  60 B serving as another heat generation part. The first heat generator group  60 A is a first group of the resistive heat generators  59 , which are other than the resistive heat generators  59  on the ends of the plurality of resistive heat generators  59  arranged in a longitudinal direction of the base  50 . The second heat generator group  60 B is a second group of the resistive heat generators  59 , which are arranged on the ends and distinct from the resistive heat generators  59  of the first heat generator group  60 A. The first heat generator group  60 A and the second heat generator group  60 B are separately controllable to generate heat. Specifically, each of the resistive heat generators  59  constructing the first heat generator group  60 A (i.e., the resistive heat generators  59  other than the resistive heat generators  59  arranged on the ends) is connected, through a first power supply line  62 A, to a first electrode  61 A provided on a first longitudinal end side of the base  50 . Each of the resistive heat generators  59  constructing the first heat generator group  60 A is also connected, through a second power supply line  62 B, to a second electrode  61 B provided on a second longitudinal end side of the base  50  opposite the first longitudinal end side of the base  50  on which the first electrode  61 A is provided. On the other hand, each of the resistive heat generators  59  constructing the second heat generator group  60 B (i.e., the resistive heat generators  59  on the ends) is connected, through a third power supply line  62 C or a fourth power supply line  62 D, to a third electrode  61 C (different from the first electrode  61 A) provided on the first longitudinal end side of the base  50 . Like each of the resistive heat generators  59  of the first heat generator group  60 A, each of the resistive heat generators  59  arranged on the ends is also connected to the second electrode  61 B through the second power supply line  62 B. 
     The electrodes  61 A to  61 C are connected to a power supply  64  via the connector  70  described above and supplied with power from the power supply  64 . A switch  65 A as a switching unit is disposed between the electrode  61 A and the power supply  64 . Turning the switch  65 A on and off can switch whether or not a voltage is applied to the electrode  61 A. Similarly, a switch  65 C as a switching unit is disposed between the electrode  61 C and the power supply  64 . Turning the switch  65 C on and off can switch whether or not a voltage is applied to the electrode  61 C. A control circuit  66  controls ON and OFF of these switches  65 A and  65 C and timing of power supply to the heater  22 . The control circuit  66  performs these controls based on detection results of various sensors in the image forming apparatus  1 . For example, the control circuit  66  determines a sheet passing timing based on detection results of the sensors provided at the entrance and the exit of the fixing nip N and determines whether or not the heater  22  is supplied with electric power and switching timings of the switches  65 A and  65 C. 
     Applying a voltage to the first electrode  61 A and the second electrode  61 B energizes the resistive heat generators  59  other than the end resistive heat generators  59 , and the first heat generator group  60 A generates heat alone. On the other hand, applying a voltage to the second electrode  61 B and the third electrode  61 C energizes the end resistive heat generators  59 , and the second heat generator group  60 B generates heat alone. When a voltage is applied to all the first to third electrodes  61 A to  61 C, the resistive heat generators  59  of both the first heat generator group  60 A and the second heat generator group  60 B (i.e., all the resistive heat generators  59 ) generate heat. For example, the first heat generator group  60 A generates heat alone to fix a toner image on a sheet P having a relatively small width conveyed, such as a sheet P of A4 size (sheet width: 210 mm) or a smaller sheet P. By contrast, the second heat generator group  60 B generates heat together with the first heat generator group  60 A to fix a toner image on a sheet P having a relatively large width conveyed, such as a sheet P larger than A4 size (sheet width: 210 mm). As described above, the heater  22  can generate heat generation areas corresponding to the sheet widths. 
     One approach to further downsize the image forming apparatus and the fixing device is downsizing the heater, which is one of the components disposed inside a loop formed by the fixing belt. That is, downsizing the heater in a short-side direction of the heater can downsize the fixing belt and, as a result, downsize the fixing device and the image forming apparatus. Note that the short-side direction of the heater is a direction indicated by arrow Y in  FIG. 10 , a direction intersecting the longitudinal direction of the heater  22  along the surface of the heater  22  on which the first heat generator group  60 A and the second heat generator group  60 B are provided in  FIG. 10 , and a direction orthogonal to the longitudinal direction of the heater  22  and different from a thickness direction of the heater  22  that is orthogonal to the sheet surface of  FIG. 10 . Specifically, the following three methods are considered as examples of methods to downsize the heater in the short-side direction of the heater. 
     A first method is downsizing the heat generator group (i.e., resistive heat generators) in the short-side direction of the heater. However, downsizing the heat generator group in the short-side direction of the heater narrows a heating span over which the fixing belt is heated, resulting in an increase in the temperature peak of the heater to maintain the same amount of heat applied to the fixing belt as the amount of heat applied before the heating span is narrowed. The increase in the temperature peak of the heater may cause the temperature of an overheating detector such as a thermostat or a fuse disposed on a back surface of the heater to exceed a heat resistant temperature. Alternatively, the increase in the temperature peak of the heater may cause malfunction of the overheating detector. In addition, the increase in the temperature peak of the heater also reduces the efficiency of heat conduction from the heater to the fixing belt. Therefore, the increase in the temperature peak of the heater is unfavorable from the viewpoint of energy efficiency. As described above, downsizing the heat generator group in the short-side direction of the heater is hardly adopted. 
     A second method is downsizing, in the short-side direction of the heater, parts of the heater that are not the heat generator groups, the electrodes, and the power supply lines. However, this method shortens a distance between the heat generator group and the power supply line or between the electrode and the power supply line, thus failing to secure the insulation. Considering the structure of the current heater, it is difficult to further shorten the distance between the heat generator group and the power supply line or between the electrode and the power supply line. 
     The remaining third method is to reduce the size of the power supply line in the short-side direction of the heater. This method has room for implementation as compared with the above two methods. However, reducing the size of the power supply line in the short-side direction increases the resistance value of the power supply line. Therefore, an unintended shunt may occur on a conductive path of the heater. In particular, if a resistance value of the heat generator group is reduced to increase the heat generation amount generated by the heat generator to speed up the image forming apparatus, the resistance value of the power supply lines and the resistance value of the heat generator group get relatively close to each other. In such a situation, an unintended shunt tends to occur. In order to prevent such an unintended shunt, the power supply lines may be upsized in a thickness direction of the heater (i.e., direction intersecting the longitudinal and short-side directions of the heater) while being downsized in the short-side direction of the heater. Such a configuration secures the cross-sectional area of the power supply lines and prevents an increase in resistance value of the power supply lines. However, in such a case, the screen printing of the power supply lines is difficult, resulting in a change of the way of forming the power supply lines. Therefore, thickening the power supply lines is hardly adopted as a solution. In conclusion, in order to downsize the heater in the short-side direction of the heater, the power supply lines are downsized in the short-side direction of the heater in anticipation of an increase in resistance value, while a measure is taken against the unintended shunt that may be caused by downsized power supply lines. 
     Hereinafter, referring now to  FIGS. 11 to 14 , a description is given of the unintended shunt and adverse effects of the unintended shunt in the heater  22  described above. 
     In the heater  22  illustrated in  FIG. 11 , applying the voltage to the first electrode  61 A and the second electrode  61 B typically generates a current that flows through the first power supply line  62 A, passes through each of the resistive heat generators  59  other than the resistive heat generators  59  located on the both ends of the heater  22 , and then flows through the second power supply line  62 B, and the resistive heat generators  59  of the first heat generator group  60 A alone generate heat. 
     However, as illustrated in  FIG. 12 , the unintended shunt occurs in current paths when increase in resistance values of the power supply lines to downsize the heater  22  as described above and decrease in resistance values of the heat generator groups to increase the heat generation amount of the heater  22  decrease the differences between the resistance values of the power supply lines and the heat generator groups. Specifically, part of the current passing through the second resistive heat generator  59  from the left in  FIG. 12  does not flow to the second electrode  61 B from a branch X of the second power supply line  62 B to which the current flow from the second resistive heat generator  59 , but flows opposite side of the second electrode  61 B from the branch X. The shunted current then passes through the resistive heat generator  59  arranged on the left end in  FIG. 12  and further passes through the third power supply line  62 C, the third electrode  61 C, the fourth power supply line  62 D, and the resistive heat generator  59  arranged on the right end in  FIG. 12  in this order. Finally, the current joins the second power supply line  62 B. 
     As described above, in the heater  22  illustrated in  FIG. 12 , a shunted current path E 3  through which the unintended shunt flows includes a part of the second power supply line  62 B extending from the branch X to the left in  FIG. 12 , the resistive heat generators  59  on the ends constructing the second heat generator group  60 B, the third electrode  61 C, the third power supply line  62 C, and the fourth power supply line  62 D. 
     The above-described unintended shunt may occur when the first heat generator group  60 A is energized as long as the heater  22  includes a conductive path including at least a first conductive portion E 1 , a second conductive portion E 2 , and the shunted current path E 3 . The first conductive portion E 1  connects the first heat generator group  60 A and the first electrode  61 A. The second conductive portion E 2  extends from the first heat generator group  60 A in a first direction S 1  (i.e., to the right in FIG.  12 ) of a longitudinal direction of the heater  22  to connect the first heat generator group  60 A and the second electrode  61 B. The shunted current path E 3  separates from the second conductive portion E 2  in a second direction S 2  (i.e., to the left in  FIG. 12 ) opposite the first direction S 1  and is connected to the second conductive portion E 2  or the second electrode  61 B without passing through the first conductive portion E 1 . In the present embodiment, the shunted current path E 3  includes the second heat generator group  60 B and the third electrode  61 C. However, the unintended shunt may occur even on a conductive path without the second heat generator group  60 B or the third electrode  61 C, or a conductive path provided with a conductor other than the second heat generator group  60 B and the third electrode  61 C. 
     The unintended shunt is a current flowing through an unexpected path and causes heat generation of the power supply lines in the unexpected path, and the heat generation causes a variation in the temperature distribution of the heater  22 . For example, in the heater  22  illustrated in  FIG. 13 , 20% of a current from the first electrode  61 A flows equally through each of the resistive heat generators  59  of the first heat generator group  60 A.  FIG. 13  illustrates a case in which 5% of a current passing through the second resistive heat generator  59  from the left in  FIG. 13  flows from the branch X to the third electrode  61 C, and the table in  FIG. 13  illustrates heat generation amounts in each of the power supply lines in each block that is separated so as to include each resistive heat generator  59 . 
     Since the portion of each power supply line extending in the short-side direction of the heater  22  is relatively short and therefore the heat generation amount generated in the shorter portion is relatively small, the heat generation amount in the shorter portion is eliminated. The table illustrated in  FIG. 13  simply indicates the calculated heat generation amounts generated in a longer portion of each power supply line extending in the longitudinal direction of the heater  22 . Specifically, the table illustrates calculated heat generation amounts in portions extending in the longitudinal direction of the heater  22  in the first power supply line  62 A, the second power supply line  62 B, and the fourth power supply line  62 D. Since a heat generation amount (W) is represented by the following equation (1), each of the heat generation amounts indicated in the table of  FIG. 13  is calculated as the square of a current (I) flowing through each of the power supply lines for convenience. Therefore, the numerical values of the heat generation amounts indicated in the table of  FIG. 13  are merely values calculated simply and may be different from the actual heat generation amount.
 
 W=R×I 2,  (1)
 
     where W represents the heat generation amount, R represents the resistance, and I represents the current. 
     A description is given of a specific calculation method of the heat generation amounts illustrated in  FIG. 13 . In the first block in  FIG. 13 , a proportion of a current flowing through the fourth power supply line  62 D to a current flowing through the first power supply line  62 A is 5%, and a proportion of the current flowing through the first power supply line  62 A is expressed as 100%. Therefore, the total heat generation amount generated by the power supply lines  62 A and  62 D in the first block is expressed as 10025, which is the total value of the square of 100 (i.e., 10000) and the square of 5 (i.e., 25). In the second block, a proportion of a current flowing through the first power supply line  62 A is 80%, a proportion of a current flowing through the second power supply line  62 B is 5%, and a proportion of a current flowing through the fourth power supply line  62 D is 5%. Therefore, the total heat generation amount of the power supply lines  62 A,  62 B, and  62 D in the second block is 6450 (6400+25+25), which is the sum of the squares of the above-described proportions of the currents. The heat generation amounts in other blocks are similarly calculated. 
       FIG. 14  is a graph based on the table of  FIG. 13 . The x-axis represents blocks in  FIG. 13 , and the y-axis represents the total heat generation amounts described above in the blocks. As illustrated in  FIG. 14 , the above-described unintended shunt affects the total heat generation amount in each block, and the distribution of the total heat generation amounts becomes a lateral unsymmetrical shape with respect to the fourth block located in the center of the heat generation area. 
     Similarly, when all the heat generator groups are energized, a difference of currents flowing through the conductive portions occurs, and the distribution of the total heat generation amounts in the longitudinal direction of the heater  22  becomes unsymmetrical shape. That is, since downsizing the heater  22  limits an arrangement of the electrodes and the conductive portions, designing the distribution of the heat generation amounts in the longitudinal direction of the heater  22  to be a lateral symmetrical shape is difficult. Speeding up the image forming apparatus as described above increases the currents flowing through the conductive portions and, as a result, increases the difference between currents flowing through the left blocks and the right blocks. The difference can not be ignored. Next, a description is given of a case when all the heat generator groups are energized. 
     As illustrated in  FIG. 15 , the difference between the case when all the heat generator groups are energized and the case when the first heat generator group is energized is that a current having a proportion of 20% to the current flowing through the first power supply line  62 A flows through each of the resistive heat generators  59  at both ends and each of the power supply lines  62 C and  62 D connected to the resistive heat generators at both ends. The value of the current flowing through the power supply line  62 A is the same as that in the case when the first heat generator group is energized. In the first block in  FIG. 15 , a proportion of a current flowing through the fourth power supply line  62 D to the current flowing through the first power supply line  62 A is 20%, and the proportion of the current flowing through the first power supply line  62 A is expressed as 100%. Therefore, the total heat generation amount generated by the power supply lines  62 A and  62 D in the first block is expressed as 10400, which is the total value of the square of 100 (i.e., 10000) and the square of 20 (i.e., 400). In the second block, a proportion of a current flowing through the first power supply line  62 A is 80%, a proportion of a current flowing through the second power supply line  62 B is 20%, and a proportion of a current flowing through the fourth power supply line  62 D is 20%. Therefore, the total heat generation amount of the power supply lines  62 A,  62 B, and  62 D in the second block is 7200 (6400+400+400), which is the sum of the squares of the above-described proportions of the currents. The heat generation amounts in other blocks are similarly calculated. 
     As illustrated in  FIG. 16 , the distribution of the total heat generation amounts becomes the lateral unsymmetrical shape with respect to the fourth block located in the center of the heat generation area. In particular, the second power supply line  62 B is connected to all resistive heat generators  59 , and a proportion of a current flowing through downstream portion of the power supply line  62 B, that is, the power supply line  62 B in the seventh block to the current flowing through the first power supply line  62 A in the first block becomes 120%. Such a large current value causes a difference between heat generation amounts in right and left portions of the power supply line. 
     Such an asymmetrical variation in the heat generation amount of the power supply lines causes a longitudinal unevenness in temperature of the heater  22 . When the temperature of the heater  22  varies in the longitudinal direction of the heater  22 , the glossiness of an image fixed on a portion of the sheet P corresponding to the higher temperature portion of the heater  22  is higher than the glossiness of an image fixed on a portion of the sheet P corresponding to the lower temperature portion of the heater  22 . In short, the entire image exhibits the unevenness in glossiness, leading to a deterioration in image quality. In the present embodiment, lengths of the blocks are designed to be the same so that the heater  22  can uniformly heat the sheet P regardless of the size of the sheet P. 
     In the present embodiment, the following measures are taken to prevent disadvantages caused by the longitudinal unevenness in temperature of the heater  22 , such as the unevenness in glossiness or an unevenness in a fixing property when one of the heat generator groups is energized to fix the image on the small sheet and when all the heat generator groups are energized to fix the image on the large sheet. 
     As illustrated in  FIG. 17 , one of flanges  321  and  322  supports one end of the fixing belt  20  in the longitudinal direction of the fixing belt  20 , and the other one of the flanges  321  and  322  supports the other end of the fixing belt  20  in the longitudinal direction. Two independent pressing devices press flanges  321  and  322 , respectively. The pressing devices press the flanges  321  and  322 , and the flanges  321  and  322  press the fixing belt  20  against the pressure roller  21  to form the fixing nip N. That is, the pressing devices press the fixing belt  20 . 
     Originally, a pressing force FL applied to the flange  321  and a pressing force FR applied to the flange  322  are set to be the same. This setting of the pressing forces is referred to as a uniform pressure condition. A pressure condition between the fixing belt  20  and the pressure roller  21  is changed, which is described in detail below. 
       FIG. 18A  is an explanatory view illustrating the heater  22  and a longitudinal positional relationship of parts in the fixing device  9  depicted in  FIG. 2  and the small sheet when the small sheet passes through the fixing device  9 . The heater  22  is depicted in the upper part of  FIG. 18A , and the parts in the fixing device  9  and the small sheet are depicted in the lower part of  FIG. 18A . As a result, the longitudinal positional relationship is illustrated.  FIG. 18B  is a graph illustrating a temperature distribution in the longitudinal direction of the fixing belt  20 . In  FIG. 20B , T means temperature of the fixing belt  20 . The sheet P passing between the fixing belt  20  and the pressure roller  21  in  FIG. 18A  is the small sheet which the heater  22  can heat, for example, the A4 size sheet. 
     As illustrated in the upper part of  FIG. 18A , the first heat generator group  60 A of the heater  22  is energized corresponding to the small sheet. In this case, as described above, one end portion of the heater  22  in the longitudinal direction that is the left end portion of the heater  22  in  FIG. 18A  generates a larger amount of heat than the other end portion of the heater, and as illustrated in  FIG. 18B , temperatures T of the fixing belt  20  include the highest temperature in a left part in  FIG. 18B . Note that the heat generation amounts in the end portions of the heater  22  alone are measured to determine whether “one end portion of the heater  22  in the longitudinal direction generates the larger amount of heat than the other end portion of the heater”. 
     The pressing devices according to the present embodiment change the pressing forces applied to the flanges  321  and  322  under the above-described uniform pressure condition based on the distribution of temperatures T of the fixing belt  20  or the distribution of heat generation amounts of the heater  22 . Specifically, as illustrated in the lower part of  FIG. 18A , the pressing device changes the pressing force applied to the flange  321  supporting the one end portion of the fixing belt  20  in the longitudinal direction thereof from the pressing force FL to a pressing force FL 1  smaller than the pressing force FL. Additionally, the pressing device maintains the pressing force applied to the flange  322  supporting the other end portion of the fixing belt  20  to be the same as the pressing force FR. Hereinafter, the above-described setting of the pressing forces is referred to as a first pressure condition. That is, the pressing force applied to the flange  321  is set smaller than the pressing force applied to the flange  322 . As a result, a nip pressure in a part of the nip N corresponding to the portion of the heater  22  that generates the larger amount of heat than the other portion of the heater  22  in the longitudinal direction becomes relatively smaller than a nip pressure in the other part of the nip N. The nip pressure may be replaced a pressure contact force between the fixing belt  20  and the pressure roller  21  at the nip N. In addition, under the first pressure condition, a nip width in the part of the nip N corresponding to the portion of the heater  22  that generates the larger amount of heat than the other portion of the heater  22  in the longitudinal direction becomes relatively smaller than a nip width in the other part of the nip N. The nip width is a width of the nip N in a direction orthogonal to the longitudinal direction of the heater  22  that is also a conveyance direction of the sheet P in the nip N. Accordingly, the above-described condition can prevent the disadvantage caused by the temperature difference between the one end portion and the other end portion in the longitudinal direction of the heater  22 . That is, the above-described condition can reduce the difference in the fixing property between the one end portion and the other end portion in the longitudinal direction of the heater  22  and the unevenness in glossiness in the longitudinal direction. That is, unevenness of the image or the unevenness in glossiness of the image on the sheet can be reduced. 
     On the other hand, as illustrated in  FIGS. 19A and 19B , when all the heat generator groups are energized to fix the image on the large sheet such as a A3 size sheet, the other end portion of the heater  22  in the longitudinal direction in  FIG. 19A  generates a larger amount of heat than the one end portion of the heater, and as illustrated in  FIG. 19B , temperatures T of the fixing belt  20  include the highest temperature in a right part in  FIG. 19B . In this case, the pressing force applied to the flange  321  is set to be the same as the pressing force FL of the uniform pressure condition, and the pressing force applied to the flange  322  is changed from the pressing force FR to a pressing force FR 1  smaller than the pressing force FR. Hereinafter, the above-described setting of the pressing forces is referred to as a second pressure condition. That is, the pressing force applied to the flange  322  is set smaller than the pressing force applied to the flange  321 . As a result, the nip pressure in the part of the nip N corresponding to the portion of the heater  22  that generates the larger amount of heat than the other portion of the heater  22  in the longitudinal direction becomes relatively smaller than the nip pressure in the other part of the nip N. In addition, under the second pressure condition, the nip width in the part of the nip N corresponding to the portion of the heater  22  that generates the larger amount of heat than the other portion of the heater  22  in the longitudinal direction becomes relatively smaller than the nip width in the other part of the nip N. Accordingly, the above-described condition can prevent the disadvantage caused by the temperature difference between the one end portion and the other end portion in the longitudinal direction of the heater  22 . That is, the above-described condition can reduce the difference in the fixing property between the one end portion and the other end portion in the longitudinal direction of the heater  22  and the unevenness in glossiness in the longitudinal direction. 
     To obtain the pressing force and the nip pressure, a pressure distribution measurement system may be used. The pressure distribution measurement system can measure a pressure in the nip N. The nip pressure can be obtained by dividing the pressing force by an area applied the pressing force. Specifically, a pressure distribution measurement system (I-SCAN, manufactured by Nitta Corporation) or the like can be used. 
     The nip width may be measured as follows. First, a solid black image is formed on the sheet by another image forming apparatus in advance, and the sheet with the solid black image is passed through the fixing device. Then, while the sheet is being passed through the fixing device, the fixing device is forcibly stopped and stopped for 10 seconds, and then the sheet on which the solid black image is formed is pulled out. As a result, a glossy portion is formed on the solid black image. The glossy portion has the same width as the nip width. Measuring the width of the glossy portion gives the nip width. Alternatively, the nip width may be measured as follows. First, an overhead projector (OHP) sheet is inserted into the nip N of the fixing device, and the contact state of the OHP sheet in the nip is continued for a certain period of time. Then, the OHP sheet is pulled out from the nip, and a trace having the nip width is formed on the OHP sheet. Measuring the width of the trace gives the nip width. 
     Next, with reference to  FIG. 20 , a specific embodiment of a switching timing between the above-described pressure conditions is described below. 
     As illustrated in  FIG. 20 , power is supplied to the image forming apparatus  1  and the fixing device  9  in step S 0 . In step S 1 , the pressing device presses the fixing belt  20  against the pressure roller  21  under the uniform pressure condition. 
     In step S 2 , a controller in the image forming apparatus  1  receives a print instruction and confirms a size of the sheet to be printed. In step S 3 , the controller starts print operations, that is, image forming operations. It should be noted that the above-described print operations (i.e. the image forming operations) include various kinds of operations for printing since the controller in the image forming apparatus  1  receives the print instruction. For example, the various kinds of operations include heating the fixing belt to the fixing temperature and rotating various kinds of rollers to convey the sheet. The print operations include operations until the last printed sheet is ejected to the outside of the image forming apparatus and the image forming apparatus finishes the various kinds of operations for printing. 
     When the controller starts the print operations, the controller controls the heater  22  to heat and maintain the fixing belt  20  to the target temperature so that the fixing device  9  can operate. After the controller starts the print operations, the controller controls the pressing device to change the pressure condition to press the fixing belt  20  based on the size of the sheet to be printed. Specifically, the controller determines whether the sheet size is small in step S 4 , and, as described above, when the sheet size is small, the controller sets the pressing device to the first pressure condition in step S 5 A. When the sheet size is large, the controller sets the pressing device to the second pressure condition in step S 5 B. In the present embodiment, the controller sets the pressing device to either the first pressure condition or the second pressure condition but may set the pressing device to the uniform pressure condition for printing based on print conditions such as the sheet size. The timing at which the pressing device changes the pressure condition after the controller starts the print operations may be, for example, a predetermined timing until the sheet firstly enters the fixing device such as a timing immediately after the start of the print operations. 
     While the pressing device maintains the pressure condition set in step S 5 A or step S 5 B, the sheet passes through the fixing device  9 . When all print operations are performed in step S 6 , that is, when the fixing device  9  completes fixing operations on all the sheets and the print operations are completed, the controller controls the pressing device to change the pressure condition to the uniform pressure condition in step S 7 . 
     Changing the pressure condition in the pressing device based on the size of sheet that passes through the fixing device  9  can uniform the fixing property of the image on the sheet from one end to the other end of the sheet in the longitudinal direction of the fixing belt and, as a result, reduce the unevenness of the image or the unevenness in glossiness of the image on the sheet. In addition, setting the pressing device to the uniform pressure condition except when the printing operations are performed reduces the time when the lateral deviation of the pressure applied to the fixing belt  20  occurs. This reduces the lateral deviation of abrasion of the fixing belt  20  and the pressure roller  21 . 
     Next, with reference to  FIG. 21 , a different embodiment of a switching timing between the pressure conditions is described below. 
     In the flowchart of the embodiment illustrated in  FIG. 21 , after the controller starts the print operations in step S 3 , the controller does not change the pressure condition, that is, sets the pressing device to the uniform pressure condition until the Bth sheet (B is a predetermined number) passes through the fixing device in steps S 11 A and S 11 B. Note that the time at which the Bth sheet passes through the fixing device is defined as the time at which a sensor disposed near the outlet of the fixing nip N detects the trailing end of the Bth sheet. After the Bth sheet passes through the fixing device, the controller controls the pressing device to change the pressure condition according to the sheet size in step S 5 A or step S 5 B. After the change of the pressure condition, similar to the embodiment illustrated in  FIG. 20 , the controller controls the pressing device to change the pressure condition to the uniform pressure condition after the all print operations are completed.  FIG. 21  illustrates the case in which the number of sheets to be printed is B or more. When the number of sheets to be printed is less than B, the image forming apparatus ends the print operations without changing the pressure condition from the uniform pressure condition. 
     Immediately after the controller starts the printing operations, the heater  22  and the fixing belt  20  have small lateral temperature differences. Accordingly, setting the pressure condition to the uniform pressure condition until the Bth sheet passes through the fixing device as in the present embodiment enables the pressing device to press the fixing belt  20  under the uniform pressure condition during a time period when uneven glossiness and uneven fixing property of the image is unlikely to occur. That is, the time during which the pressure deviation occurs in each of the fixing belt  20  and the pressure roller  21  in the present embodiment is shorter than that in the embodiment illustrated in  FIG. 20  in which the pressure condition is changed immediately after the start of the print operations as described above, and the present embodiment can reduce the lateral deviation of abrasion of the fixing belt  20  and the pressure roller  21 . 
     In the above-described embodiment, the controller controls the pressing device to change the pressure condition after the Bth sheet passes through the fixing device. However, the present disclosure is not limited to this, and providing a sensor at a position corresponding to a timing such as after the Bth sheet is ejected outside the image forming apparatus or after the Bth sheet passes through the entrance of the fixing device enables selecting such a timing. Alternatively, the controller may control the pressing device to change the pressure condition according to the sheet size after a predetermined time C has passed since the controller starts the print operations. The above-described case also sets the pressing device to the uniform pressure condition during the time period when uneven glossiness and uneven fixing property of the image is unlikely to occur and reduces the lateral deviation of abrasion of the fixing belt  20  and the pressure roller  21 . A start timing to measure the above-described time C is not limited to the timing at which the controller starts the print operations. The start timing may be when the first sheet passes through a registration roller, when the first sheet reaches the fixing device, or the like. The optimum values of B sheets and the time C can be selected according to the productivity of the image forming apparatus, the thermal capacity of the fixing belt, the linear velocity of the sheet, the sheet thickness, etc. For example, B sheets may be set to 2 sheets, and the time C may be set to 10 seconds. 
     Next, with reference to  FIG. 22 , an embodiment is described in which the controller changes the pressure condition based on temperatures detected by the temperature detectors. 
     As illustrated in  FIG. 22 , the fixing device according to the present embodiment includes temperature detectors  41   a  and  41   b  facing the fixing belt  20  to detect a temperature of the surface of the fixing belt  20  on each of one portion of the fixing belt  20  and the other portion of the fixing belt  20  which are far from each other in the longitudinal direction of the fixing belt  20 . The longitudinal direction of the fixing belt  20  is also the longitudinal direction of the heater  22  and the direction orthogonal to the sheet conveyance direction. As the temperature detectors  41   a  and  41   b , for example, a thermistor may be adopted, and other temperature detectors may be appropriately used. 
     In the present embodiment, the temperature detectors  41   a  and  41   b  are disposed at positions corresponding to one end portion and the other end portion in the longitudinal direction of the small sheet P that passes through the fixing nip in the longitudinal direction of the heater  22 . In other words, the temperature detectors  41   a  and  41   b  are disposed at the positions corresponding to the second block and the sixth block, respectively. 
     The controller determines whether the controller controls the pressing device to change the pressure condition depending on whether or not the difference between the temperature Ta detected by the temperature detector  41   a  and the temperature Tb detected by the temperature detector  41   b  exceeds the set temperature difference threshold T 1 . In the present embodiment, the controller uses the temperature detection results detected by the temperature detectors  41   a  and  41   b  to determine the pressure condition for the small sheet that passes through the fixing device  9 . 
     Specifically, as illustrated in  FIG. 23 , when the controller receives the print instruction in step S 2  and starts the print operations for the small sheet in step S 3 , the controller obtains the temperature detection results Ta and Tb detected by the temperature detectors  41   a  and  41   b  at a predetermined time intervals, calculates the difference Ta−Tb between the temperature detection results Ta and Tb, and determines whether the difference Ta−Tb is equal to or greater than the temperature difference threshold T 1  in step S 41 . When the difference Ta−Tb is equal to or greater than the temperature threshold T 1  (Yes in step S 41 ), the controller controls the pressing device to change the pressure condition from the uniform pressure condition to the first pressure condition, and the pressing device reduces the pressing force applied to the one end portion of the fixing belt  20  in the longitudinal direction thereof. In other words, the controller controls one pressing device nearer to the temperature detector that detects a higher temperature among the temperature detectors  41   a  and  41   b  than the other pressing device to reduce the pressing force applied to the portion of the fixing belt  20 . After the change of the pressure condition, similar to the embodiment illustrated in  FIG. 20 , the controller controls the pressing device to change the pressure condition to the uniform pressure condition after the all print operations are completed. When the difference Ta−Tb does not exceed the temperature threshold T 1  (No in step S 41 ), the controller determines whether the controller completes the print operations in step S 43 . As a result, the all print operations are completed. 
     The temperature detectors may be disposed at positions corresponding to both end portions of the large sheet in the width direction of the large sheet. For example, as illustrated in  FIG. 24 , the temperature detectors  41   a  and  41   b  are disposed at positions corresponding to one end portion and the other end portion in the longitudinal direction of the large sheet P. In other words, the temperature detectors  41   a  and  41   b  are disposed at the positions corresponding to the first block and the seventh block, respectively. In the present embodiment, the controller uses the temperature detection results detected by the temperature detectors  41   a  and  41   b  to determine the pressure condition for the large sheet that passes through the fixing device  9 . 
     Specifically, as illustrated in  FIG. 25 , when the controller receives the print instruction in step S 2  and starts the print operations for the large sheet in step S 3 , the controller obtains the temperature detection results Ta and Tb detected by the temperature detectors  41   a  and  41   b  at a predetermined time intervals, calculates the difference Ta−Tb between the temperature detection results Ta and Tb, and determines whether the difference Ta−Tb is equal to or greater than a temperature difference threshold T 2  in step S 42 . When the difference Ta−Tb is equal to or greater than the temperature threshold T 2  (Yes in step S 42 ), the controller controls the pressing device to change the pressure condition from the uniform pressure condition to the second pressure condition, and the pressing device reduces the pressing force applied to the other end portion of the fixing belt  20  in the longitudinal direction thereof. In other words, the pressing device reduces the pressing force applied to the portion of the fixing belt  20  at which one of the temperature detectors  41   a  and  41   b  detects a higher temperature than the other one of the temperature detectors  41   a  and  41   b.    
     As described above, the temperature detectors  41   a  and  41   b  to detect the temperatures of the fixing belt  20  enables changing the pressure condition at a more appropriate timing. Accordingly, the above-described embodiment can prevent the disadvantage caused by the temperature difference between the one portion and the other portion of the heater  22  in the longitudinal direction. That is, the unevenness in glossiness of the image and the uneven fixing property of the image on the sheet can be efficiently reduced. In addition, the time during which the pressure deviation occurs in each of the fixing belt  20  and the pressure roller  21  in the present embodiment is shorter than that in the embodiment illustrated in  FIG. 20 , and the present embodiment can reduce the lateral deviation of abrasion of the fixing belt  20  and the pressure roller  21 . 
     The temperatures T 1  and T 2  are preferably set to 20 deg or less in order to effectively prevent the unevenness in glossiness of the image and the uneven fixing property. The temperatures T 1  and T 2  are set in consideration of the temperature detection errors of the temperature detectors  41   a  and  41   b , the error of the arrangement, the variation of the sheet conveyance position with respect to the fixing nip, and the error of the arrangement of the resistive heat generators  59 . That is, in order to avoid detection errors due to these factors, it is more preferable to set the temperatures T 1  and T 2  to about 10 deg. 
     As described above, the image forming apparatus according to the present embodiment can change the pressure conditions at each timing for each of the large sheet and the small sheet. In one image forming apparatus, the condition for changing to the first pressure conditions and the condition for changing to the second pressure condition may be common or different. For example, the controller may change the pressure condition to the first pressure condition immediately after the controller starts the printing operations for the small sheet but, when the large sheet is printed, the controller may change the pressure condition after the Bth sheet passes through the fixing device  9 . Depending on how the temperature difference occurs, the condition for changing the pressure condition may be appropriately selected. The controller may change the pressure condition only for one of the sizes of sheets. 
     As a timing different from the timing described above to change the pressure condition, the controller may change the pressure condition while the sheet passes through the fixing device (i.e. the sheet P passes through the fixing nip N in  FIG. 2 ) and set the uniform pressure condition before or after the sheet P passes through the fixing device. The above-described timing can further reduce the time during which the pressure deviation occurs, without impairing the effect of preventing uneven glossiness of the image and the uneven fixing property of the image. 
     In the above embodiments, the pressing device presses the portion of the fixing belt  20  (that is, the one of the flanges that supports the portion of the fixing belt  20 ) corresponding to the portion of the heater  22  generating the larger amount of heat than the other portion of the heater  22  with the pressing force smaller than the pressing force of the uniform pressure condition. However, conversely, the pressing device may press the one of the flanges that supports the portion of the fixing belt  20  corresponding to the portion of the heater  22  generating the smaller amount of heat than other portion of the heater  22  with the pressing force larger than the pressing force of the uniform pressure condition. 
     In the above embodiments, when the controller completes all the print operations, the controller controls the pressing devices to return the pressure condition to the uniform pressure condition. However, the timing to return the pressure condition to the uniform pressure condition is not limited to this and may be set immediately after the sheet is lastly ejected from the main body of the image forming apparatus  1  in  FIG. 1  (that is, immediately after the sheet is lastly ejected to the outside of the image forming apparatus  1  in  FIG. 1 ) or immediately after the sheet lastly passes through the fixing device (that is, immediately after the trailing edge of the sheet P passes through the fixing nip N in  FIG. 2 ). The above-described timing can further reduce the time during which the pressure deviation occurs in the fixing belt  20  and the pressure roller  21 , without impairing the effect of preventing uneven glossiness of the image and the uneven fixing property of the image. 
     Next, the pressing device to press each of the flanges  321  and  322  and change the pressing force applied to each of the flanges  321  and  322  is described. 
     As illustrated in  FIG. 26A , the fixing device  9  includes the pressing device  80 A to press the flange  321  disposed on one end of the fixing belt  20  in the longitudinal direction of the fixing belt  20 . The pressing device  80 A includes a spring  33  as a biasing member, a pressure lever  81  as a pressing unit, and a cam  82  as a pressing force adjuster. 
     One end of the spring  33  is coupled to the flange  321 , and the other end of the spring  33  is coupled to the pressure lever  81 . 
     The pressure lever  81  has a fulcrum  81   a  at one longitudinal end thereof. The fulcrum  81   a  is fixed to the frame of the fixing device  9  (for example, the side wall  28  in  FIG. 3 ), and the pressure lever  81  is rotatably provided around the fulcrum  81   a  (see the double-headed arrow in  FIG. 26A ). The other longitudinal end of the pressure lever  81  contacts the cam  82 . The spring  33  is coupled to a surface of the pressure lever  81  opposite the right surface of the pressure lever  81  in  FIG. 26A  on which the cam  82  contacts. 
     The cam  82  is provided rotatably around a cam shaft  82   a . The cam shaft  82   a  is coupled to the drive control mechanism  83 . 
     The drive control mechanism  83  includes a motor  84  that applies a rotational drive force to the cam shaft  82   a  and a controller  85  that controls the motor. 
     The cam  82  presses the one end of the pressure lever  81 , and the pressing force is transmitted to the flange  321  via the spring  33  and presses the fixing belt  20  against the pressure roller  21 . 
     As illustrated in  FIG. 26B , similar to the pressing device  80 A to press the flange  321 , the fixing device  9  includes the pressing device  80 B to press the flange  322  disposed on the other end of the fixing belt  20  in the longitudinal direction of the fixing belt  20 . The pressing device  80 B has basically the same configuration as the pressing device  80 A. The cams  82 A and  82 B provided on the pressing devices  80 A and  80 B have a common cam shaft  82   a . The drive control mechanism  83  gives a driving force to the cam shaft  82   a  and rotates the cams  82 A and  82 B by the same phase. In the present embodiment, the two cams  82 A and  82 B are mounted in a phase shift of 120 degrees relative to the cam shaft  82   a . The drive control mechanism  83  that rotates the cam shaft  82   a  includes a pulse motor that drives the cam shaft  82   a  at intervals of 120 degrees. Each pressure lever  81  is independently rotatable about each fulcrum  81   a.    
     The pressing device  80 A presses the flange  321 , the pressing device  80 B presses the flange  322 , and the flanges  321  and  322  press the fixing belt  20  against the pressure roller  21  to form the fixing nip N. 
     The drive control mechanism  83  rotates the cam shaft  82   a  to change the pressing forces applied to the flanges  321  and  322 . That is, rotating each cam  82  about the cam shaft  82   a  changes a surface at which the cam  82  contacts the pressure lever  81  to change the pressing force. 
     In the uniform pressure condition, as illustrated in  FIGS. 26A and 26B , the pressing devices  80 A and  80 B set the both cams  82 A and  82 B so that long radius portions of the both cams  82 A and  82 B (each of which has a radius R 1 ) contact the pressure lever  81 . As a result, the pressing devices  80 A and  80 B apply the same pressing forces FL and FR to the flanges  321  and  322 , respectively. The cam  82 A of the pressing device  80 A and the cam  82 B of the pressing device  80 B have different rotational phases, and short radius portions of the cams  82 A and  82 B contact the pressure lever  81  at different timings. Note that  FIGS. 26A and 26GB  are views seen from the same direction. 
     As illustrated in  FIGS. 27A and 27B , rotating the cam shaft  82   a  by a predetermined rotation amount (i.e. in the present embodiment, 120 degrees clockwise from the phase illustrated in  FIG. 26A ) changes the pressure condition from the uniform pressure condition to the first pressure condition. Specifically, the short radius portion (i.e. a portion having a radius R 2 ) of the cam  82  in the pressing device  80 A contacts the pressure lever  81 , and the long radius portion of the cam  82  in the pressing device  80 B contacts the pressure lever  81 . Changing the surface at which the cam  82  of the pressing device  80 A contacts the pressure lever  81  from the long radius portion to the short radius portion reduces the pressing force of the cam  82  on the pressure lever  81  and decreases a spring load of the spring  33  that acts on the flange  321 . That is, the pressing force applied to the flange  321  becomes small. On the other hand, the pressing force of the pressing device  80 B that presses the flange  322  does not change. As a result, the pressing forces of the pressing devices  80 A and  80 B are set to be the pressing forces FL 1  and FR, respectively. According to the displacement of the pressure lever  81  in the lateral direction in  FIGS. 26A and 27A , the amount of expansion or contraction of the spring  33  changes. 
     As illustrated in  FIGS. 28A and 28B , rotating the cam shaft  82   a  by a predetermined rotation amount (i.e. in the present embodiment, 240 degrees clockwise from the phase illustrated in  FIG. 26A ), which is different from the rotation amount for changing the first pressure condition, changes the pressure condition to the second pressure condition. Specifically, the long radius portion of the cam  82  in the pressing device  80 A contacts the pressure lever  81 , and the short radius portion of the cam  82  in the pressing device  80 B contacts the pressure lever  81  to set the pressing force FL in the pressing device  80 A and the pressing force FR 1  in the pressing device  80 B. 
     Changing the phases of the cams  82 A and  82 B in the pressing devices  80 A and  80 B enables a configuration in which the common cam shaft  82   a  rotates the cams  82 A and  82 B to change the pressing forces in the pressing devices  80 A and  80 B. Sharing the cam shaft  82   a  between the pressing devices  80 A and  80 B can reduce the driving force of the pressing devices  80 A and  80 B and prevent an occurrence of deviation in the rotational phase between the cams  82 A and  82 B. 
     In the above embodiments, the pressing device corresponding to the portion of the heater that generates the larger amount of heat than the other portion of the heater in the longitudinal direction of the heater reduces the pressing force. However, the pressing device corresponding to the portion of the heater that generates the smaller heat generation amount than the other portion may increase the pressing force. That is, the pressing device  80 B may increase the pressing force to set the first pressure condition, and the pressing device  80 A may increase the pressing force to set the second pressure condition. 
     As an embodiment,  FIGS. 29A and 29B  illustrates the pressing device  80 A and the pressing device  80 B that increase the pressing force as described above. The difference from the pressing devices in the other embodiments described above is that the circumferential range of the short radius portion (i.e. the portion having the radius R 2 ) of each of the cams  82 A and  82 B is wider than the circumferential range of the long radius portion (i.e. the portion having the radius R 1 ). Similar to the other embodiments, the phase of the cam  82 A in the pressing device  80 A is different from the phase of the cam  82 B in the pressing device  80 B by 120 degrees.  FIGS. 29A and 29B  illustrate the pressing devices  80 A and  80 B under the uniform pressure condition. Rotating the cam shaft  82   a  illustrated in  FIGS. 29A and 29B  clockwise by 120 degrees causes the long radius portion of the cam  82 B in the pressing device  80 B to contact the pressure lever  81 . As a result, the pressing force FR applied by the pressing device  80 B is changed to be larger than the pressing force FL applied by the pressing device  80 A. Alternatively, rotating the cam shaft  82   a  illustrated in  FIGS. 29A and 29B  clockwise by 240 degrees causes the long radius portion of the cam  82 A in the pressing device  80 A to contact the pressure lever  81 . As a result, the pressing force FL applied by the pressing device  80 A is changed to be larger than the pressing force FR applied by the pressing device  80 B. 
     In the above embodiments, the pressing devices  80 A and  80 B change the pressing forces. However, each of the pressing devices  80 A and  80 B may change the pressing force to change a pressing amount of the fixing belt  20  to the pressure roller  21  to change the fixing nip width. 
     For example, as illustrated in  FIG. 30 , the pressure lever  81  in the pressing device  80 A according to the present embodiment includes a pressure portion  81   b  instead of the spring  33 . The pressure portion  81   b  projects toward the flange  321  and contacts the flange  321 . The pressing device  80 B basically has the same configuration. 
     In the pressing device including the spring  33  described above, the displacement of pressure lever  81  is replaced and absorbed by the amount of compression of the spring  33 . On the other hand, in the present embodiment, the flange  321  moves by an amount corresponding to the displacement of the pressure lever  81  in the lateral direction in  FIG. 30 , and the displacement of the pressure lever  81  changes a state in which the fixing belt  20  presses against the pressure roller  21 . That is, the width of the fixing nip N changes. 
       FIG. 30  illustrates the cam  82 A including the short radius portion with a narrow circumferential range. However, as illustrated in  FIG. 29 , each of the cams  82 A and  82 B may include the long radius portion with a narrow circumferential range to increase the width of the fixing nip N on a portion of the fixing belt near the portion of the heater that generates smaller heat the other portion of the heater in the longitudinal direction of the heater. 
     In the above-described embodiments, one of the pressing devices corresponding to the one portion of the heater that generates the larger amount of heat than the other portion of the heater generates the smaller pressing force than the other pressing device corresponding to the other portion of the heater, which prevents the disadvantage caused by the temperature difference of the heater  22  and the fixing belt  20  in the longitudinal direction. That is, the fixing device according to the present disclosure prevents uneven glossiness of the image and uneven fixing property of the image. Accordingly, speeding up and downsizing the image forming apparatus can be achieved. 
     The pressing devices in the above embodiments press the flanges supporting the fixing belt. However, as illustrated in  FIG. 31 , the pressing device may press the shaft  21   d  of the pressure roller  21  to press the pressure roller  21  against the fixing belt  20 . Although the pressing device presses the shaft  21   d  of the pressure roller  21  in  FIG. 31 , the pressing device may press a bearing supporting the shaft of the pressure roller  21 . 
     The Embodiments of the present disclosure are particularly suitable for the heater downsized in the short-side direction. Specifically, it is preferable for the embodiments to be applied to the heater  22  illustrated in  FIG. 32  in which a ratio (R/Q) of the short-side dimension R of the resistive heat generators  59  to the short-side dimension Q of the heater  22  (i.e. the base  50 ) is not less than 25%. It is more preferably for the embodiments to be applied to the heater  22  having the ratio (R/Q) of 40% or more in the short-side direction. A larger effect can be expected by applying the embodiments to the small heater  22  as described above. 
     In order to decrease the variation in the temperature of the heater  22  described above, a resistive heat generator having a PTC characteristic may be used. PTC defines a property in which the resistance value increases as the temperature increases. Therefore, for example, a heater output decreases under a given voltage when the temperature increases. The heat generator having the PTC property starts quickly with an increased output at low temperatures and prevents overheating with a decreased output at high temperatures. For example, if a temperature coefficient of resistance (TCR) of the PTC is in a range of from about 300 ppm/° C. to about 4,000 ppm/° C., the heater  22  is manufactured at reduced costs while retaining a resistance required for the heater  22 . The TCR is preferably in a range of from about 500 ppm/° C. to about 2,000 ppm/° C. 
     The TCR can be calculated using the following equation (2). In the equation (2), T 0  represents a reference temperature, T 1  represents a freely selected temperature, R 0  represents a resistance value at the reference temperature T 0 , and R 1  represents a resistance value at the selected temperature T 1 . For example, in the heater  22  described above with reference to  FIG. 7 , the TCR is 2,000 ppm/° C. from the equation (2) when the resistance values between the first electrode  61 A and the second electrode  61 B are 10Ω (i.e., resistance value R 0 ) and 12Ω (i.e., resistance value R 1 ) at 25° C. (i.e., reference temperature T 0 ) and 125° C. (i.e., selected temperature T 1 ), respectively.
 
TCR=( R 1− R 0)/ R 0/( T 1− T 0)×106  (2)
 
     The heater to which the embodiments of the present disclosure are applied is not limited to the heater  22  including block-shaped (or square-shaped) resistive heat generators  59  as illustrated in  FIG. 7 . For example,  FIGS. 33A and 33B  are plan views of heaters  22 V and  22 W as variations of the heater  22 . The embodiments are applicable to the heaters  22 V and  22 W including resistive heat generators  59  having a shape in which a straight line is folded back as illustrated in  FIGS. 33A and 33B . The embodiments are also applicable to a heater including resistive heat generators having another shape. In  FIGS. 33A and 33B , portions filled with gray are the resistive heat generators  59 . In  FIG. 33A , the heater  22 V has power supply lines extending in a direction intersecting the longitudinal direction of the heater  22 V from the power supply line  62 A or  62 D extending in the longitudinal direction. On the other hand, in  FIG. 33B , the heater  22 W has the resistive heat generators  59  having portions extending in the direction intersecting the longitudinal direction of the heater  22 W from the power supply line  62 A or  62 D extending in the longitudinal direction. 
     The embodiments of the present disclosure are also applicable to fixing devices as illustrated in  FIGS. 34 to 36 , respectively, other than the fixing device  9  described above. Referring now to  FIGS. 34 to 36 , a description is given of some variations of the fixing devices. 
     First, the fixing device  9  illustrated in  FIG. 34  includes a pressurization roller  90  opposite the pressure roller  21  with respect to the fixing belt  20 . The fixing belt  20  is sandwiched by the pressurization roller  90  and the heater  22  and heated by the heater  22 . On the other hand, a nip formation pad  91  is disposed inside the loop of the fixing belt  20  and opposite the pressure roller  21 . The stay  24  supports the nip formation pad  91 . The fixing belt  20  is sandwiched by the nip formation pad  91  supported by the stay  24  and the pressure roller  21  to form the nip N between the fixing belt  20  and the pressure roller  21 . 
     The fixing device  9  illustrated in  FIG. 34  also includes the pressing devices as described in the above embodiments. The pressing device presses one of the fixing belt  20  and the pressure roller  21  against the other one of the fixing belt  20  and the pressure roller  21  or may press both the fixing belt  20  and the pressure roller  21  so that the fixing belt  20  and the pressure roller  21  press each other. One of the pressing devices corresponding to the one portion of the heater that generates the larger amount of heat than the other portion of the heater generates the smaller pressing force than the other pressing device corresponding to the other portion of the heater. As a result, the nip pressure in the part of the nip N corresponding to the portion of the heater  22  that generates the larger amount of heat than the other portion of the heater  22  in the longitudinal direction becomes relatively smaller than the nip pressure in the other part of the nip N. In addition, the nip width in the part of the nip N corresponding to the portion of the heater  22  that generates the larger amount of heat than the other portion of the heater  22  in the longitudinal direction becomes relatively smaller than the nip width in the other part of the nip N. Accordingly, the above-described fixing device  9  can prevent the disadvantage caused by the temperature difference between the one end portion and the other end portion in the longitudinal direction of the heater  22 . That is, the above-described fixing device can reduce the difference in the fixing property between the one end portion and the other end portion of the image in the longitudinal direction of the image and the unevenness in glossiness of the image in the longitudinal direction. That is, unevenness of the image or the unevenness in glossiness of the image on the sheet can be reduced. 
     Next, a description is given of in the fixing device  9  illustrated in  FIG. 35 , which does not include the above-described pressurization roller  90 . The fixing device  9  in  FIG. 35  includes the heater  22  formed to be arc having a curvature of the fixing belt  20  to keep a circumferential contact length between the fixing belt  20  and the heater  22 . Other parts of the fixing device  9  illustrated in  FIG. 35  are the same as the fixing device  9  illustrated in  FIG. 34 . 
     Finally, the fixing device  9  illustrated in  FIG. 36  is described. The fixing device  9  includes a heating assembly  92 , a fixing roller  93  that is a rotator and a fixing member, and a pressure assembly  94  that is a facing member. The heating assembly  92  includes the heater  22 , the heating unit  19 , which are described in the above embodiments, and the heating belt  120 . The fixing roller  93  includes a core  21   a , an elastic layer  21   b , and a release layer  21   c . The core  21   a  is a solid core made of iron. The elastic layer  21   b  coats the circumferential surface of the core  21   a . The release layer  21   c  coats an outer circumferential surface of the elastic layer  21   b . In addition, the fixing device  9  includes a pressure assembly  94  opposite the heating assembly  92  via the fixing roller  93 . The pressure assembly  94  includes a nip formation pad  95 , a stay  96 , and a pressure belt  97 . The nip formation pad  95  and the stay  96  are inside the loop of the pressure belt  97 . The pressure belt  97  is rotatable. The sheet P passes through the fixing nip N 2  between the pressure belt  97  and the fixing roller  93  and is applied to heat and pressure, and the image is fixed on the sheet P. 
     In the fixing device  9  illustrated in  FIG. 36 , the heating assembly  92  heats the fixing roller  93 . When the heater  22  generates a difference in the heat generation amount between one portion and the other portion of the heater  22  in the longitudinal direction (i.e. a depth direction in  FIG. 36 ), the fixing roller  93  also has a temperature difference between one portion and the other portion of the fixing roller  93  in the longitudinal direction of the fixing roller  93 . 
     Accordingly, the fixing device  9  illustrated in  FIG. 36  also includes the pressing devices that press one of the fixing roller  93  as the rotator (i.e. the fixing member) and the pressure assembly  94  as an opposite member against the other one of the fixing roller  93  and the pressure assembly  94  or may press both the fixing roller  93  and the pressure assembly  94  so that the fixing roller  93  and the pressure assembly  94  press each other. One of the pressing devices corresponding to the one portion of the heater that generates the larger amount of heat than the other portion of the heater generates the smaller pressing force than the other pressing device corresponding to the other portion of the heater. As a result, the nip pressure in the part of the nip N corresponding to the portion of the heater  22  that generates the larger amount of heat than the other portion of the heater  22  in the longitudinal direction becomes relatively smaller than the nip pressure in the other part of the nip N. In addition, the nip width in the part of the nip N corresponding to the portion of the heater  22  that generates the larger amount of heat than the other portion of the heater  22  in the longitudinal direction becomes relatively smaller than the nip width in the other part of the nip N. Accordingly, the above-described fixing device  9  can prevent the disadvantage caused by the temperature difference between the one end portion and the other end portion in the longitudinal direction of the heater  22 . That is, the above-described fixing device  9  can reduce the difference in the fixing property between the one end portion and the other end portion of the image in the longitudinal direction of the image and the unevenness in glossiness of the image in the longitudinal direction. That is, unevenness of the image or the unevenness in glossiness of the image on the sheet can be reduced. 
     A layout of the electrodes and the like arranged on the base  50  of the heater  22  is not limited to the above embodiments, and the present disclosure may be applied to the heater in which a temperature difference occurs between one portion and the other portion of the heater in the longitudinal direction. 
     For example,  FIG. 37  illustrates an example of another heater to which the present disclosure is applied. All electrodes of the heater  22  illustrated in  FIG. 37  are arranged on one portion in the longitudinal direction, which is different from the above-described embodiments. That is, the second electrode  61 B and other electrodes of the heater  22  in  FIG. 37  is disposed on one end portion in the longitudinal direction of the heater  22 , which is different from the heater  22  in  FIG. 10 . As illustrated in  FIG. 37 , since the second electrode  61 B is disposed on one end portion of the heater  22  in the longitudinal direction, the power supply line directly connected to the second electrode  61 B extends to the other end portion of the heater  22  in the longitudinal direction and turns back to resistive heat generators  59  to be connected to all resistive heat generators  59 . In the present embodiment, the power supply line that connects the second electrode  61 B and all resistive heat generators  59  includes the second power supply line  62 B that is connected to all resistive heat generators  59  and extends to a turning back position on the other end portion of the heater  22  and a fifth power supply line  62 E as the conductor extending from the turning back position to the second electrode  61 B on the one end portion of the heater  22  in the longitudinal direction of the heater  22 . 
     The temperature difference in the longitudinal direction as described above occurs in the above heater  22  of  FIG. 37  when the first heat generator group  60 A is energized and when the first heat generator group  60 A and the second heat generator group  60 B are energized. 
     When only the first heat generator group  60 A is energized, the unintended shunt occurs and flows toward the third power supply line  62 C, as illustrated in  FIGS. 38 and 39 . As a result, the distribution of the total heat generation amounts becomes unsymmetrical shape in the lateral direction with respect to the fourth block located in the center of the heat generation area, and the heat generation amount in the one end portion of the heater in the longitudinal direction is larger than the heat generation amount in the other end portion of the heater. When the first heat generator group  60 A and the second heat generator group  60 B are energized, as illustrated in  FIGS. 40 and 41 , the distribution of the total heat generation amounts becomes unsymmetrical shape in the lateral direction with respect to the fourth block, and the heat generation amount in the other end portion of the heater in the longitudinal direction is larger than the heat generation amount in the one end portion of the heater. 
     Similar to the above-described embodiments, one of the pressing devices corresponding to the one portion of the heater that generates the larger amount of heat than the other portion of the heater in the longitudinal direction of the heater generates the smaller pressing force than the other pressing device corresponding to the other portion of the heater. As a result, the nip pressure and the nip width in the part of the nip N corresponding to the portion of the heater  22  that generates the larger amount of heat than the other portion of the heater  22  in the longitudinal direction becomes relatively smaller than the nip pressure and the nip width in the other part of the nip N. Accordingly, the above-described embodiment can prevent the disadvantage caused by the temperature difference between the one portion and the other portion of the heater  22  in the longitudinal direction. That is, the above-described embodiment can reduce the difference in the fixing property between the one end portion and the other end portion of the image in the longitudinal direction of the image and the unevenness in glossiness of the image in the longitudinal direction. That is, unevenness of the image or the unevenness in glossiness of the image on the sheet can be reduced. 
     A heating device according to the present disclosure is not limited to the fixing device described in the above embodiments. The heating device according to the present disclosure is also applicable to, for example, a heating device such as a dryer to dry ink applied to the sheet, a coating device (a laminator) that heats, under pressure, a film serving as a covering member onto the surface of the sheet such as paper, and a thermocompression device such as a heat sealer that seals a seal portion of a packaging material with heat and pressure. Applying the present disclosure to the above heating device can prevent the disadvantage caused by the temperature difference between the one end portion and the other end portion in the longitudinal direction of the heater. 
     The sheets P serving as recording media may be thick paper, postcards, envelopes, plain paper, thin paper, coated paper, art paper, tracing paper, overhead projector (OHP) transparencies, plastic film, prepreg, copper foil, and the like. 
     The above-described embodiments are illustrative and do not limit the present disclosure. Thus, numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the present disclosure, the present disclosure may be practiced otherwise than as specifically described herein. Further, features of components of the embodiments, such as the number, the position, and the shape are not limited the embodiments and thus may be preferably set.