Patent Publication Number: US-9429889-B2

Title: Image heating apparatus

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image heating apparatus suitable for use as a fixing device (apparatus) to be mounted in an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer, and relates to the image forming apparatus in which the image heating apparatus is mounted. 
     In the image forming apparatus in which the image heating apparatus is mounted, when continuous printing is performed using a small-sized recording material having a width smaller than a maximum-width recording material (sheet) usable in the image heating apparatus, a non-sheet-passing portion temperature rise is generated. This is a phenomenon in which the temperature in a region (non-sheet-passing portion) through which the small-sized sheet passes with respect to a longitudinal direction of a fixing nip rises. 
     As one of methods for suppressing this non-sheet-passing portion temperature rise, in Japanese Laid-Open Patent Application (JP-A) 2003-317898, a method in which a high heat-conductive sheet having high thermal conductivity is sandwiched between a heater supporting member and a ceramic heater has been proposed. 
     It has been turned out that in order to cause the high heat-conductive sheet to sufficiently exhibit the proper performance in suppressing the rise in the non-sheet-passing-portion temperature, there is a need to bring the sheet into contact with the heater at high pressure. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished in view of the above-described problem, and a principal object of the present invention is to provide an image heating apparatus capable of applying pressure sufficiently to a high heat-conductive sheet. 
     Another object of the present invention is to provide the image heating apparatus having high positional accuracy of the high heat-conductive sheet relative to a heater. 
     According to an aspect of the present invention, there is provided an image heating apparatus comprising: a heater; a supporting member for supporting the heater; and a high heat-conductive sheet sandwiched between a part of the heater and the supporting member. A recording material on which an image is formed is heated by heat from the heater. The supporting member includes a bearing surface contacting the sheet so as to apply pressure between the heater and the sheet and includes an opposing portion opposing a part of the heater not sandwiching the sheet. In a state in which the pressure is applied between the heater and the sheet, the thickness of the sheet is not less than the height of a stepped portion between the bearing portion and the opposing portion. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an image forming apparatus in Embodiment 1. 
         FIG. 2  is a schematic cross-sectional view of a principal part of a fixing device (image heating apparatus). 
         FIG. 3  is a schematic first view of the principal part of the fixing device which is partly omitted in midstream. 
       In  FIG. 4 , (a) to (d) are illustrations of a structure of a heater (heat generating element). 
         FIG. 5  is a partly enlarged view of  FIG. 2 . 
         FIG. 6  is a block diagram of a control system. 
         FIG. 7  is a control circuit diagram of the heater. 
       In  FIG. 8 , (A) to (D) are illustrations of a pressing method of the heater and a high heat-conductive sheet. 
         FIG. 9  is a graph showing a relationship between a pressure and a contact thermal resistance of the heater and the high heat-conductive sheet. 
       In  FIG. 10 , (A) and (B) are illustrations showing a compression ratio of the high heat-conductive sheet. 
       In  FIG. 11 , (A) to (C) are illustrations of a modified example of a heater supporting member. 
       In  FIG. 12 , (A) to (C) are illustrations of a pressing method of a heater and a high heat-conductive member in Embodiment 2. 
       In  FIG. 13 , (A) to (C) are illustrations of a pressing method of a heater and a high heat-conductive member in Embodiment 3. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiment 1 
     (1) Image Forming Apparatus 
       FIG. 1  is a schematic cross-sectional view of an example of an image forming apparatus  100  in which an image heating apparatus according to the present invention is mounted as a fixing device  200 . This image forming apparatus  100  is a laser printer using electrophotographic recording technology, and forms an image, on a sheet (sheet-like recording material) P, corresponding to electrical image information inputted from a host device  500  ( FIG. 6 ) such as a personal computer into a controller  101 , and then prints out the sheet. 
     When a print signal is generated, a scanner unit  21  emits laser light modulated, depending on the image information, and scans a photosensitive member  19 , which is electrically charged to a predetermined polarity by a charging roller  16  and which is rotationally driven in the counterclockwise direction indicated by an arrow in  FIG. 1 . As a result, an electrostatic latent image is formed on the photosensitive member  19 . To this electrostatic latent image, a toner (developer) is supplied from a developing device  17 , so that a toner image, depending on the image information, is formed on the photosensitive member  19 . On the other hand, the sheets P stacked in a sheet-feeding cassette  11  are fed one by one by a pick-up roller  12 , and then are fed toward a registration roller pair  14  by a roller pair  13 . 
     Then, the sheet P is fed to a transfer position from the registration roller pair  14  in synchronism with timing when the toner image on the photosensitive member  19  reaches the transfer position formed between the photosensitive member  19  and a transfer roller  20 . In a process in which the sheet P passes through the transfer position, the toner image is transferred from the photosensitive member  19  onto the sheet P. Therefore, the sheet P is heated by the fixing device  200 , so that the toner image is heat-fixed on the sheet P. The sheet P carrying thereon the fixed toner image is discharged onto a tray  31  at an upper portion by roller pairs  26  and  27 . 
     The image forming apparatus  100  includes a cleaner  18  for cleaning the photosensitive member  19  and a motor  30  for driving the fixing device  200  and the like. The photosensitive member  19 , the charging roller  16 , the scanner unit  21 , the developing device  17 , the transfer roller  20 , and the like, which are described above, constitute an image forming portion. The photosensitive member  19 , the charging roller  16 , the developing device  17  and the cleaner  18  are constituted as a process cartridge  15  detachably mountable to a main assembly of the printer in a collective manner. An operation and image forming process of the above-described image forming portion are well known, and therefore will be omitted from detailed description. 
     The laser printer  100  in this embodiment uses a plurality of sheet sizes. Specifically, the laser printer  100  is capable of printing the image on sheets having the plurality of sheet sizes, including a letter paper size (about 216 mm×279 mm), an A4 paper size (210 mm×297 mm) and A5 paper size (148 mm×210 mm). 
     The printer basically feeds the sheet in a short-edge feeding manner (in which a long edge of the sheet is parallel to a (sheet) feeding direction) by center-line basis feeding, and a largest size (in width) of compatible regular sheet sizes (listed in a catalogue) is about 216 mm in width of the letter paper. This sheet having the largest width size is defined as a large-sized paper (sheet). Sheets (A4-sized paper, A5-sized paper and the like) having paper widths smaller than this sheet are defined as a small-sized paper. 
     The center-line basis feeding of the sheet P is such that even when any large and small (width) sheets capable of being passed through the printer are used, each of the sheets is passed through the printer in a manner in which a center line of the sheet with respect to a widthwise direction is aligned with a center (line) of a sheet feeding path with respect to the widthwise direction. 
     (2) Fixing Device (Image Heating Apparatus) 
     (2-1) Brief Description of Device Structure 
       FIG. 2  is a schematic cross-sectional view of a principal part of a fixing device  200  in this embodiment.  FIG. 3  is a schematic first view of the principal part of the fixing device  200  which is partly omitted in midstream. In  FIG. 4 , (a) to (d) are illustrations of a structure of a heater (heat generating element).  FIG. 5  is a partly enlarged view of  FIG. 2 .  FIG. 6  is a block diagram of a control system. 
     With respect to the fixing device  200  and constituent elements thereof in this embodiment, a front side (surface) is a side (surface) when the fixing device  200  is seen from a sheet entrance side thereof, and a rear side (surface) is a side (surface) (sheet exit side) opposite from the front side. Left and right are left (one end side) and right (the other end side) when the fixing device  200  is seen from the front side. Further, an upstream (side) and a downstream (side) are those with respect to a sheet feeding direction X. 
     A longitudinal direction (widthwise direction) and a sheet width direction of the fixing device are directions substantially parallel to a direction perpendicular to the feeding direction X of the sheet P (or a movement direction (movable member movement direction) of a film which is a movable member). A short direction of the fixing device is a direction substantially parallel to the feeding direction X of the sheet P (or the movement direction of the film). 
     The fixing device  200  in this embodiment is an on-demand fixing device of a film (belt) heating type and a tension-less type. The fixing device  200  roughly includes a film unit  203  including a flexible cylindrical (endless) film (belt)  202  as the movable member, and includes a pressing roller (elastic roller: rotatable pressing member)  208 , having a heat-resistant property and elasticity, as a nip-forming member. 
     The film unit  203  is an assembly of a heater  300  as a heating member, a high heat-conductive member  220 , a heater supporting member  201 , a pressing stay  204 , regulating members (flanges)  205  (L, R) for regulating shift (lateral deviation) of the film  202 , and the like. 
     The film  202  is a member for conducting method to the sheet P, and has a composite structure consisting of a cylindrical base layer (base material layer), an elastic layer formed on an outer peripheral surface of the base layer, a parting layer as a surface layer formed on an outer peripheral surface of the elastic layer, and an inner surface coating layer formed on an inner peripheral surface of the base layer. A material for the base layer is a heat-resistant resin such as polyimide or metal such as stainless steel. 
     Each of the heater  300 , the high heat-conductive member  220 , the heater supporting member  201  and the pressing stay  204  is a long member extending in a left-right direction of the fixing device. The film  202  is externally fitted loosely onto an assembly of the stay  204  and the heater supporting member  201  on which the heater  300  and the high heat-conductive member  220  are supported. The regulating members  205  (L, R) are mounted on one end portion and the other end portion of the pressing stay  204  in one end side and the other end side of the film  202 , so that the film  202  is interposed between the left and right regulating members  205 L and  205 R. 
     The heater  300  is a ceramic heater in this embodiment. The heater  300  has a basic structure including a ceramic substrate having an elongated thin plate shape and a heat generating element (heat generating resistor) which is provided on a surface of this substrate in one side of the substrate and which generates heat by energization (supply of electric power) to the heat generating element, and is a low-thermal-capacity heater increased in temperature with an abrupt rising characteristic by the energization to the heat generating element. A specific structure of the heater  300  will be described in (3) below in detail. 
     The heater supporting member  201  is a molded member formed of the heat-resistant resin, and is provided with a heater-fitting groove  201   a  along a longitudinal direction of the member at a substantially central portion with respect to a circumferential direction of the outer surface of the member. The high heat-conductive member  220  and the heater  300  are fitted (engaged) into and supported by the heater-fitting groove  201   a . In the groove  201   a , the high heat-conductive member  220  is interposed between the heater supporting member  201  and the heater  300 . The high heat-conductive member  220  will be described in (3) specifically. 
     The heater supporting member  201  not only supports the high heat-conductive member  220  and the heater  300  but also functions as a guiding member for guiding rotation of the film  202  externally fitted onto the heater supporting member  201  and the pressing stay  204 . 
     The pressing stay  204  is a member having rigidity, and is a member for providing a longitudinal strength to the heater supporting member  201  by being pressed against an inside (back side) of the resin-made heater supporting member  201  and for rectifying the heater supporting member  201 . In this embodiment, the pressing stay  204  is a metal-molded material having an U-shape in cross section. 
     Each of the regulating members  205  (L, R) is a molded member formed of the heat-resistant resin so that the regulating members  205  (L, R) have a bilaterally symmetrical shape, and have the functions of regulating (limiting) movement (thrust movement) along the longitudinal direction of the heater supporting member  201  during the rotation of the film  202  and of guiding an inner peripheral surface of a film end portion during the rotation of the film  202 . That is, each of the regulating members  205  (L, R) includes a flange portion  205   a , for receiving (stopping) the film end surface, as a first regulating (limiting) portion for regulating the thrust movement of the film  202 . Further, each of the regulating members  205  (L, R) includes an inner surface guiding portion  205   b  as a second regulating portion for guiding an inner surface of the film end portion by being fitted into the film end portion. 
     The pressing roller  208  is an elastic roller having a composite layer structure including a metal core  209  formed of a material such as iron or aluminum, an elastic layer  210  formed, of a material such as a silicone rubber, around the metal core in a roller shape, and a parting layer (surface layer)  210   a  coating an outer peripheral surface of the elastic layer  210 . 
     The pressing roller  208  is provided so that each of rotation center shaft portions  209   a  in left and right end portion sides is rotatably supported in the associated one of left and right side plates  250  (L, R) of a fixing device frame via the associated one of bearing members (bearings)  251  (L, R). The right-side shaft portion  209   a  is provided concentrically integral with a drive gear G. To this drive gear G, a driving force of the motor  30  controlled by a controller  101  via a motor driver  102  is transmitted via a power transmitting mechanism (not shown). As a result, the pressing roller  208  is rotationally driven as a rotatable driving member at a predetermined peripheral speed in the clockwise direction of an arrow R 208  in  FIG. 2 . 
     On the other hand, the film unit  203  is disposed on and substantially parallel with the pressing roller  208  while keeping a heater-disposed portion side of the heater supporting member  201  downward, and is disposed between the left and right side plates  250  (L, R). Specifically, a vertical guiding groove  205   c  provided in each of the left and right regulating members  250  (L, R) of the film unit  203  engages with an associated vertical guiding slit  250   a  provided in each of the left and right side plates  250  (L, R). 
     As a result, the left and right regulating members  205  (L, R) are supported by the left and right side plates  250  (L, R), respectively, so as to be vertically slidable (movable) relative to the left and right side plates  250  (L, R), respectively. That is, the film unit  203  is supported by and vertically slidable relative to the left and right side plates  250  (L, R). The heater-disposed portion of the heater supporting member  201  of the film unit  203  opposes the pressing roller  208  via the film  202 . 
     Further, pressure-receiving portions  205   d  of the left and right regulating members  205  (L, R) are pressed at a predetermined pressing force (pressure) by left and right pressing mechanisms  252  (L, R), respectively. Each of the left and right pressing mechanisms  252  (L, R) is a mechanism including, e.g., a pressing spring, a pressing lever or a pressing cam. That is, the film unit  203  is pressed against the pressing roller  208  at the predetermined pressing force, so that the film  202  on the heater-disposed portion of the heater supporting member  201  is press-contacted to the pressing roller  208  against the elasticity of the elastic (material) layer  210  of the pressing roller  208 . 
     As a result, the heater  300  contacts the inner surface of the film  202 , so that a nip N having a predetermined width with respect to a film movement direction (movable member movement direction) is formed between the film  202  and the pressing roller  208 . That is, the pressing roller  208  forms the nip N via the film  202  in combination with the heater  300 . 
     The heater  300  exists on the heater supporting member  201  at a position corresponding to the nip N and extends in the longitudinal direction of the heater supporting member  201 . In the fixing device  200  in this embodiment, the heater  300  and the heater supporting member  201  constitute a back-up member contacting the inner surface of the film  202 . Further, the pressing roller  208  forms the nip N via the film  202  in combination with the back-up member ( 300 ,  201 ). In this way, the heater  300  is provided inside the film  202 , and is press-contacted to the film  202  toward the pressing roller  208  to form the nip N. 
     (2-2) Fixing Operation 
     The fixing operation of the fixing device  200  is as follows. The controller  101  actuates the motor  30  at a predetermined control timing. From this motor  30  to the pressing roller  208 , a rotational driving force is transmitted. As a result, the pressing roller  208  is rotationally driven at a predetermined speed in the clockwise direction of the arrow R 208 . 
     The pressing roller  208  is rotationally driven, so that at the nip N, a rotational torque acts on the film  202  by a frictional force with the film  202 . As a result, the film  202  is rotated, by the rotation of the pressing roller  208 , in the counterclockwise direction of an arrow R 202  around the heater supporting member  201  and the pressing stay  204  at a speed substantially corresponding to the speed of the pressing roller  208  while being slid in close contact with the surface of the heater  300  at the inner surface thereof. Onto the inner surface of the film  202 , a semisolid lubrication is applied, thus ensuring a sliding property between the outer surface of each of the heater  300  and the heater supporting member  201  and the inner surface of the film  202  in the nip N. 
     Further, the controller starts energization (supply of electric power) from a power supplying portion (power controller)  103  to the heater  300 . The power supply from the power supplying portion  103  to the heater  300  is made is made via an electric connector  104  mounted in a left end portion side of the film unit  203 . By this energization, the heater  300  is quickly increased in temperature. 
     The temperature increase (rise) is detected by a thermistor (temperature detecting element)  211  provided in contact with the high heat-conductive member  220  contacting the back surface (upper surface) of the heater  300 . The thermistor  211  is connected with the controller  101  via an A/D converter  105 . The film  202  is heated at the nip N by heat generation of the heater  300  by the energization. 
     The controller  101  samples an output from the thermistor  211  at a predetermined period, and the thus-obtained temperature information is reflected in temperature control. That is, the controller  101  determines the contents of the temperature control of the heater  300  on the basis of the output of the thermistor  211 , and controls the energization to the heater  300  by the power supplying portion  103  so that a temperature of the heater  300  at a portion corresponding to the sheet-passing portion is a target temperature (predetermined set temperature). 
     In a control state of the fixing device  200  described above, the sheet P on which an unfixed toner image t is carried is fed from the image forming portion toward the fixing device  200 , and then is introduced into the nip N. The sheet P is supplied with heat from the heater  300  via the film  202  in a process in which the sheet P is nipped and fed through the nip N. The toner image t is melt-fixed as a fixed image on the surface of the sheet P by the heat of the heater  300  and the pressure at the nip N. That is, the toner image on the sheet (recording material) is heated and fixed. The sheet P coming out of the nip N is curvature-separated from the film  202  and is discharged from the device  200 , and then is fed. 
     The controller  101  stops, when the printing operation is ended, the energization from the power supplying portion  103  to the heater  300  by an instruction to end the fixing operation. Further, the controller stops the motor  30 . 
     In  FIG. 3 , A is a maximum heat generation region width of the heater  300 . B is a sheet-passing width (maximum sheet-passing width) of the large-sized paper, and is a width equal to or somewhat smaller than the maximum heat generation region width A. In this embodiment, the maximum sheet-passing width B is about 216 mm (short edge feeding) of the letter paper. A full length of the nip N formed by the film  202  and the pressing roller  208  (i.e., a length of the pressing roller  208 ) is a width larger than the maximum heat generation region width A of the heater  300 . 
     (3) Heater  300   
     In  FIG. 4 , (a) is a schematic plan view of the heater  300  which is partly cut away in one surface side (front surface side), (b) is a schematic plan view of the heater  300  in the other surface side (back surface side), (c) is a sectional view at (c)-(c) position in (b) of  FIG. 4 , and (d) is a sectional view at (d)-(d) position in (b) of  FIG. 4 . 
     In this embodiment, the heater  300  is the ceramic heater. Basically, the heater  300  includes a heater substrate  303  formed by ceramic in an elongated thin plate shape, heat generating resistors (heat generating members  301 - 1  and  301 - 2  provided along the longitudinal direction of the substrate in one surface side (front surface side) of the heater substrate  303 , and an insulating (surface) protecting layer  304  which covers the heat generating resistors. 
     The heater surface  303  is a ceramic substrate, formed of, e.g., Al 2 O 3  or AlN in an elongated thin plate shape, extending in a longitudinal direction crossing with (perpendicular to) a sheet-passing direction at the nip N. Each of the heat generating resistors  301 - 1  and  301 - 2  is formed by pattern-coating an electric resistance material paste of, e.g., Ag/Pd (silver/palladium) by screen printing and then by baking the paste. In this embodiment, the heat generating resistors  301 - 1  and  301 - 2  are formed in strip shape, and the two heat generating resistors are formed to be parallel to each other along the longitudinal direction of the substrate with a predetermined interval therebetween on the substrate surface with respect to the short direction of the substrate. 
     In one end side (left side) of the heat generating resistors  301 - 1  and  301 - 2 , the heat generating resistors are electrically connected to electrode portions (contact portions) C1 and C2, respectively, via electroconductive members  305 . Further, in the other end side (right side) of the heat generating resistors  301 - 1  and  301 - 2 , the heat generating resistors are electrically connected in series by an electroconductive member  305 . Each of the electroconductive members  305  and the electrode portions C1 and C2 is formed by pattern-coating the electroconductive material paste such as Ag by the screen printing or the like and then by baking the paste. 
     The surface protecting layer  304  is provided so as to cover a whole of the heater substrate surface except for the electrode portions C1 and C2. In this embodiment, the surface protecting layer  304  is formed of glass by pattern-coating a glass paste by the screen printing or the like and then by baking the paste. The surface protecting layer  304  is used for protecting the heat generating resistors  301 - 1  and  301 - 2  and for maintaining electrical insulation. 
     The electric power is supplied to between the electrode portions C1 and C2, so that each of the heat generating resistors  301 - 1  and  301 - 2  connected in series generates heat. The heat generating resistors  301 - 1  and  301 - 2  are made to have the same length. The length region of these heat generating resistors  301 - 1  and  301 - 2  constitutes the maximum heat generation region width A. A center-basis feeding line (phantom line) O for the sheet P is located at a position substantially corresponding to a bisection position of the maximum heat generation region width A of the heater  300 . 
     The heater  300  is fitted into the heater fitting groove  201   a  of the heater supporting member  201  so that the front surface thereof is directed upward and so that the high heat-conductive member  220  is interposed between the heater back surface and the heater supporting member  201  in the groove  201   a , and thus is supported by the heater supporting member  201 . The high heat-conductive member  220  is a member for suppressing a non-sheet-passing portion temperature rise during continuous sheet passing of the small-sized paper, and is interposed between the heater back surface and the heater supporting member  201  by being sandwiched between the heater back surface and a bearing surface of the groove  201   a.    
     In  FIG. 4 , (a) shows a state in which the high heat-conductive member  220  having a size and a shape such that the high heat-conductive member  220  covers a range longer than at least the heat generation region of the heat generating resistors  301 - 1  and  301 - 2  is disposed superposedly on the heater substrate back surface. The high heat-conductive member  220  is disposed at the heater substrate back surface so as to cover at least a region corresponding to the maximum heat generation region width A of the heater  300 . 
     The high heat-conductive member  220  is sandwiched and interposed between the heater back surface and the bearing surface of the groove  201   a  in a state in which the heater  300  is fitted into the heater fitting groove  201   a  of the heater supporting member  201  with the upward front surface and is thus supported by the heater supporting member  201 . Further, the high heat-conductive member  220  is sandwiched and pressed between the heater supporting member  201  and the heater  300  by the pressing force of the above-described pressing mechanisms  252  (L, R). 
       FIG. 5  is an enlarged view of  FIG. 2  in a region where the film  202  and the pressing roller  208  contact each other. The sheet P and the pressing roller  208  are omitted from illustration. The inner surface of the film  202  and the (front) surface of the surface protecting layer  304  of the heater  300  contact each other to form the nip N between the film  202  and the pressing roller  208 . 
     The high heat-conductive member  220  is a member higher in thermal conductivity than the heater  300 . In this embodiment, as the high heat-conductive member  220 , an anisotropic heat-conductive member (high heat-conductive sheet) higher in thermal conductivity with respect to a planar (surface) direction than the heater substrate  303  is used. 
     Compared with the heater substrate  303 , as a material having a high thermal conductivity with respect to the planar direction, it is possible to use a flexible sheet-shaped member or the like using, e.g., graphite. The high heat-conductive member  220  in this embodiment is the flexible sheet-shaped member using graphite as the material therefor, and the thermal conductivity with respect to a sheet surface direction thereof is higher than the thermal conductivity of the heater  300 . In this embodiment, as the high heat-conductive member  220 , the graphite sheet of 1000 V/mK in thermal conductivity with respect to the planar direction, 15 W/mK in thermal conductivity with respect to a thickness direction, 70 μm in thickness and 1.2 g/cm 3  in density was used. The thickness of the graphite sheet suitable for use in this embodiment is 60 μm to 1 mm. 
     A thermistor (temperature detecting element)  211  and a protecting element  212 , such as a thermoswitch, a temperature fuse or a thermostat, in which a switch is provided are contacted to the high heat-conductive member  220 , and are configured to receive the heat from the heater  300 , via the high heat-conductive member  220 , fitted into and supported by the heater fitting groove  201   a  of the heater supporting member  201 . The thermistor  211  and the protecting element  212  are pressed against the high heat-conductive member  212  by an urging member (not shown) such as a leaf spring. 
     The thermistor  211  and the protecting element  212  are positioned and disposed in one end side and the other end side, respectively, with respect to the center basis feeding line O as a boundary as shown in (b) of  FIG. 4 . Further, both the thermistor  211  and the protecting element  212  are disposed in the passing region of a minimum-sized sheet P capable of passing through the fixing device  200 . The thermistor  211  is the temperature detecting element for temperature-controlling the heater  300  as described above. The protecting element  212  is connected in series to an energization circuit to the heater  300  as shown in  FIG. 6 , and operates when the heater  300  is abnormally increased in temperature to interrupt an energization line to the heat generating resistors  301 - 1  and  301 - 2 . 
     (4) Electric Power Controller for Heater  300   
       FIG. 7  shows an electric power controller for the heater  300  in this embodiment, in which a commercial AC power source  401  is connected to the printer  100 . The electric power control of the heater  300  is effected by energization and interruption of a triac  416 . The electric power supply to the heater  300  is effected via the electrode portions C1 and C2, so that the electric power is supplied to the heat generating resistors  301 - 1  and  301 - 2  of the heater  300 . 
     A zero-cross detecting portion  430  is a circuit for detecting zero-cross of the AC power source  401 , and outputs a zero-cross (“ZEROX”) signal to the controller (CPU)  101 . The ZEROX signal is used for controlling the heater  300 , and as an example of a zero-cross circuit, a method described in JP-A 2011-18027 can be used. 
     An operation of the triac  416  will be described. Resistors  413  and  417  are resistors for driving the triac  416 , and a photo-triac coupler  415  is a device for ensuring a creepage distance for insulation between a primary side and a secondary side. The triac  416  is turned on by supplying the electric power to a light-emitting diode of the photo-triac coupler  415 . A resistor  418  is a resistor for limiting a current of the light-emitting diode of the photo-triac coupler  415 . By controlling a transistor  419 , the photo-triac coupler  415  is turned on and off. 
     The transistor  419  is operated by a “FUSER” signal from the controller  101 . A temperature detected by the thermistor  211  is detected by the controller in such a manner that a divided voltage between the thermistor  211  and a resistor  411  is inputted as a “TH” signal into the controller  101 . In an inside process of the controller  101 , on the basis of a detection temperature of the thermistor  211  and a set temperature for the heater  300 , the electric power to be supplied is calculated by, e.g., PI control. Further, the electric power is converted into control level of a phase angle (phase control) and wave number (wave number control) which correspond to the electric power to be supplied, and then the triac is controlled depending on an associated control condition. 
     For example, in the case where the fixing device  200  is in a thermal runaway state by a breakdown, of the electric power controller, such as short circuit of the triac  416 , the protecting element  212  operates, and interrupts the electric power supply to the heater  300 . Further, in the case where the controller  101  detects that the thermistor detection temperature (“TH” signal) is a predetermined temperature or more, the controller  101  places a relay  402  in a non-energization state, and thus interrupts the electric power supply to the heater  300 . 
     (5) Pressing Method of Heater and High Heat-Conductive Sheet 
     In  FIG. 8 , (A) to (D) are schematic views for illustrating a pressing method of the heater  300  and the high heat-conductive sheet  220  and a shape of the heater supporting member  201 . 
     The high heat-conductive sheet  220  is provided between the heater supporting member  201  and the heater  300 . The high heat-conductive sheet  220  is sandwiched between the heater supporting member  201  and the heater  300  in a pressed state by the pressing force of the above-described pressing mechanisms  252  (L, R). 
     The heater supporting member  201  includes a first bearing surface  306  for supporting the high heat-conductive sheet  220  and the heater  300  and a second bearing surface (opposing portion)  307  opposing the heater  300 . Further, a height a of a stepped portion between the first bearing surface  306  and the second bearing surface  307  is constituted so as to be smaller than the thickness of the high heat-conductive sheet  300 . That is, the supporting member  201  is provided with the bearing surface  306  contacting the sheet  220  so as to apply the pressure to between the heater  300  and the sheet  220  and the opposing portion  307  opposing a surface where the supporting member  201  contacts a sheet-contactable surface without via the sheet  220 . Incidentally, as shown in  FIG. 8 , with respect to the recording material movement direction X (rotational direction R 202  of the film  202 ), a width L 220  of the sheet  220  is narrower than a width L 303  of the heater  300 . 
     This structure will be described specifically. In  FIG. 8 , (A) is the schematic view of the heater  300  in the (front) surface side, and (B) is a sectional view showing a cross-section of the heater  300  in a region B, as a central portion with respect to the longitudinal direction of the heater  300 , of (A) of  FIG. 8 . 
     The heater supporting member  201  includes the stepped portion, having the height a, between the bearing surface  306  and the bearing surface  307 , and the high heat-conductive sheet  220  is sandwiched between an inside of the stepped portion (height: a) of the heater supporting member  201  and is adjusted to a distance depending on a compression ratio of the high heat-conductive sheet  220  after the pressure application. 
     In  FIG. 8 , (C) is a sectional view of a cross-section of the heater  300  in a region C, of (A) of  FIG. 8 , where the protective element  212  is contacted to the high heat-conductive sheet  220 . 
     In  FIG. 8 , (D) is a sectional view of a cross-section of the  300  in a region D, of (A) of  FIG. 8 , where the thermistor  211  is contacted to the high heat-conductive sheet  220 . 
     As shown in (B) to (D) of  FIG. 8 , the heater supporting member  201  has the bearing surface  306 , at a position perpendicular to each of heat generation regions of the heat generating resistors  301 - 1  and  301 - 2 , where the high heat-conductive sheet  220  and the heater substrate  303  are contacted to each other by the heater supporting member  201 . That is, in each of the cross-sections in (B) to (D) of  FIG. 8 , the heat generation region HE1 and the region of the bearing surface  306  overlaps with the bearing surface  306  with respect to the direction X. 
     Further, the heater supporting member  201  includes the stepped portion (height: a) between the bearing surface  306  and the bearing surface  307 , and in an area of the stepped portion (height: a), the sheet  220  is disposed. As a result, the positional relationship of the high heat-conductive sheet  220  relative to the heater substrate  303  can be fixed. That is, as shown in (B) of  FIG. 8 , with respect to the direction X, the two bearing surfaces (opposing portions)  307  are provided, and thus two surfaces  307   p , which are side surfaces of the two bearing surfaces  307 , exist. During assembling of the fixing device, when the sheet  220  is inserted into between the two side surfaces  307   p , the position of the sheet  220  with respect to the direction X is substantially determined. Further, when also the heater  300  is inserted into between two surfaces  201   p , the position of the heater  300  with respect to the direction X is substantially determined. Accordingly, even in the fixing device using the sheet having the width L 220  narrower than the width L 303  of the heater, the positional relationship between the heater  300  and the sheet  220  can be substantially determined, so that the temperature non-uniformity eliminating function of the sheet  220  can be effectively used. 
     Further, the depth or height (distance) a of the stepped portion of the heater supporting member  201  is adjusted to a magnitude depending on a degree of compression of the sheet  220  after the sheet  220  is pressed springs  252 L and  252 R, so that the sheet  220  and the heater substrate  303  can be contacted to each other at a certain pressure. As a result, heat generation of the heat generating resistors  301 - 1  and  301 - 2  can be efficiently conducted to the sheet  220 . 
     The relationship between the height a of the stepped portion of the heater supporting member  201  and the thickness of the sheet  220  described above will be described with reference to  FIG. 9 .  FIG. 9  shows the relationship of the contact thermal resistance and the pressure between the sheet  220  and the heater substrate  303 .  FIG. 9  shows that the heat conduction from the heater to the sheet cannot be nearly obtained. That is, a predetermined pressure is needed for obtaining the heat conduction between the sheet  220  and the heater substrate  303 . 
     In  FIG. 10 , (A) and (B) show a relationship between the compression ratio of the sheet  220  and the stepped portion (height: a) between the bearing surfaces  306  and  307  of the heater supporting member  201 . In  FIG. 10 , (A) shows the sheet  220  and the heater supporting member  201  when the sheet  220  is not pressed. The stepped portion between the bearing surfaces  306  and  307  of the heater supporting member  201  is a, and the thickness of the sheet  220  under no-pressure application is x. At this time, a relationship between the height a of the stepped portion, between the bearing surfaces  306  and  307  of the heater supporting member  201 , and the thickness x of the sheet  220  in a non-pressure state is a&lt;x. 
     In  FIG. 10 , (B) shows the sheet  220 , the heater supporting member  201  and the heater  300  when the sheet  220  is pressed by the springs  252 L and  252 R. The thickness of the sheet  220  having the pressure application is y. At this time, the height a of the stepped portion between the bearing surfaces  306  and  307  of the heater supporting member  201  satisfies: a≦y. That is, the height a of the stepped portion between the first bearing surface  306  and the second bearing surface  307  is equal to or small than the thickness y after the sheet  220  is pressed. 
     For example, when the pressure at the bearing surface  306  is 1000 (gf/cm 2 ), and a thickness compression ratio of the sheet  220  at this time is 8%, the thickness of the sheet  220  after the pressure application is 0.92×x. Therefore, the height a of the stepped portion between the bearing surfaces  306  and  307  satisfies a≦0.92×x. 
     In this way, the sheet  220  is contacted to the heater substrate  303  in a compression state, i.e., the sheet thickness is not less than the height of the stepped portion between the bearing surface  306  and the opposing portion  307  in the state in which the pressure is applied to between the heater and the sheet, so that a dimensional tolerance of the heater  220  with respect to the thickness direction can be absorbed, and thus the sheet  220  and the heater substrate  303  can be contacted to each other at a predetermined pressure. 
     In  FIG. 11 , (A) to (C) show modified embodiments. In  FIG. 11 , each of a heater supporting member  701  in (A), a heater supporting member  702  in (B) and a heater supporting member  703  in (C) includes a first bearing surface  706 , a bearing surface  708  where the heater supporting member opposes the sheet and is recessed from the sheet relative to the bearing surface  706 , and a second bearing surface (opposing portion)  707 . 
     Also in these examples, a constitution in which the height a of the stepped portion between the first bearing surface  706  and the second bearing surface  707  is smaller than the thickness of the sheet  220  after the sheet  220  is pressed is employed. 
     This constitution will be specifically described. Each of the heater supporting member  701  of (A) of  FIG. 11 , the heater supporting member  702  of (B) of  FIG. 11 , and the heater supporting member  703  of (C) of  FIG. 11  includes the bearing surface  708 . For that reason, heat dissipation from the sheet  220  toward the heater supporting member can be suppressed. 
     Incidentally, the (planar) area of the bearing surface  706  of the heater supporting member  701  is smaller than the (planar) area of the bearing surface  306  of the heater supporting member  201  by the (planar) area of the bearing surface  708 . Therefore, in the case where the heater supporting members  701  and  201  are pressed by the same force, the pressure by the bearing surface  706  is higher than the pressure by the bearing surface  306 . 
     For example, the case where the area of the bearing surface  706  is ⅔ of the area of the bearing surface  306  and the pressure by the bearing surface  306  is 1000 (gf/cm 2 ) will be considered. In this case, the pressure by the bearing surface  706  is 1500 (gf/cm 2 ). At this time, when the compression ratio of the sheet  220  is about 11% and the thickness of the sheet  220  in the non-pressure application state, the thickness of the sheet  220  after the pressure application is about 0.89×x. Therefore, the height a of the stepped portion between the bearing surfaces  706  and  707  satisfied: a≦0.89×x. 
     Embodiment 2 
     Embodiment 2 in which the heater supporting member for the heater  300  to be mounted in the fixing device  200  is changed will be described. Constituent elements similar to those in Embodiment 1 will be omitted from illustration. In this embodiment, each of the bearing surface and the opposing portion of the heater supporting member has curvature (crown shape) with respect to a longitudinal direction (of the supporting member) perpendicular to the film movement direction of the heater. Further, the height of stepped portion between the bearing surface and the opposing portion is substantially the same over the longitudinal direction of the supporting member. 
     This constitution will be specifically described. In  FIG. 12 , (A) is a perspective view of a heater supporting member  801 . A surface  806  is a sheet pressing surface where the heater supporting member  801  presses the sheet, and a surface  807  is an opposing portion (opposing surface) opposing a sheet-contactable surface of the sheet without via the sheet. 
     The heater supporting member  801  has the crown shape with respect to the longitudinal direction of the heater substrate (or the longitudinal direction of the supporting member), so that each of the bearing surfaces  806  and  807  is a surface having certain curvature with respect to the longitudinal direction. 
     The crown shape is a shape capable of generating uniform pressure in the nip with respect to the longitudinal direction. 
     In  FIG. 12 , (B) is a sectional view of a cross-section in the neighborhood of a longitudinal end portion (B) in (A) of  FIG. 12 . The heater supporting member  801  has the stepped portion (height: a) between the bearing surfaces  806  and  807 , and the sheet  220  is sandwiched between an inside of the stepped portion and the heater  300 . The depth of the stepped portion of the heater supporting member  801  is not more than the thickness of the sheet  220  after the pressure application as described above with reference to  FIG. 10 . 
     In  FIG. 12 , (C) is a sectional view of a cross-section in the neighborhood of a longitudinal central portion (C) in (A) of  FIG. 12 . The bearing surfaces  806  and  807  in (C) of  FIG. 12  are lower than the bearing surfaces  806  and  807  in (B) of  FIG. 12  correspondingly to the curvature of the heater supporting member  801 . 
     Incidentally, the pressure of the bearing surface  806  in the area (C) is equal in value to the pressure of the bearing surface  806  in the area (B) since the pressure of the heater supporting member  801  having the crown shape is uniform with respect to the longitudinal direction of the heater supporting member  801 . Therefore, the height a of the stepped portion of the heater supporting member  801  in the area (C) is equal in value to the height a of the stepped portion of the heater supporting member  801  in the area (B). That is, the height a of the stepped portion is substantially the same over the depth of the supporting member. 
     As shown in this embodiment, the constitution of the present invention is applicable to also the heater supporting member  801  having the crown shape. 
     Embodiment 3 
     Embodiment 3 in which the heater supporting member to be mounted in the fixing device  200  is changed will be described. Constituent elements similar to those in Embodiment 1 will be omitted from illustration. 
     In  FIG. 13 , (A) is a perspective view of a heater supporting member  901  or  902  in this embodiment. The supporting members  901  and  902  are merely different in crown shape from each other and therefore the perspective view of (A) of  FIG. 13  is common to the supporting members  901  and  902 . Each of the heater supporting members  901  and  902  has the crown shape with respect to the longitudinal direction. 
     In this embodiment, a height of the stepped portion between bearing surfaces  906  and  907  of each of the heater supporting members  901  and  902  is a. The heater supporting members  901  and  902  are different in longitudinal distribution of the height (distance) a of the stepped portion. 
     In  FIG. 13 , (B) shows the relationship between the stepped portion distance (height) a and the longitudinal position of the heater  300 . 
     In (B) of  FIG. 13 , a rectilinear line indicated by a solid line  801  shows a distribution of the depth a of the heater supporting member  801  in Embodiment 2. 
     In (B) of  FIG. 13 , a rectangular line indicated by a dotted line  901  shows a distribution of the depth a of the heater supporting member  901  in this embodiment. The depth a from each of points (f) and (g) toward an associated end portion side is smaller than the depth a in an area between the points (f) and (g) by a certain length. Further, a curved line indicated by a broken line  902  shows a distribution of the depth a of the heater supporting member  902  in this embodiment, and the depth a is gradually decreased from a longitudinal center of the heater supporting member  902 . 
     In  FIG. 13 , (C) is a graph showing a relationship between the pressure applied to the bearing surface  906  and the longitudinal position of the heater. A rectilinear line indicated by a solid line  801  shows the pressure of the bearing surface  806  of the heater supporting member  801 , and the pressure is constant with respect to the heater longitudinal direction. 
     On the other hand, a rectangular line indicated by a dotted line  901  in (C) of  FIG. 13  shows the pressure of the bearing surface  906  of the heater supporting member  901 , and the pressure in areas from each of the longitudinal points (f) and (g) toward the associated end portion side is higher than the pressure in the area between the longitudinal points (f) and (g). This is because the pressure concentrates at a portion (end portion) where the depth a is small. 
     In (C) of  FIG. 13 , a curved line indicated by a broken line  902  shows the pressure of the bearing surface  906  of the heater supporting member  902 , and the pressure gradually increases from the longitudinal center toward the end portion sides. This is because the depth a decreases with a position toward the end portion, and therefore the pressure gradually increases with the position closer to the end portion. 
     In this way, with respect to each of the supporting members  901  and  902 , the pressure applied to the bearing surface  906  in the neighborhood of the longitudinal end portion of the heater is higher than the pressure applied to the bearing surface  906  at the longitudinal central portion of the heater. 
     As a result, from the relationship of the contact thermal resistance between the heater  300  and the sheet  220 , the contact thermal resistance between the heater  300  and the sheet  220  is lower in the neighborhood of the longitudinal end portions of the heater than at the longitudinal central portion of the heater. For that reason, the heat at the longitudinal end portions of the heater can be efficiently conducted to the sheet  220 , so that a temperature distribution non-uniformity of the heater can be alleviated. 
     Incidentally, the shape the heater supporting (holding) members  901  and  902  is merely an example of a shape for increasing the pressure in the neighborhood of the longitudinal end portions of the heater, but is not limited to the shape described in this embodiment. 
     The image heating apparatus in the present invention includes, in addition to the apparatus for heating the unfixed toner image (visualizing agent image, developer image) to fix or temporarily fix the image as a fixed image, an apparatus for heating the fixed toner image again to improve a surface property such as glossiness. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 237911/2013 filed Nov. 18, 2013, which is hereby incorporated by reference.