Abstract:
A fixing device for fixing a toner image formed on a sheet includes a cylindrical film; a heater having a first surface contacting an inner surface of the film; an opposed member forming a press-contact portion with the film therebetween; a supporting member for supporting a second surface of the heater opposite the first surface; and a heat conduction member contacting the second surface. In a generatrix direction of the film, an end portion of a contact region between the heat conduction member and the second surface is at or inside an end portion of the press-contact portion. In a region from an end portion of the contact region to an outside of an end portion of the press-contact portion with respect to the direction, the supporting member includes a first region not contacting the second surface and a second region contacting the second surface outside the first region.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image fixing apparatus (image heating apparatus) which is desirable as a fixing device to be mounted in an image forming apparatuses such as an electrophotographic copying machine, an electrophotographic laser beam printer, etc. 
     There are various fixing apparatuses (devices) which are mountable in an electrophotographic copying machine, an electrophotographic copying machine, and the like. One of them has been known as a fixing device of the heating film type. A fixing device of the heating-film type has a heater, a film, a pressure roller, etc. The heater has a ceramic substrate, and a heat generating resistor placed on the substrate. The film is cylindrical and rotates in contact with the heater. The pressure roller forms a nip by sandwiching the film between itself and the heater. In operation, a sheet of a recording medium, on which an unfixed toner image is present, is conveyed through the nip while being heated. Thus, the toner image on the sheet of a recording medium is thermally fixed to the sheet. 
     One of the merits which a fixing device of the heating-film type has is that it is relatively short in the length of time it takes for its temperature to rise to a level at which fixation is possible, after power begins to be supplied to its heater. Thus, a printer which employs a fixing device of the heating film is relatively short in the length of time (FPOT: First Print Out Time) it takes for the printer to output the first image after the inputting of a print start command. Another merit which a fixing device of this type has is that it is small in the amount of electrical power it consumes while it is kept on standby for a print start command. 
     By the way, it has been known that as a substantial number of small prints are continuously outputted by a printer having a fixing device of the heating-film type, with the same chronological intervals as that for large prints, the out-of-sheet-path portion of the heater, that is, the portion of the heater, which is outside the sheet path, excessively increases in temperature. As the out-of-sheet-path portion of the heater excessively increases in temperature (out-of-sheet-path temperature increase), the components, such as the pressure roller, of the fixing device sometimes are thermally damaged. Thus, it is desired that a printer, which employs a fixing device of the heating-film type, is structured so that when it is used for continuously outputting a substantial number of small prints, it is widened in print interval, compared to when it is used for continuously outputting a substantial number of large prints, in order to prevent the out-of-sheet-path portion of the heater from excessively increasing in temperature. 
     However, a widening print interval reduces the output of the image forming apparatus, that is, the print count per unit length of time. Thus, it is desired to make the output (print count per unit length of time) of a printer having a fixing device of the heating-film type when a substantial number of small prints are continuously outputted, the same as that when a substantial number of large prints are continuously outputted. 
     Referring to  FIG. 11 , Japanese Laid-open Patent Application H11-84919 discloses a heating apparatus (device) having a metallic plate  14   a  between its heater  12  and a thermally insulative supporting member  11 . With the placement of the metallic plate  14   a , which is a thermally conductive member, between thermally insulative member  11  and the heater  12 , it is possible to prevent the problem that the heater becomes non-uniform in temperature. In other words, it is possible to reduce the difference in temperature between the portion of the heater, which is outside the recording medium path (out-of-sheet-path portion), and the portion of the heater, which is within the recording medium path (sheet-path portion), which occurs as the “out-of-sheet-path temperature increase” occurs. However, if the metallic plate  14   a  is long enough to extend from one end of the heater to the other as shown in  FIG. 11 , the end portions of the sheet-path portion of the heater, in terms of the direction parallel to the generatrix of the film, are reduced in temperature, and therefore, the portions of the film, which correspond to the end portions of the sheet-path portion of the heater, are reduced in temperature, which possibly will cause an image forming apparatus to output images which are unsatisfactory in fixation. 
     Japanese Laid-open Patent Application H11-84919 proposes also a fixing device which is provided with multiple metallic plates  14   a . In the case of this fixing device, the metallic plates  14   a  are fitted in the recesses of the thermally insulative member  11 . 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a fixing device for fixing a toner image formed on a recording material, on a recording material, the fixing device comprising a cylindrical film; a heater having a first surface contacting an inner surface of the film; an opposed member cooperating with the heater to form a press-contact portion with the film interposed therebetween; a supporting member for supporting a second surface of the heater opposite the first surface; and a heat conduction member contacting the second surface. With respect to a generatrix direction of the film, an end portion of a contact region between the heat conduction member and the second surface is at or inside an end portion of the press-contact portion. In a region from an end portion of the contact region to an outside of an end portion of the press-contact portion with respect to the generatrix direction, the supporting member includes a first region not contacting the second surface and a second region contacting the second surface outside the first region. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of the fixing device in the first embodiment of the present invention, at a vertical plane which is perpendicular to the lengthwise direction of the fixing device and coincides in position to the center of the fixing device in terms of the lengthwise direction of the fixing device. 
         FIG. 2  is a front view of the fixing device (minus its center portion in terms of its lengthwise direction) in the first embodiment. 
         FIG. 3  is a plan view of the heater of the fixing device in the first embodiment, as seen from the side of its surface on which the heating film slides. 
         FIG. 4  is a drawing for showing the positional relationship among the supporting member, thermally conductive member, thermistor, and temperature fuses of the fixing device in the first embodiment. 
         FIG. 5(   a ) is a sectional view of the fixing device in the first embodiment, at a plane indicated by a pair of arrow marks I in  FIG. 1 ;  FIG. 5(   b ) is a plan view of the supporting member as seen from the side of its surface to which the thermally conductive member is attached; and  FIG. 5  ( c ) is a drawing for illustrating the positional relationship among the supporting member, the thermally conductive member, the heater, the film, and the pressure roller of the fixing device, in the adjacencies of one of the lengthwise end of the pressure roller. 
         FIG. 6(   a ) is a drawing for illustrating the direction in which heat conducts through the heater and thermally conductive member; and  FIG. 6(   b ) is a drawing for illustrating the direction in which heat conducts through the heater and thermally conductive member, in the adjacencies of one of their lengthwise ends. 
         FIG. 7(   a ) is a drawing for illustrating the power supply connector; and  FIG. 7(   b ) is a drawing for illustrating the clip. 
         FIG. 8(   a ) is a drawing for illustrating the state of contact between the heater and thermally conductive member of the fixing device in the first embodiment, in the adjacencies of one of the lengthwise ends of the thermally conductive member; and  FIG. 8(   b ) is a drawing for illustrating the state of contact between the heater and the thermally conductive member of a comparative fixing device, in the adjacencies of one of the lengthwise ends of the thermally conductive member. 
         FIG. 9  is a drawing which shows the structure of the heater, the supporting member, the thermally conductive member, and the pressure roller of the fixing device in the third embodiment of the present invention, in the adjacencies of one of the lengthwise end portions of the fixing device. 
         FIG. 10  is a drawing illustrating the characteristic features of the fixing device in the second embodiment of the present invention. 
         FIG. 11  is a drawing illustrating the structure of the heater and thermally conductive member of a fixing device in accordance with the prior art. 
         FIG. 12(   a ) is a drawing illustrating the structure of the pressure stay, the supporting member, the thermally the conductive member, the heater, and the flange, of the fixing device in the first embodiment, as seen from the direction parallel to the recording medium conveyance direction; and  FIG. 12(   b ) is a sectional view of the combination of the pressure stay, the supporting member, the thermally conductive member, and the heater, of the fixing device in the first embodiment, at a plane which is perpendicular to the lengthwise direction of the fixing device and corresponds in position to the center of the fixing device in terms of the lengthwise direction of the fixing device. 
         FIG. 13  is a schematic sectional view of one of the modified versions of the fixing device in the first embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention is described in detail with reference to appended drawings. The preferable embodiments of the present invention, which will be described hereafter, are some of the most preferable of the embodiments of the present invention. However, they are not intended to limit the present invention in scope. That is, the present invention is also applicable, within the gist of the present invention, to various known image heating apparatuses (devices) which are different in structure from those in the following embodiments. 
     Embodiment 1 
     In this embodiment, an image heating apparatus in accordance with the present invention is described as a fixing apparatus (fixing device) of the heating-film type, which is mountable in an image forming apparatus such as a printer, a copying machine, and the like, which is based on electrophotographic recording technologies. 
     (1) Structure of Fixing Device  30   
       FIG. 1  is a sectional view of the fixing device  30  in this embodiment, at a plane which is perpendicular to the lengthwise direction of the fixing device  30  and corresponds in position to the center of the fixing device  30  in terms of the lengthwise direction of the fixing device  30 .  FIG. 2  is a front view of the fixing device  30  (minus its center portion in terms of its lengthwise direction) of the fixing device in this embodiment. In  FIG. 2 , for the sake of descriptive discretion, the film  36  is drawn as if it is transparent, and the components which are within the hollow of the film  36  and are contoured with solid lines, along with the lateral plates  34  and  35  of the frame  33 .  FIG. 3  is a front view of the heater  37  as seen from its side on which the film slides.  FIG. 4  is a drawing which shows the positional relationship among the supporting member  38 , the thermally conductive member  39 , the thermistor  61 , and the temperature fuse  43 . 
     Fixing device  30  has a film unit  31 , and a pressure roller  32  as a pressure applying member. The film unit  31  has: a cylindrical film  36  as a cylindrical rotational member; the heater  37 , as a heating member, which is in the form of a piece of plate; and a supporting member  38  which supports the heater  37 . Further, the film unit  31  has: a thermally conductive member  39  which is between the heater  37  and supporting member  38 ; a pressure stay  40  as a reinforcing member for reinforcing the supporting member  38 ; and a pair of flanges  41  and  42 , as regulating members, which regulate the film  36  in terms of the movement in the lengthwise direction of the fixing device  30 . 
     The pressure roller  32 , the film  36 , the heater  37 , the supporting member  38 , the thermally conductive member  39 , and the pressure stay  40  are disposed so that their lengthwise direction becomes perpendicular to the recording medium conveyance direction. The film  36  is disposed in parallel to the pressure stay  32 . The heater  37 , the supporting member  38 , the thermally conductive member  39 , and the pressure stay  40  are disposed within the hollow of the film  36 . The flanges  41  and  42  are disposed at the lateral edges of film  36 , one for one. 
     The shape of the supporting member  38  is such that its cross-section at a plane parallel to the direction (which hereafter may be referred to as widthwise direction) parallel to the recording medium conveyance direction is roughly semicircular ( FIG. 1 ). It is rigid, heat resistant, and thermally insulative. The material for the supporting member  38  is a liquid polymer resin. The pressure roller  32  side of the supporting member  38  is provided with a groove  38   a , which extends in the lengthwise direction of the supporting member  38 . It is in this groove  38   a  that the thermally conductive member  39  and the heater  37  are fitted, being thereby supported by the supporting member  38 . Further, the opposite surface of the supporting member  38  from the pressure roller  32  is provided with a pair of guiding portions  38   b  and  38   c , which are roughly arc-shaped and are on the upstream and downstream sides, respectively, in terms of the recording medium conveyance direction. It is by these guiding portions  38   b  and  38   c  that the film  36  is properly held while the film  36  is rotated. 
     The pressure stay (reinforcing member)  40  is used for increasing the film unit  31  in rigidity (resistance to bending). It is formed by bending a long piece of stainless steel, which is 1.6 mm in thickness, in such a manner that its cross section at a plane parallel to the widthwise direction of the fixing device  30  becomes shaped like a letter U. The pressure stay  40  is disposed between the guiding portions  38   b  and  38   c  of the supporting member  38 . 
     Referring to  FIG. 3 , the heater  37  has a substrate  37   a , the lengthwise direction of which is parallel to the generatrix of the film  36 . The “direction of the generatrix of the film  36 ” means the direction indicated by an arrow mark L in  FIG. 2 . It is the same as the above-described lengthwise direction of the fixing device  30 . The material for the substrate  37   a  is a ceramic such as alumina, aluminum nitrate, etc. The heater  37  has also a pair of long and narrow heat generating members  37   b , which are on the surface (which hereafter may be referred to as film sliding surface) of the substrate  37   a , which faces inward of the film loop. Each heat generating resistor  37   b  is formed of silver/palladium alloy or the like, by screen printing or the like. It extends in the lengthwise direction of the substrate  37   a.    
     Further, the film unit  31  is provided with a pair of electrical contacts  37   c , which are on the film sliding surface of the substrate  37   a . The electrical contacts  37   c  are in contact with the pair of heat generating resistors  37   b , one for one. The heat generating resistors  37   b  are connected in series through an electrically conductive pattern  37   e  formed of silver or the like. 
     Further, film unit  31  is provided with a gloss coat  37   d , as a protective layer, which covers each heat generating resistor  37   b , the electrically conductive pattern  37   e , and a part of each electrical contact  37   c , which are on the film sliding surface of the substrate  37   a . Not only does the glass coat  37   d  protect the heat generating resistors  37   b , but also, reduces the friction between the inward surface of the film  36  and the heater  37 . 
     The above-mentioned heater  37  is fitted in the groove  38   a  of the supporting member  38  so that it extends in the direction parallel to the generatrix of the film  36 , and also, so that the heater  37  contacts the film  36  by its glass coat  37   d.    
     The substrate  37   a  of the heater  37  is in the form of a rectangular parallelepiped. It is 270.0 mm in length, 5.8 mm in width, and 1.0 mm in thickness. The material for the substrate  37   a  is alumina. The dimension of the heat generating resistor  37   b  in terms of the lengthwise direction is 22.0 mm. Each heat generating resistor  37   b  is 18Ω in electrical resistance. 
     Referring to  FIG. 4 , the supporting member  38  is provided with a pair of through holes  38   a  and  38   b , which are in the adjacencies of the lengthwise center of the supporting member  38 . It is in the through hole  38   a  that a thermistors  61 , as a temperature detection element, is disposed. It is in the through hole  38   b  that a temperature fuse  43 , as a circuit breaking element, is disposed in contact with the thermally conductive member  39 . That is, the thermistor  61  and the temperature fuse  43  are in contact with the thermally conductive member  39  so that the heat from the heater  37  is sensed by the thermistor  61  and the temperature fuse  43  through the thermally conductive member  39 . 
     In order to stabilize the thermistor  61  in the state of contact between the thermistor  43  and heater  37 , the thermistor  61  is disposed with the placement of a piece of ceramic paper or the like between its housing and the heater  37 , and is covered with a piece of dielectric tape such as a piece of polyamide tape. 
     The temperature fuse  43  is such a component that detects the abnormal heat generation of the heater  37 , based on the abnormal temperature increase of the heater  37 , and interrupts the power supply to the heater  37 . It has a cylindrical metallic housing, and a fusible element which melts at a preset temperature and is disposed in the cylindrical metallic housing. As the fusible element of the temperature fuse  43  is melted away by the abnormal temperature increase of the heater  37 , the temperature fuse  43  interrupts the power supply to an electrical circuit EC ( FIG. 3 ) for supplying the heater  37  with electrical power. The temperature fuse  43  is attached to the thermally conductive member  39  with the placement of thermally conductive grease between itself and thermally conductive member  39 , in order to prevent the malfunction of the fixing device  30 , which is attributable to the separation of the temperature fuse  43  from the heater  37 . 
     The film  36  is loosely fitted around the combination of the supporting member  38  (which supports the heater  37 ), the thermally conductive member  39 , and the pressure stay  40 . The film  36  has: a cylindrical substrate; an elastic layer formed on the peripheral surface of the substrate; and a parting layer formed on the outward surface of the elastic layer. The film  36  in this embodiment is 18.0 mm in internal diameter. The substrate is 60 μm in thickness, and is formed of polyamide. The elastic layer is roughly 150 μm in thickness, and is formed of silicone rubber. The parting layer is 15 μm in thickness, and is formed of a piece of PFA resin tube. 
     The material for the flanges  41  and  42  is liquid polymer resin. The flange  41  has a plate-like regulating portion  41   a  ( FIG. 2 ) which regulates the film  36  in terms of the movement in the lengthwise direction of the fixing device  30 . More specifically, as the film  36  deviates in the lengthwise direction of the fixing device  30 , it comes into contact with the regulating portion  41   a , being thereby preventing from deviating further. The regulating portion  41   a  supports the pressure stay  40  and the supporting member  38  at one of the lengthwise ends of the fixing device  30 . It has a pair of vertical grooves  41   a   1 , which are in its upstream and downstream end surfaces, respectively. The lateral plate  34  (one of lateral plates) of the frame  33  is fitted in this vertical groove  41   a   1 , making it possible for the flange  41  to move relative to the pressure roller  32 . 
     The flange  41  has also an arc-like guide portion  41   b , which is in the surface of the regulating portion  41   a , which faces the inward surface of the film  36 . Not only does the guide portion  41   b  support the pressure stay  40  and the supporting member  38  at one of the lengthwise ends of the fixing device  30 , but also, guides the film  36  with its outward surface, as the film  36  is rotationally moved. Moreover, the flange  41  has a spring seat  41   c , which is a part of the outward surface of the regulating portion  41   a , that is, the opposite surface of the flange  41  from the film  36 . 
     Further, the flange  41  has also a plate-like regulating portion  42   a  ( FIG. 2 ) which regulates the deviation of the film  36  in the lengthwise direction of the fixing device  30 . That is, as the film  36  deviates in the lengthwise direction of the fixing device  30 , it comes into contact with the regulating portion  42   a , being thereby prevented from deviating further. The regulating portion  42   a  supports the pressure stay  40  and the supporting member  38  at the other end of the fixing device  30 . The regulating portion  42   a  is provided with a pair of vertical grooves  42   a   1 , which are in the upstream and downstream surfaces of the regulating portion  42   a , in terms of the recording medium conveyance direction. The other lateral plate  35  of the frame  33  is fitted in this vertical groove  42   a   1 , making it possible for the flange  42  to be moved relative to the pressure roller  32 . 
     Further, the flange  42  has an arc-like guide portion  42   b , which is in the surface of the regulating portion  42   a , which faces the inward surface of the film  36 . Not only does this guide portion  42   b  support the pressure stay  40  and the supporting member  38  at one of the lengthwise ends of the fixing device  30 , but also, guides the film  36  with its outward surface, as the film  36  is rotationally moved. Moreover, the flange  42  has a spring seat  42   c , which is a part of the outward surface of the regulating portion  41   a , that is, the opposite surface of the regulating portion  41   a  from the film  36 . 
     The pressure roller  32  has a metallic core  32   a , an elastic layer  32   b , and a parting layer  32   c . The elastic layer  32   b  covers the peripheral surface of the metallic core  32   a , except for the shaft portions  32   a   1  and  32   a   2  of the metallic core  32   a , that is, the lengthwise end portions of the metallic core  32   a . The parting layer  32   c  covers the outward surface of the elastic layer  32   b . The material for the elastic layer  32   b  is silicone rubber, fluorinated rubber, or the like. The material for the parting layer  32   c  is PFA (tetrafluoroethylene, perfluoroalkylvinylether copolymer), PTFE (polytetrafluoroethylene), FEP (tetrafluoroethylene/hexafluoropropylene copolymer), or the like. 
     The structure of the pressure roller  32  in this embodiment is as follows: It has metallic core  32   a , an elastic layer  32   b  as the first elastic member layer, and a parting layer  32   c . The metallic core  32   a  is formed of stainless steel, and is 11.0 mm in external diameter. The elastic layer  32   b  covers the virtual entirety of the peripheral surface of the metallic core  32   a . The parting layer  32   c  covers the outward surface of the elastic layer  32   b . More specifically, the elastic layer  32   b  is silicone rubber layer formed on the peripheral surface of the metallic core  32   a  to a thickness of roughly 3.5 mm, by injection molding. The parting layer  32   b  is roughly 40 μm in thickness, and is formed by covering the elastic layer  32   b  with a piece of PFA resin tube. 
     The pressure roller  32  is 18.0 mm in external diameter. From the standpoint of formation of a satisfactory nip N, and also, durability, the hardness (measured by ASKER-C hardness gauge, under 9.8 N of weight) of the pressure roller  32  is desired to be in a range of 40°-70°. In this embodiment, the hardness of the pressure roller  32  is 54°. The length of the elastic layer  32   b  of the pressure roller  32  is 226.0 mm. 
     Referring to  FIG. 2 , the shaft portions  32   a   1  and  32   a   2  of the metallic core  32   a  of the pressure roller  32  are rotatably supported by the lateral plates  34  and  35 , with the placement of a pair of bearings  50  and  51  between the shaft portions  32   a   1  and  32   a   2  and the lateral plates  34  and  35 , respectively. A driving gear G is fixed to the shaft portion  32   a   2 . 
       FIG. 12(   a ) shows the pressure stay  40 , the supporting member  38 , the thermally conductive member  39 , the heater  37 , the spring seat  41   c  of the flange  41 , and the spring seat portion  42   c  of the flange  42 , as seen from the recording medium conveyance direction. It shows the structure of the fixing device  30 . The spring seating portions  41   c  and  42   c  are under the pressure generated by a pair of compression springs  54  and  55  disposed between themselves and a pair of pressure application arms  52  and  53 , respectively, in the vertical direction, which is perpendicular to the generatrix of the film  36 . The spring seat portions  41   c  and  42   c  transmit the force they receive from the compression springs  54  and  55 , to the supporting member  38  through the pressure stay  40 . The supporting member  38  presses the heater  37  against the peripheral surface of the pressure roller  32 , with the presence of the thermally conductive member  39  between itself and the heater  37 , and the film  36  between the heater  37  and the peripheral surface of the pressure roller  32 , forming thereby the nip N ( FIG. 1) . 
     The pressure stay  40  is in contact with the supporting member  38  across the entirety of the supporting member  38  in terms of the lengthwise direction. The lengthwise ends of the area of contact between the pressure stay  40  and supporting member  38  are on the outward side of the lengthwise ends of the nip N. 
     The supporting member  38  makes no contact, except for its contact with the opposite surface of the heater  37  from the surface of the heater  37 , which is in contact with the film  36 , and its contact with the film  36  by its contacting surface  38   b .  FIG. 12(   b ) is a cross-sectional view of the combination of the pressure stay  40 , supporting member  38  (minus the contacting surface  38   b ), the thermally conductive member  39 , and the heater  37 , at a plane perpendicular to the lengthwise direction. Referring to  FIG. 12(   b ), the fixing device  30  is structured so that the supporting member  38  makes no contact with the heater  37 , except for its contact with the heater  37 . This structural arrangement can improve the state of contact between the thermally conductive member  39  and the heater  37 . 
     In this embodiment, the total contact pressure between the surface of the pressure roller  32  and the surface of the film  36  is 180 N, and the width of the nip N is roughly 6.2 mm. Further, the inward surface of the film  36  is coated with heat resistant grease, to reduce the friction between the film  36  and the heater  37 , and also, the friction between the film  36  and the supporting member  38 . 
     (2) Thermally Fixing Operation of Fixing Device  30   
     The rotational force of the output shaft of the motor M is transmitted to the driving gear G, whereby the pressure roller  32  is rotated in the direction indicated by an arrow mark in  FIG. 1 . Thus, the film  36  is rotated by the rotation of the pressure roller  32  in the direction indicated by an another arrow mark in  FIG. 1 , sliding on the heater  37 , with the inward surface of the film  36  sliding on the glass coat layer  37   d  of the heater  37 . 
     As a control portion  100  ( FIG. 3 ) made up of a CPU and memories such as a RAM, a ROM, and the like, turns on a triac  101 , electric power is supplied from a commercial power source  102  to the heat generating resistor  37   b  of the heater  37  through the power supply connector  46 , which will be described later. Consequently, the heat generating resistor  37   b  generates heat, increasing thereby the heater in temperature. The triac  101  is controlled by the control portion  100  so that the temperature detected by the thermistor  61  for monitoring the temperature of the substrate  37   a  remains a preset fixation temperature (target level). 
     A sheet P of a recording medium ( FIG. 1 ) on which an unfixed toner image t is present is introduced into the nip N while being guided by an entrance guide  60 . In the nip N, the heat from the heater  37 , and the pressure in the nip N, are applied to the toner image t on the sheet P while the sheet P is conveyed, remaining pinched by the pressure roller  32  and the film  36 , through the nip N. Consequently, the toner image t is thermally fixed to the sheet P. After the fixation of the toner image t to the sheet P, the sheet P is discharged from the nip N. The dimension (width) of the widest sheet P, in terms of the lengthwise direction, which is conveyable through the fixing device  30  in this embodiment is 216.0 mm. 
     (3) Position of Lengthwise End of Pressure Roller  32 , Heater  37 , Thermally Conductive Member  39 , and Supporting Member  38   
     Next, referring to  FIG. 5 , the characteristic features of the fixing device  30  in this embodiment are described in detail.  FIG. 5(   a ) is a sectional view of the fixing device  30  at a plane I-I, as seen from the direction indicated by a pair of arrowheads.  FIG. 5(   b ) is a plan view of the supporting member  38 , as seen from the heater  37  side, after the attachment of the thermally conductive member  39  to the supporting member  38 .  FIG. 5(   c ) is a drawing for illustrating the positional relationship among the supporting member  38 , the thermally conductive member  39 , the heater  37 , the film  36 , and the pressure roller  32 , at one of their lengthwise end portions. In  FIG. 5(   a ), the power supply connector  46  and clip  47  are contoured with a solid line, for descriptive discretion. In  FIG. 5(   b ), the power supply connector  46  is not shown, for descriptive discretion. 
     Hereafter, the surface of the heater  37 , which is in contact with the film  36 , will be referred to as the first surface, whereas the opposite surface of the heater  37  from the first surface will be referred to as the second surface. Referring to  FIG. 5(   c ), the fixing device  30  is structured so that, in terms of the direction parallel to the generatrix of the film  36 , the end C of the portion of the thermally conductive member  39 , which is in contact with the second surface of the heater  37 , is on the inward side of the end D of the nip N. Further, the fixing device  30  is structured so that the inward end E of the surface  38   b  (second surface) of the supporting member  38 , which is in contact with the second surface of the heater  37 , is on the outward side of the end D of the nip N. Further, the fixing device  30  is structured so that the area (first area) of the supporting member  38 , which is between the end C and end E does not contact the second surface of the heater  37 . The end D of the nip N coincides in position with the end of the pressure roller  32 . 
     The thermally conductive member  39  in this embodiment is a piece of aluminum plate which is uniform in thickness. It is 0.3 mm in thickness. From the standpoint of minimizing the out-of-sheet-path temperature increase, that is, the phenomenon that when a substantial number of small prints are continuously outputted, the out-of-sheet-path portions of the heater  37  excessively increases in temperature, the area of contact between the thermally conductive member  39  and the heater  37  is desired to be no less in length than the heat generating resistor  37   b  of the heater  37 . However, in consideration of the temperature drop which occurs across the lengthwise end portions of the sheet-path portion ( FIG. 2 ) of the heater  37 , the length of the area of contact between the thermally conductive member  39  and the heater  37  is desired not to be excessive. 
     Regarding the shape of the area of contact between the thermally conductive member  39  and the heater in this embodiment, the area of contact is straight and rectangular. The length of the area of contact in terms of the direction parallel to the generatrix of the film  36  is 222.0 mm which is the same as the length of the heat generating resistor  37   b  of the heater  37 , and the width of the area of contact in terms of the direction perpendicular to the generatrix of the film  36  is 5.0 mm. Since the pressure roller  32  is 226.0 mm in length, the lengthwise end C of the thermally conductive member  39  is on the inward side of the end D by 2.0 mm. 
     The thermally conductive member  39  has a pair of bent portions  39   a   1  and  39   a   2 , which are at the ends of the thermally conductive member  39 , one for one, in terms of the direction parallel to the generatrix of the film  36 . The bent portions  39   a   1  and  29   a   2  are inserted into, and remain in, a pair of holes  38   a   1  and  38   a   2 , with which the end portions of the supporting member  38 , in terms of the direction parallel to the generatrix of the film  36 , is provided. The holes  38   a   1  and  38   a   2  are for absorbing the difference in coefficient of linear expansion between the thermally conductive member  39  and the supporting member  38 , and are made slightly larger in size than the bent portions  39   a   1  and  39   a   2 . As the material for the thermally conductive member  39 , not only a metallic substance such as aluminum and copper, but also, graphite, which are higher in thermal conductivity than the substrate  37   a  of the heater  37 , can be used. 
     At this time, the role of the thermally conductive member  39  is described. The role of the thermally conductive member  39  is to keep the heater  37  uniform in temperature to prevent the problem that as the out-of-sheet-path temperature increase occurs due to the continuous outputting of a substantial number of small prints, such components as the heater  37 , the film  36 , the supporting member  38 , the pressure roller  32 , etc., of the fixing device  30  are thermally damaged. 
     Next, referring to  FIG. 6 , how the heater  37  is kept uniform in temperature is described.  FIG. 6(   a ) is a drawing for illustrating the heat flow through the heater  37  and the thermally conductive member  39  in terms of the direction parallel to the generatrix of the film  36 .  FIG. 6(   b ) is for illustrating the heat flow in the lengthwise end portion of the heater  37  and that of the thermally conductive member  39 . 
     Alumina, which is used as the material for the substrate  37   a  of the heater  37 , is roughly 26 W/mK in thermal conductivity. In comparison, aluminum, which is used as the material for the thermally conductive member  39 , is roughly 230 W/mK, which is higher than that of the substrate  37   a.    
     Let&#39;s think about a case in which a portion H of the substrate  37   a , in terms of the direction parallel to the generatrix of the film  36 , became higher in temperature than the rest of the substrate  37   a . In such a case, heat flows not only in the substrate  37   a  in the direction indicated by an arrow mark A, which is parallel to the generatrix of the film  36 , but also, from the substrate  37   a  to the thermally conductive member  39  through the area of contact between the substrate  37   a  and the thermally conductive member  39 . Further, the heat having flowed into the thermally conductive member  39  flows in the thermally conductive member  39  in the direction indicated by the stem portion of arrow marks B, which is parallel to the generatrix of the film  36  to return to the substrate  37   a . Thus, the heater  37  is made uniform in heat (temperature) by this circular flow of heat, whereby the out-of-sheet-path temperature increase is minimized. 
     However, the heater  37  is made uniform in heat (temperature) even when the largest sheets of the recording medium, in terms of the direction perpendicular to the recording medium conveyance direction, are conveyed (which hereafter may be referred to as widest sheet P). Thus, if the thermally conductive member  39  is made long enough to extend into the portions of the heater  37 , which do not contact the film  36 , heat is likely to escape from the area of the heater  37 , which has the heat generating resistor  37   b , to the area of the heater  37 , which does not have the heat generating resistor  37   b . Thus, it is possible that when sheets P of the recording medium, which are the same in width as the sheet passage of the fixing device  30 , are conveyed through the nip N, the portions of toner image, which correspond in position to the lateral edge portions of the sheet, will be unsatisfactorily fixed. 
     Referring to  FIG. 6(   b ), in a case where only the portions of the heater  37 , which correspond to the adjacencies of the lengthwise end of the thermally conductive member  39 , become high in temperature, heat primarily flows toward only one end (left end in  FIG. 6(   a )) since the thermally conductive member  39  is on only one side (left side). That is, it is unlikely for heat to escape to an area other than where the thermally conductive member  39  is present. Therefore, the out-of-sheet-path temperature increase can be minimized, while preventing the temperature decrease of the end portions of the sheet-path portion of the heater  37 , by structuring the fixing device  30  so that the thermally conductive member  39  is on the inward side of the end D of the pressure application area of the pressure roller  32 , instead of across the entirety of the area of contact between the substrate  37   a  of the heater  37  and the film  36 . 
     Next, referring to  FIG. 7 , the power supply connector  46  and clip  47  as holding members are described about their structure.  FIG. 7(   a ) is a drawing for illustrating the power supply connector  46 , and  FIG. 7(   b ) is a drawing for illustrating the clip  47 . 
     In this embodiment, the heater  37  is held to the supporting member  38 , by both of its lengthwise ends, in terms of the direction parallel to the generatrix of the film  36 , with the use of the power supply connector  46  or the clip  47 . 
     Referring to  FIG. 7(   a ), the power supply connector  46  has a housing  46   a  and a contact terminal  46   b . The housing  46   a  is U-shaped, and is formed of a resinous substance. It sandwiches the heater  37  and the supporting member  38  from the outward side of the combination of the heater  37  and the supporting member  38 , preventing thereby the end portions of the heater  37 , in terms of the direction parallel to the generatrix of the film  36 , from moving in the thickness direction of the heater  37 . As for the contact terminal  46   b , it is an integral part of the housing  46   a . It elastically contacts the electrical contact  37   c  of the heater  37 , establishing electrical contact between itself and the heater  37 , while maintaining a present amount of contact pressure between itself and the heater  37 . Further, it is in contact with a wire bundle  48 , which is in contact with an unillustrated commercial power supply and triac. 
     In this embodiment, the contact terminal  46   b  of the power supply connector  46  is an integral part of the housing  46   a  of the contact terminal  46   b . However, the power supply connector  46  may be structured so that its housing and contact terminal are physically independent from each other. 
     Next, referring to  FIG. 7(   b ), the clip  47  is shaped like a letter U, and is formed of a piece of a metallic plate. It elastically sandwiches the combination of the heater  37  and the supporting member  38 , from the outward side of the combination to prevent the lengthwise end portions of the heater  37 , in terms of the direction parallel to the generatrix of the film  36 , from moving in the thickness direction of the heater  37 . 
     Further, the power supply connector  46  and the clip  47  are for regulating both of the lengthwise ends of the heater  37 , in terms of the direction parallel to the generatrix of the film  36 , to prevent them from moving in the thickness direction of the film  36 . They are structured so that they allow the lengthwise ends of the heater  37  to move in the direction parallel to the surface of the substrate  37   a  of the heater  37 , on which the film  36  slides. In other words, they are structured to prevent the problem that as the heater  37  is thermally expanded, and/or is deformed when it is subjected to pressure, or separated, it is unnecessarily subjected to stress. 
     In this embodiment, an end support surface  38   b  of the supporting member  38 , to which the heater  37  is held by the power supply connector  46  and the clip  47 , is 7.0 mm in width. In terms of the direction parallel to the generatrix of the film  36 , the end E of this end support surface  38   b  is on the outward side of the nip N formed by the pressure roller  32 , and is 15.0 mm apart from the position D of the pressure application area of the pressure roller  32 . The dimension of the end support surface  38   b , in terms of the thickness direction of the heater  37 , is 0.3 mm. 
     Next, referring to  FIG. 8 , the mechanism which ensures that the heater  37  and thermally conductive member  39  remain in contact with each other at their lengthwise ends, regardless of the component tolerance of the fixing device  30 , is described. 
       FIG. 8(   a ) is a drawing for illustrating the state of contact between the end portion of the thermally conductive member  39 , and the heater  37 , at one of the lengthwise ends of the heater  37 , in the fixing device  30  in this embodiment.  FIG. 8(   b ) is a drawing for illustrating the state of contact between the end portion of the thermally conductive member  39 , and the heater  37 , at one of the lengthwise end portions of the heater  37 , in the comparative fixing device  30 . In  FIG. 8(   b ), the power supply connector  46  is not shown for descriptive discretion. 
     In this embodiment, in terms of the lengthwise direction of the heater  37 , the center portion of the heater  37  is in contact with the thermally conductive member  39 , being thereby supported by the thermally conductive member  39 . The contact surface  38   b  of the supporting member  38  protrudes by 0.3 mm toward the pressure roller  32 , relative to a surface  38   s  ( FIG. 5(   c )) of the supporting member  38 , which is in contact with the thermally conductive member  39 , whereas the thermally conductive member  39  is 0.3 mm in thickness. Thus, if the components of the fixing device  30  have no error in their dimensions, the end support surface  38   b  will be level with the surface  39   s  of the thermally conductive member  39 , which is in contact with the heater  37 , as shown in  FIG. 5(   c ). 
     Next, referring to  FIG. 8(   a ), a case in which the contact surface  38   b  of the supporting member  38  protrudes by 0.1 mm toward the pressure roller  32  relative to the surface  39   s  of the thermally conductive member  39 , which is in contact with the heater  37 , because of component tolerance, that is, there is a step between the two surfaces  38   b  and  39   s , is discussed. 
       FIG. 8(   b ) shows the structure of a comparative fixing device, which is structured so that the outward end C of the thermally conductive member  39  is on the inward side of the end D of the pressure application area of the pressure roller  32 , and the inward end E of the area  38   b  of contact of the supporting member  38  is between the outward end C of the thermally conductive member  39 , and the end D of the nip N. Referring to  FIG. 8(   b ), the area  38   b  of contact of the supporting member  38  causes the heater  37  to deform toward the pressure roller  32 . This deformation sometimes reaches as far as the area of the heater  37 , which corresponds in position to the end C of the thermally conductive member  39 , causing the heater  37  to separate from the thermally conductive member  39 . This separation of the end portion of the thermally conductive member  39  from the heater  37  is problematic in that it makes it impossible for the thermally conductive member  39  to make satisfactorily uniform in the amount of heat, the portion of the heater  37 , which corresponds to the end portion of the thermally conductive member  39 , in terms of the lengthwise direction of the thermally conductive member  39 . 
     Next, referring to  FIG. 8(   a ), a case in which the aforementioned step occurred in the fixing device  30 , described with reference to  FIG. 5(   c ), is described. As the heater  37  is pressed by the contact surface  38   b  toward the pressure roller  32 , it deforms as if it rotates about the end D of the nip N. One of the reasons for the occurrence of this rotational deformation is that the fixing device  30  is structured so that the area (first area) of the supporting member  38 , which is between the end C and end E, does not contact the second surface of the heater  37 , and therefore, is likely to be easily deformed. The second reason is that on the inward side of the end D of the nip N, the heater  37  is subjected to a force F, shown in  FIG. 8(   a ), being enabled to resist the force which works in the direction to deform the heater  37 , whereas on the outward side of the end D, it is not subjected to the force F, being thereby likely to be easily deformed. Thus, the deformation of the heater  37  does not reach to the portion of the heater  37 , which corresponds in position to the end C of the thermally conductive member  39 . Therefore, it is ensured that the end portion of the thermally conductive member  39 , and the heater  37 , remain in contact with each other. By the way, in  FIG. 8(   a ), the end C of the thermally conductive member  39  is positioned on the inward side of the end D of the nip N, in consideration of the tolerance in component dimension. However, the fixing device  30  may be structured so that the end C coincides with the end D. 
     As described above, according to this embodiment, it is possible to ensure that the heater  37  and the thermally conductive member  39  remain in a better state of contact with each other regardless of the tolerance in component dimension. In other words, this embodiment is effective to keep the heater  37  uniform in temperature. 
     In this embodiment, the thermally conductive member  39  is a single piece of thermally conductive substance. However, it may be made up of multiple pieces of thermally conductive substance. 
     Further, in this embodiment, the fixing device  30  is structured so that the heater  37  forms the nip N, with the pressure roller  32 , with the presence of the film  36  between itself and the pressure roller  32 . However, the fixing device  30  may be structured so that the heater  37  and a fixation roller  300  form a heating-pressing portion X, with the presence of the film  36  between the heater  37  and the fixation roller  300  (modified version of first embodiment) as shown in  FIG. 13 . In the case of this modified version of the first embodiment, the fixation roller  300  is heated by the film  36 , in the heating-pressing portion X, and the fixation roller  300  is placed in contact with the pressure roller  400  to form the nip N through which recording medium is conveyed. 
     Embodiment 2 
     Next, another example of fixing device  30  in accordance with the present invention is described. The fixing device  30  in this embodiment is the same in structure as the fixing device  30  in the first embodiment, except for the structure of its pressure roller  32 . 
       FIG. 10  is a drawing for illustrating the characteristic features of the fixing device  30  in this embodiment, more specifically, the positional relationship among its supporting member  38 , the thermally conductive member  39 , the heater  37 , the film  36 , and the lengthwise end of the pressure roller  32 . In  FIG. 10 , the power supply connector  46  is not shown for descriptive discretion. 
     Referring to  FIG. 10 , the pressure roller  32  is provided with a rubber ring  32   d , as the second opposing member, which is on the outward side of the elastic layer  32   b  (first opposing member) for forming the nip N. In terms of the lengthwise direction, a gap  32   a   1  is provided between the elastic layer  32   b  and rubber ring  32   d . The rubber ring  32   d  is the same in material as the elastic layer  32   b . It is 18 mm in external diameter, and 5 mm in width. The rubber ring  32   d  presses on the heater  37 , between the outward end C of the thermally conductive member  39 , and the inward end E of the contact surface  38   b . Therefore, the heater  37  is subjected to a force F shown in  FIG. 10 . This force F can prevent the heater  37  from deforming in the adjacencies of the end C of the thermally conductive member  39 , and therefore, can improve the fixing device  30  in the state of contact between the heater  37  and thermally conductive member  39  across the lengthwise end portion of the thermally conductive member  39 . 
     In terms of the lengthwise direction, the end C of the thermally conductive member  39  is the same in position as the end of the nip N (elastic layer  32   b ), or on the inward side of the end of the nip N (elastic layer  32   b ). Further, the supporting member  38  has the first area which extends from the end C of the thermally conductive member  39  to the outward side of the end D of the rubber ring  32   d , and the second area which is in contact with the second surface, on the outward side of the first area. Here, the “second surface” means the opposite surface of the heater  37  from the surface of the heater  37 , which is in contact with the film  36 . 
     In the case of the fixing device  30  in this embodiment, the pressure roller  32  is provided with the rubber ring  32   d  in addition to the elastic layer  32   b . Therefore, it is ensured that the heater  37  remains in a better state of contact with the thermally conductive member  39 . Also in the case of the fixing device  30  in this embodiment, the amount of the force F for preventing the deformation of the heater  37  can be optionally set by adjusting the rubber ring  32   d  in external diameter. In other words, even if the tolerance set for the components of the fixing device  30  is relatively large, the state of contact between the heater  37  and the thermally conductive member  39  can be improved by adjusting the external diameter of the rubber ring  32   d.    
     The characteristic effects of the fixing device  30  in this embodiment are as follows: The heat from the heater  37  is likely to escape from the lengthwise ends of the pressure roller  32 , through the parting layer  32   c  (surface layer), the elastic layer  32   b , and the metallic core  32   a . Thus, if the elastic layer  32   b  of the pressure roller  32  is simply increased in size in the lengthwise direction to form the rubber ring  32   d , it is likely for the portions of the heater  37 , which correspond in position to the lengthwise end portions of the pressure roller  32 , to decrease in temperature, and therefore, it is possible that the portions of the toner image, which are on the lateral edge portion of a sheet of the recording medium, will be unsatisfactorily fixed. 
     In comparison, in the case of the fixing device  30  in this embodiment, the gap  32   a   1  is provided between the elastic layer  32   b  and the rubber ring  32   d  of the pressure roller  32 . That is, the elastic layer  32   b  and the rubber ring  32   d  are not in contact with each other. Therefore, this structural arrangement makes it more difficult for the heat in the lengthwise end portions of the elastic layer  32   b  to escape than the structural arrangement which simply extends the elastic layer  32   b . Therefore, it makes it less likely for the lengthwise end portions of the heater  37  to decrease in temperature. 
     As described above, this embodiment can keep the heater  37  and the thermally conductive member  39  in the better state of contact with each other to minimize the temperature reduction which occurs to the lengthwise end portions of the heater  37 . That is, it is more effective to keep the heater  37  uniform in temperature in terms of the lengthwise direction. 
     Incidentally, instead of providing the gap  32   a   1  as in this embodiment, the portion of the metallic core  32   a , which corresponds to the gap  32   a   1 , may be increased in external diameter to provide a portion like a stair step. 
     Embodiment 3 
     The fixing device in this embodiment is the same in structure as the one in the first embodiment, except for its portions which are described next. Therefore, the general structure of this fixing device is not described.  FIG. 9  is a drawing of one of the lengthwise end portions of another modified version of the fixing device  30  in the first embodiment. It is for illustrating the positional relationship among the supporting member  38 , the thermally conductive member  39 , the heater  37 , the film  36 , and the pressure roller  32 , at one of the lengthwise ends of the fixing device. In  FIG. 9 , the power supply connector  46  is not shown for descriptive discretion. 
     The fixing device  30  structured so that the inward end E of the contact surface  38   b  (second area) of the supporting member  38  is between the end C of the thermally conductive member  39  and the end D of the nip N may be modified in structure as follows. 
     Referring to  FIG. 9 , it may be structured so that the contact surface  38   b  of the supporting member  38  is offset from the area of contact between the thermally conductive member  39  and the heater  37 . This structural arrangement can absorb the component tolerance by the offset, and therefore, can ensure that the heater  37  remains in the better state of contact with the thermally conductive member  39 . 
     As described above, the fixing device  30  in this embodiment can ensure that the heater  37  remains in contact with the thermally conductive member  39  regardless of component tolerance. Therefore, it is greater in its effectiveness to keep the heater  37  uniform in temperature with the use of the thermally conductive member  39 . 
     By the way, the fixing devices in the first, second, and third embodiments were for heating the unfixed toner image t on the sheet P of a recording medium to fix the toner image to the sheet P. However, the present invention is also applicable to an image heating device for heating a fixed image on a sheet P of a recording medium to increase the image in gloss. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims priority from Japanese Patent Applications Nos. 151617/2013 and 128147/2014 filed Jul. 22, 2013 and Jun. 23, 2014, respectively, which are hereby incorporated by reference.