Patent Publication Number: US-8983353-B2

Title: Fixing apparatus and image forming apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to fixing apparatuses which fix a toner to a recording medium with heat and pressure and image forming apparatuses such as a facsimile machine, a printer, a copying machine, an equipment unit having these multiple functions, etc., using an electrophotographic scheme, an electrostatic recording scheme, etc., that include the fixing apparatuses. 
     BACKGROUND ART 
     As image forming apparatuses such as a copying machine, a printer, etc., various image forming apparatuses using an electrophotographic scheme have been devised, and have become well known in the art. An image forming process includes a process such that an electrostatic latent image is formed on a surface of a photoconductive drum, which is an image-bearing body; the electrostatic latent image on the photoconductive drum is developed by a toner, which is a developer, etc., to produce a visualized image; the developed image is transferred to a recording sheet with a transferring apparatus to bear the image thereon; and the toner image on the recording sheet is fixed with a fixing apparatus which uses pressure, heat, etc. 
     In this fixing apparatus, a pressurizing member and a fixing member that include opposing rollers or belts or a combination thereof abut against each other to form a nip portion; a recording sheet is put into the nip portion, and heat and pressure are applied to fix the toner image onto the recording sheet. 
     An example of the fixing apparatus includes a known technique which uses, as a fixing member, a fixing belt which is stretched by multiple roller members (for example, see Patent Document 1). Such an apparatus using the fixing belt includes a fixing belt (an endless belt)  204  as a fixing member, multiple roller members  202 ,  203  which stretch and support the fixing belt  204 , a heater  201  which is installed inside the roller member  202  of the multiple roller members  202 ,  203 , a pressurizing roller (a pressurizing member)  205 , etc. (see  FIG. 19 ). The heater heats the fixing belt  204  via the roller members  202 ,  203 . Then, a toner image on a recording medium which is carried to a nip portion formed between the fixing belt  204  and the pressurizing roller  205  is fixed onto the recording medium with heat and pressure received at the nip portion (Belt fixing scheme). 
     Moreover, for a fixing apparatus for use in the above-described image forming apparatus, there is a fixing apparatus which includes a fixed member which slidingly contacts an inner face of a fixing member which is a rotor. 
     For example, Patent document 2 discloses a film-heating fixing apparatus, wherein a heat-resistant film (a fixing film)  213  is sandwiched between a ceramic heater  211  as a heating body and a pressurizing roller  212  as a pressurizing member to form a fixing nip portion N, and a recording material with an unfixed toner image to be fixed being formed and carried thereon is introduced between the pressurizing roller  212  and the film  213  of the fixing nip portion N and is caused to be sandwiched and carried with the film  213  therebetween, so that, in the nip portion N, heat of the ceramic heater  211  is provided to the recording material via the film  213  and pressurizing of the fixing nip portion N fixes the unfixed toner image onto a face of the recording material by heat and pressure (see  FIG. 20 ). This type of film-heating fixing apparatus allows configuring an on-demand type apparatus using a low heat capacity material as a film and a ceramic heater and also allows powering on the ceramic heater as a heat source to be heated to a predetermined fixing temperature only when executing image formation of an image forming apparatus, so that there are advantages that a waiting time is short from when the power of the image forming apparatus is turned on to when image forming can be executed (Quick start), and power consumed at standby is significantly small (power conservation), etc. 
     Moreover, in Patent documents 3 and 4 is proposed a pressurizing-belt image fixing apparatus, including a rotatable heating fixing roller, a surface of which rotatable heating fixing roll elastically deforms; an endless belt (a pressurizing belt) which allows travelling while being in contact to the heating fixing roller and a belt nip which is non-rotatably arranged inside the endless belt to abut the endless belt against the heating fixing roll and through which a recording sheet is caused to pass between the endless belt and the heating fixing roller; and a pressurizing pad which elastically deforms the surface of the heating fixing roller. According to this fixing scheme, a lower pressurizing member is arranged to be a belt and a contact area between a sheet and a roller is widened, making it possible to drastically improve heat conduction efficiency, suppress energy consumption, and realize a reduced size. 
     However, with the above-described fixing apparatus of Patent document 1, which is suitable for an increased speed of the apparatus relative to an apparatus using a fixing roller, there are limitations on reducing warm-up time (time required to reach a temperature at which printing is possible) and first-print time (from the time at which a request for printing is received to the time at which sheet discharging is completed via print preparation and printing operations). 
     On the other hand, the fixing apparatus of Patent document 2 makes it possible to shorten the warm-up time and the first print time due to a lowered heat capacity and to realize an apparatus of a reduced size. However, the fixing apparatus of Patent document 2 has problems with durability and with belt temperature stability. In other words, phenomena occur such as abrasion which is due to sliding of an inner face of a belt; a ceramic heater which is insufficient as a heat source; operating for a long time causing a face on which continuous friction is repeated to become rough, so that frictional resistance increases and travelling of the belt becomes unstable, or driving torque of the fixing apparatus increasing, so that, as a result, slipping of a transfer sheet for forming an image occurs; or stress on a drive gear increases, causing a failure such that the gear is damaged (Problem 1). 
     Moreover, with the film-heating fixing apparatus, there is a problem that, as the belt is locally heated at the nip portion, the belt temperature becomes lowest when the rotating belt returns to a nip inlet, so that, especially when high-speed rotating is conducted, fixing failure is likely to occur (Problem 2). 
     On the other hand, Patent document 3 discloses measures of improving the problem of slidability between a fixed member and an inner face of a belt using a polytetra-fluoroethylene (PTFE) impregnated glass fiber sheet (PTFE impregnated glass cloth) as a low friction sheet-shaped sliding material on a surface layer of a pressure pad. However, with such a pressurizing-belt fixing apparatus (Patent documents 3 and 4), there is a problem that heat capacity of the fixing roller is large and temperature rise is slow, so that time required for warming up is long (Problem 3). 
     For the problems 1 to 3 as described above, Patent document 5 discloses a fixing apparatus, wherein generally pipe-shaped opposing members (a metal heat conductor, a heating member, a supporting member) arranged on the inner peripheral side of an endless fixing belt and a ceramic heater which is arranged on the inner peripheral side of the opposing members and which heats the opposing members are provided, making it possible to heat the whole fixing belt, shorten warm-up time and first-print time, and overcome a shortage of heat capacity at the time of high-speed rotation. 
     However, with the fixing apparatus of Patent document 5, the nip portion which is formed by pressing, onto the fixing belt side, a pressurizing roller which is a pressurizing member is supported by a metal heat conductor, a nip width and pressure in the nip portion becomes unstable. 
     Thus, in the Patent document 6 is proposed the feature of providing, in order to maintain the state, shape, position, etc., of a pipe-shaped supporting member and a nip portion with a pressurizing roller and a fixing belt, nip forming members (an abutting member, a fixed member) and a reinforcing member such that they correspond to a part at which the nip portion is formed. 
     PATENT DOCUMENTS 
     
         
         
           
             Patent document 1 JP11-2982A 
             Patent document 2 JP4-44075A 
             Patent Document 3 JP8-262903A 
             Patent Document 4 JP10-213984A 
             Patent Document 5 JP2007-334205A 
             Patent Document 6 JP2010-96782A 
           
         
       
    
     However, as the supporting member is a thin metal pipe and has a concave portion for arranging nip forming members, when the fixing belt slides, load (stress) becomes concentrated at the concave portion to cause deforming of the supporting member, which deforming causes local overheating of the supporting member and slipping of the fixing belt, causing problems of decrease in energy conserved, decrease in durability of parts, and image quality anomalies. 
     SUMMARY OF THE INVENTION 
     Means for Solving the Problems 
     In view of the problems of the related art as described above, an object of the present invention is to provide a fixing apparatus which prevents a supporting member from being deformed due to sliding of a fixing member, which has an improved durability of components and energy conservation, and from which a good image is obtained. 
     The present invention which is provided in order to solve the above-described problem is as shown below. In parentheses are shown corresponding parts, letters, etc., in embodiments of the present invention. 
     (1) A fixing apparatus (a fixing apparatus  20 ,  FIGS. 2 to 9 ,  FIGS. 14 to 16 ), includes: 
     a fixing member (a fixing belt  21 ) for a rotatable endless belt; 
     a pressurizing member (a pressurizing roller  31 ) which is arranged to be abuttable against the fixing member on an outer peripheral side of the fixing member; 
     a supporting member (a supporting member  60 ) which is a C type pipe member and which has a concave portion (a nip concave portion  61 ) with an opening (an opening  69 ) thereof arranged to be a bottom face side, the C type pipe member being fixedly provided inside the fixing member and an outer peripheral face of the C type pipe member being in slidable contact with an inner peripheral face of the fixing member; 
     a heating unit (a heating unit  25 ) which heats the supporting member; 
     a nip forming member (a nip forming member  26 ) which is contained in a concave portion of the supporting member that is on an inner peripheral side of the fixing member, and which abuts against the pressurizing member via the fixing member to form a nip portion; and 
     a deform preventing unit (an external holding member  70 , an internal holding member  71 ) which reinforces the concave portion to prevent deforming of the supporting member due to sliding of the fixing member. 
     (2) The fixing apparatus as recited in (1) in the above ( FIGS. 14 and 15B ), wherein the deform preventing unit includes an internal holding member (an internal holding member  71 ) which has an L-shaped angle shape or which has a groove shape with a cross section which is U-shaped and lengths of both of which sleeve portions differ, the internal holding member being installed from an outer peripheral side of the supporting member to inside the concave portion, wherein 
     when the internal holding member has the groove shape, an outer face of longer one (a sleeve portion  71   S1 ) of the sleeve portions is abutted against a side wall (a side wall  67 ) on a nip portion inlet (IN) side of the concave portion, and wherein 
     when the internal holding member has the angle shape, an outer side of one of bent faces is abutted against the side wall on the nip portion inlet side of the concave portion. 
     (3) The fixing apparatus as recited in (1) or (2) in the above ( FIGS. 14 and 15B ), wherein the deform preventing unit includes an external holding member (an external holding member  70 ) which has an L-shaped angle shape or which has a groove shape with a cross section which is U-shaped and sleeve portions whose lengths differ, the external holding member being installed from an inner peripheral side of the supporting member to outside the concave portion, wherein 
     when the external holding member has the groove shape, an inner face of a longer one (a sleeve portion  71   S2 ) of the sleeve portions is abutted against a side wall (a side wall  67 ) on a nip portion outlet (OUT) side of the concave portion, and wherein 
     when the external holding member has the angle shape, an inner side of one of bent faces is abutted against the side wall on the nip portion outlet side of the concave portion. 
     (4) The fixing apparatus as recited in (3) in the above ( FIG. 16 ), wherein the deform preventing unit includes the groove-shaped internal holding member and the groove-shaped external holding member, and wherein 
     a distance from a corner portion (C IN ) at which an outer peripheral face of the supporting member on the nip portion inlet side of the supporting member and a side wall of the concave portion cross to a tip of the sleeve portion is shorter for the internal holding member (d 1 ) than for the external holding member (d 2 ). 
     (5) The fixing apparatus as recited in (3) or (4) in the above ( FIG. 16 ), wherein the deform preventing unit includes the groove-shaped internal holding member and the groove-shaped external holding member, and wherein 
     a distance from a corner portion (C OUT ) at which an outer peripheral face of the supporting member on the nip portion outlet side of the supporting member and a side wall of the concave portion cross to a tip of the sleeve portion is shorter for the external holding member (d 4 ) than for the internal holding member (d 3 ). 
     (6) The fixing apparatus as recited in (4) or (5) in the above ( FIGS. 17 and 18 ), wherein the internal holding member and the external holding member include a reverse mounting preventing unit (a square hole  70   h , a convex portion  71   a ) which prevents a direction of mounting the holding members in a direction of rotation of the fixing member from being a reverse direction. 
     (7) An image forming apparatus (an image forming apparatus  1 ,  FIG. 1 ), includes the fixing apparatus (the fixing apparatus  20 ) as recited in any one of (1) to (6). 
     According to the fixing apparatus of the present invention, a deform preventing unit reinforces a concave portion of a supporting member to prevent the supporting member from deforming due to sliding of a fixing member, so that an anticipated shape of the supporting member is maintained, a sliding state between the supporting member and the fixing member is properly maintained, making it possible to improve energy conservation and durability of parts, and properly pass the recording medium through a nip section. 
     The image forming apparatus according to the present invention includes the fixing apparatus of the present invention, making it possible to stably form a good image over a long term. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration diagram which shows an image forming apparatus provided with a fixing apparatus according to an embodiment of the present invention; 
         FIG. 2  is a central longitudinal sectional view of a fixing apparatus according to an embodiment of the present invention; 
         FIG. 3  is a central longitudinal sectional view showing a breakdown of a supporting member, an external holding member, and an internal holding member of the fixing apparatus according to the present invention; 
         FIG. 4  is a perspective view of the supporting member of the fixing apparatus according to an embodiment of the present invention; 
         FIG. 5  is a schematic front view which shows dimensions of the supporting member of the fixing apparatus according to an embodiment of the present invention; 
         FIG. 6  is a perspective view showing the fixing apparatus according to the present invention with nip forming members removed; 
         FIG. 7  is a perspective view showing the back side of the nip forming members of the fixing apparatus according to an embodiment of the present invention; 
         FIG. 8  is a perspective view of a reinforcing member of the fixing apparatus according to an embodiment of the present invention; 
         FIG. 9  is a perspective view of a flange member of the fixing apparatus according to an embodiment of the present invention; 
         FIG. 10  is a cross-sectional diagram showing a heating unit which includes planar heating bodies. 
         FIG. 11  is a graph indicating a relationship between peripheral length difference, and friction and temperature when a fixing belt with a diameter of 30 mm and the supporting member shown in  FIG. 5  are used. 
         FIG. 12  is a perspective view of the supporting member, the flange member, and side plates of the fixing apparatus according to an embodiment of the present invention; 
         FIG. 13  is an explanatory diagram of deforming in the supporting member of the fixing apparatus of the present invention; 
         FIG. 14  is a cross-sectional diagram which shows a first part of how the internal holding member and the external holding member are installed in the supporting member at a nip concave portion; 
         FIGS. 15A and 15B  are cross-sectional diagrams which indicate the features of the external holding member and the internal holding member; 
         FIG. 16  is a cross-sectional diagram which shows a second part of how the internal holding member and the external holding member are installed in the supporting member at the nip concave portion; 
         FIGS. 17A ,  17 B, and  17 C are perspective views showing the feature of one end portion in an axial direction of the external holding member and the internal holding member; 
         FIG. 18  is a top view which indicates how the external holding member and the internal holding member are fitted into each other; 
         FIG. 19  is a schematic configuration diagram which shows a fixing apparatus of a related-art belt fixing scheme; and 
         FIG. 20  is a schematic configuration diagram which shows a fixing apparatus of a related-art film heating scheme. 
     
    
    
     DESCRIPTION OF THE REFERENCE NUMERALS 
       1  Image forming apparatus 
       3  Exposing unit 
       4 Y,  4 M,  4 C,  4 K Image forming units 
       5 Y,  5 M,  5 C,  5 K Photoconductor drums 
       12  Paper-supply unit 
       18  Maximum external diameter between heating region and separating region 
       19  Fixedly attaching member 
       20  Fixing apparatus 
       21  Fixing belt 
       21   a  Base material 
       21   b  Mold release layer 
       21   c  Coating film 
       22  Reflector plate 
       23  Reinforcing member 
       23   a  Body 
       23   b  Receiving projection 
       25  Heating unit 
       26  Nip forming member 
       26   a  Body 
       26   b  Supporting projection 
       26   c  Center line 
       27  Nip portion 
       28  Flange member 
       28   a  Cylindrical section 
       28   b  Flange section 
       28   c  Brim 
       29  Film member 
       31  Pressurizing roller (pressurizing member) 
       32  Center axle 
       33  Elastic layer 
       34  Mold release layer 
       42  Side plate 
       60  Supporting member 
       60   a  Coating film 
       61  Nip concave portion 
       62  Introducing region 
       63  Heating region 
       63   a  Arc center of heating region 
       64  Separating region 
       64   a  Arc center of separating region 
       65  Recess region 
       66  Intermediate region 
       67  Side wall 
       68  Base wall 
       69  Opening 
       70  External holding member 
       70   a  Mounting section 
       70   h  Square hole 
       70 S 0 ,  71 S 0  Base portions 
       70 S 1 ,  70 S 2 ,  71 S 1 ,  71 S 2  Sleeve portions 
       71  Internal holding member 
       71   a  Convex section 
       75  Charging unit 
       76  Developing unit 
       77  Cleaning unit 
       78  Intermediate transfer belt 
       79 Y,  79 M,  79 C,  79 K Primary transfer bias roller 
       80  Intermediate transfer cleaning unit 
       82  Secondary transfer backup roller 
       83  Cleaning backup roller 
       84  Tension roller 
       85  Intermediate transfer unit 
       89  Secondary transfer roller 
       97  Paper-supplying roller 
       98  Registration roller pair 
       99  Paper-discharge roller pair 
       100  Stacking unit 
       101  Bottle container 
       102 Y,  102 M,  102 C,  102 K Toner bottle 
       201  Heater 
       202 ,  203  Roller members 
       204  Fixing belt 
       205 ,  212  Pressurizing roller 
       211  Ceramic heater 
       213  Film 
     CIN, COUT Corner portion 
     IN Nip inlet side of nip convex portion 
     N Fixing nip portion 
     OUT Nip outlet side of nip concave portion 
     P Recording medium 
     T Toner image 
     Mode for Carrying Out the Invention 
     A description is given below with regard to embodiments of the present invention with reference to the drawings. 
     First, an image forming apparatus  1  according to an embodiment of the present invention is described with reference to  FIG. 1 . 
     As shown in  FIG. 1 , the image forming apparatus  1  is a tandem-type color printer. Four toner bottles  102 Y,  102 M,  102 C, and  102 K which correspond to colors (yellow, magenta, cyan, and black) are removably provided at a bottle container  101  which is located at an upper part of the image forming apparatus body  1 . Thus, these four toner bottles  102 Y,  102 M,  102 C, and  102 K are replaceable by a user, etc. 
     An intermediate transfer unit  85  is arranged at a lower part of a bottle container  101 . Image forming units  4 Y,  4 M,  4 C, and  4 K which correspond to the colors yellow, magenta, cyan, and black are installed together such that they oppose an intermediate transfer belt  78  of the intermediate transfer unit  85 . 
     At the image forming units  4 Y,  4 M,  4 C, and  4 K are respectively provided photoconductor drums  5 Y,  5 M,  5 C, and  5 K. Moreover, surrounding each of the photoconductor drums  5 Y,  5 M,  5 C, and  5 K are a charging unit  75 , a developing unit  76 , a cleaning unit  77 , a neutralizing unit (not shown), etc. Then, image forming processes (charging step, exposing step, developing step, transferring step, and cleaning step) are performed on the photoconductor drums  5 Y,  5 M,  5 C, and  5 K, so that images of the respective colors are formed on the photoconductor drums  5 Y,  5 M,  5 C, and  5 K. 
     The photoconductor drums  5 Y,  5 M,  5 C, and  5 K are rotationally driven in a clockwise direction in  FIG. 1  with a driving motor (not shown). Then, surfaces of the photoconductor drums  5 Y,  5 M,  5 C, and  5 K are uniformly charged at locations of the corresponding charging units  75 . (This is a charging step.) Then, the surfaces of the photoconductor drums  5 Y,  5 M,  5 C, and  5 K reach locations of irradiation of corresponding laser lights L emitted from an exposing unit  3 , at which locations scan exposing is conducted to form electrostatic latent images. (This is an exposing step.) 
     Then, the surfaces of the photoconductor drums  5 Y,  5 M,  5 C, and  5 K reach locations opposing the corresponding developing units  76 , at which locations the electrostatic latent images are developed, so that a toner image for each color is formed. (This is a developing step.) Then, the surfaces of the photoconductor drums  5 Y,  5 M,  5 C, and  5 K reach locations opposing the intermediate transfer belt  78  and respective primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K, at which locations the toner images on the photoconductor drums  5 Y,  5 M,  5 C, and  5 K are transferred onto the intermediate transfer belt  78 . (This is a primary transferring step.) Here, a small amount of untransferred toner remains on the photoconductor drums  5 Y,  5 M,  5 C, and  5 K. 
     Then, the surfaces of the photoconductor drums  5 Y,  5 M,  5 C, and  5 K reach locations opposing the corresponding cleaning units  77 , at which location untransferred toner which remains on the photoconductor drums  5 Y,  5 M,  5 C, and  5 K is mechanically collected by cleaning blades of the cleaning units  77 . 
     Finally, the surfaces of the photoconductor drums  5 Y,  5 M,  5 C, and  5 K reach locations opposing corresponding neutralizing units (not shown), at which locations remaining electric potentials on the photoconductor drums  5 Y,  5 M,  5 C, and  5 K are removed. Thus, a series of image forming processes which is conducted on the photoconductor drums  5 Y,  5 M,  5 C, and  5 K is completed. 
     Then, the toner image of each color that is formed on the respective photoconductor drum  5  via the developing step is transferred onto the intermediate transfer belt  78  in a superposed manner. In this way, a four-color color image is formed on the intermediate transfer belt  78 . Here, the intermediate transfer unit  85  includes the intermediate transfer belt  78 , the four primary bias rollers  79 Y,  79 M,  79 C,  79 K, a secondary transfer backup roller  82 , a cleaning backup roller  83 , a tension roller  84 , an intermediate transfer cleaning unit  80 , etc. The intermediate transfer belt  78  is stretched and supported by the three rollers  82 - 84 , and is also endlessly moved in an arrow direction in  FIG. 1  by rotational driving of the roller  82 . 
     The four primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K respectively put the intermediate transfer belt  78  between the four primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K and the photoconductor drums  5 Y,  5 M,  5 C, and  5 K to form primary transfer nips. Then, a transfer bias of a polarity which is reverse to a polarity of the toner is applied to the primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K. Then, the intermediate transfer belt  78  travels in the arrow direction to successively pass the primary transfer nips for the respective primary transfer bias rollers  79 Y,  79 M,  79 C, and  79 K. Thus, the toner images of the colors that are on the corresponding photoconductor drums  5 Y,  5 M,  5 C, and  5 K are primary transferred onto the intermediate transfer belt  78  in the superposed manner. 
     Then, the intermediate transfer belt  78  having the four-color toner image transferred thereon reaches a location which opposes a secondary transfer roller  89 , at which location the secondary transfer backup roller  82  sandwiches the intermediate transfer belt  78  between the secondary transfer backup roller  82  and the secondary transfer roller  89  to form a secondary transfer nip. Then, toner image of four colors that is formed on the intermediate transfer belt  78  is transferred onto a recording medium P which is carried to the location of the secondary transfer nip. Then, untransferred toner which has not been transferred onto the recording medium P remains on the intermediate transfer belt  78 . Then the intermediate transfer belt  78  reaches the location of the intermediate transfer cleaning unit  80 . Then, at this location, untransferred toner on the intermediate transfer belt  78  is collected. In this way, a series of transfer processes performed on the intermediate transfer belt  78  is completed. 
     Here, the recording medium P which is carried to the location of the secondary transfer nip is what is carried from a paper supply unit  12  which is arranged at a lower part of the apparatus main body  1  via a paper-supplying roller  97 , a registration roller pair  98 , etc. More specifically, the recording medium P such as a transfer paper is stored in the paper supply unit  12  with multiple overlaid sheets of the recording medium P being stored. Then, when the paper-supplying roller  97  is rotationally driven in a counterclockwise direction in  FIG. 1 , the topmost recording medium P is supplied between rollers of the registration roller pair  98 . 
     The recording medium P which is carried to the registration roller pair  98  stops at a location of a roller nip of the registration roller pair  98  which has stopped rotational driving. Then, in alignment with the timing of the color image on the intermediate transfer belt  78 , the registration roller pair  98  is rotationally driven, so that the recording medium P is carried towards the secondary transfer nip. In this way, a desired color image is transferred onto the recording medium P. 
     Then, the recording medium P, onto which the color image has been transferred at the location of the secondary transfer nip, is carried to a location of a fixing apparatus  20 . Then, the color image transferred onto the surface is fixed onto the recording medium P with heat and pressure due to a fixing belt  21  and a pressurizing roller  31  at this location. Then, the recording medium P is discharged out of the apparatus  1  via a location which is in between rollers of a paper-discharge roller pair  99 . The transferred medium P which is discharged out of the apparatus by the paper-discharge roller pair  99  is stacked on a stacking unit  100  as an output image. In this way, a series of image forming processes in the image forming apparatus  1  is completed. 
     Next, a configuration of the fixing apparatus  20  according to the present embodiment is described. 
     As shown in  FIG. 2 , the fixing apparatus  20  includes the endless-shaped fixing belt  21  which is rotatable and flexible; the pressurizing member  31  which is provided outside the fixing belt  21  in a radial direction and presses the fixing belt  21  toward inside the fixing belt  21  in the radial direction; a nip forming member  26  which is provided inside the fixing belt  21  in the radial direction and which puts in the fixing belt  21  between the nip forming member  26  and the pressurizing member  31  to press the fixing belt  21  together with the pressurizing member  31 ; a nip portion  27  which puts, between the fixing belt  21  and the pressurizing member  31 , the recording medium P which bears a toner image T; a tubular (generally circular cylindrical, pipe-shaped) supporting member  60  (also called a heating member) which is provided on the inner peripheral side of the fixing belt  21  and which also rotatably supports the fixing belt  21 , a heating unit  25  which heats the supporting member  60  to transfer heat to the fixing belt  21 , a reinforcing member  23  which attaches the supporting member  60  to the image forming apparatus  1 , flange members  28  which are provided at each end in the longitudinal direction of the fixing apparatus  20 , and side plates  42 , which are frames shown in  FIG. 12 , each of which supports the flange member  28  provided at the corresponding end. 
     The fixing belt  21 , which has a cylindrical shape with an inner diameter of 30 mm, includes a base material  21   a  made of steel and which has a thickness of 30-50 μm, a mold release layer  21   b  which is formed on the front face side of the base material  21   a , and a coating film  21   c  which is formed on the back face side of the base material  21   a . The material which forms the base material  21   a  is not limited to steel, so that a metal material with high heat conductivity, such as cobalt, nickel, stainless steel, an alloy thereof, etc., or a synthetic resin material such as polyimide, etc., may be used. 
     The mold release layer  21   b  is provided for increasing mold releasability against the toner image T on the recording medium P. The mold release layer  21   b  is arranged to be made of PFA (tetrafluoroethylene/fluoroalkyl vinyl ether copolymer resin) with a thickness of 10-50 μm. The material which forms the mold release layer  21   b  is not limited to PFA, so that PTFE (tetrafluoroethylene resin), polyimide, polyether imide, PES (polyether sulfide), etc., may be used. The mold release layer  21   b  is provided to secure mold releasability against the toner image T. 
     The coating film  21   c  is provided in order to make frictional resistance with the supporting member  60  small. The coating film  21   c  is arranged to be Teflon (registered trademark) coating. The material which forms the coating film is not limited to Teflon (registered trademark), so that surface coating such as glass coating, DLC (Diamond-like carbon), plating, etc., may be used. 
     As shown in  FIGS. 3-5 , the supporting member  60  is arranged to be a pipe which is made of metal such as steel, which has a thickness of 0.1-1 mm, and which has a generally C-shaped cross section. The supporting member  60  includes a nip concave portion which houses the nip forming member  26  to form a part of the nip portion  27 , an introducing region  62  which is provided on the upstream side in the rotating direction of the fixing belt  21  of the nip concave portion  61 , a heating region  63  which is provided in continuation with the introducing region  62 , a separating region  64  which is formed on the downstream side in the rotating direction of the fixing belt  21  of the nip concave portion  61 , a recess region  65  which is flat and which is provided in continuation with the separating region  64 , and an intermediate region  66  which is formed in continuation with the recess region  65  on the downstream side of the recess region  65  in the rotational direction and which is formed in continuation with the heating region  63 . The supporting member  60  is formed by press molding. 
     The heating region  63 , which has a cross section of an arc shape with a radius of 14.5 mm and which is in continuation with the nip concave portion  61  on the upstream side in the rotating direction of the nip concave portion  61 , is arranged to be a region which is heated by the heating unit  25 . Moreover, arc center  63   a  of the heating region  63  is arranged such that it is separated 3.4 mm to the upstream side in the recording medium carrying direction relative to a center line  26   c  in the recording medium carrying direction (shown with a whited out arrow in  FIG. 2 ) of the nip forming member  26 . In this way, the fixing belt  21  is pulled towards the downstream side in the recording medium carrying direction, so that it becomes more difficult for the fixing belt  21  to separate from the heating region  63 . Moreover, the inner face of the supporting member  60 , the heating region  63  in particular, is coated black. In this way, the transfer of radiant heat from the heating unit  25  is improved. 
     The introducing region  62  has a cross section shape formed such that a distance from the arc center  63   a  of the heating region  63  is smaller than a radius 14.5 mm of the heating region  63 . In other words, the introducing region  62 , which has a flat shape with a small curvature, is arranged such that it is in continuation with the nip concave portion  61  and the heating region  63 . In this way, floating, from the supporting member  60 , of the fixing belt  21  in the vicinity of the nip portion  27  is suppressed. 
     The separating region  64 , having an arc-shaped cross section with a radius of 13 mm, which is smaller than a radius of 14.5 mm of the heating region  63 , is arranged to be a region at which the recording medium P is separated from the fixing belt  21  by rapidly separating the fixing belt  21  from the recording medium P which has passed through the nip portion  27 . Moreover, the arc center  64   a  of the separating region  64  is arranged such that it is 2.7 mm away to the downstream side in a recording medium carrying direction and 2 mm away to the nip portion  27  side relative to the arc center  63   a  of the heating region  63 . In this way, the maximum external diameter  18 , which connects the arc centers  63   a  and  64   a  of the heating region  63  and the separating region  64 , becomes the maximum external diameter of the supporting member  60  and is 30.86 mm, thus becoming larger than the inner diameter of 30 mm of the fixing belt  21 . In this way, the fixing belt  21  is pulled between the heating region  63  and the separating region  64 , so that it becomes more difficult for the fixing belt  21  to separate from the heating region  63 . Moreover, the peripheral length difference L 2 -L 1  is arranged to be 0.7 mm when the external peripheral length of the supporting member  60  in which the nip forming member  26  is installed is set to L 1  and the internal peripheral length of the fixing belt  21  is set to L 2 . 
     The intermediate region  66  has an arc shaped cross section with the same radius and the same center  63   a  as the heating region  63 . In this way, the heating region  63  and the intermediate region  66  may be formed with the same curvature, so that the supporting member  60  is easily worked. 
     The recess region  65  is formed between the intermediate region  66  and the separating region  64  on a plane which is 11.5 mm away from the arc center  64   a  of the separating region  64  on the downstream side in the recording medium carrying direction. In this way, the supporting member  60  and the fixing belt  21  become contact-less in the recess region  65 , so that frictional resistance is reduced. 
     As shown in  FIG. 2 , an external face of the supporting member  60  is coated with a coating film  60   a . The coating film  60   a  is provided in order to make frictional resistance with the fixing belt  21  small. The coating film  60   a  is arranged to be Teflon (registered trademark) coating. The material which forms the coating film  60   a  is not limited to Teflon (registered trademark), so that surface coating such as glass coating, DLC, plating, etc., may be used. Moreover, grease is applied between the supporting member and the fixing belt  21 . In this way, frictional resistance between the supporting member  60  and the fixing belt  21  becomes small. 
     As shown in  FIG. 3 , the nip concave portion  61  includes a pair of side walls  67  that extends in parallel towards inside the supporting member  60 , a bottom wall  68  which links tips of each of the side walls  67 , and an opening  69  which is formed at the bottom wall  68 . An external holding member  70  which is provided outside the nip concave portion  61 , or inside the supporting member  60  and which is generally U-shaped, for example, and an internal holding member  71  which is provided inside the nip concave portion  61 , or outside the supporting member  60  and which is generally U-shaped, for example, are installed to the nip concave portion  61 . The external holding member  70  and the internal holding member  71  nip the bottom wall  68  and the side wall  67  of the nip concave portion  61  of the supporting member  60  to screw them. The installing of the external holding member  70  and the internal holding member  71  maintains the shape of the nip concave portion  61 . Moreover, a mounting portion  70   a  is formed at each end of the external holding member  70  in the longitudinal direction. The mounting portion  70   a  is fixed to the holding member  60  by the flange member  28 . 
     As shown in  FIGS. 2 ,  6 , and  7 , the nip forming member  26  is provided inside the internal holding member  71 . The nip forming member  26  is arranged to be made of a heat-resistant resin material such as PAI (polyamideimide), polyimide resin, or LCP (liquid crystal polymer) and have a generally square bar shape in the longitudinal direction of the supporting member  60 . The nip forming member  26  includes a body  26   a  which opposes the pressurizing member  31 , a supporting projection  26   b  which is abutted against and supported by the reinforcing member  23  at the back face of the body  26   a , and a film member  29  which is provided surrounding the body  26   a.    
     When the body  26   a  is pressed by the pressurizing member  31 , the supporting projection  26   b  is abutted against and supported by the reinforcing member  23 , so that being pushed by the pressurizing member  31  is prevented. A face on the pressurizing member  31  side of the nip forming member  26  is formed in a planer shape. It may also be arranged to be shaped in such a concave shape as to align with the surface of the pressurizing member  31 . 
     The film member  29 , which is made of a PTFE textile fabric, reduces frictional resistance with the fixing belt  21 . The film member  29 , which is wrapped around the body  26   a , is fastened such that it is placed between the body  26   a  and a fixedly attaching member  19  screwed in the vicinity of the supporting projection  26   b . The nip forming member  26  is fixed to the holding member  60  by the flange member  28 . 
     As shown in  FIGS. 2 and 8 , the reinforcing member  23  includes a highly rigid metal-made body  23   a , which has a generally square bar shape along the longitudinal direction of the supporting member  60 , a receiving projection  23   b  which abuts against the supporting projection  26   b  of the nip forming member  26 , and a reflector plate  22  which opposes the heating unit  25 . The receiving projection  23   b  abuts against the supporting projection  26   b  of the nip forming member  26  and supports from behind the nip forming member  26  which is pressed by the pressurizing member  31 . The reflector plate  22  reflects radiant heat from the heating unit  25  to reduce quantity of heat which is transferred towards the body  23   a  of the reinforcing member  23 . The reinforcing member  23  is fixed to the holding member  60  by the flange member  28 . 
     The heating unit  25 , which is a line-shaped heating body provided inside the supporting member  60  along the longitudinal direction thereof, is arranged to be a halogen heater in the present embodiment. The heating unit  25  is provided inside the heating region  63 . In this way, the heating region  63  becomes a radiant region in which heat from the heating unit  25  is radiated without being blocked by the reinforcing member  23 . Moreover, a temperature sensor which detects a temperature of the fixing belt  21  is provided at an appropriate location of the heating region  63 . 
     As shown in  FIG. 9 , the flange member  28 , which includes a cylindrical section  28   a , one of which is inserted into an inner diameter section of each end of the supporting member  60  in an axial direction and which holds a shape in the vicinity of the corresponding end of the supporting member  60 , and a flange section  28   b  which is securely installed on a corresponding side plate of the fixing apparatus  20 , holds the nip forming member  26 , the external holding member  70 , the reinforcing member  23 , and the heating unit  25  to fix them. Moreover, the flange member  28  regulates movement of the fixing belt  21  in an axial direction with a brim  28   c.    
     As described above, while the holding member  60  is arranged to have a predetermined cross sectional shape in order to obtain predetermined functions such as adhering to the fixing belt  21  to efficiently heat the fixing belt  21  in the heating region  63 , maintain separability of the recording medium P in the separating region  64 , etc., it is a thin metal pipe, so that there is a tendency for the working shape to vary and for it to be slided on by the fixing belt  21  to be deformed to undermine the anticipated functions. Thus, the holding of the external peripheral face of the cylindrical section  28   a  of the flange member  28  is arranged such that a shape in the vicinity of the end of the supporting member  60  becomes the shape as described above and anticipated functions are obtained in a stable manner. Thus, the clearance between the external peripheral face of the cylindrical section  28   a  and the inner peripheral face of the end of the supporting member  60  is arranged to be no more than 0.15 mm. 
     The pressurizing member  31 , which is a pressurizing roller with an external diameter of 30 mm, includes a pipe-shaped center axle  32  which is made of metal, an elastic layer  33  which is made of heat-resistant silicone rubber and which is provided therearound, and a mold release layer  34  which is made of PFA and which is formed on the surface. The elastic layer  33  is arranged to have a thickness of 2-3 mm. The mold release layer  34  is formed such that it coats a PFA tube of a thickness of 50 μm. Moreover, a heating body such as a halogen heater may be built into the center axle  32  as needed. 
     Moreover, the pressurizing member  31  is pushed to the nip forming member  26  by a pressurizing mechanism (not shown) via the fixing belt  21 . The pressurizing member  31  is pressed to the nip forming member  26  via the fixing belt  21  to form the nip portion  27 . The pressurizing member  31  is rotated (an arrow direction in  FIG. 2 ) by a driving mechanism (not shown) while it presses the fixing belt  21 . The fixing belt  21  rotates with the above-described rotation and, at the same time, the recording medium P is carried while being pressed at the nip portion  27 . 
     Next, the operation is described. 
     A user operates an operating panel, a computer, etc., to issue a request for printing. When an image forming apparatus receives an output signal due to the above-described request for printing, the pressurizing member  31  is rotated by the driving mechanism and the fixing belt  21  rotates with the pressurizing member  31  as well. 
     Here, with the arc center  63   a  of the heating region  63  being located on the upstream side in the recording medium carrying direction relative to a center line in the recording medium carrying direction of the nip forming member  26 , the fixing belt  21  is pulled to the downstream side in the recording medium carrying direction, or the opposite side of the heating region  63 , so that adhesion between the fixing belt  21  and the supporting member  60  in the heating region  63  increases, and, at the same time, it becomes more difficult for the fixing belt  21  to separate from the supporting member  60 . Moreover, the heating region  63  has a cross sectional shape which is an arc shape with a radius of 14.5 mm which is generally the same as a radius of 15 mm of the fixing belt  21 , so that, in the heating region  63 , the fixing belt  21  adheres to the supporting member  60  with almost no transformability acting on the fixing belt  21 , so that adhesion between the supporting member  60  and the fixing belt  21  increases. Moreover, with the maximum external diameter  18  of 30.86 mm which is between the heating region  63  and the separating region  64  being larger than the inner diameter of 30 mm of the fixing belt  21 , the fixing belt  21  is pulled between the heating region  63  and the separating region  64 , so that adhesion between the fixing belt  21  and the supporting member  60  in the heating region  63  increases, and, at the same time, it becomes more difficult for the fixing belt  21  to separate from the supporting member  60 . Due to these reasons, the fixing belt  21  adheres to the supporting member  60  to slide on the supporting member  60  in the heating region  63 . 
     On the other hand, in synchronization with a rotation of the pressurizing member  31 , an electric current is sent to the heating unit  25 , which generates heat. The heat of the heating unit  25  is radiated to the heating region  63 , so that the heating region  63  is rapidly heated. Rotation of the pressurizing member  31  and heating by the heating unit  25  do not have to start at the same time, so that a time difference may be provided as needed. Then, a temperature of the fixing belt  21  is detected with a temperature sensor, the nip portion  27  is raised in temperature to a temperature necessary for fixing, after which feeding of the recording medium P is started while the temperature is maintained. With the recording medium P which passed through the nip portion  27 , a toner image T of the recording medium P is fixed by pressure and heat of the nip portion  27 . 
     As described above, according to an image forming apparatus of the present embodiment, adhesion between the fixing belt  21  and the supporting member  60  in the heating region  63  increases, and, at the same time, it becomes more difficult for the fixing belt  21  to separate from the supporting member  60 , so that heat conductivity from the supporting member  60  to the fixing belt  21  increases, suppressing overheating of the supporting member  60  to make it possible to prevent degradation of the coating films  60   a  and  21   c . Moreover, an increased adhesion between the supporting member  60  and the fixing belt  21  makes it possible to shorten warm-up time and first print time and to improve energy conservation. 
     Then, according to the present embodiment, the separation area  64  has a cross sectional shape of an arc with a radius which is smaller than a radius of the heating region  63 , so that the fixing belt  21  is rapidly separated from the recording medium P. Thus, separability from the fixing belt  21  at the recording medium P after passing through the nip portion  27  may be improved. 
     Moreover, according to the present embodiment, the peripheral length difference L 2 -L 1  is arranged to be 0.5-0.9 mm when the external peripheral length of the supporting member  60  in which the nip forming member  26  is installed is set to L 1  and the internal peripheral length of the fixing belt  21  is set to L 2  ( FIG. 11 ). Here, when the peripheral length difference exceeds 0.9 mm, the fixing belt  21  is loosely wrapped around the supporting member  60 , so that the fixing belt  21  floats to cause an overheated portion in a part of the supporting member  60 , making it easier for durability of the coating film to be deteriorate. Moreover, when the peripheral length difference is less than 0.5 mm, the fixing belt  21  is tightly wrapped around the supporting member  60 , so that friction between the fixing belt  21  and the supporting member  60  increases to make it more difficult for the fixing belt  21  to rotate, making it more likely for the pressurizing member  31  and the recording medium P to slip against the fixing belt  21 . Therefore, when the peripheral length difference L 2 -L 1  is 0.5-0.9 mm as in the present embodiment, the fixing belt  21  never floats from the supporting member  60 , making it possible to prevent overheating of the supporting member  60  and to prevent slipping of the recording medium P without the coiling of the fixing belt  21  around the supporting member  60  being too tight. 
     Moreover, according to the present embodiment, the fixing belt  21  is pulled between the heating region  63  and the separating region  64 , so that the adherence of the fixing belt  21  and the supporting member  60  in the heating region  63  increases even when the fixing belt  21  is stopped. In this way, when the fixing apparatus  20  which has been stopped is activated to statically heat the fixing belt  21 , the fixing belt  21  can be efficiently heated without overheating the supporting member  60 . 
     Moreover, according to the present embodiment, with the heating unit  25  being a line-shaped heating body which is provided inside along the longitudinal direction of the supporting member  60 , the line-shaped heating body has a simple installation structure, making it possible to simplify the configuration of the fixing apparatus  20 . Moreover, the inner face of the supporting member  60  is coated black, so that the radiation rate at the supporting member  60  is improved, making it possible to shorten warm-up time and first-print time and to improve energy conservation. 
     Furthermore, the introducing region  62  is provided between the heating region  63  and the nip forming member  26  with a cross-sectional shape having a distance from the arc center  63   a  of the heating region  63  being smaller than the radius 14.5 mm of the heating region  63 , making it possible to prevent in the introducing region  62  the fixing belt  21  from floating from the external peripheral face of the supporting member  60  and to prevent the supporting member  60  from overheating. 
     Then, according to the present embodiment, the intermediate region  66  has a cross-sectional shape which is an arc shape with the same radius and center  63   a  as the heating region  63 , making it is possible to form the heating region  63  and the intermediate region  66  with the same curvature. Thus, the supporting member  60  may be easily worked and the manufacturing cost may be reduced. 
     Moreover, according to the present embodiment, with a flat recess region  65  being provided between the intermediate region  66  and the separating region  64 , the supporting member  60  and the fixing belt  21  become contact-less in the recess region  65 , so that frictional resistance therebetween decreases such that it becomes even smaller than the frictional resistance between the fixing belt  21  and the recording medium P, making it possible to suppress slipping of the recording medium P against the fixing belt  21 . Moreover, material for forming the supporting material  60  may be shortened, making it possible to reduce material cost. 
     Furthermore, according to the present embodiment, the inner face of the fixing belt  21  and the outer face of the supporting member  60  are coated with coating films  21   c  and  60   a , respectively, and grease is applied therebetween, so that frictional resistance of a sliding portion between the supporting member  60  and the fixing belt  21  decreases such that it becomes smaller than the frictional resistance between the fixing belt  21  and the recording medium P, making it possible to suppress slipping of the recording medium P against the fixing belt  21 . 
     In the image forming apparatus according to the present embodiment, while the peripheral length difference L 2 -L 1  is arranged to be 0.7 mm when the outer peripheral length of the supporting member  60  in which the nip forming member  26  of the fixing apparatus  20  in the present invention is set to L 1  and the inner peripheral length of the fixing belt  21  is set to L 2 , it is not limited thereto. 
     In other words, when the difference between the inner peripheral length of the fixing belt  21  and the outer peripheral length of the supporting member  60  exceeds 0.9 mm, the fixing belt  21  is loosely wrapped around the supporting member  60 , so that the fixing belt  21  may float to cause an overheated portion in a part of the supporting member  60 , so that durability of the coating film may be reduced. Moreover, when the difference between the inner peripheral length of the fixing belt  21  and the outer peripheral length of the supporting member  60  is less than 0.5 mm, the fixing belt  21  is tightly wrapped around the supporting member  60 , so that friction between the fixing belt  21  and the supporting member  30  may increase, making it more difficult for the fixing belt  21  to rotate and making it more likely for the pressurizing member  31  and the recording medium P to slip against the fixing belt  21 . 
     Thus, it suffices for the peripheral length difference between the inner peripheral length of the fixing belt  21  and the outer peripheral length of the supporting member  60  to be 0.5-0.9 mm, more preferably 0.6-0.8 mm, and most preferably 0.7 mm, making it possible to prevent overheating of the supporting member  60  while suppressing slipping of the recording medium P within the above-described ranges. Depending on whether the coating films  21   c ,  60   a  and the grease are present, or on the shape and dimensions of each element, the peripheral length difference is not limited to 0.5-0.9 mm, so that it may be set appropriately. 
     Moreover, in the image forming apparatus of the present embodiment, while the intermediate region  66  of the supporting member  60  of the fixing apparatus  20  of the present invention is arranged to have an arc-shaped cross-section having the same radius and the same center  63   a  as the heating region  63 , it is not limited thereto, so that it may be arranged to have a cross-sectional shape having a distance from the arc center  63   a  of the heating region  63  that is smaller than the radius of the heating region  63  as long as it does not interfere with the reinforcing member  23 , for example. In this case, with the supporting member  60  and the fixing belt  21  becoming contact-less in the intermediate region  66 , frictional resistance therebetween decreases such that it becomes even smaller than the frictional resistance between the fixing belt  21  and the recording medium P, making it possible to suppress slipping of the recording medium P against the fixing belt. Moreover, material for forming the supporting material  60  may be shortened, making it possible to reduce material cost. 
     Moreover, in the image forming apparatus according to the present embodiment, while the fixing belt  21  of the fixing apparatus  20  is arranged to have a diameter of 30 mm, it is not limited thereto, so that the diameter may be set to 15-120 mm, or more specifically 25 mm, for example. 
     Furthermore, in the image forming apparatus according to the present embodiment, the heating unit  25  of the fixing apparatus  20  is arranged to be a line-shaped heating body such as a halogen heater, it is not limited thereto, so that it may be arranged to be a plane-shaped heating body which is provided so as to contact the inner peripheral face along the longitudinal direction of the supporting member  60  as shown in  FIG. 2  in an imaginary line. 
     As shown in  FIG. 10  for example, the plane-shaped heating body includes a flexible heating sheet  52   s  having a predetermined width and length corresponding to the axial and peripheral directions of the fixing belt  21 . The heating sheet  52   s  includes a base layer  52   a  which has insulating properties, a resistive heating layer  52   b  at which conductive particles are dispersed in a heat-resistive resin, and an electrode layer  52   c  which supplies electric power to the resistive heating layer  52   b . On the base layer  52   a , an insulating layer  52   d  is provided which insulates around the electrode layer  52   c , which is a different feeding system neighboring the resistive heating layer  52   b , and which insulates between an edge portion of the heating sheet  52   s  and the outside. Moreover, the plane-shaped heating body, which is connected to the electrode layer  52   c  at the end of the heating sheet  52   s , includes an electrode terminal which supplies power provided from a feeding line to the electrode layer  52   c . The plane-shaped heating body is not limited to the above-described configuration, so that a different configuration may be adopted. 
     The plane-shaped heating body is applied and the line-shaped heating body is omitted, so that the heating region  63  becomes a contacting region at which heat is conducted from the heating unit  25  which includes the plane-shaped heating body. In this way, the plane-shaped heating body may efficiently heat the supporting member  60 , making it possible to shorten the warm up time and the first print time and to improve energy conservation. 
     Alternatively, the heating unit  25  may be provided outside or inside the supporting member  60 , so that it is an inductive coil which inductively heats the supporting member  60 . In this case, the heating region  63  is an opposing region which is inductively heated opposing the heating unit  25 . In this way, with the inductive heating, elements other than the supporting member  60  are not directly heated, so that, unlike the line-shaped heating body, an element other than the supporting member  60 , such as the reinforcing member  23 , for example, is not heated, making it possible to efficiently heat the supporting member  60 . 
     EXAMPLE 
     Using the supporting member  60  with the dimensions and shape shown in  FIG. 5  under the same conditions as the above-described embodiment, various measurements were made, varying only the peripheral length difference L 2 -L 1  between the inner peripheral length L 2  of the fixing belt  21  and the outer peripheral length L 1  of the supporting member  60  in which the nip forming member is installed. The measurements were made for the relationship between the surface temperature of the supporting member  60  and the peripheral distance difference and the relationship between the friction of the fixing belt  21  and the supporting member  60  and the peripheral length difference. 
     The results are shown in  FIG. 11 . As shown in  FIG. 11 , when the peripheral length difference exceeds 0.9 mm, the surface temperature of the supporting member  60  exceeded a predetermined temperature limit value. In other words, it is supposed that, when the peripheral length difference exceeds 0.9 mm, the fixing belt  21  is loosely wrapped around the supporting member  60 , so that the fixing belt  21  floats to cause an overheated portion in a part of the supporting member  60  to exceed the temperature limit. Thus, it has been revealed that, when the peripheral length difference exceeds 0.9 mm, overheating of the supporting member  60  leads to durability of the coating film  60   a  becoming more likely to be reduced. 
     Moreover, when the peripheral length difference is less than 0.5 mm, friction between the supporting member  60  and the fixing belt  21  exceeded a predetermined limit value. In other words, it is supposed that, when the peripheral length difference is less than 0.5 mm, the fixing belt  21  is tightly wrapped around the supporting member  60 , so that the frictional force between the fixing belt  21  and the supporting member  60  increases, so that slipping limit value of the recording medium P and the pressurizing member  31  has been exceeded. Thus, it has been revealed that, when the peripheral length difference is less than 0.5 mm, it has become more difficult for the fixing belt  21  to rotate and it has become more likely for the pressurizing member  31  and the recording medium P to slip against the fixing belt  21 . 
     On the basis of these results, it has been revealed that the peripheral length difference between the inner peripheral length of the fixing belt  21  and the outer peripheral length of the supporting member  60  is 0.5-0.9 mm, more preferably 0.6-0.8 mm and most preferably 0.7 mm. In this way, it is verified that slipping of the recording medium P may be suppressed while preventing overheating of the supporting member  60 . 
     Now, in the fixing apparatus  20 , in order to efficiently conduct heat from the supporting member  60  to the fixing belt  21 , it is important that the supporting member  60  and the fixing belt  21  are in proper contact, impacting energy conservation in its turn. In other words, when the pressure of the fixation belt  21  being in contact with the support member  60  is too high, rotational load of the fixing belt  21  increases, causing a problem of slipping of the fixing belt  21 . Then, when slipping of the fixing belt  21  occurs, diffusion of heat by rotating ceases to occur, causing inflammation and decrease in durability of parts due to local heat supply. Moreover, transportability of the recording medium decreases and deviation occurs in heat supply to cause image quality anomalies such as deviation in gloss, fixing failure, etc. On the other hand, heat transfer efficiency decreases when the pressure of the fixing belt  21  being in contact with the supporting member  60  is too low, and, as it is, there is a lack of temperature of the fixing belt  21 , so that heat supply from the heating unit  25  increases, causing reduction in energy conservation. Moreover, the supporting member  60  overheats, impacting the durability of the coating film  60   a  of the surface. Thus, as described previously, in order to achieve predetermined performance such as being able to adhere to the fixing belt  21  in the heating region  63  to efficiently heat the fixing belt  21 , the supporting member  60  is arranged to have a predetermined cross-sectional shape. 
     However, there is a tendency for the supporting member  60  to undermine anticipated functions by being deformed by sliding contact with the fixing belt  21  or by being deformed by being thermally affected by the heating unit  25  since it is a thin metal pipe. In particular, the supporting member  60  is provided with a nip concave portion  61 , which is bent such that the opening  69  of a C shaped pipe member as shown in  FIG. 13  is on the bottom face side in order to arrange the nip forming member  26  (not shown); when the fixing belt  21  rotates and slides at the time of driving the apparatus (or when the supporting member  60  thermally expands due to being heated by the heating unit  25 ), load concentrates at the nip concave portion  61  of the supporting member  60 ; and a corner portion, at which the side wall  67  of the nip concave portion  61  and the outer peripheral face of the supporting member  60  at each of inlet and outlet sides of the nip portion of the supporting member  60  cross, ends up getting deformed in the direction of rotation of the fixing belt  21  from a state of the proper position  61   a  which is shown in a solid line to a state of a deformed position  61   b  which is shown in a dotted line. Moreover, the deforming increases as the supporting member  60  is made thinner in order to increase heat transfer efficiency. In this way, when the supporting member  60  deforms, the peripheral length difference between the inner peripheral length of the fixing belt  21  and the outer peripheral length of the supporting member  60  changes, so that it becomes no longer possible to achieve anticipated performance in the supporting member  60 , causing slipping of the fixing belt  21  and local overheating of the supporting member  60 , leading to problems of reduction in energy conservation, decrease in durability of parts, image quality anomalies, etc. 
     Thus, in the present invention, in order to solve the problem described above, a deform preventing unit is provided which reinforces the nip concave portion  61  to prevent the supporting member  60  from deforming due to sliding of the fixing belt  21 . 
     More specifically, as shown in  FIG. 14 , the deform preventing unit preferably includes the groove-shaped internal holding member  71  with a cross section having a U shape and each of sleeve portions having different lengths, the groove-shaped internal holding member  71  being installed from the outer peripheral side of the supporting member  60  to inside the nip concave portion  61 . 
     In other words, as shown in  FIG. 15B , the internal holding member  71 , which is a groove-shaped member having a U-shaped cross section, includes a sleeve portion  71   S1  and a sleeve portion  71   S2 , which are bent 90 degrees in the same direction at corresponding ends in the lateral direction of a base portion  71   S0  that corresponds to the direction of rotation of the fixing belt  21 . 
     Here, with a length L S11  of the sleeve portion  71   S1  being arranged to be longer than a length L S12  of the sleeve portion  71   S2  ( FIG. 15B ), the base portion  71   S0  is fixed to the bottom wall  68 , so that the outer face of the sleeve portion  71   S1 , which is a longer sleeve portion, is abutted against the inner face of the side wall of the nip portion inlet IN side of the nip concave portion  61  of the supporting member  60  ( FIG. 14 ). 
     In order to suppress deforming of the corner portion at which the side wall  67  of the nip concave portion  61  and the outer peripheral face of the supporting member  60  cross, it is more advantageous to have a larger area of contact between the side wall  67  and the sleeve portion above-described corner portion to the tip of the sleeve portion  71   S1  is preferably no more than 1.5 mm, and is preferably no more than 1.0 mm. 
     On the other hand, the sleeve portion  71   S2  does not contribute to suppressing deforming, due to sliding of the fixing belt  21 , of the corner portion at which the side wall  67  of the nip concave portion  61  and the outer peripheral face of the supporting member  60  on the nip portion outlet OUT side of the nip concave portion  61  cross. This is because the sleeve portion  71   S2  is arranged on the side which is opposite the direction of deforming of the corner portion. Therefore, for the sleeve portion  71   S2 , the area of contact with the side wall  67  is not needed as much as for the sleeve portion  71   S1 . Moreover, considering that the supporting member  60  is for being heated by the heating unit  25 , so that the heat is conducted to the fixing belt  21 , it is desirable that heat capacity is as small as possible for the internal holding member  71 , which is installed in the supporting member  60 . Moreover, for the aspect of mounting, it is desirable to make each member small in order to avoid interference among the parts. Thus, the sleeve portion  71   S2  preferably has a length which is shorter than that of the sleeve portion  71   S1 , so that, for example, the distance from the corner portion at which the side wall  67  of the nip concave portion  61  and the outer peripheral face of the supporting member  60  cross to the tip of the sleeve portion  71   S2  is preferably no less than 2.0 mm, and is more preferably no less than 2.5 mm. 
     In this way, the internal holding member  71  is used to suppress increase in cost and heat capacity (consumed power), and, at the same time, as shown in  FIG. 13 , the sleeve portion  71   S1  of the internal holding member  71  supports the side wall  67  on the nip portion inlet IN side of the nip concave portion  61 , making it possible to prevent deforming, due to sliding of the fixing belt  21 , of the corner portion at which the side wall  67  of the nip concave portion  61  and the outer peripheral face of the supporting member  60  cross. 
     The internal holding member  71  may be arranged to have an L-shaped angle shape, including only the base portion  71   S0  and the sleeve portion  71   S1  and not including the sleeve portion  71   S2  in  FIG. 15B . In this case as well, the base portion  71   S0  is fixed to the base wall  68 , so that an outer face (one of bent faces) of the sleeve portion  71   S1  is abutted against the side wall on the nip portion inlet IN side of the nip concave portion  61  of the supporting member  60 , making it possible to prevent deforming of the corner portion at which the side wall  67  of the nip concave portion  61  and the outer peripheral face of the supporting member  60  cross. 
     Moreover, as shown in  FIG. 14 , the deform preventing unit preferably includes the groove-shaped external holding member  70  with the cross section having a U shape and each of sleeve portions having different lengths, the groove-shaped external holding member  70  being installed from the inner peripheral side of the supporting member  60  to outside of the nip concave portion  61 . 
     In other words, as shown in  FIG. 15A , the external holding member  70 , which is a groove-shaped member having a U-shaped cross section, includes a sleeve portion  70   S1  and a sleeve portion  70   S2 , which are bent 90 degrees in the same direction at corresponding ends in the lateral direction of a base portion  70   S0  that corresponds to the direction of rotation of the fixing belt  21 . 
     Here, with a length L S02  of the sleeve portion  70   S2  being arranged to be longer than a length L S01  of the sleeve portion  70   S1  ( FIG. 15A ), the base portion  70   S0  is fixed to the bottom wall  68 , so that the inner face of the sleeve portion  70   S2 , which is a longer sleeve portion, is abutted against the outer face of the side wall of the nip portion outlet OUT side of the nip concave portion  61  of the supporting member  60  ( FIG. 14 ). 
     In order to suppress deforming of the corner portion at which the side wall  67  of the nip concave portion  61  and the outer peripheral face of the supporting member  60  cross, it is more advantageous to have a larger area of contact between the side wall  67  and the sleeve portion  70   S2 , so that, for example, the distance from the above-described corner portion to the tip of the sleeve portion  70   S2  is preferably no more than 1.5 mm, and is more preferably no more than 1.0 mm. 
     On the other hand, the sleeve portion  70   S1  does not contribute to suppressing deforming, due to sliding of the fixing belt  21 , of the corner portion at which the side wall  67  of the nip concave portion  61  and the outer peripheral face of the supporting member  60  on the nip portion inlet IN side of the nip concave portion  61  cross. This is because the sleeve portion  70   S1  is arranged on the side which is opposite the direction of deforming of the above-described corner portion. Therefore, for the sleeve portion  70   S1 , the area of contact with the side wall  67  is not needed as much as for that of the sleeve portion  70   S2 . Moreover, considering that the supporting member  60  is to be heated by the heating unit  25 , so that the heat is conducted to the fixing belt  21 , it is desirable that heat capacity is as small as possible also for the external holding member  70 , which is installed in the supporting member  60 . Moreover, for the aspect of mounting, it is desirable to make each member small in order to avoid interference among the parts. Thus, the sleeve portion  70   S1  preferably has a length which is shorter than that of the sleeve portion  70   S2 , so that, for example, the distance from the corner portion at which the side wall  67  of the nip concave portion  61  and the outer peripheral face of the supporting member  60  cross to the tip of the sleeve portion  70   S1  is preferably no less than 2.0 mm, and is more preferably no less than 2.5 mm. 
     In this way, the external holding member  70  is used to suppress increase in cost and heat capacity (consumed power), and, at the same time, as shown in  FIG. 13 , the sleeve portion  70   S2  of the external holding member  70  supports the side wall  67  on the nip portion outlet OUT side of the nip concave portion  61 , making it possible to prevent deforming, due to sliding of the fixing belt  21 , of the corner portion at which the side wall  67  of the nip concave portion  61  and the outer peripheral face of the supporting member  60  cross. 
     The external holding member  70  may be arranged to have an L-shaped angle shape, including only the base portion  70   S0  and the sleeve portion  70   S2  and not including the sleeve portion  70   S1  in  FIG. 15A . In this case as well, the base portion  70   S0  is fixed to the base wall  68 , so that an inner face (one of the bent faces) of the sleeve portion  70   S2  is abutted against the outer face of the side wall on the nip portion outlet OUT side of the nip concave portion  61  of the supporting member  60 , making it possible to prevent deforming of the corner portion at which the side wall  67  of the nip concave portion  61  and the outer peripheral face of the supporting member  60  cross. 
     Moreover, the term installing herein means being directly or indirectly fixed to the supporting member  60  such that reinforcing of the nip concave portion  61  is possible and may be any appropriate fixing method such as screwing, adhering, etc. For example, in  FIG. 14  the external holding member  70  and the internal holding member  71  are screwed to each other, having placed the bottom wall  68  and the side wall  67  of the nip concave portion  61  therebetween, so that they are fixed to the supporting member  60 , reinforcing the nip concave portion  61 . 
     Moreover, when using the groove-shaped internal holding member  71  and the groove-shaped external holding member  70  as described above and as shown in  FIG. 16 , it is preferable that the distance d 1  from the corner portion C IN  at which the side wall  67  of the nip concave portion  61  and the outer peripheral face of the supporting member  60  on the nip portion inlet side of the supporting member  60  cross to the tip of the sleeve portion  71   S1  of the internal holding member  71  is shorter than the distance d 2  from the corner portion C IN  to the tip of the sleeve portion  70   S1  of the external holding member  70 . 
     In this way, deforming of the corner portion C IN  may be prevented while suppressing increase of cost and heat capacity (consumed power) on the nip portion inlet side of the supporting member  60 . 
     Since it is possible that the corner portion C IN  of the supporting member  60  deforms in a direction which is opposite to the direction of rotation of the fixing belt  21  due to pressing of the fixing belt  21  on the nip portion inlet side, it is more preferable for the external holding member  70  to have a cross section having the sleeve portion  70   S1  to have a U-shaped groove shape than to have a L-shaped angle shape. 
     Moreover, when using the groove-shaped internal holding member  71  and the groove-shaped external holding member  70  as described above, as shown in  FIG. 16 , it is preferable that the distance d 4  from the corner portion C OUT  at which the side wall  67  of the nip concave portion  61  and the outer peripheral face of the supporting member  60  on the nip portion outlet side of the supporting member  60  cross to the tip of the sleeve portion  70   S2  of the external holding member  70  is shorter than the distance d 3  from the corner portion C OUT  to the tip of the sleeve portion  71   S2  of the internal holding member  70 . 
     In this way, deforming of the corner portion C OUT  may be prevented while suppressing increase of cost and heat capacity (consumed power) on the nip portion outlet side of the supporting member  60 . 
     Since it is possible that the corner portion C OUT  of the supporting member  60  deforms in a direction which is opposite the direction of rotation of the fixing belt  21  due to pressing of the fixing belt  21  on the nip portion inlet side, it is more preferable for the internal holding member  71  to have a cross section having the sleeve portion  71   S2  to have a U-shaped groove shape than to have an L-shaped angle shape. 
     Moreover, the support member  60  is molded to a C type pipe shape having the nip concave portion  61  by bending a stainless plate with a plate thickness of approximately 0.1 mm, the opening  69  of the nip concave portion  61  tends to open due to springing back of the plate material. In the present invention, a groove-shaped internal holding member  71  and a groove-shaped external holding member  70  may be used as a deform preventing unit to prevent deforming due to springing back to maintain proper shape of the nip concave portion  61 . 
     Moreover, the groove-shaped internal holding member  71  and the groove-shaped external holding member  70  in the deform preventing member have different lengths for each of the sleeve portions as described above, direction of mounting in the direction of rotation of the fixing belt  21  when mounting to the supporting member  60  becomes important. Thus, it is preferable for the groove-shaped internal holding member  71  and the groove-shaped external holding member  70  to have a reverse-mounting preventing unit which prevents the direction of mounting on the supporting member  60  in the direction of rotation of the fixing belt  21  being in the reverse direction. The specific configurations are explained with reference to  FIGS. 17 and 18 . 
       FIGS. 17A and 17B  are perspective diagrams showing a configuration of one end portion in the axial direction (longitudinal direction) of the groove-shaped external holding member  70  and the grooved-shaped internal holding member  71 . Of these,  FIG. 17A  shows a configuration of an end of the external holding member  70 ,  FIG. 17B  shows a configuration of an end of the internal holding member  71 , and  FIG. 17C  shows a configuration of an end at which the internal holding member  71  is fitted into the external holding member  70 . 
     First, the external holding member  70  includes, at each end in the axial direction thereof as a mounting portion  70   a  which is fixed to the supporting member  60  by the flange member  28 , a portion which is bent in a step shape in the outside circumferential direction of the supporting member  60 , but, only at the one end side, as shown in  FIG. 17A , a square hole  70   h  is provided in the mounting portion  70   a  at the step-bent portion thereof. 
     Moreover, as shown in  FIG. 17B , the internal holding member  71  includes a convex portion  71  at only one of the ends in the axial direction thereof, the convex portion  71  projecting in the axial direction. The convex portion  71   a  at the internal holding member  71  can be fitted into the square hole  70   h  of the external holding member  70  as shown in  FIG. 17C . 
     Here, as shown in  FIG. 18 , in the external holding member  70 , the square hole  70   h  is provided at one end (shown on the left side) in the axial direction thereof and is not provided at the other end (shown on the right side). Thus, the convex portion  71   a , which is provided at only one end in the axial direction of the internal holding member  71  may be fitted into only the mounting portion  70   a  on the one end side in the axial direction of the external holding member  70 , so that a combination of sleeve portions of the internal holding member  71  and the external holding portion  70  may be arranged to be a proper one, i.e., a combination of the sleeve portions  70   S1  and  71   S1  at the one end side in the direction of rotation of the fixing belt  21  and a combination of the sleeve portion  70   S2  and the sleeve portion  71   S2  at the other end side, making it possible to prevent the direction of mounting to the supporting member  60  in the direction of rotation of the fixing belt  21  from becoming the reverse direction. 
     The present invention has been described with the embodiments shown in the drawings. However, the present invention is not limited to those embodiments shown therein. Thus, modifications may be made thereto within the scope a skilled person would have arrived at, such as other embodiments, additions, changes, deletions, etc., and are to be included in the scope of the present invention in any of the modes thereof as long as they achieve the operation and advantages of the present invention. 
     The present application is based on and claims the benefit of priority of Japanese Patent Application No. 2010-278149 filed on Dec. 14, 2010.