Abstract:
An image heating apparatus includes an endless belt having a heat generation layer; a driving rotatable member forming a nip; a pressing pad for pressing the endless belt toward the driving rotatable member; an electric energy supply device for supplying electric energy to the heat generation layer; and a shut-off element configured to break electric connection between the electric energy supply device and the heat generation layer when a temperature of the endless belt reaches a predetermined upper limit temperature, wherein the shut-off element is disposed contacted with the endless belt in a region, with respect to a widthwise direction of the endless belt, outside a passing region of a maximum width of the sheet usable with the apparatus and inside of a heat generating region of the heat generation layer.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image heating apparatus (device) for heating an image on a sheet of a recording medium. More specifically, it relates to an image heating apparatus (device) usable for a copying machine, a printer, a facsimile machine, a multifunction machine capable of functioning as two or more of the preceding machines, etc., for example. 
     In recent years, in the field of an electrophotographic image forming apparatus, fixing apparatuses (devices) which are substantially smaller in thermal capacity than conventional fixing devices have been proposed in order to satisfy users of an electrophotographic image forming in terms of how quickly the apparatus starts up. One of the methods proposed to reduce a fixing apparatus (device) in thermal capacity is to use an endless belt as the fixing member (which hereafter may be referred to simply as belt) for the fixing apparatus. 
     For example, Japanese Laid-open Patent Application proposes a belt-based fixing apparatus which uses an endless belt having a heat generating resistor layer formed of a substance which generates heat as electric current flows through it. This fixing apparatus can heat the entirety of its fixation belt while keeping the belt stationary. Thus, it is substantially smaller than conventional fixing apparatuses, in terms of the amount of heat transferred from the belt to its pressure applying member, which forms a nip between itself and the belt. Therefore, it is significantly smaller than conventional fixing apparatuses, in the length of time required to warm up the belt. Further, its fixation belt does not need to be circularly moved while it is warmed up. Thus, its fixation belt is significantly longer in service life than the fixation belt of any of the conventional fixing apparatuses. In other words, it is substantially longer in service life than any of the conventional fixing apparatuses. 
     However, the belt is very thin and very small in thermal capacity. Thus, if any of various components of the fixing apparatus, which are not directly involved in the driving of the belt, are in contact with the belt, in the area in which the belt comes into contact with a sheet of the recording medium, the portion of the belt, which is in contact with the component which is not directly involved in the driving of the belt, is likely to more slowly increase in temperature than the rest of the belt, being therefore likely to remain lower in temperature than the rest of the belt. Thus, it is possible that a fixing apparatus, such as the one disclosed in the aforementioned patent application, will output a print suffering from such fixation problems that a part or parts of the image are lower in glossiness than the rest, that toner is missing from a part of parts of the image, and/or the like problem. In order to prevent these problems from occurring to a belt-based fixing apparatus such as the aforementioned one, the apparatus has to be extended in the length of warm-up time, contradicting the desire to reduce the apparatus in the length of warm-up time. 
     More concretely, one of the members of a conventional belt-based fixing apparatus, which is placed in contact with the fixation belt, is a current blocking element such as a thermal switch which is for blocking the current flow to the heat generation layer of the fixation belt if the belt excessively increases in temperature. 
     On the other hand, a current blocking element is required to block the current flow to the heat generation layer as quickly as possible if the fixation belt excessively increases in temperature. Therefore, the occurrence of the above-described problems is related to the positioning of the current blocking element(s). 
     SUMMARY OF THE INVENTION 
     Thus, the primary object of the present invention is to provide an image heating apparatus, the current blocking elements of which properly function without interfering with the image heating function of its image heating endless belt. 
     According to an aspect of the present invention, there is provided an image heating apparatus comprising: an endless belt provided with a heat generation layer for heating an image on a sheet; a driving rotatable member cooperative with the endless belt to nip and feed the sheet therebetween and configured to drive the endless belt; a pressing pad configured to press the endless belt toward the driving rotatable member; an electric energy supply device configured to supply electric energy to the heat generation layer; and a shut-off element configured to break electric connection between the electric energy supply device and the heat generation layer when a temperature of the endless belt reaches a predetermined upper limit temperature, wherein the shut-off element is disposed contacted with the endless belt in a region, with respect to a widthwise direction of the endless belt, outside a passing region of a maximum width of the sheet usable with the apparatus and inside of a heat generating region of the heat generation layer. 
     These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of the image forming apparatus in the first embodiment of the present invention, and shows the general structure of the apparatus. 
         FIG. 2  is a schematic sectional view of the fixing apparatus (device) in the first embodiment, at a vertical plane perpendicular to the recording medium conveyance direction of the apparatus, as seen from the right-hand side of the apparatus. 
         FIG. 3  is a sectional view of the fixing device in the first embodiment, at the vertical plane corresponding to a line ( 3 )-( 3 ) in  FIG. 2 , as seen from the front side of the device. 
         FIG. 4  is a sectional view of the fixing device in the first embodiment, at the vertical plane corresponding to a line ( 4 )-( 4 ) in  FIG. 2 , as seen from the front side of the device. 
         FIG. 5  is a perspective view of the belt of the fixing device in the first embodiment, and shows the structure of the belt. 
         FIG. 6  is a drawing for describing the structure of the belt backing member of the fixing device in the first embodiment. 
         FIG. 7  is an exploded perspective view of one of the lengthwise end portions of the belt unit of the fixing device in the first embodiment. 
         FIG. 8  is a front view of the fixing device in the first embodiment when the fixation belt and pressure roller of which are separated. 
         FIG. 9  is the flowchart of the fixing device in the first embodiment. 
         FIG. 10  is a drawing for showing the changes in the temperature of the fixation belt and pressure roller of the fixing device in the first embodiment, which occur during the period between the starting of the warming up of the fixation belt and pressure roller, and the starting of the fixation, and those which occur during the period thereafter. 
         FIG. 11  is a drawing for showing the changes in the temperature of the fixation belt and pressure roller of a comparative fixing device, which occur during the period between the starting of the warming up of the fixation belt and pressure roller, and the starting of the fixation, and those that occur during the period thereafter. 
         FIG. 12  is a flowchart of the operational sequence of the fixing device in the second embodiment of the present invention. 
         FIG. 13  is a drawing for showing the changes in the temperature of the fixation belt and pressure roller of the fixing device in the second embodiment, which occur during the period between the starting of the warming up of the fixation belt and pressure roller, and the starting of the fixation, and those which occur during the period thereafter. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     (1) Typical Image Forming Apparatus 
       FIG. 1  is a schematic sectional view of the image forming apparatus in the first embodiment of the present invention, and shows the general structure of the apparatus. The image forming apparatus in this embodiment is an electrophotographic full-color laser beam printer of the tandem-intermediary transfer type. The apparatus has four image formation stations, more specifically, the first to fourth image formation stations Pa, Pb, Pc and Pd, which are horizontally aligned in tandem. The four image formation stations form yellow, magenta, cyan, and black toner images, one for one, through a process for forming a latent image and a process for developing the latent images. 
     The image formation stations Pa, Pb, Pc and Pd have their own image bearing member. That is, they have electrophotographic photosensitive member  3   a ,  3   b ,  3   c , and  3   d , on which toner images of the aforementioned colors are formed one for one. Further, the image forming apparatus has an intermediary transfer member (intermediary transfer belt)  8 , which is positioned adjacent to the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d.    
     The toner images, different in color, formed on the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , one for one, are transferred (primary transfer) onto the intermediary transfer belt  8 , and then, are transferred together (secondary transfer) from the intermediary transfer member  8  onto a sheet P of the recording medium. After the toner images are transferred (secondary transfer) onto the sheet P, the sheet P is introduced into the fixing device (fixing means)  9 , which is an image heating device. Then, it is subjected to heat and pressure by the fixing device  9 , and then, is discharged, as a finished print, into the external delivery tray  18  of the apparatus. 
     The image formation stations Pa, Pb, Pc and Pd have also drum charging devices  2   a ,  2   b ,  2   c  and  2   d , developing devices  1   a ,  1   b ,  1   c , and  1   d , primary transfer charging devices  7   a ,  7   b ,  7   c  and  7   d , and cleaners  4   a ,  4   b ,  4   c  and  4   d , which are in the adjacencies of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , respectively. Further, the image forming apparatus has laser scanners  5   a ,  5   b ,  5   c  and  5   d , which are above the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , respectively. 
     The photosensitive drums  3   a ,  3   b ,  3   c  and  3   d  are rotated in the counterclockwise direction indicated by arrow marks, at a preset peripheral velocity. As they are rotated, they are uniformly charged (primary charging) to preset polarity and potential level by the drum charging devices  2   a ,  2   b ,  2   c  and  2   d , respectively. The uniformly charged (areas of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d  are scanned by (exposed to) beams La, Lb, Lc and Ld of laser light outputted by the laser scanners  5   a ,  5   b ,  5   c  and  5   d , respectively, while being modulated with image formation signals. Consequently, latent images which reflect the image formation signals are effected on the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , respectively. 
     Each of the laser scanners  5   a ,  5   b ,  5   c  and  5   d  consists of a light source, a polygonal mirror, etc., and scans the uniformly charged area of the peripheral surface of the corresponding photosensitive drum  5  by rotating the polygonal mirror while deflecting thereby the beam of laser light emitted by the light source. The beam of laser light deflected by the polygonal mirror is focused by an f-θ lens upon the generatrix of the corresponding photosensitive drum  3  (photosensitive drums  3   a ,  3   b ,  3   c  and  3   d ). Consequently, a latent image which reflects the image formation signals is effected upon the peripheral surface of the photosensitive drum  3 . 
     The developing devices  1   a ,  1   b ,  1   c  and  1   d  are kept filled with a preset amount of yellow, magenta, cyan, and black toners (as developers) by toner supplying devices  6   a ,  6   b ,  6   c  and  6   d , respectively. The developing devices  1   a ,  1   b ,  1   c  and  1   d  develop the latent images on the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , into visible images, that is, toner images made of yellow, magenta, cyan and black toners, respectively. 
     The intermediary transfer member  8  is an endless belt which is suspended and kept tensioned by three parallel rollers  13 ,  14  and  15 . It is rotated (circularly moved) in the clockwise direction indicated by an arrow mark, at roughly the same speed as the peripheral velocity of each of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d.    
     On the peripheral surface of the photosensitive drum  3   a  of the first image formation station Pa, a toner image of yellow color (first color) is formed. As the photosensitive drum  3   a  is rotated, the yellow toner image is transferred (primary transfer) onto the intermediary transfer member  8  while it is conveyed through the nip (primary transfer nip) between the peripheral surface of the photosensitive drum  3   a  and intermediary transfer member  8 . This transfer is made by the electric field generated by the primary transfer bias applied to the intermediary transfer member  8  from the primary transfer charging device  7   a , and the pressure in the nip. 
     Similarly, the toner images of magenta (second color), cyan (third color), and black (fourth color) formed on the photosensitive drums  3   b ,  3   c  and  3   d , respectively, are sequentially transferred in layers onto the intermediary transfer member  8 . Consequently, a full-color image of the intended original full-color image is synthetically effected on the intermediary transfer member  8 . 
     A reference code  11  stands for the secondary transfer roller, which is kept pressed against the roller  14 , that is, one of the three parallel rollers  13 ,  14  and  15  (by which intermediary transfer member  8  is suspended and kept tensioned), with the presence of the intermediary transfer member  8  between itself and the roller  14 , forming thereby the secondary transfer nip between itself and the intermediary transfer member  8 . 
     Meanwhile, one of the sheets P of the recording medium (which may be referred to simply as sheets P) is fed into the main assembly of the image forming apparatus, while being separated from the rest, from a sheet feeder cassette  10 , and is delivered through a sheet passage  16 , a sheet passage  17 , a pair of registration rollers  12 , and a pre-transfer guide, to the secondary transfer nip, which is the area of contact between the intermediary transfer member  8  and secondary transfer roller  14 , with a preset timing. Further, the secondary transfer bias is applied to the secondary transfer roller  11  from a bias application power source with preset timing. Consequently, the full-color toner image synthetically effected on the intermediary transfer member  8  through the process of transferring in layers the monochromatic color images, different in color, from the photosensitive drums  3   a ,  3   b ,  3   c , and  3   d , onto the intermediary transfer member  8 , are transferred together (secondary transfer) onto the sheet P. 
     After the sheet P of the recording medium has received the synthetically effected full-color image, in the secondary transfer nip, the sheet P is separated from the intermediary transfer member  8 , and is introduced into the fixing device  9  through the sheet passage  17   a . In the fixing device  9 , the sheet P and the unfixed full-color toner image thereon are subjected to heat and pressure. Consequently, the unfixed full-color toner image becomes fixed to the sheet P. Then, the sheet P is discharged as a finished color print into the external delivery tray  18  of the image forming apparatus. 
     After the completion of the primary transfer, the residual toner, that is, the toner remaining on the peripheral surface of each of the photosensitive drums  3   a ,  3   b ,  3   c  and  3   d , is removed by the cleaners  4   a ,  4   b ,  4   c  and  4   d , respectively, to prepare the photosensitive drums  3  for the formation of the next latent images. The toner and the like contaminants remaining on the intermediary transfer member  8  are wiped away by the cleaning web (unwoven cloth)  19  as they are made to come into contact with the web  19  by the movement of the intermediary transfer member  8 . 
     (2) Fixing Device  9   
     In the following description of the embodiments of the present invention, the lengthwise direction of the fixing device  9  (which functions as image heating device), and the structural components of the fixing device  9 , is the direction perpendicular to the recording medium conveyance direction in terms of the plane which coincides with the recording medium passage. Their widthwise direction is the direction parallel to the recording medium conveyance direction. The front side of the fixing device  9  is the side where the recording medium entrance is present, and the rear side of the fixing device  9  is the opposite side (recording medium exit side) of the fixing device  9  from the front side. Their left and right sides are their left and right sides as seen from the front side of the fixing device  9 . Further, their upstream and downstream sides are in terms of the recording medium conveyance direction. 
       FIG. 2  is a schematic sectional view of the fixing apparatus (device)  9  in the first embodiment, at a vertical plane perpendicular to the recording medium conveyance direction of the fixing device  9 , as seen from the right-hand side of the fixing device  9 .  FIG. 3  is a sectional view of the fixing device  9  in the first embodiment, at the vertical plane corresponding to a line ( 3 )-( 3 ) in  FIG. 2 , as seen from the front side of the device.  FIG. 4  is a sectional view of the fixing device  9  in the first embodiment, at the vertical plane corresponding to a line ( 4 )-( 4 ) in  FIG. 2 , as seen from the front side of the fixing device  9 . The fixing device  9  in this embodiment is a fixing device of the belt type. It employs an endless and circularly movable belt  20  as an image heating means for heating a sheet P of the recording medium, and an image t on the sheet P, by coming into contact with the sheet P and the image t thereon. The belt  20  is endless and rotatable (circularly movable). It has a heat generation layer (heat generating section) made up of such a substance that generates heat as electric current is flowed through it. 
     The fixing device  9  in this embodiment is structured so that when a sheet P of the recording medium is conveyed through the fixing device  9 , the centerline of the sheet P in terms of the direction perpendicular to the recording medium conveyance direction, coincides with the centerline of the recording medium passage of the fixing device  9 . That is, in terms of the direction perpendicular to the recording medium conveyance direction, the centerline of a sheet P of the recording medium coincides with the centerline of the recording medium passage, regardless of the dimension of the sheet P in terms of the direction perpendicular to the recording medium conveyance direction. Referring to  FIG. 4 , a reference code O stands for the centerline of the recording medium passage (theoretical line) as the positional reference line for recording medium conveyance. The width of a sheet P of the recording medium is the dimension of the sheet P in terms of the direction perpendicular to the recording medium conveyance direction a, in the plane which coincides with the recording medium passage. Referring also to  FIG. 4 , a reference code Wmax stands for the path (track) of the widest sheet of the recording medium properly conveyable through the fixing device  9  (width of the recording medium conveyance passage). A reference code Wmin stands for the path (track) of the narrowest sheet of the recording medium properly conveyable through the fixing device  9  (path (track) of narrowest sheet). 
     Designated by a reference code  21  is a belt assembly, the lengthwise direction of which coincides with the left-and-right direction of the fixing device  9 . Designated by a reference code  22  is a pressure roller, which functions as a pressure application member (rotatable member), and the lengthwise direction of which coincides with the left-and-right direction of the fixing device  9 . The belt assembly  21  and pressure roller  22  are positioned between the left and right lateral plates  30 L and  30 R of the frame (chassis) of the fixing device  9 , being vertically stacked roughly in parallel to each other. They are pressed upon each other by a preset amount of pressure, forming thereby a fixing nip N, between them, which has a preset dimension in terms of the recording medium conveyance direction a. 
     The pressure roller  22  is a multilayer roller having a metallic core  22   a , an elastic layer  22   b , and a parting layer  23   c . The metallic layer  22   a  is formed of stainless steel. The elastic layer  22   b  is formed of silicone rubber, and covers virtually the entirety of the peripheral surface of the metallic core  22   a . The parting layer  22   c  is a piece of tube made of PFA resin, and covers the elastic layer  22   b . It is roughly 50 μm in thickness. The pressure roller  22  is rotatably supported by the left and right lateral plates  30 L and  30 R, by the left and right lengthwise ends of its metallic core  22   a , with the placement of a pair of bearings  31  between the left and right ends of the metallic core  22   a  and the left and right lateral walls  30 L and  30 R, respectively. 
     A reference code G stands for a pressure roller driving gear, which is coaxially fitted around the right end portion of the metallic core  22   a . It is to this gear G that a rotational driving force is transmitted through a driving force transmitting means (unshown) from a motor M which is under the control of a control circuit (CPU, controller)  100 , which functions as the means for controlling the apparatus. As the rotational driving force is transmitted to the gear G, the pressure roller  22  is rotated in the counterclockwise direction indicated by an arrow mark R 22  in  FIG. 2 . 
     The belt unit  21  has an endless (cylindrical) belt  20  (which hereafter may be referred to simply as belt  20 ), which functions as an image heating member. The widthwise direction of the belt  20  coincides with the left-and-right direction of the fixing device  9 . The belt unit  21  has also a belt backing member  23 , which functions as a pressure pad. The belt backing member  23  is within the loop which the belt  20  forms, and guides the belt  20  from within the belt loop. Further, the belt unit  21  has a belt backing member supporting stay  24 , which is on the inward side of the belt backing member  23  with reference to the belt loop. Further, the belt unit  21  has left and right flanges  25 L and  25 R, with which the left and right lengthwise ends of the supporting stay  24  are fitted, respectively. 
     1) Belt  20   
     Next, the structure of the belt  20  is described with reference to  FIGS. 5(   a )- 5 ( d ).  FIG. 5(   a ) is an external perspective view of the belt  20 , and  FIG. 5(   b ) is a schematic sectional view of the belt  20 , at a vertical plane perpendicular to the center portion of the belt  20  in terms of the direction perpendicular to the recording medium conveyance direction a, and shows the laminar structure of the belt  20 .  FIGS. 5(   c ) and  5 ( d ) are schematic sectional views of the left and right end portion of the belt  20 , respectively, and show the laminar structure of the lengthwise end portions of the belt  20 . Basically, the belt  20  in this embodiment is a laminar belt having four layers, that is, a substrate layer  20   a , a heat generation layer  20   b , an elastic layer  20   c , and a parting layer  20   d , listing from the inward side of the belt  20 . It is flexible, and small in thermal capacity. 
     As the material for the substrate layer  20   a , a heat resistant substance, for example, polyimide, polyimide-amide, PEEK, PTFE, PFA, FEP or the like resin, can be used. From the standpoint of providing the belt  20  with a certain amount of rigidity, SUS, nickel, or the like metallic substance can be used as the material for the substrate layer  20   a . For the purpose of enabling the fixing device  9  to start up quickly, the substrate layer  20   a  needs to be small in thermal capacity. Thus, it is desired to be no more than 50 μm in thickness. However, from the standpoint of strength, it is desired to be no less than 20 μm in thickness. In this embodiment, a cylindrical polyimide belt, which is 30 μm in thickness and 25 mm in diameter, was used as the substrate layer  20   a . Incidentally, in a case where an electrically conductive substance is used as the material for the substrate layer  20   a , it is desired that an electrically insulating layer formed of polyimide or the like is provided between the substrate layer  20   a  and heat generation layer  20   b.    
     The heat generation layer  20   b  is formed of a heat generating resistor, which is made by dispersing carbon particles (as electrically conductive particles) in polyimide resin. It is formed on the peripheral surface of the substrate layer  20   a  by uniformly coating the substrate layer  20   a  with the heat generating resistor to a thickness of roughly 10 μm. The value of the overall electrical resistance of the heat generation layer  20   b  is 10.0Ω. Therefore, the amount by which electric power is consumed by the heat generation layer  20   b  (amount by which heat is generated by heat generation layer  20   b ) when 100 V of commercial alternating voltage is applied to the heat generation layer  20   b  is 1,000 W. The value for the electrical resistance of the heat generation layer  20   b  has only to be set according to the amount of heat required of the fixing device  9 , the amount of voltage to be applied to the fixing device  9 , or the like factor. It can be adjusted by adjusting the ratio between the carbon particles and polyimide resin. 
     The elastic layer  20   c  is 300 μm in thickness. As for the material for the elastic layer  20   c , silicon rubber which is 10 degree in hardness (JIS-A) and 1.3 W/m·K in thermal conductivity was used. As the material for the parting layer  20   d , a piece of PFA tube which is 20 μm in thickness was used. The parting layer  20   d  may be formed by coating the elastic layer  20   c  with PFA. The method of forming the parting layer  20   d  with the use of a piece of PFA tube and the method of forming the parting layer  20   d  by coating the elastic layer  20   d  with PFA resin can be selectively used according to the thickness, mechanical and electrical strength which are required of the parting layer  20   d . Incidentally, in this embodiment, the parting layer  20   d  was adhered to the elastic layer  20   c  with the use of an adhesive made of silicone resin. 
     Referring to  FIG. 4 , designated by a reference code W 20  is the width (belt width) of the belt  20 , that is, the dimension of the belt  20  in terms of the direction X. The belt width W 20  is greater than the width of the path Wmax of the widest sheet of the recording medium conveyable through the fixing device  9 , in terms of the direction perpendicular to the recording medium conveyance direction, and designated by a reference code W 20   b  is the dimension of the heat generation layer  20   b  (heat generation range of heat generation layer), which is greater than the width, in terms of the direction perpendicular to the recording medium conveyance direction a, of the path Wmax of the widest sheet of the recording medium conveyable through the fixing device  9 , and is less than the belt width W 20 . Designated by a reference code WN is the dimension of the nip N in terms of the direction X in  FIG. 4 , which is equal to the dimension of the elastic layer  22   b  of the pressure roller  22  in terms of the lengthwise direction of the fixing device  9 . This dimension WN of the nip N is greater than the dimension W 22   b  of the heat generation layer  20   b , and is less than the belt width W 20 . 
     In this embodiment, in order to ensure that the toner is reliably fixed even across the left and right edge portions of the widest sheet of the recording medium, the fixing device  9  is structured so that, in terms of the direction perpendicular to the recording medium conveyance direction a, the left and right end portions of the heat generation layer  20   b  extend by 10 mm beyond the path Wmax of the widest sheet of the recording medium, in terms of direction perpendicular to the recording medium conveyance direction a. 
     The left and right end portions of the substrate layer  20   a  are fitted with the first and second power supply electrodes  20   e L and  20   e R, which are ring-shaped and are in contact with the inward surface of the substrate layer  20   a . The material for the power supply electrodes  20   e L and  20   e R is an electrically conductive substance which contains silver/palladium. 
     The first and second power supply electrodes  20   e L and  20   e R are in contact with the left and right ends of the heat generation layer  20   b , which is on the peripheral surface of the substrate layer  20   a , through the electrically conductive coated layers  20   f L and  20   f R on the left and right edge portions of the substrate layer  20   a . Thus, as voltage is applied between the first and second power supply electrodes  20   b L and  20   b R, the entirety of the heat generation layer  20   b  generates heat, whereby the entirety of the heating range of the belt  20  is heated by the heat generation layer  20   b.    
     As described above, the belt  20  is a laminar belt, having at least the substrate layer  20   a , the heat generating resistor layer  20   b , and the parting layer  20   c , listing from the inward side of the belt  20 . It has also: the first and second power supply electrodes  20   e L and  20   e R and the first and second electrically conductive coated layers  20   f L and  20   f R, as the power supplying portions for providing electrical connection between the electrical power supplying portion  102  (electrical power source), which will be described later, and the heat generating resistor layer  20   b.    
     The belt backing member  23  which functions as a pressure pad is in the form of a trough which is roughly semicircular in cross section. It is rigid, heat resistant, and thermally nonconductive. It extends in the left-and-right direction. It supports the belt  20  from within the loop which the belt  20  forms. Further, it guides the belt  20  as the belt  20  is circularly moved. Moreover, it functions as the member for pressing the belt  20  upon the pressure roller  22 . 
     From the standpoint of energy conservation, the material for the belt backing member  23  is desired to be a substance which is small in the amount of thermal conduction between itself and belt backing member supporting stay  24 . For example, it is desired to be heat resistant glass, or heat resistant resin such as poly-carbonate, liquid polymer, or the like. In this embodiment, the belt backing member  23  is provided with the electrically conductive portions to supply the heat generation layer  20   b  with the electric power from the electrical power source  102 . Therefore, it is mandatory that the material for the belt backing member  23  is electrically insulating. In this embodiment, the material for the belt backing member  23  is Sumikasuper E5204L (product of Sumitomo Chemistry Co., Ltd.). 
       FIG. 6(   a ) is an external perspective view of the belt backing member  23 .  FIGS. 6(   b ) and  6 ( c ) are the top and bottom plan views, respectively, of the belt backing member  23 . The belt backing member  23  has left and right arms  23   a L and  23   a R, which extend outward beyond the left and right openings of the belt  20 , respectively. The left and right arms  23   a L and  23   a R are provided with the first and second electrically conductive portions  23   b L and  23   b R, which are on the bottom surfaces of the left and right arms  23   a L and  23   a R. 
     The first and second electrically conductive portions  23   b L and  23   b R are the portions of the belt backing member  23 , which face the first and second power supply electrode portions  20   e L and  20   e R, respectively, which are on the inward side of the loop which the belt  20  forms. They are long enough, in terms of the widthwise direction the belt  20 , to extend into the area across which the belt backing member  23  presses the pressure roller  22 , that is, the fixation nip N, with the presence of the belt  20  between itself and pressure roller  22 . 
     The portion of the belt backing member  23 , which corresponds in position to the base side of the left arm  23   a L, is provided with a through hole  23   c  (perpendicular to top and bottom surface of belt backing member  23 ), through which safety elements such as a thermo-switch, a temperature fuse, or the like are put. 
     3) Belt Backing Member Supporting Stay  24   
     The belt backing member supporting stay  24  (which hereafter may be referred to simply as support stay  24 ) is a rigid member, and extends in the left-and-right direction of the fixing device  9 . It is U-shaped in cross section, and is placed in such an attitude that its opening faces downward. It is placed within the belt backing member  23  to support the belt backing member  23 . The material for the support stay  24  is desired to be such a substance that is unlikely to bend even if it is subjected to a large amount of pressure. In this embodiment, SUS 304 was used as the material for the support stay  24 . The belt backing member supporting stay  24  has arms  24   a L and  24   a R, which extend outward of the belt  20  through the left and right openings of the belt  20 , respectively. The arms  24   a L and  24   a R are positioned above the left and right arms  23   a L and  23   a R of the belt backing member  23 , respectively. 
     4) Flanges  25 L and  25 R 
     The flanges  25 L and  25 R are symmetrical to each other in shape, and are symmetrically positioned with reference to the center of the fixing device  9  in terms of the lengthwise direction of the fixing device  9 . They are solidly attached to the left and right arms  24   a L and  24   a R, respectively.  FIG. 7  is an exploded perspective view of the right end portion of the belt backing member  23 , and shows the right flange  25 R, right arm  24   a R of the support stay  24 , and right arm  23   a R of the belt backing member  23 , and right power supply connector  26 R (which will be described later). The left end portion of the belt backing member  23  and the left end portion of the support stay  24  are the same in structure as the right counterparts shown in  FIG. 7 . The belt  20  is loosely fitted around the combination of the belt backing member  23  and support stay  24 , between the left and right flanges  25 L and  25 R. 
     5) Mechanism for Moving Belt Backing Member 
     The belt unit  21  consisting of the belt backing member  23 , support stay  24 , belt  20 , left flange  25 L, and right flange  25 R. It is positioned between the left and right lateral plates  30 L and  30 R, in parallel to the pressure roller  22 , in such an attitude that the belt backing member  23  is under the support stay  24 . The left and right flanges  25 L and  25 R are in engagement with the left and right lateral plates  30 L and  30 R of the frame of the fixing device  9 . More specifically, the left and right flanges  25 L and  25 R are provided with a vertical groove (unshown), and the left and right lateral plate  30 L and  30 R are provided with vertical slits  30   a L and  30   a R, respectively. Further, the vertical grooves (unshown) of the left and right flanges  25 L and  25 R are in engagement with the vertical edge portions of the vertical slits  30   a L and  30   a R of the lateral plate  30 L and  30 R of the flanges  25 L and  25 R, respectively. Therefore, the belt unit  21  is vertically movable (slidable) between the left and right lateral plates  30 L and  30 R of the frame of the fixing device  9 . 
     Designated by reference codes  27 L and  27 R are left and right mechanisms for moving (shifting) the belt backing member  23 , which are on the outward side of the left and right lateral plates  30 L and  30 R of the frame of the fixing device  9 . The belt backing member moving (shifting) mechanism in this embodiment is a combination of a solenoid switch and a plunger. More specifically, the plunger  27   a L of the left mechanism (device)  27 L for moving (shifting) the belt backing member  23  is attached to the outward extension  25   a L of the left flange  25 L, and the plunger  27   a R of the right mechanism  27 R for moving (shifting) the belt backing member  23  is solidly attached to the outward extension  25   a R of the right flange  25 R. The left and right mechanisms (devices)  27 L and  27 R are turned on or off by the control circuit  100 . 
     In this embodiment, when the left and right mechanisms (devices)  27 L and  27 R are off, their plungers  27   a L and  27   a R are kept at their bottom positions by the compression springs (unshown) in the fixing device  9 . The pressure applied to the plungers  27   a L and  27   a R by the compression springs acts on (presses) the belt backing member  23  through the left and right flanges  25 L and  25 R and belt backing member  23 . 
     Therefore, the downwardly facing surface of the belt backing member  23  applies a preset amount of pressure to the upwardly facing surface of the pressure roller  22  against the elasticity of the elastic layer  2   b  of the pressure roller  22 . In this embodiment, the amount of pressure applied by the aforementioned compression springs per lengthwise end of the pressure roller  22  is 156.8 N; the total amount of pressure applied to the pressure roller  22  is 313.6 N (32 kgf). Thus, the fixation nip N, which has a preset width in terms of the recording medium conveyance direction a, is formed between the belt  20  and pressure roller  22 . Further, the first and second power supply electrodes  20   e L and  20   e R of the belt  20  are in contact with the first and second electrically conductive portions  23   b L and  23   b R of the belt backing member  23 , establishing thereby electrical connection between the electrodes  20   e L and  20   e R and first and second electrically conductive portions  23   b L and  23   b R, at the left and right ends of the nip N. 
     On the other hand, as the left and right mechanism (devices)  27 L and  27 R for moving the belt backing member  23  are turned on, the plungers  27   a L and  27   a R are lifted into their top positions against the compression springs in the fixing device  9 , and kept in the top positions. Thus, the pressure being applied to the pressure roller  22  by the belt backing member  23  is removed, and the belt backing member  23  is moved away from the pressure roller  22 . That is, as long as the left and right mechanism (devices)  27 L and  27 R for moving the belt backing member  23  are kept turned on, the belt  20  is kept separated from the pressure roller  22 . 
     In the case of the fixing device  9  in this embodiment, even while the belt  20  is kept separated from the pressure roller  22 , the first and second power supply electrodes  20   e L and  20   e R of the belt  20  are kept in contact with the first and second electrically conductive portions  23   b L and  23   b R of the belt backing member  23  to maintain the electrical connection between the first and second power supply electrodes  20   e L and  20   e R of the belt  20 , and the first and second electrically conductive portions  23   b L and  23   b R of the belt backing member  23 , in order to keep on supplying the heat generation layer  20   b  with electric power. 
     In this embodiment, the combination of the solenoid and plunger was used as the left and right mechanism (devices)  27 L and  27 R as the mechanism for placing the belt  20  in contact with, or separating the belt  20  from, the pressure roller  22 . However, the application of the present invention is not limited to a fixing device, whose mechanism (devices) for moving the belt backing member  23  is a combination of a solenoid switch and a plunger. That is, the present invention is applicable to a fixing device, the mechanism of which for vertically moving the belt backing member  23  is not a combination of a solenoid and a plunger, as long as the mechanism can vertically move the belt backing member  23  as described above. For example, the present invention is applicable to a fixing apparatus (device) whose mechanism for moving the belt backing member  23  is a combination of a pressure application arm, a compression spring, and a cam, a combination of a rack and a pinion gear, or the like. 
     6) Power Supply System for Heat Generation Layer  20   b  of Belt  20   
     The left and right arms  23   a L and  23   a R of the belt backing member  23  are fitted with power supply connectors  26 L and  26 R, respectively. Thus, the electrode portion  26   a  ( FIG. 7 ) of the left connector  26 L is pressed upon the first power supply electrode  20   e L, enabling electric current to flow between the electrode portion  26   a  and first power supply electrode  20   e L, and the electrode portion  26   a  of the right connector  26 R is pressed upon the second power supply electrode  20   e R, enabling electric current to flow between the electrode portion  26   a  and the second power supply electrode  20   e R. 
     The vertical through hole  23   c  of the belt backing member  23  is fitted with the safety element  40 . The safety element (current blocking element, thermal fuse) used in this embodiment is a thermo-switch CH-16 (product of Wako Electronics Co., Ltd.). Hereafter, the safety element  40  will be referred to as the thermo-switch  40 . It is the belt backing member  23  that is fitted with the thermo-switch  40 ; the thermo-switch  40  is inserted into the vertical through hole (opening)  23   c  of the belt backing member  23 , from the inward side of the belt backing member  23 , in such a manner that its heat sensitive portion is exposed on the belt side of the belt backing member  23 . 
     Thus, the thermo-switch  40  is kept pressed downward by the resiliency of the compression springs (pressure applying members)  42  installed compressed between the spring holder  41  (which is on the inward side of the belt backing member  23 , and the top end of the thermo-switch  40 . Therefore, the heat sensitive portion of the thermo-switch  40  is kept pressed upon the inward surface of the belt  20 . That is, the fixing device  9  is structured so that the thermo-switch  40  is attached to the belt backing member  23  which is the pressure applying member for keeping the belt  20  pressed in the preset direction, as described above. Therefore, it is ensured that the thermo-switch  40  remains in contact with the belt  20 . 
     One of the two electrical terminals of the thermo-switch  40  is in electrical connection to the electrical terminal  26   a  of the left connector  26 L through an electrical wire  43   a . The other electrical terminal of the thermo-switch  40  is in electrical connection to the electrical power source (commercial power source)  102  through an electrical wire  43   b . Further, the electrical terminal  26   a  of the right connector  26 R is in electrical connection to the power supply source  102  through an electrical wire  43   c . The amount of electrical power supplied from the power supply source  102  to the heat generation layer  20   b  is controlled by a triac  101 , which is under the control of the control circuit  100 . 
     Therefore, an electrical power supply circuit, through which electric power is transmitted from the electric power source  102  to the heat generation layer  20   b  through wire  42   b  R thermo-switch  40  R electrical terminal of left connector  26 LR first electrically conductive portion  23   b L R first electrical power supply electrode  20   e L R electrically conductive coated layer  20   f L, is formed. Further, another electrical power supply circuit is formed between the heat generation layer  20   b  and electrical power source  102 : heat generation layer  20   b  R electrically conductive coated layer  20   f R R second power supply electrode  20   e R R electrode  26   a  of right connector  26 R R electrical wire  42   c  R electrical power source  102 . 
     Thus, as electric power is supplied from the electrical power source  102  to the heat generation layer  20   b  of the belt  20  through the above described electrical power transmission circuit, the heat generation layer  20   b  of the belt  20  generates heat across its entire range in terms of both the circumferential direction and widthwise direction of the belt  20 . Therefore, virtually the entirety of the belt  20  is heated. 
     As the temperature of the belt  20  reaches a preset level (upper limit), the thermo-switch  40  interrupts the electrical connection between the heat generating resistor layer  20   b  and electrical power source  102 . That is, the thermo-switch  40  is an electrical element for blocking the electrical power supply circuit through which electrical power is supplied to the heat generation layer  20   b . In terms of the widthwise direction of the belt  20 , the thermo-switch  40  is kept in contact with the belt  20 , at a position B which is within the heat generation range W 22   b  of the heat generation resistor layer  20   b  of the belt  20 , and outside the path Wmax of the widest sheet of the recording medium properly conveyable through the fixing device  9 . 
     Referring to  FIG. 2 , designated by a reference code TH is a thermistor as a temperature sensor for detecting the temperature of the belt  20 . In this embodiment, the thermistor TH is supported by the frame  30  of the fixing device  9 , in the adjacencies of the outward surface of the belt  20 ; there is no contact between the thermistor TH and belt  20 . It plays the role of detecting the surface temperature of the belt  20  without contacting the surface of the belt  20 . The thermistor TH is for detecting the temperature of the portion of the belt  20 , which remains within the recording medium path, in terms of the direction perpendicular to the recording medium conveyance direction a, even when the narrowest sheet of the recording medium is conveyed through the fixing device  9 . Therefore, it is positioned in the adjacencies of the outward surface of the belt  20 , with no contact between itself and belt  20 , so that, in terms of the direction perpendicular to the recording medium conveyance direction a, its position corresponds to the centerline O (recording medium conveyance reference line) of the recording medium passage of the fixing device  9 . 
     The thermistor TH is in connection to the control circuit (controller)  100  through an A/D converter  103 . The controller  100  samples the output of the thermistor TH with a preset frequency, and reflects the obtained temperature information upon the controlling of the amount by which electrical current is supplied to the heat generation layer  20   b  of the belt  20 . That is, the control circuit  100  sets the amount of current to be supplied to the heat generation layer  20   b  of the belt  20 , based on the output of the thermistor TH, and controls the amount by which current is supplied to the heat generation layer  20   b  from the electrical power source  102 . In other words, in the case of the fixing device  9  in this embodiment, the control circuit  100  controls the amount of current supplied to the heat generation layer  20   b , based on the temperature level for fixing a toner image t to a sheet P of the recording medium, in such a manner that the temperature of the belt  20  detected by the thermistor TH remains stable at 160° C., which is the temperature level at which the toner image t is to be heated. 
     If the belt  20  happens to abnormally increase in temperature for some reason or other, that is, the control circuit  100  determines, based on the information it receives from the thermistor TH, that the belt  20  is abnormally high in temperature, the control circuit  100  stops supplying the heat generation layer  20   b  with electric power. Further, in this embodiment, the fixing device  9  is provided with the thermo-switch  40 . Therefore, even if the control circuit  100  gets out of order, the belt  20  is prevented by the thermo-switch  40  from continuing to excessively increasing in temperature. That is, the fixing device  9  in this embodiment is provided with double layers of safety measure. 
     7) Operational Sequence of Fixing Device 
     Next, referring to  FIG. 9 , the operational flow of the fixing device  9  in this embodiment is described. While the fixing device  9  is kept on standby, the fixation motor M is kept turned off. Therefore, the pressure roller  22  remains stationary. Further, the power supply  102  is kept turned off. Thus, the belt  20  is not heated. Further, the left and right mechanisms (devices)  27 L and  27 R for moving the belt backing member  23  are not supplied with electrical current. Thus, the belt unit  21  is kept pressed against the pressure roller  22 . 
     As the control circuit  100  receives a print start command from a user through the control panel of the image forming apparatus, or from the input device of the host apparatus  200  such as a personal computer while the fixing device  9  is in the above described state, the control circuit  100  makes the image formation station(s) start an image forming operation (Step A 1 ). 
     Regarding the fixing device  9 , the control circuit  100  turns on the electrical power source  102  to start supplying electrical power to the heat generation layer  20   b  of the belt  20  through the aforementioned power supply system (Step A 2 ). At this point in time, the motor M is still kept turned off, and therefore, the pressure roller  22  is not rotated, and therefore, the belt  20  remains stationary. 
     While the fixing device  9  is in the above described state, as the belt  20  is heated and the temperature of the belt  20  detected by the thermistor TH reaches the preset level (120° in this embodiment) (Step S 3 ), the formation of a toner image on the photosensitive drum  3  in the image formation station(s) is started (Step A 4 ). Then, a sheet P of the recording medium on which an unfixed toner image t is borne, is conveyed to the fixing device  9 . The control circuit  100  turns on the motor M with such timing that the sheet P reaches a point which is 20 mm from the fixation nip N (Step A 5 ). In other words, the pressure roller  22  begins to be rotated in the counterclockwise direction indicated by an arrow mark R 22  in  FIG. 2 , at a preset peripheral velocity (Step A 6 ). 
     The timing with which the pressure roller  22  begins to be driven is the same as the timing with which the photo interrupter P 1  is turned off (its output signals is blocked) because a recording medium detection flag F of the entrance guide  44  of the fixing device  9  is moved by the pressure from the sheet P. The temperature of the belt  20  detected by the thermistor TH at this point in time is 160° C., which is the same as the target temperature level of the belt  20  (image heating temperature level). 
     As the pressure roller  22  begins to rotate, rotational force is transmitted from the pressure roller  22  to the belt  20  by the friction between the peripheral surface of the pressure roller  22  and the outward surface of the belt  20 , in the nip N. Thus, the belt  20  circularly moves around the combination of the belt backing member  23  and support stay  24  in the clockwise direction indicated by an arrow mark R 20 , with the inwardly facing surface of the belt  20  sliding on the downwardly facing surface of the belt backing member  23 , at roughly the same speed as the peripheral velocity of the pressure roller  22 . 
     As the belt  20  is circularly moved, the belt backing member  23  guides the belt  20 . Further, the leftward or rightward deviation of the belt  20  with reference to the lengthwise direction of the belt backing member  23 , which tends to occur as the belt  20  is circularly moved, is controlled by the left and right flanges  25 L and  25 R, respectively. The inward surface of the belt  20  is coated with grease (lubricant), which bears the role of minimizing the amount of the wear of the inward surface of the belt  20 , which is attributable to the friction between the downwardly facing surface of the belt backing member  23  and the inward surface of the belt  20 . 
     A sheet P of the recording medium, on which an unfixed toner image t is borne, is guided into the nip N by the guide  44 , and is conveyed through the nip N while remaining pinched by the belt  20  and pressure roller  22 . That is, the sheet P moves with the belt  20  through the nip N, with its image bearing surface remaining in contact with the outward surface of the belt  20 . While the sheet P is conveyed through the nip N, the heat generated in the heat generation layer  20   b  of the belt  20  is given to the sheet P, whereby the unfixed toner image t on the sheet P is melted, and becomes fixed to the sheet P. As the sheet P is conveyed out of the nip N, it is separated by the curvature of the belt  20  (belt backing member  23 ), and is discharged from the fixing device  9  by the pair of discharge rollers  45  of the fixing device  9 . 
     The control circuit  100  counts the number of the subsequent sheets P of the recording medium with the use of the combination of the sheet detection flag F and photo-interrupter P 1 . As soon as a sheet P of the recording medium is detected in the single print mode by the combination of the sheet detection flag F and photo-interrupter P 1 , or the last sheet P of the recording medium in the continuous printing mode, in which a preset number of prints are to be outputted, is counted by the combination, the control circuit  100  calculates the length of time it takes for the trailing edge of the last sheet P to be moved through the nip N, based on the point in time at which the leading edge of the last sheet P is detected by the combination, speed at which the last sheet P is being conveyed, and the measurement of the last sheet P in terms of the recording medium conveyance direction. 
     As soon as the control circuit  100  determines, based on the above described calculation, the point in time at which the single sheet P of the recording medium in the single printing mode, or the last sheet P of the recording medium in the continuous printing mode, comes out of the nip N (Step A 7 ), it quickly stop supplying the heat generation layer  20   b  with electrical current (Step A 8 ). Thereafter, it turns off the motor M to stop rotating the pressure roller  22  (Step A 9 ), ending thereby the printing operation (Step A 10 ). 
     In this connection, the fixing device  9  may be structured so that the sheet P of the recording medium in the single print mode, or the last sheet P in the continuous printing mode, that is, the printing mode in which a preset number of sheets P of recording prints are continuous conveyed, is detected by positioning a sheet sensor at the sheet exit of the nip N. 
     As described above, the fixing device  9  in this embodiment is structured so that during the warm-up period in which the heat generation layer  20   b  of the belt  20  is supplied with electric current to heat the belt  20  until the belt temperature reaches the preset temperature level (fixation temperature), the belt  20  is not rotated. That is, the fixing device  9  is structured so that the belt  20  is heated, without being circularly moved, until the temperature level detected by the thermistor TH reaches 160° C., which is the image heating temperature level, and immediately before the first sheet (or only sheet) of the recording medium in an image forming operation reaches the nip N. Therefore, the length of time it takes for the temperature of the belt  20  to reach its target level from when it begins to be heated, that is, the warm-up time, is roughly 6 seconds. In other words, the fixing device  9  is very short in warm-up time. 
       FIG. 10  shows the changes in the temperature of the belt  20  and the pressure roller  22  of the fixing device  9  in this embodiment, which occur during the period in which the fixing device  9  is warmed up (started up), and also, during the actual fixing operation. In comparison,  FIG. 11  shows the changes in the temperature of the belt  20  and pressure roller  22  of a comparative fixing device ( 9 ), which occurs during the period in which the fixing device ( 9 ) is warmed up (started up), and also, the during the actual fixing operation. In the case of the comparative fixing device ( 9 ), the pressure roller ( 22 ) is continuously rotated even while the belt ( 20 ) is warmed up. In  FIGS. 10 and 11 , the solid lines represent the changes in the temperature of the fixation belt, and the broken lines represent the changes in the temperature of the pressure roller. 
     In the case of the fixing device  9  in this embodiment, the pressure roller  22  is not rotated during the warm-up period. Therefore, it is difficult for heat to transfer from the belt  20  to the pressure roller  22 . Thus, the pressure roller  22  hardly increases in temperature during the warm-up period. That is, almost the entirety of the heat generated in the heat generation layer  20   b  is used for heating the belt  20 . Therefore, it takes a significantly shorter length of time to warm up the belt  20  (to startup fixing device  9 ) in this embodiment than the belt ( 20 ) of the comparative fixing device ( 9 ). That is, the fixing device  9  in this embodiment is structured so that during the warm-up period, the belt  20  is heated without being driven. Therefore, not only is the fixing device  9  in this embodiment shorter in warm-up time, but also, longer in service life, than the comparative fixing device. 
     It takes about six seconds for the fixing device  9  ( FIG. 10 ) in this embodiment to warm up. In the case of the comparative fixing device ( FIG. 11 ), the pressure roller  22  is rotated while the fixing device is warmed up, that is, the belt is circularly moved. Therefore, the comparative fixing device is greater in the amount of heat transferred from the belt to the pressure roller during the warm-up period, being therefore greater in the amount the pressure roller  22  increases in temperature. Thus, the length of time it takes to increase the temperature of the belt to the target level, or the fixation level, that is, the length of time necessary to warm up the fixing device, is about 15 seconds. 
     In addition, in the case of the fixing device  9  in this embodiment, its belt is not circularly moved, that is, the pressure roller  22  is not rotated, while it is warmed up. Therefore, it is smaller in the amount of minute particles and the like attributable to the frictional wear of the inward surface of the belt  20  and belt backing member  23 , being therefore smaller in the amount of torque necessary to drive the device, than the comparative fixing device. 
     Next, referring to  FIG. 4 , the positional relationship, in terms of the widthwise direction of the belt  20  (X direction in  FIG. 4 ), among the thermo-switch (safety element)  40 , thermistor TH, and heat generation layer  20   b  of the belt  20  is described. The thermo-switch  40  is serially connected between the electrical power source  102  (which functions as the device for supplying the heat generation layer  20   b  of the belt  20  with electric power) and the heat generation layer  20   b.    
     Therefore, if the belt  20  abnormally increases in temperature because of the malfunctioning of the control circuit (CPU)  100 , temperature detecting means, etc., the internal bimetal of the thermo-switch  40  interrupts the power supply to the heat generation layer  20   b  by melting. In the case of the thermo-switch  40  in this embodiment, as the temperature of its heat sensitive portion reaches 220° C., its internal bimetal breaks the electrical connection between its electrodes by melting. 
     It is desired that the thermo-switch  40  is placed in such a position that ensure that the thermo-switch  40  detects the abnormal increase in the temperature of the belt  20 . More specifically, it is desired that the heat sensitive portion of the thermo-switch  40  is placed in contact with the belt  20 , within the heat generation range W 22   b  of the belt  20  in terms of the widthwise direction of the belt  20 . This positioning of the thermo-switch  40  is for ensuring that the thermo-switch  40  quickly responds to the abnormal increase in the temperature of the belt  20 , and interrupts the power supply to the heat generation layer  20   b.    
     Referring to  FIG. 4 , in the case of the fixing device  9  in this embodiment, the width W 20   b  of the heat generation layer  20   b  of the belt  20  is such that the belt  20  extends by 10 mm beyond each of the edges of the path Wmax of the widest sheet P of the recording medium usable with the fixing device  9 . Further, in terms of the widthwise direction of the belt  20 , the thermo-switch (current blocking element, safety element, safety device)  40  is positioned between the edge of the path Wmax of the widest sheet P of the recording medium properly conveyable through the fixing device  9  and the corresponding edge of the heat generation range W 20   b  of the heat generation layer  20   b , with the heat sensitive portion of the thermo-switch  40  being in contact with the inward surface of the belt  20 . The safety element, such as the thermo-switch  40 , is characterized in that if electric current flows through an electric circuit by an amount greater than a preset amount, due to some anomaly or the other, it breaks the circuit by being melted by the Joule heat generated by the current which flows through the element. 
     An expression “to place the thermo-switch  40  outside the path Wmax of the widest sheet of the recording medium conveyable through the fixing device  9 ” means that, in terms of the direction (Y direction in  FIG. 4 ) perpendicular to the widthwise direction of the belt  20 , the thermo-switch  40  is outside the path Wmax of the widest sheet P of the recording medium conveyable through the fixing device  9  as shown in  FIG. 4 . Further, an expression “placing the thermo-switch  40  on the inward side of the heat generation range W 20   b  of the heat generation layer  20   b ” means that, in terms of the direction (Y direction in  FIG. 4 ) perpendicular to the width direction of the belt  20 , the thermo-switch  40  is within the heat generation range W 20   b  of the heat generation layer  20   b.    
     Therefore, the thermo-switch  40  can play the role of blocking the current supply to the heat generation layer  20   b  as the temperature of the belt  20  abnormally increases. Therefore, even if the malfunction of the control circuit  100  and/or temperature detecting means TH, or the like problem, prevents the control circuit  100 , temperature detecting means TH, and/or the like, from blocking of the current supply to the heat generation layer  20   b , the current supply to the heat generation layer  20   b  can be blocked. 
     Moreover, even though the portion of the belt  20 , which is in contact with the heat sensing portion of the thermo-switch  40 , remains lower in temperature than the other portions of the belt  20 , its position B is outside the path Wmax of the widest sheet P of the recording medium conveyable through the fixing device  9 . Therefore, it does not occur that the fixing device  9  outputs a print, the image of which is lower in gloss than a normal print, and/or that the fixing device  9  fails to properly fix an unfixed toner image on a sheet P of the recording medium. 
     That is, the fixing device  9  in this embodiment is structured so that the thermo-switch  40  or the like safety element (current blocking element)  40 , is placed in contact with the belt  20 , outside the path of the unfixed toner image, and within the heat generation range of the heat generating resistor layer  20   b.    
     Because the fixing device  9  in this embodiment is structured as described above, it can deals with such a situation that the control circuit  100 , which controls the voltage applied to the heat generation resistor layer  20   b  fails; the temperature detecting means for detecting the temperature of the belt  20  fails; and/or the like problem occurs. That is, as the temperature of the belt  20  reaches a preset limit (upper limit), the fixing device  9  in this embodiment can break the electrical connection between the heat generating resistor layer  20   b  and electrical power source  102 . In other words, it does not uncontrollably increase in temperature. Further, it does not suffer from the problem that the portion of its belt, which is within the path of an unfixed toner image, becomes, and/or remains, lower in temperature across a certain portion. Therefore, it does not outputs a print, the image on which is lower in gloss than the image on a normal print, a print which is unsatisfactory in fixation, and/or the like print. 
     The thermistor TH in this embodiment is an infrared temperature sensor A2TPMI (product of Perling Elmer, Co., Ltd.). It is widely known that this type of thermistor TH can measure the temperature of an object without being placed in contact with the object, and also, that it is excellent in term of responsiveness. 
     The current supply to the heat generation layer  20   b  of the fixing device  9  in this embodiment is controlled based on the difference between the temperature of the belt  20  detected by the thermistor TH and the target temperature level for the belt  10 , which is 160° C. Therefore, in order to keep the temperature of the belt  20  constant, across its portion within the recording medium path, while the fixing device  9  is in operation, it is obvious that it is desired that the thermistor TH is positioned within the path Wmin of the narrowest sheet of the recording medium. In this embodiment, therefore, the thermistor TH is positioned so that in terms of the widthwise direction of the belt  20 , its position coincides with the centerline O of the recording medium passage, and also, so that it is positioned in the adjacencies of the outward surface of the belt  20 , with the presence of a preset amount of distance from the outward surface of the belt  20 . 
     Therefore, it is ensured that the thermistor TH in this embodiment can accurately measure the temperature of the portion of the belt  20 , which is within the recording medium path, even when the narrowest sheet P of the recording medium is conveyed for fixation through the fixing device  9 . Further, since there is no physical contact between the thermistor TH in this embodiment and belt  20 , it does not occur that the belt  20  remains lower in temperature across its certain area during the warm-up period. 
     That is, the fixing device  9  in this embodiment is structured so that the temperature detecting means TH for detecting the temperature of the belt  20  is placed within the path of the narrowest sheet of the recording medium conveyable through the fixing device  9 . Therefore, not only can the temperature detecting means TH accurately detect the temperature of the portion of the belt  20  within the recording medium path, regardless of the recording medium size, but also, does not make lower in temperature, the portion of the belt, which corresponds in position to the temperature detecting means TH. Thus, the fixing device  9  in this embodiment does not output a print, the image on which is lower in gloss than the image on a normal print, and/or does not unsatisfactorily fix an unfixed toner image. 
     As described above, in the case of the fixing device  9  in this embodiment, even though the belt  20  is warmed up while the pressure roller  22  is kept stationary, the problems that occur to a portion or portions of an unfixed toner image, which correspond in position to a portion or portions of the belt  20 , which are lower in temperature than the rest, that is, the problem that a part or parts of the fixed toner image are lower in gloss than the rest, and/or the problem that a fixing device fails to satisfactorily fix an unfixed toner image, do not occur. 
     Embodiment 2 
     The fixing device  9  in this embodiment is structured so that the belt  20  is kept separated from the pressure roller  22  while it is warmed up. Therefore, it is even shorter in the length of the warm-up time than the fixing device  9  in the first embodiment. Otherwise, this embodiment is the same as the first embodiment, in terms of the structure of the image forming apparatus, structure of the fixing device, positioning of the thermo-switch  40  and thermistor TH, temperature setting, and the like factors. Thus, the fixing device in this embodiment is not described in order not to repeat the same description. 
     Next, referring to  FIG. 12 , the operational sequence of the fixing device  9  in this embodiment is described. While the fixing device  9  is kept on standby, the fixation motor M is kept turned off, and therefore, the pressure roller  22  remains stationary. Further, the electrical power source  102  is kept turned off, and therefore, the belt  20  is not heated. As the control circuit  100  receives a print start command from a user through the control panel  104  of the image forming apparatus, or from the input device of the host apparatus  200  such as a personal computer while the fixing device  9  is in the above described state, the control circuit  100  makes the image formation station(s) start an image forming operation (Step B 1 ). 
     At this point of time, if the mechanism (devices)  27 L and  27 R for moving the belt backing member  23  is off, the control circuit  100  turns on the mechanisms  27 L and  27 R. That is, if the belt  20  and pressure roller  22  are kept pressed upon each other, the control circuit  100  separates the belt  20  from the pressure roller  22  as shown in  FIG. 8  (Steps B 2  and B 30 . 
     Next, the control circuit  100  turns on the electric power source  102  to begin supplying the heat generation layer  20   b  of the belt  20  with electric power through the aforementioned power supply system (Step B 4 ). As the belt  20  is heated, and the temperature T 1  of the belt  20  detected by the thermistor TH reaches 120° C. (Step B 5 ), the control circuit  100  starts the formation of a toner image on the drum  3  of each image formation station (Step B 6 ). 
     Then, a sheet P of the recording medium, on which an unfixed toner image t is borne, is conveyed to the fixing device  9 . Then, the control circuit  100  stops supplying the left and right devices  27 L and  27 R for moving the belt backing member  23 , with electric power, with such a timing that the sheet of the recording medium P arrives at a point which is 20 mm upstream from the fixation nip N in terms of the recording medium conveyance direction (Step B 7 ). That is, the control circuit  100  allows the belt  20  to be pressed upon the pressure roller  22  (Step B 8 ). Then, it turns on the motor M. Thus, the pressure roller  22  begins to be driven (Step B 9 ). That is, the pressure roller  22  begins to be rotated in the counterclockwise direction indicated by the arrow mark  22  in  FIG. 2 , at a preset peripheral velocity. 
     The steps thereafter, that is, Steps B 11 -B 13 , are the same as Steps A 7 -A 10 , and therefore, are not described here. 
       FIG. 13  shows the changes which occurred to the temperature of the belt  20  and pressure roller  22  during the period between the starting of the warming up of the belt  20  and pressure roller  22 , and after the beginning of fixation. The solid lines represent the changes in the temperature of the fixation belt, and the broken lines represent the changes in the temperature of the pressure roller. 
     In the case of the fixing device  9  in this embodiment, the belt  20  is kept separated from the pressure roller  22  during the warm-up time. Therefore, heat does not transfer from the belt  20  to the pressure roller  22  during the warm-up time. That is, all the heat generated in the heat generation layer  20   b  of the belt  20  is used for heating the belt  20 . Therefore, the fixing device  9  in this embodiment is substantially shorter in the length of the warm-up time than the fixing device  9  in the first embodiment. Incidentally, the length of time it takes to warm up the fixing device  9  in this embodiment is roughly four seconds. 
     As described above, the fixing device  9  in this embodiment is structured so that while the belt  20  is warmed up, the belt  20  is kept separated from the pressure roller  22 . Therefore, all the heat generated in the heat generation layer  20   b  of the belt  20  is used for heating the belt  20 . Therefore, the fixing device  9  in this embodiment is substantially shorter in the length of the warm-up time than the fixing devices in the first embodiment. Further, it is structured so that the belt  20  is pressed upon the pressure roller  22  immediately before a sheet P of the recording medium reaches the nip N. Therefore, the decrease in the temperature of the belt  20  as the belt  20  is pressed upon the pressure roller  22  is minimized. 
     [Miscellanies] 
     1) The application of the present invention is not limited to a fixing device, the pressure applying member of which for worming the nip N between itself and the belt  20 , is in the form of a roller. That is, the present invention is also applicable to a fixing device, the pressure applying member of which is a circularly movable endless belt. 
     2) The application of the present invention is not limited to a fixing device structured so that when a sheet of the recording medium is conveyed through the fixing device, the centerline of the sheet, in terms of the direction perpendicular to the recording medium conveyance direction, coincides with the centerline of the recording medium passage of the fixing device. That is, the present invention is also applicable to a fixing device structured so that when a sheet of the recording medium is conveyed through the fixing device, one of the lateral edges of the sheet, in terms of the recording medium conveyance direction, coincides with the corresponding edge of the recording medium passage of the fixing device. 
     3) The choice of an apparatus as which the image heating apparatus in accordance with the present invention is used is not limited to a fixing device for fixing an unfixed image formed on a sheet of the recording medium, to the sheet. That is, the image heating apparatus in accordance with the present invention is also effective as a heating apparatus which applies heat and pressure to a permanently fixed image, or a temporarily fixed image, on a sheet of the recording medium, in order to modify (improve) the image in surface properties, such as gloss. 
     4) The choice of an apparatus with which the present invention is compatible is not limited to an image forming apparatus, the image formation stations of which are electrophotographic. For example, the present invention is also compatible with an image forming apparatus, the image formation stations of which uses an electrostatic recording method or a magnetic recording method. Further, the choice of an apparatus with which the present invention is compatible is not limited to an image forming apparatus of the transfer type. That is, the present invention is also compatible with an image forming apparatus which directly forms an unfixed image on a sheet of the recording medium. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 231335/2011 filed Oct. 21, 2011 which is hereby incorporated by reference.