Abstract:
An image heating apparatus includes: an endless belt; a rotatable member configured to cooperate with the endless belt to form the nip; a back-up member including a heater and an electrode portion connected with the heater; an electric energy supply member mounted to the back-up member; a regulating member mounted to the back-up member to regulate movement of the endless belt in a longitudinal direction of the belt; and a covering member for covering the electrode portion when the endless belt is inserted and removed in a longitudinal direction in the state that the electric energy supply member and the regulating member are dismounted from the back-up member.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image heating apparatus which heats the toner image on a sheet of recording medium. 
     It has been a common practice to provide an electrophotographic image forming apparatus with a fixing apparatus (device) for fixing the toner image formed on a sheet of recording medium to the sheet of recording medium. 
     An example of a fixing apparatus employed by an electrophotographic image forming apparatus is disclosed in Japanese Laid-open Patent Application Sho63-313182. This fixing apparatus employs a fixing film (endless belt). More concretely, it is provided with the fixation film, a heater (which is thin and flat in shape), and a pressure roller. It is structured so that the fixation film is sandwiched by the heater and pressure roller, forming a nip in which a sheet of recording medium is conveyed while being heated. 
     There is disclosed another fixing device such as the above described one in Japanese Laid-open Patent Application Hei10-171276. This fixing apparatus is structured so that its fixation film is replaceable. Therefore, if it becomes unsatisfactory in performance because of the deterioration of its fixation film, all that is necessary is to replace the fixation film; it is unnecessary to replace the entirety of the fixing device. 
     More specifically, this fixing apparatus is provided with a portion through which electric power is supplied to its heater, and a power supplying connector (power supplying member) which can be connected to, or disconnected, from the portion through which the electric power is supplied to the heater, and from which electric wire extends. Here, referring to  FIG. 15 , the procedure for replacing the fixation film of this fixing apparatus is described. 
     Referring to  FIG. 15 , when it is necessary to replace the fixation film  100 , first, the power supply connector  112  is to be disengaged from the electrode portion  105   a  of the heater  105 , in order to prevent the power supply connecter  112  from interfering with the operation for replacing the fixation film (Step 1). Then, the flange  102  (regulating member) which regulates the fixation film  100  in movement in terms of the lengthwise direction of the fixation film  100  is to be removed (Step 2). Then, the fixation film  100  is to be extracted in the direction parallel to its lengthwise direction (Step 3). 
     In other words, this fixing apparatus is structured as described above, from the standpoint of reducing a fixing apparatus in the consumption of natural resource, and also, in operational cost. It is possible, however, that such a structural arrangement as the one described above will suffer from the following problems. 
     That is, when the fixation film is extracted from the fixing apparatus, the lubricant with which the inward surface of the fixation film is coated will adhere to (transfer onto) the electrode portion of the heater. As the lubricant adheres to the electrode portion of the heater, there is not going to be established proper electrical connection between the electrode portion of the heater and the electric power supplying portion of the apparatus main assembly. Therefore, it is possible that after the replacement of the fixation film, the fixation film will not be satisfactorily heated. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided an image heating apparatus comprising an endless belt configured to heat a toner image on a sheet in a nip, wherein an inner surface of said endless belt is coated with a lubricant; a rotatable member configured to cooperate with said endless belt to form the nip; a back-up member cooperate with said rotatable member to sandwich said endless belt, said back-up member including a heater configured to heat said endless belt, and an electrode portion connected with said heater; an electric energy supply member dismountably mounted to said back-up member to supply electric power to said heater through said electrode portion; a regulating member dismountably mounted to said back-up member and configured to regulate movement of said endless belt in a longitudinal direction thereof; and a covering member configured and positioned to cover said electrode portion when said endless belt is inserted and removed in the longitudinal direction in a state that said electric energy supply member and said regulating member are dismounted from said back-up member. 
     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 
       In  FIG. 1 , (a) is a plan view of the image heating apparatus (device) in the first embodiment of the present invention, as seen from the direction of one of the lengthwise ends of the apparatus, (more specifically, lengthwise end from which fixation film is to be extracted), and in  FIG. 1 , (b) is a plan view of the same portion of the image heating apparatus (device) as the one shown in  FIG. 1( a ) , as seen from direction perpendicular to the lengthwise direction of the apparatus. 
       In  FIG. 2 , (a) is a plan view of the entirety of the image heating apparatus in the first embodiment, as seen from the direction from which a sheet of recording medium is fed into the apparatus, and in  FIG. 2 , (b) is a plan view of the image heating apparatus (device), minus its electrode protecting member, in the first embodiment, as seen from the direction perpendicular to the lengthwise direction of the apparatus. 
         FIG. 3  is a plan view of the electrode portion of the heating member of the image heating apparatus (device) in the first embodiment, as seen from the underside of the apparatus. 
         FIG. 4  is a plan view of the lengthwise end of the fixing apparatus in the first embodiment, from which the fixation film is to be extracted, as seen from the underside of the apparatus, when the electrode protecting member of the apparatus is in the first state (when protecting electrode). 
         FIG. 5  is a plan view of the combination of the fixation belt and heating unit in the first embodiment, as seen from the direction perpendicular to the lengthwise direction of the apparatus, when the electrode protecting member is in the first state (when protecting electrode), into which it has been moved by the fixation belt (endless member) as the fixation belt is extracted from the fixing device. 
         FIG. 6  is a plan view of the combination of the fixation film, heating unit, power supplying member, and electrode protecting member, when the electrode protecting member is in its second position (in which it expose (does not protect) electrode). 
       In  FIG. 7 , (a) is a schematic plan view of the combination of the heating unit, electrode protecting member, and film insertion facilitating member  114  of the image heating apparatus (device) in the second embodiment, when the power supplying member is engaged with, or disengaged from, the heating unit, as seen from the lengthwise direction of the fixing device (fixation film), and in  FIG. 7 , (b) is the lengthwise end of the fixation unit, from which the fixation film is inserted, as seen from the direction perpendicular to the fixation film insertion direction. 
       In  FIG. 8 , (a) is a schematic plan view of the combination of the heating unit, electrode protecting member, and film insertion facilitating member  114  of the image heating apparatus (device) in the third embodiment, when the power supplying member is engaged with, or disengaged from, the heating unit, as seen from the lengthwise direction of the fixing device (fixation film), and in  FIG. 8 , (b) is the lengthwise end of the fixation unit, from which the fixation film is inserted, as seen from the direction perpendicular to the fixation film insertion direction. 
         FIG. 9  is a schematic perspective view of the portion of the fixing device in the fourth embodiment, which is directly involved in the replacement of the endless film. 
         FIG. 10  is an exploded perspective view of the portion of the fixing device in the fourth embodiment, which is directly involved in the replacement of the endless film, and shows how the components of this portion can be disassembled. 
         FIG. 11  is a schematic sectional view of the portion of the fixing device in the fourth embodiment, which is directly involved in the replacement of the fixation film, when the electrode protecting member is in the second state (exposing electrode portion) while the power supplying member is attached. 
         FIG. 12  is a partially exploded schematic sectional view of the portion of the fixing device in the fourth embodiment, which is directly involved in the replacement of the fixation film, when the electrode protecting member is in the first position (protecting (covering) electrode portion), while the power supplying member is detached. 
         FIG. 13  is a perspective view of one of the lengthwise end portions of the fixation film supporting portion of the fixing device in the first embodiment. 
         FIG. 14  is a schematic sectional view of the image forming apparatus in the first embodiment, which employs an image heating apparatus (device) in accordance with the present invention. 
       In  FIGS. 15 , (1)-(3) are drawings illustrating the procedure for replacing the fixation film. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, some of the preferred embodiments of the present invention are described in detail with reference to the appended drawings. 
     Embodiment 1 
     Image Forming Apparatus 
       FIG. 14  shows the overall structure of the full-color laser beam printer as an example of a typical image forming apparatus to which the present invention is applicable. The image forming apparatus  200  is made up of an external shell  201 , a scanner unit  6 , a development unit, etc. It employs four process cartridges  1   a ,  1   b ,  1   c  and  1   d  which are for forming yellow, magenta, cyan, and black monochromatic images, respectively. The four process cartridges  1  are installed in the external shell  201  of the main assembly of the apparatus  200  so that they are positioned in parallel. Each process cartridge  1  is provided with a photosensitive drum (which hereafter will be referred to simply as drum) as an image bearing member which is rotatable in the clockwise direction by a drum driving means (unshown). Each process cartridge is also provided with a charging device and a development unit. The image forming apparatus  200  is structured so that after the proper installation of each cartridge into the external shell  201  of the image forming apparatus  200 , the charging device, scanner unit  6 , development unit, and the like processing means will be in the adjacencies of the drum, listing from the upstream side in terms of the rotational direction of the drum  1 . 
     Further, the image forming apparatus  200  is provided with electrostatic transferring devices  2   a ,  2   b ,  2   c  and  2   d , and a cleaning blade, which are positioned in the listed order in terms of the moving direction of the intermediary transfer belt  2 . 
     The scanner unit  6  projects a beam of laser light upon the peripheral surface of each of the drums a, b, c and d while modulating the beam according to the information of the image to be formed, to form an electrostatic latent image on the peripheral surface of the drum. The scanner unit  6  is on the bottom side of the drums a, b, c and d in terms of the gravity direction. As development bias is applied to the development roller, which is positioned so that its peripheral surface opposes the peripheral surface of the corresponding drum, a toner image, which reflects the latent image, is formed on the peripheral surface of each of the drums a, b, c and d. 
     The electrostatic transferring devices  2   a ,  2   b ,  2   c  and  2   d  are devices for transferring the toner images on the drums a, b, c and d, respectively, onto a sheet P of transfer medium as recording medium. The image forming apparatus  200  has the intermediary transfer belt  2 , which is circularly moved. The image forming apparatus  200  is structured so that when the four process cartridges  1   a ,  1   b ,  1   c  and  1   d  are properly positioned for image formation, in the main assembly of the image forming apparatus, the outward surface of the intermediary transfer belt  2  is in contact with the peripheral surface of each of the four drums a, b, c and d. The toner images on the drums a, b, c and d, one for one, are sequentially transferred onto the intermediary transfer belt  2  (primary transfer). Each of the electrostatic transferring devices  2   a ,  2   b ,  2   c  and  2   d  is provided with a transfer roller. The image forming apparatus  200  is structured so that the four transfer rollers are positioned in parallel, within the loop (belt loop) which the intermediary transfer belt  2  form, and oppose the drums a, b, c and d, one for one. 
     To these transfer rollers, bias which is positive in polarity is applied, providing thereby the belt  2  with positive electric charge, during the primary transfer. Thus, the toner images on the drums a, b, c and d, which are negative in polarity, are transferred, by the electrical field generated by the electric charge, onto the belt  2 , with which the drums are in contact. Then, the toner images on the belt  2  are circularly moved with the belt  2  in the counterclockwise direction in the drawing, and are transferred onto a sheet P of recording medium by the secondary transferring device  3 . The sheets P of recording medium are stored by multiple number in the sheet feeder cassette  4  in the sheet feeding-conveying portion  8  of the apparatus main assembly. The sheets P in the sheet feeder cassette  4  are fed into the apparatus main assembly one by one while being separated from the rest, and conveyed to the secondary transferring device  3 . 
     After the transfer of the toner images on the drums a, b, c and d onto the sheet P of recording medium, the sheet P is conveyed through the fixing device  40 , which functions as an image heating device, with which the image forming apparatus  200  is provided. While the sheet P is conveyed through the fixing device  40 , it and the multiple toner images thereon, different in color, are subjected to heat and pressure, whereby the toner images are fixed to the surface of the sheet P. After the thermal fixation of the toner images to the sheet P, the sheet P is discharged from the apparatus main assembly of the image forming apparatus  200  by the pair of discharge rollers  11 , with which the apparatus main assembly is provided. 
     A referential code  47  in  FIG. 2  stands for the sheet passage through which a sheet P of recording medium is conveyed when the image forming apparatus  200  is operated in the two-sided printing mode. When the image forming apparatus  200  is in the two-sided printing mode, each sheet P of recording medium is guided upward through the recording medium conveyance passage  48 , as it comes out of the fixing device  40 . Then, as the trailing edge of the sheet P arrives at the sheet reversal point, the sheet conveyance passage  48  is switched in its sheet conveyance direction so that the sheet P is conveyed in the opposite direction from the direction in which the sheet P has been conveyed. Consequently, the sheet P enters the two-sided printing mode sheet passage  47 . Then, the sheet P is conveyed into the recording medium conveyance passage  48  for the second time, and is conveyed to the secondary transfer station  3  while remaining upside down. 
     Thus, a toner image (toner images) is transferred onto the second surface of the sheet P of recording medium. After the sheet P is conveyed out of the secondary transferring station, it is guided into the fixing device  40 . After being conveyed through the fixing device  40 , that is, after the image formation on both surfaces of the sheet P, the sheet P is discharged from the apparatus main assembly of the image forming apparatus  200  as it is when the image forming apparatus  200  is in the one-sided printing mode. 
     (Fixing Device) 
     Next, the fixing device  40  as an image heating apparatus (device) is concretely described.  FIG. 2( b )  is a sectional view of the fixing device  40  in this embodiment, as seen from the direction parallel to the lengthwise direction of the fixing device  40 .  FIG. 2( a )  is a plan view of the fixing device  40  as seen from the direction indicated by an arrow mark in  FIG. 2( b ) . The fixing device  40  is provided with a heater  105 , a fixation film  100 , and a pressure roller  101 . The heater  105 , which is a heat generating member, is in the form of a thin rectangular parallelepiped. The fixation film  100  is an endless belt, and is heated by the heater  105 . The pressure roller  101  is a rotational member. It forms a nip between itself and the fixation belt  100 . After the transfer of a toner image (toner images) onto each sheet P of recording medium, the sheet P is conveyed through this nip between the pressure roller  101  and fixation film  100  while remaining sandwiched by the pressure roller  101  and fixation film  100 . As the sheet P is conveyed through this nip, it and the toner images thereon are heated. Consequently, the toner image (images) on the sheet P become fixed to the sheet P. 
     The fixing device  40  in this embodiment employs an image heating method which heats a sheet of recording medium and the unfixed toner image therein through a fixation film. More specifically, it has the fixation film  100  as a rotational heating member, and the pressure roller  101  as a pressure applying member. The fixation film  100  is cylindrical, and is made up of a thin metallic substrative layer. The pressure roller  101  is rotationally driven by a driving mechanism (motor). It functions as the driver roller for circularly moving the fixation film  100 . That is, the fixing device  40  is structured so that the fixation film  100  is circularly moved by the rotation of the pressure roller  101 . 
     Further, the fixing device  40  has a regulating member  102  for regulating the fixation film in its movement in its lengthwise direction, that is, the lengthwise direction of the fixing device  40 . Not only does the regulating member  102  regulate the fixation film  100  in the movement in the lengthwise direction, but also, function as a guiding member for guiding the fixation film  100  in the circular movement. Hereafter, the regulating member  102  is referred to as a fixation film track regulating member  102 . Further, the fixing device  40  has a film-backing member, upon which the fixation film  100  is pressed by the pressure roller  101  to form a nip between the fixation film  100  and pressure roller  101 , while allowing the fixation film  100  to slide on the film-backing member by its inward surface. 
     The film-backing member is made up of the heater  105 , a pressure bearing member  103 , and a stay  104 . It is disposed within the loop which the fixation film  100  forms. The fixing device  40  has also an electric power supplying member  112  (power supplying member, hereafter) for supplying the heater (electrically resistive heat generating layer)  105  with electric power. Hereafter, the combination of the fixation film  100 , pressure bearing member  103 , stay  104 , fixation film track regulating member  102 , and power supplying member  112  will be referred to as a fixation unit. The pressure bearing member  103 , stay  104 , and heater  105  are disposed within the loop which the fixation belt  100  forms. The power supplying member  112  is connected to the heater  105 . 
     Next, the details of each of the abovementioned components of the fixing device  40  are given. The fixation film  100  is a cylindrical heat resistant member through which heat is transmitted to a sheet P of recording medium from the heater  105 . It is loosely fitted around the fixation film track regulating member  102 . The smaller in thermal capacity the fixation film  100 , the quicker the fixing device  40  starts up. Thus, in order to reduce the fixation film  100  in thermal capacity, it is desired that the fixation film  100  is no more than 100 μm, preferably, no more than 20 μm, in thickness. 
     As the material for the substrative layer of the fixation film  100  in this embodiment, such metal as SUS is used. Incidentally, instead of employing a fixation film such as the one in this embodiment which is made up of the substrative layer and elastic layer, a single-layer film made up of PTFE, PFA, FEP, or the like, which is heat resistant, may be employed as the fixation film  100 . Further, the fixation film  100  may be a cylindrical multilayer film, which is made up of a substrative layer formed of polyimide, polyamide-imide, PEEK, PES, PPS, or the like, and a surface layer formed on the outward surface of the substrative layer, of PTFE, PFA, FEP, or the like. 
     The pressure roller  101  is made up of a metallic core formed of a metallic substance such as iron, and a heat resistant elastic layer formed (molded) on the peripheral surface of the metallic core, of silicone rubber, fluorinated rubber, fluorinated resin, or the like. It has also a surface layer as a parting layer. As the material for the parting layer, one among fluorinated resin, silicone resin, fluorinated silicone rubber, fluorinated rubber, silicone rubber, PFA, PTFE, FEP, and the like, which are superior in parting property, and heat resistant, may be selected. The lengthwise end portions of the metallic core are fitted with a pair of supporting members, one for one, made of a heat resistant resin such as liquid polymer, and are rotatably supported. 
     The fixation film track regulating member  102  is made of a heat resistant resin such as PPS, LCP, or the like. The fixation film  100  is loosely fitted around the fixation film track regulating member  102 , being thereby supported by the fixation film track regulating member  102  in such a manner that the fixation film  100  can be circularly moved. The fixation film track regulating member  102  flanges so that it can regulate the fixation film  100  in position in terms of the lengthwise direction of the film  100  by its flange portions. That is, the fixation film track regulating member  102  is such a member that guides the fixation film  100  as the fixation film  100  is circularly moved; as the film  100  shifts in its lengthwise direction, and comes into contact with the fixation film track regulating member  102 . 
     The fixing device  40  in this embodiment is structured so that the fixation film track regulating member  102  can be detached from, or attached to, the main structure of the fixing device  40 , by being moved away from, or toward, the main structure of the fixing device  40  in the lengthwise direction of the fixation film  100 . 
     The pressure bearing member  103  is a heat resistant and thermal insulating member. It is roughly semicircular in cross section, and its lengthwise direction is perpendicular to the recording medium conveyance direction. As the material for the pressure bearing member  103 , one of the electrically insulating and heat resistant substances such as phenol resin, polyimide resin, polyamide resin, polyamide-imide resin, PEEK resin, PPS resin, PFA, PTFE resin, LCP resin, and the like is used. It plays the role of backing up the fixation film  100  as the pressure roller  101  is pressed upon the fixation film  100  to form a nip N, to provide the fixing device  40  with the nip N having a preset amount of internal pressure. It plays also the role of ensuring that the fixation film  100  is reliably moved. 
     The stay  104  is a metallic member. It is kept pressed upon the pressure bearing member  103  to provide the pressure bearing member  103  with such strength that prevents the pressure bearing member  103  from being bent in the direction perpendicular to its lengthwise direction, and also, to keep the pressure bearing member  103  correct in attitude. 
     Basically, the heater  105  is made up of a ceramic substrate, which is in the form of a thin and long rectangular parallelepiped, and a layer of an electrically resistive substance (heat generating layer, hereafter), which generates heat as electric current is flowed through it. The heat generating layer is formed on the substrate. The heater  105  is very small in thermal capacity. Thus, as electric current is flowed through its heat generating layer, it very quickly increases in temperature. Referring to  FIG. 2 , the pressure bearing member  103  is provided with a groove which extends in the lengthwise direction of the pressure bearing member  103 . It is positioned so that its groove faces the abovementioned nip N. The ceramic substrate of the heater  105  is fitted in this groove of the pressure bearing member  103 . That is, the heater  105  is supported by the pressure bearing member  103  so that its heat generating layer faces the fixation film  100 . 
     The heater  105  is provided with a protective layer  105   b  for protecting the heat generating layer of the heater  105 , which is the surface layer of the heater  105 . The protective layer  105   b  is formed of glass, for example. In the case of the fixing device  40  in this embodiment, it is this protective layer  105   b  that is the surface of the heater  105  on which the fixation film  100  slides. 
     One of the lengthwise end portions of the heater  105  is provided with an electrode portion  105   a  ( FIG. 3 ) through which the heat generating layer of the heater  105  is supplied with electric power. Electric power is supplied to the electrically resistive heat generating layer by the placement of the power supplying member  112  ( FIG. 1( a ) ) in contact with the electrode portion  105   a . The power supplying member  112  has a contacting portion (electrical contact)  112   a  (shown in  FIG. 1( a ) ), which is within the power supplying member  112 . During a fixing operation (image heating operation), the electrically resistive heat generating layer is provided with electric power from the contact portion  112   a  through the electrode portion  105   a . The power supplying member  112  is fixed to the pressure bearing member  103  in such manner that when it is in connection to the heater  105 , it is kept pressed upon the electrode portion  105   a  of the heater  105 . 
     The fixation film  100  is circularly moved by the rotation of the pressure roller  101 , with its inward surface being kept in contact with the above-described pressure bearing member  103 , and the surface (protective layer  105   b , in particular) of the heater  105 . The pressure roller  101  is pressed against the pressure bearing member  103 , with the fixation film  100  being pinched between the pressure roller  101  and pressure bearing member  103 , by a very large amount of pressure, for example 300 N. Thus, in order to minimize the friction between the inward surface of the fixation film  100  and the surface of the pressure bearing member  103 , and the friction between the inward surface of the fixation film  100  and the surface of the heater  105 , the inward surface of the fixation film  100  (or surface of the pressure bearing member  103 , and surface of heater  105 , on which inward surface of fixation film  100  slides) is coated with lubricant. 
     (Method for Replacing Fixation Film) 
     Next, the procedure for replacing the fixation film  100  is described. This procedure is the same as the one described previously with reference to  FIG. 15 . That is, if it becomes necessary for the fixation film  100  of the fixing device  40  to be replaced because of its deterioration, first, the fixation unit of the fixing device  40  is to be moved out of the fixing device  40 . Next, the power supplying member  112  is to be removed from the fixation unit (Step 1). Then, the fixation film track regulating member  102  is to be disengaged from the fixation unit (Step 2). Lastly, the fixation film  100  is removed from the fixation unit in its lengthwise direction (Step 3). This is how the deteriorated fixation film  100  is to be extracted from the fixing device  40 . All that is necessary to fit the fixing device  40  with a new fixation film (replacement fixation film)  100  is to carry out the above-described steps in the reverse order. 
     It is possible here that in Step 3, that is, when the fixation film  100  is extracted, the lubricant on the inward surface of the fixation film  100  will adhere to the electrode portion  105   a  of the heater  105 , as described before. In the case of the fixing device  40  in this embodiment, therefore, it is provided with an electrode protecting member  113 , which will be described next. 
     (Electrode Protecting Member) 
     Referring to  FIG. 1 , the electrode protecting member  113 , which functions as an electrode covering member, is shaped like a rectangular sheet of paper. It is disposed at the lengthwise end of the fixation unit, which corresponds in position to the electrode portion  105   a  of the heater  105 . It is positioned so that it opposes the electrode portion  105   a . It is fixed in position relative to the pressure bearing member  103 , by being sandwiched by the pressure bearing member  103  and stay  104 . 
     Next, referring to  FIG. 1( a ) , the electrode protecting member  113  has a movable portion  113   a , in addition to an immovable portion  113   b  by which the electrode protecting member  113  is held to the fixation unit. This movable portion  113   a  is the very portion of the electrode protecting member  113  that protects (covers) the electrode portion  105   a  of the heater  105 . Incidentally, the drawings other than  FIG. 1( a ) , which show the electrode protecting member  113 , are primarily for showing the movement of the movable portion  113   a.    
       FIG. 3  shows an example of the shape of the electrode portion  105   a  of the heater  105 . Referential codes Dh and Lh stand for the width and length of the portion of the electrode portion  105   a  of the heater  105 , which is not covered with the protective layer  105   b , that is, the portion of the electrode portion  105   a , with which the contact portion  112   a  ( FIG. 1( a ) ) of the power supplying member  112  is placed in contact to supply the heater  105  with electric power. Next, referring to  FIG. 4 , referential codes Dc and Lc stand for the width and length of the movable portion  113   a  of the electrode protecting member  113 . It is desired that the widths Dh and Dc, and the lengths Lh, and Lc satisfy the following conditions (inequities). 
     To begin with, in order for the electrode protecting member  113  to be able to completely cover the electrode portion  105   a  of the heater  105 , it is desired that following relationships are satisfied: Dc&gt;Dh, Lc&gt;Lh. With these relationships being satisfied, it is possible to satisfactorily prevent the problem that when the fixation film  100  is removed from the fixation unit, the lubricant adheres to the electrode portion  105   a  of the heater  105 . 
     As for the length Lc, it is desired to be long enough for the edge portion of the movable portion  113   a  of the electrode protecting member  113  to extend beyond the peripheral surface of the fixation film  100  after the disengagement of the power supplying member  112 . As long as this relationship is satisfied, the electrode protecting member  113  will block the fixation film removal path, that is, the path through which the fixation film  100  is to be removed from the fixation unit by a user. 
     The reason why the fixing device  40  (fixation unit) is to be structured as described above is that as long as the electrode protecting member  113  is positioned to block the fixation film removal path, the fixation film  100  is guided by the electrode protecting member  113  when it is removed, and therefore, it is easier for a user (operator) to remove the fixation film  100 . More specifically, as the fixation film  100  is pulled outward of the fixation unit in the lengthwise direction of the fixation film  100 , the electrode protecting member  113  is pushed by the fixation film  100  (being thereby forced to retract into the inward side (hollow) of the fixation belt  100  as shown in  FIG. 4 ) before the fixation film  100  can be removed. Thus, it is ensured that when the fixation film  100  is removed from the fixation unit, the electrode portion  105   a  of the heater  105  is always protected by the electrode protecting member  113 . 
     Lastly, to describe the largest value for Dc and Lc, it is after the disengagement of the power supplying member  112 , that is, when the lengthwise end portion of the fixation unit is in the state shown in  FIG. 3 , that the electrode protecting member  113  is allowed to move outward of the portion Dc of the movable portion  113   a , in the lengthwise direction of the fixation film  100 . Therefore, the portion Dc is desired to be smaller than the distance from the outward edge of the electrode protecting member  113  in terms of the lengthwise direction of the fixation unit, to the corresponding edge of the fixation film  100 . Further, the value of the portion Lc is desired to be such that the movable portion  113   a  of the electrode protecting member  113  can be moved into the inward side (hollow) of the fixation film  100  to put the fixation unit in the state shown in  FIG. 5 , for example. 
     With the above-described relationships being satisfied, unless the electrode protecting member  113  is retracted into the inward side (hollow) of the fixation film  100  in terms of the radius direction of the fixation film  100 , the fixation film  100  cannot be moved away from the fixing device  40 . In other words, because the fixing device  40  is structured so that as the fixation film  100  is moved outward of the fixing device  40  in the lengthwise direction of the fixation film  100  to be extracted from the fixing device  40 , the movable portion  113   a  of the electrode protecting member  113  is retracted into the inward side (hollow) of the fixation film  100  in terms of the radius direction of the fixation film  100 , by the fixation film  100 . Therefore, it is ensured that when the fixation film  100  is extracted from the fixing device  40 , the electrode portion  105   a  of the heater  105  remains entirely covered (protected) with the electrode protecting member  113 . 
     Further, as the power supplying member  112  is engaged with the electrode portion  105   a  of the heater  105  after the insertion of the fixation film  100  into the fixing device  40 , the movable portion  113   a  of the electrode protecting member  113  is pushed by the power supplying member  112 , being thereby displaced as shown in  FIG. 6 . That is, the fixing device  40  is structured so that the movable portion  113   a  of the electrode protecting member  113  can be pivotally moved from the position (which is referred to as “first position”), in which its movable portion  113   a  covers the entirety of the electrode portion  105   a  of the electrode  105 , to the position  113   a ′ (which is referred to as “second position”), in which its movable portion  113   a  allows the power supplying member  112  to be connected to the electrode portion  105   a . That is, it is when the movable portion  113   a  of the electrode protecting member  113  is in the position  113   a ′ that electric power can be supplied from the contact portion  112   a  of the power supplying member  112  to the heater  105  through the electrode portion  105   a  of the heater  105 . 
     Embodiment 2 
     Next, referring to  FIG. 7 , the second embodiment of the present invention is described.  FIG. 7( b )  shows the state of the fixation film supporting portion of the fixing device  40  in this embodiment, as seen from the direction from which the fixation film  100  is inserted into the fixing device  40 .  FIG. 7( a )  is a schematic plan view of the same portion of the fixing device  40  as that shown in  FIG. 7( b ) , as seen from the direction indicated by an arrow mark in  FIG. 7( b ) . As is evident from the drawings, the fixation film supporting portion is provided with a film insertion facilitation member  114 , which is a movable member for facilitating the insertion of a replacement fixation film  100  into the fixing device  40 . The film insertion facilitation member  114  has a film insertion facilitating portion  114   a  which is the very portion of the film insertion facilitation member  114  that facilitates the insertion of the fixation film  100 , and a pressing portion  114   b  for causing the electrode protecting member  113  to retract. 
     This insertion facilitating portion  114   a  is shaped so that it is tilted downward and downstream relative to the fixation film insertion direction, and is positioned so that it can facilitate the insertion of the fixation film  100 . Therefore, as an operator attempts to insert a replacement fixation film  100  into the fixing device  40 , the replacement fixation film  100  comes into contact with the insertion facilitation portion  114   a , applying thereby a force F 1  to the insertion facilitation portion  114   a , as shown in the drawing. Since the insertion facilitating portion  114   a  is tilted relative to the fixation film insertion direction as described above, the insertion facilitating portion  114   a  is subjected to a component F 1 ′ of the force F 1 . Consequently, insertion facilitating portion  114   a  bends in a manner to retract inward of the cylindrical fixation film  100 . 
     In addition, the pressing portion  114   b , which also is an integral part of the insertion facilitation member  114 , like the insertion facilitation portion  114   a , is made to come into contact with the electrode protecting member  113 , and press the electrode protecting member  113 , causing thereby the electrode protecting member  113  to retract inward of the cylindrical fixation film  100  along with the pressing portion  114   b.    
     Therefore, it is ensured that as an attempt is made by an operator to insert the fixation film  100  into the fixing device  40 , the electrode protecting member  113  is made to retract inward of the cylindrical fixation film  100  in terms of the radius direction of the fixation film  100 . That is, it is ensured that when the fixation film  100  is inserted into the fixing device  40 , the electrode protecting member  113  is always between the lubricant coated on the inward surface of the fixation film  100  and the electrode portion  105   a  of the heater  105 . Therefore, the simple insertion of the fixation film  100  into the fixing device  40  by a person who happens to have to replace the fixation film  100  is all that is necessary to prevent the problem that when the fixation film  100  in the fixing device  40  is replaced, the lubricant adheres to the electrode portion  105   a  of the heater  105 . 
     Moreover, the insertion facilitating portion  114   a  of the insertion facilitating member  114  is tilted downward and downstream relative to the fixation film insertion direction. Therefore, it is possible to prevent the problem that the edge of the fixation film  100  squarely collides with the insertion facilitating member  114 , and the components in the adjacencies thereof. Therefore, it is possible to reduce the damages to which the edge of the replacement fixation film  100  is subjected during its insertion. Incidentally, when the power supplying member  112  is engaged with the electrode portion  105   a  of the heater  105  after the completion of the insertion of the new fixation film  100 , the electrode protecting member  113  and insertion facilitating member  114  are in the position  113   a ′ and  114 ′, respectively. Therefore, the electrode protecting member  113  and insertion facilitating member  114  do not interfere with the engagement of the power supplying member  112  and the electrode portion  105   a  of the heater  105  to supply the heater  105  with electric power. 
     Embodiment 3 
     Next, referring to  FIG. 8 , the third embodiment of the present invention is described.  FIG. 8( b )  is a schematic plan view of the lengthwise end portion of the fixation film supporting member (unit) in the third embodiment, as seen from the direction from which the fixation film  100  is inserted into the fixing device  40 .  FIG. 8( a )  is a schematic plan view of the same portion of the fixing film supporting unit as the one shown in  FIG. 8( b )  as seen from the direction indicated by an arrow mark in  FIG. 8( b ) . In the second embodiment, the insertion facilitating member  114  is physically independent from the electrode protecting member  113 . In this embodiment, a part  113   c  of the electrode protecting member  113  is given the function of facilitating the insertion of the fixation film  100 . 
     That is, in this embodiment, the electrode protecting member  113  is provided with the insertion facilitating portion  113   c , which is on the outward side of the electrode protecting member  113  in terms of the lengthwise direction of the fixing device  40 . That is, the insertion facilitating portion  113   c  is a part of the upstream end portion of the electrode protecting member  113  in terms of the direction in which a new (replacement) fixation film  100  is inserted into the fixing device  40 . The insertion facilitating portion  113   c  is shaped so that it is tilted downward and downstream relative to the fixation film insertion direction. Therefore, as the fixation film  100  is inserted into the fixing device  40 , first, it comes into contact with the insertion facilitating portion  113   c , and presses on the insertion facilitating portion  113   c  by a force F 1 . Thus, the insertion facilitating portion  113   c  is subjected to a component F 1 ′ of the force F 1 . Thus, the insertion facilitating portion  113   c  is pushed upward, being thereby made to retract into the hollow of the fixation film  100  by the force F 1 ′. 
     Therefore, simple insertion of the fixation film  100  into the fixing device  40  is all that is necessary to prevent the lubricant from adhering to the electrode portion  105   a  of the heater  105  while the fixation film  100  is inserted into the fixing device  40 , as it is in the second embodiment. 
     Embodiment 4 
     Next, referring to  FIGS. 9-13 , the fourth embodiment of the present invention is described.  FIG. 9  is a schematic perspective view of the portion of the fixing device  40  in this embodiment, which is directly involved in the replacement of the fixation film.  FIG. 10  is an exploded perspective view of the portion of the fixing device in this embodiment, which is directly involved in the replacement of the fixation film. It shows how this portion can be disassembled.  FIG. 11  is a schematic sectional view of the portion of the fixing device in this embodiment, which is directly involved in the replacement of the fixation film.  FIG. 12  is a partially exploded schematic sectional view of the portion of the fixing device in this embodiment, which is directly involved in the replacement of the fixation film  100 . 
     Referring to  FIG. 10 , the first step to be carried out to replace the fixation film  100  in the fixing device  40  is to disengage the power supplying member  112  from the fixation film supporting unit, in the direction indicated by an arrow mark in the drawing, because the power supplying member  112  is on the downstream side of the flange portion  102   b  of the fixation film track regulating member  102 , in terms of the direction in which the fixation film  100  is to be extracted from the fixing device  40 . 
     The next step to be taken is to remove the fixation film track regulating member  102  (in the direction indicated by arrow mark A). This step is necessary because the flange portion  102   b  for regulating the movement of the fixation film  100  in the direction parallel to the rotational axis of the fixation film  100  is a part of the fixation film track regulating member  102 . 
     Referring to  FIG. 11 , the fixation film track regulating member  102  is coupled with the stay  104  in such a manner that it can be moved relative to the fixation unit in the direction indicated by the arrow mark A as described above. The fixation film track regulating member  102  is provided with protrusion  102   d , which protrudes toward the pressure bearing member  103 . The pressure bearing member  103  is provided with a slide-guide  103   c , which guides the protrusion  102   d  of the fixation film track regulating member  102 , guiding thereby the fixation film track regulating member  102 . Thus, the fixation film track regulating member  102  can be moved in the direction indicated by the arrow mark A, being guided by the stay  104  and the slide-guide  103   c  of the pressure bearing member  103 , and remaining in the same attitude as the one in which it was set. 
     Next, the role of a shutter  122  is described, which is the same as the role of the above described electrode protecting member  113  which protects the electrode portion  105   a  of the heater  105  when the fixation film track regulating member  102  is removed. Referring to  FIG. 10 , the protrusion  102   d , with which the fixation film track regulating member  102  is provided is coupled with the slide-guide  103   c , with which the pressure bearing member  103  is provided, and also, with the slit  122   a  with which the shutter  122  is provided. 
     The shutter  122  is a thin and flexible member. It is made of resin, metal, etc. It is held by the shutter guide  103   a , with which the pressure bearing member  103  is provided, in such a manner that it can be moved along the outward surface of the fixation film  100  in the circumferential direction of the fixation film  100 . Therefore, as the shutter  122  comes into contact with the protrusion  102   d  of the fixation film track regulating member  102 , it is pushed by the protrusion  102   d , being thereby moved relative to the pressure bearing member  103  in the direction indicated by an arrow mark C. The movement of the shutter  122 , which is caused by the removal of the fixation film track regulating member  102 , will be described later. 
     When the power supplying member  112  is in connection to the fixation unit ( FIG. 11 ), the shutter  122  is in its retreat (home position). Therefore, the surface of the electrode portion  105   a  of the heater  105  remains exposed, being therefore enable to supply the heater  105  with electric power. That is, it is in contact with the contact portion  112   a  of the power supplying member  112 . Further, the protrusion  102   d  ( FIG. 10 ) of the fixation film track regulating member  102  remains fitted in the slit  122   a  of the shutter  122 , regulating thereby the shutter  122  in its movement in the rotational direction of the fixation film  100 . 
     Next, the movement of the shutter  122 , which is caused by the movement of the fixation film track regulating member  102  when the fixation film track regulating member  102  is removed from the fixation unit, is described. As the fixation film track regulating member  102  is moved in the direction indicated by an arrow mark A in  FIG. 10 , the protrusion  102   d  of the fixation film track regulating member  102  comes out of the fixation film track regulating member positioning portion  122   b  of the slit  122   a , and moves into the slanted portion  122   c  of the slit  122   a.    
     As the fixation film track regulating member  102  is moved further in the direction indicated by the arrow mark A, with the protrusion  102   d  remaining in the slanted portion  122   c  of the slit  122   a , the portion of the shutter  122 , which has the slanted portion  122   c  of the slit  122   a , is pushed by the protrusion  102   d  of the fixation film track regulating member  102 . Thus, the shutter  122  moves in the direction indicated by an arrow mark C in  FIGS. 10 and 11 . 
     That is, as the fixation film track regulating member  102  is moved in the direction indicated by the arrow mark A in  FIG. 10 , the shutter  122  is moved in the direction indicated by the arrow mark C in  FIG. 10 , by the movement of the fixation film track regulating member  102 , with the protrusion  102   d  of the fixation film track regulating member  102  remaining in the fixation film track regulating member insertion opening portion  122   d  of the slit  122   a . Thus, the shutter  122  is moved into the position ( FIG. 12 ) in which it covers (protects) the electrode portion  105   a  of the heater  105 . That is, the shutter  122  is disengaged from the pressure bearing member  103 , making it possible for the fixation film track regulating member  102  to be removed from the fixation unit. 
     With the positional relationship among the abovementioned components being kept as described above, it is ensured that when the fixation film  100  is extracted from the fixing device  40 , the electrode portion  105   a  is protected by the shutter  122 . That is, when the inward surface of the fixation film  100  passes over the electrode portion  105   a , the electrode portion  105   a  is protected from the adhesion of the contaminants (lubricant) such as grease. 
     (Cleaning Member) 
     Referring to  FIGS. 11 and 12 , the surface of the shutter  122 , which faces the electrode portion  105   a  of the heater  105 , is provided with a cleaning member (cleaning portion)  123 . As the shutter  122  is moved, the cleaning member  123  comes into contact with the surface of the electrode portion  105   a , and wipes clean the surface of the electrode portion  105   a  as it moves with the shutter  122 . The cleaning member  123  is made of a heat resistant substance, for example, unwoven cloth made of aramid fiber, foamed silicone rubber, PPS or the like resin. 
     Next, the effects of the cleaning member  123  are described in detail. As described above, in order to remove the fixation film  100  from the fixation unit, the power supplying member  112  has to be removed from the fixation unit. The power supplying member  112  is in contact with the electrode portion  105   a  of the heater  105  to supply the heater  105  with electric power during an image heating period, and is kept pressed against the pressure bearing member  103 . Therefore, when the power supplying member  112  is engaged with, or disengaged from, the electrode portion  105   a , the power supplying member  112  and electrode portion  105   a  rub against each other, producing thereby particled shavings or the like. If the particled shavings are present between the electrode portion  105   a  and power supplying member  112 , it is possible that the electrical connection between the electrode portion  105   a  and power supplying member  112  will become abnormal. 
     In this embodiment, however, when the shutter  122  is moved along the pressure bearing member  103 , the cleaning member  123  removes the contaminants on the electrode portion  105   a  by moving with the shutter  122  while remaining in contact with the electrode portion  105   a . Therefore, it is ensured that the electrical connection between the electrode portion  105   a  and power supplying member  112  remains satisfactory. 
     The cleaning member  123  in this embodiment is a sheet of foamed silicone rubber, unwoven cloth, or the like. However, this embodiment is not intended to limit the present invention in terms the shape of the cleaning member  123 . That is, all that is required of the cleaning member  123  in terms of shape is that the cleaning member  123  is shaped so that it can easily remove the contaminants on the electrode portion  105   a . For example, the cleaning member  123  may be different in shape from the one in which it is in this embodiment, in such a manner that its cleaning edge can be placed in contact with the electrode portion  105   a  at an acute angle. 
     Referring to  FIG. 2 , in the above described first to fourth embodiments of the present invention, a pair of power supplying members  112  are disposed at the lengthwise ends of the fixation film  100 , one for one, and so are the electrode protecting member (shutter)  122 . This structural arrangement was made to enable the fixation film  100  to be extracted from the fixing device  40  in either direction in terms of its lengthwise direction. However, these embodiments are not intended to limit the present invention in the direction, in terms of the lengthwise direction of the fixation film  100 , in which the fixation film  100  is to be extracted from the fixing device  40 . That is, the fixing device  40  may be designed so that the fixation film  100  can be extracted only in one direction in terms of the lengthwise direction of the fixation film  100 , and only the lengthwise end of the fixation unit, from which the fixation film  100  is to be extracted, is provided with the electrode protecting member (shutter)  122 . 
     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. 270304/2012 filed Dec. 11, 2012, which is hereby incorporated by reference.