Patent Publication Number: US-6671488-B2

Title: Image heating apparatus

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image heating apparatus such as a thermal fixing device mounted in an image forming apparatus, for example, a copying machine or a printer. In particular, it relates to an image heating apparatus having a rotational member, which contacts a recording medium on which an image is borne, and a regulating member for regulating the movement of the rotational member in the direction parallel to its generatrix direction. 
     In the image forming portion of an image forming apparatus which employs an image formation process, for example, an electrophotographic process and an electrostatic recording process, an unfixed toner image in accordance with the image formation information for an intended image is directly formed on, or transferred onto, a recording medium (transfer medium, printing paper, photosensitive paper, electrostatic recording paper, and the like). This unfixed toner image on the recording medium is thermally fixed to the recording medium by a thermal fixing apparatus; in other words, the unfixed image is turned into a permanent image by a thermal fixing apparatus. As for a thermal fixing apparatus, a heating apparatus employing the so-called roller heating method is widely used. A roller heating type thermal fixing apparatus comprises a heat roller (fixing roller) as a heating member, and a pressure roller as a pressure applying member. The two rollers are rotated while kept pressed against each other, by the application of a predetermined amount of pressure. In operation, while a recording medium bearing an unfixed toner image is conveyed through the nip formed between the heat and pressure rollers, the unfixed image is fixed to the recording medium; it is turned into a permanent image. 
     In recent years, due to the concerns regarding “quick start” and energy conservation, a film heating type heating apparatus has been put to practical use, which has been disclosed in, for example; Japanese Laid-open Patent Applications 63-313182, 2-157878, 4-44075, and 4-204980. 
     A film heating type heating apparatus comprises: a heating member, for example, a ceramic heater; a pressure roller as a pressure applying member; and a heat resistant resin film (which hereinafter will be referred to as fixation film). The pressure roller is kept pressed against the heating member, forming a compression nip (which hereinafter will be referred to as fixation nip). In operation, a recording medium bearing an unfixed toner image is introduced into the fixation nip, and conveyed through the fixation nip, together with the fixation film, with the recording medium pinched between the fixation film and pressure roller, while the heating member, or the ceramic heater, conducts heat to the recording medium through the fixation film. As a result, the unfixed image on the recording medium is fixed to the surface of the recording medium, by the heat and compressive force in the fixation nip. 
     A film heating type heating apparatus is capable of becoming ready to heat by the time a recording medium reaches the heating apparatus, even if power begins to be supplied to the heater after the reception of a print signal by the image forming apparatus. In other words, while a film heating type heating apparatus is kept on standby, the heater of a film heating type heating apparatus does not need to be supplied with power. Thus, from the standpoint of energy conservation, a film heating type heating apparatus is an excellent thermal fixing apparatus in that it does not waste energy. 
     Replacing the resin fixation film, as a heating member, of a film heating type heating apparatus, with a thin metallic sleeve, the base layer of which is formed of a metal superior in thermal conductivity to the resin fixation film, improves the fixation performance of the fixing apparatus, enabling the fixing apparatus to satisfactorily cope with the increase in the speed of an image forming apparatus. 
     The fixation film, or sleeve, tends to shift in the thrust direction. Thus, a film heating type heating apparatus in accordance with the prior arts is provided with a regulating member, such as a sleeve end flange, for regulating this shifting of the fixation film in the thrust direction, more specifically, for catching the fixation film by one of its edges. With the provision of this type of structural arrangement, the fixation film which is rotated while being kept in contact with the flat surface of the heater, is repeatedly flexed while one of its edges is remaining in contact with the corresponding sleeve end flange. Eventually, the edge portions of the metallic film develop cracks. This phenomenon is more severe in the case that a metallic sleeve (fixation film formed of thin sheet of metallic material) comprising a base layer formed of metallic material superior in thermal conductivity to resin fixation film, is used as a heating member. 
     More specifically, as the edge of a fixation film is placed in contact with a sleeve end flange due to the shifting of the fixation film, it is subjected to such force that acts in the direction to increase the diameter of the edge portion of the fixation film, that is, in a manner to expand the edge portion of the fixation film like the end of a trumpet. This force is greater in the case of a metallic fixation film, or sleeve, for the following reason. That is, a metallic sleeve is supported from inside by a flange, being therefore not regulated in terms of its expansion in its radius direction. Thus, its edge portion is more likely to be deformed in the form of the end portion of a trumpet by the above described force. Obviously, the edge of a metallic sleeve rubs against a sleeve end regulation flange as does any fixation film. Therefore, a metallic fixation film (sleeve) is more likely to develop cracks along the edges. 
     In addition, if the metallic base layer of a metallic sleeve is excessively thick, it takes a long time for the temperature of the heater to be raised to a level at which satisfactory fixation is possible, after a printing operation is initiated at room temperature; it takes too long for the heater temperature to rise to a predetermined level; the waiting time is long. 
     In other words, if the metallic base layer of a metallic sleeve is excessively thick, it is difficult to reduce the length of the time (first print time) from the reception of a print signal to the completion of the first print. 
     Further, if a metallic sleeve is thick, it is inferior in elastic deformation which is essential for the metallic sleeve to contact the heater surface, without leaving any gap between the two. Therefore, it is difficult for the heat from the heater to efficiently conduct to the metallic sleeve. 
     Further, the thicker the metallic sleeve, the higher the pressure which must be applied to ideally place the metallic sleeve in contact with the heater surface. In other words, in the case of a relatively thick metallic sleeve, it must be forcefully bent. Therefore, it does not stand long use; it breaks down due to fatigue. 
     As will be evident from the above description, a structural arrangement which is capable of making a thin metallic sleeve in accordance with the prior arts, satisfactory in terms of thermal and mechanical strength, has not been found. 
     As for a method for solving the above described problem, it is possible to provide a metallic sleeve with edge reinforcement members. However, a metallic sleeve must be able to elastically deform in order to conform to the flat heater surface, and it has been virtually impossible to form an edge reinforcement member which does not interfere with the elastic deformation of a metallic sleeve, which is necessary for the metallic sleeve to conform to the flat heater surface. More specifically, when an edge reinforcement member is secured to the edge of a metallic sleeve by gluing or the like method, it must be able to flex with the metallic sleeve. However, in the case of a film heating type heating apparatus in accordance with the prior arts, in which the metallic sleeve is in contact with the flat heater surface from one end of the heater to the other in terms of the lengthwise direction of the heater, if a part of the edge reinforcement member is on the inward surface of the metallic sleeve, it comes into contact (interferes) with the heater surface, resulting in the frictional damage to the edge reinforcement member and/or heater surface. 
     It is also possible to make such a structural arrangement that the portions of the heater surface correspondent in position to the edge portions of a metallic sleeve, that is, the portions fitted with an edge portion reinforcement member, are kept away from the inward surface of the metallic sleeve, in order to prevent the above described problem. However, it is extremely difficult to process a heater, in particular, a thin ceramic heater with a fixed thermal capacity in order to give it such a structure. Also, it is extremely costly. In other words, the idea is not practical. Further, from the standpoint of the reliability of current flow, it is extremely difficult to provide the electrically conductive portion of a heating member, or the like, on a substrate, with areas which are not level with the other portion of the conductive portion. 
     As will be evident from the above, it is difficult to realize a thermal fixing apparatus which employs a metallic sleeve, is satisfactory in fixing performance, and also is durable. So far, a means for realizing such a thermal fixing apparatus has not been provided. 
     SUMMARY OF THE INVENTION 
     The present invention was made in consideration of the above described problem. Thus, its primary object is to provide an image heating apparatus superior in the durability of its rotational member. 
     Another object of the present invention is to provide an image heating apparatus which is compact in the measurement in terms of the direction parallel to its generatrix direction. 
     According to an aspect of the present invention, an image heating apparatus comprises: 
     a rotational member which contacts an image bearing recording medium; 
     a regulating member for regulating the movement of said rotational member in the direction parallel to the direction of the generatrix of the rotational member; 
     wherein said regulating member is secured so that it does not rotate, and is provided with a surface which faces peripheral surface of the lengthwise end portion of said rotational member. 
     According to another aspect of the present invention, an image heating apparatus comprises: 
     a rotational member which contacts an image bearing recording medium; 
     a first regulating member for regulating the movement of said rotational member in the direction parallel to the direction of the generatrix of the rotational member, said first regulating member being enabled to rotate following the rotation of said rotational member; and 
     a second regulating member for regulating the movement of the said first regulating member in the direction parallel to the generatrix direction, said second regulating member being nonrotationally secured; 
     wherein said first regulating member is in the form of a cylindrical ring, and said second regulating member is shaped like a “cylindrical” pillar with a missing segment. 
     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 structure thereof. 
     FIG. 2 is a schematic sectional view of the thermal fixing apparatus in the first embodiment of the present invention. 
     FIG. 3 is a schematic front view of the thermal fixing apparatus in the first embodiment of the present invention, in which certain portions have been intentionally left out to show the interior thereof. 
     FIG. 4 is a perspective view of the bearing, and the recess in the side wall of the boxy frame of the fixing apparatus, in which the bearing is fitted. 
     FIG. 5 is a schematic drawing for describing how the fixing member is mounted in the boxy frame of the fixing apparatus. 
     FIG. 6 is a drawing for showing the structure of the stationary protective cap. 
     FIG. 7 is a schematic front view of the thermal fixing apparatus in the second embodiment of the present invention, in which certain portions have been intentionally left out to show the interior thereof. 
     FIG. 8 is a drawing for showing the structure of the rotational protective ring. 
     FIG. 9 is a sectional drawing for describing the behavior of the metallic sleeve. 
     FIG. 10 is a drawing for showing the structure of the rotational protective ring in the fifth embodiment of the present invention. 
     FIG. 11 is a sectional view of a protective cap, the internal surface which is given a curvature in terms of the thrust direction. 
     FIG. 12 is a sectional view of a thermal fixing apparatus, in which the lengthwise end portion of the fixation nip overlaps with the protective cap. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     &lt;Embodiment 1&gt; 
     (1) Example of Image Forming Apparatus 
     FIG. 1 is a schematic sectional view of an example of an image forming apparatus, for showing the general structure thereof. The image forming apparatus in this embodiment is a transfer type laser beam printer which employs an electrophotographic image formation process. 
     A referential code  1  designates a photoconductive drum, which comprises a cylindrical base formed of aluminum or nickel, and a layer of photoconductive substance such as OPC, amorphous selenium, amorphous silicon, or the like, formed on the peripheral surface of the cylindrical base. 
     The photoconductive drum  1  is rotationally driven at a predetermined peripheral velocity in the clockwise direction indicated by an arrow mark. As it is rotated, first, its peripheral surface is uniformly charged to predetermined polarity and potential level by a charge roller  2  as a charging apparatus. 
     Next, the uniformly charged peripheral surface of the photoconductive drum  1  is exposed to a laser beam  3   a,  which is emitted in a manner to scan the peripheral surface of the photoconductive drum  1 , from a laser scanner  3  while being turned on or off in accordance with the image formation data. As a result, an electrostatic latent image is formed on the peripheral surface of the photoconductive drum  1 . 
     This electrostatic latent image is developed (visualized) into a toner image by a developing apparatus. As for the method for developing an electrostatic latent image, there are the jumping developing method, two-component developing method, FEED developing method, and the like. In many cases, these developing methods are used as means for developing, in reverse, an electrostatic latent image formed by exposure. 
     A toner image, or a visual image, is transferred by a transfer roller  5 , as a transferring apparatus, from the photoconductive drum  1  onto a recording medium P, which is delivered, with a predetermined timing, to the transfer station. 
     More concretely, in order to align the theoretical line on the peripheral surface of the photoconductive drum  1 , from which the formation of a latent image begins, with the theoretical line on the recording medium P, which will coincide with the leading edge of a toner image, in terms of the recording medium conveyance direction, after the transfer of the toner image onto the recording medium P, the arrival of the leading edge of the recording medium P is detected by a sensor  8 . After being delivered to the transfer station with the predetermined timing, the recording medium P is conveyed through the transfer station, while being sandwiched by the peripheral surfaces of the photoconductive drum  1  and transfer roller  5 . 
     After the transfer of the toner image onto the recording medium P, the recording medium P is conveyed to a thermal fixing apparatus  6 , in which the toner image is thermally fixed to the recording medium P; the toner image is turned into a permanent image. 
     Meanwhile, the residual toner particles, that is, the toner particles remaining on the peripheral surface of the photoconductive drum  1  after the toner image transfer, are removed from the peripheral surface of the photoconductive drum  1  by a cleaning apparatus. 
     (2) Thermal Fixing Apparatus  6   
     FIG. 2 is a sectional view of the thermal fixing apparatus  6 , at a plane perpendicular to the lengthwise direction of the apparatus  6 . FIG. 3 is a schematic front view of the thermal fixing apparatus  6 , in which certain portions have been intentionally left out in order to show the interior thereof. 
     Designated by a referential code  10  is a fixing member (heater assembly) as a heating member, and designated by a referential code  20  is an elastic pressure roller as a pressure applying member. The fixing member  10  and pressure roller  20  are disposed so that their peripheral surfaces are kept pressed against each other, forming a nip, or a fixation nip N, in which a recording medium is heated. 
     The fixing member  10  comprises a heater  11  as a heating member, a thermally nonconductive stay/holder  12 , a metallic sleeve  13  (fixation sleeve), a sleeve end protection cap  15 , and the like. 
     The heater  11  is secured to the bottom surface of the thermal nonconductive stay/holder  12 . The metallic sleeve  13  is loosely fitted around the thermally nonconductive stay/holder  12 . The protective cap  15  is fitted around the lengthwise end of the thermally nonconductive stay/holder  12 , to regulate the edge of the metallic sleeve  13  (both edges of the metallic sleeve  13  are fitted a pair of the protective caps  15 , one for one). In other words, the protective cap  15  is a regulating member for regulating the movement of the metallic sleeve  13  in its lengthwise direction. 
     The elastic pressure roller  20  as a pressure applying member is made up of a metallic core  21 , and an elastic layer  22  formed of heat resistant rubber, for example, silicon rubber and fluorinated rubber, or foamed silicon rubber, around the peripheral surface of the metallic core  21 . It may also be provided with a releasing layer  23  formed of PFA, PTFE, FEP, or the like, which is coated over the elastic layer  22 . 
     Designated by a referential code  30  are the front and rear walls of the boxy frame of the fixing apparatus. The pressure roller  20  and fixing member  10  are placed in the boxy frame, being held between these walls  30 . More specifically, each of the front and rear walls  30  of the boxy frame is provided with a recess (FIGS. 4 and 5) with a width of Lb, the deepest end of which is fitted with a bearing  32  formed of heat resistant resin such as PEEK, PPS, liquid crystal polymer, or the like. The pressure roller  20  is rotationally borne by the bearing  32 , with the end portion of the metallic core  21  of the pressure roller  20  fitted in the groove of the bearing  32 . 
     As will be described later, the fixing member  10  is disposed between the front and rear walls  30  of the boxy frame, being located on the top side of the pressure roller, with the lengthwise outward end portions  15   c  of the protective caps  15  fitted in the grooves  31  of the front and rear walls  30  of the boxy frame, one for one. 
     Further, at each longitudinal end of the fixing member  10 , a compression spring  17  is disposed in the compressed state between the spring seat portion  15   d  of the protective cap  15 , and a stationary spring seat  40 . The resiliency of the compression springs  17  generates a predetermined amount of pressure, keeping the fixing member  10  pressed upon the peripheral surface of the pressure roller  20 , on the top side, against the elasticity of the metallic sleeve  13  and the elasticity of the pressure roller  20 . Therefore, a nip, or the fixation nip N, with a predetermined width, is formed and maintained between the pressure roller  20  and the fixing member  10 . In the fixation nip N, in which the fixing member  10  and pressure roller  20  are kept pressed against each other, the metallic sleeve  13  is sandwiched between the heater  11  and elastic pressure roller  20 , being deformed in a manner to conform to the flat bottom surface of the heater  11 , with the inward surface of the metallic sleeve  13 , in terms of the radius direction of the metallic sleeve  13 , contacting the flat bottom surface of the heater  11 , with no gap between the two surfaces. 
     Designated by a referential code G is a drive gear attached to one of the lengthwise ends of the metallic core  21  of the pressure roller  20 . As the driving force from a driving portion M is transmitted to this driving gear G, the pressure roller  20  is rotationally driven in the counterclockwise direction indicated by an arrow mark in FIG. 2, at a predetermined peripheral velocity. As the pressure roller  20  is rotationally driven, friction occurs between the pressure roller  20  and the metallic sleeve  13  of the fixing member  10 , in the fixation nip N. As a result, the metallic sleeve  13  is rotationally driven by the friction, in the clockwise direction in FIG. 2, around the thermally nonconductive stay/holder  12 , with the internal surface of the metallic sleeve  13  sliding on the bottom surface of the heat  11 ; the metallic sleeve  13  is driven by the rotation of the pressure roller  20  (pressure roller driving system). 
     Since the metallic sleeve rotates while sliding on the heater  11  and thermally nonconductive stay/holder  12  within the loop of the metallic sleeve, it is necessary to reduce the friction between the fixation film  13  and the heater  11 , and the friction between the fixation film  13  and thermally nonconductive stay/holder  12 . For this reason, the bottom surface of the heater  11 , and the outwardly facing surface of the thermally nonconductive stay/holder  12 , in terms of the radius direction of the fixation film  13 , are coated with a small amount of lubricant, such as heat resistant grease or the like, making it possible for the metallic sleeve  13  to smoothly rotate. 
     The heater  11  heats the fixation nip N, in which a toner image T on the recording medium P is fixed to the recording medium P by being melted. 
     While the metallic sleeve  13  is rotated by the rotation of the pressure roller  20 , and after the heater temperature has been raised to a predetermined level by the power supplied to the heater  11 , the recording medium P bearing an unfixed toner image T is conveyed between the metallic sleeve  13  and pressure roller  20 , in the fixation nip N, being guided by the fixing apparatus entrance guide  24 . While the recording medium P is conveyed through the fixation nip N, the unfixed toner image T on the recording medium P is fixed to the recording medium P by being heated by the heat from the heater  11 , through the metallic sleeve  13 . After passing through the fixation nip N, the recording medium P separates from the peripheral surface of the metallic sleeve  13 , and is discharged into an unshown delivery tray, being guided by an unshown heat resistant exit guide of the fixing apparatus. 
     a) Metallic Sleeve  13   
     The metallic sleeve  13  is a sleeve with a small thermal capacity. In order to enable the metallic sleeve  13  to quickly start up, the base layer of the metallic sleeve  13  is formed of heat resistant and highly thermally conductive metallic substance, such as aluminum, nickel, copper, zinc, or the like, or metallic alloy, such as stainless steel, or the alloys of the preceding metals. The overall thickness of the metallic sleeve  13  is made to be no more than 200 μm. On the other hand, in order to make the metallic sleeve  13  strong and durable enough to realize a durable thermal fixing apparatus, the overall thickness of the metallic sleeve  13  needs to be no less than 30 μm. Thus, the overall thickness of the metallic sleeve  13  is desired to be no less than 30 μm and no more than 200 μm. 
     In addition, in order to prevent offset and to ensure that recording medium is properly released from the peripheral surface of the metallic sleeve  13 , the metallic sleeve  13  is provided with a surface layer formed of heat resistant resin superior in releasing properties, such as fluorinated resin or silicon resin. As for the examples of fluorinated resin, there are PTFE (polytetrafluoroethylene), PFA (copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether), FEP (copolymer of tetrafluoroethylene and hexafluoropropylene), FTFE (copolymer of ethylene and tetrafluoroethylene), CTFE (polychlorotrifluoroethylene), and PVDF (polyvinylidene fluoride). The surface layer may be formed of the mixture of the above listed resins. As for the coating method, first, the external surface of the base member of the metallic sleeve  13  is etched, and then, is painted with one of the aforementioned resins to formed the releasing layer, by dipping, power spray, or the like method. Instead of coating, a tube formed of one of the aforementioned resins may be slipped over the base member of the metallic sleeve  13 . Further, after blasting the external surface of the metallic base member, the releasing layer may be placed thereon by painting, after the base member is painted with primer, that is, adhesive. 
     Further, the internal surface of the metallic sleeve  13 , which makes contact with the heater  11 , may be covered with a layer of highly lubricous substance such as fluorinated resin, polyimide, polyamide, or the like. 
     b) Heater  1   
     The heater  11  heats the fixation nip N, in which the toner image T on the recording medium P is permanently fixed to the recording medium P by being melted. It generates heat as electrical current is flowed through it. It comprises a substrate, and a layer of heat generating conductor. The substrate is formed of electrically highly insulative ceramic such as alumina, aluminum nitride, or the like, or formed of heat resistant resin such as liquid crystal polymer or the like. The heat generating layer is coated on the surface of the heat resistant substrate, and is formed of heat generating conductor such as Ag/Pd (silver/palladium alloy), RuO 2 , Ta 2 N, or the like, by screen printing or the like method. It is approximately 10 μm in thickness, and approximately 1-5 mm in width. 
     The heater  11  may be a metallic heater comprising a metallic substrate, an insulative layer, and a heat generating conductor layer, wherein the insulative layer and heat generating layer are placed in layers on the surface of the metallic substrate, on the side opposite to where the fixation nip is formed, in the listed order. In this case, the metallic substrate may be incurvated on the fixation nip side. 
     Electric power is supplied to the heat generating conductor layer from an unshown power supplying portion, through an unshown connector. On the back side of the heater substrate, a heat detection element  14  such as a thermistor for detecting the temperature of the heater, the temperature of which is increased by the heat generated by the heat generating conductor layer, is disposed. The duty ratio, frequency, and the like, of the voltage applied to the heat generating conductor layer through unshown electrode portions located at the lengthwise ends of the heater, are properly controlled according to the signals from this temperature detection element  14 . This is for keeping constant the internal temperature of the fixation nip N so that the heat necessary for fixing the toner image T on the recording medium P is given to the recording medium P. The DC current from the temperature detection element  14  is flowed to an unshown temperature control portion through an unshown connector, with the interposition of an unshown current flowing portion and DC electrodes. 
     Further, the heat generating conductor layer of the heater  11  is coated, on the fixation nip side, with a thin protective layer formed of glass, fluorinated resin, or the like substance, in order to make the layer resistant to the rubbing by the metallic film. 
     Incidentally, when aluminum nitride, or the like, which is superior in abrasion resistance and thermal conductivity, is used as the material for the substrate, the surface of the substrate on which the heat generating conductor layer is present, may be on the side opposite to the fixation nip N. 
     In order to reduce the amount of the bending stress to which the metallic sleeve  13  is subjected, the fixation nip side of the heater  11  may be incurvated so that its center portion, in terms of the recording medium conveyance direction, recesses toward the center of the metallic sleeve  13 . With the provision of this structural arrangement, the fixing member would be further improved in durability. 
     c) Thermally Nonconductive Stay/holder  12   
     The thermally nonconductive stay/holder  12  plays the roles of supporting the heater  11 , guiding the rotation of the metallic sleeve  13 , and pressing the pressure roller  20 , the role of a thermally insulating member for preventing heat from radiating in the direction opposite to the fixation nip N, or the like role. It is rigid, heat resistant, and thermally nonconductive. It is formed of liquid crystal polymer, phenol resin, PPS, PEEK, or the like. 
     d) Sleeve Edge Protection Member 
     The sleeve edge protection member  15  as a regulating member regulates the metallic sleeve  13  at the lengthwise (direction parallel to the generatrix of the metallic sleeve  13 ) edges. It is provided with a means for preventing the edge portions of the metallic sleeve  13  from being spread, that is, a means for reinforcing the edge portions of the metallic sleeve  13 . 
     The sleeve edge protection member  15  in this embodiment is in the form of a cap and is nonrotational; it is of a stationary type. In FIG. 6, (a) is an external view; (b) is an internal view; (c) is a side view; (d) is a plan view; and (e) is a sectional view. 
     Referring to FIG.  6 ( e ), the metallic sleeve  13  has an external diameter of Lf. The portion of the peripheral surface of the metallic sleeve  13 , adjacent to each lengthwise edge of the metallic sleeve  13 , makes contact with the internal surface of the cylindrical portion of the protective cap  15 . A referential code D stands for the width of this protective member contacting portion of the metallic sleeve  13 . 
     The protective cap  15  is formed of a heat resistant substance such as PPS, liquid crystal polymer, phenol resin, or the like. It has a sleeve end confining portion  15   a,  which has an internal diameter of Lc, which is slightly greater than the external diameter Lf of the metallic sleeve  13 . The aforementioned portion of the metallic sleeve  13  with the width of D fits in this sleeve end confining portion  15   a.  As the portion of the metallic sleeve  13  with the width of D, is inserted into the sleeve end confining portion  15   a  of the protective cap  15  with the internal diameter of Lc, the edge of the metallic sleeve  13  comes into contact with the sleeve catching portion  15   b  of the protective cap  15 . 
     The sleeve catching portion  15   b  is tapered so that its internal diameter gradually reduces toward the outward side of the metallic sleeve  13  in terms of the lengthwise direction of the metallic sleeve  13 . The internal diameter Ls of the outward edge of the sleeve catching portion, that is, the point at which the internal diameter of the sleeve catching portion is smallest, is sufficiently smaller than the external diameter Lf of the metallic sleeve  13 . Incidentally, the internal surface of the sleeve catching portion  15   b  may be given such a curvature as shown in FIG.  11 . 
     The lateral wall of the protective cap  15  is provided with a substantial gap, which faces the fixation nip N. It has two sections different in diameter: section with a diameter of Lc and section with a diameter of Ls which is smaller than Lc. This configuration in which the lateral wall is provided with the above described substantial gap facing the fixation nip N is suitable for a stationary protective cap  15 , that is, a protective cap  15  which does not rotate in its circumferential direction. In other words, the aforementioned problem regarding the interference between the protective cap  15  and heater  11  can be easily solved by providing the lateral wall of the stationary protective cap  15  with a substantial gap which is proper in size and position. The provision of the gap allows the sleeve end confining portion  15   a  of the protective cap  15  to overlap with the lengthwise end portions of the fixation nip N (FIG.  12 ). Thus, the distance between the two protective cap  15  can be reduced, which makes it possible to reduce the dimension of a heating apparatus in terms of its lengthwise direction. 
     As for the means for immovably securing the protective cap  15 , any means may be used as long as it immovably secure the protective cap  15 . Incidentally, the spring seat portion  15   d  of the protective cap  15 , which catches the pressure generated by the resiliency of the compression spring  17  may be independent from the protective cap  15 . 
     The stationary type protective cap  15  has an anchoring portion  15   c  with a width of La, which is located between the protective portion  15   a  and a second regulating portion  15   e,  and is fitted in the recess of the wall  30  of the boxy frame of the apparatus. The spring seat portion  15   d  has a spring regulating portion  15   f,  which regulates the position of the compression spring  17  provided to generate a predetermined amount of contact pressure between the fixing member  10  and pressure roller  20 . 
     The stationary protective cap  15  is provided with a substantial gap, the position of which corresponds to that of the fixation nip N. Further, the heater  11 , and the heater supporting stay portion  12   a  (FIG. 3) of the thermally nonconductive stay/holder  12 , are disposed in such a manner that they align with the substantial gap of the stationary protective cap  15 , in terms of the direction perpendicular to the recording medium conveyance direction. Thus, the stationary protective cap  15  does not interfere with the heater  11 , at the lengthwise ends of the heater  11 . 
     Referring to FIG. 5, the aforementioned anchoring portion  15   c  of the stationary protective cap  15  which constitutes the lengthwise end portion of the fixing member  10  is fitted in the recess  31  of the front (rear) wall  30  of the boxy frame of the fixing apparatus. 
     As a result, the fixing member  10  becomes supported by the front (rear) wall  30  of the boxy frame of the fixing apparatus. The width Lb of the recess of the front (rear) wall  30  of the boxy frame of the fixing apparatus is virtually the same as, or slightly greater than the width La of the anchoring portion  15   c  (FIG.  6 ( d )). Therefore, the anchoring portion  15   c  of the stationary protective cap  15  can be smoothly fitted in the recess of the front (rear) wall  30  of the boxy frame of the fixing apparatus. Therefore, the shifting of the protective cap  15  in the direction parallel to the thrust direction of the metallic sleeve  13  is prevented by the boxy frame of the fixing apparatus. 
     As a result, the shifting of the metallic sleeve  13  in its thrust direction is regulated by the pair of stationary protective caps  15 . 
     Each protective cap  15  catches the metallic sleeve  13  by the peripheral surface and edge, with the portions  15   a  and  15   b,  confining the end portion of the metallic sleeve  13 , with the internal surfaces of the portion  15   a  and  15   b,  preventing thereby the end portion of the metallic sleeve  13  from widening in the form of the end of a trumpet. Therefore, the metallic sleeve  13  is prevented from breaking at its lengthwise edges. 
     &lt;Embodiment 2&gt; 
     In this embodiment, the sleeve end protection member as a regulating member is of a rotational type. FIG. 7 is a schematic front view of a thermal fixing apparatus  6 , the sleeve end protection member of which is rotational. In the drawing, the portions of the fixing apparatus  6  nonessential to the description of this embodiment have been left out. The structural members and portions of the fixing apparatus  6  in FIG. 7, which are similar to those of the fixing apparatus in FIG. 1, are given the same referential codes as those in FIG. 1, to avoid the repetition of the same descriptions. 
     The sleeve end protection member  15  (first regulating member) for regulating the metallic sleeve  13  by the lengthwise edge is in the form of a wide ring. It is rotationally fitted around the inward portion, or the protective member supporting portion, of a sleeve end flange  15 A (second regulating member). 
     FIG.  8 ( a ), FIG.  8 ( b ), FIG.  8 ( c ), FIG.  8 ( d ), and FIG.  8 ( e ) are an external view, an internal view, a side view, a plan view; and a sectional view, respectively, of the combination of the sleeve end flange  15 A and protective ring  15 . 
     Referring to FIG.  8 ( e ), the metallic sleeve  13  in this embodiment has an external diameter of Lf. The end portion of the peripheral surface of the metallic sleeve  13  contacts the internal surface of the rotational protective ring  15 . This portion of the peripheral surface has a width of D in terms of the lengthwise direction of the metallic sleeve  13 . 
     The rotational protective ring  15  is formed of a heat resistant substance such as PPS, liquid crystal polymer, phenol resin, or the like. It has a sleeve end confining portion  15   a,  which has an internal diameter of Lc, which is slightly greater than the external diameter Lf of the metallic sleeve  13 . The aforementioned portion of the metallic sleeve  13  with the width of D fits in this sleeve end confining portion  15   a.  As the portion of the metallic sleeve  13  with the width of D is inserted into the sleeve end confining portion  15   a  of the rotational protective ring  15  with the internal diameter of Lc, the edge of the metallic sleeve  13  comes into contact with the sleeve catching portion  15   b  of the rotational protective ring  15 . The sleeve catching portion  15   b  is tapered so that its internal diameter gradually reduces from the metallic sleeve side toward the outward side of the metallic sleeve  13  in terms of the lengthwise direction of the metallic sleeve  13 . The internal diameter Ls of the outward edge of the sleeve catching portion, that is, the point at which the internal diameter of the sleeve catching portion  15   b  is smallest, is sufficiently smaller than the external diameter Lf of the metallic sleeve  13 . In other words, the rotational protective ring  15  has two sections different in diameter: Section with a diameter of Lc and section with a diameter of Ls which is smaller than Lc. Incidentally, the internal surface of the sleeve catching portion  15   b  may be given such a curvature as shown in FIG.  11 . 
     When the rotational protective ring  15  is employed in place Of the stationary protective cap  15  in the first embodiment, the rotational protective ring  15  is provided with a flange engaging portion  15   g,  which has the internal diameter of Ls, which is the same as that of the smallest internal diameter portion of the sleeve catching portion  15   b.  The sleeve end flange  15 A is formed of the same heat resistant resin as the one used for the rotational protective ring  15 . The sleeve end flange  15 A comprises a rotational protective ring supporting portion  15   h  with a diameter of Le, which is smaller than the internal diameter Ls of the flange engaging portion  15   g  of the rotational protective ring  15 . Thus, the flange engaging portion  15   g  of the sleeve end flange  15 A can be loosely fitted around the rotational protective ring supporting portion  15   h  of the sleeve end flange  15 A, being thereby rotationally supported by the sleeve end flange  15 A. Further, the sleeve end flange  15 A is provided with a regulating portion  15   i,  which regulates the shifting of the rotational protective ring  15  in its thrust direction. 
     The sleeve end flange  15 A, around which the rotational protective ring  15  is loosely fitted, is provided with an anchoring portion  15   c  with a width of La, which is located between the regulating portion  15   i  and second regulating portion  15   e.  The spring seat portion  15   d  has a spring regulating portion  15   f,  which regulates the position of the compression spring  17  provided to generate a predetermined amount of contact pressure between the fixing member  10  and pressure roller  20 . 
     The portion of the sleeve end flange  15 A, which would interfere with the fixation nip N if the sleeve end flange  15 A were virtually cylindrical, has been removed. Thus, the sleeve end flange  15 A does not interfere with the heater  11 , and the heater supporting stay  12   a  of the thermally nonconductive stay/holder  12 , at their lengthwise ends. 
     The anchoring portion  15   c  of the sleeve end flange  15 A, which constitutes the lengthwise end portion of the fixing member  10  is fitted in the recess  31  of the front (rear) wall  30  of the boxy frame of the fixing apparatus. As a result, the fixing member  10  becomes supported by the front (rear) wall  30  of the boxy frame of the fixing apparatus. Therefore, the shifting of the sleeve end flange  15 A in the direction parallel to the thrust direction of the metallic sleeve  13  is prevented by the boxy frame of the fixing apparatus. As a result, the shifting of the metallic sleeve  13  in its thrust direction is regulated by the pair of sleeve end flanges  15 A and the pair of rotational protective rings  15 . 
     Next, referring to FIGS.  9 ( a ) and  9 ( b ), the behaviors of the portions of the metallic sleeve  13  correspondent in position to the lengthwise center and edge portions of the fixing member  10 , and the behaviors of the heater and the like, will be described with reference to the rotational protective ring  15 . FIG.  9 ( b ) is a sectional view of the heating apparatus, at the lengthwise center portion (plane B—B in FIG. 7) of the fixing member  10 , and shows the relationship among the various components thereof. As a given portion of the metallic sleeve  13  in terms of the circumferential direction enters between the pressure roller  20  and heater  11 , it is deformed in a manner to perfectly conform to the surface of the heater  11 , by the compressive pressure which acts on the metallic sleeve  13 , between the pressure roller  20  and heater  11 , as shown in the drawing. The rest of the metallic sleeve  13 , that is, the portion outside the fixation nip N, acts in a manner to keep the metallic sleeve  13  cylindrical. 
     Next, referring to FIG.  9 ( a ), the behavior of the end portions of the metallic sleeve  13  outside the range of the pressure roller  20  (which is correspondent to the plane A—A in FIG.  7 ), in terms of the lengthwise direction, and the members in the adjacencies thereof, will be described. The metallic sleeve  13  is relatively high in rigidity. Therefore, as the lengthwise center portion of the metallic sleeve  13  deforms in a certain way, the lengthwise end portions of the metallic sleeve  13  deform in the same way. Thus, as a given center portion of the metallic sleeve  13  in terms of the circumferential direction enters the fixation nip N due to the rotation of the metallic sleeve  13 , the portion of each lengthwise end portions of the metallic sleeve  13  correspondent in position to this portion in the fixation nip N, also deforms in the same way. 
     On the other hand, the portion of rotational protective ring  15  forms of resin, more specifically, the sleeve end confirming portion  15   a  of the rotational protective ring  15 , the internal surface of which contacts a portion of the peripheral surface of the end portion of the metallic sleeve  13 , in terms of the circumferential direction, remains cylindrical. Therefore, the portion of each lengthwise end portion of the metallic sleeve  13 , correspondent in position to the portion of the metallic sleeve  13  in the fixation nip N, in terms of the circumferential direction, remains separated from the internal surface of the sleeve end confining portion  15   a  of the sleeve end flange  15 A. Thus, the rotational protective ring  15  can be supported by the sleeve end flange  15 A in such a manner that the rotational protective ring  15  holds a certain distance from the heater surface. With this arrangement, the rotational protective ring  15  does not interfere with the heater  11 , and the heater supporting stay  12   a.    
     The rotational protective ring  15  is rotationally supported by the sleeve end flange  15 A. Therefore, the rotational protective ring  15  is made to rotate with the metallic sleeve  13  by the friction between a given range of the peripheral surface of the end portion of the metallic sleeve  13 , in terms of the circumferential direction, and the portion of the internal surface of the sleeve end confining portion  15   a  which is in contact with this range of the metallic sleeve  13 , and the friction between the edge of the metallic sleeve  13  and the internal surface of the sleeve edge catching portion  15   b  of the rotational protective ring  15 . Thus, the elastically deformed portion of the metallic sleeve  13  never rubs against a stationary member, drastically prolonging the time it takes for the problems such as the tearing of the edge portion of the metallic sleeve  13  to occur. 
     &lt;Embodiment 3&gt; 
     A thermal fixing apparatus comprising a stationary sleeve protective member such as the protective cap in the first embodiment, and a thermal fixing apparatus comprising a rotational sleeve protective member such as the protective ring in the second embodiment, were evaluated in terms of fixing performance, startup speed, and durability. 
     The basic structural arrangement common among the actual thermal fixing apparatuses used in the tests is as follows: As for the heater  11 , aluminum nitride was used as the material for the heater substrate. As for the heat generating conductor layer, a mixture of silver/palladium allay as a conductive component, phosphate glass as a matrix component, organic solvent, binder, dispersant, and the like, was printed on the heater substrate surface, on the side opposite to the fixation nip N, by screen printing, and then, was sintered at 400° C. The surface of the aluminum nitride substrate, on the fixation nip N side, was coated with a 10 μm thick protective layer of slippery glass, by screen printing. 
     The metallic sleeve  13  comprised a 50 μm thick stainless steel cylinder, as a base member, with an internal diameter of 30 mm, a 5 μm thick layer of primer, and a 10 μm thick layer of PFA resin coated on the prior layer by dipping. The external diameter of the finished metallic sleeve  13  was 30.13 mm. 
     The sleeve end protection members  17  were formed of liquid crystal polymer. The internal diameter of the portion of the protective member  17 , which makes contact with the peripheral surface of the metallic sleeve  13  was 31.6 mm at where it was smallest. Two types of protective members  17  were prepared: stationary type (Test A), and rotational type (Test B) rotationally attached to the sleeve end flange. 
     As for the pressure roller  20 , the peripheral surface of an aluminum core with a diameter of 20 mm was covered with a 5 mm thick layer of silicon rubber, and then, was wrapped with a tube of PFA as a surface layer. 
     In the tests, image forming apparatuses were adjusted so that the recording medium conveyance speed became 200 mm/sec. 
     In Comparative Test 1, a fixing film, such as the one described regarding the fixing apparatus in accordance with the prior arts, which had a polyimide film as the base layer, was used in place of the metallic sleeve  13 . In order to ensure that the fixing film was satisfactory in terms of thermal conductivity, the fixing film was made using the following method: The 50 μm thick base layer was formed of polyimide containing BN filler by 30% in volume. This base layer was coated with a 5 μm thick primer layer. Then, a 10 μm thick layer of PFA resin was coated on the primer layer by dipping. The finished fixing film was virtually identical in shape and external diameter to the metallic sleeve  13 . The shifting of the fixing film in its thrust direction was regulated using a method in accordance with the prior arts; it was regulated by the surface of a sleeve end flange, which was perpendicular to the lengthwise direction of the sleeve. As for the driving of the fixing film, the pressure roller was driven, and the fixing film was made to follow the rotation of the pressure roller. 
     In Comparative Test 2, the metallic sleeve  13  was used, and the shifting of the metallic sleeve  13  in its thrust direction was regulated using a method in accordance with the prior arts; it was regulated by the surface of a sleeve end flange, which was perpendicular to the lengthwise direction of the sleeve. 
     In order to verify the fixing apparatus performance, the fixing apparatuses were tested regarding the following aspects: 
     1) fixing performance: an unfixed toner image was formed on a recording medium, and the temperature at which the unfixed toner image could be satisfactorily fixed was measured while varying the heater temperature. 
     2) startup speed: the temperature of the fixing nip was measured three second after power began to be supplied to the heat generating conductive layer of the heater. 
     3) durability: the number of cut sheets of paper, as recording media, to which an unfixed toner image was satisfactorily fixed before the damages to the fixing film or metallic sleeve was confirmed, was counted. 
     The results of the tests were shown in Table 1. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Required 
                 Nip temp. 
                 No. of 
               
               
                   
                 temp. 
                 after 3 sec. 
                 sheets 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 EMB. A 
                 190° C. 
                 182° C. 
                 600,000-1,000,000 
               
               
                 EMB. B 
                 190° C. 
                 182° C. 
                 ≧1,000,000 
               
               
                 COMP. 
                 213° C. 
                 169° C. 
                 500,000-700,000 
               
               
                 EX. 1 
               
               
                 COMP. 
                 190° C. 
                 182° C. 
                 5,000-50,000 
               
               
                 EX. 2 
               
               
                   
               
            
           
         
       
     
     As is evident from the above table, with the employment of the metallic sleeve  13 , which was superior in thermal conductivity to the fixing film formed of resin, the thermal efficiency could be drastically improved. The metallic sleeve  13  was superior to the resin fixing film also in startup speed, reducing the time it took before the first print was made. 
     Regarding the durability, compared to the fixing apparatus in Comparative Test 2, the fixing apparatus in Tests A and B, in which the metallic sleeve  13  superior in rigidity was used, and the shifting of the metallic sleeve  13  in its thrust direction was regulated by the protective member  17  in accordance with the present invention, was far superior in durability, for the following reason. That is, with the provision of the protective member  17  in accordance with the present invention, as the metallic sleeve  13  shifted in its thrust direction, each lengthwise end portion of the metallic sleeve  13  was caught with the internal surface of the sleeve end confining portion  15   a  of the protective member  17 , by the peripheral surface, and each edge of the metallic sleeve  13  was caught by sleeve catching portion  15   b  of the protective member  17 . Therefore, the load, which acted in the direction to widen the lengthwise end portion of the metallic sleeve  13  as the edge of the fixing film contacted the protective member  17 , was cancelled by the sleeve end confining portion  15   a  and sleeve end catching portion  15   b  of the protective member  17 . 
     As is evident from Comparative Tests  1  and  2 , as the edge of the metallic sleeve higher in rigidity was caught by the flat surface of the sleeve end flange, which was perpendicular to the lengthwise direction of the sleeve, and the lengthwise end portion of the metallic sleeve was caught with the sleeve confining portion of the sleeve end flange, by the internal surface, the end portion of the metallic sleeve was subjected to such load that acted in the direction to expand the end portion. Thus, the metallic sleeve, which was higher in rigidity was less durable than the flexible resin fixing film. 
     In the case of Test B in which the protective ring in the second embodiment was rotationally attached to the sleeve end flange, the rotational protective ring  15  was caused to rotate following the rotation of the metallic sleeve (end portion). Therefore, it did not occur that the edge of the metallic sleeve was subjected to the large load generated by the friction between the edge of the metallic sleeve and the sleeve end flange in accordance with the prior arts. In other words, with the provision of the structural arrangement in the second embodiment, the edge of the metallic sleeve is far less likely to tear, drastically improving the durability of the metallic sleeve. 
     In the case of Test A in which the stationary protective cap was employed as a sleeve end protecting member, the stationary protective cap doubled as a sleeve end flange, making it possible to reduce the lengthwise dimension of the fixing apparatus. In other words, the employment of the protective cap in the first embodiment does not invite increase in the size of a thermal fixing apparatus. 
     &lt;Embodiment 4&gt; 
     Next, the fourth embodiment of the present invention will be described. The general structure of the image forming apparatus in this embodiment is the same as that in the first embodiment shown in FIG.  1 . Further, the structure of the thermal fixing apparatus in this embodiment is the same as that in the second embodiment, in which the rotational protective ring was used as a sleeve end protecting member. Therefore, their descriptions will not be given here. 
     In this embodiment, the frictional resistance of the peripheral surface of the metallic sleeve was made greater across the end portions of the metallic sleeve which contact the rotational protective ring  15 , than across the center portion which did not contact the rotational protective ring  15 . As a result, the peripheral velocity at which the rotational protective ring  15  rotated following the rotation of the metallic sleeve  13  became much closer to the peripheral velocity at which the metallic sleeve  13  rotated. In other words, with this arrangement, it is possible to provide a highly durable high speed fixing apparatus. 
     More specifically, in this embodiment, the frictional resistance of the peripheral surface of the lengthwise end portions of the metallic sleeve  13 , which had the width of D, that is, the end portions of the metallic sleeve  13 , which were inserted into the sleeve end sleeve end confining portion  15   a,  was made greater than that of the lengthwise center portion of the metallic sleeve  13 , which contacted the recording medium on which an unfixed toner image had been formed. 
     To described in more detail, the end portions of the metallic sleeve  13  having the width of D (in lengthwise direction) were not provided with the aforementioned releasing layer; only the center portion of the metallic sleeve  13  was provided with the releasing layer. In other words, across the lengthwise end portions, the base member of the metallic sleeve  13 , or the primer layer, was left exposed; heat resistant fibers were planted; or heat resistant substance greater in frictional resistance than the releasing layer was painted or sprayed. 
     With the provision of the above described arrangement, as the metallic sleeve  13  rotated following the rotation of the pressure roller  20 , the rotational protective ring  15  was rotated at a peripheral velocity far closer to the peripheral velocity of the metallic sleeve  13  than otherwise, due to the increase in the friction between the internal surface of the rotational protective ring  15 , which contacted the peripheral surface of the metallic sleeve  13  due to the deformation of the metallic sleeve  13 , and the peripheral surface of the metallic sleeve  13 , at the lengthwise ends of the metallic sleeve  13 . 
     In particular, when the friction coefficient μ between the peripheral surface of the metallic sleeve  13  and the internal surface of the rotational protective ring  15 , and the friction coefficient μ′ between the internal surface of the rotational protective ring  15 , and the sleeve end flange  15 A, satisfy the following relationship: μ&gt;&gt;μ′, in other words, the greater the difference between μ and μ′, the closer to the peripheral velocity of the metallic sleeve  13  the peripheral velocity of the rotational protective ring  15 . 
     In other words, with the provision of the above described arrangement, the friction between the edge of the metallic sleeve  13  and the sleeve end catching portion  15   b  of the rotational protective ring  15  is further reduced, improving therefore the durability of the metallic sleeve  13 . 
     In order to confirm the above discovery, the following test was carried out. The apparatus used for this test was the same as the one used in the third embodiment, except that the surface condition of the lengthwise end portions was varied. 
     Evaluated in this test Were the ratio of the rotation of the rotational protective ring  15  to that of the metallic sleeve  13  (100% when two component are rotating at the same peripheral velocity), and the durability of the metallic sleeve  13  measured in terms of the number of copies successfully made before the damages began to appear at the edges of the metallic sleeve  13 . 
     The results of the test are given in Table 2. The tested surface conditions of the lengthwise end portions of the metallic sleeve  13  were: when the bare surface of the metallic base member was exposed across the lengthwise end portions of the metallic sleeve  13 ; when the bare surface of the primer layer was exposed across the lengthwise end portions of the metallic sleeve  13 ; and when the lengthwise end portions of the metallic sleeve  13  were covered with the same releasing layer as the one covering the lengthwise center portion of the metallic sleeve  13 . The friction coefficient between the rotational protective ring  15  and sleeve end flange  15 A was kept at 0.3. 
     The friction coefficient in Table 2 represents the friction coefficient between the metallic sleeve  13  and the rotational protective ring  15 . 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Bare 
                 Bare 
                 Releasing 
               
               
                   
                 metal 
                 primer 
                 layer 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Friction 
                 1.5 
                 0.8 
                 0.4 
               
               
                 coefficient 
               
               
                 Rotation 
                 98% 
                 90% 
                 75% 
               
               
                 ratio 
               
               
                 No. of 
                 ≧3,000,000 
                 2,500,000-3,000,000 
                 1,000,000-1,500,000 
               
               
                 sheets 
               
               
                   
               
            
           
         
       
     
     As is evident from Table 2, the greater the difference between the friction between the lengthwise end portions of the metallic sleeve  13  and the rotational protective ring  15 , and the friction between the rotational protective ring  15  and sleeve end flange  15 A, the closer to the peripheral velocity of the metallic sleeve  13  the peripheral velocity of the rotational protective ring  15 , and also the more durable the metallic sleeve  13 . 
     &lt;Embodiment 5&gt; 
     Next, the fifth embodiment of the present invention will be described. The general structure of the image forming apparatus in this embodiment is the same as that in the first embodiment shown in FIG.  1 . Further, the structure of the thermal fixing apparatus in this embodiment is the same as that in the second embodiment, in which the rotational protective ring was used as a sleeve end protecting member. In this embodiment, however, the measurement of the rotational protective ring in terms of the lengthwise direction of the metallic sleeve was reduced to prevent the increase in the apparatus size, while providing a means for making a highly thermally conductive metallic sleeve last for a long period of time. 
     Referring to FIG.  10 ( a ), in this embodiment, the lengthwise end portion of the metallic sleeve, which has the width (in terms of the lengthwise direction of the metallic sleeve) of D, was caught by the internal surface of the sleeve end confining portion  15   a  of the rotational protective ring  15  with the internal diameter of Lc, as in the second embodiment. Further, the edge of the metallic sleeve  13  was caught by the sleeve end catching portion  15   b  of the rotational protective ring  15 , the internal diameter of which gradually reduces toward the outward side of the apparatus, also as in the second embodiment. 
     The essential characteristic of this embodiment is how the rotational protective ring  15  is rotationally supported by the rotational protective ring supporting portion of the sleeve end flange  15 A. Referring to FIGS.  10 ( a ) and  10 ( b ), the internal diameter Lg of the rotational protective ring supporting portion  15   j  of the sleeve end flange  15 A is made the same as, or slightly greater than, the external diameter Ld of the rotational protective ring  15 , and the rotational protective ring  15  is supported by the internal surface of the rotational protective ring supporting portion  15   j  of the sleeve end flange  15 A, by the peripheral surface. In other words, the peripheral surface of the sleeve end confining portion  15   a  of the rotational protective ring  15 , into which the lengthwise end portion of the metallic sleeve  13  with the width D is fitted, slides on the internal surface Of the rotational protective ring supporting portion  15   j  of the sleeve end flange  15 A. Therefore, in terms of the lengthwise direction, the range, across which the rotational protective ring  15  contacts the metallic sleeve  13 , overlaps with the range across which the rotational protective ring  15  contacts the sleeve end flange  15 A. In other words, with the provision of this structural arrangement, it is possible to reduce the lengthwise measurement of the fixing apparatus, without causing the edge of the metallic sleeve  113 , and the sleeve end flange  15 A, to interfere with each other. Therefore, it is possible to prevent the problem that the edge of the metallic sleeve  13  cracks as it continuously rubs against the member for regulating the lengthwise shifting of the metallic sleeve  13 , without inviting the increase in the size of a fixing apparatus. 
     Further, in order to allow the rotational protective ring  15  to more smoothly slides on the internal surface of the rotational protective ring supporting portion of the sleeve end flange  15 A, lubricant such as grease or the like may be interposed between the external surface of the rotational protective ring  15  and the internal surface of the rotational protective ring supporting portion of the sleeve end flange  15 A. Further, a bearing or the like may be interposed between the two surfaces in order to minimize the friction between the two surfaces so that the force which interferes with the rotation of the rotational protective ring  15  caused by the rotation of the metallic sleeve  13  is minimized. 
     With the provision of the above described structural arrangement, the amount of torque necessary to rotate the rotational protective ring  15  become smaller, allowing the rotational protective ring  15  to rotate more easily following the rotation of the metallic sleeve  13 . Therefore, it is possible to provide a highly durable fixing apparatus. 
     As described above, according to an aspect of the present invention, a sleeve protecting cap is disposed at at least one of the lengthwise end of a metallic sleeve. The protective cap catches the end portion the metallic sleeve with its internal surface, by the peripheral surface as well as the edge, confining the lengthwise end portion of the metallic sleeve  13 . Therefore, the length wise end portion of the metallic sleeve  13  is prevented from widening like the end of a trumpet. 
     Therefore, the metallic sleeve  13  is prevented from breaking from the edge. 
     According to another aspect of the present invention, a sleeve protecting ring is rotationally attached to a sleeve end flange. This rotational protective ring is rotatable relative to a metallic sleeve. The portion of the protective ring, which catches the metallic sleeve by the edge in terms of the thrust direction, is tapered so that its internal diameter gradually reduces toward the lengthwise end of the apparatus. The portion of the rotational protective ring, on the metallic sleeve side, is structured so that the peripheral surface of the lengthwise end portion of the metallic sleeve contacts the internal surface of this portion of the rotational protective ring. With the provision of the above described structural arrangement, the rotational protective ring does not interfere with the heater surface, for the following reason. That is, as a given portion of the metallic sleeve, in terms of the circumferential direction, enters between the heater and pressure roller, the this portion of the metallic sleeve is deformed in a manner to conform to the surface of the heater, perfectly contacting the surface of the heater. As a result, the portion of the lengthwise end portion of the metallic sleeve, correspondent in the circumferential direction to the portion of the metallic sleeve between the heater and pressure roller, becomes separated from the internal surface of the rotational protective ring. Consequently, the portion of the lengthwise end portion of the metallic sleeve other than the portion correspondent to the portion between the heater and pressure roller is placed perfectly in contact with the internal surface of the rotational protective ring, by the deformation of the metallic sleeve, with no interference between the rotational protective ring and heater surface. Therefore, the rotational protective ring rotates following the rotation of the metallic sleeve, preventing the problem that the edge Of the metallic sleeve cracks as it continuously rubs against the member for regulating the shifting of the metallic sleeve in its lengthwise direction. In other words, according to this second aspect of the present invention, it is possible to provide a highly durable fixing system, in which heat is efficiently conducted to the recording medium being conveyed through the fixation nip after the formation of an unfixed toner image on the recording medium, and in which the problem that the metallic sleeve is damaged at its edges does not occur for a long period of time. Further, the rotational protective ring is prevented by the sleeve end flange fixed to the boxy frame of the fixing apparatus, from shifting in the thrust direction, preventing therefore the metallic sleeve, the shifting of which in the thrust direction is regulated by the rotational protective ring, from shifting relative to the boxy frame of the apparatus, in the thrust direction. 
     According to another aspect of the present invention, the sleeve edge protective member is structured so that the lengthwise end portion of the metallic sleeve contacts the internal surface of the rotational protective ring, and the rotational protective ring contacts the sleeve end flange by the peripheral surface, reducing therefore the measurement of the apparatus in terms of the lengthwise direction. Thus, it is possible to make a fixing apparatus highly durable without increasing its size, making it possible to realize a compact and durable fixing apparatus. 
     Further, in order to make the friction resistance of the peripheral surface of the lengthwise end portion of the metallic sleeve greater than that of the peripheral surface of the lengthwise center portion of the metallic sleeve, the peripheral surface of the lengthwise end portion of the metallic sleeve is roughened; the releasing layer is not formed across the lengthwise end portion of the metallic sleeve; or the lengthwise end portion of the metallic sleeve is coated with a substance high in frictional resistance than the releasing layer. With this arrangement, it is possible to cause the rotational protective ring fitted around the lengthwise end portion of the metallic sleeve, to rotates at a peripheral velocity much closer to the peripheral velocity at which the metallic sleeve rotates, reducing the amount of the friction between the lengthwise end portion of the metallic sleeve and the sleeve end catching portion of the rotational protective ring. Therefore, it is possible to realize a far more durable fixing system. 
     &lt;Miscellanies&gt; 
     1) A structural arrangement may be made to cause the metallic sleeve  13  to shift only in one direction so that the stationary sleeve edge protection cap  15 , or sleeve edge protection ring  15 , needs to be disposed at only one lengthwise edge of the metallic sleeve, that is, the edge toward which the metallic sleeve shifts. 
     2) The heater  11  may be replaced with an electromagnetic induction based heating member, for example, an iron piece or the like, in which heat is electromagnetically generated by the function of the magnetic field generated by an exciter coil assembly. Also, a metallic sleeve, through the entirety of which heat can be generated by electromagnetic induction, may be employed, as a heating/fixing film, by an image fixing apparatus. 
     3) Not only is a heating apparatus in accordance with the present invention usable as a thermal image fixing apparatus, but also usable as an image heating apparatus for improving the surface properties, such as glossiness, by heating a recording medium bearing an image, an image heating apparatus for temporarily fixing an image, a heating apparatus through which an object in the form of a sheet is conveyed to be dried, laminated, or pressed for wrinkle removal, a heating apparatus for drying an ink image in an ink jet printer or the like, or the like heating apparatus, which is obvious. 
     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.