Abstract:
A device configured to be heated to a temperature sufficient to fuse an image forming substance to a sheet includes a hollow roller made of a conductive material and a coil arranged in a hollow portion of the hollow roller. The coil is configured to carry an electrical current that induces a current in the conductive material of the hollow roller such that the hollow roller becomes heated so as to fuse toner to a sheet. The coil is mounted on a member that is disposed in the hollow portion of the roller. The member includes various combinations of features that permit a reliable and safe operation of the induction heating apparatus such as recess and projections formed in the member to separate respective turns in the coil, a hollow center of the member that can be ventilated with a ventilation fan, and predefined gaps being maintained between the inside surface of the hollow roller and an outer surface of the coil.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application contains subject matter related to application Ser. No. 08/383,181, filed Feb. 3, 1995, now U.S. Pat. No. 5,594,540 (Jan. 14, 1997); application Ser. No. 08/187,496, filed Jun. 20, 1995, now U.S. Pat. No. 5,426,495 (Jun. 20, 1995); application Ser. No. 07/893,050, filed Jun. 3, 1992, now U.S. Pat. No. 5,300,996 (Apr. 5, 1994), and reissue application Ser. No. 08/628,270, filed Apr. 5, 1996, all of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to devices, such as image forming devices, that include a fixing device that affixes toner, or another image forming substance, to a sheet so as to make a toner image on the sheet. 
     2. Discussion of the Background 
     Image forming apparatuses, such as electrostatic copying machines, printers and facsimiles that employ an electrophotography process also include a fixing apparatus that fixes a toner image on a transfer paper. A conventional fixing apparatus includes a heating roller having a heating element therein and a press roller that contacts the heating roller. The conventional fixing apparatus is adapted to pass the transfer paper between the heating roller and press roller such that a toner image disposed on the transfer paper becomes fixed to the transfer paper as a result of heat imparted to the toner by the heating roller and pressure applied to the toner and transfer paper by the press roller and heating roller. 
     A quality of the bond between the toner and transfer paper depends on heat conditions of the fixing apparatus. For example, as the toner is heated beyond a predetermined melting temperature, the quality of the fixing process improves because the toner melts well. However, if the toner is not heated above the predetermined temperature, the quality of the fixing process is sub-optimal because the toner only partially melts. 
     Japanese Laid-Open Pat. Application No. 53-50844 discloses an induction heating element in the form of a heating roller. As shown in FIG. 12, this heating roller includes a core  2  made of a magnetic material fixed to a shaft  1 , a coil of wire  3  wound around the core, a roller member  5  which is an induction heating member rotatably supported by the shaft  1 , and a heat-resistant and heat-insulating layer  4  arranged on an inner circumferential surface of the roller member. In the heating roller, a current (generally, 5 to 15 A) from a commercial power supply  8  is supplied to the coil via leads  6  and  7  to generate an induced current in the roller member  5 . This induced current flows in the presence of an internal resistance in the roller member  5 , which, according to the Joule effect, produces thermal energy, and thus heat, as a result of the induced current flow in the roller member  5 . 
     In the induction heating system of FIG. 12, the coil  3  is arranged inside the roller member  5  and a high voltage is applied to the coil so as to supply a high current during a fixing operation in an attempt to heat the toner to a sufficient temperature. In addition, the roller member  5  covering the coil is made of a wire made of a conductive material having an internal resistance such that, when subjected to a high current, the wire itself produces heat, albeit a small amount. So respective windings in the coil  3  do not short-out to adjacent windings or to other conductive bodies, the wire is coated with an insulating layer. However, if a portion between the coil  3  is subjected to too much heat, there is a risk that a part of the insulating layer will deteriorate, thereby causing adjacent windings to short-out. 
     Generally, available insulating materials that are suitable for coating the wire are expensive, and the present inventors have identified that avoiding this expense by employing a structural alternative would be desirable, if possible. Furthermore, avoiding special steps for coating the wire with the special insulating materials would also be desirable. 
     FIG. 13 shows another conventional induction heating roller as disclosed in Japanese Laid-Open Patent Application No. 58-209887. This induction heating roller includes a hollow roller  231  and a supporting member  232  which supports the hollow roller  231 . A solid core portion  234  is included and an induction coil  233  is mounted on an outer periphery of the solid core portion  234 . A supporting shaft  236  which protrudes from each side of the core portion  234  rotatably supports a hollow shaft portion  238  of the hollow roller  231  via a bearing  237 . Further, on the supporting shaft  236 , there is provided a lead wire  239 , one end of which is connected to the induction coil  233 . The lead wire  239  is led out of the supporting shaft  236  to connect to a power supply (not shown). In addition, a jacket  241  is put on the supporting member  232  and a cylindrical thermal insulating material  242  is concentrically wound around the induction coil  233 . 
     In this induction heating roller, a refrigerant is circulated through the jacket  241 , as shown, to cool the supporting member  232 , thereby preventing the induction coil  233  from receiving conduction heat from the hollow roller  231 . In addition, the thermal insulating material  242  is used to intercept radiation heat and convection heat generated by the hollow roller  231 , thereby preventing the induction coil  233  from being exposed to the heat. Thus, the induction heating roller of FIG. 13 addresses the concern of overheating the induction coil  233  by a combining an active cooling mechanism with sufficient thermal insulating material. 
     As recognized by the present inventors, the conventional heating roller of FIG. 13 is an expensive approach for solving the problem because this structure is complex in that (1) the thermal insulating material is wound around the induction coil, (2) the jacket is put on the supporting member, (3) the thermal insulating material is used and (4) the refrigerant is used. Another limitation with the device of FIG. 13, is that a copy operation start up period is relatively long because the refrigerant initially absorbs much of the thermal energy. 
     Another induction heating roller is disclosed in Japanese Granted Utility Model Application No. 57-52874, in which a supporting member is configured to support a hollow roller having an iron core therethrough. An induction coil is mounted about an outer periphery of the iron core, and the iron core supports the hollow roller via a bearing. At a precise location about the outer surface of the iron core, an electrical insulating spacer is provided for preventing a short-circuit to occur between the coil and the iron core. Other electrical insulating spacers, of a different type, are inserted about the core and between respective windings of the wire so as to prevent the windings from short-circuiting. 
     As recognized by the present inventors, a limitation with this conventional heating roller is that the process for forming the spacers on the heat roller is complex and thus expensive. Furthermore, this type of roller cannot be manufactured as quickly as other heating rollers, which is a significant manufacturing liability. 
     In the above-mentioned heating roller, the iron core is made of a magnetic material, although alternatively a bobbin made of a heat-resistant material may be used instead of this iron core. When the bobbin is used, and when the heating roller becomes hot, there is a risk that the bobbin shape will become deformed, perhaps in an eccentric shape. As a result of the deformation, a problem occurs in that the coil wound around the bobbin comes into contact with the roller member, thus resulting in the creation of an electrical leakage current or the like. The deformation problem becomes particularly pronounced when the induction heating roller is 15-50 millimeters in diameter and 1-2 millimeters in thickness and used in an image forming apparatus because it is difficult to maintain adequate gap-control between the hollow roller and the coil. 
     Furthermore, as the heating roller becomes bent as a result of pressure being applied thereto from the press roller, a problem occurs that, especially when the heating roller rotates at high velocity, the heating roller comes into contact with the coil, thereby resulting in electrical leakage. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of this invention is to provide a novel heating roller adapted for use in a fixing device that overcomes the above-mentioned limitations of existing methods and systems. 
     Another object of the present invention is to provide a safe induction heating roller, a safe roller heating apparatus that employs the induction heating roller, and a safe image forming apparatus that employs the roller heating apparatus, each of which minimize a risk of short-circuiting a coil wire. 
     Yet another object of the present invention is to provide a safe induction heating roller, roller heating apparatus, and image forming apparatus which prevent from occurring an electrical leak caused by a coil electrically connecting with a roller member. 
     Still another object of the present invention is to provide an induction heating roller which may be relatively simple to manufacture at a low-cost, yet avoid the possibility of damaging the induction coil as a result of heat-induced stress. 
     It is still a further object of the present invention to provide an induction heating roller that does not require a significant warm-up time so that a copy operation may be speedily initiated after energizing the heating roller. 
     The above and other objects and novel features of the present invention are achieved in a device configured to fuse an image forming substance to a sheet. The device includes a hollow roller made of a conductive material and a coil arranged in a hollow portion of the hollow roller. The coil is configured to carry an electrical current that induces a current in the conductive material of the hollow roller such that the hollow roller becomes heated so as to fuse toner to the sheet. The coil is mounted on a member that is disposed in the hollow portion of the roller. The member includes various combinations of features that permit a reliable and safe operation of an induction heating apparatus such as recesses and projections formed in the member to separate respective turns in the coil, a hollow center of the member that can be ventilated with a ventilation fan, and predefined distances set between the inside surface of the hollow roller and an outer surface of the coil. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 is a cross-sectional view of a fixing apparatus including an inductive heating roller according to a first embodiment of the present invention; 
     FIG. 2 is a partial cross-sectional view of the inductive heating roller according to the first embodiment; 
     FIG. 3 is a partial cross-sectional view of a bobbin portion and a coil portion of the inductive heating roller according to the first embodiment; 
     FIGS. 4 and 5 are partial cross-sectional views of a bobbin portion and a coil portion of an induction heating roller according to a second embodiment of the present invention; 
     FIG. 6 is a side view of a bobbin portion of an induction heating roller according to a third embodiment of the present invention; 
     FIG. 7 is a cross-sectional diagram of an induction heating roller according to a fourth embodiment according of the present invention; 
     FIG. 8 is a partial cross-sectional diagram of a large-diameter portion of a bobbin of the induction heating roller of FIG. 7; 
     FIG. 9 is a partial cross-sectional view of the inductive heating roller according to a fifth embodiment of the present invention; 
     FIG. 10 is a perspective view of an end of a bobbin of the inductive heating roller of FIG. 9; 
     FIG. 11 is a cross-sectional view of an inductive heating roller according to a sixth embodiment of the present invention; 
     FIG. 12 is a cross-sectional view of a conventional inductive heating roller; and 
     FIG. 13 is a partial cross-sectional view of another conventional heating roller. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof, a fixing apparatus of an image forming apparatus that includes the inductive heating roller of the present invention. 
     In FIG. 1, a fixing apparatus  10  of the image forming apparatus includes an induction heating roller  11 , a press roller  12  that contacts the heating roller  11  by a pressing element (not shown), a cleaning roller  13  which contacts the heating roller  11  and removes toner or paper dust attached to the heating roller  11 . A blade  14  is positioned to contact the cleaning roller  13  so as to scrape away deposits such as used toner that remains attached to the cleaning roller  13 . Felt  17  is provided for coating a releasing agent  21  on an outer circumferential surface of the heating roller  11  and a releasing agent blade  18  is provided for scraping away an excess amount of the releasing agent  21  coated by the felt  17 . A pick-off pawl  19  is provided for separating a transfer paper P whose toner image formed thereon has been fixed by the heating roller  11  and the press roller  12 . A pair of discharging rollers  20   a  and  20   b  is provided for discharging the transfer paper P separated by the pick-off pawl  19 . 
     Near a surface of the heating roller  11 , there is arranged a thermistor  26  for detecting a temperature of the heating roller  11 . The temperature information detected by the thermistor  26  is input to a power supply device (not shown) that controls an amount of power applied to the heating roller  11  based on the temperature information from the thermistor  26  so as to maintain the temperature of the heating roller  11  at a predetermined temperature. 
     The press roller  12  includes a core metal  120  made of metal, for example, an aluminum alloy, and a rubber layer  121  made of silicon rubber formed around the outer circumferential surface of the core metal  120 . 
     The heating roller  11  includes a hollow roller  30  which forms an outer circumferential portion of the heating roller  11 , and a core portion  40  arranged inside the roller  30 . The heating roller  11  may be configured to have a diameter in a range of 15-50 millimeters in diameter and 1-2 millimeters in thickness, where the actual size will correspond with the requirements of the image forming apparatus or fixing device in which the heating roller  11  is used. 
     As shown in FIG. 2, the core portion  40  of the heating roller  11  includes a bobbin  41  made of a heat-resistant and electrical insulating resin, for example, a synthetic material like nylon or polyester that has been doped with a flame-retarding material. The bobbin  41  has wrapped about a peripheral surface (as will be discussed) an induction coil  42  that is powered by current from electrodes  43   a  and  43   b , which connect to lead wires  44   a  and  44   b  respectively. The bobbin  41  is formed in a cylindrical shape, the central portion of which is formed as a large-diameter portion  410  in an axial direction, and small-diameter portions  411   a  and  411   b  are formed at both ends of the large-diameter portion  410 , as shown. 
     The outer ends of the small-diameter portions  411   a  and  411   b  are fixed to side plates (not shown) of the fixing apparatus  10 , respectively. In the small-diameter portions  411   a  and  411   b , cylindrical electrodes  43   a  and  43   b  are respectively arranged as shown. At the ends of the electrodes  43   a  and  43   b  that connect to the large-diameter portion  410 , fixing members  47 , described later, are electrically connected to the electrodes. The other ends of the electrodes  43   a  and  43   b  have lead wires  44   a  and  44   b  are secured thereto by screws  45 , as shown. The lead wires  44   a  and  44   b  are connected to the power supply device, not shown, where the power supply provides a predetermined amount of power via the lead wires  44   a  and  44   b.    
     On the surface of the large-diameter portion  410  of the bobbin  41 , a continuous spiral slot  46  is formed, extending from one end of the large-diameter portion  410  to the other end thereof. As shown in FIG. 2, and in more detail in FIG. 3, the slot  46  has almost the same shape as the outer peripheral shape of a wire that forms the induction coil  42  so that the wire fits neatly into the slot  46 . A depth of the slot  46  is shown to be almost the same as a radius of a wire of the induction coil  42 . The spacing between the turns of the slot  46  is set to approximately 0.3 mm, as shown in FIG.  3 . The surface of the large-diameter portion  410  of the bobbin  41  may be described as having recessed and projection features formed thereon where the recesses are defined by the slot  46  and the projections are defined by areas therebetween. 
     Around the large-diameter portion  410 , the induction coil  42  is wound along the slot  46 . Both ends of the induction coil  42  are terminated by the fixing members  47  that are disposed on the outer circumferential surface at end portions of the large-diameter portion  410 . The fixing members  47  are made of a conductive material and are connected with the electrodes  43   a  and  43   b  as described above. 
     On the small-diameter portions  411   a  and  411   b  of the bobbin  41 , support tubes  32   a  and  32   b  are rotatably supported via respective roller bearings  31 . The support tubes  32   a  and  32   b  have flanges  33   a  and  33   b , respectively, which oppose each other. A cylindrical roller  34  is arranged between the flanges  33   a  and  33   b  such that both ends of the roller  34  are mated with the flanges  33   a  and  33   b , respectively, and the roller  34  is fixed to the flanges  33   a  and  33   b  with screws (not shown). 
     The roller  34  includes a core metal member  340  made of a conductive magnetic member, for example, iron, stainless steel or the like, and a releasing agent layer  341  which is made of a resin which is formed on the outer circumferential surface of the core metal member  340 . The resin allows the toner T to be more easily released from the roller  34 . The hollow roller  30  includes the roller  34  and the support tubes  32   a  and  32   b , as shown. 
     A gear  35  is fixed to the support tube  32   a , and a drive gear (not shown) is mated with the gear  35 . When the gear  35  is rotated by the drive gear, the hollow roller  30  rotates around the outer periphery of the bobbin  41 . 
     In light of the above discussion about the structure of the heating roller  11 , a description of how the heating roller  11  operates will now be provided. Because the wire of the induction coil  42  is set in the spiral slot  46 , a relative, lateral movement of the wire is restricted and thus short circuiting of adjacent turns in the coil  42  is avoided even though the induction coil  42  is subjected to a variety of operational conditions. Therefore, even if the conductive portions of the wire become exposed as a result of an outer insulating coat becoming damaged or fused, the exposed portions will not touch one another, thereby avoiding a short-circuit event and improving the safety of the apparatus. In addition, even when an inexpensive wire is used, which generally has a relatively low heat resistance, it is possible to prevent short-circuiting caused by adjacent wire turns contacting one another, and therefore the induction coil  42  can reliably be used, while the cost of the wire is reduced. 
     Furthermore, because the bobbin  41  requires a generally continuous groove to be formed therein during manufacturing, it is possible to manufacture the bobbin  41  with a straight-forward manufacturing process. Although the continuous spiral slot  46  is formed on the surface of the large-diameter portion  410  in this embodiment, the slot  46  may be formed with discontinuous sections, as long as the groove is configured to maintain a separation between adjacent wire turns (i.e., windings). 
     An operation of the fixing apparatus  10  is described below. As shown in FIG. 1, when a transfer paper P having a toner image formed thereon is conveyed to the fixing apparatus  10 , the power supply device provides a high current, drawn from a commercial power source, to the induction coil  42  via lead wires  44   a  and  44   b , electrodes  43   a  and  43   b , and fixing members  47  (FIG.  2 ). By this current being supplied to the induction coil  42 , another current is induced in the roller  34 , and the heating roller  11  is heated to a predetermined temperature as a result of the Joule effect from this induced current. Thereafter, the transfer paper P is inserted between the heating roller  11  and the press roller  12  where the paper P is heated to the predetermined temperature and the toner T become affixed thereto. Once the toner T is affixed to the transfer paper P, the transfer paper P is separated from the surface of the heating roller  11  by the pick-off pawl  19 , which is spring biased, as shown, and then conveyed by way of a pair of the discharging rollers  20   a  and  20   b  to be discharged from the fixing apparatus  10 . 
     A second embodiment will be described with respect to FIGS. 4 and 5. In the second embodiment, a shape of a large-diameter portion  2410  of a bobbin  241  is different from that of the first embodiment. Therefore, an explanation will be generally directed to the shape of the large-diameter portion  2410 , and a discussion of the features of the second embodiment that are common with the first embodiment will be omitted. 
     As shown in FIG. 4, on the surface of the large-diameter portion  2410  of the bobbin  241 , a spiral projection streak  50  is integrally formed from one end of the large-diameter portion  2410  to the other end thereof. A slot  51  is formed by a gap between neighboring turns of the projection streak  50 ; in other words, opposing faces of the neighboring turns of the projection streak  50  and an outer circumferential surface of the large-diameter portion  2410  define the slots therebetween. Along the slot  51 , an induction coil  242  is wound around the large-diameter portion  2410 . 
     In FIG. 5, assuming that W (mm) indicates a width of the slot  51 , H (mm) indicates a distance from an outer circumferential surface of the induction coil  242  in the slot  51  to an outer edge of the projection streak  50 , and R (mm) indicates a diameter of a wire of the induction coil  242 . The width W of the slot  51  and the distance H (mm) are set according to the relationships, 
     W (mm)&gt;R (mm), and 
     H (mm)≧3 mm. 
     In these conditions, a depth H1 (mm) of the slot  51  satisfies a relationship of 
     H1 (mm)≧3 mm+R (mm). 
     According to the above constitution, even if the bobbin  241  vibrates, a movement of the wire of the induction coil  242  is not sufficient to remove the wire from the slot  51 . Therefore, safety standards such as those set by Underwriters Laboratory (UL), the Canadian Standard Association (CSA), or the like can be satisfied and the apparatus can be operated safely. In addition, although the slot  51  is formed by the projection streaks  50  in this embodiment, the same effect can be obtained by extending the depth of the slot  46  in the first embodiment. Thus, a variant of the second embodiment is a combination of the first embodiment and the structure shown in FIGS. 4 and 5. 
     Regarding the fabrication process of the heating roller, the projection streak  50  and the bobbin  241  are both made of resin. Accordingly, the projection streak  50  and the bobbin  241  may be formed in a one piece mold using an injection molding process. 
     FIG. 6 illustrates a third embodiment that will be described below. In this third embodiment, a shape of a large-diameter portion  3410  of a bobbin  341  is different from that of the first embodiment. Therefore, an explanation will be generally directed to the shape of the large-diameter portion  3410 , and features of the third embodiment that are common with the first embodiment will be omitted. 
     As shown in FIG. 6, on the surface of the large-diameter portion  3410  of the bobbin  341 , numerous protrusions  55  are arranged radially from the center of the large-diameter portion  3410  at a fixed interval. The numerous protrusions  55  are spirally arranged on the surface of the large-diameter portion  3410  so as to spirally guide the wire of the induction coil  342 . The wire of the induction coil  342  is wound on the surface of the large-diameter portion  3410  by threading the wire between the protrusions  55  so that neighboring turns of the wire of the induction coil  42  do not contact one another. A height of each protrusion  55  is sufficiently high to prevent respective turns in the induction coil  342  from jumping over the protrusions  55  when the wire is jostled, vibrated or subjected to thermal contraction/expansion. In addition, the wire of the induction coil  342  is wound on the surface of the large-diameter portion  3410  so that neighboring turns of the wire do not contact one another. Accordingly, the protrusions  55  need not be arranged at predetermined intervals, but may also be arranged at varying spacings (e.g., random spacing and/or varying spacings that follow a predetermined pattern) provided that the height and distance between the spacings is sufficient to maintain the separation of adjacent windings. 
     According to the above constitution, even if the bobbin  341  vibrates, a movement of the wire of the induction coil  3420  is reliably avoided. Therefore, safety standards such as those set by UL, CSA, or the like can be satisfied and the safety of an apparatus can be maintained. Furthermore, as with the second embodiment, because the protrusions  55  and the bobbin  341  are made of resin, they may be jointly formed in a single mold in an injection molding process. 
     FIGS. 7 and 8 illustrate a fourth embodiment that will be described below. In the fourth embodiment, a shape of a core portion  4440  is different from that of the first embodiment and therefore an explanation will be made here only for the shape of the core portion  4440 . Therefore, an explanation will be generally directed to the shape of the core portion, and features of the fourth embodiment that are common with the first embodiment will be omitted or simplified. 
     The heating roller  4110 , as shown in FIG. 7, includes a hollow roller  4300  which forms an outer circumferential portion of the heating roller  4110 , and a core portion  4400  arranged inside the roller  4300 . The core portion  4400  includes a cylindrical bobbin  4410 , an induction coil  4420 , and lead wires  44   a  and  44   b  that provide electrical power to the induction coil  4420 . 
     The bobbin  4410  includes a large-diameter portion  4104  formed in the central portion, in an axial direction of the bobbin  4410 , and small-diameter portions  411   a  and  411   b  formed at both ends of the large-diameter portion  4104 , respectively. Flange portions  4440  are formed near a central portion and at both ends of the large-diameter portion  4104 . The flange portions  4440  are provided as projecting portions for maintaining a gap between the hollow roller  4300  and the induction coil  4420 , even if the hollow roller  4300  becomes deformed. 
     As shown in FIG. 8, on the surface of the large-diameter portion  4104 , a continuous spiral slot  4460  is formed from one end of the large-diameter portion  4104  toward the other end thereof. The shape of the slot  4460  is almost the same shape as a lower half portion of the outer peripheral shape of the wire of the induction coil  4420 . Around the large-diameter portion  4104 , the induction coil  4420  is wound so as to sit in the slot  4460 . As shown in FIG. 7, both ends of the induction coil  4420  connect to the lead wires  44   a  and  44   b  embedded at the both ends of the large-diameter portion  4104 , respectively. The lead wires  44   a  and  44   b  connect to a power supply device (not shown) and are passed through an inside of the small-diameter portions  411   a  and  411   b . The amount of electrical power supplied is controlled by the power supply device. 
     From a viewpoint of safety, the bobbin  4410  is made of heat-resistant insulating resin, for example, a synthetic resin, such as nylon, polyester or the like which has a flame retardant material applied thereto. The bobbin  4410  includes the large-diameter portion  4104 , the small-diameter portions  411   a  and  411   b  and the flange portions  4440 , all of which may be formed using resin molding manufacturing processes. 
     Referring to FIG. 8, a height of an edge of the flange portion  4440  and a shape of the slot  4460  will be discussed. Assuming that H11 (mm) indicates a height of the edge of the flange portion  4440  having a tip end  4440   a , (i.e., a distance from the outer circumferential surface of the induction coil  4420  to the tip end  4440   a  of the flange portion  4440 ), H12 (mm) indicates a depth of the slot  4460 , and R (mm) indicates a diameter of the wire of the induction coil  42 . The height of the edge of the flange portion  4440  and the depth of the slot  4460  are set so as to satisfy relationships of 
     H11 (mm)≧3 (mm) and 
     H12 (mm)=(½)×R (mm), respectively. 
     A gap H13 between the tip end  4440   a  of the flange portion  4440  and the inner circumferential surface of the roller  4340  is set to a distance in which both members do not contact each other even when the hollow roller  4430  rotates. In this embodiment, the gap H13 is set in the range of 0.5 to 1 (mm). 
     Although the bobbin  4410  is made of heat-resistant insulating resin so as to have resistance to heat of the heating roller  4110 , eccentricity or deformation may occur over a period of time in the bobbin  4410  if the heat of the heating roller  4110  affects the bobbin  4410  when the bobbin  4410  is subject to vibration as well as heat. 
     In this embodiment, to counteract an eccentricity or deformation of the bobbin  4410 , a plurality of the flange portions  4440  are integrated in the large-diameter portion  4104 . The tip end  4440   a  of the flange portions  4440  contacts the inner circumferential surface of the roller  4340  when the eccentricity occurs, resulting in prevention of further eccentricity or deformation, thus preserving a gap between the outer circumferential surface of the induction coil  4420  and the inner circumferential surface of the roller  4340 . Consequently, the induction coil  4420  is prevented from contacting the inner circumferential surface of the roller  4340 . Accordingly, the risk of electrical current leakage is reduced relative to the risk of current leakage if the gap were not preserved by the flange  4440 . Because H11 (mm) is 3 (mm) or longer, even if the bobbin  4410  does experience some eccentricity or deformation, or even if the hollow roller is bent by a pressure from the press roller  121  (FIG.  1 ), the induction coil  4420  is reliably prevented from contacting the inner circumferential surface of the roller  4340 . Accordingly, safety standards such as those levied by the UL, CSA or the like can be satisfied and the safety of the present apparatus is improved over conventional apparatuses. 
     Because the wire of the induction coil  4420  is wound along the slot  4460  formed on the surface of the large-diameter portion  4104 , a relative movement of turns of the wire of the induction coil  4420  is restricted, thereby preventing the turns from contacting one another. Therefore, even if the conductive portions of the wire are exposed as a result of fusing or degradation of the insulating layer of the wire, it is possible to prevent short-circuiting caused by a contact between neighboring windings, resulting in improved safety. 
     In this embodiment, although the flange portions  4440  each have a shape of a circular plate, the flange portions  4440  may include fragments (such as prongs). In addition, a plurality of protrusions shaped like raised fragments can be arranged on the surface of the large-diameter portion  4104  as the projecting portions. 
     FIGS. 9 and 10 illustrate a fifth embodiment that is described below. Common elements in the first and fifth are represented in FIGS. 9 and 10 by the same reference numerals, and thus, the explanation of the common members is omitted here. 
     As shown in FIG. 9, on the both ends of the large-diameter portion  5410 , holes  5410   a  and  5410   b  are formed which connect the small diameter portions  411   a  and  411   b , respectively. On the large-diameter portion  5410  of the bobbin  5541 , the induction coil  5420  is wound from the hole  5410   a  toward the hole  5410   b.    
     On the large-diameter portion  5410  around which the induction coil  5420  is wound, spacers  55   a ,  55   b , and  55   c  for forming a gap between the large-diameter portion  5410  and the inner circumferential surface of the roller are loosely fitted so as to be slidable over the outer circumferential surface of the induction coil  5420 . The spacers  55   a ,  55   b , and  55   c  are arranged near the center and at the both ends of the large-diameter portion  5410 , respectively. The spacers  55   a ,  55   b , and  55   c  have similar flange shapes and are made of a material having excellent heat resistance and wear-resistance, for example, a material of wear-resistance improved PI (polyphenyiene sulfide) or PPC (polymide resins). 
     At the both ends of the induction coil  5420 , there are provided fixing fragments  53   a  and  53   b  which also serve as electrodes. The fixing fragments  53   a  and  53   b  are arranged in the positions corresponding to the holes  5410   a  and  5410   b  and are fastened on the large-diameter portion  5410  with conductive screws  54   a  and  54   b . Thus, the fixed fragments  53   a  and  53   b  are electrically connected to the electrodes  50   a  and  50   b  via the screws  54   a  and  54   b.    
     Inside the small-diameter potions  411   a  and  411   b , cylindrical electrodes  50   a  and  50   b  are arranged, respectively. As shown in FIG. 9, at inner ends of the electrodes  50   a  and  50   b  screw matching portions are provided with which screws  54   a  and  54   b  are matched, respectively. At the outer ends of the electrodes  50   a  and  50   b , there are provided screw portions  50   c  on which lead wires  51   a  and  51   b  are fastened with nuts  52 , respectively. The lead wires  51   a  and  51   b  are connected to the power supply device, which is not shown, and the amount of power supplied thereto is controlled by the power supply device. 
     According to the above-mentioned configuration, if the spacers  55   a ,  55   b , and  55   c  contact the inner circumferential surface of the roller  340  due to a vibration of the bobbin  5541 , the spacers  55   a ,  55   b , and  55   c  rotate around the bobbin  5541  while sliding over the outer circumferential surface of the induction coil  5420 . This sliding action inhibits the generation of a noise caused by a contact between components and wearing of the outer peripheral ends of the spacers  55   a ,  55   b , and  55   c.    
     The bobbin  5541  may be molded using a heat-resistant resin such as PPS (polyphenyiene sulfide), PEEK (poly-ether ether ketone), PES (poly ether), PI (polymide resins), and a liquid crystal polymer. The same effects obtained in the above embodiment can also be obtained when other heat-resistant resins are used. 
     FIG. 11 illustrates a sixth embodiment that will be described below. In the sixth embodiment, a core portion  6400  is different from that of the first embodiment, and thus, the different features will be described and a discussion of common features will be simplified or omitted. 
     In FIG. 11, a hollow roller  6300  is made of a magnetic material such as iron and has a releasing layer (not shown) made of Teflon resin, silicone rubber, fluororubber or the like on the outer circumferential surface thereof. A core member  72  supports the hollow roller  6300  and has a central hole  73 , an induction coil supporting portion  74  and hollow roller supporting shafts  75  and  76  which are formed at both ends of the induction coil supporting portion  74 . The hollow roller supporting shafts  75  and  76  rotatably support hollow shaft portions  78  and  79  of the hollow roller  6300  via a bearing  31 . 
     The induction coil supporting portion  74  supports an induction coil  80 . A distance in a radial direction from an outer circumferential surface  81  of the induction coil  80  to an inner circumferential surface  82  of the hollow roller  6300  is set to be in the range of 3 to 8 mm. The induction coil  80  is energized by an external power supply (not shown) by way of an energizing harness  83 . In addition, a ventilating fan  84  is fixed on the hollow roller supporting shaft  75  so as to face the central hole  73 . The induction coil supporting portion  74  is equipped with a temperature detector  85  that contacts the induction coil  80 , and produces a detection signal that is sent to a controller, (not shown, although may be implemented as a digital signal processing device such as a microprocessor) by a bus  86  to control the operation of the ventilating fan  84 . 
     In addition, it is possible, in this embodiment, that the core member  72  is made of a resin, which was noted in above-mentioned embodiments, as well as a metal. 
     In this embodiment, since a gap distance in a radial direction from the outer circumferential surface  81  of the induction coil  80  to the inner circumferential surface  82  of the hollow roller  6300  is 3 mm or greater, it is possible to prevent an adverse effect caused by heat radiation to affect the hollow roller  6300 . Therefore, the induction coil  80  is prevented from being heated excessively, thereby preventing the induction coil  80  from being disconnected or damaged due to short-circuiting. Furthermore, the structure of this embodiment is such that it is possible to provide a low-cost induction heating roller having a simple structure. 
     Further since the gap distance is 8 mm or less, it is possible to prevent a reduction of a heat conversion efficiency of induction heating. Namely, when adapted for use in a fixing device, to which an image forming apparatus supplies at 400−2k watts of power, the heating roller can be kept at high temperatures (generally near 200 degrees centigrade) at such that the toner image is reliably fixed on the transfer paper. 
     In addition, the ventilating fan  84 , which is equipped with the hollow roller supporting shaft  75 , is arranged to face the central hole  73  of the core member  72 . The fan  84  does not operate until a predetermined temperature is detected by the temperature detector  85  which is arranged on the induction coil supporting portion  74  and contacts the induction coil  80 . 
     When the temperature exceeds the predetermined temperature and a detection signal is output, the ventilating fan  84  starts to operate so that a cooling fluid (e.g., air) circulates through the central hole  73  of the core member  72  to cool the induction coil  80  via the core member  72 . Thus the induction coil  80  is prevented from being heated excessively, thereby preventing the induction coil  80  from being disconnected or damaged due to short-circuiting. Further, at warm-up time, since the temperature of the induction coil  80  is lower than the predetermined temperature, the ventilating fan  84  is controlled by the controller to not operate, thereby avoiding unnecessary cooling and faster start-up time. As a consequence, the induction coil  80  is prevented from being disconnected or damaged due to short-circuiting, and accordingly it is possible to provide a low-cost induction heating roller having a simple structure. 
     In above-mentioned embodiment, the hollow roller  6300  is shaped to have tier portions, but the hollow roller  6300  may instead be shaped without the tier portions for ventilation considerations. 
     Regarding the controller for controlling the fan  84 , or even the power supply, the controller may be implemented using a conventional general purpose microprocessor programmed according to the teachings of the present specification, as will be appreciated to those skilled in the relevant art(s). Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will also be apparent to those skilled in the relevant art(s). 
     The present invention thus also includes a computer-based product which may be hosted on a storage medium and include instructions which can be used to program a computer to perform a process in accordance with the present invention. The storage medium can include, but is not limited to, any type of disk including floppy disk, optical disk, CD-ROMS, and magneto-optical disks, ROMS, RAMs, EPROMs, EEPROMs, flash memory, magnetic or optical cards, or any type of media suitable for storing electronic instructions. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.