Abstract:
An induction heating fixing apparatus including a heating roller having an outer cylinder, a coil arranged inside the outer cylinder to generate an induction magnetic flux, and a first insulating cylinder member arranged between the outer cylinder and the coil. A second insulating cylinder member is provided coaxial arrangement with the first insulating cylinder member and includes a spiral rib in substantially tight contact with the first insulating cylinder member to remove heat from the inside of the heating roller during rotation of the second insulating cylinder member. The coil is wound on a tubular member having an outer circumferential surface provided with radial holes which communicate with the interior of the tubular member and thereby to outside of the heating roller also to remove heat from the heating roller.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of priority to Japanese Patent Application Nos. 10-086611 filed Mar. 31, 1998; 10-086612 filed Mar. 31, 1998; and 11-021992 filed Jan. 29, 1999, the entire contents of which are incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fixing apparatus having a heating roller using an induction heating system. 
     2. Discussion of the Background 
     A known fixing apparatus used for image forming apparatuses such as a photocopying machine, a printer, and a facsimile, includes a heating roller using an induction heating system. In general in such a fixing apparatus having an induction coil is installed inside of a cylinder which forms the circumference of the heating roller, and an induction magnetic flux is generated by sending a high frequency alternating current to the induction coil. An induction current is generated in an electroconductive layer of the cylinder of the heating roller by the induction magnetic flux, and the heating roller is heated by joule heat generated by the induction current. 
     Because the induction coil is installed inside the heating roller, the fixing apparatus having the heating roller using the induction heating system has a problem in that the temperature of the induction coil is raised by radiant heat emitted from the cylinder of the roller and heated beyond a heat-proof temperature of the insulating cover of the coil. The fixing apparatus having the heating roller using the induction heating system has another problem in that fixing efficiency is not good, because heat generated in the cylinder of the heating roller is dispersed in the inside of the heating roller. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of this invention is to overcome the above-described and other problems with background apparatuses, by providing a fixing apparatus having a heating roller using an induction heating system, which is capable of preventing temperature rise of an induction coil and improving fixing efficiency. 
     This and other objects are achieved according to a preferred embodiment of the present invention by providing a novel fixing apparatus having a heating roller using an induction heating system, and which includes an insulating member arranged between a cylinder of the heating roller that is the heating unit of the heating roller and a coil arranged inside the cylinder to generate an induction magnetic flux. 
     According to the invention, the insulating member may further include a heat-absorbing member on the outer surface of the insulating member. 
     According to another preferred embodiment, in a fixing apparatus having a heating roller using an induction heating system, a tubular member on which a coil is wound to generate an induction magnetic flux is arranged inside the heating roller and an outer circumferential surface of the tubular member communicates with the outside of the heating roller by a plurality of openings provided in the tubular member. 
     Further, in a fixing apparatus having a heating roller of an induction heating system, according to still another embodiment of the present invention, an insulating cylinder member is provided inside a cylinder of the heating roller substantially tightly contacting the inner circumference of the cylinder. The length of the insulating cylinder member in the axial direction of the cylinder member may be made longer than the length of the cylinder of the heating roller in the axial direction of the cylinder. 
     In addition, a spiral rib may be provided on the insulating cylinder member such that an upstream pitch of the spiral rib is shorter than a downstream pitch of the spiral in the direction of air flow caused by the rib when the cylinder member is rotated. 
     Furthermore, at least one additional insulating cylinder member may be provided inside the insulating cylinder member substantially tightly contacting the cylinder. The lengths of the insulating cylinder member contacting the cylinder and the at least one additional insulating cylinder member in respective axial directions may be made larger than the length of the cylinder of the heating roller in the axial direction of the cylinder. 
     Furthermore, an optional cylinder member among the at least one additional insulating cylinder member may be configured to rotate integrally with the cylinder of the heating roller and a spiral rib may be provided on the rotative cylinder member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present 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 thereof when considered in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic sectional drawing illustrating the main part of a fixing apparatus according to an embodiment of the present invention; 
     FIG. 2 is a schematic drawing illustrating a fixing operation of the fixing apparatus; 
     FIG. 3 is a schematic sectional drawing illustrating the main part of a fixing apparatus according to another embodiment of the present invention; 
     FIG. 4 is a perspective side view of a fixing shaft (bobbin) of the heating roller of the fixing apparatus illustrated in FIG. 3; 
     FIG. 5 is a schematic sectional drawing illustrating a fixing apparatus according to still another embodiment of the present invention; 
     FIG. 6 is a front view illustrating an inner cylinder member having a spiral rib, which is provided inside the heating roller of the fixing apparatus illustrated in FIG. 5; 
     FIG. 7 is a schematic sectional drawing illustrating the heating roller of the fixing apparatus illustrated in FIG. 5; 
     FIG. 8 is a front view illustrating the shape of a stopper to fix the cylinder and the outer cylinder member of the fixing apparatus illustrated in FIG. 5; a 
     FIG. 9 is a partial sectional view illustrating the parts of the cylinder and the outer cylinder member where the cylinder and the outer cylinder member are engaged and fixed; and 
     FIG. 10 is a schematic drawing illustrating an image forming apparatus according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Next, preferred embodiments of the present invention will be described with reference to attached drawings, wherein like reference numerals designate identical or corresponding parts throughout several views. 
     FIG. 1 is a sectional outlined view showing the main part of a fixing apparatus according to an embodiment of the present invention. In this figure, a pressurizing roller  2  is pressed to a heating roller  1  of the fixing apparatus. The heating roller I uses an induction heating system and includes a cylinder  20  forming the circumference of the roller I and a core unit  10  provided inside of the cylinder  20 . 
     The cylinder  20  forming the circumference of the roller  1  is made of a magnetic material such as, for example, stainless steel or iron, and is rotatively supported by bearings  22  and  22 . A gear  21  is engaged and fixed to the end of the cylinder  20  and meshed with a driving gear (not shown) to receive a driving force such that the cylinder  20  of the roller  1  is rotated. A release layer including a fluorine resin is provided on the outside surface of the cylinder  20 . 
     The core unit  10  installed inside of the cylinder  20  of the heating roller  1  includes a fixing shaft  11 , and an induction coil  13  wound around the fixing shaft  11 , and leads  14  and connected to the induction coil  13 . The induction coil  13  receives a high frequency current from a power source (not shown) via the leads  14  and  15 . The fixing shaft  11  is held with brackets  16  and  17  installed on a side board (not shown) of the fixing apparatus and is not rotative. 
     The fixing shaft  11  on which the induction coil  13  is wound has a through hole  18  penetrating the center thereof in the axial direction of the shaft  11 . In addition, a cooling fan  3  is installed outside of the shaft  11  at one side near the end of the through hole  18 . The cooling fan may be omitted, if the fixing shaft  11  can be resistant to the maximum temperature of the induction coil  13 , such as for example, 300° C. In this embodiment, the fixing shaft  11  is made from a resin or ceramics. Further, an insulating unit  30  is installed covering the induction coil  13  of the core unit  10 . The insulating unit  30  includes an insulating member  31  including, for example, a heat resistant resin, and a heat absorbing member  32 , such as for example, a felt member, which is installed around the outer circumference of the insulating member  31 . For the insulating member  31 , a heat constriction tube made of silicon rubber or silicon rubber containing fluorine resin, or a heat resistance member made of silicon rubber or a fluorine resin, may be used. The insulating member  31  is installed tightly contacting the upper surface of the induction coil  13 . The felt member  32  is adhered to the insulating member  31  with a heat resistant adhesive. In this embodiment, belt member  32  is a felt-like felt spirally wound around the outside of the insulating member  31 . Besides, in this embodiment, both ends of the felt member  32  are located outside of both sides of the insulating member  31  (outside in the axial direction of the shaft  11  of the roller  1 ) and these felt ends are adhered to the fixing shaft  11  with PPS resin. 
     In the fixing apparatus constituted as described above, an induction magnetic flux is generated by supplying a high frequency current to the induction coil  13  of the core unit  10  and an induction current is sent to the cylinder  20  made of a magnetic material by the induction magnetic flux. Joule heat is generated in the cylinder  20  by the induction current and thereby the cylinder  20  is heated. 
     As illustrated in FIG. 2, the heating roller  1  is rotatively driven clockwise and the pressurizing roller  2  is pressed to the heating roller  1  to be rotated counterclockwise. A recording sheet S on which a toner image T is loaded for fixing is fed between the heating roller  1  and the pressurizing roller  2  and is conveyed from the right-hand direction to the left-hand direction in the figure, and the toner image T is fixed on the recording sheet S by heat and pressure. In FIG. 2, illustration has been omitted for the core unit  10  and the insulating unit  30  inside the heating roller  1 . In the background heating roller using an induction heating system, when the heating roller is heated, the temperature of the induction coil may rise gradually due to a radiated heat arrived from the cylinder of the heating roller up to a temperature to break an insulation film of the coil. In the above embodiment, when the heating roller I is heated by a fixing operation, the temperature of the inside surface of the cylinder  20  rises up to about  180 - 200 ° C. and a radiant heat is emitted from the internal surface of the cylinder  20  to the inside of the cylinder  20 . However, in this embodiment, the insulating unit  30  is installed covering the induction coil  13  of the core unit  10  of the heating roller  1  and the insulating member  31  of the insulating unit  30  reduces the effect of radiated heat from the cylinder  20  to the coil  13 . Therefore, temperature rise of the induction coil  13  can be prevented and also a defect causing breakdown of the insulating film of the coil  13  can be prevented. 
     In addition, the outer circumferential surface of the insulating member  31  has the felt  32  as a heat absorbing member which reduces the effect of heat on the induction coil  13  by absorbing heat arriving from the cylinder  20  of the heating roller  1 . In addition, the heat absorbing member  32  prevents temperature fall of the cylinder  20  of the heating roller I to increase fixing efficiency. 
     Further, in the embodiment, the insulating unit  30  is installed tightly contacting the induction coil  13  of the core unit  10 . Thus, a gap between the core unit  10  and the cylinder  20 of the heating roller I can be made small, such as for example, 3 min, and thereby the cylinder  20  can be heated efficiently by the induction coil  13 , and as a result the start-up time of the apparatus can be shortened. 
     Next, another embodiment of a fixing apparatus according to the present invention is described below. 
     In the embodiment illustrated in FIG.  3  and FIG. 4, the pressurizing roller  2  is pressed to the heating roller  1  of the fixing apparatus. The heating roller  1  uses the induction heating system, and a core unit  10  is installed inside a cylinder  20  of the roller  1 . Besides, an insulating unit  30  is installed covering the core unit  10 . The cylinder  20  which forms the circumference of the heating roller  1  is made of a magnetic material such as, for example, stainless steel or iron, and is rotatively supported by bearings  22  and  22 . A gear  21  is engaged and fixed to the end of the cylinder  20  and meshed with a driving gear (not shown) to receive a driving force such that the cylinder  20  of the roller  1  is rotated. A release layer including a fluorine resin is provided on the outside surface of the cylinder  20 . 
     The core unit  10  installed inside the cylinder  20  of the heating roller  1  includes a fixing shaft  11 , an induction coil  13  wound around the fixing shaft  11 , and leads  14  and  15  connected to the induction coil  13 . The induction coil  13  receives a high frequency current from a power source (not shown) via the leads  14  and  15 . The fixing shaft  11  is held with brackets  16  and  17  installed on a side board (not shown) of the fixing apparatus and is not rotative. 
     As illustrated in FIG.  3  and FIG. 4, the fixing shaft  11  on which the induction coil  13  is wound has a through hole  18  penetrating the center thereof in the axial direction of the shaft  11  and a plurality of connecting holes  19  connecting the central through hole  18  to the outer circumference of the fixing shaft  11 . 
     The insulating unit  30  includes an insulating cylinder member  31  which is made of, for example, a heat resistant resin, and a felt member  32  which is a heat absorbing member, installed around the outer circumferential surface of the cylinder member  31 . The cylinder member  31  is substantially in tight contact with the fixing shaft  11  via the induction coil  13  which is wound around the fixing shaft  11 . 
     In the fixing apparatus as above constituted, an induction magnetic flux is generated by supplying a high frequency current to the induction coil  13  of the core unit  10  and an induction current is induced in the cylinder  20  made of a magnetic material by the induction magnetic flux. Joule heat is generated in the cylinder  20  by the induction current and the cylinder  20  is thereby heated. 
     As illustrated in FIG. 2, the heating roller  1  is rotatively driven clockwise and the pressurizing roller  2  is pressed to the heating roller  1  to be rotated counterclockwise. A recording sheet S on which a toner image T is loaded for fixing is fed between the heating roller  1  and the pressurizing roller  2  and is conveyed from the right-hand direction to the left-hand direction in FIG. 2, and the toner image T is fixed on the recording sheet S by heat and a pressure. In the FIG. 2, illustration has been omitted of the core unit  10  and the insulating unit  30  inside the heating roller  1 . 
     In the heating roller using an induction heating system, when the heating roller is heated, the temperature of the induction coil may rise gradually due to heat radiated from the cylinder  20  of the roller up to a temperature to cause breakdown of an insulating film of the coil. In the above embodiment of the present invention, when the heating roller  1  is heated by a fixing operation, the temperature of the inside surface of the cylinder  20  rises up to about 180-200° C. and a radiant heat is emitted from the inside surface of the cylinder  20  to the inside of the cylinder  20 . 
     However, in the above embodiment, the fixing shaft  11  of the core unit  10  has a central through hole  18  and a plurality of connecting holes  19  connecting the central through hole  18  to the outer circumference of the fixing shaft  11 . The induction coil  13  is wound around such fixing shaft  11  in an appropriate interval. With this configuration, heat around the coil  13  can be released via the connecting hole  19  and the central through hole  18  to the outside of the heating roller  1 , the coil  13  can be thereby cooled, and temperature rise of the induction coil  13  when heating the heating roller  1  can be prevented. 
     Furthermore, in this embodiment, the insulating unit  30  is installed covering the core unit  10 . The cylinder member  31  of the insulating unit  30 , which is made of a heat resistant resin, reduces the effect of radiated heat from the cylinder  20  of the heating roller  1  to the coil  13 , and prevents heated air inside the roller I from flowing in the core unit  10 . Therefore, temperature rise of the induction coil  13  can be surely prevented and a defect such as breakdown of an insulating film of the coil  13  can be prevented. 
     Furthermore, the felt member  32  serving as a heat absorbing member is adhered to the surface (outer circumferential surface) of the cylinder member  31  to reduce the effect of heat on the induction coil  13  by absorbing the heat radiated from the cylinder  20  of the heating roller  1 . 
     Thus, in the fixing apparatus of the above embodiment, temperature rise of the induction coil  13  and a defect such as breakdown of an insulating film of the coil  13  are surely prevented. 
     Next, another embodiment of a fixing apparatus according to the present invention is described below with reference to FIG.  5 . 
     In FIG. 5, the pressurizing roller  2  is pressed to the heating roller  1  of the fixing apparatus. The heating roller  1  using an induction heating system has a core unit  10  inside a cylinder  20  of the roller  1 . 
     The cylinder  20  which forms the circumference of the heating roller  1  is made of such magnetic material as, for example, stainless steel or iron, and is rotatively supported by bearings  22  and  22 . A gear  21  is engaged and fixed to the end of the cylinder  20  and meshed with a driving gear (not shown) to receive a driving force such that the cylinder  20  of the roller I is rotated. A release layer including a fluorine resin is provided on the outside surface of the cylinder  20 . 
     The core unit  10  installed in the cylinder  20  of the heating roller  1  includes a fixing shaft  11 , an induction coil  13  wound around the fixing shaft  11 , and leads  14  and  15  connected to the induction coil  13 . The fixing shaft  11  around which the induction coil  13  is wound is formed of a non-magnetic material and has a through hole  18  through the center in the axial direction of the shaft  11 . The leads  14  and  15  supplying a high frequency current to the induction coil  13  are wires penetrating from the central through hole  18  to the outer circumference of the fixing shaft  11  and are configured to send a high frequency current from a power source (not shown) to the induction coil  13 . The fixing shaft  11  is held with brackets  16  and  17  installed on a side board (not shown) of the fixing apparatus and is not rotative. 
     In the inside of the cylinder  20 , two insulating cylinder members  41  and  42  are provided so as to rotate integrally with the cylinder  20 . In this embodiment, the cylinder members  41  and  42  are made of a heat resistant resin, such as for example, polyester resin. 
     As illustrated in FIG.  5  and FIG. 7, the outside cylinder member  41  is installed substantially in tight contact with the inner circumference of the cylinder  20  to rotate together with the cylinder  20 . On the other hand, the outer circumference of the inner cylinder member  42  has, as illustrated in FIG. 6, a spiral rib  43 . The rib  43  is substantially in tight contact with the outside cylinder member  41  and thereby the inner cylinder member  42  also rotates together with the cylinder  20  and the outside cylinder member  41 . The width (the length in the axial direction of the shaft  11 ) of the cylinder members  41  and  42  is made larger than the width (the length in the axial direction of the shaft  11 ) of the cylinder  20 . FIG. 7 is a sectional view of the heating roller  1 , in which the cylinder member  41  is installed tightly contacting the inside of the cylinder  20 , and the cylinder member  42  having the rib  43  is installed in the inside of the cylinder member  41 . Also, the fixing shaft  11  is provided in the inside of the cylinder member  42  and the induction coil  13  is wound around the outer circumference of the fixing shaft  11 . 
     In the fixing apparatus with the above configuration, an induction magnetic flux is generated by supplying a high frequency current to the induction coil  13  of the core unit  10  and an induction current is induced in the cylinder  20 , which is made of a magnetic material, by the induction magnetic flux. Joule heat is generated in the cylinder  20  by the induction current and the cylinder  20  is thereby heated. In this embodiment, because the cylinder member  41  is installed substantially tightly contacting the cylinder  20 , heat generated in the cylinder  20  does not disperse inside the roller  1  and efficiently conducts to the surface of the cylinder  20 , and thereby fixing efficiency can be improved. 
     As illustrated in the FIG. 2, the heating roller  1  is rotatively driven clockwise in the figure and the pressurizing roller  2  is pressed to the heating roller  1  to be rotated counterclockwise in the figure. A recording sheet S on which a toner image T is loaded for fixing is fed between the heating roller  1  and the pressurizing roller  2  and is conveyed from the right-hand direction to the left-hand direction in the figure, and the toner image T is fixed on the recording sheet S by heat and pressure. In the FIG. 2, illustration has been omitted for the core unit  10  and the cylinder members  41  and  42  inside the heating roller  1 . 
     In the conventional heating roller using an induction heating system, when the heating roller is heated by induction, the temperature of the induction coil may rise gradually due to a radiated heat arrived from the cylinder of the roller up to a temperature to cause breakdown of an insulating film of the coil. In the above embodiment, when the heating roller I is heated by a fixing operation, the temperature of the inside surface of the cylinder  20  rises up to about 180-200° C. and a radiant heat is emitted from the internal surface of the cylinder  20  to the inside of the cylinder  20 . 
     In this embodiment, the resin-made outer cylinder member  41  is installed inside the cylinder  20  tightly contacting the cylinder  20  to insulate the heat radiated by the cylinder  20 . Further, the insulating resin-made inner cylinder member  42  is installed in the inside of the outside cylinder member  41  with a  2  mm space between them. As a result, the inside cylinder member  42  is exposed both to heat radiated by the outer cylinder member  41  tightly contacting the cylinder  20  and to heat radiated by the outer cylinder member  41  and conducted through air. In this case, the heat conducted through air is of very low level and a great part of the heat received by the inside cylinder member  42  is radiated heat, and therefore, heat supply to the inside cylinder member  42  is greatly reduced. In addition, because the inside cylinder member  42  is made of an electrically insulating resin, even when the induction coil  13  is broken, an electric current from the coil  13  does not flow to the inside of the apparatus. Thus, temperature rise of the induction coil  13  is suppressed and a defect such as breakdown of an insulating film of the coil  13  can be prevented, and thereby a safe fixing apparatus can be realized. 
     Furthermore, in the above embodiment, the spiral rib  43  is provided between double cylinder members  41  and  42 . The spiral rib  43  with a height of 2 mm is formed integrally with the inner cylinder  42  and then engaged with the outer cylinder member  41  at an engaging part, not illustrated, to rotate integrally with the outer cylinder member  41 . Alternatively, as another example, the outer cylinder member  41  may be adhered to fix to the spiral  43  of the inner cylinder member  42  by applying an adhesive to the tip of the spiral  43 . On the basis of such structure, the rib  43  is rotated by rotation of the heating roller  1 , and thereby air between the cylinder members  41  and  42 , which is heated to a high temperature by the heat of the cylinder  20 , is exhausted when the rib  43  is rotated. Further, warm air  1  resident in the space between the cylinder members  41  and  42  can be surely sent to an exhausting direction by making the spiral rib  43  to contact respective cylinder members  41  and  42  tightly. Furthermore, as described above, the width of the cylinder members  41  and  42  is larger than the width of the cylinder  20 , and by exhausting warmed air between the cylinder member  41  and  42  outside the heating roller  1 , cooling efficiency of the heating roller  1  is increased. According to this structure, temperature rise of the induction coil  13  is further prevented and a defect such as breakdown of an insulating film of the coil  13  can be more surely prevented. 
     The pitch (the axial distance between adjacent ribs) of the spiral rib  43  of the inner cylinder member  42  is not constant, and as illustrated in FIG. 6, in the direction of air flow  20  (from left-hand to right-hand in FIG. 6) which is caused by the rib  43  in rotating the cylinder member  42 , an upstream (that is, the entrance side) pitch is made short and a downstream (i.e., the exit side) pitch is made relatively longer. When air flows in the space between the outer cylinder member  41  and the inner cylinder member  42 , more heated air flows to the exit side to make the temperature of the cylinder members  41  and  42  higher at the exit side. However, as described above, because the pitch of the rib  43  at the entrance side is made short and the pitch at the exit side is made relatively longer and the volume of air sent by one rotation of the longer pitch is greater than that of the shorter pitch, the temperature rise of the cylinder members  41  and  42  at the exit side is suppressed. 
     Meanwhile, as illustrated in FIG.  5  and FIG. 9, the cylinder  20  of the heating roller  1  and the outer cylinder member  41  are fixed with a stopper  44 . In the above embodiment, the resin-made outer cylinder member  41  has a coefficient of linear expansion larger than that of the cylinder  20 , and under a normal temperature, the outer cylinder members  41  can be attachable to and detachable from the cylinder  20 , and when heated, the outer cylinder member  41  tightly contacts the cylinder  20 . The stopper  44  is shaped, as illustrated in FIG. 8, in a ring shape (with a cut edge) having an internal projection  44   a  in a position of both ends opposite to each other. Further, as illustrated in FIG. 9, the cylinder  20  and the outside cylinder member  41  have cutaway portions  20   a  and  41  a, respectively. The outer cylinder member  41  is inserted into the cylinder  20 , respective cutaway portions  20   a  and  41   a  are positioned, the stopper  44  is engaged with the cylinder  20  to engage the projection  44   a  with the cutaway portions  20   a  and  41   a , and thereby the outer cylinder member  41  and the cylinder  20  are engaged with each other and fixed. In addition, the stopper  44  also works as the thrust stopper for the cylinder  20 . That is, as illustrated in FIG. 9, removal of the cylinder  20  to the right direction of the cylinder  20  is prevented by the stopper  44  engaged with the cylinder  20  contacting a bearing  22 . The thrust in the opposite direction is stopped, as illustrated in the FIG. 1, by the driving gear  21  fixed to the cylinder  20 . 
     As described above, the thrust of the cylinder  20  is stopped by the stopper  44  engaged with the cylinder  20 . Therefore, the cylinder  20  can be easily pulled for removal by removing the stopper  44 . Besides, removing the stopper  44  allows release of assemblage of the cylinder  20  with the outside cylinder member  41  and therefore, the cylinder member  41 , which is engaged with and in tight contact with the cylinder  20  without use of any adhesive, can be easily removed. Thus, disassembling of the heating roller  1  is made easy and the heating roller  1  as configured above is suitable for recycling. 
     The present invention has been described so far by way of illustrated embodiments. The invention may be practiced in other forms without departing from the spirit or essential characteristics thereof. For example, a polyester resin is used as the cylinder members  41  and  42  in the above embodiment, but resins and other materials (e.g., a silicon rubber) having a heat resistance against the temperature of the heating roller can be used. It is needless to say that the material used for cylinder members  41 ,  42  must be a material not heated by induction. 
     Furthermore, three or more cylinder members can be installed inside the cylinder of the heating roller. In this case, all the cylinder members to be rotated may have spiral ribs, or one or an optional number of cylinder members may have a spiral rib. The rib may be provided in either outside or inside of the cylinder member. For example, in the above embodiment, the rib  43  is provided on the outer circumference of the inner cylinder member  42 . However, the rib may be provided on the inner circumference of the outer cylinder member  41 . Naturally, ribs can be provided on both the outer and inner circumferences of the inner cylinder member  42 . In addition, the height of the rib may be optionally selected. In the above embodiment, the rib  43  provided on the outer circumference of the inner cylinder member  42  is engaged and substantially tightly contacting with the outer cylinder member  41 . The height of the rib  43  may be made lower than the distance between the cylinder members. In this case, using the rib is impossible for transmission of the rotation of the cylinder  20  to the inner cylinder member and therefore another connecting member may be used for connection between respective cylinder members. 
     Further, among a plurality of the cylinder members, only the outermost cylinder member, which is in tight contact with the cylinder  20 , may be configured to be rotated integrally with the cylinder  20  and the other inner cylinder member not be rotated. For example, in the above embodiment, the outer cylinder member  41  tightly contacts the cylinder  20  and thus, naturally rotates integrally with the cylinder  20 . A spiral rib is provided on the inner circumference of the outer cylinder member  41  and the height of the rib is adjusted to a height not reaching the inner cylinder member  42 . In this case, the inner cylinder member  42  is fixedly installed to inhibit rotation following the cylinder  20  (and the outer cylinder member  41 ). Heated air between the cylinder members  41  and  42  is exhausted to the outside of the roller  1  by rotation of the spiral rib provided on the surface of the inner circumference of the outer cylinder member  41 . Also, temperature rise of the induction coil  13  can be prevented by heat insulation by the inner fixed cylinder member  42 . When three or more cylinder members are installed inside the cylinder  20  of the roller  1 , not only the cylinder member tightly contacting the cylinder  20 , but also an optional number of cylinder members can be configured so as to be rotated integrally with the cylinder  20 . In this case, the spiral rib can be used for transmission of rotation of the cylinder  20  to the inner cylinder members or another connector member may be installed. 
     Also, a heat-insulating effect is also yielded when only the outermost cylinder member tightly contacting the cylinder  20  is installed. If the spiral rib is provided on the inner surface of the cylinder member, temperature rise of the induction coil can be suppressed by exhausting heated air inside the roller  1  by way of the spiral rib. Naturally, combined use with other cylinder member increases the suppressing effect on temperature rise of the induction coil. 
     As described above, in a fixing apparatus having a heating roller using an induction heating system according to the preferred embodiments of the present invention, the insulating member installed between the cylinder of the heating roller, which is the heating unit of the fixing apparatus, and the induction coil can reduce an effect of a heat radiated from the cylinder of the roller on the induction coil and also prevent flowing of heated air inside the roller to the induction coil. Therefore, temperature rise of the induction coil can be surely prevented. 
     Further, installing a heat absorbing member on the outer surface of the insulating member can reduce the effect of the heat radiated from the cylinder of the roller on the induction coil by absorbing the heat radiated by the cylinder of the heating roller. 
     Furthermore, a plurality of holes opened in a tubular member, around which the induction coil is wound, and connecting the outer circumferential surface of the tubular member to the outside of the heating roller allows cooling the induction coil and preventing the temperature rise of the induction coil when the heating roller is heated. 
     The insulating cylindrical member installed substantially tightly contacting the inside of the cylinder of the heating roller prevents dispersion of a heat generated by the cylinder in the inside of the roller and allows efficient conduction of the heat to the surface of the cylinder to improve fixing efficiency. 
     Furthermore, provision of at least one additional insulating cylinder member inside the insulating cylinder tightly contacting the cylinder improves insulation of the heat from the cylinder and can further suppress the temperature rise of the induction coil. 
     Still furthermore, the outer insulating cylinder member tightly contacting the cylinder and the inner insulating cylinder member provided inside the outer insulating cylinder member that are longer than the cylinder allows sending air heated by the cylinder to the outside of the heating roller efficiently and suppressing the temperature rise of the induction coil. 
     The spiral rib provided on the outer insulating cylinder member tightly contacting the cylinder and the rotative insulating cylinder member provided inside of the outer insulating cylinder member allows exhausting heated air in the inside of the roller to the outside of the roller by the rotation of the heating roller, cooling the inside of the heating roller efficiently, and preventing more surely the temperature rise of the induction coil. 
     The fixing apparatus according to the preferred embodiments of the present invention as described above can be applied in various types of image forming apparatus, including for example a copying machine, a printer, a facsimile and the like. 
     FIG. 10 illustrates a digital copying machine as an exemplary construction of an image forming apparatus according to the present invention, using a fixing apparatus having a heating roller using an induction heating system. 
     In FIG. 10, a digital copying machine  100  includes an image reading device  111 , a printing device  1   12  and an automatic document feeding device  113 . The automatic document feeding device  113  separates each of the original document sheets set in the automatic document feeding device  113  from each other one by one and feeds the separated original document sheet on a contact glass  114  so as to be positioned in a reading position. 
     The original document on the contact glass  114  is lighted by way of the illuminating lamp  115  and the reflecting mirror  116 , and the light reflected by the original document is imaged on a charge-coupled device (CCD)  122  by a lens  121  via the first mirror  117 , the second mirror  118 , the third mirror  119 , and a color filter  120 . The CCD  122  converts the  5  received light image to electrical signals and outputs analogue image signals representing the read image of the original document. 
     The analog image signals outputted from the CCP device  122  are converted into digital image signals by an analog-to-digital converter (not shown). When an image is formed in the printing device  112 , after a photoconductor drum  125  as an image carrier is driven by a drive unit (not shown) and the surface of the photoconductor drum  125  is uniformly charged by a charging device  126 , the above digital image signals are sent to a semiconductor circuit board (not shown) and a latent image is formed on the surface of the photoconductor drum  125  according to the digital image signals with an image exposure operation performed by a laser beam scanning device  127 . 
     The latent image on the photoconductor drum  125  is then developed with toner to a visible toner image by a developing device  128 . A recording sheet is fed to a registration roller  136  from a selected one of sheet cassettes  133 ,  134  and  135  and is fed toward the photoconductor drum  125  at a timing to register the leading edge of the recording sheet with the leading edge of a toner image formed on the surface of the photoconductor drum  125 .  20  The toner image on the photoconductor drum  125  is transferred onto the recording sheet with a transfer device  130 . The recording sheet carrying the toner image is separated from the photoconductor drum  125  with a separating device  131  and is conveyed by a conveying device  137  to a fixing apparatus  138 , where the toner image is fixed onto the recording sheet. The recording sheet carrying the fixed toner image is then discharged onto an exit tray  139 . The surface of the photoconductor drum  125  is cleaned with a cleaning device  132  after the recording sheet is separated such that residual toner is removed from the surface of the photoconductor drum  125 . 
     In FIG. 10, the fixing apparatus  138  includes a pressurizing roller  2  and a heating roller  1  configured as described above and as illustrated in FIG. 1 or in FIGS. 3 to  9 . The heating roller  1  is rotatively driven clockwise in the drawing and the pressuring roller  2  is pressed to the heating roller  1  to be rotated counterclockwise in the drawing. The recording sheet carrying a toner image thereupon is fed between the heating roller  1  and the pressurizing roller  2  and thereby the toner image is fixed on the recording sheet. In FIG. 10, illustration has been omitted for the core unit  10 , the insulating unit  30  and other elements inside the heating roller. 
     Numerous additional 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 present invention may be practiced otherwise that as specifically described herein.