Abstract:
An induction heating unit enabling to attaching coil on coil attaching surface of bobbin with a high degree of accuracy, and a fixing device capable of maintaining the distance between the coil and the member to be heated at a high degree of accuracy, ensuring uniformity of temperature distribution of the member to be heated, rising temperature at a high speed, and enhancing quality of fixation. In an induction heating unit  30  including a magnetizing coil  39  attached on a coil bobbin  38  which is disposed outside of annular body  19  to be heated, the unit including: a layer of elastic adhesive  44   a  provided on the outer surface of the coil bobbin  38 ; a magnetizing coil  39  provided on the layer of elastic adhesive  44   a , the magnetizing coil  39  being wound beforehand following an attaching surface; and a layer of elastic adhesive  44   b  provided on the magnetizing coil  39.

Description:
This application is based on application No. 2006-324148 filed in Japan on Nov. 30, 2006, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an induction heating unit used as a heat source of fixing device in an image forming apparatus such as copying machine, printer, FAX and complex machine thereof, particularly to a coil heating structure. The present invention also relates to a fixing device using the induction heating unit and a method for attaching coil for the induction heating unit. 
     Conventionally, a heating roller system using a halogen heater has been adopted as a heat source for fixing device of image forming apparatus. From an energy-conservation point of view, an induction heating system having a high heat transfer efficiency has been focused on and developed for practical use. In the fixing device using the induction heating system, a heat transfer efficiency same as or higher than the heating roller system is obtained so that the device can be quickly started up. Especially, an induction heating unit in which magnetic flux generated by coil is directed to heating layer through core material such as ferrite is compact and efficiency. 
     The fixing device has a fixing roller and a press roller which surfaces come into contact with each other and are rotatable. The fixing roller comprises at least five layers of in order from inside support layer, sponge layer, electromagnetic induction heating layer, resilient layer, and releasing layer. Due to magnetic flux generated by a magnetic flux generating means provided around the fixing roller, the electromagnetic induction heating layer of the fixing roller produces heat. When a recording material with an unfixed toner image supported is conveyed and sandwiched in the nip portion between the fixing roller and the press roller, the unfixed toner image on the recording material is melted due to the heat of the heated electromagnetic induction heating layer and fixed to the recording material. 
     In such fixing device of electromagnetic induction heating system, in order to ensure quality of fixation, it is required to heat the member to be heated in uniform distribution of temperature and rise the temperature at high speed. However, if heat capacity of the member to be heated is made lower in order to obtain a high speed temperature rising property, it would be difficult to ensure a uniformity of the temperature distribution. In order to uniform the temperature distribution, it is necessary to maintain a distance between the coil and the member to be heated with a high degree of accuracy. 
     As a bobbin to which the coil is attached is metal molded, the geometry of the coil attaching surface has high accuracy. So the distance between the coil attaching surface of the bobbin and the member to be heated can be maintained in a high degree of accuracy. Therefore, it is important to attaching the coil on the coil attaching surface. 
     As a method for attaching the coil on the bobbin, as shown in  FIG. 13 , there has been a method of directly attaching coil  54  on the coil attaching surface of a bobbin  53  disposed outside of a fixing belt  52  which is provided around a heating roller  51 . In this method, loose of the coil  54  makes it difficult to ensure the accuracy of the distance between the coil  54  and the heating roller  51  that is a member to be heated. Further, thermal expansion at high temperature and vibration causes the coil  54  to shift and flip-flop. Especially, in the case that the unit is small and winding number of the coil  54  is low and that the coil  54  is attached in multiple stages, it is difficult to ensure the distance between the coil  54  and the heating roller  51  and also uniform the temperature distribution of the heating roller  51 . 
     As shown in  FIG. 14 , there has been also a method comprising steps of bonding coil  54  to a bobbin  53  with a silicon adhesive tape  55  and winding PFA tape  56  partly on the coil  54 , preventing the coil  54  from raveling. However, in this method, raveling of the coil  54  of parts other than the parts on which the PFA tape  56  is wound may be caused. 
     The patent document 1 discloses a method in a fixing device for induction heating a fixing roller from inside, as shown in  FIG. 15 , the method comprising steps of directly winding coil  58  on a bobbin  57  and impregnating the coil  58  with modified silicon resin  59  to bond the coil  58  to bobbin  57 , preventing the coil  58  from raveling. The patent document 2 discloses a method in a fixing device for induction heating a fixing roller from inside, the method comprising steps of forming litz wire in a coil wound shape, press forming the coil, and solidifying the coil with adhesive and so on. However, these methods have disadvantages that torsional deformation is generated in the coil due to lack of adhesion force; accuracy of the distance between the coil and the member to be heated becomes worsen; temperature distribution does not become uniform; and noise due to slack of the coil is generated.
     Patent Document 1; JP2002-174971A   Patent Document 2; JP2002-373774A   

     SUMMARY OF THE INVENTION 
     In view of the problems described above, it is an object of the present invention to provide an induction heating unit enabling to attaching coil on coil attaching surface of bobbin with a high degree of accuracy, a method for attaching coil for the induction heating unit and a fixing device capable of maintaining the distance between the coil and the member to be heated at a high degree of accuracy, ensuring uniformity of temperature distribution of the member to be heated, rising temperature at a high speed, and enhancing quality of fixation. 
     In order to attain the above object, according to a first aspect of the present invention, there is provided an induction heating unit comprising a magnetizing coil attached on a coil bobbin which is disposed outside of annular body to be heated, the unit comprising: 
     a layer of elastic adhesive provided on the outer surface of the coil bobbin; 
     a magnetizing coil provided on the layer of elastic adhesive, the magnetizing coil being wound beforehand following an attaching surface; and 
     a layer of elastic adhesive provided on the magnetizing coil. 
     In the specification, the phrase of “annular body to be heated” means a roller having a circular cross section, a belt having a circular cross section provided on the outer surface of the roller, and an endless belt supported in two rollers and so on. The term of “attaching surface” means a surface on the coil bobbin on which the coil is attached. The term of “elastic adhesive” means an adhesive which maintains an elasticity like a rubber even after being hardened. 
     Preferably, the unit further comprises: 
     an insulating sheet provided on a part of the layer of elastic adhesive on the magnetizing coil; 
     a layer of elastic adhesive provided on the insulating sheet; 
     a first demagnetizing coil provided on the layer of elastic adhesive, the demagnetizing coil being wound beforehand following an attaching surface; and 
     a layer of elastic adhesive provided on the first demagnetizing coil. 
     Preferably, the unit further comprises: 
     an insulating sheet provided on a part of the layer of elastic adhesive on the first demagnetizing coil; 
     a layer of elastic adhesive provided on the insulating sheet; 
     a second demagnetizing coil provided on the layer of elastic adhesive, the demagnetizing coil being wound beforehand following an attaching surface; and 
     a layer of elastic adhesive provided on the second demagnetizing coil. 
     According to a second aspect of the present invention, there is provided an induction heating unit comprising a magnetizing coil attached on a coil bobbin which is disposed outside of annular body to be heated, wherein a magnetizing coil wound beforehand following an attaching surface is bonded on the outer surface of the coil bobbin by an elastic adhesive and wherein an elastic adhesive is applied on the outer surface of the magnetizing coil. 
     Preferably, an insulating sheet is provided on a part of the elastic adhesive on the magnetizing coil; a first demagnetizing coil wound beforehand following an attaching surface is bonded on the insulating sheet by an elastic adhesive; and an elastic adhesive is applied on the outer surface of the first demagnetizing coil. 
     Preferably, an insulating sheet is provided on a part of the elastic adhesive on the first demagnetizing coil; a second demagnetizing coil wound beforehand following an attaching surface is bonded on the insulating sheet by an elastic adhesive; and an elastic adhesive is applied on the outer surface of the second demagnetizing coil. 
     Preferably, the magnetizing coil, the first demagnetizing coil and the second demagnetizing coil are formed by winding ritz wire on a winding jig having a winding surface which follows the attaching surface thereof. 
     Preferably, the magnetizing coil, the first demagnetizing coil and the second demagnetizing coil are held in the shape of the outer surface of the attaching surface thereof by self fusion bonding. 
     According to a third aspect of the present invention, there is provided a fixing device comprising:
         an induction heating unit;   a fixing roller as a member to be heated; and   a press roller which comes into press contact with the fixing roller to form a fixing nip portion for sandwiching a recording medium.       

     According to a fourth aspect of the present invention, there is provided a method for attaching coil on a coil bobbin for an induction heating unit, comprising steps of: 
     preparing a magnetizing coil wound beforehand following a surface shape of the coil bobbin; 
     applying an elastic adhesive uniformly on the outer surface of the coil bobbin; 
     setting the magnetizing coil on the elastic adhesive; 
     pressing the outer surface of the magnetizing coil uniformly; 
     heating the coil bobbin and the magnetizing coil to harden the elastic adhesive; and 
     applying an elastic adhesive on the outer surface of the magnetizing coil. 
     According to the induction heating unit in the first aspect of the present invention, as the magnetizing coil wound beforehand following an attaching surface is provided on the layer of elastic adhesive, it is possible to maintain an accuracy of the distance between the coil and the member to be heated and make the temperature distribution of the member to be heated uniform. It is also possible to eliminate a creaking noise due to a difference of thermal expansion between the magnetizing coil and the coil bobbin. 
     According to the induction heating unit in the second aspect of the present invention, as the magnetizing coil wound beforehand is bonded on the coil bobbin by the elastic adhesive, it is possible to maintain an accuracy of the distance between the coil and the member to be heated and make the temperature distribution of the member to be heated uniform. It is also possible to eliminate a creaking noise due to a difference of thermal expansion between the magnetizing coil and the coil bobbin. 
     According to the fixing device in the third aspect of the present invention, as the device has the induction heating unit enabling to maintain an accuracy of the distance between the coil and the member to be heated and make the temperature distribution of the member to be heated uniform, it is possible to enhance the quality of fixation. 
     According to the method for attaching coil in the fourth aspect of the present invention, as the magnetizing coil wound beforehand is bonded on the coil bobbin by the elastic adhesive, it is possible to maintain an accuracy of the distance between the coil and the member to be heated and make the temperature distribution of the member to be heated uniform. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further objects and advantages of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which: 
         FIG. 1  is a sectional view showing inner constitution of an image forming apparatus provided with a fixing device comprising an induction heating unit according to the present invention; 
         FIG. 2  is a sectional view of the fixing device comprising an induction heating unit according to a first embodiment of the present invention; 
         FIG. 3  is a front view of the induction heating unit of  FIG. 2 ; 
         FIG. 4  is a fragmental perspective view of the induction heating unit of  FIG. 3 ; 
         FIG. 5  is sectional views showing attaching procedure of magnetizing coil; 
         FIG. 6  is a sectional view showing attaching structure of the magnetizing coil of  FIG. 3 ; 
         FIG. 7  is a graph showing quantity of warpage of the fixing roller; 
         FIG. 8  is a graph showing temperature distribution of the fixing roller; 
         FIG. 9  is a fragmental perspective view of the induction heating unit according to a second embodiment of the present invention; 
         FIG. 10  is a front view of the induction heating unit of  FIG. 9 ; 
         FIG. 11  is a sectional view showing attaching structure of the magnetizing coil of  FIG. 6 ; 
         FIG. 12  is a graph showing temperature distribution of the fixing roller before and after endurance printing; 
         FIG. 13  is a sectional view of a fixing device comprising conventional induction heating unit; 
         FIG. 14  is a sectional view showing attaching structure of conventional magnetizing coil; and 
         FIG. 15  is a sectional view showing another attaching structure of conventional magnetizing coil. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a schematic construction of the image forming apparatus  10  provided with a fixing device according to the present invention. The image forming apparatus  10  is provided with an intermediate transfer belt  11  in the nearly center of the inside. The intermediate transfer belt  11  is supported on the outer circumference of rollers  12   a ,  12   b  and driven to rotate in the direction of arrow A. Beneath the lower horizontal portion of the intermediate transfer belt  11 , four imaging units  13 Y,  13 M,  13 C,  13 K corresponding to each color of yellow (Y), magenta (M), cyan (C) and black (K) are disposed along the intermediate transfer belt  11 . Inside the intermediate transfer belt  11  are disposed first transfer rollers  15 Y,  15 M,  15 C,  15 K which are opposed to photosensitive drums  14 Y,  14 M,  14 C,  14 K of the imaging units  13 Y,  13 M,  13 C,  13 K via the intermediate transfer belt  11 . A second transfer roller  16  comes into contact with the part supported by the drive roller  12   a  of the intermediate transfer belt  11 . The nip portion between the second transfer roller  16  and the intermediate belt  11  is a second transfer area  17 . In the paper path  18  on the down stream side of the second transfer area  17 , a fixing roller  19  and a press roller  20  are disposed. The press contact portion of the fixing roller  19  and the press roller  20  is a fixing nip area  21 . 
     In the lower portion of the image forming apparatus  10 , four kinds of first, second, third and fourth paper feed portions  22   a ,  22   b ,  22   c ,  22   d  are provided. The first paper feed portion  22   a  and the second paper feed portion  22   b  are disposed up and down. The third paper feed portion  22   c  and the fourth paper feed portion  22   d  are disposed right and left below the second paper feed portion  22   b . The paper stacked and contained in each of the paper feed portions  22   a ,  22   b ,  22   c ,  22   d  is fed one by one from the upper most one to the paper passage  23 . A circulation passage  24  is formed on the side portion of the image forming apparatus  10 . The paper with one surface printed which switchbacks at a paper discharge roller  25  is conveyed downward through the circulation passage  24  and then conveyed upward again through the paper passages  23 ,  18  in a state that the unprinted surface faces the side of the intermediate transfer belt  11 . A manual paper feed unit  26  is disposed below the circulation passage  24 . The paper fed from the manual paper feed unit  26  is conveyed upward through the paper passage  23 . Numerals  27 Y,  27 M,  27 C,  27 K denote toner cartridges for feeding toner of each color to the imaging units  13 Y,  13 M,  13 C,  13 K. Numeral  28  shows a control unit for controlling the imaging units  13 Y,  13 M,  13 C,  13 K. 
     Next, brief operation of the image forming apparatus  10  having the above construction will be described. Color print data obtained by reading an image at an image reading portion  29  or image data outputted from a personal computer or so is given a predetermined signal processing at the control unit  28  and transmitted to each of the imaging units  13 Y,  13 M,  13 C,  13 K as image signals for each color of yellow (Y), magenta (M), cyan (C) and black (K). In each of the imaging units  13 Y,  13 M,  13 C,  13 K, a laser light modulated by the image signal is projected on the photosensitive drums  14 Y,  14 M,  14 C,  14 K to form an electrostatic latent image. The latent image formed on each of the photosensitive drums  14 Y,  14 M,  14 C,  14 K is developed by the respective developing unit to form a toner image of yellow, magenta, cyan, black on the photosensitive drums  14 Y,  14 M,  14 C,  14 K. The toner images of yellow, magenta, cyan, black are superimposed and first transferred on the moving intermediate transfer belt  11  under the operation of the first transfer rollers  15 Y,  15 M,  15 C,  15 K. The superimposed toner image formed on the intermediate transfer belt  11  reaches the second transfer area  17  as the intermediate transfer belt  11  moves. In the second transfer area  17 , the superimposed toner image is second transferred on a paper fed from the paper feed portions  22   a ,  22   b ,  22   c ,  22   d  or the manual paper feed unit  26  under the operation of the second transfer roller  16 . Then, the paper on which the toner image is second transferred reaches the fixing nip area  21 . In the fixing nip area  21 , the toner image is fixed on the paper under the operation of the fixing roller  19  and the press roller  20 . The paper P on which the toner image is fixed is discharged on the discharge tray  7  through the discharge roller  25 . 
       FIG. 2  shows a detailed sectional view of a fixing device  31  according to a first embodiment of the present invention. The fixing device  31  comprises the fixing roller  19 , the press roller and an induction heating unit  30 . 
     The fixing roller  19  is consist of a core  32  comprising a stainless steel pipe, a silicon sponge layer  33  covered on the core  32  and a fixing belt  34  attached on the silicon sponge layer  33 . The fixing belt  34  comprises an endless electrocast sleeve, silicon rubber and PFA tube covered on the sleeve. 
     The press roller  20  is consist of a core  35  comprising a steel pipe, a silicon sponge layer  36  covered on the core  35  and a PFA tube  37  covered on the silicon sponge layer  36 . 
     The induction heating unit  30 , as shown in  FIGS. 3 ,  4 , comprises a coil bobbin  38 , a magnetizing coil  39  and a core  40 . 
     The coil bobbin  38  has a plate like shape that curves in a circular arc pattern along the outer surface of the fixing roller  19  so as to cover the substantially half surface of the fixing roller  19  and extends in the axial direction of the fixing roller  19 . An oblong or track shape of rib  41  extending in the longitudinal direction is formed on the middle of the outer surface of the coil bobbin  38 . As the coil bobbin  38 , liquid crystal polymer, PPS (polyphenylen sulfide), PEEK® (trade mark of Victrex plc), or phenol resin can be used but is not limited to these. The profile tolerance of the outer surface of the coil bobbin  38  to the center of the fixing roller  19  is less than 0.4. The profile tolerance is a difference between maximum and minimum of radius from the center of the fixing roller  19  at measurement positions that are provided at a predetermined pitch both in the longitudinal direction and the circumferential direction of the coil bobbin  38 . 
     The magnetizing coil  39  is consisted of a litz wire comprising twisted and bound element wires. As shown in  FIG. 3 , the magnetizing coil  39  has a construction wound around the rib  41  of the coil bobbin  38 . The construction for attaching the magnetizing coil  39  will be described in detail hereinafter. Considering that the litz wire receives heat transmission, the litz wire coated with heat resistant resin is preferably used. 
     The core  40  comprises a plurality of magnetic bodies each of which has a bar like shape bended and extended in the circumference direction of the coil bobbin  38 . The magnetic bodies are disposed at a predetermined pitch in the longitudinal direction of the coil bobbin  38  and attached to the coil bobbin  38  to cover the outer surface of the magnetizing coil  39 . 
     A method for attaching the magnetizing coil  39  onto the coil bobbin  38  will be described below. First, a winding jig not shown comprising a winding surface having the same shape as the outer surface of the coil bobbin  38 , a rib having the same shape as the rib  41  of the coil bobbin  38 , and a flange extending along the winding surface on both sides of the rib is prepared. 
     Using the winding jig, the litz wire is wound and heated so that the wound litz wire is self fusion bonded and held in the shape of the outer surface of the coil bobbin  38  to form the magnetizing coil  39  as shown in  FIG. 4 . 
     Then, as shown in  FIG. 5(   a ), a heat resistant elastic adhesive  44   a  is uniformly applied on the outer surface of the coil bobbin  38 . The magnetizing coil  39  is set on the coil bobbin  38  so that the rib  41  the coil bobbin  38  is inserted into the inner diameter portion of the magnetizing coil  39 . As shown in  FIG. 5(   b ), the outer surface of the magnetizing coil  39  is uniformly pressed using a press jig  45 . Thus, the magnetizing coil  39  follows and adheres in a contact state to the outer surface of the coil bobbin  38 . After that, the elastic adhesive  44   a  is hardened so that the magnetizing coil  39  does not depart from the coil bobbin  38 . When the magnetizing coil  39  is removed from the magnetizing coil  39 , warpage due to residual stress is apt to occur. However, as the elastic adhesive  44   a  is hardened during pressure using the pressure jig  45 , the warpage is corrected. A departure of less than 0.1 mm from the outer surface of the coil bobbin  38  of the magnetizing coil  39  is acceptable. Subsequently, as shown in  FIG. 5(C) , an elastic adhesive  44   b  is applied on the outer surface of the magnetizing coil  39  and hardened, preventing the magnetizing coil  39  from raveling. Lastly, as shown in  FIG. 5(   d ), the core  40  is attached on the coil bobbin  38 . 
       FIG. 6  shows a sectional view of the induction heating unit by attaching the magnetizing coil  39  on the coil bobbin  38  as described above. The induction heating unit  30  comprises a layer of elastic adhesive  44   a  provided on the outer surface of the coil bobbin  38 ; the magnetizing coil  39  provided on the layer of elastic adhesive  44   a , the magnetizing coil  39  wound beforehand following the attaching surface; and a layer of elastic adhesive  44   b.    
     In the conventional method for winding coil directly on a coil bobbin, it has been necessary to form the coil bobbin larger without any protrusions in order to prevent the coil during the wind from interfering with each part of the coil bobbin. On the other hand, in the present invention, as the magnetizing coil  39  wound beforehand by the winding jig is bonded to the coil bobbin, even if there is a protrusion in the vicinity of the attaching surface, the protrusion does not get in the way of attaching the coil. Thus, in comparison with the conventional construction in which the coil is wound directly on the coil bobbin, the present invention is possible to downsize the device. 
     The elastic adhesive  44   a ,  44   b  is necessary to have a heat resistance of at lest more than 180° C. Also, it is important that the elastic adhesive  44   a ,  44   b  has rubber elasticity after hardening. In an example, RVT rubber KE3418 (Shin-Etsu Chemical Co., Ltd.) was used. Also, one-component system, two-component system and more than three-component system of silicon rubber, and LTV type, RTV type or HTV type of silicon rubber can be used. 
     The self fusion bonding of the magnetizing coil  39  is performed by fusion bonding the enamel layer on the surface of the litz wire at the temperature of 180 to 220° C. to hold the wire in the curved shape of the coil bobbin  38 . 
     When using the induction heating unit  30 , it is necessary to set a margin of heating temperature in order to prevent the magnetizing coil  39  from being heated to a temperature of more than fusion-bonding temperature of the enamel layer. It is possible to cope with a speed up of the image forming apparatus  10  by air cooling the magnetizing coil  39  or whole of the induction heating unit  30  when the magnetizing coil  39  becomes more than fusion bonding temperature in use. 
     As the coil bobbin  38  and the magnetizing coil  39  are different in thermal expansion coefficient, the difference of thermal expansion is caused due to the heat generated in use of the image forming apparatus  10 . However, the elastic adhesive  44   a  allows the magnetizing coil  39  to follow the expansion of the coil bobbin  38 , absorbing the difference of thermal expansion. Thus, departure of the magnetizing coil  39  would be never caused and accuracy of the distance between the magnetizing coil  39  and the fixing roller  19  that is a member to be heated could be ensured, reducing generation of noise due to slack of the magnetizing coil  39 . 
     Quantity of thermal expansion of the coil bobbin  38  and the magnetizing coil  39  when heating the induction heating unit  30  from normal temperature 20° C. to actual normal temperature 200° C. was calculated. Where, material and linear thermal coefficient of the coil bobbin  38  and the magnetizing coil  39  of the induction heating unit  30  were as shown in Table 1 and dimension in the axial direction were 360 mm. As a result, as shown in Table 1, difference of the quantity of the thermal expansion between the coil bobbin  38  and the magnetizing coil  39  was 1.4 mm. In the experiment, almost same result was obtained. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Linear thermal 
                 Quantity of 
               
               
                   
                   
                 coefficient 
                 thermal 
               
               
                   
                 Material 
                 (10 −5  ° C.) 
                 expansion (mm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Coil Bobbin 
                 A130 
                 4.4 
                 2.4 
               
               
                   
                 Magnetizing 
                 Copper 
                 1.9 
                 1.0 
               
               
                   
                 Coil 
               
               
                   
                   
               
             
          
         
       
     
     The difference of the quantity of the thermal expansion between the coil bobbin  38  and the magnetizing coil  39  causes the coil bobbin  38  to warp.  FIG. 7  shows quantity of warpage (curve of “▴” in the figure) of the inner surface of the coil bobbin  38  of the induction heating unit  30  according to the present invention in which the elastic adhesive (KE3417) was used for bonding the coil bobbin  38  and the magnetizing coil  39  and quantity of warpage (curve of “           ” in the figure) of the inner surface of the coil bobbin of conventional induction heating unit in which non-elastic adhesive was used.
     As clear from the figure, the quantity of warpage of the inner surface of the coil bobbin in the case that non-elastic adhesive was used was 0.7 mm. On the other hand, the quantity of warpage of the inner surface of the coil bobbin  38  in the case that elastic adhesive (KE3417) was used according to the present invention was 0.2 mm. 
     The warpage of the coil bobbin  38  causes the distance between the magnetizing coil  39  and the fixing roller  19  to change and affects the temperature distribution in the axial direction of the surface of the fixing roller  19 .  FIG. 8  shows temperature distribution (curve of thick line in the figure) of the fixing roller  19  of the induction heating unit  30  according to the present invention in which the elastic adhesive (KE3417) was used for bonding the coil bobbin  38  and the magnetizing coil  39  and temperature distribution (curve of thin line in the figure) of the fixing roller of the conventional induction heating unit in which non-elastic adhesive was used. 
     As clear from the figure, the temperature distribution of the surface of the fixing roller in the case that non-elastic adhesive was used as in the conventional unit was that both ends portion was higher by 7° C. than the center. On the other hand, the temperature distribution of the surface of the fixing roller  19  in the case that elastic adhesive (KE3417) was used according to the present invention was substantially flat and had a maximum difference of 2° C. 
     The difference of the quantity of thermal expansion between the coil bobbin  38  and the magnetizing coil  39  and the temperature change between the low-temperature time and the high-temperature time generate a “creaking” noise from the contact portion of the coil bobbin  38  and the magnetizing coil  39 . Table 2 shows existence or nonexistence of the creaking noise of the induction heating unit  30  according to the present invention in which the elastic adhesive (KE3417) was used for bonding the coil bobbin  38  and the magnetizing coil  39  and existence or nonexistence of the creaking noise of the conventional induction heating unit in which non-elastic adhesive was used. The test was performed by 5 minutes observation after endurance printing and entrusted to a tester. 
     As clear from the table, in the case that non-elastic adhesive was used as in the conventional unit, the bonding was hard and the creaking noise was easily generated. The creaking noise began to occur from 24 hours endurance printing and occurred frequently at 1000 hours endurance printing. On the other hand, in the case that elastic adhesive (KE3417) was used according to the present invention, no creaking noise was occurred even at 1000 hours endurance printing. 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Elastic adhesive was 
                 Non-elastic adhesive 
               
               
                 Endurance 
                 used 
                 was used 
               
               
                 printing time 
                 (present invention) 
                 (conventional unit) 
               
               
                   
               
             
             
               
                   0 hr 
                 none 
                 none 
               
               
                  24 hr 
                 none 
                 rarely 
               
               
                  100 hr 
                 none 
                 occasionally 
               
               
                 1000 hr 
                 none 
                 frequently 
               
               
                   
               
             
          
         
       
     
       FIG. 9  is an exploded perspective view of an induction heating unit  30 A according to a second embodiment of the present invention.  FIG. 10  is a front view of the unit. In the induction heating unit  30 A, first demagnetizing coils  46  are provided on both ends portion of the magnetizing coil  39  and second demagnetizing coils  47  are provided on the first demagnetizing coils  46 . If the length of the magnetizing coil  39  corresponds to the maximum paper size A, the length of the first demagnetizing coil  46  is decided so that the length between the first demagnetizing coils  46  is substantially same as the paper size B smaller than the paper size A. Similarly, the length of the second demagnetizing coil  47  is decided so that the length between the second demagnetizing coils  47  is substantially same as the paper size C smaller than the paper size B. 
     The same method as the first embodiment can be adopted until the magnetizing coil  39  is attached on the coil bobbin  38 . The first demagnetizing coil  46  is wound using a winding jig in the same manner as the magnetizing coil  39 , heated and self fusion bonded and held in the shape of the outer surface of the magnetizing coil  39  that is an attaching surface. On the other hand, as shown in  FIG. 11 , an insulating sheet  48   a  is set on an elastic adhesive  44   b  applied in the magnetizing coil  39 . Then, a heat resistant elastic adhesive  44   c  is uniformly applied on the insulating sheet  48   a  to bond the wound first demagnetizing coil  46 . The first demagnetizing coil  46  is uniformly pressed using a press jig and applied with an elastic adhesive  44   d . Further, in the same manner, an insulating sheet  48   b , an elastic adhesive  44   e . the second demagnetizing coil  47  and an elastic adhesive  44   f  are provided. The most upper surface is protected by a glass cloth tape, a heat resistant tape not shown. As the insulating sheet  48   a ,  48   b , a Nomex sheet (made by NITTO DENKO CORPORATION) is preferable but a polyimide sheet, a fluorine sheet or so is possible if insulation property and heat resistance are ensured. 
     The induction heating unit  30 A comprises: a layer of elastic adhesive  44   a  provided on the outer surface of the coil bobbin  38 ; the magnetizing coil  39  provided on the layer of elastic adhesive  44   a , the magnetizing coil  39  being wound beforehand following the attaching surface; a layer of elastic adhesive  44   b  provided on the magnetizing coil  39 ; an insulating sheet  48   a  provided on the elastic adhesive  44   b ; a layer of elastic adhesive  44   c  provided on the insulating sheet  48   a ; the first demagnetizing coil  46  provided on the layer of elastic adhesive  44   c , the first demagnetizing coil  46  being wound beforehand following the attaching surface; a layer of elastic adhesive  44   d  provided on the first demagnetizing coil  46 ; an insulating sheet  48   b  provided on the elastic adhesive  44   d ; a layer of elastic adhesive  44   e  provided on the insulating sheet  48   b ; the second demagnetizing coil  47  provided on the layer of elastic adhesive  44   e , the first demagnetizing coil  47  being wound beforehand following the attaching surface; and a layer of elastic adhesive  44   f  provided on the second demagnetizing coil  47 . 
     In general, in the case that the paper size is small in comparison with the length of the fixing roller, when continuously passing the paper, fixing operation causes the heat to be drawn from the center portion of the fixing roller though which the small size of paper passes, reducing the temperature of the fixing roller. In order to compensate for this, control to increase the temperature of the fixing roller is performed. However, at the both ends portion through which no paper passes, the fixing operation does not cause decrease of temperature. Thus, the temperature of the both ends increase in comparison with the center portion. 
     So, in the induction heating unit  30 A of the second embodiment, switches for turning on and off current to the first demagnetizing coils  47  and the second demagnetizing coils  47  are closed in response to the paper size to generate a counter electromotive force in the demagnetizing coils  46 ,  47 . Thus, a reverse magnetic field is generated in a direction that change of the magnetic field from the magnetizing coil  39  is prevented, reducing the magnetic field generated from the magnetizing coil  39  at the portions that the demagnetizing coil  46 ,  47  are disposed. As a result, it is possible to prevent increase of the temperature of the fixing roller  19  only at the range of demagnetizing coils  46 ,  47 . As the first demagnetizing coils  46  and the second demagnetizing coils  47  are attached in the same manner as the magnetizing coil  39 , no warpage and no creaking noise occurs, enabling to obtain a uniform temperature distribution of the fixing roller  19 . Also, error and decrease of paper passing speed due to temperature anomaly of the fixing roller  19  that are generated in the case that the demagnetizing rollers  46 ,  47  are not used would not be caused. 
       FIG. 12  shows a temperature distribution of the fixing roller  19  before and after 1500,000 endurance printing by the image forming apparatus having the fixing device using the induction heating unit  30 A according to the second embodiment. 
     As clear from the figure, the temperature distribution before and after the endurance printing was not remains unchanged. This shows that fixing quality can be ensured even after the endurance printing. 
     Although the present invention has been fully described by way of the examples with reference to the accompanying drawing, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications otherwise depart from the spirit and scope of the present invention, they should be construed as being included therein.