Patent Publication Number: US-7907882-B2

Title: Image heating apparatus

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image heating apparatus of an electromagnetic (magnetic) induction heating type suitably used as an image heating fixing apparatus (device) to be mounted in an image forming apparatus, such as a copying machine, a printer, or a facsimile machine, for effecting image formation through an electrophotographic system, an electrostatic recording system, a magnetic recording system, or the like. 
     As the image heating apparatus, it is possible to use a fixing device for fixing or temporarily fixing an unfixed image on a recording material, a glossiness-enhancing device for enhancing glossiness of an image fixed on the recording material by heating the image, and the like device. 
     In the image forming apparatus, a fixing device is provided in order to fix an unfixed toner image formed on the recording material as a fixed image. As the fixing device, in recent years, those of the electromagnetic induction heating type in which a heating medium such as a heating roller is heated by Joule heat generated by the action of electromagnetic induction have received attention from the viewpoint of energy saving. 
     Particularly, in a constitution in which a heating belt having an endless shape is used as the heating medium, the heating belt has a thermal capacity smaller than that of the heating roller, so that a rise in temperature is rapid and therefore electric energy consumption can be further reduced. 
     For example, Japanese Laid-Open Patent Application (JP-A) Hei 08-076620 discloses a heating device of the electromagnetic induction heating type in which a magnetic field is applied to an endless belt-like electroconductive heat generating member by a magnetic field generating means and a material to be heated which is brought into intimate contact with the belt is heated by heat generated by eddy current generated in an electroconductive heat generating layer. The magnetic field generating means is formed integrally with a means for urging the belt to form a nip and is disposed inside the endless belt. 
     JP-A Hei 07-295414 discloses a fixing device in which the magnetic field generating means is disposed along an outer peripheral surface of a fixing member (heat generating member), so that an induction (exciting) coil as the magnetic field generating means is liable to dissipate heat. 
     In the fixing device in which the magnetic field generating means is disposed along the outer peripheral surface of the fixing member, as described in JP-A 2004-341164, a length of the coil with respect to its longitudinal direction is shorter than that of the fixing member. 
     On the other hand, in order to downsize the image forming apparatus, it is preferable that the longitudinal direction length of the fixing member is decreased. As a result, a distance between an end of an image area and an end portion of the fixing member is decreased. For this reason, in order to ensure a temperature at an end portion of the image area, there is need to provide the coil with the longitudinal direction length equal to or longer than the longitudinal direction length of the fixing member. 
     However, in such a constitution, magnetic flux concentrates at the end portion of the fixing member correspondingly to the increment of the longitudinal direction length of the coil, so that the temperature of the fixing member at its end portion is increased. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to provide an image heating apparatus capable of reducing a degree of temperature rise caused to magnetic flux concentration at a metal belt end portion. 
     According to an aspect of the present invention, there is provided an image heating apparatus comprising: 
     a coil for generating magnetic flux; 
     a rotatable heat generating member, having an electroconductive layer which generates heat by the magnetic flux, for heating an image on a recording material, wherein the coil has a length longer than that of the heat generating member with respect to a rotational axis direction of the heat generating member; and 
     a magnetic member, provided oppositely to the coil at an end position of the heat generating member, having AC magnetic permeability of 1000 or more at 100 kHz. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional right side view of a principal part of a fixing device in Embodiment 1. 
         FIG. 2  is a partly omitted schematic front view of the fixing device. 
         FIG. 3  is a partly omitted schematic longitudinal sectional front view of the fixing device. 
         FIG. 4  is a schematic view showing a layer structure of a fixing belt (heat generating member). 
         FIG. 5(   a ) is an exploded perspective view showing a left flange member, a left end portion of a stay, and a left end portion of a guiding member, and  FIG. 5(   b ) is an exploded perspective view showing a right flange member, a right end portion of the stay, and a right end portion of the guiding member. 
         FIG. 6  is a schematic plan view of a coil assembly. 
         FIG. 7  is a block diagram of a control system. 
         FIG. 8  is a schematic perspective view of a magnetic member. 
         FIG. 9  is a schematic view for illustrating a relationship between a longitudinal direction length of a coil and a longitudinal direction length of a belt. 
         FIG. 10  is a graph showing a distribution of a temperature of the belt along the longitudinal direction of the belt in the case where a longitudinal central portion of the belt is heated from room temperature to 190° C. by driving fixing devices in Embodiment 1, Comparative Embodiment 1, and Comparative Embodiment 2. 
         FIG. 11  is a schematic sectional view showing a portion at which the belt end portion is covered with the magnetic member. 
         FIG. 12  is a schematic sectional view showing a portion at which the belt end portion is covered with a belt end portion abutting member of a non-magnetic material (PPS) in Comparative Embodiment 1. 
         FIG. 13  is a schematic view showing a state of the magnetic flux with respect to the longitudinal direction of the belt in Comparative Embodiment 1. 
         FIG. 14  is a graph showing a change in hardness with the lapse of an idling time in Embodiment 1 and Comparative Embodiment 1. 
         FIG. 15(   a ) is a schematic view showing a constitution in which the magnetic member is disposed in contact with an end portion side surface of the belt, and  FIG. 15(   b ) is a schematic view showing a constitution in which the magnetic member is disposed close to the end portion side surface of the belt. 
         FIG. 16  is a schematic view showing a relationship among the longitudinal direction length of the coil, the longitudinal direction length of a coil core, and the longitudinal direction length of the belt in a fixing device in Embodiment 2. 
         FIG. 17  is a graph showing a distribution of a temperature of the belt along the longitudinal direction of the belt in the case where a longitudinal central portion of the belt is heated from room temperature to 190° C. by driving fixing devices in Embodiment 2 and Comparative Embodiment 3. 
         FIG. 18  is a schematic longitudinal sectional showing a schematic structure of an embodiment of an image forming apparatus. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinbelow, the present invention will be described specifically based on embodiments with reference to the drawings. In the present invention, the following embodiments are preferred embodiments of the present invention but the present invention is not limited to constitutions described in the following embodiments. That is, within the scope of the present invention, the constitution described in the following embodiments are substitutable by other known constitutions. 
     Embodiment 1 
     (1) Image Forming Station 
       FIG. 2  is a longitudinal schematic view showing a general structure of an electrophotographic full-color printer as an example of an image forming apparatus in which the image heating apparatus according to the present invention is mounted as a fixing device. First, a schematic structure of an image forming station (portion) will be described. 
     This printer performs an image forming operation depending on image information inputted from an external host device  200  communicatably connected with a control circuit portion (control board: CPU)  100  including a control portion, thus being capable of forming a full-color image on a recording material P and then outputting the full-color image. 
     The external host device  200  is a computer, an image reader, or the like. The control circuit portion  100  as the control portion sends signals to and receives signals from the external host device  200 . Further, the control circuit portion  100  sends signals to and receives signals from various devices for image formation to manage image forming sequence control. 
     An endless and flexible intermediary transfer belt  8  (hereinafter referred also simply to as a belt) is stretched between a secondary transfer opposite roller  9  and a tension roller  10  and is rotatable driven at a predetermined speed in a counterclockwise direction indicated by an arrow By rotation of the roller  9 . A secondary transfer roller  11  presses the belt  8  against the secondary transfer opposite roller  9 . A (press)-contact portion between the belt  8  and the secondary transfer roller  11  constitutes a secondary transfer portion. 
     First to fourth (four) image forming stations  1 Y,  1 M,  1 C and  1 Bk are disposed in line under the belt  8  along a belt movement direction with a predetermined interval. Each of the image forming stations is an electrophotographic process mechanism of a laser exposure type and includes a drum-type electrophotographic photosensitive member  2  (hereinafter simply referred to as a drum) as an image bearing member to be rotationally driven at a predetermined speed in a clockwise direction indicated by an arrow. Around the drum  2 , a primary charger  3 , a developing device  4 , a transfer roller  5  as a transfer means, and a drum cleaning device  6  are disposed. The transfer roller  5  is disposed inside the intermediary transfer belt  8  and presses the lower-side belt portion of the belt  8  against the drum  2 . A (press)-contact portion between the drum  2  and the belt  8  constitutes a primary transfer portion. A laser exposure device  7  for each of the drums  2  of the respective image forming stations is constituted by a laser emitting means for emitting light correspondingly to a time-serial electric digital pixel signal of image information to be provided, a polygonal mirror, a reflection mirror, and the like. 
     The control circuit portion  100  causes each image forming station to perform an image forming operation on the basis of a color-separated image signal inputted from the external host device  200 . As a result, at the first to fourth image forming stations  1 Y,  1 M,  1 C and  1 Bk, color toner images of yellow, cyan, magenta, and black are formed, respectively, on surfaces of associated rotating drums  2 . Electrophotographic image forming principle and process for forming a toner image on the drum  2  are well known in the art, thus being omitted from description. 
     The toner images formed on the drums  2  at the respective image forming stations are successively transferred onto an outer surface of the belt  8 , in a superposition manner, which is rotationally driven in the same direction as the rotational directions of the respective drums  2  at a speed corresponding to the rotational speeds of the respective drums  2 . As a result, on the surface of the belt  8 , unfixed full-color toner images are synthetically formed in a superposition manner of the above-described four toner images. 
     With predetermined sheet feeding timing, a sheet-feeding roller  14  at a stage selected from a vertical multi-stage sheet-feeding cassettes  13 A,  13 B, and  13 C in which various recording material P having different widths are stacked and accommodated is driven. As a result, one sheet of the recording material P stacked and accommodated in the sheet-feeding cassette at the selected stage is separated and fed to be conveyed to registration rollers  16  through a vertical conveying path  15 . When a manual sheet feeding mode is selected, a sheet-feeding roller  18  is driven. As a result, one sheet of the recording material placed and set on a manual sheet feeding tray (multi-purpose tray)  17  is separated and fed to be conveyed to the registration rollers  16  through the vertical conveying path  15 . 
     The registration rollers  16  timing-convey the member P so that a leading end of the recording material P reaches the secondary transfer portion in synchronism with timing when a leading end of the above-described full-color toner images on the rotating belt  8  reaches the secondary transfer portion. As a result, at the secondary transfer portion, the full-color toner images on the belt  8  are secondary-transferred collected onto the surface of the recording material P. The recording material P coming out of the secondary transfer portion is separated from the surface of the belt  8  and guided by a vertical guide  19  into the fixing device  20  as the image heating apparatus. By this fixing device  20 , the above-described toner images of a plurality of colors are melted and mixed to be fixed on the surface of the recording material as a fixed image. The recording material coming out of the fixing device  20  is sent onto a sheet discharge tray  23  as a full-color image formed product by sheet discharge rollers  22  through a conveying path  21 . 
     The surface of the intermediary transfer belt  8  after the separation of the recording material at the secondary transfer portion is subjected to removal of residual deposited matter such as secondary transfer residual toner or the like by a belt cleaning device  12  to be cleaned, thus being repeatedly subjected to image formation. 
     In the case of a monochromatic print mode, only the four image forming station  1 Bk for forming the black toner image is actuated. In the case where a both-side print mode is selected, a recording material which has been subjected to printing on a first surface is sent onto the sheet discharge tray  23  by the sheet discharge rollers  22 . Immediately before a trailing end of the recording material passes through the sheet discharge rollers  22 , rotation of the sheet discharge rollers  22  is reversed in direction. As a result, the recording material is subjected to switch black to be introduced into a re-conveying path  24 . Thus, the recording material is conveyed again to the registration rollers  16  in a reversed state. Thereafter, similarly as in the case of the first surface printing, the recording material is conveyed to the fixing device  20  through the secondary transfer portion, thus being sent onto the sheet discharge try  23  as a both-side image formed product. 
     (2) Fixing Device  20   
     In the following description, with respect to the fixing device  20  or members constituting the fixing device, a front surface is a surface at which the fixing device is viewed from a recording material entrance side and a rear surface is a surface (recording material exit side) opposite from the front surface. Left and right are those in the case where the fixing device is viewed from the recording material entrance side. Further, the longitudinal direction is a rotational axis direction of the rotatable heat generating member generated by heat generating magnetic flux or a direction parallel to the direction. A short direction is a direction perpendicular to the longitudinal direction. An upstream side and a downstream side are those with respect to a recording material conveying direction. A sheet passing width is a dimension of the recording material with respect to a direction perpendicular to the recording material conveying direction in a plane of the recording material. 
     The fixing device  20  in this embodiment is the image heating apparatus of the electromagnetic heating type in which the magnetic field generating means is provided outside the fixing member.  FIG. 1  is a schematic cross-sectional right side view of a principal port of the fixing device  20 .  FIG. 2  is a partly omitted schematic front view of the fixing device, and  FIG. 3  is a partly omitted schematic longitudinal sectional front view of the fixing device  20 . 
     The fixing device  20  includes a belt assembly  31 , as the fixing member, disposed and held between left and right opposite side plates  51 L and  51 R of a device frame (chassis)  50  at both longitudinal end portions of the belt assembly  31 . The fixing device  20  further includes a pressing roller  32 , as a rotatable pressing member, disposed and held between the left and right opposite side plates  51 L and  51 R at both longitudinal end portions of the pressing roller  32 . The belt assembly  31  and the pressing roller  32  press-contact each other to form a nip (fixing nip) N, between the pressing roller  32  and a rotatable heat generating member  34  generated by magnetic flux on the belt assembly  31  side, having a predetermined width with respect to a recording material conveying direction. Further, the fixing device  20  includes an exciting coil assembly  33 , as the magnetic field generating means, disposed and held between the side plates  51 L and  51 R on the side 180 degrees opposite from the pressing roller  32  side with respect to the belt assembly  31 . The exciting coil assembly  33  is oppositely disposed outside the heat generating member  34  of the belt assembly  31  with a predetermined spacing. 
     1) Belt Assembly  31   
     The belt assembly  31  includes the fixing belt  34 , as the heat generating member generated by heat through the magnetic flux and configured to heat the image on the recording material by the generated heat, which is cylindrical and has flexibility (flexible endless belt; hereinafter, referred simply to as a belt). The belt  34  has a magnetic portion (electroconductive layer) which generates heat through electromagnetic induction heating when the magnetic portion passes through an area in which a magnetic field (magnetic flux) generated from the coil assembly  33  is present. 
     The belt assembly  31  includes a belt guide member  35  which is inserted into an disposed inside the belt  34  in a semi-arcuate cross-sectional shape and has heat resistivity and rigidity. The belt assembly  31  also includes a rigid pressing stay  36  inserted into and disposed inside the guide member  35  in an inverted U-like cross-sectional shape. The belt assembly  31  further includes a magnetic core (magnetic shield core disposed inside the belt  34 )  37 , disposed in an inverted U-like cross-sectional shape so as to cover the outside of the stay  36 . Further, the belt assembly  31  includes a left flange member  38 L and a right flange member  38 R mounted on a left end portion side and a right end portion side, respectively, of the stay  36 . 
       FIG. 4  is a schematic view showing a layer structure of the belt  34  in this embodiment. The belt  34  is a member having a four-layer composite layer structure constituting of a cylindrical base layer  34   a , an inner layer  34   b  provided at an inner peripheral surface of the base layer  34   a , and an elastic layer  34   c  and a parting layer  34   d  which are successively laminated on an outer peripheral surface of the base layer  34   a , thus having flexibility as a whole. 
     The base layer  34   a  is an electroconductive layer of a magnetic member which generate heat through electromagnetic induction heating, i.e., an electromagnetic induction heating layer which generates an induced current (eddy current) by the action of the magnetic field of the coil assembly  33  to generate heat by Joule heat. In this embodiment, as the base layer  34   a , a 50 μm thick Ni (nickel) electro-formed layer having a diameter of 30 mm is used. The base layer  34   a  may preferably be thin in order to improve a quick start property but requires a certain degree of thickness in consideration of an efficiency of electromagnetic induction heating, so that the base layer  34   a  may preferably have a thickness of approximately 10-100 μm. 
     The inner surface layer  34   b  is provided to ensure slidability with a member contacting the inner surface of the belt. In this embodiment, a 15 μm-thick polyimide (PI) layer is used as the inner surface layer  34   b . When the inner surface layer is excessively thick, the inner surface layer adversely affects thermal responsiveness of a temperature detecting means such as a thermistor or the like provided in contact with the inner surface of the belt and adversely affects the quick start property, so that the inner surface layer may preferably have a thickness of approximately 10-100 μm. 
     The elastic layer  34   c  may preferably have a thickness as small as possible in order to improve the quick start property but requires a certain degree of thickness in order to achieve such an effect that the belt surface is softened to encompass and melt the toner. Therefore, the elastic layer  34   c  may preferably have a thickness of approximately 10-1000 μm. In this embodiment, a 400 μm-thick rubber layer having a rubber hardness (JIS-A) of 10 degrees and a thermal conductivity of 0.8 W/m·K is used. 
     As the parting layer  34   d , it is possible to use a PFA tube or a PFA coating. The PFA coating can be decreased in thickness, thus being superior in material to the PFA tube in terms of a large effect of encompassing the toner. On the other hand, the PFA tube is superior to the PFA coating in terms of mechanical and electrical strength, so that it is possible to properly use the PFA tube and the PFA coating depending on the situation. In order to transfer heat to the recording material as much as possible, in either case, the parting layer d may preferably be thinner but may desirably have a thickness of approximately 10-100 μm in consideration of abrasion by the use of the fixing device. In this embodiment, a 30 μm-thick PFA tube is used. 
     The guide member  35  backs up and rotationally guides the belt  34 , and the belt  34  is externally engaged loosely with the guide member  35 . As the guide member  35 , a heat-resistant resin material can be used and in this embodiment, polyphenylene sulfide (PPS). In this embodiment, the guide member  35  has a thickness of 3 mm. 
     The stay  36  has the function of pressing the guide member  35  and supporting the magnetic core  37 . The stay  36  has the function of suppressing bending of the guide member  35  at the time when the belt assembly  31  and the pressing roller  32  press-contact each other. In this embodiment, the stay  36  is constituted by SUS. 
     The magnetic core  37  is disposed inside the belt  34  and opposes the coil assembly  33  through the belt  34  and adjusts the magnitude of induced magnetic field exerted from the coil assembly  33  to the belt  34 . The magnetic core  37  has the function of improving a heat generating efficiency of the belt  34 . Further, the magnetic core  37  also has the function of suppressing warming of the stay  36  through the induction heating by covering an outer surface of the stay  36  as the metallic material to block the magnetic flux toward the stay  36 . As the magnetic core  37 , a material having high magnetic permeability and low loss is used. The magnetic core  37  is used for enhancing an efficiency of a magnetic circuit and for magnetic shielding with respect to the stay  36 . As a typical example of the material for the magnetic core  37 , ferrite core is used. 
     left and right flange members  38 L and  38 R have the function of lateral deviation (movement) toward the left direction or the right direction along the longitudinal portion of the guiding member  35  during the rotation of the belt  34 .  FIG. 5(   a ) is an exploded perspective view showing the left flange member  38 L, the left end portion of the stay  36 , and the left end portion of the guiding member  35 , and  FIG. 5(   b ) is an exploded perspective view showing the right flange member  38 R, the right end portion of the stay  36 , and the right end portion of the guiding member  35 . 
     Each of the left and right flange members  38 L and  38 R includes a disk-like flange portion  38   a  facing an associated left (or right) end portion of the belt  34  and includes a pressure-receiving portion  38   b  which covers an associated left (or right) end portion of the stay  36  from above and is fitted on the end portion. Each of the flange members  38 L and  38 R further includes a vertical guide groove  38   c  provided to front and rear side surfaces of the pressure-receiving portion  38   b . The left and right flange members  38 L and  38 R are generally constituted by a high heat-resistant resin material such as PPS (polyphenylene sulfide) or LCP (liquid crystal polymer). In this embodiment, the left and right flange members  38 L and  38 R are a molded product of PPS. To inner surfaces of the flange portions  38   a  of the flange members  38 L and  38 R, magnetic members  39 L and  39 R which are formed of a magnetic material and also function as a belt end portion abutting member for preventing lateral deviation with respect to the longitudinal direction of the belt  34  by receiving the end portion of the belt  34  are attached. The magnetic members  39 L and  39 R will be described later. The left and right flange members  38 L and  38 R are engaged, at the guide grooves  38   c , with vertical guide slit portions  52 L and  52 R, respectively, provided to the left and right opposite side plates  51 L and  51 R of the device frame  50 . As a result, the left and right flange members  38 L and  38 R are guided by the guide slit portions  52 L and  52 R, respectively, thus being disposed slidably (movably) in a direction toward the pressing roller  32  and its opposite direction with respect to the left and right opposite side plates  51 L and  51 R. 
     Inside the belt  31 , a thermistor  40  as a first temperature detecting means for detecting the belt temperature in order to control the temperature of the belt  34  is disposed. This thermistor  40  is caused to elastically contact the inner surface of the belt  34  at its temperature detecting portion by a spring property of an elastic member  41  while a base portion thereof is held at an end portion of the elastic member  41  fixed to the guide member  35  at the other end. The thermistor  40  is caused to contact a portion which is a belt portion corresponding to the inside of an image forming area and at which an amount of heat generation of the belt  34  by the coil assembly  33  is largest, i.e., a portion at which the amount of heat generation at the inner surface of the belt member  31   a  with respect to the belt rotational direction is largest. 
     Further, inside the belt  31 , a thermo-switch  42  as a second temperature detecting means for detecting the belt temperature is disposed. 
     This thermo-switch  42  is caused to elastically contact the inner surface of the belt  34  at its temperature detecting portion by a spring property of an elastic member  43  while a base portion thereof is held at an end portion of the elastic member  43  fixed to the guide member  35  at the other end. The thermo-switch  42  is caused to contact a portion at which an amount of heat generation of the belt  34  by the coil assembly  33  is largest, i.e., a portion at which an amount of heat generation at the inner surface of the belt  34  with respect to the belt rotational direction is largest. 
     2) Pressing Roller  32   
     The pressing roller  32  as the pressing member is decreased in hardness by providing an elastic layer  32   b  of a silicone rubber or the like to a core metal  31   a . In order to improve a surface property, at an outer peripheral surface of the pressing roller  32 , a fluorine-containing resin material layer  32   c  of PTFE, PFA, FEP, or the like may also be provided as a parting layer. 
     The pressing roller  32  in this embodiment as an outer diameter of 30.06 mm. The core metal  32   a  has a radius of 8.5 mm and is a solid member of SUS. The elastic layer  32   b  is formed of a silicone rubber in a thickness of 6.5 mm. The parting layer  32   c  is a PFA tube having a thickness of 30 μm. 
     The pressing roller  32  are rotatably supported and disposed between the left and right opposite side plate,  51 L and  51 R through bearing members  44 L and  44 R at both (left and right) end portions of its core metal  32   a . At the right end of the core metal  32   a , At the right end of the core metal  32   a , a drive gear G is fixedly provided. 
     Between the pressure-receiving portion  38   b  of the left flange member  38 L of the belt assembly  31  and a left spring receptor  53 L provided to the device frame  50  and between the pressure-receiving portion  38   b  of the right flange member  38 R and a right spring receptor  53 R, urging springs  54 L and  54 R are provided, respectively, in a compressed state. A predetermined expansion force F of the left and right urging springs  54 L and  54 R acts on the guiding member  35  through the pressure-receiving portions  38   b  of the left and right flange members  38 L and  38 R and through the stay  36 . As a result, the guiding member  35  press-contacts the belt  34  to press the pressing roller  32  against elasticity of the elastic layer  32   b , so that a nip N with a predetermined width with respect to the recording material conveying direction is formed between the belt  34  and the pressing roller  32 . 
     3) Exciting coil assembly  33   
     c) Exciting Coil Assembly  33   
     The coil assembly  33  is curved along the outer peripheral surface of the cylindrical belt  34  in a substantially semicircular range in cross section. The coil assembly  33  is disposed in parallel with the belt assembly  31  with respect to their longitudinal directions with a predetermined spacing between its inner surface and the outer surface of the belt  34  on an opposite side from the pressing roller  32  side with respect to the belt assembly  31 . The coil assembly  33  is disposed between the left and right opposite side plates  51 L and  51 R of the device frame  50  through the supporting members  55 L and  55 R on its left and right sides.  FIG. 6  is a schematic plan view of the coil assembly  33 . The coil assembly  33  includes the magnetic field generating coil (exciting coil for generating magnetic flux)  33   a  for generating induced current in the base layer  34   a  of the belt  34  and includes a magnetic coil core (magnetic core)  33   b . The coil  33   a  and the coil core  33   b  are prepared by resin molding or accommodated in a casing (not shown). The coil  33   a  is supplied with high-frequency electric power of 10-2000 kW. As the coil  33   a , a so-called Litz wire consisting of a plurality of enameled wire strands woven together is used in order to increase a conductor surface area for the purpose of suppressing the temperature rise of the coil. As a coating for the coil  33   a , a heat-resistant coating is used. The coil core  33   b  is formed of a material having high magnetic permeability and low loss. The coil core  33   b  is used for enhancement of the efficiency of the magnetic circuit and for magnetic shielding. As a typical magnetic core, ferrite core can be used. A necessary property of the core used as such a part of the fixing device is high magnetic permeability. Herein, the high magnetic permeability refers to an AC magnetic permeability of 1000 or more at least at 100 kHz. The AC magnetic permeability of 1000 means that the resultant core has a conducting power for lines of magnetic force 1000 times higher than that of the air layer, thus being suitable for the core material for creating a magnetic path. 
     4) Fixing Operation 
       FIG. 7  is a block diagram of a control system. The control circuit portion  100  drives a fixing device drive motor M with predetermined timing on the basis of an image formation start signal input from the external host device  200 . A driving from this motor M is transmitted to the drive gear G through a power transmitting system (not shown), so that the pressing roller  32  is rotationally driven in the counterclockwise direction indicated by the arrow in  FIG. 1  at a predetermined speed. By the rotation of the pressing roller  32 , a frictional force is generated between the surface of the pressing roller  32  and the surface of the belt  34  in the fixing nip N, thus exerting a rotational force on the belt  34 . As a result, the belt  34  is rotated around the outer surface of the guiding member  35  by the pressing roller  32  at the substantially same rotational speed as that of the pressing roller  32  in the counterclockwise direction indicated by the arrow while intimately sliding on the guiding member  35  in the nip at its inner surface. 
     Further, the control circuit portion  100  turns on an electromagnetic induction heating driving circuit (exciting circuit or high-frequency converter)  101 . As a result, the high-frequency current is caused to flow from an AC power source  102  to the coil  33   a  of the coil assembly  33 , so that the base layer  34   a  of the belt  34  generates heat through the induction heating by the magnetic field generated by the coil  33   a . By the heat generation of the base layer  34   a , the rotating belt  34  is increased in temperature. Then, the temperature of the belt  34  is detected by the thermistor  40 , so that electrical information on the detecting temperature is input into the control circuit portion  100  through the A/D converter  103 . The control circuit portion  100  controls the electromagnetic induction heating driving circuit  101  so that the belt temperature is increased and kept at a predetermined temperature (fixing temperature) on the basis of the detected temperature information from the thermistor  31   e . That is, the control circuit portion  100  controls the electric power supply from the AC power source  102  to the coil  33   a . The thermo-switch  42  is inserted in series into an electric energy supplying circuit for supplying electric energy to the coil  33   a  and is actuated, when the temperature of the belt  34  exceeds a predetermined acceptable temperature, to interrupt the electric power supply to the coil  33   a.    
     In the above-described manner, the pressing roller  32  is driven and the belt  34  is temperature-controlled so as to increase in temperature up to the predetermined fixing temperature. Then, in this state, the recording material P having thereon unfixed toner images t is introduced into the fixing nip N with a toner image carrying surface directed toward the belt  34  side. The recording material P intimately contacts the outer peripheral surface of the belt  34  in the fixing nip N and is nip-conveyed through the fixing nip N together with the belt  34 . As a result, heat of the belt  34  is applied to the recording material P and the recording material P is subjected to application of the nip pressure, so that the unfixed toner images t are heat-fixed to the surface of the recording material P as a fixed image. The recording material P having passed through the fixing nip N is separated from the outer peripheral surface of the belt  34  to be conveyed to the outside of the fixing device. 
     5) Fixing Members  39 L and  39 R 
     As described above, to the inner surfaces of the flange portions  38   a  of the left and right flange members  38 L and  38 R, the magnetic members  39 L and  39 R which are formed of the magnetic material in the cylindrical shape. The magnetic members contains ferrite or iron and has the AC magnetic permeability of 1000 or more at least at the 100 kHz. 
     The AC magnetic permeability was measured by using a vibrating sample magnetometer (“VSM-5”, mfd. by TOEI INDUSTRY CO. LTD.). In this measuring apparatus, a sample placed in a uniform magnetic field is vibrated at a constant frequency of 80 Hz with an amplitude of 0.5 mm and an electromotive force induced in a detection coil disposed in the neighborhood of the sample is detected by using a lock-in amplifier to measure a magnetic property of the sample. In this embodiment, the uniform magnetic field was changed for measurement from zero (Oe) to 3000 (Oe) by 100 (Oe). 
     In this embodiment, the magnetic members  39 L and  39 R function as the belt end portion abutting member for preventing the lateral deviation with respect to the longitudinal direction of the belt  34 . That is when the belt  34  is moved toward the left side along the longitudinal portion of the guiding member  35  during the rotation of the belt  34 , the left magnetic member  39 L receives (stops) the side surface of the left side end portion of the belt  34 , thus preventing leftward deviation of the belt  34 . Further, when the belt  34  is moved toward the right side along the longitudinal portion of the guiding member  35  during the rotation of the belt  34 , the right magnetic member  39 R receives (stops) the side surface of the right side end portion of the belt  34 , thus preventing rightward deviation of the belt  34 . 
     In this embodiment, with respect the rotational axis direction of the belt member, the end portion of the magnetic member is located outside the end portion of the belt member and the magnetic member covers the end portion of the belt member. However, in the present invention, the magnetic member is not necessarily required to completely cover the end portion of the belt member. In the present invention, the end portion of the belt member refers to an area which is other than a sheet passing area of the recording material with a maximum width passable in the direction perpendicular to the recording material conveying direction and is within 20 mm from the end of the belt member. In this area, at least a part of the magnetic member is only required to be located. 
       FIG. 8  is a schematic perspective view of the left and right magnetic members  39 L and  39 R in this embodiment. Each of the left and right magnetic members  39 L and  39 R includes a disk-like (cylindrical) portion  39   a  substantially corresponding to the flange portion  38   a  of the associated one of the left and right flange members  38 L and  38 R and includes an inward projection edge portion  39   b  providing along the outer circumference of the disk-like portion  38   a . In this embodiment, each of the left and right flange members  38 L and  38 R themselves was constituted by a 1.5 mm-thick ferrite core. In this embodiment, the ferrite core having the AC magnetic permeability of 1800 at about 100 kHz was used. An amount of projection of the projection edge portion  39   b  is 2.5 mm. The left and right magnetic members  39 L and  39 R are provided and fixed with an adhesive to the inner side surfaces of the flange portions  38   a  of the left and right flange members  38 L and  38 R at associated ones of the outer side surfaces thereof. Further, the left end portion of the belt  34  is caused to enter the inside of the projection edge portion  39   b  of the left magnetic member  39 L, so that the side surface and the outer peripheral surface of the left end portion of the belt  34  is covered with the left magnetic member  39 L. Similarly, the right end portion of the belt  34  is caused to enter the inside of the projection edge portion  39   b  of the right magnetic member  39 R, so that the side surface and the outer peripheral surface of the right end portion of the belt  34  is covered with the right magnetic member  39 R. In this embodiment, the portions each in the range of 2.5 mm from the end of each of the left and right end portions of the belt  34  are covered with the left and right magnetic members  39 L and  39 R, respectively. The inner surface of the disk-like portion  39   a  of each of the left and right magnetic members  39 L and  39 R constitutes an abutting surface with respect to the end portion side surface of the belt  34 . 
       FIG. 9  is a schematic view showing a length relationship between a longitudinal direction length L 1  of the coil  33   a  and a longitudinal direction length L 3  of the belt  34 . The longitudinal direction is the rotational axis direction of the heat generating member. Further, the longitudinal direction length of the coil is a distance between the both ends of the coil. In this embodiment, L 1  is 370 mm and L 3  is 340 mm, so that L 1 &gt;L 3  is satisfied, the longitudinal direction length L 2  of the coil core  33   b  is 330 mm. The belt  34  was rotated at a speed of 321 mm/s. In this embodiment, L 1 &gt;L 3  is satisfied but a similar effect can also be obtained even in the constitution of L 1 =L 2 . 
     As Comparative Embodiment 1, in the constitution of the fixing device, the magnetic members  39 L and  39 R as the belt end portion abutting member were changed to non-magnetic members  39 L′ and  39 R′ formed of PPS. 
     As Comparative Embodiment 2, in the constitution of the fixing device, in addition to the constitution of Comparative Embodiment 1, the longitudinal direction length L 1  of the coil  33   a  was 370 mm and the longitudinal direction length L 3  of the belt  34  was changed to 380 mm, so that L 3 &gt;L 1  was satisfied. 
     Table 1 shows the constitutes of the fixing devices in Embodiment 1, Comparative Embodiment 1 and Comparative Embodiment 2. Further, a distribution of temperature with respect to the longitudinal direction of the belt  34  in the case where each of the fixing devices in Embodiment 1, Comparative Embodiment 1 and Comparative Embodiment 2 is driven to increase the temperature of the belt  34  at its longitudinal central portion to 190° C. is shown in  FIG. 10 . 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 EMB. 
                 Relationship 
                 L1(coil) 
                 L3(belt) 
                 Material 
               
               
                   
               
             
            
               
                 EMB. 1 
                 L1 &gt; L3 
                 370 mm 
                 340 mm 
                 Ferrite 
               
               
                 COMP. EMB. 1 
                 L1 &gt; L3 
                 370 mm 
                 340 mm 
                 PPS 
               
               
                 COMP. EMB. 2 
                 L3 &gt; L1 
                 370 mm 
                 380 mm 
                 PPS 
               
               
                   
               
            
           
         
       
     
     In Embodiment 1, as shown in  FIG. 11 , at the left and right end portions of the belt  34 , the magnetic field generated by the coil  33   a  passes through the left and right magnetic members  39 L and  39 R, so that the temperature rise at the belt end portions is suppressed ( FIG. 10 ).  FIG. 11  is a schematic sectional view showing a portion at which the belt end portion is covered with the associated one of the left and right magnetic members  39 L and  39 R. 
     In Comparative Embodiment 1, the magnetic field generated by the coil  33   a  concentrates particularly at the belt end portions as shown in  FIG. 12 , so that the temperature at the belt end portions is increased ( FIG. 10 ).  FIG. 12  is, similarly as in  FIG. 11 , a schematic view showing a portion at which the belt end portion is covered with the associated one of the belt end portion abutting members  39 L′ and  39 R′ of the non-magnetic material (PPS). A state of the magnetic field with respect to the longitudinal direction of the belt in Comparative Embodiment 1 is shown in  FIG. 13 , from which it is understood that the magnetic flux concentrates at the belt end portions. 
     Similarly, also in Embodiment 1, the magnetic flux also concentrates at the belt end portions but the concentrated magnetic flux passes through the magnetic members  39 L and  39 R formed of the magnetic material as the belt end portion abutting member, so that the temperature rise at the belt end portions is of no problem. 
     In Embodiment 1 and Comparative Embodiment 2, the uniform temperature distribution with respect to the longitudinal direction is realized in the substantially similar manner. However, compared with Embodiment 1, in Comparative Embodiment 2, the longitudinal direction length L 3  of the belt  34  is longer than the longitudinal direction length L 1  of the coil  33   a , so that there is a disadvantage that the fixing device in Comparative Embodiment 2 requires much electric power during the copying due to the increased longitudinal direction length L 3 . 
     Further, with respect to Embodiment 1 and Comparative Embodiment 1, when idling of each of the fixing devices in Embodiment 1 and Comparative Embodiment 1 is continued while keeping the temperature of the belt  34  at its longitudinal central portion at 190° C., a hardness of the belt  34  is changed as shown in  FIG. 14 . From  FIG. 14 , it is understood that there is a difference in hardness of the belt  34  particularly at the belt end portions between the belts  34  in Embodiment 1 and Comparative Embodiment 1. This may be attributable to thermal deterioration of the elastic layer  34   b  of the belt  34  in Comparative Embodiment 1. Here, the hardness of the belt  34  is a measured value by a micro-rubber hardness meter (trade name: “MD-1 (C type)”, mfd. by KOBUNSHI KEIKI CO., LTD.) using a probe of hemisphere type (1 mm in diameter). 
     As in Embodiment 1, in the case where the positions of the left and right end portions of the belt  34  are regulated by abutting the belt  34  against the abutting members  39 L and  39 R, it is not preferable that a strength of the belt at its end portions is lowered. The constitution in Embodiment 1 is effective also from the viewpoint of no occurrence of the thermal deterioration at the belt end portions. 
     That is, when the length of the coil  33   a  is made longer than that of the belt  34  in order to prevent the change in temperature at the end portions of the belt  34 , the magnetic flux density is increased at the end portions of the belt  34 , thus increasing the belt temperature at the end portions. By preparing the end portion abutting members  39 L and  39 R for the belt  34  with the magnetic member, the concentration of the magnetic flux at the end portions of the belt  34  is avoided, so that the end portion temperature rise is suppressed and the thermal deterioration at the end portions of the belt  34  is also suppressed. 
     Thus, the image heating apparatus of the electromagnetic induction heating type in which the magnetic field generating means  33  is provided outside the belt  34  in Embodiment 1 is capable of suppressing excessive temperature rise at the end portions of the heat generating member  34  and the thermal deterioration of the heat generating member  34  while achieving energy saving. 
     In Embodiment 1, the left and right magnetic members  39 L and  39 R also function as the belt end portion abutting member. Therefore, the end portion side surfaces and the end portion outer peripheral surfaces of the belt  34  are covered with the magnetic members  39 L and  39 R. The left and right magnetic members  39 L and  39 R may also have a constitution in which they are disposed in contact with the end portion side surfaces of the belt  34  without functioning as the belt end portion abutting member as shown in  FIG. 15(   a ). Further, as shown in  FIG. 15(   b ), the left and right magnetic members  39 L and  39 R may also have a constitution in which they are disposed close to the belt  34  without contacting the end portion side surfaces of the belt  34 . In this case, a distance a between the end portion side surface of the belt  34  and the associated magnetic member  39 L ( 39 R) may preferably be about 3.0 mm or less. An effect similar to that in Embodiment 1 can also be achieved in the constitutions shown in  FIGS. 15(   a ) and  15 ( b ). 
     Embodiment 2 
     In this embodiment, the image forming stations are similar to those in Embodiment 1. With reference to  FIG. 16 , a constitution of the fixing device in this embodiment will be described. The fixing device in this embodiment have the same constitution as that in Embodiment 1 except that the longitudinal direction length L 2  of the coil core  33   b  is changed to 350 mm. That is, the longitudinal direction L 1  of the coil  33   a  is 370 mm, the longitudinal direction length L 2  of the coil core  33   b  is 350 mm, and the longitudinal direction length L 3  of the belt  34  is 340 mm, i.e., L 1 &gt;L 2 &gt;L 3 . The belt  34  was rotated at the speed of 321 mm/s similarly as in Embodiment 1. 
     As Comparative Embodiment 3, in the fixing device in Embodiment 3, L 1 =370 mm, L 2 =330 mm, and L 3 =340 mm were set. That is, L 1 &gt;L 3 &gt;L 2  is satisfied. 
     Table 2 shows the constitutes of the fixing devices in Embodiment 2 and Comparative Embodiment 3. Further, a distribution of temperature with respect to the longitudinal direction of the belt  34  in the case where each of the fixing devices in Embodiment 2 and Comparative Embodiment 3 is driven to increase the temperature of the belt  34  at its longitudinal central portion to 190° C. is shown in  FIG. 17 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Length (mm) 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 EMB. 
                 Relationship 
                 L1 
                 L2 
                 L3 
                 Material 
               
               
                   
               
               
                 EMB. 2 
                 L1 &gt; L2 &gt; L3 
                 370 
                 350 
                 340 
                 Ferrite 
               
               
                 COMP. EMB. 2 
                 L1 &gt; L3 &gt; L2 
                 370 
                 330 
                 340 
                 Ferrite 
               
               
                   
               
            
           
         
       
     
     Compared with Comparative Embodiment 3, in Embodiment 2, the longitudinal direction length of the coil core  33   b  is made longer than the belt  34 , so that it is understood that the temperature at the belt end portions are kept at a higher level (closer to 190° C.). 
     Therefore, compared with the length relationship of L 1 &gt;L 3 &gt;L 2  (Comparative Embodiment 3), it is found that the length relationship of L 1 &gt;L 2 &gt;L 3  (Embodiment 2) is preferable in order to realize a uniform temperature distribution along the longitudinal direction of the belt  34 . This may be attributable to a stronger magnetic field exerted on the belt  34  in Embodiment 2 compared with that in Comparative Embodiment 3. 
     Thus, the image heating apparatus of the electromagnetic induction heating type in which the magnetic field generating means  33  is provided outside the heat generating member  34  in Embodiment 2 is capable of suppressing excessive temperature rise at the end portions of the heat generating member  34  and the thermal deterioration of the heat generating member  34  while achieving energy saving. 
     In the above-described Embodiments 1 and 2, the belt member is used as the heat generating member  34  but a similar effect can also be obtained by using a thin film member as the heat generating member  34 . Further, in the above-described embodiments, the magnetic member has the cylindrical shape but the similar effect can also be obtained even when the magnetic member does not have a complete cylindrical shape. Further, the similar effect can also be obtained by employing the magnetic member having a substantially cylindrical shape with a partly lacking portion. 
     The image heating apparatus of the present invention can be used as not only the image heating fixing apparatus as in the embodiments described above but also, e.g., the image heating apparatus for modifying a surface property such as glossiness or the like by heating the recording material on which the image is carried, the image heating apparatus for effecting temporary fixation, and the like. 
     As described hereinabove, according to the present invention, it is possible to reduce a degree of the temperature rise at the end portions of the heat generating member even when the coil length is longer than the length of the heat generating member. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 296462/2008 filed Nov. 20, 2008, which is hereby incorporated by reference.