Patent Publication Number: US-2009232567-A1

Title: Fixing device, cylindrical heat fixing roll and image forming device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2008-062890 filed on Mar. 12, 2008. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a fixing device, cylindrical heat fixing roll used in the fixing device and an image forming device. 
     SUMMARY 
     A first aspect of the present invention is a fixing device including: a magnetic field generating unit that generates a magnetic field; a cylindrical fixing member that is disposed so as to face to the magnetic field generating unit, generates heat due to electromagnetic induction of the magnetic field, and includes a heat generating layer that has a thickness thinner than a skin depth, and a temperature sensing layer which has a magnetic permeability change starting temperature, in a temperature range from a set fixing temperature to a heat resisting temperature, at which a magnetic permeability starts to decrease continuously, and which contacts with a face of the heat generating layer opposite to the magnetic field generating unit; and a pressure rotating body that is brought into pressure contact with an outer peripheral surface of the fixing member so as to deform. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a general view of an image forming device in accordance with a first exemplary embodiment of the invention; 
         FIG. 2A  is a cross sectional view of a fixing device in accordance with the first exemplary embodiment of the invention; 
         FIG. 2B  is a cross sectional view of a fixing roll in accordance with the first exemplary embodiment of the invention; 
         FIG. 3  is a connection diagram of a control circuit and a energizing circuit in accordance with the first exemplary embodiment of the invention; 
         FIG. 4  is a schematic view showing a relation between a magnetic permeability and a temperature; 
         FIG. 5  is a graph showing a temperature distribution of a fixing roll in accordance with the first exemplary embodiment of the invention; 
         FIG. 6  is a cross sectional view of a fixing device in accordance with a second exemplary embodiment of the invention; 
         FIGS. 7A and 7B  are schematic views showing a state in which a magnetic field passes through a fixing roll in accordance with the second exemplary embodiment of the invention; and 
         FIG. 8  is a graph showing a temperature distribution of the fixing roll in accordance with the second exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A description will be given of exemplary embodiments of a fixing device and an image forming device in accordance with the present invention with reference to the accompanying drawings. 
     A printer  10  serving as the image forming device is shown in  FIG. 1 . The printer  10  is structured such that a light scanning device  54  is fixed to a casing  12  constructing a main body of the printer  10 , and a control unit  50  controlling the operations of each of sections of the light scanning device  54  and the printer  10  is provided at a position which is adjacent to the light scanning device  54 . 
     The light scanning device  54  is structured such as to scan a light beam emitted from a light source (not shown) by a rotating polygon mirror, reflect by plural optical parts such as reflection mirrors, and emit light beams  60 Y,  60 M,  60 C and  60 K corresponding to respective toners of yellow (Y), magenta (M), cyan (C) and black (K). The light beams  60 Y,  60 M,  60 C and  60 K are introduced to respective photoconductive bodies  20 Y,  20 M,  20 C and  20 K. 
     A sheet tray  14  storing recording sheets P is provided in a lower side of the printer  10 . A pair of registration rolls  16  regulating a position of a lead edge portion of the recording sheet P are provided above the sheet tray  14 . Further, an image forming unit  18  is provided in a center portion of the printer  10 . The image forming unit  18  is provided with four photoconductive bodies  20 Y,  20 M,  20 C and  20 K mentioned above and these photoconductive bodies are arranged vertically in a row. 
     Charging rollers  22 Y,  22 M,  22 C and  22 K charging surfaces of the photoconductive bodies  20 Y,  20 M,  20 C and  20 K are provided in an upstream side in a rotating direction of the photoconductive bodies  20 Y,  20 M,  20 C and  20 K. Further, developing devices  24 Y,  24 M,  24 C and  24 K developing respective toners Y, M, C and K on the photoconductive bodies  20 Y,  20 M,  20 C and  20 K are provided in a downstream side in a rotating direction of the photoconductive bodies  20 Y,  20 M,  20 C and  20 K. 
     On the other hand, a first intermediate transfer body  26  comes into contact with the photoconductive bodies  20 Y and  20 M, and a second intermediate transfer body  28  comes into contact with the photoconductive bodies  20 C and  20 K. Further, a third intermediate transfer body  30  comes into contact with the first intermediate transfer body  26  and the second intermediate transfer body  28 . A transfer roll  32  is provided at a position opposing to the third intermediate transfer body  30 . The recording sheet P is transported between the transfer roll  32  and the third intermediate transfer body  30 , and the toner image on the third intermediate transfer body  30  is transferred to the recording sheet P. 
     A fixing device  100  is provided in a downstream of a sheet transport path  34  in which the recording sheet P is transported. The fixing device  100  has a fixing roll  102  and a pressure roll  104 , and fixed the toner image to the recording sheet P by heating and pressurizing the recording sheet P. The recording sheet P on which the toner image is fixed is output to a tray  38  provided in an upper portion of the printer  10  by a sheet transport roll  36 . 
     A description will be given here of an image formation of the printer  10 . 
     When the image formation is started, the surfaces of the photoconductive bodies  20 Y to  20 K are uniformly charged by the charging rollers  22 Y to  22 K, respectively. The surfaces of the charged photoconductive bodies  20 Y to  20 K are irradiated with light beams  60 Y to  60 K corresponding to an output image emitted from the light scanning device  54 , and electrostatic latent images corresponding to the respective color separation images are formed on the photoconductive bodies  20 Y to  20 K. The developing devices  24 Y to  24 K selectively apply the toners of the respective colors, that is, Y to K to the electrostatic latent images, and the toner images of the colors Y to K are formed on the photoconductive bodies  20 Y to  20 K. 
     Thereafter, the toner image of the magenta is primarily transferred to the first intermediate transfer body  26  from the photoconductive body  20 M for magenta. Further, the toner image of the yellow is primarily transferred to the first intermediate transfer body  26  from the photoconductive body  20 Y for yellow, and is superposed on the toner image of the magenta on the first intermediate transfer body  26 . 
     On the other hand, the toner image for the black is primarily transferred to the second intermediate transfer body  28  from the photoconductive body  20 K for black in the same manner. Further, the toner image of the cyan is primarily transferred to the second intermediate transfer body  28  from the photoconductive body  20 C for cyan, and is superposed on the toner image of the black on the second intermediate transfer body  28 . 
     The magenta and yellow toner images primarily transferred to the first intermediate transfer body  26  are secondarily transferred to the third intermediate transfer body  30 . On the other hand, the black and cyan toner images primarily transferred to the second intermediate transfer body  28  are secondarily transferred to the third intermediate transfer body  30 . Here, the previously secondarily transferred magenta and yellow toner images and the cyan and black toner images are superposed, and a full color toner image of three colors and black is formed on the third intermediate transfer body  30 . 
     The secondarily transferred full color toner image reaches a nip portion between the third intermediate transfer body  30  and the transfer roll  32 . The recording sheet P is transferred to the nip portion from the registration roll  16  in synchronization with the timing thereof, and a full color toner image is thirdly transferred (finally transferred) onto the recording sheet P. 
     The recording sheet P is thereafter sent to the fixing device  100  and passes through the nip portion between the fixing roll  102  and the pressure roll  104 . At this time, the full color toner image is fixed on the recording sheet P due to the operation of the heat and the pressure applied from the fixing roll  102  and the pressure roll  104 . The recording sheet P is output to the tray  38  by the sheet transport roll  36  after fixing, and the full color image formation onto the recording sheet P is finished. 
     Next, a description will be given of the fixing device  100  in accordance with the exemplary embodiment. 
     As shown in  FIG. 2A , the fixing device  100  is provided with a casing  120  in which an opening for carrying out an entry or an exit of the recording sheet P is formed. The endless fixing roll  102  rotating in a direction of an arrow A is provided in an inner portion of the casing  120 . A gear (not shown) is bonded to both end portions of the fixing roll  102 . 
     A bobbin  108  constructed by an insulative material is arranged at a position opposing to an outer peripheral surface of the fixing roll  102 . The bobbin  108  is formed as an approximately circular arc shape that follows the outer peripheral surface of the fixing roll  102 , and a convex portion  108 A is protruded from an approximately center portion of an opposite surface to the fixing roll  102 . An interval between the bobbin  108  and the fixing roll  102  is set about 1 to 3 mm. 
     An excitation coil  110  generating a magnetic field H by being energized is wound plural times in an axial direction (the direction perpendicular to the surface of the drawing of  FIG. 2A ) around the convex portion  108 A of the bobbin  108 . A magnetic body core  112  formed approximately in a circular arc shape so as to follow the circular arc shape of the bobbin  108  is arranged at a position opposing to the excitation coil  110 , and is supported to the bobbin  108 . 
     On the other hand, the pressure roll  104  rotating in a driven manner in a direction of an arrow B with respect to the rotation of the fixing roll  102  is brought into pressure contact with the outer peripheral surface of the fixing roll  102 . The pressure roll  104  is structured such that a silicon rubber elastic layer and a PFA release layer are coated around a cored bar  106  made of a metal such as steel. Further, the pressure roll  104  is formed as a concave shape that follows the outer peripheral surface of the fixing roll  102 , in a nip portion  117  corresponding to a contact portion with the fixing roll  102 . 
     A separating pawl (not shown) having a lead edge portion directed to the fixing roll  102  side is provided in the vicinity of an outlet side in a transport direction of the recording sheet P of the nip portion  117 , and prevents the recording sheet P mounting the toner T thereon from being drawn to the fixing roll  102  side at a time when the recording sheet P passes through the nip portion  117 . Accordingly, the recording sheet P transported from a direction of an arrow IN is output to a direction of an arrow OUT. 
     A thermistor  18  measuring a temperature of the surface of the fixing roll  102  is provided in a contact manner in a region which does not oppose to the excitation coil  110  and a region which is close to the output side of the recording sheet P, on the surface of the fixing roll  102 . A contact position of the thermistor  118  comes to an approximately center portion in the axial direction of the fixing roll  102  (the direction perpendicular to the surface of the drawing of  FIG. 2A ), in such a manner as to prevent a measured value from being varied in accordance with a magnitude of a size of the recording sheet P. The thermistor  118  measures a temperature of the surface of the fixing roll  102  on the basis of a variation of a resistance value in correspondence to a heat quantity given from the surface of the fixing roll  102 . 
     As shown in  FIG. 3 , the thermistor  118  is connected to a control circuit  134  provided in an inner portion of the control unit  50  (refer to  FIG. 1 ) mentioned above via a wiring  132 . Further, the control circuit  134  is connected to an energizing circuit  138  via a wiring  136 , and the energizing circuit  138  is connected to the excitation coil  110  mentioned above via wirings  140  and  142 . 
     Here, the control circuit  134  measures the temperature of the surface of the fixing roll  102  on the basis of a quantity of electricity sent from the thermistor  118  so as to compare the measured temperature with a previously stored set fixing temperature (170° C. in the exemplary embodiment). Further, in the case that the measured temperature is lower than the set fixing temperature, it drives the energizing circuit  138  so as to excite the excitation coil  110 , and generates a magnetic field H (refer to  FIG. 2A ) serving as a magnetic circuit. On the other hand, in the case that the measured temperature is higher than the set fixing temperature, it stops the energizing circuit  138 . 
     The energizing circuit  138  is driven or stopped based on an electric signal sent from the control circuit  134 , and is configured to send, or stop sending, an alternating current having a predetermined frequency to the excitation coil  10  via the wirings  140  and  142 . 
     Next, a description will be given of the structure of the fixing roll  102 . Note that, in the exemplary embodiment, the fixing roll  102  has a structure which does not deform in a concave shape while deforming at the nip portion  117 , or a structure in which it is not necessary to independently provide a pressing member at an inner peripheral surface of the fixing member for forming the nip portion opposing the pressure rotating body, and the fixing belt has a structure which is deformed in a concave shape, or a structure in which it is necessary to independently provide a pressing member at an inner peripheral surface of the fixing member for forming the nip portion opposing the pressure rotating body. 
     The fixing roll  102  is constructed by a temperature sensing layer  130  having a thickness 200 μm (between 150 and 200 μm) from an inner side toward an outer side, a heat generating layer  128  having a thickness 10 μm (between 2 and 20 μm), an elastic layer  126  having a thickness 400 μm, and a release layer  124  having a thickness 30 μm, as shown in  FIG. 2B , and they are laminated and integrated. A diameter of the fixing roll  102  is set to 30 mm. 
     The temperature sensing layer  130  is positioned in a base layer for maintaining strength of the fixing roll  102 , and employs a metal magnetically soft material including an alloy of at least one of steel, nickel, chrome, silicone, boron, niobium, copper, zirconium or cobalt, or the like. Further, the temperature sensing layer  130  employs a material having a magnetic permeability change starting temperature, at which a magnetic permeability starts to continuously decrease, in a temperature range from a set fixing temperature (a fixing temperature necessary for the fixing roll  102 ) of the fixing device  100  to a heat resisting temperature (a temperature at which deformation begins due to heat) of the heat generating layer  128  (or the fixing roll  102 ). Note that, the magnetic permeability change starting temperature is a temperature at which the magnetic permeability (measured in accordance with JIS-C2531) starts to decrease continuously, and means a point at which a penetration amount of the magnetic flux of the magnetic field starts changing, as shown in  FIG. 4 . 
     On the basis of the above, the exemplary embodiment sets a heat resisting temperature to 240° C. and a fixing temperature to 170° C. and as the temperature sensing layer  130 , employs an iron-nickel alloy having a magnetic permeability change starting temperature of about 200° C. A specific resistance of the temperature sensing layer  130  is 70×10 −8 Ω or more. 
     The temperature sensing layer  130  comes to a ferromagnetic material at a lower temperature than the magnetic permeability change starting temperature, and makes the magnetic field H (refer to  FIG. 2A ) mentioned above intrude. Further, if it exceeds the magnetic permeability change starting temperature, the magnetic permeability thereof starts to decrease so as to come close to that of a non-magnetic material (a paramagnetic material), and a magnetic flux penetrating amount of the magnetic field H is increased. 
     Here, in order to sufficiently generate a temperature sensing function of the temperature sensing layer  130 , it is necessary to set a skin depth δ indicating a depth at which the magnetic field H may intrude at a lower temperature than the magnetic permeability change starting temperature to a thickness of the temperature sensing layer  130  or less. The skin depth  6  is given by the following formula (1). 
     
       
         
           
             
               
                 
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     In the formula (1), ρ indicates a specific resistance, f indicates a frequency (an electromagnetic induction heating frequency), and μ r  indicates a relative magnetic permeability (room temperature). Here, determining the relative magnetic permeability, for example, achieving δ≦200 μm on the basis of the formula (1), under a necessary condition ρ≧70×10 −8  Ωm, f≧20 kHz, at least the relative magnetic permeability μ r ≧230 is required. The temperature sensing layer  130  in accordance with the exemplary embodiment is previously set to a high magnetic permeability in accordance with a heat treatment (an annealing) in such a manner that the relative magnetic permeability μ r  becomes 230 or more. The specific resistance ρ is determined in accordance with a method of JIS K6911. 
     Note that, since the specific resistance of the temperature sensing layer  130  is sufficiently high, for example, at a thickness of 200 μm, and when the electromagnetic induction heating frequency is from 20 kHz to 100 kHz, the temperature sensing layer  130  does not readily generate heat in comparison with the heat generating layer  128 . Accordingly, the temperature sensing layer  130  rarely generates excessive heat to an extent that affects the heat generating temperature of the heat generating layer  128 . Further, the temperature sensing layer  130  is designed to be 150 μm or more to secure sufficient strength of the fixing roll  102 . 
     On the other hand, the heat generating layer  128  employs a metal material generating heat due to electromagnetic induction action that eddy current flows in such a manner as to generate a magnetic field canceling the magnetic field H (refer to  FIG. 2A ) mentioned above. As the metal material mentioned above, for example, there may be employed gold, silver, copper, aluminum, zinc, tin, lead, bismuth, beryllium, antimony or an alloy thereof Further, in order to shorten a warm-up time of the fixing device  100 , the thickness of the heat generating layer  128  may be made as thin as possible, and if the heat generating layer  128  employs a non-magnetic metal material in which the thickness is between 2 and 20 μm, and the specific resistance is 2.7×10 −8  Ωcm or less, the heat generating amount which is necessary in the range of alternating frequency between 20 kHz and 100 kHz which a general-use power supply can utilize may be efficiently obtained. In the exemplary embodiment, copper is used and the thickness is set to 10 μm to obtain favorable heat generating efficiency and low costs. Since this thickness is sufficiently thinner than the skin depth δ, this allows the magnetic flux to pass through. 
     The elastic layer  126  employs a silicone rubber or a fluorine-contained rubber to obtain excellent elasticity and heat resistance, and employs silicone rubber in the exemplary embodiment. 
     The release layer  124  is provided for weakening an adhesive force with the toner T (refer to  FIG. 2A ) fused on the recording sheet P and making the recording sheet P be easily peeled off from the fixing, roll  102 . In order to obtain an excellent surface mold release characteristic, a fluorine-contained resin, a silicone resin or a polyimide resin is used as the release layer  124 , and a tetrafluoroethylene perfluoro alkoxy ethylene copolymerization resin (PFA) is used in the exemplary embodiment. 
     Next, a description will be given of an operation of the first exemplary embodiment in accordance with the invention. 
     As shown in  FIGS. 1 to 3 , the recording sheet P to which the toner T is transferred is sent to the fixing device  100  via the image forming step of the printer  10 . In the fixing device  100 , the fixing roll  102  starts its rotational drive in the direction of the arrow A, and the pressure roll  104  rotates in the direction of the arrow B in accordance therewith. At this time, the energizing circuit  138  is driven on the basis of an electric signal from the control circuit  134 , and alternating current is supplied to the excitation coil  110 . 
     If the alternating current is supplied to the excitation coil  110 , generation and extinction of the magnetic field H serving as the magnetic circuit are repeated at the periphery of the excitation coil  110 . Further, if the magnetic field H traverses the heat generating layer  128  of the fixing roll  102 , eddy current (not shown) is generated in the heat generating layer  128  such that a magnetic field that impedes changes in the magnetic field H arises. 
     The heat generating layer  128  generates heat in proportion to the skin resistance of the heat generating layer  128  and the magnitude of the eddy current flowing through the heat generating layer  128 , whereby the fixing roll  102  is heated. The temperature of the surface of the fixing roll  102  is detected by the thermistor  118 , and in the case that the temperature does not reach the set fixing temperature 170° C., the control circuit  134  drives and controls the energizing circuit  138  so as to apply alternating current having a predetermined frequency to the excitation coil  110 . Further, in the case that the temperature reaches the set fixing temperature, the control circuit  134  stops the control of the energizing circuit  138 . 
     Subsequently, as shown in  FIG. 2A , the recording sheet P fed to the fixing device  100  is heated and pressed by the fixing roll  102  in which the heat generating layer  128  generates heat so as to reach the predetermined set fixing temperature (170° C.). and the pressure roll  104 , and the toner image T is fixed onto the surface of the recording sheet P. The recording sheet P exited the fixing device  100  is output to the tray  38  by the sheet transport roll  36 . 
     Here, a description will be given of a case that the recording sheet P having a small size is continuously supplied so as to be fixed, and the recording sheet having a large size is subsequently supplied. 
     As shown in  FIGS. 2A and 5 , in the fixing roll  102 , a region through which the recording sheet P having a small size passes is set to C, a region through which the recording sheet P having a large size passes is set to B+C+D, and a region through which the recording sheet P does not pass and in which the excitation coil  110  is not arranged is set to A and E. Further, G 1  is a temperature graph of the fixing roll in a case when the temperature sensing layer  130  is not provided, or the magnetic permeability change starting temperature of the temperature sensing layer  130  is not between 170° C. and 240° C., and G 2  is a temperature graph of the fixing roll  102  in accordance with the exemplary embodiment. 
     First of all, if the recording sheet P having the small size is continuously supplied so as to be fixed, the heat quantity is absorbed by the recording sheet P and the temperature of the fixing roll  102  becomes lower than the set fixing temperature (170° C.), in the passing region C of the recording sheet P in the fixing roll  102 . 
     The control circuit  134  (refer to  FIG. 3 ) controls the energizing circuit  138  in such a manner as to approximate the temperature of the fixing roll  102  to the set fixing temperature, on the basis of a difference between the temperature detected by the thermistor  118  and the set fixing temperature, and the heat generating layer  128  generates heat. Due there to, the temperature of the fixing roll  102  rises. Since the heat quantity is not absorbed by the recording sheet P, in the non-passing regions B and D (including A and E) of the recording sheet P in the fixing roll  102 , the temperature further rises due to the heat generation of the heat generating layer  128 . 
     In the case of using the fixing roll in which the temperature sensing layer  130  is not provided, or the magnetic permeability change starting temperature of the temperature sensing layer  130  does not exist between 170° C. and 240° C., there is not provided with the means for reducing the temperature rise in the non-passing regions B and D of the recording sheet P. Accordingly, the temperature difference between the passing region C of the recording sheet P and the non-passing regions B and D thereof in the fixing roll becomes larger as shown by the graph G 1 . 
     If the temperature difference between the passing region C of the recording sheet P and the non-passing regions B and D thereof becomes large, an excessive heat quantity is given to the end portion (the regions B and D) of the recording sheet P at a time of fixing the recording sheet P having the large size after the recording sheet P having the small size, there is generated a so-called hot offset phenomenon that a part of the toner T remains in the fixing roll side, and a fixing unevenness of the image is generated. 
     In the fixing device  100  in accordance with the exemplary embodiment, the temperature rises in the non-passing regions B and D of the recording sheet P, and, when the temperature of the surface of the fixing roll  102  becomes higher than the set fixing temperature 170° C. and exceeds the magnetic permeability change starting temperature 200° C., the relative magnetic permeability of the temperature sensing layer  130  in the non-passing regions B and D starts to approach  1 . At this time, the skin depth δ determined by the formula (1) mentioned above becomes larger than the thickness of the temperature sensing layer  130 , and the magnetic flux of the magnetic field H passes through the temperature sensing layer  130 . Accordingly a magnetic flux density of the magnetic field H is reduced and an eddy current caused by the electromagnetic induction is reduced, whereby the heat generating quantity of the heat generating layer  128  is lowered in the non-passing region of the recording sheet P. 
     As mentioned above, in the non-passing regions B and D of the recording sheet P in the fixing roll  102 , the fixing temperature is lowered at ΔT 1  in comparison with when the temperature sensing layer  130  is not provided, and the temperature rise is lowered. 
     Note that, in the passing region C of the recording sheet P in the fixing roll  102 , since the temperature of the temperature sensing layer  130  does not reach the magnetic permeability change starting temperature 200° C., and temperature sensing layer  130  remains as a ferromagnetic material, the heat generating quantity of the heat generating layer  128  is not lowered, and the temperature rises up to the set fixing temperature 170° C. 
     As described above, since the temperature difference is lowered between the passing region C of the recording sheet P and the non-passing regions B and D thereof in the fixing roll  102 , and the temperature distribution becomes as shown in the graph G 2 , the hot offset phenomenon mentioned above is not likely to be generated at an end portion of the recording sheet P having the large size even if the recording sheet P having the large size is fixed after fixing the recording sheet P having the small size. 
     Further, a shape of the nip portion  117  is convex to the pressure roll  104  side, and an outer shape of the fixing roll  102  comes to a held state. Accordingly, a compression force or a tension force is hard to be applied to the heat generating layer  128  of the fixing roll  102 . 
     Next, a description will be given of a second exemplary embodiment of the fixing device and the image forming device in accordance with the invention with reference to the accompanying drawings. Note that, the same reference numerals are applied to parts which are basically the same as those of the first exemplary embodiment mentioned above, and a description thereof will be omitted. 
     A fixing device  150  is shown in  FIG. 6 . An endless pressure belt  152  is used in the fixing device  150  in place of the pressure roll  104  of the fixing device  100  in accordance with the first exemplary embodiment. The pressure belt  152  is structured such that a release layer made of PFA having a thickness 30 μm is coated on an endless base layer made of a polyimide having a thickness 100 μm. 
     A support member  154  is provided inside the pressure belt  152  so as to extend along a width direction of the pressure belt  152 . The support member  154  is structured such that a concave portion  154 A is formed in an opposing side to a nip portion  119  with which the fixing roll  102  and the pressure belt  152  come into contact, and a circular arc portion  154 B is formed in an opposite side to the concave portion  154 A. 
     Note that, the support member  154  is provided with a support shaft (not shown) in both ends portions in a longitudinal direction, and the support shaft is fixed to a casing  120 . Further, a discoid cap (not shown) provided with a bearing inserted on the exterior of the support shaft is attached to both end portions of the pressure belt  152 , and the pressure belt  152  is rotatably pivoted. The pressure belt  152  is rotated in accordance with a rotation of the fixing roll  102 . 
     A pressure pad  156  made of a heat resisting resin (two layers) such as a liquid crystal polymer (LCP) is bonded to the concave portion  154 A of the support member  154 . The pressure pad  156  comes into contact with an inner peripheral surface of the pressure belt  152 , and presses the nip portion  119 . 
     On the other hand, a induction body  114  is provided at a position opposing to the excitation coil  110  inside the fixing roll  102  in the fixing device  150  so as to be in non-contact with the fixing roll  102 . The induction body  114  is made of aluminum that is a non-magnetic body, is formed in a circular arc shape along the fixing roll  102 . and is fixed to the casing  120  in both ends. Further, a resistance value of the induction body  114  is 2.7×10 −8  Ωm or less. Note that, the position of the induction body  114  is set to a position that induces the magnetic flux of the magnetic field H in the case that the magnetic flux of the magnetic field H passes through the fixing roll  102 . The induction body  114  is spaced from the fixing belt  102  by 1.5 mm (1 to 5 mm). 
     Next, a description will be given of an operation of the second exemplary embodiment in accordance with the invention. 
     As shown in  FIG. 6 , the recording sheet P to which the toner T is transferred is sent to the fixing device  150  via the image forming step. In the fixing device  150 , the fixing roll  102  starts its rotational drive in the direction of the arrow A, and the pressure belt  152  rotates in the direction of the arrow B in accordance therewith. At this time, the energizing circuit  138  is driven on the basis of an electric signal from the control circuit  134  (refer to  FIG. 3 ), and alternating current is supplied to the excitation coil  110 . 
     If the alternating current is supplied to the excitation coil  110 , generation and extinction of the magnetic field H serving as the magnetic circuit are repeated at the periphery of the excitation coil  110 . Further, if the magnetic field H traverses the heat generating layer  128  of the fixing roll  102 , eddy current (not shown) is generated in the heat generating layer  128  such that a magnetic field that impedes changes in the magnetic field H arises. 
     The heat generating layer  128  generates heat in proportion to the magnitude of the skin resistance of the heat generating layer  128  and the eddy current flowing through the heat generating layer  128 , whereby the fixing roll  102  is heated. The temperature of the surface of the fixing roll  102  is detected by the thermistor  118 , and in the case that the temperature does not reach the set fixing temperature 170° C., the control circuit  134  drives and controls the energizing circuit  138  so as to apply alternating current having a predetermined frequency to the excitation coil  110 . Further, in the case that the temperature reaches the set fixing temperature, the control circuit  134  stops the control of the energizing circuit  138 . 
     Subsequently, the recording sheet P fed to the fixing device  150  is heated and pressed by the pressure belt  152  and the fixing roll  102  which has reached a predetermined set fixing temperature (170° C.) due to heat generated by the heat generating layer  128 , and the toner image T is fixed onto the surface of the recording sheet P. The recording sheet P exited the fixing device  150  is output to the tray  38  by the sheet transport roll  36 . 
     Here, a description will be given of a case that the recording sheet P having a small size is continuously supplied so as to be fixed, and the recording sheet having a large size is subsequently supplied, in the fixing device  150 . 
     As shown in  FIGS. 6 and 8 , in the fixing roll  102 , a region through which the recording sheet P having the small size passes is set to C, a region through which the recording sheet P having the large size passes is set to B+C+D, and a region through which the recording sheet P does not pass and in which the excitation coil  110  is not arranged is set to A and E. Further, G 3  is a temperature graph of the fixing roll in the case that the temperature sensing layer  130  is not provided, or the magnetic permeability change starting temperature of the temperature sensing layer  130  does not exist between 170° C. and 240° C., and G 4  is a temperature graph of the fixing roll  102  in accordance with the exemplary embodiment. 
     First of all, if the recording sheet P having the small size is continuously supplied so as to be fixed, the heat quantity is absorbed by the recording medium P and the temperature of the fixing roll  102  becomes lower than the set fixing temperature (170° C.), in the passing region C of the recording sheet P in the fixing roll  102 . 
     The control circuit  134  (refer to  FIG. 3 ) controls the energizing circuit  138  in such a manner as to approximate the temperature of the fixing roll  102  to the set fixing temperature, on the basis of a difference between the temperature detected by the thermistor  118  and the set fixing temperature, and the heat generating layer  128  generates heat. Due thereto, the temperature of the fixing roll  102  rises. Since the heat quantity is not absorbed by the recording sheet P, in the non-passing regions B and D (including A and E) of the recording sheet P in the fixing roll  102 , the temperature further rises on the basis of the heat generation of the heat generating layer  128 . 
     In the case of using the fixing roll in which the temperature sensing layer  130  is not provided, or the magnetic permeability change starting temperature of the temperature sensing layer  130  does not exist between 170° C. and 240° C., there is not provided with means for reducing the temperature rise in the non-passing regions B and D of the recording sheet P. Accordingly, the temperature difference between the passing region C of the recording sheet P and the non-passing regions B and D thereof in the fixing roll becomes larger as shown by the graph G 3 , and if the continuous sheet feeding is carried over, the temperature exceeds the heat resisting temperature so as to break the elastic layer  126  and the release layer  124  of the fixing roll  102 . 
     If the temperature difference between the passing region C of the recording sheet P and the non-passing regions B and D thereof becomes large, an excessive heat quantity is given to the end portion (the regions B and D) of the recording sheet P at a time of fixing the recording sheet P having the large size after the recording sheet P having the small size, there is generated a so-called hot offset phenomenon that a part of the toner T remains in the fixing roll side, and a fixing unevenness of the image is generated. 
     In the fixing device  150  in accordance with the exemplary embodiment, the temperature rises in the non-passing regions B and D of the recording sheet P, and, when the temperature of the surface of the fixing roll  102  becomes higher than the set fixing temperature 170° C. and exceeds the magnetic permeability change starting temperature 200° C., the relative magnetic permeability of the temperature sensing layer  130  in the non-passing regions B and D starts to approach  1 . At this time, the skin depth δ determined by the formula (1) mentioned above becomes larger than the thickness of the temperature sensing layer  130 , and the magnetic flux of the magnetic field H passes through the temperature sensing layer  130 . 
     Here,  FIG. 7A  shows a case that the temperature of the temperature sensing layer  130  is the magnetic permeability change starting temperature or less, and  FIG. 7B  shows a case that the temperature of the temperature sensing layer  130  is the magnetic permeability change starting temperature or more of the temperature sensing layer  130 . 
     As shown in  FIG. 7A , in the case that the temperature of the temperature sensing layer  130  is the magnetic permeability change starting temperature or less, a magnetic field H 1  passing through the heat generating layer  128  makes an intrusion into the temperature sensing layer  130  so as to form a closed magnetic path, and strengthens the magnetic field H 1 , because the temperature sensing layer  130  is constituted by a ferromagnetic material. Accordingly, the heat generating quantity of the heat generating layer  128  can be sufficiently obtained. 
     On the other hand, as shown in  FIG. 7B , in the case that the temperature of the temperature sensing layer  130  is the magnetic permeability change starting temperature or more, a magnetic field H 2  passes through the temperature sensing layer  130  and the magnetic field H 2  is weakened. The magnetic field H 2  passes through the temperature sensing layer  130 , and is thereafter directed to the induction body  114 . In the induction body  114  a closed magnetic path is formed from the magnetic body core  112  to the induction body  114 , and eddy current flows on the basis of an operation of the magnetic field H 2  however, the heat generating amount is small because its resistance is small. Further, since it does not come into contact with the fixing roll  102 , it does not raise the temperature of the fixing roll  102 . 
     Here, since the eddy current is reduced in the heat generating layer  128  in contrast to the increase of the eddy current in the induction body  114 , the heat generating amount is reduced. As mentioned above, the heat generating amount of the heat generating layer  128  decreases in the non-passing region of the recording sheet P, and compared to when the temperature sensing layer  130  is not provided, the fixing temperature decreases at ΔT 2 , and the temperature rise decreases in the non-passing regions B and D of the recording sheet P in the fixing roll  102 . 
     Note that, since the temperature rise of the non-passing regions B and D is lowered in the case that the induction body  114  is provided, ΔT 2  of the second exemplary embodiment becomes larger than ΔT 1  of the first exemplary embodiment. 
     As described above, since the temperature difference is lowered between the passing region C of the recording sheet P and the non-passing regions B and D thereof in the fixing roll  102 , and the temperature distribution becomes as shown in the graph G 4 , the hot offset phenomenon mentioned above is not likely to be generated at an end portion of the recording sheet P having a large size, even if the recording sheet P having a large size is fixed after fixing the recording sheet P having a small size. 
     Further, a shape of the nip portion  119  is convex at a pressure belt  152  side, and an outer shape of the fixing roll  102  is maintained. Accordingly, a compression force or a tension force is not likely to be applied to the heat generating layer  128  of the fixing roll  102 , and a durability is higher in comparison with when the fixing belt is used. 
     Note that, the invention is not limited to the exemplary embodiment mentioned above. 
     The printer  10  may be structured such as to use a liquid developer in addition to a dry type electrophotographic system using a solid developer. Further, a thermo couple may be used in place of the thermistor  118  as a detecting device of the temperature of the fixing roll  102 . 
     The attaching position of the thermistor  118  is not limited to the surface of the fixing roll  102 , but may be set to an inner peripheral surface of the fixing roll  102 . In this case, the surface of the fixing roll  102  is not likely to be worn. Further, the thermistor  118  may be attached to the surface of the pressure roll  104 . 
     The excitation coil  110  may be arranged inside the fixing roll  102 . 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.