Patent Publication Number: US-8971744-B2

Title: Image heating apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image heating apparatus used for an image forming apparatus, such as copiers and laser beam printers, employing an image-forming process, such as electrophotography and electrostatic recording. 
     2. Description of the Related Art 
     Some image heating apparatuses used for an image forming apparatus may have a cylindrical belt, a nip portion forming member in contact with an inner surface of the belt, and a pressure member for forming a nip portion through the belt together with the nip portion forming member. The image heating apparatus described above may generally heat a toner image while conveying a recording material which bears the toner image by the nip portion. The image heating apparatus described above may be adapted to have the belt formed thin so that heat capacity can be small, thereby allowing for the merit of a shorter period needed to warm up the image heating apparatus. 
     And now, recently, there has been required a technology for further shortening the warm-up period. Japanese Patent Application Laid-Open No. 2007-57827 discloses an apparatus configured in a manner that, at warm-up operation, a fixing belt having an electrically conductive layer is spaced apart from a pressure member, and the fixing belt is brought into contact with the pressure member after the fixing belt reaches a fixable temperature. 
     However, in the case of the image heating apparatus disclosed in Japanese Patent Application Laid-Open No. 2007-57827, the fixing belt is locally heated due to electromagnetic induction, and accordingly the fixing belt has to be configured so that it can be driven to rotate for warming the entire circumference of the fixing belt at warm-up even if the fixing belt is spaced apart from the pressure member. If the fixing belt, as shown in Japanese Patent Application Laid-Open No. 2007-57827, is driven using an end cap attached to an end of the fixing belt, and a drive gear, then the fixing belt needs to have rigidity to some extent. However, an increase in thickness and rigidity of the fixing belt leads to a larger heat capacity of the fixing belt, resulting in a longer warm-up time. Accordingly, even if the fixing belt is spaced apart from the pressure member at warm-up in a configuration in which the fixing belt is locally heated, a shortening effect of the warm-up time may be unfortunately reduced. 
     SUMMARY OF THE INVENTION 
     A purpose of the present invention is to provide an image heating apparatus configured so that a belt is not easily deprived of heat by a pressure roller at warm-up of the image heating apparatus, and the belt rises rapidly in temperature. 
     Another purpose of the present invention is to provide an image heating apparatus for conveying a recording material which bears a toner image by a nip portion and heating the toner image, including a tubular belt with a heat generating layer electrified to self-generate heat over the entire circumference; a nip portion forming member that contacts an inner surface of the belt; and a pressure roller that forms the nip portion through the belt together with the nip portion forming member, the pressure roller driven by a driving source and driving the belt to rotate in the nip portion, wherein in the image heating apparatus for a predetermined period from the start of warming up the image heating apparatus, a contact area between the belt and the pressure roller is smaller than a contact area for a period to heat a toner image, or the belt does not contact the pressure roller, and the belt stops rotating, and wherein for a period from a completion of the predetermined period to the start of a period to heat a toner image, the contact area between the belt and the pressure roller is made to be equal to the contact area for the period to heat a toner image and the belt is rotated. 
     A further purpose of the present invention is to provide an image heating apparatus for conveying a recording material which bears a toner image by a nip portion and heating the toner image, including a belt having a cylinder shape with a heat generating layer heated by electromagnetic induction over the entire circumference; a nip portion forming member that contacts an inner surface of the belt; and a pressure roller that forms the nip portion through the belt together with the nip portion forming member, the pressure roller driven by a driving source and driving the belt to rotate in the nip portion, wherein in the image heating apparatus for a predetermined period from the start of warming up the image heating apparatus, a contact area between the belt and the pressure roller is smaller than a contact area for a period to heat a toner image, or the belt does not contact the pressure roller, and the belt stops rotating, and wherein for a period from a completion of the predetermined period to the start of a period to heat a toner image, the contact area between the belt and the pressure roller is made to be equal to the contact area for the period to heat a toner image and the belt is rotated. 
     The present invention can provide an image heating apparatus configured so that a belt is not easily deprived of heat by a pressure roller at warm-up of the image heating apparatus, and the belt rises rapidly in temperature. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  schematically illustrates a configuration of an image heating apparatus in a vertical-sectional view. 
         FIG. 1B  schematically illustrates a configuration of an enlarged power supply area of a heat generating layer of a belt. 
         FIG. 2  schematically illustrates a configuration of the image heating apparatus in a cross-sectional view. 
         FIG. 3  schematically illustrates one example of a configuration of an image forming apparatus. 
         FIG. 4  illustrates a layer configuration of the heat generating layer of a fixing belt in a cross-sectional view. 
         FIG. 5  illustrates a configuration of an electrical contact portion according to a first exemplary embodiment in a cross-sectional view. 
         FIG. 6  is a flowchart illustrating operations of a fixing apparatus according to the first exemplary embodiment. 
         FIG. 7A  schematically illustrates a fixing apparatus according to a second exemplary embodiment in a vertical-sectional view. 
         FIG. 7B  schematically illustrates the fixing apparatus according to the second exemplary embodiment in a cross-sectional view. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings. 
     A first exemplary embodiment is described. 
     (Image Forming Apparatus) 
       FIG. 3  schematically illustrates one example of a configuration of an image forming apparatus which mounts an image heating apparatus according to an exemplary embodiment of the present invention. This image forming apparatus is a laser beam printer (hereinafter, called a “printer”) which forms, using electrophotography, an image on a recording material such as a recording paper and an OHP sheet. The printer shown in this exemplary embodiment is configured so that a controller (not shown) executes a predetermined control sequence for image forming in response to a print command output by an external apparatus (not shown) such as a host computer, and performs a predetermined operation for image forming according to this control sequence for image forming. The controller includes a central processing unit (CPU) and a memory such as a ROM and a RAM, and the memory stores the control sequence for image forming and various types of programs needed for image forming. 
     The printer as the image forming apparatus according to this exemplary embodiment includes an image forming portion for forming a toner image on a recording material, and a fixing portion (hereinafter, called a “fixing apparatus”) used as an image heating apparatus for heating and fixing an unfixed toner image on the recording material. When the control sequence for image forming is executed, first, in the image forming portion, an electrophotographic photosensitive member of drum type 1 (hereinafter called a “photosensitive drum  1 ”) used as an image bearing body is driven to rotate at a predetermined peripheral speed (process speed) in the direction shown by the arrow. An outer periphery (surface) of this photosensitive drum  1  is, then, electrically charged uniformly by a charged roller  2  used as a charged member. 
     Subsequently, to the charged surface of this photosensitive drum  1 , scan exposure of laser beams is applied, which laser beams are on/off controlled by an optical scanning apparatus  3  depending on image information, and on the charged surface of this photosensitive drum  1 , an electrostatic latent image is formed based on the image information. This electrostatic latent image is then developed as a toner image by a development apparatus  4  using toner (developer). 
     On the other hand, a recording material P fed from a feeding cassette (not shown) by a recording material conveyance mechanism (not shown) is conveyed to a transfer nip portion between the surface of the photosensitive drum  1  and an outer periphery (surface) of a transfer roller  5  used as a transfer member. This recording material P is sandwiched in the transfer nip portion between the surface of the photosensitive drum  1  and the surface of the transfer roller  5  and conveyed, so that the toner image on the surface of the photosensitive drum  1  is transferred onto the recording material P by the transfer roller  5  in the conveyance process of the recording material P. As the result, the recording material P bears the toner image. 
     The recording material P which bears the toner image is introduced into a fixing apparatus  7 , which applies heat and a pressure to the toner image to fix, under heat, on the recording material P. The recording material P on which the toner image is fixed under heat is ejected onto a receiver tray (not show) by a recording material ejection mechanism (not shown). 
     Residual toner which stays behind on the surface of the photosensitive drum  1  after the toner image is transferred is removed by a cleaning blade  6   a  of a cleaning apparatus  6  and used for subsequent image forming. 
     (Fixing Apparatus) 
     A fixing apparatus used as an image heating apparatus according to an exemplary embodiment of the present invention will be described below. The term “longitudinal direction”, as used relative to the fixing apparatus and a member for forming the fixing apparatus, is the direction perpendicular to a recording material conveyance direction in a surface of the recording material (the direction of the axis of rotation of a fixing belt described below). The term “lateral direction” is the direction parallel to the recording material conveyance direction in the surface of the recording material. Further, the term “length” is the dimension in the longitudinal direction, and “width” is the dimension in the lateral direction. 
       FIGS. 1 and 2  illustrate a configuration of a fixing apparatus  7  used as an image heating apparatus according to an exemplary embodiment. The fixing apparatus  7  is a device configured in a manner that a fixing belt having an electrification heat-generating resistance layer (heat generating layer) described later, self-generates heat. A fixing belt  11 , which is a rotating body for heat generation having rotatable flexibility, is configured so that its inner periphery is rotatably supported by a belt guide  13  which is a nip portion forming member, and its side end plane is also rotatably supported by a right and left flange  14 . And, an inner periphery of the fixing belt is in contact with the belt guide  13 , and an outer periphery of the fixing belt  11  is in contact with a pressure roller  12  which is a pressure member. 
     The pressure roller  12  forms a fixing nip portion N through the fixing belt  11  together with the belt guide  13 . In the fixing nip portion N, the recording material P which bears an unfixed toner image is heated while conveyed to fix the unfixed toner image on the recording material P. Note that a thermistor  18  used as a temperature detecting member for detecting a temperature in a sheet-passing area of the recording material P abuts on the inner periphery of the fixing belt  11  to control a temperature of the fixing belt  11 . 
     In this exemplary embodiment, the belt guide  13 , while sandwiching the fixing belt  11  between it and the pressure roller  12 , is pressed against the pressure roller  12  through the flange  14  by a pressure spring  15  which is a compression spring, at a force of about 118 N (about 12 Kgf) in total pressure. Also, the pressure roller  12  is driven by a pressure roller drive gear to rotate counterclockwise as shown by the arrow in  FIG. 3 . This rotation of the pressure roller  12  applies a force to the fixing belt  11  in the fixing nip portion, and the fixing belt  11  is accordingly driven to rotate. 
     (Fixing Belt) 
     Then, a fixing belt according to this exemplary embodiment will be described with reference to  FIGS. 2 and 4 . The cylindrical fixing belt  11  is loosely fitted outside of a belt guide  13  described later, and has an excess peripheral length. Referring to  FIG. 4 , a configuration of the fixing belt  11  will be described in detail.  FIG. 4  illustrates a layer configuration of a heat generating layer of the fixing belt in a cross-sectional view. 
     The fixing belt  11 , as shown in  FIG. 4 , has a cylindrical heat generating layer  11   a  which is electrified to generate heat. The heat generating layer  11   a  has a resin material  11   a   1  and an electrically conductive filler  11   a   2  dispersed in the resin material  11   a   1 . The resin material  11   a   1  is formed of a heat resistant resin such as polyimide, polyamideimide, polyether ether ketone (PEEK), polyether sulfone (PES), and polyphenylene sulfide (PPS). The electrically conductive filler  11   a   2  has an anisotropic shape and the longitudinal direction thereof oriented in a peripheral direction of the belt. For the electrically conductive filler, a carbon nanomaterial such as carbon nanofiber, carbon nanotube and carbon microcoil, and a metal microparticle or a metal oxide microparticle are used. 
     A proportion of the electrically conductive filler to the resin material  11   a   1  may be 30 to 60% by weight percentage. Note that the heat generating layer used in this exemplary embodiment is formed by dispersing carbon nanotube having a length of 150 μm in polyimide. In  FIG. 4 , the electrically conductive filler  11   a   2  is dispersed in the resin material and exists in the heat generating layer in a random manner. However, the electrically conductive filler has the longitudinal axis thereof oriented in the peripheral direction of the belt. 
     In the fixing belt  11  in this exemplary embodiment, since the electrically conductive filler is oriented in the peripheral direction of the belt, anisotropy can be imparted to sheet resistance ohm/square of the resistive heat generating layer  11   a . That is, let sheet resistance (surface resistance) of the heat generating layer  11   a  in the longitudinal direction be R 1 , and sheet resistance (surface resistance) of the heat generating layer  11   a  in the peripheral direction be R 2 , the relation of R 1 &gt;R 2  can hold. Therefore, the electrical sheet resistance R 1  of the heat generating layer  11   a  in the longitudinal direction is larger than the sheet resistance R 2  of the heat generating layer  11   a  in the peripheral direction. Note that a proportion between the sheet resistances R 1  and R 2  may be determined from the measurement result, as described below. 
     That is, the belt is cut open in a generatrix direction in a part of the peripheral direction of the belt  11  to be formed in a rectangular sheet, and further, the long side of the rectangular sheet is cut to have an equal length to that of the short side, forming a square shape. Then, on the two opposing sides of the square, terminals for measuring a resistance value are attached to measure (there are two sets of the opposing sides, and each of them is intended for measurement). A method for orienting the electrically conductive filler (dispersed material) in the peripheral direction of the heat generating layer  11   a  includes, for example, a method in which a cylindrical mold while rotating is beam-coated with a polyimide precursor solution having an electrically conductive filler dispersed therein. 
     In addition, if the image forming apparatus is operated using a commercial power, electric power applied to the fixing belt  11  may be 100 W to 1500 W, considering a power supply capacity, a printing speed, and a start-up time of the fixing apparatus. Accordingly, a resistance value measured between both ends of the heat generating layer  11   a  in the longitudinal direction (the direction of the axis of rotation) (that is, between feeding electrodes) may be in the range of 5Ω to 100Ω. Also, the heat generating layer  11   a  may be 30 to 200 μm in thickness, considering the range of the resistance value (5Ω to 100Ω) and strength of the fixing belt  11 . 
     On the outer periphery of the heat generating layer  11   a , a release layer  11   b  (surface release layer) ( FIG. 4 ) is provided to ensure releasability from a toner image T ( FIG. 4 ) which the recording material P bears. The release layer  11   b  is formed of a heat resistant fluorine resin, such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), and tetrafluoroethylene-hexafluoropropylene copolymer (FEP). And, the release layer  11   b  is bonded to the outer periphery of the heat generating layer  11   a  through a primer layer (not shown). In this release layer  11   b , carbon or electrical resistance control substance having ion conductive property (organophosphorous acid, antimony pentoxide, titanic oxide) may be dispersed. 
     (Electrode Member) 
     As shown in  FIGS. 1A and 1B , electrode members  16  (electrical contact portion) for supplying electric power to the heat generating layer  11   a  are connected in areas  11   a R,  11   a L (hereinafter, called a “power supply area”) outside of a sheet-passing area in both ends of this heat generating layer  11   a  in the longitudinal direction, at predetermined positions of the heat generating layer  11   a  in the peripheral direction. The power supply areas  11   a R,  11   a L of the heat generating layer  11   a  may be coated with an electrically conductive material such as Ag. 
     A configuration of the electrode member  16  will be described with reference to  FIG. 5 .  FIG. 5  schematically illustrates the electrode member  16  in a cross-sectional view. The electrode member  16  is provided on a metal core  12   a  of the pressure roller  12  and the electrode member  16  includes an elastic layer  16   b  formed of insulating silicone rubber and disposed coaxially with the pressure roller  12 , and an electrically conductive layer formed by coating the outside of the elastic layer  16   b  with an electrically conductive material  16   a  such as Ag. Also, the elastic layer  16   b  has a hollow central portion, which is fitted to a metal core  12   a  of the pressure roller  12 . Also, as shown in  FIGS. 1A and 1B , the magnitude relation between a diameter A 2  of the electrode member  16  and a diameter A 1  of the pressure roller  12  is A 2 &gt;A 1 . In addition, this electrode member  16  is fed from an AC power supply through a sliding contact  21 . 
     (Belt Guide Used as Nip Portion Forming Member) 
     The belt guide  13  is formed using a high heat resistive resin such as polyimide, polyamideimide, polyether ether ketone (PEEK), polyphenylene sulfide (PPS) and liquid crystal polymer, and a composite material such as any combinations of these resins with ceramics, metal and glass. In this exemplary embodiment, liquid crystal polymer was used as a material of the belt guide  13 . This belt guide  13  is configured so that both ends of the belt guide  13  in the longitudinal direction are supported on a unit flame of the fixing apparatus  7  through the flanges  14 R,  14 L. 
     (Pressure Roller) 
     The pressure roller  12  includes, as shown in  FIG. 2 , a metal core  12   a , an elastic body layer  12   b  provided on an outer periphery of the metal core  12   a , and a release layer  12   c  which is the outermost layer provided on an outer periphery of the elastic body layer  12   b . In this exemplary embodiment, for the metal core  12   a , an aluminum metal core was used, for the elastic body layer  12   b , silicone rubber was used, and the release layer  12   c  was formed by coating coatetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA). The pressure roller  12  is disposed below and parallel to the fixing belt  11 , and both ends of the metal core  12   a  in the longitudinal direction are rotatably supported on the unit flame through a bearing (not shown). 
     (Operation of Fixing Apparatus for Predetermined Period from Start of Warm-Up and for Period to Heat Toner Image) 
     Referring to  FIGS. 1A and 1B , a description will be provided about a predetermined period from the start of warming up the fixing apparatus which is a previous stage to a period to heat a toner image. The phrase “predetermined period from the start of warm-up”, as used herein, means a period in which the fixing belt is forced to generate heat after a print signal is received until a temperature of the fixing belt reaches a predetermined temperature. The phrase “period to heat a toner image” is a period to convey and heat the recording material which bears a toner image in the fixing nip portion N. First, as shown in  FIG. 1A , for the predetermined period from the start of warming up the fixing apparatus after the print signal is transmitted, the fixing belt  11  and the pressure roller  12  are not in contact with each other, except for contact portions outside of the sheet-passing area on the sides of both ends in the longitudinal direction (spaced apart state). That is, as shown in  FIGS. 1A and 1B , in the sheet-passing area, a pressure control device  200  prevents a pressure applied by a pressure device  100 , and the pressure spring  15  is released from compression, resulting in a decreased pressure applied to the flange  14  by the pressure spring  15 . 
     Even if the fixing belt  11  and the pressure roller  12  are spaced apart from each other in the sheet-passing area, the electrode members  16  provided in both ends of the fixing belt and the pressure roller are in contact with the power supply areas  11   a L,  11   a R in both ends of the fixing belt in the longitudinal direction. It is because the diameter A 2  of the electrode members  16  is larger than the diameter A 1  of the pressure roller  12 , and the electrode members  16  are in contact with the power supply areas  11   a L,  11   a R, with the elastic layer thereof being forced to be elastically deformed. 
     The electrode members  16  are in contact with the heat generating layer  11   a  (power supply area) for all of the predetermined period from the start of warm-up, a period from the completion of the predetermined period to the start of the period to heat a toner image, and the period to heat a toner image, and the AC power supply can feed the heat generating layer  11   a  of the fixing belt through the sliding contact  21 . 
     Further, for the period from the completion of the predetermined period from the start of warm-up to the start of the period to heat a toner image, a contact area of the belt with the pressure roller is made equal to the contact area for the period to heat a toner image, and the belt is rotated, subsequently the recording material is introduced into the fixing nip portion. 
     (Arrangement of Belt Guide Relative to Fixing Belt for Predetermined Period from Start of Warm-Up) 
     The fixing belt  11  and the belt guide  13  are also spaced apart from each other in the sheet-passing area, but outside of the sheet-passing area, they are in contact with each other. The details will be described with reference to  FIG. 1B .  FIG. 1B  schematically illustrates an enlarged area shown by the dotted line B in  FIG. 1A  in a longitudinal sectional view. The belt guide  13  has a depressed portion at a position opposing to the electrode member  16 , and into this depressed portion, an elastic member  17  (first elastic member) is inserted. 
     When the pressure spring  15  is released from compression through the elastic member  17  by the pressure control device  200  working as a pressure release mechanism, only the fixing belt  11  and the elastic member  17  are in contact with each other, and in other areas in the longitudinal direction, the fixing belt  11  and the belt guide  13  are spaced apart from each other. For the predetermined period from the start of warm-up, the contact area of the fixing belt with the belt guide decreases relative to the contact area for the period to heat a toner image. That is, the fixing belt and the belt guide spaced apart from each other in the sheet-passing area are in contact with each other only in the areas opposing to the elastic members  17  on the sides of both ends of the fixing belt outside of the sheet-passing area. 
     For the period to heat a toner image, the pressure applied by the pressure spring  15  forces the elastic member  17  to be elastically deformed, so that the belt guide  13  and the fixing belt  11  are brought into contact with each other over the entire area in the longitudinal direction. 
     Note that for the predetermined period from the start of warm-up, the fixing belt and the belt guide can be spaced apart from each other using a detachment device  300  over the entire area including the outside of the sheet-passing area. 
     (Heating and Fixing Operations) 
     Next, heating and fixing operations of the fixing apparatus will be described with reference to a flowchart in  FIG. 6 . When the controller receives a print command at S 101 , the AC power supply begins to electrify the heat generating layer  11   a  of the fixing belt  11  through the electrode members  16  (S 102 ). Accordingly, the heat generating layer  11   a  generates heat over the entire circumference, and the fixing belt  11  rapidly rises in temperature. The temperature of the fixing belt  11  is detected by a temperature detecting member  18  such as a thermistor disposed in contact with or near the inner surface of the heat generating layer  11   a  (S 103 ). This temperature detecting member  18  is supported on the unit flame or the belt guide through a predetermined bracket. 
     When it is detected at S 104  that the temperature of the fixing belt  11  reaches a predefined and predetermined temperature, then the process proceeds to S 105 . At S 105 , the pressure spring  15  is compressed to press the fixing belt against the pressure roller. At this time, the elastically deformable electrode members  16  are elastically deformed due to the pressure applied to the fixing belt  11 , and the fixing belt accordingly abuts against the pressure roller. Also, at S 105 , at the same time, a motor M shown in  FIG. 2  is driven to rotate. The rotation of an output shaft of the motor M is transmitted to the metal core  12   a  of the pressure roller  12  through a predetermined gear train (not shown). Accordingly, the pressure roller  12  is driven to rotate at a predetermined peripheral speed (process speed) counterclockwise as shown by the arrow. 
     The rotation of the pressure roller  12  is transmitted to the fixing belt  11  by a frictional force produced between the surface of the pressure roller  12  and the surface of the fixing belt  11  in the fixing nip portion N. Accordingly, the rotation of the pressure roller  12  drives the fixing belt  11  to rotate while the inner periphery (inner surface) of the heat generating layer  11   a  of the fixing belt  11  is in contact with the outer periphery of the belt guide  13 . While the motor M is driven to rotate and the heat generating layer  11   a  is electrified, the recording material P which bears an unfixed toner image T is introduced into the fixing nip portion N, with its plane which bears the toner image upward (S 106 ). 
     This recording material P is sandwiched between the surface of the fixing belt  11  and the surface of the pressure roller  12 , and conveyed in the fixing nip portion N. In this conveyance process, the toner image T on the recording material P is heated by the fixing belt  11  and melts, and then the toner image is pressed in the fixing nip portion N to be fixed under heat on the recording material P. And, the recording material P on which the toner image T is fixed under heat is conveyed from the fixing nip portion N to a recording material ejection mechanism. At this time, the controller takes in an output signal from the temperature detecting member  18  (temperature detection signal), and based on this signal, controls the electrical power so that the fixing belt  11  can maintain the predetermined fixing temperature (target temperature). 
     Next, if the print signal subsequently comes, the processing at S 106  is repeated, and if there is no print signal, the process proceeds to S 108  to stop electrifying the fixing belt and applying the pressure to the fixing belt. 
     (Comparison with Comparative Examples) 
     The fixing apparatus in this exemplary embodiment is viewed as an example 1 and an example 2, and a fixing apparatus for comparative study is viewed as a comparative example 1 and a comparative example 2, and comparison study on them will be described. The fixing belt has all the same components in the example 1, the example 2, the comparative example 1 and the comparative example 2, and includes, as shown in  FIG. 4 , a two-layer configuration composed of the heat generating layer  11   a  and the release layer  11   b . For the heat generating layer  11   a , polyimide having the thickness of 60 μm was used. Further, for the electrically conductive filler to be dispersed in the heat generating layer  11   a , carbon nanofiber (150 μm in length) was used. The carbon nanofiber has its longitudinal axis oriented in the peripheral direction of the belt. 
     A proportion of the electrically conductive filler (carbon nanofiber) to the resin material  11   a   1  formed of polyimide is 40% by weight. In this heat generating layer  11   a , a ratio of sheet resistance R 1  in the longitudinal direction to sheet resistance R 2  in the peripheral direction was 1.6:1. For the release layer  11   b , PFA having the thickness of 10 μm is coated on the outer periphery of the heat generating layer  11   a . The fixing belt has φ 24 mm in inner diameter, and 230 mm in length. The power supply areas  11   a R,  11   a L of the both ends of the heat generating layer  11   a  in the longitudinal direction, except for the release layer  11   b , are coated with Ag. The resistance value measured between both ends of the heat generating layer  11   a  of the fixing belt in the longitudinal direction was 15Ω. 
     The pressure roller includes also all the same components in the example 1, the example 2, the comparative example 1 and the comparative example 2, and has φ 25 mm in outer diameter. And, the elastic layer was formed of silicone rubber on the outer periphery of the metal core of aluminum, and the outer periphery of this elastic layer was coated with a PFA resin to form the release layer. 
     &lt;Comparison of Warm-Up Time&gt; 
     In heating and fixing operations according to the example 1, for the predetermined period from the start of warm-up, the fixing belt and the pressure roller are not in contact with each other, and the fixing belt and the belt guide are also not in contact with each other, and further the pressure roller is not rotating. Under these conditions, the fixing belt is supplied with a fixed electric power of 800 W, and when the temperature detecting element disposed on the inner surface of the fixing belt senses the temperature of 180° C., then a pressure of 118 N is applied so that the contact area of the belt guide with the fixing belt and the contact area of the fixing belt with the pressure roller are equal to those for the period to heat a toner image. At the same time, the pressure roller is driven to rotate. 
     Also, in heating and fixing operations according to the example 2, for the predetermined period from the start of warm-up, the belt guide and the pressure roller are pressed under a weak pressure of 50 N so that the contact area of the fixing belt with the pressure roller is smaller than the contact area for the period to heat a toner image. Under this condition, the fixing belt is supplied with a fixed electric power of 800 W, and when the temperature detecting element disposed on the inner surface of the fixing belt senses the temperature of 180° C., then a pressure of 118 N is applied so that the contact area of the belt guide with the fixing belt and the contact area of the fixing belt with the pressure roller are equal to those for the period to heat a toner image. At the same time, the pressure roller is driven to rotate. 
     On the other hand, in heating and fixing operations according to the comparative example 1, for the predetermined period from the start of warm-up, the contact area of the fixing belt with the pressure roller is equal to the contact area for the period to heat a toner image. Also, the contact area of the fixing belt with the belt guide is equal to the contact area for the period to heat a toner image. Under these conditions, the fixing belt is supplied with a fixed electric power of 800 W, and when the temperature detecting element disposed on the inner surface of the fixing belt senses the temperature of 180° C., the pressure roller is driven to rotate. 
     Also, in heating and fixing operations according to the comparative example 2, for the predetermined period from the start of warm-up, the contact area of the fixing belt with the pressure roller is equal to the contact area for the period to heat a toner image. Also, the contact area of the fixing belt with the belt guide is equal to the contact area for the period to heat a toner image. Under these conditions, the fixing belt is supplied with a fixed electric power of 800 W, and at the same time, the pressure roller is driven to rotate. 
     Note that the fixing apparatuses according to the examples 1 and 2, and the comparative examples 1 and 2 have the sheet resistance R 1  of the heat generating layer  11   a  in the peripheral direction lower than the sheet resistance R 2  of the heat generating layer  11   a  in the longitudinal direction. Accordingly, if the heat generating layer  11   a  is electrified from both ends of the heat generating layer  11   a  in the longitudinal direction through the electrode members  16 , a current flowing in the heat generating layer  11   a  has a tendency to flow around in the peripheral direction, so that a distribution of generated heat is uniformed in the peripheral direction. As the result, even if the heat generating layer  11   a  is electrified when the fixing belt is not rotating, the fixing belt can generate heat uniformly over the entire circumference. 
     Under the conditions described above, each of the fixing apparatuses was warmed up, and after a predetermined time elapsed from the start of electrification, the recording material which bore an unfixed toner image was introduced into the fixing nip portion to verify whether poor fixing was present or not. The result is shown in table 1. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Time period from start of electrification to 
               
               
                   
                 introduction of recording material 
               
            
           
           
               
               
               
               
               
            
               
                   
                 1.5 sec. 
                 1.8 sec. 
                 2.5 sec. 
                 4 sec. 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 1 
                 Absence of 
                 Absence of 
                 Absence of 
                 Absence of 
               
               
                   
                 poor fixing 
                 poor fixing 
                 poor fixing 
                 poor fixing 
               
               
                 Example 2 
                 Presence of 
                 Absence of 
                 Absence of 
                 Absence of 
               
               
                   
                 poor fixing 
                 poor fixing 
                 poor fixing 
                 poor fixing 
               
               
                 Comparative 
                 Presence of 
                 Presence of 
                 Absence of 
                 Absence of 
               
               
                 example 1 
                 poor fixing 
                 poor fixing 
                 poor fixing 
                 poor fixing 
               
               
                 Comparative 
                 Presence of 
                 Presence of 
                 Presence of 
                 Absence of 
               
               
                 example 2 
                 poor fixing 
                 poor fixing 
                 poor fixing 
                 poor fixing 
               
               
                   
               
            
           
         
       
     
     In the case of the comparative example 2, for the predetermined period from the start of warm-up, while the contact area of the fixing belt with the pressure roller and the contact area of the fixing belt with the belt guide are equal to those for the period to heat a toner image in the sheet-passing area, the pressure roller is driven to rotate, thereby the fixing belt is driven to rotate. Accordingly, the heat is easily conducted from the fixing belt to the pressure roller and the belt guide in the fixing nip portion. Furthermore, because the rotation of the fixing belt and the pressure roller caused the heat to be conducted from the fixing belt to the entire surface of the pressure roller, the fixing belt rose slowly in temperature, leading to occurrence of poor fixing even if the recording material was fed after 2.5 sec. elapsed from the start of electrification. 
     Next, in the case of the comparative example 1, for the predetermined period from the start of warm-up, the contact area of the fixing belt with the pressure roller and the contact area of the fixing belt with the belt guide are similar to the comparative example 2. However, because the fixing belt was not rotating, the fixing belt rose more rapidly in temperature than in the case of the comparative example 2. Poor fixing did not occur if the recording material was fed after 2.5 sec. elapsed from the start of electrification. However, unevenness in fixable property occurred with a period of the circumference of the fixing belt if the recording material was fed after 1.8 sec. elapsed, and it was determined to be poor fixing. 
     Next, the comparative example 2 differs from the comparative example 1 in that for the predetermined period from the start of warm-up, while the contact area of the fixing belt with the pressure roller and the contact area of the fixing belt with the belt guide in the sheet-passing area are decreased relative to those for the period to heat a toner image (in a weak pressure applied state), the fixing belt is electrified. 
     In the case of the example 2, for the predetermined period from the start of warm-up, the heat conduction from the fixing belt to the pressure roller and the belt guide is not likely to occur compared with the case of the comparative example 1. Accordingly, the fixing belt more rapidly rose in temperature than in the case of the comparative example 1. Poor fixing did not occur even if the recording material was fed after 1.8 sec. elapsed from the start of electrification. However, poor fixing began to appear when the recording material was fed after 1.5 sec. elapsed. 
     In the case of the example 1, for the predetermined period from the start of warm-up, in the sheet-passing area, the fixing belt and the pressure roller are not in contact with each other and the fixing belt and the belt guide are not in contact with each other. For the predetermined period from the start of warm-up, the fixing belt was not deprived of a large quantity of heat due to heat conduction, and the fixing belt could generate heat, so that poor fixing did not occur even if the recording material was fed after 1.5 sec. elapsed from the start of electrification. 
     Therefore, from the foregoing, in this exemplary embodiment, for the predetermined period from the start of warm-up, while the contact area of the fixing belt with the pressure roller and the contact area of the fixing belt with the belt guide are decreased relative to those for the period to heat a toner image, the fixing belt is electrified, thereby allowing the fixing belt to rise more rapidly in temperature. Also, in this exemplary embodiment, the fixing belt can be at once spaced apart/contacted from/with the pressure roller and from/with the belt guide by integrally stretching or compressing the respective elastic members disposed between the fixing belt and the pressure roller and between the fixing belt and the belt guide. Further, by providing respectively simple elastic members at a position of both ends of the fixing belt and the pressure roller between them and at a position of both ends of the fixing belt and the belt guide between them, the operation for the predetermined period from the start of warming up the fixing apparatus can be stably achieved in an easy manner. 
     Note that the fixing belt used in this exemplary embodiment has the two-layer configuration including the heat generating layer and the release layer (surface layer), but it may have an elastic layer as an intermediate layer, the elastic layer consisting of silicone rubber or the like and disposed between the heat generating layer and the release layer. Further, without imparting anisotropy to the shape of the filler dispersed in the electrified heat generating layer of the fixing belt, the similar effect can be achieved. 
     Furthermore, if the fixing belt generates heat over the entire circumference for the predetermined period from the start of warm-up, the fixing belt needs not to be rotated when the fixing belt and the pressure roller are spaced apart from each other. Accordingly, the fixing belt needs not to have a larger rigidity, and a thin fixing belt having a small heat capacity can accordingly be used. As the result, a configuration can be provided in which the warm-up period can be advantageously shortened. Also, the fixing belt may not be rotated for the predetermined period from the start of warm-up, which can give the fixing belt a long life. 
     It was described that the predetermined period from the start of warm-up is the period until the temperature of the temperature detecting member for detecting the temperature of the fixing belt reaches the predetermined temperature, but the predetermined period may be a period until a predetermined time elapses from the start of warm-up. 
     A second exemplary embodiment will be described. Hereinafter, the second exemplary embodiment of the present invention will be described with reference to  FIGS. 7A and 7B . This exemplary embodiment differs from the first exemplary embodiment in that a leaf spring is used as the electrical contact portion member, and has configurations of the fixing belt  11  and the pressure roller  12  similar to the first exemplary embodiment, and repeated description will accordingly be omitted. Also, to components and parts similar to the first exemplary embodiment, similar symbols are assigned.  FIG. 7A  schematically illustrates a main portion of a fixing apparatus in the longitudinal direction.  FIG. 7B  is a schematic cross-sectional view, including a electrical contact portion. In addition,  FIGS. 7A and 7B  illustrate a standby state at start of the fixing apparatus, and the pressure spring  15  is released from compression by the pressure control device  200  ( FIG. 1A ) used as a pressure release mechanism. Accordingly, the fixing belt  11  and the pressure roller  12  are spaced apart from each other in the sheet-passing area. 
     In this exemplary embodiment, a method for feeding the heat generating layer  11   a  of the fixing belt  11  includes sliding and pressing leaf springs  20  shown in  FIG. 7B . The leaf springs  20  are disposed at positions abutting against outer surfaces of the power supply areas  11   a l,  11   a R in both ends of the fixing belt shown in  FIG. 7A  (outside of the sheet-passing area). Also, as shown in  FIG. 7B , a direction in which the leaf spring  20  applies a pressure is a direction opposing to the belt guide  13 , and an opposing direction to a direction in which the fixing belt  11  applies a pressure to the pressure roller  12  using the pressure spring  15 . 
     Further, while the fixing belt  11  is pressed against the pressure roller  12 , the strength of the spring pressure of the pressure spring  15  is set to be larger than that of the leaf spring  20 . Also, an end of the leaf spring  20  on the side opposing to an end abutting against the fixing belt  11  is fixedly supported on an unshown flame of the fixing apparatus through an insulating layer. 
     Furthermore, for the predetermined period from the start of warm-up, while the pressure is not applied by the pressure spring  15 , the fixing belt  11  and the pressure roller are spaced apart from each other because the leaf spring  20  presses the fixing belt  11  in a direction in which the fixing belt  11  and the pressure roller  12  are spaced apart from each other. Further, similarly to the first exemplary embodiment, the fixing belt  11  abuts against elastic members  17  disposed in both ends of the belt guide  13  shown in  FIG. 7B  (outside of the sheet-passing area), and the fixing belt  11  and the belt guide  13  are spaced apart from each other. 
     (Heating and Fixing Operations of Fixing Apparatus) 
     When the controller receives a print command, the AC power supply begins to electrify the heat generating layer  11   a  of the fixing belt  11  through the leaf spring  20 . This causes the heat generating layer  11   a  to generate heat, and the fixing belt  11  then rises rapidly in temperature. The temperature of the fixing belt  11  is detected by the temperature detecting member  18  such as a thermistor disposed in contact with or near the inner surface of the heat generating layer  11   a . When it is detected that the temperature of the fixing belt  11  reaches the predetermined temperature, the pressure spring  15  is compressed to press the fixing belt against the pressure roller. 
     At this time, the leaf spring  20  is pressed by the fixing belt  11  to be elastically deformed, and the fixing belt accordingly abuts against the pressure roller. Further, at the same instant, the pressure roller  12  is driven to rotate. The rotation of the pressure roller  12  is transmitted to the fixing belt  11  due to a frictional force generated between the surface of the pressure roller  12  and the surface of the fixing belt  11  in the fixing nip portion N. This forces the fixing belt  11  to be driven to rotate following the rotation of the pressure roller  12 , with the inner periphery (inner surface) of the heat generating layer  11   a  of the fixing belt  11  being in contact with the outer periphery of the belt guide  13 . While the heat generating layer  11   a  is electrified, the recording material P which bears an unfixed toner image T is introduced into the fixing nip portion N, with the surface on which bears the toner image upward. 
     This recording material P is sandwiched between the surface of the fixing belt  11  and the surface of the pressure roller  12  and conveyed in the fixing nip portion N. In this conveyance process, the toner image T on the recording material P is heated by the fixing belt  11  and melts, and the toner image T is then pressed in the fixing nip portion N to be fixed under heat on the recording material P. Subsequently, the recording material P on which the toner image T is fixed under heat is conveyed from the fixing nip portion N to the recording material ejection mechanism. At this time, the controller takes in an output signal (temperature detecting signal) from the temperature detecting member  18 , and based on this output signal, controls the electrical power so that the fixing belt  11  can maintain a predetermined fixing temperature (target temperature). 
     From the foregoing, also in this exemplary embodiment, the similar effect to the first exemplary embodiment can be achieved. 
     Other Exemplary Embodiment 
     The fixing apparatus for fixing under heat an unfixed toner image on a recording material has been described above, but the present invention is not limited to this. That is, for example, this fixing apparatus can be used as an apparatus for heating and temporarily fixing an unfixed toner image on a recording material, or as an apparatus for heating a toner image already fixed under heat on a recording material to impart glazing to a surface of the toner image. 
     Further, in the first and second exemplary embodiment, the fixing belt which generates heat from electrification has been described as a fixing belt. A fixing apparatus in the form of induction heat generation can be also used, which fixing apparatus generates heat in a manner that a fixing belt having an electrically conductive layer and an exciting coil are provided to form an electromagnetic field, and the electromagnetic field produces eddy currents over the entire circumference of the fixing belt, which causes the fixing belt to generate heat. Furthermore, for the predetermined period from the start of warm-up, the fixing belt remaining at rest can be used. 
     Note that the pressure control device  200  and the detachment device  300  are not limited to the foregoing, and the operation for spacing apart may be performed in a manner that for the predetermined period from the start of warm-up, the ends of the belt guide and the pressure roller are moved obliquely relative to the fixing belt by using the other ends of them in the longitudinal direction as a supporting point. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2011-166701, filed Jul. 29, 2011, which is hereby incorporated by reference herein in its entirety.