Patent Publication Number: US-2022236676-A1

Title: Fixing device and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a fixing device and an image forming apparatus. 
     2. Description of the Related Art 
     There are image forming apparatuses such as a printer, a copier, and a facsimile machine. The image forming apparatus (for example, the printer) includes an image bearing body. A surface of the image bearing body is uniformly charged by a charging member. The surface of the image bearing body is exposed with light emitted by an exposure device so that an electrostatic latent image is formed thereon. The electrostatic latent image is developed by a developer bearing body to form a toner image. Then, a sheet transported from a sheet cassette is introduced between the image bearing body and a transfer member, and the toner image is transferred to the sheet by the transfer member. Then, the sheet is transported to a fixing device or a fuser. The fixing device fixes the toner image to the sheet, so that printing is completed. 
     The fixing device includes a fixing belt as an annular belt, a heater as a heating member disposed inside the fixing belt, and a pressure roller pressed against the heater via the fixing belt. In the fixing device, a heat diffusion member is disposed to face an inner circumferential surface of the fixing belt so that heat generated by the heater is easily transmitted to the sheet via the fixing belt (for example, Patent Reference 1). 
     Patent Reference 1: Japanese Patent Application Publication No. 2020-3611 
     However, in the conventional fixing device, heat cannot be transferred stably to the fixing belt when printing is performed at high speed, and an image quality may be degraded. 
     SUMMARY OF THE INVENTION 
     An object of the present disclosure is to provide a fixing device and an image forming apparatus capable of stabilizing heat transfer to an annular belt and improving an image quality. 
     A fixing device of the present disclosure includes an annular belt, a heating member disposed to face an inner circumferential surface of the annular belt, and a heat storage member disposed to face a surface of the heating member on a side opposite to the annular belt. A thermal diffusivity of the heat storage member is lower than that of the annular belt. 
     Since the heat storage member is disposed on the side of the heating member opposite to the fixing belt, and the thermal diffusivity of the heat storage member is lower than that of the annular belt, sufficient heat can be stored in the heat storage member, and a decrease in the temperature of the heat storage member can be prevented. 
     Therefore, heat can be sufficiently transferred to the annular belt. Further, heat taken away from the annular belt when the sheet passes a fixing nip can be replenished by the heat storage member via the heating member. Thus, the temperature of the annular belt can be made uniform and a gloss difference in an image can be reduced. As a result, the image quality can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, 
         FIG. 1  is a cross-sectional view of a main part of a fixing device in an embodiment; 
         FIG. 2  is a schematic diagram of a printer in the embodiment; 
         FIG. 3  is a perspective view of the main part of the fixing device in the embodiment; 
         FIG. 4  is a front view of the main part of the fixing device in the embodiment; 
         FIG. 5  is a cross-sectional view taken along line V-V in  FIG. 4 ; 
         FIG. 6  is an exploded perspective view of the main part of the fixing device in the embodiment; 
         FIG. 7  is a plan view of a heater in the embodiment; 
         FIG. 8  is a cross-sectional view of a fixing belt in the embodiment; 
         FIG. 9  is a perspective view of a pressure roller in the embodiment; 
         FIG. 10  is a cross-sectional view taken along line X-X in  FIG. 9 ; 
         FIG. 11  is a diagram for explaining properties of a heat storage member when the number of polyimide films is varied; 
         FIG. 12  is a diagram for explaining a measurement method of a glossiness; 
         FIG. 13  is a diagram illustrating a relationship between a heat capacity per unit area of the polyimide film and a gloss difference; 
         FIG. 14  is a diagram illustrating a relationship between the heat capacity per unit area of the polyimide film and a heater-back temperature rising rate; 
         FIG. 15  is a diagram for explaining a measurement method of a thermal diffusivity; 
         FIG. 16  is a diagram illustrating an example of a state of disposing a thermistor in the embodiment; 
         FIG. 17  is a plan view illustrating another example of the state of disposing the thermistor in the embodiment; and 
         FIG. 18  is a diagram illustrating still another example of the state of disposing the thermistor in the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present disclosure will be described in detail below with reference to the drawings. Herein, a fixing device and a printer as an image forming apparatus will be described. 
       FIG. 2  is a schematic diagram of a printer  10  in the embodiment. In  FIG. 2 , a +X axis direction is a rearward direction of the printer  10 , while a −X axis direction is a frontward direction of the printer  10 . A +Y axis direction is a left direction of the printer  10 , while a −Y axis direction is a right direction of the printer  10 . A +Z-axis direction is an upward direction of the printer  10 , while a −Z axis direction is a downward direction of the printer  10 . 
     In  FIG. 2 , reference numeral  10  denotes the printer, reference numeral Cs denotes a housing of the printer  10 , and reference numeral Bd denotes a main body of the printer  10 . The housing Cs serves as an enclosure of the printer  10 . The main body Bd is also referred to as an apparatus main body. 
     A sheet cassette  11  as a medium storage container is provided at a lower part of the apparatus main body Bd. Sheets P as media are stacked and stored in the sheet cassette  11 . A feed mechanism (not shown) is disposed adjacent to a front end of the sheet cassette  11 . The sheets P are separated one by one by the feed mechanism, and each sheet P is fed to a sheet transport path Rt 1  as a medium transport path. Then, the sheet P is transported along the sheet transport path Rt 1  by a pair of feed rollers m 1  as a first transport member. Further, the sheet P is transported to an image forming section Q 1  disposed downstream of the pair of feed rollers m 1 . 
     The image forming section Q 1  includes a plurality of (in this example, four) image forming units  16 Bk,  16 Y,  16 M, and  16 C of respective colors (for example, black, yellow, magenta, and cyan), LED heads  23  as exposure devices, and a transfer unit u 1 . 
     Each of the image forming units  16 Bk,  16 Y,  16 M, and  16 C includes a toner cartridge Ct as a developer storage container, a photosensitive drum  21  as an image bearing body, a charging roller  22  as a charging member, a developing roller  24  as a developer bearing body, and a cleaning blade  25  as a first cleaning member. The toner cartridge Ct stores a toner as a developer. The photosensitive drum  21  is rotatably provided. The charging roller  22  is rotatably provided in contact with the photosensitive drum  21 . The developing roller  24  is rotatably provided in contact with the photosensitive drum  21 . The cleaning blade  25  is provided so that its tip end is in contact with the photosensitive drum  21 . A surface of the photosensitive drum  21  is uniformly charged by the charging roller  22 , and is exposed with light emitted by the LED head  23  so that an electrostatic latent image (or a latent image) is formed on the surface of the photosensitive drum  21 . The toner supplied from the toner cartridge Ct adheres to the photosensitive drum  21  by the developing roller  24 , so that the electrostatic latent image is developed. As a result, a toner image as a developer image of each color is formed on the surface of each photosensitive drum  21 . 
     The transfer unit u 1  includes a driving roller r 1  as a first roller, a driven roller r 2  as a second roller, a transfer belt  26  as a belt member, transfer rollers  28  as transfer members, and a cleaning blade  29  as a second cleaning member. The transfer belt  26  is stretched over the driving roller r 1  and the driven roller r 2  so that the transfer belt  26  is movable. The transfer roller  28  is rotatably provided to face the photosensitive drum  21  via the transfer belt  26 . The cleaning blade  29  is provided so that its tip end is in contact with the transfer belt  26 . When a belt motor (not shown) as a driving source for transfer is driven, the driving roller r 1  rotates to cause the transfer belt  26  to move. 
     As the sheet P reaches the image forming section Q 1 , the sheet P is transported through transfer portions between the photosensitive drums  21  of the image forming units  16 Bk,  16 Y,  16 M, and  16 C and the corresponding transfer rollers  28  by the movement of the transfer belt  26 . While the sheet P is transported as above, toner images of the respective colors are transferred to the sheet P in an overlapping manner by the transfer rollers  28 , and a color toner image is formed on the sheet P. 
     After the toner images of respective colors are transferred to the sheet P, the toner remaining on each photosensitive drum  21  is scraped off and removed by the cleaning blade  25 . Furthermore, the toner, foreign matter and the like adhering to the transfer belt  26  are scraped off and removed by the cleaning blade  29 . 
     A fixing device  31  (or a fuser) is disposed downstream of the image forming section Q 1  along the sheet transport path Rt 1 . The fixing device  31  includes a fixing belt unit  32  as a first fixing member and a pressure roller  33  as a second fixing member or a facing member. The pressure roller  33  is disposed to face the fixing belt unit  32 . While the sheet P is transported through a fixing nip Np ( FIG. 1 ) as a fixing portion between the fixing belt unit  32  and the pressure roller  33 , the color toner image on the sheet P is heated and melted by the fixing belt unit  32 , pressurized by the pressure roller  33 , and fixed to the sheet P. The fixing nip Np will be described below. 
     The sheet P to which the color toner image is fixed in this way is ejected to the outside of the apparatus main body Bd by a pair of ejection rollers m 2  as a second transport member, and then placed on a stacker sk provided at a top wall Wt (or a top cover) of the housing Cs. 
     Next, the fixing device  31  will be described. 
       FIG. 1  is a cross-sectional view of a main part of the fixing device  31  in the embodiment.  FIG. 3  is a perspective view of the main part of the fixing device  31  in the embodiment.  FIG. 4  is a front view of the main part of the fixing device  31  in the embodiment.  FIG. 5  is a cross-sectional view taken along line V-V in  FIG. 4 .  FIG. 6  is an exploded perspective view of the main part of the fixing device  31  in the embodiment. In this regard,  FIG. 1  corresponds to an enlarged view of a part encircled by a circle I in  FIG. 5 . In  FIGS. 1, 3 to 5 , the +X axis direction is the rearward direction of the printer  10 , while the −X axis direction is the frontward direction of the printer  10 . However, hereinafter, the +X axis direction is described as a frontward direction of the fixing device  31  (i.e., a transport direction of the sheet P passing through the fixing device  31 ), while the -X axis direction is a rearward direction of the fixing device  31  (i.e., a direction opposite to the transport direction of the sheet P passing through the fixing device  31 ). The +Y axis direction is a left direction of the fixing device  31 , while the −Y axis direction is a right direction of the fixing device  31 . The +Z axis direction is an upward direction of the fixing device  31 , while the −Z axis direction is a downward direction of the fixing device  31 . 
     In  FIGS. 3 and 4 , the fixing device  31  includes a frame FL, a frame FR, levers Lv, the fixing belt unit  32 , and the pressure roller  33 . The frame FL is disposed on the left side of the printer  10  ( FIG. 2 ) to constitute a part of an enclosure of the fixing device  31 . The frame FR is disposed on the right side of the printer  10  to constitute another part of the enclosure of the fixing device  31 . The levers Lv are provided on the frames FL and FR so that each lever Lv is pivotal in a plane defined by the X-axis and the Y-axis. The fixing belt unit  32  is provided between the levers Lv. The fixing belt unit  32  is supported by the frames FL and FR so that the fixing belt unit  32  is pivotal together with the levers Lv. The pressure roller  33  is provided between the frames FL and FR below the fixing belt unit  32  and is rotatably supported by the frames FL and FR. 
     The frames FL and FR are composed of box-shaped bodies whose facing sides are opened. Each of the frames FL and FR includes a main plate Pm. Each of the frames FL and FR further includes a front piece Pf, a rear piece Pr, a top piece Pt, and a bottom piece Pb which are formed by perpendicularly bending the front, back, upper and lower edges of the main plate Pm, respectively. By fixing the bottom piece Pb to a main body frame FBd of the apparatus main body Bd with screws bt 1  as fixing members, the fixing device  31  is disposed at a predetermined position in the apparatus main body Bd. 
     On each of the frames FL and FR, the lever Lv is disposed pivotally about a shaft sh 1  within a housing space enclosed by the main plate Pm, the front piece Pf, the rear piece Pr, the top piece Pt, and the bottom piece Pb. The shaft sh 1  is provided on the main plate Pm in the vicinity of a corner of the rear piece Pr and the bottom piece Pb. The fixing belt unit  32  is held between the levers Lv. 
     In  FIG. 5 , each lever Lv is biased toward the front piece Pf side with a predetermined biasing force generated by a spring Sp as a biasing member. The spring Sp is disposed in the vicinity of the top piece Pt and extends in the front-back direction. A rear end Spr of the spring Sp is attached to the rear piece Pr of each of the frames FL and FR, and a front end Spf of the spring Sp is attached to a locking portion Lvt erected upward from a front edge of the lever Lv. 
     When the fixing device  31  does not perform fixing, the levers Lv are locked to the frames FL and FR by lever fixing members (not shown) in a state where the springs Sp are compressed against its biasing force, and thereby the levers Lv are held at a predetermined position. 
     In  FIGS. 3 and 4 , the pressure roller  33  is located in the vicinity of the bottom pieces Pb. The pressure roller  33  is supported by the main plates Pm of the frames FL and FR so that the pressure roller  33  is rotatable in contact with the fixing belt unit  32 . Gears gr 1 , gr 2  and gr 3  are disposed in mesh with each other on an outer side of the frame FR. The gear gr 1  is attached to a support shaft sh 2  of the pressure roller  33 . Rotation from a fixing motor (not shown) is transmitted to the gear gr 3 . 
     When the fixing device  31  performs fixing, the rotation from the fixing motor is transmitted to the gear gr 3  and then transmitted to the gear gr 1  via the gear gr 2 . The lock of the levers LV to the frames FL and FR by the lever fixing members is released by a releasing mechanism (not shown). This causes the lever Lv to pivot in a clockwise direction on the frame FL side (in a counterclockwise direction on the frame FR side) by the biasing force of the spring Sp as shown in  FIG. 5 . 
     Thus, the fixing belt unit  32  held between the levers Lv is pressed against the pressure roller  33 , causing the sheet P to be sandwiched between the fixing belt unit  32  and the pressure roller  33 . As the pressure roller  33  rotates, the sheet P is transported in a direction indicated by an arrow in  FIG. 3 . 
     In  FIG. 5 , the fixing belt unit  32  includes a heating unit  38  serving as a heating source of the fixing belt unit  32 , and a fixing belt  39  formed of an annular body to surround the heating unit  38 . The fixing belt  39  is also referred to as an annular belt (i.e., an endless belt), and is moved by the rotation of the pressure roller  33 . 
     In  FIG. 6 , the heating unit  38  includes components provided for heating the toner image on the sheet P. More specifically, the heating unit  38  includes a stay  41 , a holding member  43 , a heat storage member  45 , a heater  46  as a first heating member, and a heat diffusion member  47  as a second heating member. The stay  41  extends in a longitudinal direction (Y axis direction) of the heating unit  38 . Both ends of the stay  41  are fixed to the levers Lv. The holding member  43  extends in the longitudinal direction along the stay  41  and is attached to the stay  41 . The heat storage member  45 , the heater  46 , and the heat diffusion member  47  are all held by the holding member  43 . Each of the heat storage member  45 , the heater  46 , and the heat diffusion member  47  has a strip shape and extends in the longitudinal direction. 
     In the embodiment, the heater  46  generates heat so as to heat the toner image via the fixing belt  39 . The heat diffusion member  47  transfers heat generated by the heater  46  to the fixing belt  39  to thereby heat the toner image via the fixing belt  39 . The heater  46  and the heat diffusion member  47  are disposed to face an inner circumferential surface Si of the fixing belt  39 . The heat storage member  45  is disposed to face a surface of the heater  46  on a side opposite to the fixing belt  39 . The heater  46  and the heat diffusion member  47  are also referred to as a heating member. 
     The stay  41  is made of a resin material. As shown in  FIG. 6 , the stay  41  has a box shape with its lower end opened. The stay  41  has a top plate  41   t,  a front leg  41   f  hung down from a front edge of the top plate  41   t,  and a rear leg  41   r  hung down from a rear edge of the top plate  41   t.  Both ends of the stay  41  penetrate openings Wn formed in the levers Lv, and protrude toward the frames FL and FR. The protruding portions of the stay  41  are fixed to flaps Lvp by screws bt 2  as fixing members. Each flap Lvp is formed by bending an upper edge of the opening Wn of the lever Lv laterally. 
     The holding member  43  is made of a metal and has a box shape with its upper end opened. As shown in  FIG. 1 , the holding member  43  has a bottom plate  43   b,  a front arm  43   f  erected from a front edge of the bottom plate  43   b,  and a rear leg  43   r  erected from a rear edge of the bottom plate  43   b.  The holding member  43  is attached to the stay  41  by fitting the front arm  43   f  and the rear leg  43   r  to the inside of the stay  41  from below. 
     A concave portion having a rectangular shape is formed at a lower surface of the holding member  43  so as to face the inner circumferential surface Si of the fixing belt  39  and to face the pressure roller  33  via the fixing belt  39 . This concave portion serves as a housing space  51  in which the heat storage member  45 , the heater  46 , and the heat diffusion member  47  are housed in this order from above in an overlapping manner. 
     In addition, a thermistor Th is disposed on the bottom plate  43   b  of the holding member  43  so as to face the housing space  51 . The thermistor Th serves as a temperature detecting element for detecting a temperature of the heater  46  in a non-contact manner. In the embodiment, the thermistor Th is configured to detect the temperature of the heater  46  indirectly via the heat storage member  45 . However, it is also possible to directly detect the temperature of the heater  46  using a contact-type element such as a contact-type thermistor. 
     Furthermore, in order to detect a temperature of a surface layer  39   c  ( FIG. 8 ) of the fixing belt  39  as described later, a temperature sensor TH ( FIG. 5 ) as a non-contact type temperature detector is disposed in the vicinity of the shaft sh 1  in the lever Lv so that the temperature sensor TH faces the fixing belt  39 . The heater  46  is turned on and off based on a difference between a temperature detected by the temperature sensor TH (i.e., a detected temperature) and a target temperature, thereby performing a warming-up operation of the fixing device  31 . 
     As shown in  FIG. 1 , each of the heat storage member  45  and the heater  46  has a flat shape. The heat diffusion member  47  includes a contact portion  47   b  formed to contact the inner circumferential surface Si of the fixing belt  39 , and erected pieces  47   f  and  47   r  as first and second engaging portions that are erected from the front and rear edges of the contact portion  47   b,  respectively. Furthermore, grooves  53   f  and  53   r  as first and second to-be-engaged portions are formed at portions of the bottom plate  43   b  of the holding member  43  corresponding to the erected pieces  47   f  and  47   r,  respectively, in such a manner that the grooves  53 F and  53   r  are opened downward. 
     Therefore, the erected pieces  47   f  and  47   r  are inserted into the grooves  53   f  and  53   r,  respectively. Thus, the heat diffusion member  47  is disposed movably in the vertical direction in such a manner that the heat diffusion member  47  surrounds the heat storage member  45  and the heater  46 . The heat storage member  45  and the heater  46  provided between the holding member  43  and the heat diffusion member  47  are movable vertically without being restricted by the holding member  43  and the heat diffusion member  47 . 
     As described above, when the fixing belt unit  32  is pressed against the pressure roller  33 , the contact portion  47   b  of the heat diffusion member  47  is brought into contact with the fixing belt  39  so that the fixing nip Np is formed between the heat diffusion member  47  and the pressure roller  33  via the fixing belt  39 . 
     The heat diffusion member  47  is made of a metal plate of stainless steel, aluminum, an aluminum alloy, or iron or the like, so as to efficiently transfer the heat of the heater  46  to the fixing belt  39 . A surface of the metal plate facing the inner circumferential surface Si of the fixing belt  39  is coated with a material having a low coefficient of friction and high wear resistance. For example, the surface of the metal plate may be subjected to glass coating, hard chrome plating, fluorine resin coating, and polyamide-imide coating, so that the surface is not deformed by rubbing against a base material layer  39   a  ( FIG. 8 ) constituting the inner circumferential surface Si of the fixing belt  39 . 
     In the embodiment, a metal plate made of aluminum is used for the thermal diffusion member  47  and a surface of the metal plate is coated with polyamide-imide to which graphite is added. A blending ratio of graphite to polyamide-imide is 11 [weight %]. A thickness of the coating is 10 [μ]. In order to efficiently transfer the heat stored in the heat storage member  45  to the fixing belt  39 , a thermal diffusivity of the heat diffusion member  47  is preferably higher than that of the heat storage member  45 . In the embodiment, the thermal diffusivity of the heat diffusion member  47  is 4.56×10 −6  [m 2 /s]. 
     Thermal conductive grease is applied between the heater  46  and the heat diffusion member  47  to efficiently transfer the heat of the heater  46  to the heat diffusion member  47 . It is noted that thermal conductive grease may also be applied between the heat storage member  45  and the heater  46 . Furthermore, sliding grease is applied to a sliding portion between the heat diffusion member  47  and the fixing belt  39  in order to enhance slidability and prevent wear of the heat diffusion member  47  and the fixing belt  39 . 
     Next, the heater  46  will be described. 
       FIG. 7  is a plan view of the heater  46  in the embodiment. 
     In  FIG. 7 , the heater  46  includes a connector  55 , heating sections Ar 1  (i=1, 2, and 3), conducting heat generating elements  57 , and lead wires  58 . The connector  55  is disposed at one end of the heater  46 . The heating sections Ar 1  are formed at a plurality of locations on a substrate  56 , specifically five locations in the embodiment, by printing or embedding. The conducting heat generating element  57  is disposed in each heating section Ar 1  while meandering and generates heat by application of current. The lead wires  58  are disposed between the connector  55  and both ends of each conducting heat generating element  57  and supply power to the corresponding heating section Ar 1 . 
     Among the heating sections Ar 1 , the heating section An is disposed at a center of the heater  46  in the longitudinal direction. The heating sections Ar 2  are disposed on both sides of the heating section Ar 1 . The heating sections Ar 3  are disposed on both sides (more specifically, outer sides) of the heating section Ar 2 . Power is selectively supplied to the heating sections Ar 1  according to a width of the sheet P. For example, when printing is performed on a narrow sheet P such as a postcard, power is supplied only to the central heating section Ar 1 . When printing is performed on a wide sheet P such as an A 4  sheet P in a transverse feed (or an A 3  sheet P in a longitudinal feed), power is supplied to all heating sections Ar 1 , Ar 2  and Ar 3 . Since the heating sections Ar 1  are selected according to the sheet P to be used, the power can be saved. 
     In the embodiment, a boundary portion between each adjacent two of the heating sections Ar 1  is inclined in the longitudinal direction of the heater  46 , and thus it is possible to suppress a change in the temperature between the heating section Ar 1  and its adjacent boundary portion. 
     Next, the fixing belt  39  will be described. 
       FIG. 8  is a cross-sectional view of the fixing belt  39  in the embodiment. 
     In  FIG. 8 , the fixing belt  39  includes at least three layers, namely, the base material layer  39   a,  a resilient layer  39   b  formed on the base material layer  39   a,  and the surface layer  39   c  formed on the resilient layer  39   b.    
     The base material layer  39   a  has high mechanical strength and high durability so as to withstand repeated bending, buckling, or the like in order that the fixing belt  39  is able to move without breakage throughout its lifetime. 
     For this reason, in the embodiment, the base material layer  39   a  is made of polyimide (PI), and has a diameter of 30 [mm] and a thickness of 80 [μ]. For the base material layer  39   a,  it is possible to use a base material having high heat resistance so as to withstand a fixing temperature, exhibits high durability against bending, buckling, and the like, and has a predetermined Young&#39;s modulus. Examples of such a base material include SUS430, polyether ether ketone (PEEK), and the like. If necessary, fillers made of polytetrafluoroethylene (PTFE), boron nitride or the like may be added to the base material in order to enhance slidability, thermal conductivity or the like. Furthermore, electric conductive fillers containing carbon black, zinc or other metallic elements may be added to the base material so that the base material layer  39   a  has electric conductivity. 
     The resilient layer  39   b  has appropriate rubber hardness and thickness in order to form the fixing nip Np and to efficiently transfer heat from the heating unit  38  to an outer circumferential surface (toner contact surface) So of the fixing belt  39  while suppressing the dissipation of heat. If the resilient layer  39   b  is extremely thick, a uniform fixing nip Np can be formed, but a heat capacity of the resilient layer  39   b  increases, resulting in a large heat loss. Thus, the thickness of the resilient layer  39   b  is preferably 50 [μ] or thicker and 500 [μ] or thinner. The rubber hardness of the resilient layer  39   b  is preferably 20 [degrees] or higher and 60 [degrees] or lower so as to form the fixing nip Np uniformly. In the embodiment, the thickness of the resilient layer  39   b  is 300 [μ], and the rubber hardness is 20 [degrees]. Silicone rubber is used as a material of the resilient layer  39   b  in order to enhance the heat resistance of the resilient layer  39   b  so as to withstand the fixing temperature. The material of the resilient layer  39   b  is not limited to silicone rubber, and any material capable of withstanding the fixing temperature, for example, fluoro rubber may be used. 
     The surface layer  39   c  is generally desired to be thin so that the surface layer  39   c  is deformable according to deformation of the resilient layer  39   b.  However, if the surface layer  39   c  is extremely thin, wrinkles may occur on its surface due to rubbing against the pressure roller  33 , the sheet P, or the like. Thus, the thickness of the surface layer  39   c  is preferably  9  [μ] or thicker and 50 [μm] or thinner. In the embodiment, the thickness of the surface layer  39   c  is set to 20 [μ]. The surface layer  39   c  is desired to have high heat resistance so as to withstand the fixing temperature. In addition, the surface layer  39   c  is desired to have high releasability so that the toner of the toner image fixed to the sheet P is less likely to adhere to the surface layer  39   c. For this reason, the surface layer  39   c  is made of fluorine-substituted material. In the embodiment, tetrafluoroethylene-perfluoroalkoxy ethylene copolymer (PFA) is used for the surface layer  39   c.    
     Next, the pressure roller  33  will be described. 
       FIG. 9  is a perspective view of the pressure roller  33  in the embodiment.  FIG. 10  is a cross-sectional view taken along line X-X in  FIG. 9 . 
     The pressure roller  33  includes at least four layers, namely, a shaft  33   a,  a resilient layer  33   b,  an adhesive layer  33   c,  and an outer circumferential surface layer  33   d.  The shaft  33   a  has the support shaft sh 2  protruding on both ends thereof. The resilient layer  33   b  is formed on an outer surface of the shaft  33   a. The adhesive layer  33   c  is formed on an outer surface of the resilient layer  33   b.  The outer circumferential surface layer  33   d  is formed on an outer surface of the adhesive layer  33   c.  It is also possible to provide an adhesive layer between the shaft  33   a  and the resilient layer  33   b  as needed. 
     In the embodiment, the pressure roller  33  has an outer diameter of 40 [mm]. The pressure roller  33  has an inverted crown shape such that an outer diameter of a center of the pressure roller  33  in the axial direction is smaller than an outer diameter of each end of the pressure roller  33  in the axial direction by 0.2 [mm]. The pressure roller  33  has a hardness of 50 [degrees] or higher and 65 [degrees] or lower. 
     The shaft  33   a  is made of a material that withstands a pressure applied when fixing is performed. In the embodiment, a hollow shaft made of stainless steel (SUS304) is used as the shaft  33   a,  but a solid shaft may also be used as the shaft  33   a.    
     The resilient layer  33   b  is desired to have appropriate rubber hardness and thickness so as to form the fixing nip Np, as is the case with the resilient layer  39   b  of the fixing belt  39 . Further, the resilient layer  33   b  is desired to have heat storage properties sufficient to prevent dissipation of heat transferred from the fixing belt  39  to the sheet P and the toner image thereon. Although the resilient layer  33   b  may be made of solid rubber as is the case with the resilient layer  39   b  of the fixing belt  39 , the resilient layer  33   b  in the embodiment is made of a silicone sponge having foam cells for the above-described reason. In order to prevent a nip mark from being formed by pressure applied to the fixing nip Np, cell diameters of foam cells are preferably small, and an average cell diameter is preferably 20 [μ] or larger and 250 [μ] or smaller. In the embodiment, a silicone rubber having an average cell diameter of about 100 [μm] is used. The cell diameter is determined by cutting the silicone rubber in its thickness direction with a razor or the like, observing the cut surface with a CCD microscope, measuring cell diameters of  10  cells in a field of view, and then averaging the measured cell diameters. The silicone rubber has a thickness of 4 [mm]. Further, in the embodiment, in order to suppress electrostatic adhesion of paper dust or other materials to the pressure roller  33  due to electric charge accumulated in the pressure roller  33  during continuous printing, an electric conductive agent is added to the silicone rubber. It is noted that the electric conductive material is not necessarily added to the resilient layer  33   b.    
     The adhesive layer  33   c  is used to prevent peeling off of the outer circumferential surface layer  33   d  from the resilient layer  33   b  and generation of wrinkles or the like. Thus, in the embodiment, a silicone adhesive, which has high adhesive strength and excellent resistance to the fixing temperature, is used as the adhesive layer  33   c.  In order to suppress the adhesion of paper dust or other materials to the pressure roller  33  during continuous printing, an electric conductive agent may be added to the silicone adhesive, or alternatively an electric conductive adhesive may be used. 
     The outer circumferential surface layer  33   d  is generally desired to be thin so that the outer circumferential surface layer  33   d  is deformable according to deformation of the resilient layer  33   b.  However, if the outer circumferential surface layer  33   d  is extremely thin, wrinkles may occur on its surface due to rubbing against the sheet P (mainly paper), the fixing belt  39 , and the like. Thus, the thickness of the outer circumferential surface layer  33   d  is preferably 15 [μm] or thicker and 50 [μm] or thinner. In the embodiment, the thickness of the outer circumferential surface layer  33   d  is set to 30 [μm]. The outer circumferential surface layer  33   d  is also desired to have high heat resistance so as to withstand the fixing temperature. In addition, the outer circumferential surface layer  33   d  is desired to have high releasability so that the toner of the toner image fixed to the sheet P is less likely to adhere to the outer circumferential surface layer  33   d.  For this reason, the outer circumferential surface layer  33   d  is made of fluorine-substituted material. In the embodiment, tetrafluoroethylene-perfluoroalkoxy ethylene copolymer (PFA) is used for the outer circumferential surface layer  33   d.    
     As described above, in the embodiment, the heat diffusion member  47  is disposed to face the inner circumferential surface Si of the fixing belt  39  so that the heat generated by the heater  46  is easily transferred to the fixing belt  39 . 
     However, if heat cannot be sufficiently transferred from the heater  46  to the fixing belt  39  during printing at high speed, a temperature of the fixing belt  39  varies between a first turn and a second turn of the fixing belt  39 . This increases a difference in glossiness (hereinafter referred to as a gloss difference) between image portions on the sheet P formed in the first turn and the second turn of the fixing belt  39 , and thus an image quality may be degraded. 
     If a heat capacity of the fixing belt  39  is large, power consumption by the heater  46  increases, and a warming-up time increases. Thus, a standby time after the printer  10  is turned on until printing starts increases. For this reason, it is conceivable to reduce the heat capacity of the fixing belt  39  by reducing the thickness of the resilient layer  39   b  of the fixing belt  39 . However, if the heat capacity of the fixing belt  39  is set extremely small, a heat storage capacity of the fixing belt  39  decreases. In this case, a change in the temperature of the fixing belt  39  between the first turn and the second turn of the fixing belt  39  increases, and the gloss difference between the image portions formed in the first turn and the second turn of the fixing belt  39  increases. 
     Thus, in the embodiment, the heat storage member  45  is provided for the heater  46 . The heat storage member  45  replenishes heat to the fixing belt  39  whose temperature decreases due to the passage of the sheet P. In addition, a thermal diffusivity of the heat storage member  45  is lower than that of the fixing belt  39 , and thus sufficient heat can be stored in the heat storage member  45 . 
     Next, the heat storage member  45  will be described. 
     The heat storage member  45  in the embodiment is formed by laminating polyimide resin films (hereinafter referred to as “polyimide films”) onto the heater  46 . In other words, the heat storage member  45  is formed by bonding the polyimide films onto the heater  46  in an overlapping manner. Each polyimide film has a thickness of about 70 [μ]. The polyimide film is, for example, “heat resistant polyimide tape” manufactured by 3M Japan Limited. Each polyimide film is smaller than the substrate  56  of the heater  46  ( FIG. 7 ), but has a dimension of 250 [mm] in the longitudinal direction and a dimension of 15 [mm] in the widthwise direction so as to cover the heating sections Ar 1 . 
     As the number of laminated polyimide films on the heater  46  increases, a heat capacity of the heat storage member  45  increases. 
     Next, properties of the heat storage member  45  when the number of polyimide films is varied will be described. 
       FIG. 11  is a table for explaining the properties of the heat storage member  45  when the number of polyimide films is varied.  FIG. 12  is a diagram for explaining a measurement method of the glossiness.  FIG. 13  is a diagram illustrating a relationship between a heat capacity per unit area of the polyimide film and the gloss difference.  FIG. 14  is a diagram illustrating a relationship between the heat capacity per unit area of the polyimide film and a heater-back temperature rising rate.  FIG. 15  is a diagram for explaining a measurement method of the thermal diffusivity. In  FIG. 13 , the horizontal axis indicates the heat capacity per unit area, while the vertical axis indicates the gloss difference. In  FIG. 14 , the horizontal axis indicates the heat capacity per unit area, while the vertical axis indicates the heater-back temperature rising rate. In this regard,  FIG. 11  also shows properties of heat storage members of Comparative Examples  1  and  2  which are made of copper and aluminum, respectively. 
       FIG. 11  illustrates the thicknesses, the heat capacities, the heat capacities per unit area, the gloss differences, the warming-up times, the heater-back temperature rising rates, and the thermal diffusivities of the heat storage member  45  when the number of polyimide films is set to 1, 3, 4, 5, 10, 15 to 18, and 20. 
     The heat capacity of the heat storage member  45  was determined as follows. Measurements were performed by a differential scanning calorimeter (DSC) method using a DSC (“DSC6220” manufactured by SII Nano Technology Inc.) to obtain a DSC curve. A specific heat capacity Cps of the polyimide film was calculated based on the DSC curve. Then, the specific heat capacity Cps was multiplied by the weight of the heat storage member  45 , so that the heat capacity of the heat storage member  45  was obtained. 
     In the measurements, alumina was used as a reference material. A measured weight of the alumina was 10.3 [mg], and a measured weight of the polyimide film was 11.9 [mg]. The specific heat capacity Cps of the polyimide film was calculated using the following formula. 
         Cps =( H/h )·( mr/ms )· Cpr  
 
     In this formula, “H” denotes a difference between the DSC curves of the polyimide film and an empty container. “h” denotes a difference between the DSC curves of the alumina (reference material) and the empty container. “mr” denotes a weight of the alumina. “ms” denotes a weight of the polyimide film. “Cpr” denotes a specific heat capacity of the alumina. 
     In a state where the printer  10  was left in an environment at a room temperature of 25[° C.], the specific heat capacity Cps of one polyimide film was 1.03 [J/gK], the weight per polyimide film was 0.34 [g], and the dimensions of the polyimide film was 250 [mm]×15 [mm]. Thus, the heat capacity per unit area of one polyimide film was 0.0093 [J/K·cm 2 ]. 
     Therefore, when the number of the polyimide films was set to 1, 3, 4, 5, 10, 15 to 18, and 20, the heat capacities of the heat storage member  45  were 0.35 [J/K], 1.05 [J/K], 1.40 [J/K], 1.75 [J/K], 3.50 [J/K], 5.25 [J/K], 5.60 [J/K], 5.95 [J/K], 6.30 [J/K], and 7.00 [J/K], respectively. Thus, the heat capacities per unit area of the heat storage members  45  were 0.01 [J/K·cm 2 ], 0.03 [J/K·cm 2 ], 0.04 [J/K·cm 2 ], 0.05 [J/K·cm 2 ], 0.09 [J/K·cm 2 ], 0.14 [J/K·cm 2 ], 0.15 [J/K·cm 2 ], 0.16 [J/K·cm 2 ], 0.17 [J/K·cm 2 ], and 0.19 [J/K·cm 2 ], respectively. 
     The thermal diffusivity of the heat storage member  45  is the thermal diffusivity of polyimide which is the material of the film. Regardless of the number of films, the thermal diffusivity of the heat storage member  45  is 0.302×10 −6  [m 2 /s]. 
     The gloss difference was measured by transporting one sheet P of A4 size (297 [mm]×210 [mm]) (“Excellent White” manufactured by Oki Electric Industry Co., Ltd, and having a basis weight of 80 [g/m 2 ]) at a speed of 162 [mm/s] in a longitudinal feed and printing a blue image whose image density was 200 [%] (i.e., a magenta image whose image density was 100 [%] and a cyan image whose image density was 100 [%]) on the sheet P. The glossiness was measured at six measurement positions Stj (j=1, 2, . . . , 6) on the sheet P using “micro-gloss 75° ” manufactured by BYK-Gardner Inc., according to “75° specular gloss measuring method (JIS P8142)” as illustrated in  FIG. 12 . 
     The fixing belt  39  having a diameter of 30 [mm] was used in the fixing device  31  in the embodiment. A virtual boundary line LH was defined as a line extending in a lateral (widthwise) direction and distanced 94 [mm] from a leading (front) edge Egf of the sheet P. An area from the leading edge Egf of the sheet P to the boundary line LH corresponds to the first turn of the fixing belt  39 , while an area on a rear side of the boundary line LH corresponds to the second turn of the fixing belt  39 . 
     A center line LV was defined as a line extending in the longitudinal direction and distanced 105 [mm] from each of left and right edges Egs 1  and Egs 2  of the sheet P. A measurement position St 1  was defined at a position distanced 45 [mm] to the left from the center line LV and distanced 10 [mm] to the front from the boundary line LH. A measurement position St 2  was defined at a position on the center line LV and distanced 10 [mm] to the front from the boundary line LH. A measurement position St 3  was defined at a position distanced 45 [mm] to the right from the center line LV and distanced 10 [mm] to the front from the boundary line LH. A measurement position St 4  was defined at a position distanced 45 [mm] to the left from the center line LV and distanced 10 [mm] to the rear from the boundary line LH. A measurement position St 5  was defined at a position on the center line LV and distanced 10 [mm] to the rear from the boundary line LH. A measurement position St 6  was defined at a position distanced 45 [mm] to the right from the center line LV and distanced 10 [mm] to the rear from the boundary line LH. 
     Then, an average of glossinesses measured at the measurement positions St 1  to St 3  was defined as a first glossiness of an image formed by the first turn of the fixing belt  39 , and an average of glossinesses measured at the measurement positions St 4  to St 6  was defined as a second glossiness of an image formed by the second turn of the fixing belt  39 . The gloss difference was defined as a difference between the first glossiness and the second glossiness. The gloss differences when the number of polyimide films was set to 1, 3, 4, 5, 10, and 15 were 6.5, 5.8, 6.1, 4.8, 2.5, and 1.0, respectively. It is found that, when the number of polyimide films is 1, 3, or 4, the gloss difference is large. In contrast, when the number of polyimide films is 5, 10, or 15, the gloss difference is small. 
     Therefore, as the number of polyimide films increases, the heat capacity of the heat storage member  45  increases, and thus the heat storage member  45  is able to store (hold) much more heat transferred from the heater  46 . Even if the heat of the fixing belt  39  is taken away by the sheet P during the first turn of the fixing belt  39  due to the passage of the sheet P, heat stored in the heat storage member  45  is transferred to the fixing belt  39  via the heater  46 . Consequently, heat can be stably transferred to the sheet P even in the second turn of the fixing belt  39 . 
     The warming-up time was measured by detecting the temperature of the surface layer  39   c  of the fixing belt  39  using the non-contact temperature sensor TH, and measuring the time required for the temperature of the surface layer  39   c  to rise from the room temperature (30[° C.] or lower) to 140[° C.] after the heater  46  was turned on. 
     The warming-up time was measured based on the detected temperature of the surface layer  39   c  of the fixing device  31  while changing the number of polyimide films constituting the heat storage member  45 . For this purpose, a multi-input data logger (“NR-500” manufactured by Keyence Corp.) as a data monitoring device was connected to the temperature sensor TH, whereby the temperature of the surface layer  39   c  of the fixing device  31  was detected, and the warming-up time was measured. 
     The warming-up times when the number of polyimide films was set to 1, 10, and 15 were 7.73 [sec], 7.53 [sec], and 7.40 [sec], respectively. 
     When the number of polyimide films is  10 , the thickness of the heat storage member  45  is thick, and thus the thermal insulation property of the heat storage member  45  increases, as compared to when the number of polyimide films is 1. Thus, if the transfer of heat from the heater  46  to the fixing belt  39  is improved by a heat insulation effect of the heat storage member  45 , heat of the heater  46  should be quickly transferred to the fixing belt  39  and the warming-up time should be shortened when the number of polyimide films is 10. However, the warming-up time required for the detected temperature of the surface layer  39   c  to rise from the room temperature to 140[° C.] is 7.73 [sec] when the number of the polyimide films is 1, and is 7.53 [sec] when the number of polyimide films is 10. These warming-up times are substantially the same as each other. 
     From this result, it cannot be said that the transfer of heat from the heater  46  to the fixing belt  39  is improved by the heat insulation effect of the heat storage member  45 . 
     Further, for example, if the gloss difference is 5.0 or larger, an operator can visually recognize the gloss difference in a printed image on the sheet P. As can be seen from  FIG. 11 , when the number of polyimide films is 1, the gloss difference is 6.5, and the warming-up time is 7.73 [sec]. On the other hand, when the number of polyimide films is 10, the gloss difference is 2.5, and the warming-up time is 7.53 [sec]. That is, the warming-up times are substantially the same values even when the gloss differences are different from each other. Thus, it is considered that the gloss difference is not optimized by the heat insulation effect of the heat storage member  45  described above, but optimized by a heat storage effect of the heat storage member  45 , i.e., the effect that the heat storage member  45  stores heat transferred from the heater  46 . 
     This can be interpreted as follows. Even if the temperature of the fixing belt  39  decreases during the first turn, a decrease in the temperature of the heater  46  is suppressed by the heat storage member  45 . In addition, the heater  46  receives heat from the heat storage member  45 , and supplies a large amount of heat to the fixing belt  39 . That is, it is understood that the gloss difference is not only caused by a decrease in the temperature of the fixing belt  39 , but also caused by a decrease in the temperature of the heater  46 . 
     Next, the heater-back temperature rising rate will be described. 
     The heater-back temperature rising rate is a rising rate of the temperature of a backside of the heater  46 , i.e., the temperature detected by the thermistor Th. The heater-back temperature rising rate is monitored in order to prevent breakage of the holding member  43  due to its overheating by the heater  46 . 
     When the number of polyimide films was changed to 1, 10, 15 to 18, and 20, the heater-back temperature rising rates were 22.1[° C./s], 13.8[° C./s], 8.8[° C./s], 6.8[° C./s], 5.0[° C./s], 4.0[° C./s], and 2.9[° C./s], respectively. 
     It is understood that, when the number of polyimide films increases from 1 to 10, 15 to 18 and 20, the heat becomes less likely to be transferred from the heater  46  to the thermistor Th, and thus the heater-back temperature rising rate decreases. 
     The heater-back temperature rising rate is used to monitor the temperature rising rate in order to prevent the breakage of the holding member  43  due to its overheating as described above. However, the heater-back temperature rising rate is also used to check whether the heater  46  is activated or not. If the heater-back temperature rising rate is extremely low, it cannot be checked whether the heater  46  is activated or not. Therefore, the heater-back temperature rising rate is desired to be 5.0[° C./sec] or higher. 
     As shown in Comparative Example 2 in  FIG. 11 , when the heat storage member  45  is made of aluminum, heat of the heater  46  is transmitted to the thermistor Th more easily because the thermal diffusivity of aluminum is higher than that of the polyimide film, and thus it becomes easy to monitor the heater-back temperature rising rate. However, a gloss difference is 6.5 when the heat storage member  45  is made of aluminum, which degrades the image quality. This is considered to be because the thermal diffusivity of aluminum is higher than that of the fixing belt  39 , and the heat from the heater  46  is released to the outside without being stored in the heat storage member  45 . 
     When the values of the heat capacity per unit area and the gloss difference obtained while changing the number of polyimide films are plotted, the following exponential function is obtained: 
         y= 9.3454e −15.24x    
     where×is the heat capacity per unit area, and y is the gloss difference. This exponential function is indicated by line L 1  in  FIG. 13 . 
     When R 2  is defined as a coefficient of determination representing a correlation between the heat capacity per unit area and the gloss difference, R 2 =0.9628 is obtained from the above described values of the heat capacity per unit area and the gloss difference. This indicates that the correlation between the heat capacity per unit area and the gloss difference is high. 
     Furthermore, when the values of the heat capacity per unit area and the heater-back temperature rising rate are plotted, the following linear function is obtained: 
         y=− 110.44 x+ 23.415 
     where×is the heat capacity per unit area, and y is the heater-back temperature rising rate. The linear function is indicated by line L 2  in  FIG. 14 . 
     When R 2  is defined as a coefficient of determination representing a correlation between the heat capacity per unit area and the heater-back temperature rising rate, R 2 =0.9926 is obtained from the above described values of the heat capacities per unit area and the heater-back temperature rising rate. This indicates that the correlation between the heat capacity per unit area and the heater-back temperature rising rate is high. 
     In order to make the gloss difference invisible, the gloss difference is preferably 5.0 or smaller. Further, in order to check whether the heater  46  is activated or not, the heater-back temperature rising rate is preferably 5.0[° C./sec] or higher. 
     For this reason, in the embodiment, the heat capacity per unit area of the heat storage member  45  is 0.05 [J/k·cm 2 ] or more and 0.16 [J/k·cm 2 ] or less. 
     Next, the thermal diffusivity will be described. 
     First, a measurement method of the thermal diffusivity will be described. 
       FIG. 15  illustrates the heat storage member  45 , a measurement device  71  for measuring the thermal diffusivity, and a temperature detector  73 . As the measurement device  71 , “Thermo Wave Analyzer TA35” manufactured by Bethel Co., Ltd., (Hudson Laboratory) was used. The thermal diffusivity of the heat storage member  45  in its thickness direction was measured by a distance variation method using the measurement device  71 . As the temperature detector  73 , an InSb (indium antimonide) detector was used. A surface of the heat storage member  45  facing the heater  46  ( FIG. 1 ) in the fixing device  31  is defined as an irradiation surface Fa, while the other surface of the heat storage member  45  facing the thermistor Th is defined as a detection surface Fb. 
     The surface of the heat storage member  45  was applied with graphite spray, and was subjected to blackening. The measurement device  71  was placed to face the heat storage member  45  at a predetermined measurement position PO, and then the irradiation surface Fa of the heat storage member  45  was irradiated with heating light E. A semiconductor laser beam having a wavelength of 808 [nm] was used as the heating light ε. The irradiation with the laser beam was conducted with a pulse width of 10 [μs] to 100 [μs] at a beam angle of 48 [degrees]. The temperature detector  73  was placed to face the detection surface Fb at the measurement position PO. By varying the frequency of the semiconductor laser beam in a range of 3.6 [Hz] to 14.0 [Hz], a phase difference for each frequency was calculated, and the thermal diffusivity of the heat storage member  45  was calculated based on the calculated phase difference. 
     Since the heat storage member  45  is formed by laminating and bonding polyimide films onto the heater  46 , the thermal diffusivity of the heat storage member  45  is equal to the thermal diffusivity of polyimide which is the material of the film. Regardless of the number of polyimide films, the thermal diffusivity of the heat storage member  45  is 0.302×10 −6  [m 2 /s]. 
     In the embodiment, since the surface layer  39   c  of the fixing belt  39  ( FIG. 8 ) is made of tetrafluoroethylene-perfluoroalkoxy ethylene copolymer, the thermal diffusivity of the fixing belt  39  is equal to the thermal diffusivity of tetrafluoroethylene-perfluoroalkoxy ethylene copolymer, i.e., 0.41×10 −6  [m 2 /s]. 
     Therefore, a difference between the thermal diffusivity of the fixing belt  39  and the thermal diffusivity of the heat storage member  45  is preferably 0.108×10 −6  [m 2 /s] or more. 
     If a difference between the thermal diffusivity of the fixing belt  39  and the thermal diffusivity of the heat storage member  45  is less than 0.108×10 −6  [m 2 /s], heat stored in the heat storage member  45  may be dissipated during the first turn of the fixing belt  39 , and thus the amount of heat stored in the heat storage member  45  and transferred to the fixing belt  39  via the heater  46  may decrease in the second turn of the fixing belt  39 , which may increase the gloss difference. Therefore, the difference between the thermal diffusivity of the fixing belt  39  and the thermal diffusivity of the heat storage member  45  is preferably 0.108×10 −6  [m 2 /s] or more. 
     As the number of polyimide films increases, the heat capacity of the heat storage member  45  increases, thereby the heat storage member  45  is able to store much more heat transferred from the heater  46 . Even if the heat of the fixing belt  39  is taken away by the sheet P during the first turn of the fixing belt  39  due to the passage of the sheet P, heat stored in the heat storage member  45  is transferred to the fixing belt  39  via the heater  46 . Thus, the heat can be stably transferred to the sheet P even in the second turn of the fixing belt  39 . 
     As described above, in the embodiment, the heat storage member  45  is disposed on the side of the heater  46  opposite to the fixing belt  39 , and the thermal diffusivity of the heat storage member  45  is lower than that of the fixing belt  39 . Thus, the heat storage member  45  is able to store sufficient heat, and a decrease in the temperature of the heat storage member  45  can be prevented. 
     Therefore, the heat can be sufficiently transferred to the fixing belt  39 . The heat taken away from the fixing belt  39  at the fixing nip Np due to the passage of the sheet P can be replenished by the heat storage member  45  via the heater  46 . Thus, the temperature of the fixing belt  39  can be made uniform. 
     That is, a change in the temperature of the fixing belt  39  between the first and second turns of the movement (rotation) of the fixing belt  39  can be made small. Thus, it is possible to suppress a gloss difference at the boundary between the image portions formed in the first turn and the second turn of the fixing belt  39 . 
     As a result, the image quality can be improved. That is, a high quality image can be formed even when the printing is performed at high speed. 
     The heat capacity per unit area of the heat storage member  45  is 0.05 [J/K·cm 2 ] or more, and thus it is possible to reduce the gloss difference to 2.5 or smaller. Therefore, the image quality can be further improved. 
     Since the heat capacity per unit area of the heat storage member  45  is 0.16 [J/K·cm 2 ] or less, the heater-back temperature rising rate can be set to 5.0[° C./sec] or higher. Therefore, whether the heater  46  is activated or not can be checked surely. 
     In the embodiment, the heat storage member  45  is made of polyimide film(s), but other material may be used. A material having a thermal diffusivity lower than that of the fixing belt  39  can be used for the heat storage member  45 . In particular, a heat resistant resin having a thermal diffusivity lower than that of the fixing belt  39  is preferably used for the heat storage member  45 . Examples of the heat resistant resin suitable for the heat storage member  45  include, for example, a polyamide-based resin such as polyamide 6 (PA6), as well as a polyimide resin. The thermal diffusivity of polyamide 6 is 0.183×10 −6  [m 2 /s]. When polyamide 6 is used for the material of the heat storage member  45 , the thickness of the polyamide 6 is preferably adjusted so that the heat capacity per unit area of the heat storage member  45  is 0.05 [J/K·cm 2 ] or more, and 0.16 [J/K·cm 2 ] or less. 
     Even when polyamide 6 is used for the material of the heat storage member  45 , the heat stored in the heat storage member  45  may be dissipated to the outside during the first turn of the fixing belt  39 , if a difference between the thermal diffusivity of the fixing belt  39  and the thermal diffusivity of the heat storage member  45  is less than 0.108×10 −6  [m 2 /s]. In such a case, the amount of heat stored in the heat storage member  45  and transferred to the fixing belt  39  via the heater  46  may decrease in the second turn of the fixing belt  39 , which may increase the gloss difference. For this reason, the difference between the thermal diffusivity of the fixing belt  39  and the thermal diffusivity of the heat storage member  45  is preferably 0.108×10 −6  [m 2 /s] or more. 
     Therefore, in order to make the difference between the thermal diffusivities of the fixing belt  39  and the heat storage member  45  to 0.108×10 −6  [m 2 /s] or more when the thermal diffusivity of the heat storage member  45  is in a range of 0.183×10 −6  [m 2 /s] to 0.302×10 −6  [m 2 /s], the thermal diffusivity of the fixing belt  39  is set to 0.291×10 −6  [m 2 /s] or higher and 0.410×10 −6  [m 2 /s] or lower by appropriately choosing the material of the fixing belt  39  (for example, resin of the base material layer  39   a ). In this regard, 0.291×10 −6  [m 2 /s] is a sum of 0.183×10 −6  [m 2 /s] and 0.108×10 −6  [m 2 /s]. Further, 0.410×10 −6  [m 2 /s] is a sum of 0.183×10 −6  [m 2 /s] and 0.302×10 −6  [m 2 /s]. 
     Next, a state of disposing the thermistor Th will be described. 
     In the embodiment, the thermistor Th illustrated in  FIG. 1  is disposed to face the entire surface of the heat storage member  45 , but the state of disposing the thermistor Th can be modified. 
       FIG. 16  is a diagram illustrating an example of the state of disposing the thermistor Th in the embodiment.  FIG. 17  is a diagram illustrating another example of the state of disposing the thermistor Th in the embodiment.  FIG. 18  is a diagram illustrating still another example of the state of disposing the thermistor Th in the embodiment. 
     In  FIGS. 16, 17 and 18 , reference numeral  45  denotes the heat storage member, reference numeral SA denotes an entire surface of the heat storage member  45 , reference numeral  46  denotes the heater, reference numeral Th denotes the thermistor, reference numeral Tha denotes a detection surface, and reference numeral Sm denotes a detection position on the heat storage member  45  for temperature detection. 
     In the embodiment, as illustrated in  FIG. 16 , the thermistor Th is disposed so that the detection surface Tha faces the entire surface SA (specifically, a portion of the entire surface SA corresponding to the detection position Sm) of the heat storage member  45 , and the thermistor Th is configured to detect the temperature of the detection position Sm. As illustrated in  FIG. 17 , a member having a thermal diffusivity higher than that of the heat storage member  45  (i.e., polyimide film) is disposed at the detection position Sm of the heat storage member  45  to thereby form a high thermal diffusivity section Pa. The thermistor Th can be disposed so that the detection surface Tha faces the high thermal diffusivity section Pa. With this arrangement, the high thermal diffusivity section Pa easily diffuses heat from the heater  46 , and therefore the temperature of the heater  46  can be easily detected by the thermistor Th. Thus, the temperature of the heater  46  can be controlled with high accuracy. 
     Furthermore, as illustrated in  FIG. 18 , first and second portions P 1  and P 2  are formed on the entire surface SA of the heat storage member  45 . The thermal diffusivity of the first portion P 1  is higher than that of the second portion P 2 . Further, the thermal diffusivities of both of the first and second portions P 1  and P 2  are desirably higher than the thermal diffusivity of the heat storage member  45  (i.e., polyimide film). The first portion P 1  is provided at the detection position Sm. The thermistor Th can be disposed so that the detection surface Tha faces the first portion P 1 . 
     With this arrangement, it is possible to prevent occurrence of a portion where the thermal diffusivity largely changes on the entire surface SA of the heat storage member  45 . Thus, occurrence of the gloss difference in the image can be prevented. 
     In the embodiment, the heater  46  and the heat diffusion member  47  are disposed between the heat storage member  45  and the fixing belt  39 , but it is also possible to dispose only the heater  46  as a heating member between the heat storage member  45  and the fixing belt  39 . 
     In the embodiment, the laminated polyimide films are bonded to the heater  46 , but a heat storage material whose thermal diffusivity is lower than that of the fixing belt  39  may be applied to the surface of the heater  46 , or the surface of the heater  46  may be coated with such a heat storage material. 
     In the embodiment, the term “heat resistant resin” refers to a resin having a glass transition temperature higher than a temperature of the heater  46  generating heat. Because the temperature of the heater  46  can be increased up to 200[° C.], a resin having the glass transition temperature of 200[° C.] or higher is referred to as the “heat resistant resin”. 
     The printer  10  has been described in the above-described embodiment, but the present disclosure is also applicable to image forming apparatuses such as copiers, facsimile machines, and multifunction peripherals. 
     The present disclosure is not limited to the above-described embodiment, and various modifications and changes can be made to the embodiment without departing from the spirit and scope of the present disclosure as described in the following claims. 
     DESCRIPTION OF REFERENCE CHARACTERS 
       31  fixing device;  39  fixing belt;  45  heat storage member; heater; Si inner circumferential surface.