Patent Publication Number: US-11029632-B2

Title: Fixing apparatus providing a fixing apparatus capable of suppressing a temperature rise in a non-sheet-passing portion without degrading first print out time

Description:
BACKGROUND OF THE DISCLOSURE 
     Field of the Disclosure 
     The present disclosure relates to a fixing apparatus for use with an image forming apparatus, such as a copying machine, a printer, or a facsimile, which includes a function of forming an image on a recording material. 
     Description of the Related Art 
     An electrophotographic process using toner has heretofore been widely used for image forming apparatuses such as a copying machine, a printer, and a facsimile. As a fixing apparatus for use with such image forming apparatuses, a fixing apparatus having the following structure is known. That is, the fixing apparatus has a structure in which a ceramic heater provided with a pattern of heat generating resistors on a ceramic substrate is used as a heating member and a fixing film which is a rotatable cylindrical endless belt to be heated by the heating member is used. Specifically, a fixing apparatus that employs a film heating process as described below is known. That is, a recording material is brought into pressure contact by a cylindrical fixing film and a pressure roller, and the recording material bearing an image is nipped and conveyed by a pressure-contact portion (fixing nip portion) while being heated, to thereby fix a toner image onto the recording material as a fixed image. 
     The fixing apparatus that employs the film heating process as described above has a feature that a ceramic heater and a fixing film with a low heat capacity can be used, and thus the temperature of each of the ceramic heater and the fixing film can be increased to a temperature at which the fixing process can be achieved in a short period of time. Therefore, the fixing apparatus that employs the film heating process has advantages such as a reduction in wait time (quick start property: activation on demand), power saving, and suppression of a temperature rise in the main body of an image forming apparatus. 
     In the fixing apparatus that employs the film heating process, when a recording material (small-size paper) having a width narrower than that of a recording material (large-size paper) having a maximum width for printing is caused to pass in a longitudinal direction, the temperature gradually rises in a non-sheet-passing area (non-sheet-passing portion temperature rise). This temperature rise in the non-sheet-passing portion increases as the speed of printing increases, which is one of the issues for obtaining high productivity. 
     As one method for suppressing the temperature rise in the non-sheet-passing portion, a method of improving thermal conductivity in the longitudinal direction by disposing a thermal conductive member in contact with the back surface of a heating member such as a ceramic heater is known (Japanese Patent Application Laid-Open No. 11-84919). 
     However, one of the issues of a fixing apparatus having a structure in which a thermal conductive member is disposed in contact with the back surface of a heating member is an increase in First Print Out Time (FPOT) in an image forming apparatus using such a fixing apparatus. The FPOT refers to a time period since a print signal is transmitted to a printer until a first recording material is discharged from the printer. To shorten the FPOT, it is necessary to use members having a low heat capacity in the fixing apparatus. However, if the thickness of the thermal conductive member is increased to enhance the effect on the temperature rise in the non-sheet-passing portion, the heat capacity increases by that amount, resulting in an increase in heat capacity of the entire fixing apparatus. Accordingly, heat generated from the heater is easily transferred to the thermal conductive member, which leads to a deterioration in the efficiency of heat supply to the recording material. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure has been made in view of the above-described circumstances and is directed to providing a fixing apparatus capable of suppressing a temperature rise in a non-sheet-passing portion without degrading FPOT. 
     According to an aspect of the present disclosure, a fixing apparatus includes a rotatable cylindrical film, a heater including a first surface in contact with an inner peripheral surface of the film, and a second surface disposed on an opposite side of the first surface, a support member configured to support the heater, and a pressure member configured to form a nip with the heater through the film. The fixing apparatus heats a toner image at the nip, and fixes the toner image onto a recording material. The fixing apparatus also includes a thermal conductive member in contact with the second surface, and a thermal-resistant member disposed between the thermal conductive member and the support member and having a thermal conductivity lower than the thermal conductivity of the thermal conductive member. 
     Further features and aspects of the present disclosure will become apparent from the following description of example embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view illustrating a fixing apparatus according to a first example embodiment. 
         FIG. 2  is a schematic front view illustrating the fixing apparatus according to the first example embodiment. 
         FIG. 3  is an explanatory diagram illustrating an example ceramic heater according to the first example embodiment. 
         FIG. 4  is an explanatory diagram illustrating an example thermistor and an example temperature fuse according to the first example embodiment. 
         FIG. 5  is an explanatory diagram illustrating an example structure and arrangement of a thermal conductive member and a thermal-resistant sheet according to the first example embodiment. 
         FIGS. 6A and 6B  are explanatory diagrams illustrating an example heater clip and a feed connector as heater holding members, respectively, according to the first example embodiment. 
         FIG. 7  is a table illustrating a list of results of a fixing start-up time and a non-sheet-passing portion temperature rise according to the first example embodiment. 
         FIG. 8  is a graph illustrating a range in which both a reduction in fixing start-up time and suppression of the non-sheet-passing portion temperature rise are achieved according to the first example embodiment. 
         FIG. 9  is a schematic sectional view illustrating a related art fixing apparatus. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Example embodiments and various aspects of the present disclosure will be described in detail below. 
     (Outline of Example Fixing Apparatus) 
       FIG. 1  is a schematic sectional view illustrating a fixing apparatus  18 .  FIG. 2  is a schematic front view illustrating the fixing apparatus  18 . In the following description of components of the fixing apparatus  18 , a longitudinal direction (generatrix direction) corresponds to an X-axis direction in the drawings, a width direction corresponds to a Y-axis direction in which a recording material is conveyed, and a height direction corresponds to a Z-axis direction. An in-plane direction is a direction parallel to a plane formed by the X-axis and the Y-axis, and a thickness direction corresponds to the Z-axis direction. 
     The fixing apparatus  18  includes a film assembly  31 , which is a flexible rotary member including a fixing film  36 , and a pressure roller  32  which is a pressure member. The film assembly  31  and the pressure roller  32  are provided substantially in parallel to each other vertically between left and right side plates  34  of an apparatus frame  33 . 
     The pressure roller  32  includes a core metal  32   a  and an elastic layer  32   b  which is formed in a roller shape concentrically integral with the core metal  32   a  and is made of silicone rubber, fluororubber, or the like. On the elastic layer  32   b , a release layer  32   c  which is made of a perfluoroalkoxy alkane (PFA), polytetrafluoroethylene resin (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer resin (FEP), or the like is formed. In the present example embodiment, the pressure roller  32  having a structure in which the elastic layer  32   b  having a thickness of approximately 3.5 mm is formed on the core metal  32   a , which is made of stainless steel and has an outer diameter of 11 mm, by injection molding and the PFA resin tube  32 C having a thickness of approximately 40 μm is coated on the silicone rubber layer  32   b  is used. The outer diameter of the pressure roller  32  is 18 mm. The hardness of the pressure roller  32  is can be in a range from 40° to 70° with a load of 9.8 N by an ASKER-C hardness meter from the standpoint of securing a fixing nip portion N, endurance, and the like. In the present example embodiment, the hardness of the pressure roller  32  is set to 540. The length of a longitudinal rubber surface of the pressure roller  32  is 226 mm. As illustrated in  FIG. 2 , the pressure roller  32  is disposed in such a manner that the pressure roller  32  is rotatably supported between the side plates  34  of the apparatus frame  33  through a bearing member  35  at both ends in the longitudinal direction of the core metal  32   a . A drive gear G is fixed at one end of the core metal  32   a  of the pressure roller  32 . A rotary force is transmitted to the drive gear G from a drive mechanism portion (not illustrated), so that the pressure roller  32  is rotationally driven. 
     The film assembly  31  is illustrated in  FIG. 1 . The film assembly  31  includes the rotatable cylindrical fixing film  36 , a ceramic heater (hereinafter referred to as a heater)  37 , a heater holder (support member)  38 , a thermal-resistant sheet  100 , a thermal conductive member  51 , a pressure stay  40 , and left and right fixing flanges  41 . 
     The heater  37  is a heating member that heats the fixing film  36 . The heater holder  38  guides the fixing film  36  from the inside and supports the heater  37 . The thermal-resistant sheet  100  is a thermal-resistant member which is disposed on a surface where the heater  37  is not in contact with the fixing film  36 . The thermal conductive member  51  is a heat leveling member that is disposed between the thermal-resistant sheet  100  and the heater holder  38 . The film assembly  31  has a structure in which the left and right fixing flanges (regulating members)  41  regulate the movement in the longitudinal direction of the pressure stay  40  and the fixing film  36 . 
     In the present example embodiment, the fixing film  36  has an outer diameter of 18 mm in a non-deformed cylindrical state, and has a multi-layer structure in the thickness direction. The fixing film  36  includes layers, such as a base layer for maintaining the strength of the fixing film  36 , and a release layer for reducing the adhesion of soiling on the surface of the fixing film  36 . The material of the base layer is required to have a heat resistance because the base layer receives heat from the heater  37 . The material of the base layer is also required to have a sufficient strength because the base layer and the heater  37  slide against each other. Accordingly, metal, such as stainless steel or nickel, or a heat resistant resin, such as polyimide, may be used. In the present example embodiment, a polyimide resin is used as the material of the base layer of the fixing film  36 , and a carbon-based filler for improving the thermal conductivity and strength is added. Heat generated from the heater  37  is more likely to be transferred to the surface of the pressure roller  32  as the thickness of the base layer decreases, while the strength deteriorates due to a decrease in the thickness of the base layer. For this reason, the thickness of the base layer can be approximately 15 μm to 100 μm. In the present example embodiment, the thickness of the base layer is 50 μm. 
     As the material of the release layer of the fixing film  36 , fluorine resins such as PFA, PTFE, and FEP can be used. In the present example embodiment, the PFA having excellent releasability and heat resistance among the fluorine resins is used. As the release layer, a layer coated with a tube may be used, and a layer having a surface coated with coating solution may also be used. In the present example embodiment, the release layer is molded by a coating method excellent in thin molding. Although heat generated from the heater  37  is more likely to be transferred onto the surface of the fixing film  36  as the thickness of the release layer decreases, the endurance deteriorates if the thickness of the release layer is extremely small. Accordingly, the thickness of the release layer can be approximately 5 μm to 30 μm. In the present example embodiment, the thickness of the release layer is 10 μm. Although not used in the present example embodiment, an elastic layer may be provided between the base layer and the release layer. In this case, silicone rubber, fluororubber, or the like is used as the material of the elastic layer. 
     As illustrated in  FIG. 1 , the heater holder  38  is a member having a substantially semicircular trough-like shape in cross section and has rigidity, a heat-resistant property, and a heat-insulating property. The heater holder  38  is formed of a liquid crystal polymer or the like. The heater holder  38  has a function of rotationally guiding the film  36  externally fitted to the heater holder  38 , a function of adiabatically holding the heater  37 , and a function of serving as an opposed pressure member opposed to the pressure roller  32 . 
     As illustrated in  FIG. 3 , the heater  37  has a structure in which heat generating resistors  37   b  made of a silver-palladium alloy or the like are formed on a substrate  37   a  made of a ceramic material, such as alumina or aluminum nitride, by screen printing or the like, and an electrode  37   c  made of silver or the like is connected to the heat generating resistors  37   b . In the present example embodiment, the two heat generating resistors  37   b  are connected in series and have a resistance value of 18Ω. On the heat generating resistors  37   b , a glass coat  37   d  is formed to protect the heat generating resistors  37   b  and ensure slidability against the fixing film  36 . The heater  37  is disposed along the longitudinal direction at a lower surface portion of the heater holder  38 . 
       FIG. 4  is a top view illustrating a state where a safety element and a temperature detection element are mounted on the heater holder  38 . The heater holder  38  is provided with through-holes. A thermistor  42  serving as the temperature detection element and a temperature fuse  43  serving as the safety element are disposed in contact with the back surface of the thermal conductive member  51  through the through-holes, respectively. The thermistor  42  has a structure in which a housing is provided with a thermistor element through ceramic paper or the like for stabilizing a contact state with the heater  37 , and the housing is coated with an insulating material such as a polyimide tape. The thermistor  42  is an overheat protecting part that senses abnormal heat generation of the heater  37  when the heater  37  causes an abnormal temperature rise, and then blocks a primary circuit. The temperature fuse  43  incorporates a fuse element that is melted at a predetermined temperature in a metal housing having a cylindrical shape. During an abnormal temperature rise, the fuse element is fused to block the circuit. As for the size of the temperature fuse  43  according to the present example embodiment, the length of the metal housing corresponding to a portion in contact with the heater  37  is approximately 10 mm, and the width of the metal housing is approximately 4 mm. The temperature fuse  43  is located on the back surface of the thermal conductive member  51  through thermal conductive grease, thereby preventing a malfunction due to floating of the temperature fuse  43  with respect to the heater  37  from occurring. 
     When power is supplied to the heat generating resistors  37   b  from a feed portion located at an end of the heater  37 , the temperature of the heater  37  rapidly rises. Then, the heater temperature is detected by the thermistor  42 , and the supply of power to the heat generating resistors  37   b  from the feed portion is controlled by a control portion (not illustrated) so that the temperature can be controlled at a predetermined temperature. 
     The pressure stay  40  is a horizontally-long rigid member having a downward U-shaped cross section. In the present example embodiment, stainless steel with a plate thickness of 1.6 mm is used. 
     As illustrated in  FIG. 2 , the fixing film  36  is formed on the outside of the heater holder  38  in a state where the heater  37  is attached to the lower surface of the heater holder  38 , and the pressure stay  40  is inserted into the heater holder  38 . The left and right fixing flanges  41  are respectively fitted to left and right outward extending arm portions of the pressure stay  40 . In this manner, the film assembly  31  is assembled. 
     As illustrated in  FIG. 1 , the film assembly  31  is disposed substantially in parallel to the upper side of the pressure roller  32  with the side of the film assembly  31  located closer to the heater  37  facing downward, and is disposed between the left and right side plates  34  of the apparatus frame  33 . The left and right fixing flanges  41  have a structure in which vertical groove portions  41   a , which are provided to the left and right fixing flanges  41 , respectively, engage with vertical edge portions  34   b  of vertical guide slits  34   a , which are provided to the left and right side plates  34  of the apparatus frame  33 , respectively. In the present example embodiment, a liquid crystal polymer resin is used as the material of the fixing flanges  41 . 
     As illustrated in  FIG. 2 , pressure springs  45  are provided in a contracted state between pressure arms  44  and pressure portions  41   b  of the left and right fixing flanges  41 , respectively. The pressure springs  45  cause the heater  37  to be pressed against the pressure roller  32  by a predetermined pressing force with the fixing film  36  interposed therebetween through the left and right fixing flanges  41 , the pressure stay  40 , and the heater holder  38 . In the present example embodiment, the pressure of the pressure springs  45  is set so that a total pressing force of 160 N is applied by the fixing film  36  and the pressure roller  32 . This pressing brings the heater  37  into pressure contact with the pressure roller  32  with the fixing film  36  interposed therebetween against the elasticity of the fixing film  36  and the elasticity of the pressure roller  32 , so that the fixing nip portion N of approximately 6 mm is formed. At the fixing nip portion N, the fixing film  36  is sandwiched between the heater  37  and the pressure roller  32  and is deformed along a flat surface (first surface) of the lower surface of the heater  37 , and the inner surface of the fixing film  36  is in close contact with the flat surface (first surface) of the lower surface of the heater  37 . 
     Further, a rotary force is transmitted from the drive mechanism portion (not illustrated) to the drive gear G of the pressure roller  32 , so that the pressure roller  32  is rotationally driven at a predetermined speed clockwise in  FIG. 1 . Along with the rotational driving of the pressure roller  32 , the rotary force acts on the fixing film  36  due to a frictional force between the pressure roller  32  and the fixing film  36  at the fixing nip portion N. As a result, the fixing film  36  is rotated around the heater holder  38  counterclockwise in  FIG. 2  in accordance with the rotation of the pressure roller  32 , while the inner surface of the fixing film  36  slides in contact with the lower surface of the heater  37 . The inner peripheral surface of the fixing film  36  is coated with heat-resistant grease, thereby ensuring the slidability between the heater  37  and each of the heater holder  38  and the inner peripheral surface of the fixing film  36 . 
     In a state where the fixing film  36  is rotated in accordance with the rotation of the pressure roller  32  and the heater  37  is energized to increase the heater temperature to a predetermined temperature and then the heater temperature is controlled, a recording material P is introduced. An inlet guide  30  has a function of guiding the recording material P so that the recording material P having an unfixed toner image t formed thereon can be accurately guided to the fixing nip portion N. 
     When the recording material P bearing the unfixed toner image t advances between the fixing film  36  and the pressure roller  32  of the fixing nip portion N, the recording material P is nipped and conveyed together with the fixing film  36  in a state where the toner image bearing surface of the recording material P is in close contact with the outer surface of the fixing film  36 . The recording material P is heated by heat from the fixing film  36  which is heated by the heater  37  in the nipping and conveyance process, and the unfixed toner image t formed on the recording material P is heated and pressed onto the recording material P and is then melted and fixed. The recording material P which has passed through the fixing nip portion N is curvature-separated from the surface of the fixing film  36  and is then discharged and conveyed by a discharge roller pair (not illustrated). 
     The substrate  37   a  of the heater  37  has a rectangular parallelepiped shape having a longitudinal-direction length of 260 mm, a width-direction length of 5.8 mm, and a thickness of 1.0 mm, and is made of alumina. The longitudinal-direction length of each heat generating resistor  37   b  on the heater  37  is 222 mm. Also, when the recording material P of a maximum size (having a width of 216 mm in the present example embodiment) that can be used in an image forming apparatus incorporating the fixing apparatus  18  according to the present example embodiment is used, the heater  37  has a width greater than that of the recording material P so that toner can be uniformly fixed onto the recording material P. 
     Accordingly, in an area outside the width of the recording material P, heat supplied from the heater  37  is not absorbed by the recording material P and the toner thereon, and the heat is accumulated in the components such as the fixing film  36 , the heater  37 , and the heater holder  38 . When paper is used as the recording material P, in an area outside the recording material P (the area is hereinafter referred to as a non-sheet-passing portion), an excessive temperature rise is likely to occur. This phenomenon is referred to as a “non-sheet-passing portion temperature rise”. The temperature at which each member is used has an upper limit. If each member is used at a temperature higher than the upper limit, a problem such as a damage to the member is caused. For this reason, it is necessary to use each member at a temperature lower than or equal to a certain temperature. The “non-sheet-passing portion temperature rise” becomes prominent as the width of the recording material P with respect to the length of each heat generating resistor  37   b  becomes smaller. Accordingly, some measures, such as reduction of an output speed by increasing intervals between recording materials P, are required to reduce the non-sheet-passing portion temperature rise to a certain temperature or lower. Further, if the “non-sheet-passing portion temperature rise” occurs, a thermal stress is applied to the heater  37  due to a temperature difference between a sheet-passing portion and the non-sheet-passing portion, which may cause a damage to the heater  37 . 
     (Arrangement of Example Thermal-Resistant Sheet and Thermal Conductive Member) 
     In this case, the thermal conductive member  51  having a thermal conductivity higher than the thermal conductivity of the base material of the heater  37  is disposed on the back surface of the heater  37 , thereby obtaining a heat leveling effect in which temperature variations in the longitudinal direction are averaged by transferring heat from the non-sheet-passing portion which is at a high temperature to the sheet-passing portion which is at a relatively low temperature. Specifically, the thermal conductive member  51  having a thermal conductivity higher than the thermal conductivity of 32 W/m·K of the base material of the heater  37  formed of aluminum is used. Thus, heat generated outside the recording material P is also transferred to the sheet-passing portion through the thermal conductive member  51  and is then transmitted to the recording material P, so that the heat can be used more efficiently and the “non-sheet-passing portion temperature rise” can be suppressed. 
     A heat leveling member using the thermal conductive member  51  as illustrated in  FIG. 9  has heretofore been proposed. In recent years, heat to be accumulated in the non-sheet-passing portion has been increasing along with the speed-up of an image forming apparatus, and thus there is a demand for a higher heat leveling effect. A heat transport amount in the longitudinal direction of the thermal conductive member  51  is determined depending on the product of a thermal conductivity and a cross-sectional area. Therefore, in order to enhance the heat leveling effect, it is effective to increase the heat transport amount by increasing the thickness of the thermal conductive member  51 . 
     However, if the thickness of a material such as a metal plate is increased, the heat capacity also increases in proportion to an increase in thickness. When the heat capacity of the thermal conductive member  51  is increased, heat generated from the heater  37  is lost to the thermal conductive member  51  at start-up of the fixing apparatus  18 , which leads to an increase in time required for the temperature to rise to a temperature at which the fixing film  36  can be fixed. 
     Accordingly, in the present example embodiment, the thermal-resistant sheet  100  is disposed between the heater  37  and the thermal conductive member  51 . Thus, the pressing force to be applied from the heater holder  38  is sequentially transmitted to the thermal conductive member  51 , the thermal-resistant sheet  100 , and the heater  37 , so that the heater  37  can be pressed against the pressure roller  32  through the fixing film  36  and a uniform fixing pressure can be applied. On the other hand, in the structure according to the present example embodiment, the thermal resistance value of the thermal-resistant sheet  100  is increased and the heat capacity is decreased to achieve a high-speed start-up, and the non-sheet-passing portion temperature rise can be suppressed by the heat transport amount of the thermal conductive member  51  with a large cross-sectional area during the occurrence of the non-sheet-passing portion temperature rise. 
     The structure and advantageous effects of the present example embodiment will be described in detail below. The structure and arrangement of the thermal conductive member  51  and the thermal-resistant sheet  100  will be described with reference to  FIG. 5  and  FIGS. 6A and 6B .  FIG. 5  is a schematic sectional view in the longitudinal direction of a part of the film assembly  31  (the illustration of the fixing film  36 , the pressure stay  40 , and the fixing flange  41  is omitted).  FIGS. 6A and 6B  are explanatory diagrams illustrating a heater clip  47  and a feed connector  46  as heater holding members, respectively. 
     As illustrated in  FIG. 5 , the thermal conductive member  51  contacts a surface (second surface) opposite to the flat surface (first surface) of the lower surface of the heater  37 , and the thermal-resistant sheet  100  is disposed on the thermal conductive member  51  and the heater holder  38  is further disposed on the thermal-resistant sheet  100 . Thus, in the present example embodiment, the feed connector  46  and the heater clip  47 , each of which serves as a holding member provided at an end in the longitudinal direction of the heater holder  38 , form a laminated structure including the heater  37 , the thermal conductive member  51 , the thermal-resistant sheet  100 , and the heater holder  38 . The thermistor  42  and the temperature fuse  43  are disposed in contact with the back surface of the thermal conductive member  51  through the respective through-holes of the heater holder  38 . In the present example embodiment, the thermistor  42  and the temperature fuse  43  contact the thermal conductive member  51 , but instead may contact the fixing film  36 , for example, in terms of improvement in responsiveness. 
     In the present example embodiment, the longitudinal-direction length of each of the thermal conductive member  51  and the thermal-resistant sheet  100  is 222 mm, and the width-direction length of each of the thermal conductive member  51  and the thermal-resistant sheet  100  is 5.8 mm. The longitudinal-direction length is set to be equal to the length of each heat generating resistor  37   b  of the heater  37 , thereby obtaining a temperature averaging effect without deficiency or excess. The thermal conductivity and thickness of each of the thermal conductive member  51  and the thermal-resistant sheet  100  according to the present example embodiment will be described in detail below. 
     As illustrated in  FIG. 6A , the heater clip  47  formed of a metal plate curved in a U-shape is provided at one end in the longitudinal direction of the heater holder  38 . The heater clip  47  holds an end of each of the thermal conductive member  51  and the heater  37  with respect to the heater holder  38  by a spring property of the heater clip  47 . Further, the end of the heater  37  that is pressed by the heater clip  47  is movable in the in-plane direction of a heater sliding surface. This prevents an unnecessary stress from being applied to the heater  37  due to thermal expansion of the heater  37 . 
     Accordingly, the heater holder  38 , the thermal conductive member  51 , the thermal-resistant sheet  100 , and the heater  37  are not fixed to each other so as to absorb a difference in thermal expansion and bending caused due to the pressing force. The heater holder  38 , the thermal conductive member  51 , the thermal-resistant sheet  100 , and the heater  37  contact each other by the spring property of the holding member and the pressing force generated by the pressure roller  32 . 
     As illustrated in  FIG. 6B , at the other end in the longitudinal direction of the heater holder  38 , the feed connector  46  including a housing portion  46   a , which is formed of a resin with a recessed shape, and a contact terminal  46   b  is formed. The housing portion  46   a  and the contact terminal  46   b  sandwich and hold the thermal conductive member  51 , the heater  37 , and the heater holder  38 , and the contact terminal  46   b  contacts the electrode  37   c  of the heater  37  so as to establish an electrical connection therebetween. In the present example embodiment, the feed connector  46  is used as the heater holding member, but instead the function of feeding power to the heater  37  and the function as the heater holding member may be separately provided. The contact terminal  46   b  is connected to a bundle wire  48 , and the bundle wire  48  is connected to an alternate current (AC) power supply and a triac (not illustrated) (gate-controlled semiconductor switch). 
     In the present example embodiment, Kapton® (DU PONT-TORAY CO., LTD.), which is a polyimide film having a high heat-insulating property, is used as the thermal-resistant sheet  100 , and the thermal conductivity is set to 0.16 [W/mK]. The specific heat and the density of the thermal-resistant sheet  100  are 1.16 [kJ/kgK] and 2000 [kg/m 3 ], respectively. Pure aluminum is used as the thermal conductive member  51  and the thermal conductivity is set to 237 [W/mK]. The specific heat and the density of the thermal conductive member  51  are 0.905 [kJ/kgK] and 2688 [kg/m 3 ], respectively. These values are merely examples. The thermal-resistant sheet  100  may have any value as long as the thermal conductivity is less than or equal to 2 [W/mK] so as to achieve high-speed start-up, and the thermal conductive member  51  may have any value as long as the thermal conductivity is greater than or equal to 80 [W/mK] so as to suppress the non-sheet-passing portion temperature rise. 
     The thermal resistance [K/W] of each of the thermal-resistant sheet  100  and the thermal conductive member  51  is obtained by dividing the thickness of each member by the product of the thermal conductivity and the area in the plane direction. The heat capacity [J/Km 2 ] per unit area in the plane direction is obtained by integrating the specific heat, the density, and the thickness. 
     The present example embodiment has a feature in that the thermal resistance in the thickness direction of the thermal-resistant sheet  100  is higher than that of the thermal conductive member  51 , and the heat capacity in the plane direction of the thermal conductive member  51  is higher than that of the thermal-resistant sheet  100 . 
     When the above-described relationships are satisfied, heat generated from the heater  37  at the start-up can be prevented from being lost to the thermal conductive member  51  due to the high thermal resistance of the thermal-resistant sheet  100 . Accordingly, the heat capacity of the thermal conductive member  51  can be increased. Since the heat capacity of the thermal-resistant sheet  100  is low during continuous printing in which the non-sheet-passing portion temperature rise occurs, heat is transmitted to the thermal conductive member  51 , so that the non-sheet-passing portion temperature rise can be suppressed by the heat transport amount. 
     Next, advantageous effects of the present disclosure will be described with reference to  FIG. 7 . To verify the operation and advantageous effects of the present example embodiment, the thickness of each of the thermal-resistant sheet  100  and the thermal conductive member  51  was set within the range of Table 1, and the fixing start-up time and the non-sheet-passing portion temperature rise were measured by changing the thermal resistance of the thermal-resistant sheet  100  and the heat capacity of the thermal conductive member  51 . In a comparative example, a structure in which only the thermal conductive member  51  which is made of pure aluminum and has a thickness of 0.3 mm is disposed on the back surface of the heater  37  as illustrated in  FIG. 9  was used, and this structure was compared with the structure according to the present example embodiment. The fixing start-up time is a period from a time when the energization of the heater  37  and the rotation of the pressure roller  32  are started from a room-temperature state to a time when the toner image t formed on the recording material P can be fixed. The non-sheet-passing portion temperature rise is a maximum value of a surface temperature of the pressure roller  32  when 200 A4-size sheets are continuously caused to pass at a sheet passing speed of 30 sheets/minute. In the measurement of the non-sheet-passing portion temperature rise, A4-size thick paper with a grammage of 128 g/m 2  was used as evaluation paper, and an infrared thermography manufactured by FLIR Systems, Inc. was used to measure the temperature. The width of A4-size paper is 210 mm, which is shorter by 12 mm (6 mm on one side) than the width of 222 mm of the heat generation member. Accordingly, the non-sheet-passing portion temperature rise occurs on the inside of the heat generating resistors  37   b  of the heater, and the non-sheet-passing portion temperature rise occurs at both end portions on the outside of the A4-size paper. In the present example embodiment, silicone rubber used for the elastic layer of the pressure roller  32  first reaches an upper-limit service temperature, and thus the temperature of the pressure roller  32  was measured. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Thermal conductive member 51 
               
            
           
           
               
               
               
            
               
                 Heat-insulating sheet 100 
                   
                 Heat 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Thermal 
                   
                 Thermal 
                   
                 Thermal 
                   
                 capacity 
               
               
                   
                 conductivity 
                 Thickness 
                 resistance 
                   
                 conductivity 
                 Thickness 
                 [log10 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Material 
                 [W/mK] 
                 [mm] 
                 [K/W] 
                 Material 
                 [W/mK] 
                 [mm] 
                 (J/K · m 2 ] 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Kapton 
                 0.16 
                 0.03 
                 1.5 
                 pure 
                 237 
                 0.3 
                 2.86 
               
               
                   
                   
                 0.05 
                 2.9 
                 aluminum 
                   
                 1 
                 3.39 
               
               
                   
                   
                 0.1 
                 4.9 
                   
                   
                 3 
                 3.86 
               
               
                   
                   
                 0.15 
                 7.3 
                   
                   
                 5 
                 4.09 
               
               
                   
                   
                 0.2 
                 9.7 
                   
                   
                 10 
                 4.39 
               
               
                   
                   
                 0.25 
                 12.1 
               
               
                   
                   
                 0.3 
                 14.6 
               
               
                   
               
            
           
         
       
     
     As a result, in the comparative example, the fixing start-up time was 6.0 seconds and the maximum temperature of the pressure roller  32  when the temperature rise occurred in the non-sheet-passing portion was 230° C. Based on the results of the comparative example,  FIG. 7  illustrates a list of evaluation results of the start-up time and the non-sheet-passing portion temperature rise in combination of settings of the thickness of the thermal-resistant sheet  100  and the thickness of the thermal conductive member  51 . When the thickness of the thermal-resistant sheet  100  is 0.03 [mm] and the thermal resistance is 1.5 [K/W], there was no structure in which the start-up time and the temperature rise in the non-sheet-passing portion improved when the thickness of the thermal conductive member  51  is in a range from 0.3 to 10 [mm]. 
     When the thickness of the thermal-resistant sheet  100  is 0.3 [mm] and the thermal resistance is 14.6 [K/W], the heat-insulating performance of the thermal-resistant sheet  100  was too high, and thus an improvement in the effect of suppressing the non-sheet-passing portion temperature rise was not confirmed even when the thickness of the thermal conductive member  51  was set to 10 [mm]. 
     On the other hand, when the thickness of the thermal-resistant sheet  100  is in a range from 0.05 to 0.25 [mm], excellent results for both the start-up time and the temperature rise in the non-sheet-passing portion were obtained by optimizing the thickness of the thermal conductive member  51 . 
     In this regard,  FIG. 8  illustrates a line that satisfies the start-up performance satisfying the fixing property at an end portion and an allowable line for temperature rise in the non-sheet-passing portion, which were obtained by experiments. In  FIG. 8 , a horizontal axis represents a thermal resistance X [K/W] in the thickness direction of the thermal-resistant sheet  100 , and a vertical axis represents a logarithm Y [log 10 (J/K·m 2 )] of the heat capacity per unit area in the plane direction of the thermal conductive member  51 . 
     As illustrated in  FIG. 8 , as a result of experiments, it has turned out that it is necessary to set the logarithm Y [log 10 (J/K·m 2 )] of the heat capacity per unit area in the plane direction of the thermal conductive member  51  to be greater than 2.55X+2.6 so as to obtain a required start-up performance. This is considered to be because when the heat capacity of the thermal conductive member  51  with respect to the thermal resistance of the thermal-resistant sheet  100  is higher than the allowable line of the start-up performance, heat generated from the heater  37  is easily lost to the thermal conductive member  51  and thus the start-up performance is not satisfied. On the other hand, it has turned out that the non-sheet-passing portion temperature rise can be sufficiently suppressed by setting the logarithm Y [log 10 (J/K·m 2 )] of the heat capacity per unit area in the plane direction of the thermal conductive member  51  to be less than 0.09X+2.85. This is considered to be because when the heat capacity of the thermal conductive member  51  with respect to the thermal resistance of the thermal-resistant sheet  100  is lower than the allowable line for temperature rise in the non-sheet-passing portion, the effect of suppressing the non-sheet-passing portion temperature rise due to heat transport of the thermal conductive member  51  cannot be obtained and thus the non-sheet-passing portion temperature rise cannot be sufficiently suppressed. Accordingly, it has turned out that, in order to satisfy the start-up performance and the non-sheet-passing portion temperature rise performance, it is necessary for the heat capacity per unit area in the plane direction of the conductive member  51  with respect to the thermal resistance of the thermal-resistant sheet  100  to satisfy the following condition.
 
0.09 X+ 2.85&lt; Y&lt; 2.55 X+ 2.6[log 10 (J/K·m 2 )]  (A)
 
In expression (A), the thermal resistance is set to be greater than 2.0 [K/W] so that X [K/W] is set in a range in which no failure occurs due to the non-sheet-passing portion temperature rise, and the thermal resistance is set to be less than 12.5 [K/W] so that X [K/W] is set in a range that does not exceed the upper limit of the start-up time.
 
     Table 2 illustrates a list of measurement results of the start-up time and the non-sheet-passing portion temperature rise when the material and the thermal conductivity of the thermal-resistant sheet  100  are varied. As the thermal-resistant sheet  100 , not only Kapton®, but also UPILEX® (UBE INDUSTRIES, LTD.) and a mixture of polyimide and thermal conductive filler such as boron nitride carbon fiber were used. UPILEX® includes polyimide as the main material, just as in the case of Kapton®, and has a thermal conductivity of 0.29 [W/mK]. A mixture of polyimide and thermal conductive filler such as boron nitride carbon fiber, in which the amount of thermal conductive filler was adjusted as needed and the thermal conductivity was 2.0 [W/mK], was used. The measurement was carried out at the same thermal resistance by changing the thickness of the thermal-resistant sheet  100 . Each thermal-resistance sheet  100  was evaluated by using 3-mm pure aluminum as the thermal conductive member  51  and by setting the thermal conductivity to 237 [W/mK] and the heat capacity to 3.86 [log 10 (J/K·m 2 )]. When the thermal-resistant sheets  100  have the same thermal resistance, the values of the start-up time and the non-sheet-passing portion temperature rise were the same even when a heat-insulating member other than Kapton® was used, and the same advantageous effects as those of the present example embodiment were obtained. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Measurement results 
               
            
           
           
               
               
               
            
               
                   
                 Non-sheet- 
                   
               
            
           
           
               
               
               
               
            
               
                 Heat-insulating sheet 100 
                 Start- 
                 passing 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Thermal 
                   
                 Thermal 
                 up 
                 portion 
                   
               
               
                   
                 conductivity 
                 Thickness 
                 resistance 
                 time 
                 temperature 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Material 
                 [W/mK] 
                 [mm] 
                 [K/W] 
                 [s] 
                 rise [° C.] 
                 Determination 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Kapton 
                 0.16 
                 0.15 
                 7.3 
                 5.7 
                 220 
                 Effective 
               
               
                 UPILEX 
                 0.29 
                 0.3 
                 7.3 
                 5.7 
                 220 
               
               
                 PI + 
                 2 
                 2 
                 7.3 
                 5.7 
                 220 
               
               
                 boron 
               
               
                 nitride 
               
               
                 filler 
               
               
                   
               
            
           
         
       
     
     Next, Table 3 illustrates a list of measurement results of the start-up time and the non-sheet-passing portion temperature rise when iron and copper, which is metal other than pure aluminum, are used as metal materials of the thermal conductive member  51 . Iron has a thermal conductivity of 80 [W/mK], and the specific heat and the density of iron are 0.442 [kJ/kgK] and 7870 [kg/m 3 ], respectively. Copper has a thermal conductivity of 398 [W/mK], and the specific heat and the density of copper are 0.386 [kJ/kgK] and 8880 [kg/m 3 ], respectively. Accordingly, in the present example embodiment, the measurement was carried out at the same heat capacity by changing the thickness of the thermal conductive member  51 . The thermal conductive member  51  was evaluated by using Kapton® with a thickness of 150 [μm] as the thermal-resistant sheet  100  and by setting the thermal conductivity to 0.16 [W/mK] and the thermal resistance to 7.3 [K/W]. 
     At the same heat capacity of the thermal conductive member  51 , the values of the start-up time and the non-sheet-passing portion temperature rise were the same even when metal other than pure aluminum was used, and the same advantageous effects as those of the present example embodiment were obtained. 
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Measurement results 
               
            
           
           
               
               
               
               
            
               
                 Thermal conductive member 51 
                   
                 Non-sheet- 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Heat 
                   
                 passing 
                   
               
               
                   
                 Thermal 
                   
                 capacity 
                   
                 portion 
               
               
                   
                 conductivity 
                 Thickness 
                 [log10 
                 Start-up 
                 temperature 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Material 
                 [W/mK] 
                 [mm] 
                 (J/K · m 2 ] 
                 time [s] 
                 rise [° C.] 
                 Determination 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 pure 
                 237 
                 3 
                 3.86 
                 5.7 
                 220 
                 Effective 
               
               
                 aluminum 
               
               
                 iron 
                 80 
                 1.5 
                 3.84 
                 5.7 
                 220 
               
               
                 copper 
                 398 
                 2 
                 3.84 
                 5.7 
                 220 
               
               
                   
               
            
           
         
       
     
     The present example embodiment has been described above using the thermal-resistant sheet  100  including polyimide as the main material. However, as the thermal-resistant sheet  100 , a material having a low thermal conductivity and a high thermal resistance, such as PFA, PTFE, or FEP can be used. 
     While the present example embodiment has been described above using pure aluminum, iron, and copper as the material of the thermal conductive member  51 , the material is not limited to metal as described above. As long as the heat capacity falls within the range indicated by the expression (A), other metals having a high thermal conductivity and a high heat capacity, such as gold, silver, nickel, and brass, can also be used. As long as the heat capacity falls within the range indicated by the expression (A), the same advantageous effects as those described above can be obtained by using a material other than metal, such as silicone rubber or carbon graphite. 
     While the present disclosure has been described with reference to example embodiments, it is to be understood that the disclosure is not limited to the disclosed example 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. 2018-088842, filed May 2, 2018, which is hereby incorporated by reference herein in its entirety.