Patent Publication Number: US-9405243-B2

Title: Image heating apparatus and heater used in the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to image heating apparatuses configured to heat images formed on recording materials and to heaters used in the image heating apparatuses. 
     2. Description of the Related Art 
     Image heating apparatuses are provided in image forming apparatuses such as a copying machine and a printer to serve as fixing apparatuses. An image heating apparatus that includes an endless belt, a ceramic heater, which makes contact with an inner surface of the endless belt, and a pressure roller, which, along with the ceramic heater, forms a fixing nip portion with the endless belt provided therebetween, is one of such image heating apparatuses. Continuous printing on small-sized sheets with an image forming apparatus that includes such an image heating apparatus causes the temperature of an area in a lengthwise direction of the fixing nip portion where the sheets do not pass through to gradually rise (i.e., non-sheet-passing part temperature rise). An excessive rise in the temperature of a non-sheet-passing part may cause damage to parts in an apparatus, or printing on a large-sized sheet with the temperature of the non-sheet-passing part remaining high may cause toner on the area corresponding to the non-sheet-passing part of the small-sized sheets to be overheated and be offset onto the belt (i.e., high temperature offset). 
     Japanese Patent Application Laid-Open No. 2003-317898 and Japanese Patent Application Laid-Open No. 2003-007435 discuss a method of providing a thermally conductive anisotropic layer such as graphite on a ceramic heater to suppress the non-sheet-passing part temperature rise. Graphite has a layered structure of hexagonal plate crystal formed of carbon, and the layers are bonded by the van der Waals force. Graphite has higher thermal conductivity in a direction parallel to the surface of the ceramic heater (i.e., direction parallel to the plane of a covalently bonded layer in graphite). Thus, providing graphite on a ceramic substrate enables the rise in the temperature of a non-sheet-passing part of small-sized sheets to be suppressed. 
     Furthermore, graphite has low thermal conductivity in the thickness direction thereof (i.e., direction perpendicular to the plane of the covalently bonded layer in graphite). Thus, heat dissipation to a holder supporting the ceramic heater can be suppressed, and heat can be efficiently provided to paper. 
     Bringing a temperature detection member into contact with a ceramic heater to detect the temperature of the ceramic heater is a generally used method. Graphite, however, has low thermal conductivity in the thickness direction thereof. Thus, when the temperature of the ceramic heater is detected with a thermally conductive anisotropic layer such as graphite provided therebetween, there is a delay in response of the temperature detection member. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an image heating apparatus and a heater with improved responsiveness in temperature detection while alleviating the non-sheet-passing part temperature rise during fixing processing of small-sized sheets. 
     According to an aspect of the present disclosure, an image heating apparatus includes a plate-shaped heater, and a temperature detection member configured to detect a temperature of the heater. In such an image heating apparatus, a thermally conductive anisotropic sheet having greater thermal conductivity in a plane direction thereof than that in a thickness direction thereof is provided on one surface of the heater where the temperature detection member is provided. Further, the sheet is not provided at a portion of the heater where the temperature detection member is provided, or the sheet has a reduced thickness at a portion where the temperature detection member is provided compared to the thickness thereof in a surrounding area of the portion. 
     According to another aspect of the present disclosure, a heater used in an image heating apparatus includes a plate-shaped substrate. In such a heater, a thermally conductive anisotropic sheet having greater thermal conductivity in a plane direction thereof than that in a thickness direction thereof is provided, and the sheet is not provided at a portion of the substrate where a temperature detection member for detecting a temperature of the heater is provided, or the sheet has a reduced thickness at a portion where the temperature detection member is provided compared to the thickness thereof in a surrounding area of the portion. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a configuration of an image forming apparatus. 
         FIG. 2  is a sectional view of a fixing apparatus. 
         FIGS. 3A and 3B  are diagrams illustrating a ceramic heater according to a first exemplary embodiment. 
         FIG. 4  illustrates a drive circuit of a heater. 
         FIG. 5  is a sectional view illustrating the shape of a thermally conductive anisotropic member according to the first exemplary embodiment. 
         FIGS. 6A and 6B  are plan views illustrating the shape of the thermally conductive anisotropic member according to the first exemplary embodiment. 
         FIG. 7  illustrates temperature distributions in the ceramic heater. 
         FIGS. 8A and 8B  are diagrams illustrating a ceramic heater according to a second exemplary embodiment. 
         FIG. 9  is a sectional view illustrating the shape of a thermally conductive anisotropic member according to the second exemplary embodiment. 
         FIGS. 10A, 10B, 10C, and 10D  are diagrams illustrating thermal resistance in portions leading to a temperature detection member in cases where part of the thermally conductive anisotropic member is cut out and is not cut out. 
         FIG. 11  illustrates temperature distributions of the ceramic heater according to the second exemplary embodiment. 
         FIG. 12  is a sectional view illustrating the shape of a thermally conductive anisotropic member according to a third exemplary embodiment. 
         FIG. 13  is a diagram illustrating a multilayer structure of the thermally conductive anisotropic member according to the third exemplary embodiment. 
         FIGS. 14A, 14B, 14C, 14D, 14E, 14F, 14G, 14H, 14I , and  14 J are sectional views illustrating various shapes of the thermally conductive anisotropic member according to a fourth exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  illustrates a configuration of an image forming apparatus  100  that includes an image heating apparatus, which serves as a fixing apparatus according to a first exemplary embodiment. The image forming apparatus  100  includes a paper feed cassette  101 , a paper presence detection sensor  102 , and a paper size detection sensor  103 . The paper feed cassette  101  stores a recording sheet P serving as a recording material, the paper presence detection sensor  102  detects whether the recording sheet P is present, and the paper size detection sensor  103  detects the size of the recording sheet P. The image forming apparatus  100  further includes a pickup roller  104 , a paper feeding roller  105 , and a retard roller  106 . The pickup roller  104  sends out the recording sheets P stacked in the paper feed cassette  101 , the paper feeding roller  105  conveys the recording sheets P that have been sent out by the pickup roller  104 , and the retard roller  106 , which is disposed opposite the paper feeding roller  105 , feeds the recording sheets P sheet by sheet. The recording sheet P is then conveyed by registration rollers  107  at a predetermined timing. A process cartridge  108  is integrally formed of a charging roller  109 , a developing roller  110 , a cleaner  111 , and a photosensitive drum  112 , which serves as an electrophotographic photosensitive member. 
     The surface of the photosensitive drum  112  is charged uniformly by the charging roller  109 , and then an image is exposed thereon by a scanner unit  113  in accordance with an image signal. A laser diode  114  in the scanner unit  113  emits a laser beam. The laser beam is steered by a rotating polygon mirror  115  and a reflection mirror  116  to scan in a main scanning direction, and the rotation of the photosensitive drum  112  causes the laser beam to also scan in a sub-scanning direction. Thus, a two-dimensional latent image is formed on the surface of the photosensitive drum  112 . The latent image on the photosensitive drum  112  is visualized by the developing roller  110  in the form of a toner image, and the toner image is then transferred by a transfer roller  117  onto a recording sheet P that has been conveyed by the registration rollers  107 . The recording sheet P, on which the toner image has been transferred, is then conveyed to a fixing apparatus  118 , in which the recording sheet P undergoes heating/pressing processing. Thus, an unfixed toner image is fixed onto the recording sheet P. The recording sheet P is then discharged outside the image forming apparatus  100  by intermediate paper discharge rollers  119  and paper discharge rollers  120 , and thus a series of print operations ends. A pre-registration sensor  121 , a fixing paper discharge sensor  122 , and a paper discharge sensor  123  monitor the conveyance condition of the recording sheets P. 
       FIG. 2  is a sectional view illustrating the configuration of the fixing apparatus  118 . The fixing apparatus  118  includes a cylindrical fixing film (endless belt)  201 , a heater  203 , and a nip portion forming member (pressure roller)  202 . The heater  203  makes contact with an inner surface of the fixing film  201 , and the nip portion forming member  202 , along with the heater  203 , forms a nip portion  205  with the fixing film  201  provided therebetween. The nip portion  205  nips and conveys the recording sheet P carrying an image. The fixing apparatus  118  further includes a heater holder  204  formed of heat-resistant resin and a stay  206  formed of metal. The heater holder  204  holds the heater  203 , and the stay  206  is provided in parallel to the lengthwise direction of the heater holder  204  for ensuring rigidity of the heater holder  204 . The heater  203  is in contact with a temperature detection member that detects the temperature of the heater  203 . As described above, the fixing apparatus  118  includes the endless belt  201 , the heater  203 , and the nip portion forming member  202 . The heater  203  makes contact with the inner surface of the endless belt  201 , the nip portion forming member  202 , along with the heater  203 , forms the nip portion  205  with the endless belt  201  provided therebetween, and the nip portion  205  nips and conveys the recording sheet P carrying an image. The fixing apparatus  118  further includes the temperature detection member that detects the temperature of the heater  203 . The temperature detection member is provided on a second surface of the heater  203  which is opposite a first surface thereof forming the nip portion  205 . 
     A thermally conductive anisotropic member  207  is provided on a rear surface of the heater  203  (i.e., the surface (the second surface) that is opposite the surface (first surface) that forms the nip portion  205 ). In the first exemplary embodiment, the thermally conductive anisotropic member  207  is a sheet formed of graphite. Graphite has a layered structure of hexagonal plate crystal formed of carbon, and the layers are bonded by the van der Waals force. Graphite, having such a structure, has very high thermal conductivity in a direction parallel to the layer plane (sheet plane) while it has lower thermal conductivity in a direction perpendicular to the layer plane (sheet plane) than that in the direction parallel thereto. In  FIG. 2 , a direction x is a widthwise direction of the fixing apparatus  118  (i.e., the widthwise direction of the heater  203 ), a direction y is a lengthwise direction of the fixing apparatus  118  (i.e., the lengthwise direction of the heater  203 ), and a direction z is a heightwise direction of the fixing apparatus  118 . 
     As illustrated in  FIG. 2 , the graphite sheet  207  is located between the heater holder  204  and the heater  203 . The graphite sheet  207  in the first exemplary embodiment is 100 μm in thickness and has thermal conductivity of 700 W/(m·K) in a direction parallel to the sheet plane and 3 to 10 W/(m·K) in the thickness direction thereof (i.e., the direction perpendicular to the sheet plane). In the first exemplary embodiment, the heater  203  and the graphite sheet  207  are not integrated with an adhesive, but the graphite sheet  207  is simply sandwiched between the heater holder  204  and the heater  203 . When such a configuration is employed, grease (not illustrated) having high thermal conductivity may be applied between the graphite sheet  207  and the heater  203  to retain the relative position of the heater  203  and the graphite sheet  207 . 
     As described above, the graphite sheet  207  is not affixed to either the heater  203  or the heater holder  204 , and the graphite sheet  207  is simply sandwiched between the heater  203  and the heater holder  204 . In other words, the graphite sheet  207  is a separate component from the heater  203  and the heater holder  204 . The graphite sheet  207 , however, may be affixed to the heater holder  204 , and the heater  203  may be pressed toward the heater holder  204  so that the heater  203  makes contact with the graphite sheet  207 . Alternatively, the graphite sheet  207  may be affixed to the heater  203  with an adhesive having high thermal conductivity, and the heater  203 , to which the graphite sheet  207  has been affixed, may be held onto the heater holder  204  without being affixed thereto. As another alternative, the heater  203 , to which the graphite sheet  207  has been affixed, may be affixed to the heater holder  204  with an adhesive. 
       FIGS. 3A and 3B  are diagrams illustrating the heater  203  according to the first exemplary embodiment.  FIG. 3A  is a top view of the heater  203 , and  FIG. 3B  is a sectional side view of the heater  203  as viewed in the lengthwise direction thereof. 
     The heater  203  includes an insulating substrate  304 , heat generating resistors  301 ,  302 , and  303 , electrically-conductive portions  308 , electrode portions  305 ,  306 , and  307 , and a protective layer (glass)  309 . The insulating substrate  304  is formed of ceramics such as silicon carbide (SiC), aluminum nitride (AlN), and aluminum oxide (Al 2 O 3 ), the heat generating resistors  301 ,  302 , and  303  are formed by printing paste on the surface of the insulating substrate  304 , and the protective layer  309  protects the heat generating resistors  301 ,  302 , and  303 . As illustrated in  FIG. 3A , the heat generating resistors  301  and  303  are connected in parallel with the heat generating resistor  302  provided therebetween. The heat generating resistors  301  and  303  are driven by a triac  403  illustrated in  FIG. 4 , and the heat generating resistor  302  is driven by a triac  404  illustrated in  FIG. 4 . The triacs  403  and  404  can be driven independently from each other. Thus, the heater  203  of the first exemplary embodiment is a dual drive heater, which is driven by the two triacs  403  and  404  that can be driven independently from each other. 
     The resistance value of each of the heat generating resistors  301  and  303  is set so that a larger amount of heat is generated at the center of the ceramic heater  203  than that generated at ends thereof in the lengthwise direction. The resistance value of the heat generating resistor  302 , meanwhile, is set so that a larger amount of heat is generated at the ends of the ceramic heater  203  in the lengthwise direction than that generated at the center thereof. The set of the heat generating resistors  301  and  303  can be driven independently from the heat generating resistor  302 , and thus a heat generation distribution in the heater  203  can be modified, for example, in accordance with the width of a recording material. 
       FIG. 4  illustrates a heater drive circuit. The heater drive circuit includes an alternate current (AC) power supply  401 , which is connected to the heat generating resistors  301 ,  302 , and  303  through an AC filter  402 . The power supplied to the heat generating resistors  301  and  303  is controlled by controlling the drive of the triac  403 , and the power supplied to the heat generating resistor  302  is controlled by controlling the drive of the triac  404 . Bias resistors  405  and  406  drive the triac  403 , and bias resistors  407  and  408  drive the triac  404 . Phototriac couplers  409  and  410  secure a creeping distance between a primary side and a secondary side. When electric current flows through light emitting diodes of the respective phototriac couplers  409  and  410 , the triacs  403  and  404  are turned on, respectively. Resistors  411  and  412  regulate the electric current in the phototriac couplers  409  and  410 , respectively. Transistors  413  and  414  control on/off states of the phototriac couplers  409  and  410 , respectively. The transistors  413  and  414  operate in accordance with respective heater drive signals FSRD 1  and FSRD 2  transmitted from an engine controller  417  through resistors  415  and  416 , respectively. The heater drive signals FSRD 1  and FSRD 2  are set to an “H” level to turn on the triacs  403  and  404  and are set to an “L” level to turn off the triacs  403  and  404 . The “H” level, which is a voltage level of a port of the engine controller  417 , indicates a voltage level that is close to the level of the voltage supplied to the engine controller  417 . The “L” level, meanwhile, indicates a voltage level that is close to a ground potential of the engine controller  417 . A zero-cross detection circuit  418  is connected to the AC power supply  401  through the AC filter  402 . The zero-cross detection circuit  418  transmits a pulse signal (hereinafter, referred to as a “ZEROX signal”) to the engine controller  417  to notify that the commercial power supply voltage has reached or fallen below a threshold voltage. In the image forming apparatus  100 , the engine controller  417  determines timings of passing the electricity to the respective triacs  403  and  404  based on pulse edges of the ZEROX signal to control the on/off states of the triacs  403  and  404 . 
     A thermistor element  419  detects the temperature of the ceramic heater  203  at a center portion thereof in the lengthwise direction. Thermistor elements  420 ,  421 , and  422  detect the temperature of the ceramic heater  203  at end portions thereof in the lengthwise direction. The temperatures detected by the thermistor elements  419 ,  420 ,  421 , and  422  are input to the engine controller  417 . Resistors  423 ,  424 ,  425 , and  426  divide the voltages of outputs from the respective thermistor elements  419 ,  420 ,  421 , and  422 . Thus, TH1, TH2, TH3, and TH4 signals, which each have undergone voltage division and analog to digital conversion, are input to the engine controller  417 . The thermistor elements  419 ,  420 ,  421 , and  422  are negative temperature coefficient (NTC) thermistors with such properties that resistance values thereof decrease as the temperature rises. Therefore, the voltages of the TH1, TH2, TH3, and TH4 signals decrease as the temperatures of the respective thermistor elements  419 ,  420 ,  421 , and  422  rise. The temperature of the ceramic heater  203  is monitored by the engine controller  417  and is compared with a target temperature set in the engine controller  417 . Thus, the power to be supplied to the heat generating resistors  301 ,  302 , and  303  is adjusted. Through this configuration, the power supplied to the heater  203  is controlled to maintain the heater  203  at the target temperature. 
     A safety circuit  427  detects malfunctioning of the fixing apparatus  118  and forcibly stops the power supply to the ceramic heater  203 . The TH1, TH2, TH3, and TH4 signals from the respective thermistor elements  419 ,  420 ,  421 , and  422  are also input to the safety circuit  427  without passing through the engine controller  417 . The safety circuit  427  compares the temperatures detected by the thermistor elements  419 ,  420 ,  421 , and  422  with a reference temperature, which serves as a reference for determining malfunctioning of the fixing apparatus  118 . If the temperatures detected by the thermistor elements  419 ,  420 ,  421 , and  422  fall below the reference temperature, the safety circuit  427  retains an output signal SAFE at an “H” level. If the temperatures detected by the thermistor elements  419 ,  420 ,  421 , and  422  exceed the reference temperature, the safety circuit  427  sets the output signal SAFE to an “L” level to turn off a transistor  428 . 
     A relay  431 , where a primary side and a secondary side are insulated from each other, includes a switch unit, and the switch unit is disposed in a power supply path from the AC power supply  401  to the heat generating resistors  301 ,  302 , and  303 . When the transistor  428  causes electric current to flow through a built-in coil connected to the secondary side of the relay  431 , the coil is excited, and the switch unit is turned on/off. The transistor  428  is connected to the safety circuit  427  through a resistor  429 . When the fixing apparatus  118  malfunctions, the relay  431  is turned off to stop the power supply to the ceramic heater  203 . 
     A thermostatic switch  430  is in contact with the ceramic heater  203 . The contact of the thermostatic switch  430  breaks when the operating temperature thereof exceeds a predetermined temperature, shutting off the power supply to the heater  203 . The thermostatic switch  430  has its operating temperatures set such that the power supply to the heater  203  stops when the temperature of the heater  203  rises to an abnormal temperature and is used as a protective element of the fixing apparatus  118 . The thermostatic switch  430  and the relay  431  operate independently from each other when the fixing apparatus  118  malfunctions, enhancing safety of the fixing apparatus  118 . 
       FIG. 5  is a diagram illustrating the shape of the graphite sheet  207  in a temperature detection unit.  FIG. 5  illustrates the positional relationship among the ceramic heater  203 , the graphite sheet  207 , a thermistor unit (temperature detection member)  501 , which is indicated by a dotted rectangular in  FIG. 4 , and the heater holder  204 . As illustrated in  FIGS. 2 and 5 , the ceramic heater  203  is disposed such that the protective layer  309  faces the nip portion  205  and the insulating substrate  304  is in contact with the graphite sheet  207 . The thermistor unit  501  is in contact with the second surface (surface opposite the surface that faces the nip portion  205 ) of the ceramic heater  203 . The thermistor unit  501  includes a hard resin  505 , a ceramic paper  506  placed on the hard resin  505 , and the chip-sized thermistor element  419  placed on the ceramic paper  506 , all of which are then wrapped by an insulating film  507 . A heat-sensitive plate may be attached to the thermistor element  419  to collect heat to the thermistor element  419 . Such a temperature detection unit may be provided in a plurality in a single fixing apparatus  118 . In the first exemplary embodiment, thermistor units  502 ,  503 , and  504  that include the thermistor elements  420 ,  421 , and  422 , respectively, are further provided. In the first exemplary embodiment, the thermostatic switch  430  is also referred to as a temperature detection member. 
     The graphite sheet  207  has such a shape that a portion through which the temperature detection member makes contact with the heater  203  is cut out. In other words, the thermally conductive anisotropic member, which has higher thermal conductivity in a direction parallel to the second surface of the heater  203  than that in a direction perpendicular to the second surface, is provided on the second surface, but such a thermally conductive anisotropic member is not provided at a portion of the heater  203  where the temperature detection member is disposed. Although the ceramic heater  203  is disposed such that a surface of the insulating substrate  304  on which the heat generating resistors  301 ,  302 , and  303  are provided is opposite the nip portion  205  in the first exemplary embodiment, the ceramic heater  203  may instead be disposed such that a surface of the insulating substrate  304  on which the heat generating resistors  301 ,  302 , and  303  are not provided is opposite the nip portion  205 . In that case, a surface of the insulating substrate  304 , the surface that is opposite the nip portion  205  may be coated with paste such as a polyimide in order to enhance slidability between the insulating substrate  304  and the fixing film  201 . If such a configuration is employed, the graphite sheet  207  is disposed between the heater holder  204  and the protective layer  309  that is provided on a surface of the heater  203  on which the heat generating resistors  301 ,  302 , and  303  are provided. 
       FIGS. 6A and 6B  are diagrams illustrating the shape of the graphite sheet  207  in the lengthwise direction of the heater  203  according to the first exemplary embodiment.  FIGS. 6A and 6B  illustrate the graphite sheet  207  being placed on the ceramic heater  203 . 
     With reference to  FIG. 6A , the thermistor unit  501  makes contact with the ceramic heater  203  through a portion  601 . Since the graphite sheet  207  is cut out by an area corresponding to a contact area between the thermistor unit  501  and the heater  203 , the insulating substrate  304  is exposed therethrough. Similarly, the end portion thermistor units  502 ,  503 , and  504  make contact with the ceramic heater  203  through portions  602 ,  603 , and  604 , respectively, and the graphite sheet  207  is cut out by areas corresponding to respective contact areas between the thermistor units  502 ,  503 , and  504  and the heater  203 . The thermostatic switch  430  serving as the protective element makes contact with the heater  203  through a portion  605 , and the portion  605  is also cut out from the graphite sheet  207  by an area corresponding to a contact area between a heat-sensitive surface of the thermostatic switch  430  and the heater  203 . The heater  203  is nipped by a power supply connector at portions  606  and  607 , and the graphite sheet  207  is not provided at these portions  606  and  607  of the heater  203 . The electrode portions  305 ,  306 , and  307  illustrated in  FIG. 3  are provided on rear surfaces of the portions  606  and  607 , respectively. If heat from the heat generating resistors  301 ,  302 , and  303  is conducted to the portions  606  and  607 , the temperature of the connector rises excessively. Therefore, the graphite sheet  207  is not provided at the portions  606  and  607 . The graphite sheet  207 , however, is provided across almost the entire surface of the ceramic heater  203  except at the portions  606  and  607 . Providing the graphite sheet  207  advantageously allows heat at the ends of the heater  203  in the lengthwise direction to dissipate to the center portion thereof in the lengthwise direction and to suppress the non-sheet-passing part temperature rise, and keeping the area of the heater  203  where the graphite sheet  207  is not provided to a minimum brings about such an advantage to a full extent. Alternatively, as illustrated in  FIG. 6B , a line containing the portions  601 ,  602 ,  603 , and  604 , through which the thermistor units  501 ,  502 ,  503 , and  504  make contact with the heater  203 , and the portion  605 , through which the thermostatic switch  430  makes contact with the heater  203 , may be cut out. In other words, the thermally conductive anisotropic member may have an elongated shape in the lengthwise direction of the heater  203  and include portions where the temperature detection members for the heater  203  are disposed, and the portions where the temperature detection members are disposed may be cut out. In this case as well, the graphite sheet  207  is present continuously across the lengthwise direction of the heater  203 , and thus the non-sheet-passing part temperature rise can be suppressed effectively. The ceramic heater  203  may be affixed to the heater holder  204  with an adhesive, and in such a case, the graphite sheet  207  may be cut out not only at the portions  601 ,  602 ,  603 , and  604  through which the thermistor units  501 ,  502 ,  503 , and  504  make contact with the heater  203  but also at a portion where the adhesive is applied. 
     Subsequently, the calculation result of thermal resistance from the heat generating resistor  302  to the thermistor element  419  will be described. When the thermal conductivity of the graphite sheet  207  in the z direction ( FIG. 2 ) is 3 W/(m·K), the thickness of the graphite sheet  207  is 0.1 mm, and the area of a portion cut out from the graphite sheet  207 , that is, the contact area between the thermistor unit  501  and the heater  203  in the first exemplary embodiment is 10.3×4 mm 2 , thermal resistance of 8.09×10 3  K/W (Kelvin/Watt) is eliminated. The thermal resistance is calculated through an equation where thermal resistance (K/W)=thermal conductivity/distance/cross-sectional area. Cutting out a portion of the graphite sheet  207  to allow the temperature detection member to make contact with the heater  203  therethrough can eliminate a delay in thermal conduction in the thickness direction (z direction) of the graphite sheet  207 , and thus heat from the heater  203  can be conducted quickly to the thermistor element  419 . 
       FIG. 7  illustrates temperature distributions in the ceramic heater  203  while the temperature thereof rises. Cases where the graphite sheet  207  is not provided ((1)), the graphite sheet  207  is provided across the entire surface of the heater  203  ((3)), and the graphite sheet  207  is cut out by an area corresponding to the contact area between the thermistor unit  501  and the heater  203  as illustrated in  FIGS. 5, 6A, and 6B  ((2)) are compared. 
     The broken line indicates the temperature distribution in the case where the graphite sheet  207  is not provided ((1)). Since the heat generating resistors  301 ,  302 , and  303  are concentrated toward the center of the ceramic heater  203  in the x direction, a maximum temperature appears at the center and the temperature decreases toward the ends. Meanwhile, with the configuration where the graphite sheet  207  is provided across the entire surface of the heater  203  ((3)), as indicated by the dashed-dotted line, heat around the heat generating resistors  301 ,  302 , and  303 , where a maximum temperature appears, is conducted to the ends of the graphite sheet  207 . Thus, the difference in temperature between the center and the ends of the heater  203  in the x direction is reduced. When a portion of the graphite sheet  207  is cut out as in the case (2), the cut out portion can suppress heat dissipation toward the ends, where the temperature is lower, and thus the temperature at the center remains high. 
     Thus, the greater the cut out area is, the higher the temperature of the portion detected by the thermistor element  419 . In other words, responsiveness of the thermistor element  419  improves. If, however, the difference in temperature between the center and the ends increases, thermal stress increases, leading to more stress on the ceramic heater  203 . Therefore, the graphite sheet  207  is cut out only by an area corresponding to the contact area between the thermistor unit  501  and the heater  203  in the first exemplary embodiment. When the temperature rises with a temperature distribution as in the case (2), this indicates that the temperature rises quickly in the temperature detection unit. By eliminating influence of thermal resistance by an amount corresponding to the thickness of the graphite sheet  207 , the highest thermal response speed to the thermistor element  419  is achieved. With the configuration of the first exemplary embodiment, the power of 1800 W was actually supplied to the ceramic heater  203 , and the time taken for the thermistor element  419  to reach the temperature of 250° C. was compared in the cases (2) and (3). It took 2.490 seconds in the case (3) while it took 2.017 seconds in the case (2) to reach the same temperature. 
     As described thus far, cutting out a portion of the graphite sheet  207  to allow the temperature detection member to make contact with the heater  203  therethrough increases thermal response speed of the temperature detection member. As the temperature is detected more quickly, safety protective operation can be taken more quickly when protecting the fixing apparatus  118  with the engine controller  417  and the safety circuit  427 . 
     The configurations of the image forming apparatus  100  and the fixing apparatus  118  according to a second exemplary embodiment are similar to those of the first exemplary embodiment. Identical components are given identical reference numerals, and description thereof will be omitted. 
       FIGS. 8A and 8B  are diagrams illustrating the ceramic heater  203  according to the second exemplary embodiment.  FIG. 8A  is a top view of the ceramic heater  203 , and  FIG. 8B  is a sectional view of the ceramic heater  203 . 
     The second exemplary embodiment differs from the first exemplary embodiment in that the heater  203  is a single drive heater in which two heat generating resistors  801  and  802  are driven by a single triac. The insulating substrate  304  and the protective layer  309  illustrated in  FIG. 8B  are similar to those of the first exemplary embodiment, and thus description thereof will be omitted. 
       FIG. 9  is a sectional view illustrating the positional relationship among the ceramic heater  203 , the graphite sheet  207 , the thermistor unit  501 , and the heater holder  204  taken along a plane intersecting the lengthwise direction of the heater  203  and containing the thermistor unit  501 . In the second exemplary embodiment, the thickness of the graphite sheet  207  is 1 mm. The graphite sheet  207  has thermal conductivity of 700 W/(m·K) in a direction parallel to the sheet plane and 3 W/(m·K) in the thickness direction thereof. The graphite sheet  207  having a thickness of 1 mm may be formed by stacking graphite sheets each having a thickness of 100 μm. In the second exemplary embodiment as well, the graphite sheet  207  is cut out by an area corresponding to the contact area between the thermistor unit  501  and the heater  203 , as illustrated in  FIG. 9 . In addition, in the second exemplary embodiment as well, the thermostatic switch  430  and the thermistor units  502 ,  503 , and  504  used to detect the temperatures at the ends of the heater  203  are provided, and portions of the graphite sheet  207  through which these temperature detection members make contact with the heater  203  are cut out in a similar manner to that illustrated in  FIG. 9 . The shape of the graphite sheet  207  in the lengthwise direction of the heater  203  in the second exemplary embodiment is similar to the one illustrated in  FIG. 6A or 6B , and thus description thereof will be omitted. 
       FIGS. 10A, 10B, 10C, and 10D  illustrate a difference in thermal resistance between a configuration where a portion of the graphite sheet  207  is cut out and a configuration with no cutout in the graphite sheet  207 .  FIG. 10A  illustrates the case with the cutout, whereas  FIG. 10B  illustrates the case without the cutout. The dimensions are indicated in  FIGS. 10A and 10B .  FIG. 10C  illustrates the thermal conductivity and the cross-sectional areas of heat transmission paths used to calculate the thermal resistance. The thermal resistance is calculated through an equation where thermal resistance (K/W)=thermal conductivity/distance/cross-sectional area in a model where heat from the heat generating resistors  801  and  802  is conducted, in the end, to the contact surface of the thermistor unit  501  with the heater  203 . As illustrated in  FIG. 10A , the flow of heat from the heat generating resistor  801  to the thermistor element  419  is calculated separately in the x direction and the z direction. In that case, heat is conducted in two distinct directions, namely through the graphite sheet  207  and through the insulating substrate  304  in an area along the x direction where the graphite sheet  207  overlaps the insulating substrate  304  (e.g., area L1 in  FIG. 10A ). Therefore, the total thermal resistance in such an area is calculated under an assumption that each thermal resistance is connected in parallel.  FIG. 10D  illustrates a table indicating comparison results of the thermal resistance between the case illustrated in  FIG. 10A  and the case illustrated in  FIG. 10B . 
     With the configuration illustrated in  FIG. 10B , thermal resistance in the x direction is extremely small due to the effect of the graphite sheet  207 . Thermal resistance, however, is still present in the z direction in the graphite sheet  207  immediately underneath the thermistor unit  501 . Meanwhile, with the configuration illustrated in  FIG. 10A , although thermal resistance in the x direction increases at the cut out portion, thermal resistance in the z direction in the graphite sheet  207  immediately underneath thermistor unit  501  is eliminated. Thus, the total thermal resistance from the heat generating resistors  801  and  802  to the thermistor unit  501  is smaller in the configuration illustrated in  FIG. 10A  than that in the configuration illustrated in  FIG. 10B . The difference in the thermal resistance between the configuration illustrated in  FIG. 10A  and the configuration illustrated in  FIG. 10B  is a difference between the thermal resistance in the x direction and the thermal resistance in the z direction in an area L2. In other words, the speed of heat conduction to the thermistor element  419  can be increased by setting the total thermal resistance in the x direction in the area L2 to be smaller than the thermal resistance in the graphite sheet  207  in the z direction. 
     The thermal resistance above may be calculated while replacing with another parameter indicating ease of heat conduction such as thermal conductance or may be obtained through actual measurement. 
       FIG. 11  illustrates the temperature distributions in the ceramic heater  203  while the temperature thereof rises. Cases where the graphite sheet  207  is not provided at all ((1)′), the graphite sheet  207  is provided ((3)′), and the graphite sheet  207  that is cut out by an area corresponding to the contact area between the thermistor unit  501  and the heater  203  as illustrated in  FIG. 9  is used ((2)′) are compared. The broken line indicates the temperature distribution in the case where the graphite sheet  207  is not provided ((1)′). In this case, a difference in temperature between the portions corresponding to the locations of the heat generating resistors  801  and  802  and the ends of the heater  203  in the x direction (widthwise direction of the heater  203 ) is extremely large. Of course, suppression of the non-sheet-passing part temperature rise in the lengthwise direction of the heater  203 , which is a direction perpendicular to the paper plane of  FIG. 11 , cannot be expected. Meanwhile, with the configuration where the graphite sheet  207  is provided across the entire surface ((3)′) as indicated by the dashed-dotted line, heat around the heat generating resistors  801  and  802  is conducted to the ends of the heater  203 , leading to more uniform temperature throughout the heater  203 . As described with reference to  FIGS. 10A, 10B, 10C, and 10D , however, thermal resistance leading to the thermistor unit  501  is large and the responsiveness of the thermistor unit  501  is not sufficient. Therefore, as in the case (2)′ in the second exemplary embodiment, cutting out a portion of the graphite sheet  207  to allow the thermistor unit  501  to make contact with the heater  203  therethrough increases the speed of temperature detection while reducing unevenness in the temperature distribution in the widthwise direction of the heater  203 . 
     The configurations of the image forming apparatus  100  and the fixing apparatus  118  according to a third exemplary embodiment are similar to those of the first exemplary embodiment. Identical components are given identical reference numerals, and description thereof will be omitted. 
       FIG. 12  is a sectional view of the fixing apparatus  118  according to the third exemplary embodiment illustrating the heater  203  and the vicinity thereof. The thermally conductive anisotropic member in the third exemplary embodiment is thinner around a portion where the temperature detection member makes contact with the thermally conductive anisotropic member than in the remaining portion. In other words, the thickness of the thermally conductive anisotropic member at a portion where the temperature detection member is disposed thereon is less than that in the area around the aforementioned portion. Furthermore, the thermally conductive anisotropic member used in the third exemplary embodiment is not a graphite sheet but is obtained by printing paste-form graphite on the ceramic heater  203  and sintering the resulting ceramic heater  203 . Graphite layers  1200  are obtained through printing graphite multiple times and thus have a multilayer structure. The thermally conductive anisotropic member (graphite layers  1200  and graphite layer  1201 ) used in the third exemplary embodiment has a total of four layers. 
     The thermistor element  419  detects the temperature of the ceramic heater  203  through the lowermost graphite layer  1201 . Each layer in the graphite layers  1200  and  1201  is approximately 20 μm in thickness, and thus the thickness of the graphite layers  1200  and  1201  is approximately 80 μm in the area except areas where the thermistor units  501 ,  502 ,  503 , and  503  make contact therewith. 
       FIG. 13  is a diagram illustrating the multilayer structure of the graphite layers  1200  and  1201 . The lowermost layer (first layer)  1201  is formed by printing paste-form graphite across the entire surface of the heater  203  except for the portions  606  and  607  at which the heater  203  is connected to the connector. Second to fourth layers  1200  above the first layer  1201  each have the same external dimensions as the first layer  1201  and are formed by printing paste-form graphite on the first layer  1201  aside from the portions  601  to  604 , through which the thermistor units  501  to  504  make contact with the first layer  1201 , and the portion  605 , through which the thermostatic switch  430  makes contact with the first layer  1201 . 
     The graphite sheet  207  may be used, similarly to the first and second exemplary embodiments, and the thickness thereof may be made to differ between an area where the temperature detection member makes contact with the graphite sheet  207  and the remaining areas. Further, a thin thermally conductive anisotropic member may also be provided at a portion through which the temperature detection member is to make contact with the heater  203 , as in the third exemplary embodiment, and the shape of the remaining area may take on such a shape as illustrated in  FIG. 6B . 
     The configurations of the image forming apparatus  100  and the fixing apparatus  118  according to a fourth exemplary embodiment are similar to those of the first exemplary embodiment. Identical components are given identical reference numerals, and description thereof will be omitted. In the fourth exemplary embodiment, alternative examples of an area cut out from the graphite sheet  207  will be described in addition to those in the first and second exemplary embodiments. 
     As described in the first exemplary embodiment with reference to  FIG. 7 , if a maximum temperature location is close to the location of the thermistor element  419 , the responsiveness of the thermistor element  419  increases as the area to be cut out from the graphite sheet  207  increases. However, if the temperature at the center in the widthwise direction is high and the temperatures at the ends are low, thermal stress on the ceramic heater  203  increases, leading to stress on the heater  203 . Accordingly, even if the graphite sheet  207  is to be cut out at a portion where the thermistor unit  501  makes contact with the heater  203 , a configuration with as less thermal stress as possible is desirable. 
     Several patterns are illustrated in  FIGS. 14A, 14B, 14C, 14D, 14E, 14F, 14G, 14H, 14I, and 14J , and these are configured so that heat from the heat generating resistors  301 ,  302 ,  303 ,  801 , and  802  is conducted toward the ends of the heater  203  in the widthwise direction as much as possible using the graphite sheet  207 . The heater  203  includes a plurality of heat generating resistors provided on a substrate. As indicated by an area defined by the dotted lines, an area G is present in which the heat generating resistor located at the farthest end (heat generating resistor  301  or  303  in the example illustrated in  FIG. 14A ) and the graphite sheet  207  overlap each other in the widthwise direction of the heater  203 .  FIGS. 14A, 14B, and 14C  illustrate configuration examples in heat generating patterns in the first exemplary embodiment, whereas  FIGS. 14D and 14E  illustrate configuration examples in heat generating patterns in the second exemplary embodiment. With such configurations, the difference in temperature between a portion where a heat generating resistor is located and the end of the heater  203  is reduced, and stress on the heater  203  is thus alleviated.  FIGS. 14F, 14G, 14H, 14I, and 14J  illustrate configuration examples in which the thermally conductive anisotropic member of the heater is thinner around a portion where the temperature detection member makes contact with the thermally conductive anisotropic member than that in the remaining portion, and the same configurations as  FIGS. 14A, 14B, 14C, 14D, and 14E , respectively, are applied to the other parts. 
     According to the exemplary embodiments of the present invention, responsiveness in temperature detection can be improved while reducing the non-sheet-passing part temperature rise during fixing processing of small-sized sheets. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2012-255368 filed Nov. 21, 2012, which is hereby incorporated by reference herein in its entirety.