Patent Publication Number: US-2022229387-A1

Title: Heating unit

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2021-004689, which was filed on Jan. 15, 2021, the disclosure of which is herein incorporated by reference in its entirety. 
     BACKGROUND 
     The following disclosure relates to a heating unit used for a fixing device of an electrophotographic type image forming apparatus or the like. 
     In the past, there has been known a fixing device in which a rotating belt is interposed between a ceramic heater and a pressure roller. In the fixing device, the ceramic heater includes a substrate and a resistance heating element, in which one sheet-shaped heat conductive member is disposed so as to be in contact with a back surface located on an opposite side of a nip surface which is in contact with the belt. A temperature detecting member is in contact with the heater. 
     SUMMARY 
     Incidentally, in a case where the heater is configured such that the resistance heating element is provided on the substrate, a temperature difference occurs between a portion of the heater near to the resistance heating element and a portion of the heater apart from the resistance heating element. Accordingly, when the temperature detecting member is brought into contact with said one heat conductive member disposed between the temperature detecting member and the heater as in the related-art technique, it may be difficult to detect an accurate temperature by the temperature detecting member due to unevenness in the heat conductive member in temperature caused by disposition of the resistance heating element. 
     In view of the above, an object of the present disclosure is to detect the accurate temperature by the temperature detecting member. 
     In one aspect of the disclosure, a heating unit includes a heater including a substrate and a resistance heating element provided on the substrate, a temperature sensor configured to detect a temperature of the heater, an endless belt configured to rotate around the heater, a holder supporting the heater, a first heat conductive member located between the heater and the holder, the first heat conductive member including a first heater-side surface which is in contact with a back surface of the heater and a first opposite surface located on an opposite side of the first heater-side surface, the first heat conductive member having a heat conductivity higher than that of the substrate, and a second heat conductive member which is smaller than the first heat conductive heat member when viewed in an orthogonal direction orthogonal to the first opposite surface, the second heat conductive member including a second heater-side surface which is in contact with the first opposite surface and a second opposite surface located on an opposite side of the second heater-side surface. The temperature sensor is in contact with the second opposite surface of the second heat conductive member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, features, advantages, and technical and industrial significance of the present disclosure will be better understood by reading the following detailed description of the embodiments, when considered in connection with the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view of a heating unit at a position of a thermistor, 
         FIG. 2A  is a view illustrating a surface on which resistance heating elements of a heater are disposed; 
         FIG. 2B  is a view of the heater, a first heat conductive member, and second heat conductive members viewed from a back side of the heater; 
         FIG. 2C  is a view of a holder viewed from a side opposite to the heater; 
         FIG. 3A  is a perspective view of the thermistor; 
         FIG. 3B  is a perspective view of an energization interrupting member: 
         FIG. 4  is a cross-sectional view of the heating unit at a position of the energization interrupting member; 
         FIG. 5A  is a cross-sectional view of the heating unit along a longitudinal direction for explaining positioning of the second heat conductive member and the thermistor; 
         FIG. 5B  is a cross-sectional view of the heating unit along the longitudinal direction for explaining positioning of the second heat conductive member and the energization interrupting member: 
         FIG. 6  is a cross-sectional view of a heating unit at the position of the thermistor in a case where the first heat conductive member is a graphite sheet; 
         FIG. 7A  is a cross-sectional view of a heating unit along the longitudinal direction for explaining another modification of positioning of the second heat conductive member and the thermistor; 
         FIG. 7B  is a cross-sectional view of a heating unit along the longitudinal direction for explaining another modification of positioning of the second heat conductive member and the energization interrupting member; 
         FIG. 7C  is an enlarged cross-sectional view of a heating unit along the longitudinal direction for explaining further another modification of the second heat conductive member and the thermistor; 
         FIG. 8A  is a cross-sectional view of an embodiment along the longitudinal direction in which the second heat conductive member is positioned with respect to the first heat conductive member by a protruding portion of the second heat conductive member; 
         FIG. 8B  is a cross-sectional view of an embodiment along the longitudinal direction in which the second heat conductive member is positioned with respect to the first heat conductive member by a protruding portion of the first heat conductive member, 
         FIG. 9A  is a view illustrating a surface on which resistance heating elements of a heater in a modification is disposed; 
         FIG. 9B  is a view of the heater, the first heat conductive member and the second heat conductive member viewed from the back side of the heater; and 
         FIG. 9C  is a view of the heater and the holder viewed from the side opposite to the heater. 
     
    
    
     EMBODIMENTS 
     A heating unit  1  according to an embodiment is used for a fixing device of an image forming apparatus, or a device that transfers foil by heat, and the like. As illustrated in  FIG. 1 , the heating unit  1  includes a belt  3 , a heater  10 , a holder  20 , a first heat conductive member  30 , second heat conductive members  45 ,  46  (see  FIG. 4 ), a thermistor  50  as an example of a temperature sensor, and an energization interrupting member  60  as another example of the temperature sensor (see  FIG. 4 ). 
     The belt  3  is an endless belt, which is made of metal or resin. The belt  3  rotates around the heater  10  while being guided by the holder  20 . The belt  3  has an outer circumferential surface and an inner circumferential surface. The outer circumferential surface comes into contact with a sheet to be heated. The inner circumferential surface is in contact with the heater  10 . 
     The heater  10  includes a substrate  11 , resistance heating elements  12  provided on the substrate  11 , and a cover  13 . The substrate  11  is formed of a long rectangular plate made of ceramic. The heater  10  is a so-called ceramic heater. The resistance heating elements  12  are formed on one surface of the substrate  11  by printing. As illustrated in  FIG. 2A , two resistance heating elements  12  are provided in the present embodiment. The two resistance heating elements  12  are respectively disposed so as to extend in a longitudinal direction of the heater  10  (hereinafter the longitudinal direction of the heater  10  is referred to merely as a “longitudinal direction”) and so as to be spaced apart from each other in parallel in a short-side direction, of the heater  10 , orthogonal to the longitudinal direction. A conducting wire  19 A is connected to one end  12 A of each of the resistance heating elements  12 , and a terminal  18  for supplying power is provided at an end portion of the conducting wire  19 A of each of the resistance heating elements  12 . The other ends  12 B of the resistance heating elements  12  are connected to each other by a conducting wire  19 B. The number of resistance heating elements  12  is not particularly limited. It is noted that the resistance heating elements may be configured such that a resistance heating element in which a heat generation amount at the center in the longitudinal direction is higher than a heat generation amount at end portions in the longitudinal direction and a resistance heating element in which the heat generation amount at end portions in the longitudinal direction is higher than the heat generation amount at the center in the longitudinal direction are provided, and such that a heat generation distribution in the longitudinal direction is regulated by individually controlling each of the resistance heating elements. 
     The cover  13  covers the resistance heating elements  12 . The cover  13  is made of, for example, glass. The heater  10  includes a nip surface  15  which is in contact with the inner circumferential surface of the belt  3 , and a back surface  16  located on an opposite side of the nip surface  15 . 
     The holder  20  is a member supporting the heater  10 . The holder  20  includes a support portion  21  and guide portions  22 . The support portion  21  has a plate shape corresponding to a shape of the heater  10 . The support portion  21  includes a support surface  21 A which is a surface facing a side on which the heater  10  is disposed and an inside surface  21 B located on an opposite side of the support surface  21 A. As illustrated in  FIG. 2C , the support portion  21  has holder openings  25 A,  25 B, and  26  piercing through the support portion  21 . The holder opening  25 A is disposed at a center of the support portion  21  in the longitudinal direction, and has a long rectangular shape in the longitudinal direction. The holder opening  26  is disposed at one end portion of the support portion  21  in the longitudinal direction, and has a long rectangular shape in the longitudinal direction. The holder opening  25 B is disposed at the other end portion of the support portion  21  in the longitudinal direction, and has a long rectangular shape in the longitudinal direction. 
     The thermistor  50  includes two thermistors which are a first thermistor  50 A and a second thermistor  50 B. The first thermistor  50 A and the second thermistor  50 B are the same components. The first thermistor  50 A detects a temperature at a center of the heater  10  in the longitudinal direction of the heater  10 . The first thermistor  50 A is used for controlling the temperature of the heater  10  such that the temperature of the heater  10  becomes a target temperature based on the temperature detected by the first thermistor  50 A. The second thermistor  50 B detects the temperature of the heater  10  at a position nearer to an end of the heater  10  in the longitudinal direction than the position detected by the first thermistor  50 A. The second thermistor  50 B is used for detecting that the temperature is increased at the position near to the end of the heater  10 . The holder opening  25 A is disposed at a position corresponding to the first thermistor  50 A. The first thermistor  50 A and the second thermistor  50 B may not be the same component. In this case, it is preferable that the first thermistor  50 A is a member with higher accuracy in temperature detection than the second thermistor  50 B in a temperature range during printing operation. 
     The energization interrupting member  60  is a member configured to interrupt energization to the resistance heating elements  12  when the heater  10  is abnormally increased in temperature. The holder opening  26  is disposed at the position corresponding to the energization interrupting member  60 . 
     Returning to  FIG. 1 , the guide portions  22  are provided at both ends in a short-side direction of the support portion  21 . The short-side direction is a direction orthogonal to the longitudinal direction of the support portion  21 . Each of the guide portions  22  includes a guide surface  22 G extending along the inner circumferential surface of the belt  3 . Each of the guide portions  22  has a plurality of guide ribs  22 A arranged in the longitudinal direction as illustrated in  FIG. 1  and  FIG. 2C . 
     The first heat conductive member  30  is a member configured to uniformize the temperature of the heater  10  in the longitudinal direction by conducting heat in the longitudinal direction of the heater  10 . The first heat conductive member  30  is a sheet-like member, and is located between the heater  10  and the support portion  21  of the holder  20 . When the sheet as a heating target is interposed between the heating unit  1  and another pressure member, the first heat conductive member  30  is interposed between the heater  10  and the support portion  21 . The first heat conductive member  30  includes a first heater-side surface  31  which is in contact with the back surface  16  of the heater  10  and a first opposite surface  32  located on an opposite side of the first heater-side surface  31 . The first opposite surface  32  is in contact with the support surface  21 A of the support portion  21 . 
     The first heat conductive member  30  is a member in which a heat conductivity in a direction parallel to the first heater-side surface  31  (hereinafter referred to merely as a “planar direction”) is higher than a heat conductivity of the substrate  11  in the planar direction. A material of the first heat conductive member  30  is not particularly limited. For example, metals such as aluminum, aluminum alloys, and copper having high heat conductivities can be adopted. The first heat conductive member  30  may be an anisotropic heat conductive member in which the heat conductivity in the planar direction is higher than a heat conductivity in a thickness direction orthogonal to the first heater-side surface  31 . For example, a thin graphite sheet illustrated in  FIG. 6  can be adopted as the anisotropic heat conductive member. A thickness of the first heat conductive member  30  is not particularly limited either. For example, a film-like member thinner than 0.1 mm and a plate-like member thicker than 1 mm may be adopted. It is preferable that the thickness of the first heat conductive member  30  is 0.03 mm to 3 mm. 
     The second heat conductive members  45 ,  46  are members configured to uniformize the temperature at each of portions where the second heat conductive members  45 ,  46  are in contact with the first heat conductive member  30  by conducting heat in the planar direction. 
     The second heat conductive member  45  is a sheet-like member, and includes a second heater-side surface  45 F facing the heater  10  side and a second opposite surface  45 R located on an opposite side of the second heater-side surface  45 F. The second heater-side surface  45 F is in contact with the first opposite surface  32 . 
     As illustrated in  FIG. 4 , the second heat conductive member  46  also includes a second heater-side surface  46 F facing the heater  10  side and a second opposite surface  46 R located on an opposite side of the second heater-side surface  46 F in the same manner. The second heater-side surface  46 F is in contact with the first opposite surface  32 . 
     As illustrated in  FIG. 1  and  FIG. 4 , second heat conductive members  45 A.  45 B,  46  are disposed at positions respectively corresponding to the holder openings  25 A,  25 B, and the holder opening  26  when viewed in an orthogonal direction orthogonal to the first opposite surface  32  of the first heat conductive member  30 . The second heat conductive member  45  includes the second heat conductive member  45 A and the second heat conductive member  45 B. In the embodiment, the second heat conductive member  45 A and the second heat conductive member  45 B are the same component while disposed at positions different from each other. 
     In the embodiment, sizes of the second heat conductive members  45 A,  45 B, and  46  are smaller than a size of the first heat conductive member  30  when viewed in the orthogonal direction orthogonal to the first opposite surface  32 . The relationship in which “the sizes of the second heat conductive member  45 A,  45 B, and  46  are smaller than the size of the first heat conductive member  30 ” means that, in a case where any one of the second heat conductive members  45 A,  45 B, and  46  overlaps the first heat conductive member  30 , said any one of the second heat conductive members  45 A,  45 B, and  46  is entirely located inside an outline of the first heat conductive member  30  when viewed in the orthogonal direction. 
     The second heat conductive members  45 A,  45 B.  46  are members in which a heat conductivity in the planar direction is higher than the heat conductivity in the planar direction of the substrate  11 . A material of each of the second heat conductive members  45 A,  45 B.  46  is not particularly limited. For example, metals such as aluminum, aluminum alloys, and copper having high heat conductivities can be adopted. A thickness of each of the second heat conductive members  45 A,  45 B,  46  is not particularly limited either. For example, a film-like member thinner than 0.1 mm and a plate-like member thicker than 1 mm may be adopted as the second heat conductive members  45 A,  45 B,  46 . It is preferable that the thickness of each of the second heat conductive members  45 ,  46  is 0.03 mm to 3 mm. 
     Dimensions of the second heat conductive members  45 A,  45 B,  46  in the short-side direction orthogonal to the longitudinal direction are larger than a dimension of the resistance heating element  12  in the short-side direction. Then, the second heat conductive members  45 A,  45 B,  46  are located between the two resistance heating elements  12  in the short-side direction. 
     The second heat conductive member  46  has protruding portions  46 B, each of which is an example of a second protruding portion, protruding toward the energization interrupting member  60  in the thickness direction as illustrated in  FIG. 5B . The protruding portions  46 B protrude from end portions in the longitudinal direction of the second heat conductive member  46 . 
     As illustrated in  FIG. 3A , the thermistor  50  ( 50 A,  50 B) includes a support plate  51 , an urging member  52 , a film  53 , and a temperature detecting element  55 . The urging member  52  is a spongy member having elasticity, and the urging member  52  is supported by the support plate  51 . The urging member  52  has a D-shape in cross section. The temperature detecting element  55  is disposed so as to be located at a most protruding portion in the urging member  52 , and the temperature detecting element  55  is connected to not-illustrated wiring. The film  53  is disposed such that the temperature detecting element  55  is located at the most protruding portion in the urging member  52 , and the film  53  is mounted to the support plate  51  so as to be wound around the urging portion  52  and the support plate  51 . 
     As illustrated in  FIG. 3A , the film  53  has slits  53 X extending in a direction orthogonal to the longitudinal direction at both end portions of the film  53  in the longitudinal direction. Accordingly, the film  53  includes a central portion  53 A located at the center of the film  53  in the longitudinal direction and being in contact with the urging portion  52 , and protruding portions  53 B, each of which is an example of a first protruding portion, positioned at both end portions of the film  53  in the longitudinal direction. The protruding portions  53 B are portions, as illustrated in  FIG. 5A , protruding relatively to the central portion  53 A by the urging member  52  which is pushed and deformed when the thermistor  50  is mounted to the holder  20  and the thermistor  50  is pushed onto the second heat conductive member  45 A,  45 B. The second heat conductive member  45 A,  45 B are positioned with respect to the thermistor  50  in a state in which both ends of the second heat conductive member  45 A,  45 B in the longitudinal direction are engaged with the protruding portions  53 B. 
     As illustrated in  FIG. 3B , the energization interrupting member  60  is a thermostat having an interrupting mechanism formed of bimetal and located inside the thermostat, and the energization interrupting member  60  includes a case  61  accommodating the interrupting mechanism and a detector  62  protruding from the case  61  and configured to detect a temperature. As illustrated in  FIG. 5B , the second heat conductive member  46  is positioned with respect to the energization interrupting member  60  in a state in which the protruding portions  46 B are engaged with both ends of the detector  62  in the longitudinal direction. 
     As illustrated in  FIG. 1 , the first thermistor  50 A is configured such that a portion protruding from the support plate  51  enters an inside of the holder opening  25 A, and the portion protruding from the support plate  51  is in contact with the second opposite surface  45 R of the second heat conductive member  45 A through the holder opening  25 A. The urging member  52  of the first thermistor  50 A is pushed and deformed, and the temperature detecting element  55  is pushed onto the second opposite surface  45 R of the second heat conductive member  45 A. A configuration in which the second thermistor  50 B is in contact with the second opposite surface  45 R is the same as the configuration in which the first thermistor  50 A is in contact with the second opposite surface  45 R; therefore, explanation of the second thermistor  50 B is dispensed with. 
     As illustrated in  FIG. 4 , the energization interrupting member  60  is configured such that the detector  62  protruding from the case  61  enters the holder opening  26 , and the detector  62  is in contact with the second opposite surface  46 R of the second heat conductive member  46  through the holder opening  26 . 
     When viewed in the orthogonal direction orthogonal to the first opposite surface  32 , a dimension of the second heat conductive member  45  in the longitudinal direction is equal to or less than twice a dimension of a contact portion on the second opposite surface  45 R, in the longitudinal direction, with which the thermistor  50  is in contact. That is, the contact portion between the thermistor  50  and the second opposite surface  45 R is a portion on the second opposite surface  45 R which is produced by contact of the thermistor  50  with the second opposite surface  45 R. By the contact of the thermistor  50  with the second opposite surface  45 R, a contact area on the second opposite surface  45 R is produced, and the dimension of the contact portion is defined by an outline of the contact area. That is, the dimension of the contact portion in the longitudinal direction is defined by the outline of the contact area in the longitudinal direction. Moreover, a dimension of the second heat conductive member  45  in a short-side direction orthogonal to the orthogonal direction and the longitudinal direction is equal to or less than a dimension of the contact portion of the second opposite surface  45 R, in the short-side direction, with which the thermistor  50  is in contact. That is, the dimension of the contact portion in the short-side direction is defined by the outline of the contact area in the short-side direction. It is preferable that the dimension of the second heat conductive member  45  in the short-side direction is greater than a width of one resistance heating element  12  in the short-side direction, and it is preferable that the dimension of the second heat conductive member  45  in the short-side direction is greater than a distance between the adjacent two resistance heating element  12  in the short-side direction. 
     When viewed in the orthogonal direction, a dimension of the second heat conductive direction  46  in the longitudinal direction is equal to or less than twice a dimension of a contact portion of the second opposite surface  46 R, in the longitudinal direction, with which the energization interrupting member  60  is in contact. That is, the contact portion between the energization interrupting member  60  and the second opposite surface  46 R is a portion on the second opposite surface  46 R which is produced by contact of the energization interrupting member  60  with the second opposite surface  46 R. By the contact of the energization interrupting member  60  with the second opposite surface  46 R, a contact area on the second opposite surface  46 R is produced, and the dimension of the contact portion is defined by an outline of the contact area. That is, the dimension of the contact portion in the longitudinal direction is defined by the outline of the contact area in the longitudinal direction. Moreover, a dimension of the second heat conductive member  46  in the short-side direction orthogonal to the orthogonal direction and the longitudinal direction is equal to or less than a dimension of the contact portion of the second opposite surface  46 R, in the short-side direction, with which the energization interrupting member  60  is in contact. That is, the dimension of the contact portion in the short-side direction is defined by the outline of the contact area in the short-side direction. It is preferable that the dimension of the second heat conductive member  46  in the short-side direction is greater than the width of one resistance heating element  12  in the short-side direction, and it is preferable that the dimension of the second heat conductive member  46  in the short-side direction is greater than the distance between the adjacent two resistance heating element  12  in the short-side direction. 
     As illustrated in  FIG. 2C , the first thermistor  50 A is disposed so as to detect the temperature at positions in a range in which a sheet with a minimum width W 2  usable in the heating unit  1  can pass. The second thermistor  50 B is disposed so as to detect the temperature at a position in a range in which the sheet with a maximum width W 1  usable in the heating unit  1  can pass and out of the range in which the sheet with the minimum width W 2  usable in the heating unit  1  can pass (a range located on the other-end side of the minimum width W 2  in which the second thermistor  50 B can be disposed is illustrated in  FIG. 2A  as an end range AE 1 ). The energization interrupting member  60  is disposed so as to detect the temperature at a position in the range in which the sheet with the maximum width W 1  usable in the heating unit  1  can pass and out of the range in which the sheet with the minimum width W 2  usable in the heating unit  1  can pass (a range located on one-end side of the minimum width W 2  in which the energization interrupting member  60  can be disposed is illustrated in  FIG. 2A  as an end range AE 2 ). 
     Then, one ends  12 A and the other ends  12 B of the resistance heating elements  12  are located on outer sides of the maximum width W 1  and on an inner side of one end portion  38 A and the other end portion  38 B of the first heat conductive member  30  in the longitudinal direction. That is, a length of the first heat conductive member  30  is longer than a length of the resistance heating element  12  in the longitudinal direction. 
     The one end portion  38 A and the other end portion  38 B of the first heat conductive member  30  are located on outer sides of the one ends  12 A and the other ends  12 B of the resistance heating element  12  and on an inner side of one end  11 A and the other end  11 B of the substrate  11  in the longitudinal direction. That is, a length of the substrate  11  is longer than the length of the first heat conductive member  30  in the longitudinal direction. 
     Operations and effects of the above heating unit  1  will be explained. 
     The thermistor  50  is in contact with the second opposite surface  45 R of the second heat conductive member  45 , and the energization interrupting member  60  is in contact with the second opposite surface  46 R of the second heat conductive member  46 . Incidentally, if the thermistor  50  and the energization interrupting member  60  are in contact with the first opposite surface  32  of the first heat conductive member  30  directly, the thermistor  50  and the energization interrupting member  60  may be affected by temperature unevenness due to disposition of the resistance heating elements  12 . For example, in a case where the thermistor  50  and the energization interrupting member  60  are in contact with portions each corresponding to a portion located between the adjacent two resistance heating elements  12  in the short-side direction on the first opposite surface  32 , it may be difficult to detect an accurate temperature. However, the thermistor  50  and the energization interrupting member  60  are in contact with the second opposite surfaces  45 R,  46 R of the second heat conductive members  45 ,  46  without directly being in contact with the first opposite surface  32  of the first heat conductive member  30  in the embodiment; therefore, temperature unevenness due to disposition of the resistance heating elements  12  can be uniformed by the second heat conductive members  45 ,  46 . Accordingly, it is possible to detect the accurate temperature by the thermistor  50  and the energization interrupting member  60 . 
     The end ranges AE 1 , AE 2  are portions in which the temperatures of the end ranges AE 1 , AE 2  are easily increased, since heat is not deprived by the sheet with the minimum width W 2  when the sheet with the minimum width W 2  is heated. When the temperatures at the end ranges AE 1 , AE 2  are increased, heat of the heater  10  is transmitted through the first heat conductive member  30  and the second heat conductive members  45 B,  46  and flows from the end ranges AE 1 , AE 2  to the range inside the minimum width W 2 . However, since the second heat conductive members  45 B,  46  are provided in addition to the first heat conductive member  30  in the embodiment, heat conduction performance at the end ranges AE 1 , AE 2  improves. Accordingly, it is possible to suppress temperature increase at end portions in the longitudinal direction of the heater  10 . 
     Since the length of the first heat conductive member  30  is longer than the length of the resistance heating element  12 , it is possible to uniform the temperature of the heater  10  in the entire range in which the resistance heating elements  12  are disposed in the longitudinal direction of the heater  10 . 
     Since the second heat conductive members  45 ,  46  are configured such that the dimensions of the thermistor  50  and the energization interrupting member  60  in the longitudinal direction and the short-side direction are respectively equal to or less than twice the dimensions of the contact portions of the second heat conductive members  45 ,  46 , in the longitudinal direction and the short-side direction, with which the thermistor  50  and the energization interrupting member  60  are in contact, the second heat conductive members  45 ,  46  are properly small. Accordingly, it is possible to limit a range in the second heat conductive members  45 ,  46  where temperatures are to be detected to a predetermined range. 
     Since the second thermistor  50 B is disposed so as to detect the temperature at a position in the end range AE 1 , it is possible to detect temperature increase in the end range AE 1  by the second thermistor  50 B. 
     Since the energization interrupting member  60  is disposed so as to detect the temperature at the position in the end range AE 2 , it is possible to detect temperature increase in the range AE 2  by the energization interrupting member  60 . 
     Since the second heat conductive member  45  is engaged with the protruding portions  53 B of the thermistor  50 , it is possible to be properly positioned the second heat conductive member  45  with respect to the thermistor  50 . 
     Since the protruding portions  46 B of the second heat conductive member  46  are engaged with the energization interrupting member  60 , it is possible to be properly positioned the second heat conductive member  46  with respect to the energization interrupting member  60 . 
     The embodiment of the present disclosure has been explained above. The present disclosure is not limited to the above embodiment and can be achieved by being modified suitably. 
     A method for positioning the second heat conductive member may be different from one in the above embodiment. 
     For example, instead of the protruding portions of the thermistor  50 , a second heat conductive member  245  may have protruding portions  245 B, each of which is an example of a second protruding portion, at both ends in the longitudinal direction of the second heat conductive member  245 , and the protruding portions  245 B may be engaged with both end portions of the film  53  in the thermistor  50  as illustrated in  FIG. 7A . 
     Instead of the protruding portions of the second heat conductive member, the energization interrupting member  60  may have protruding portions  61 A, which is an example of a first protruding portion, at both ends in the longitudinal direction of the energization interrupting member  60 , and the protruding portions  61 A may be engaged with both end portions of a second heat conductive member  246  as illustrated in  FIG. 7B . 
     Not only the second heat conductive member  245  has the protruding portions  245 B protruding toward the thermistor  50  as in the modification illustrated in  FIG. 7A , but also a second heat conductive member  345  may have locking members  345 C protruding from protruding portions  345 B, each of which is an example of a second protruding portion, toward an inner side in the longitudinal direction in addition to protruding portions  345 B protruding toward the thermistor  50  as in a modification illustrated in  FIG. 7C . When the locking members  345 C are engaged with the film  53 , it is possible to prevent the second heat conductive member  345  from coming off unnecessarily after the film  53  is mounted to the second heat conductive member  345 . 
     Moreover, the second heat conductive member may include a positioner by which the second heat conductive member is positioned with respect to the first heat conductive member. For example, as illustrated in  FIG. 8A , the second heat conductive member  45  may include protruding portions  45 P, which is an example of a protrusion, protruding toward the heater  10  side at ends of the second heat conductive member  45  in the longitudinal direction, and the first heat conductive member  30  may include holes  30 Q, which is an example of a recess, with which the protruding portions  45 P engage. Alternatively, on the contrary, as illustrated in  FIG. 8B , the first heat conductive member  30  may include a protruding portion  30 Q which is an example of a protrusion, protruding toward a side opposite to the heater  10  side, and the second heat conductive member  45  may include a hole  45 Q, which is an example of a recess, with which the protruding portion  30 Q engages. According to these configurations, it is possible to position the second heat conductive member  45  with respect to the first heat conductive member  30 . 
     Moreover, the energization interrupting member  60  may be disposed so as to detect the temperature at a position in the range in which the sheet with the minimum width W 2  usable in the heating unit  1  can pass as in a modification illustrated in  FIG. 9C . Also in this case, it is possible to detect the accurate temperature by the thermistor  50  and the energization interrupting member  60 . The energization interrupting member  60  is disposed at the position in the range in which the sheet with the minimum width W 2  usable in the heating unit  1  can pass; therefore, it is possible to detect abnormal temperature increase of the heater  10  regardless of the size of the sheet in the width direction. 
     The numbers of the temperature sensors and the energization interrupting members are not limited. Only one temperature sensor may be provided and three or more temperature sensors may be provided. Two or more energization interrupting members may be provided and it is possible that no energization interrupting member is provided. 
     In the above embodiment, each of the first heat conductive member  30  and the second heat conductive members  45 ,  46  is formed of one sheet-like member; however, each of them may be formed of a combination of a plurality of sheet-like members. In this case, the material, heat conductivity, and the shape of the plurality of sheet-like members may be different from one another and may be the same as one another. 
     In the above embodiment, the substrate  11  of the heater  10  is formed of the long rectangular plate made of ceramic; however, the substrate  11  may be formed of a long rectangular plate made of metal such as stainless steel, which has a heat conductivity lower than that of the heat conductive member  30 . 
     Respective components explained in the above embodiment and modification examples may be arbitrarily combined to achieve the disclosure.