Patent Publication Number: US-11656565-B2

Title: Image forming apparatus configured to determine amount of power supplied to heating elements based on measured temperatures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/152,718, filed Jan. 19, 2021, which is a continuation of U.S. patent application Ser. No. 16/728,691, filed Dec. 27, 2019, now U.S. Pat. No. 10,928,755, issued on Feb. 23, 2021, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2019-021851, filed on Feb. 8, 2019, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments relate to an image forming apparatus and an image forming method. 
     BACKGROUND 
     There are on-demand heating devices such as film fixing units. Such an on-demand heating device drives a film by means of a rotating member provided with an elastic layer. In such an on-demand heating apparatus, a lubricant such as grease is applied to the film, so that torque required for driving the film is reduced. However, the viscosity of the lubricant varies depending on the temperature. Therefore, in the case where the on-demand heating device which has not been used for a while is restarted, the viscosity of the lubricant may decrease. When the viscosity of the lubricant is reduced as described above, torque required for driving the film may increase. In order to suppress the torque increase, there has been proposed a technique in which the lubricant is heated before the rotation member starts rotating in order to decrease the viscosity of the lubricant and suppress the torque increase. However, due to the decrease in viscosity, the lubricant may leak to the outside of the film to be coated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic configuration diagram of an image forming apparatus according to a first embodiment. 
         FIG.  2    is a hardware configuration diagram of an image forming apparatus according to the first embodiment. 
         FIG.  3    is a front sectional view of a fixing unit according to the first embodiment. 
         FIG.  4    is a front sectional view of a heater unit of the fixing unit according to the first embodiment. 
         FIG.  5    is a bottom view of the heater unit. 
         FIG.  6    is a top view of a heater thermometer and thermostat according to the first embodiment. 
         FIG.  7    is an electric circuit diagram of the fixing unit according to the first embodiment. 
         FIG.  8    is a flowchart illustrating processing executed by a controller in a period from the start of the pre-processing period to the execution of the post-processing in the first embodiment. 
         FIG.  9    is a flowchart illustrating processing for determining a pre-processing energization method by the controller according to the first embodiment. 
         FIG.  10    is a diagram showing a relationship between torque and temperature of the heating element set  45  in the image forming apparatus  100  according to the first embodiment. 
         FIG.  11    is a diagram illustrating a hardware configuration of an image forming apparatus according to a second embodiment. 
         FIG.  12    is a flowchart illustrating processing executed by a controller of the image forming apparatus in the period from the start of the pre-processing period to the execution of the post-processing in the second embodiment. 
         FIG.  13    is a diagram illustrating an ambient thermometer in a modification example. 
         FIG.  14    is a diagram showing an angle θ formed in the modification example. 
         FIG.  15    is a diagram showing a relationship between temperature measured by a film thermometer and temperature of the heating element set for each angle θ in the modification example. 
     
    
    
     DETAILED DESCRIPTION 
     An image forming apparatus according to an embodiment includes a fixing unit including a fixing belt having a surface to which a lubricant is applied, a heater contacting the surface of the fixing belt through the lubricant and including a plurality of heating elements arranged along a width direction of the fixing belt, the plurality of heating elements including at least a first end heating element, a central heating element, and a second end heating element, and a pressing roller capable of pressing and rotating the fixing belt, a power supply configured to supply electric power to the plurality of heating elements, and a controller configured to determine a first amount of electric power to be supplied to the central heating element and a second amount of electric power to be supplied to each of the first and second end heating elements such that the first amount of electric power is greater than the second amount of electric power, and control the power supply to supply the determined first and second amounts of electric power to the central heating element and each of the first and second end heating elements before controlling the pressing roller to rotate. 
     Hereinafter, an image forming apparatus and an image forming method according to an embodiment will be described with reference to the drawings. 
     First Embodiment 
       FIG.  1    is a schematic configuration diagram of an image forming apparatus according to a first embodiment. The image forming apparatus  100  according to the first embodiment is, for example, a multi-functional peripheral. The image forming apparatus  100  includes a housing  10 , a display  1 , a scanner unit  2 , an image forming unit  3 , a sheet supply unit  4 , a forcing unit  5 , a paper discharge tray  7 , a reversing unit  9 , a control panel  8 , and a controller  6 . The image forming unit  3  may be an apparatus for fixing a toner image or an ink jet type apparatus. The image forming apparatus  100  forms an image on a sheet S by using a developer such as toner or the like. The sheet may be, for example, printing paper or label paper. The sheet may be any material on which an image can be formed by the image forming apparatus  100 . 
     The housing  10  forms an outer shape of image forming apparatus  100 . The display  1  is an image display device such as a liquid crystal display, an organic EL (Electro Luminescence) display, or the like. The display  1  displays various information relating to the image forming apparatus  100 . The scanner unit  2  reads the image information from a sheet as the light and dark of the light. The scanner unit records the image information that has been read. The scanner unit  2  outputs the generated image information to the image forming unit  3 . The recorded image information may be transmitted to another information processing apparatus via a network. 
     The image forming unit  3  forms an output image (hereinafter referred to as a toner image) by a recording agent such as toner on the basis of the image information received from the scanner unit  2  or another external device. The image forming unit  3  transfers the toner image onto the surface of the sheet S. The image forming unit  3  heats and pressurizes the toner image on the surface of the sheet S to fix the toner image to the sheet S. The details of the image forming unit  3  will be described later. The sheet S may be supplied by the sheet supply unit  4 , or may be supplied manually by a user. 
     The sheet supply unit  4  supplies the sheet S one by one to the pressing unit  5  in accordance with the timing at which the image forming unit  3  forms the toner image  1 . The sheet supply unit  4  includes a sheet storage unit  20  and a pickup roller  21 . The sheet storage unit  20  accommodates a sheet S of a predetermined size and type. The pickup roller  21  takes out the sheets S from the sheet storage unit  20 . The pickup roller  21  supplies the taken-out sheet S to the conveying unit  5 . 
     The conveying unit  5  conveys the sheet S supplied from the sheet supply unit  4  to the image forming unit  3 . The conveying unit  5  includes a conveying roller  23  and a registration roller  24 . The conveying roller  23  conveys the sheet S supplied from the pickup roller  21  to the registration roller  24 . The conveying roller  23  presses the leading end of the sheet S in the conveying direction against the nip N of the registration roller  24 . The registration roller  24  bends the sheet S in the nip N to thereby adjust the position of the leading edge of the sheet S in the conveying direction. The registration roller  24  conveys the sheet S in accordance with the timing at which the image forming unit  3  transfers the toner image to the sheet S. 
     The details of the image forming unit  3  will be described below. The image forming unit  3  includes a plurality of image forming units  25 , a laser scanning unit  26 , an intermediate transfer belt  27 , a transfer unit  28 , and a fixing unit (or a heating device)  30 . Each of the image forming units  25  includes a photosensitive drum  25   d . Each of the image forming units  25  forms a toner image corresponding to the image information from the scanner unit  2  or from an external device on the photosensitive drum  25   d . The plurality of image forming units include image forming units  25 Y,  25 M,  25 C and  25 K, which form toner images of yellow, magenta, cyan and black toners, respectively. 
     A charger, a developing device, and the like are disposed around the photosensitive drum  25   d  of each of the image forming units  25 Y,  25 M,  25 C, and  25 K. The charging device charges the surface of the photosensitive drum  25   d . The developing device of each of the image forming units  25 Y,  25 M,  25 C, and  25 K contains developer containing one of yellow, magenta, cyan and black toners. The developing device develops the electrostatic latent image on the photosensitive drum  25   d . As a result, a toner image formed by the toner of each color is formed on the corresponding photosensitive drum  25   d.    
     The laser scanning unit  26  scans the charged photosensitive drum  25   d  with the laser beam L to expose the photosensitive drum  25   d . The laser scanning unit  26  exposes the photosensitive drums  25   d  of the image forming units  25 Y,  25 M,  25 C and  25 K of the respective colors with the respective laser beams LY, LM, LC and LK. In this manner, the laser scanning unit  26  forms an electrostatic latent image on the photosensitive drum  25   d.    
     The toner image on the surface of the photosensitive drum  25   d  is primarily transferred onto the intermediate transfer belt  27 . The transfer portion  28  transfers the toner image primarily transferred onto the intermediate transfer belt  27  onto the surface of the sheet S at the secondarily transfer position. The fixing unit  30  heats and pressurizes the toner image transferred to the sheet S to fix the toner image on the sheet S. The details of the fixing unit  30  will be described later. 
     The reversing unit  9  reverses the sheet S to form an image on the back surface of the sheet S. The reversing unit  9  reverses the sheet S discharged from the fixing unit  30  by switch-back. The reversing unit  9  conveys the reversed sheet S toward the registration roller  24 . The sheet discharge tray  7  supports the sheet S that has been ejected with an image formed thereon. The control panel  8  comprises a plurality of buttons. The control panel  8  accepts the operation of the user. The control panel  8  outputs a signal corresponding to the operation performed by the user to the controller  6  of the image forming apparatus  100 . The display  1  and control panel  8  may be integrated into a single touch panel. The controller  6  controls each of the components installed in the image forming apparatus  100 . The details of the controller  6  will be described later. 
       FIG.  2    is a hardware configuration diagram of the image forming apparatus  100  according to the first embodiment. The image forming apparatus  100  includes a CPU (Central Processing Unit)  91 , a memory  92 , and an auxiliary storage device  93  connected to each other via a bus, and executes programs. As described above, the image forming apparatus  100  includes the scanner unit  2 , the image forming unit  3 , the sheet supply unit  4 , the forcing unit  5 , the reversing unit  9 , the control panel  8 , and a communication unit  90 . 
     The CPU  91  is a component of the controller  6  and executes programs stored in the memory  92  and the auxiliary storage device  93  to control the operation of each component of the image forming apparatus  100 . The auxiliary storage device  93  is a storage device such as a magnetic hard disk device or a semiconductor storage device. The auxiliary storage  93  stores various kinds of information related to the image forming apparatus  100 . The communication unit  90  includes a communication interface for communicating with an external device. 
     The fixing unit  30  will be described in detail.  FIG.  3    is a front sectional view of the fixing unit  30  according to the first embodiment. The fixing unit  30  includes a pressing roller  30   p  and a film unit  30   h.    
     The pressing roller  30   p  forms a nip N with the film unit  30   h . The pressing roller  30   p  pressurizes the toner image on the sheet S that has entered into the nip N. The pressing roller  30   p  rotates and conveys the sheet S. The pressing roller  30   p  includes a core metal  32 , an elastic layer  33 , and a release layer (not shown). In this way, the pressing roller  30   p  can press and drive rotatably the surface of a cylindrical film  35 . 
     The core metal  32  is formed in a cylindrical shape by a metal material such as stainless steel or the like. Both end portions in the axial direction of the core metal  32  are supported to be rotatable. The core metal  32  is driven to rotate by a motor (not shown). The core metal  32  comes into contact with a cam member (not shown). The cam member is rotated to move the core metal  32  toward and away from the film unit  30   h.    
     The elastic layer  33  is formed of an elastic material such as silicone rubber. The elastic layer  33  is formed to have a constant thickness on the outer peripheral surface of the core metal  32 . The release layer (not shown) is formed of a resin material such as PFA (tetrafluoroethylene perfluoroalkyl vinyl ether copolymer). The release layer is formed on the outer peripheral surface of the elastic layer  33 . It is preferable that the hardness of the outer circumferential surface of the pressing roller  30   p  is between 40° and 70° under a load of 9.8N by an ASKER-C hardness meter. As a result, the area of the nip N and the durability of the pressing roller  30   p  are secured. 
     The pressing roller  30   p  can be moved toward and away from the film unit  30   h  by the rotation of the cam member. When the pressing roller  30   p  is brought close to the film unit  30   h  and pressed by a pressing spring, a nip N is formed. On the other hand, when the sheet S is jammed in the fixing unit  30 , the sheet S can be removed by separating the pressing roller  30   p  from the film unit  30   h . In addition, in a state in which the cylindrical film  35  is stopped to rotate, such as in a sleep state, the pressing roller  30   p  is moved away from the film unit  30   h , thereby preventing plastic deformation of the cylindrical film  35 . 
     The pressing roller  30   p  is rotated by a motor. When the pressing roller  30   p  rotates in a state where the nip N is formed, the cylindrical film  35  of the film unit  30   h  is driven to rotate. The pressing roller  30   p  conveys the sheet S in the conveying direction W by rotating the sheet S in a state in which the sheet S is placed in the nip N. 
     The film unit  30   h  heats the toner image of the sheet S that has entered the nip N. The film unit  30   h  includes the cylindrical film  35 , a heater unit  40  (more generally referred to herein as a heater), a heat conductor  49 , a support member  36 , a stay  38 , a heater thermometer  62 , a thermostat  68 , and a film thermometer  64 . 
     The cylindrical film  35  is formed in a cylindrical shape. The cylindrical film  35  includes a base layer, an elastic layer, and a release layer in this order from the inner peripheral side. The base layer is formed in a cylindrical shape by a material such as nickel (Ni) or the like. The elastic layer is laminated and arranged on the outer peripheral surface of the base layer. The elastic layer is formed of an elastic material such as silicone rubber. The release layer is laminated and arranged on the outer peripheral surface of the elastic layer. The release layer is formed of a material such as a PFA resin. 
       FIG.  4    is a front sectional view of the heater unit  40  taken along the line IV-IV in  FIG.  5   .  FIG.  5    is a bottom view of the heater unit  40  (i.e., viewed from the +z direction). The heater unit  40  includes a substrate  41 , a heating element set  45 , and a ring set  55 . 
     The substrate  41  is made of a metal material such as stainless steel, a ceramic material such as aluminum nitride, or the like. The substrate  41  is formed in an elongated rectangular plate shape. The substrate  41  is disposed radially inward of the cylindrical film  35 . In the substrate  41 , the longitudinal direction corresponds to the axial direction of the cylindrical film  35 . 
     In the present application, the x direction, the y direction, and the z direction are defined as follows. The y direction is the longitudinal direction of the substrate  41 . The y direction is parallel to the width direction of cylindrical film  35 . As will be described later, the +y direction is a direction from the central heating element  45   a  toward the first end heating element  45   b   1 . The x direction is the short direction of substrate  41 , and the +x direction is the transport direction (i.e., downstream side) of the sheet S. The z direction is the normal direction of the substrate  41 , and the +z direction is the direction in which the heating element set  45  is arranged with respect to the substrate  41 . An insulating layer  43  is formed on the surface of the substrate  41  in the +z direction by a glass material or the like. 
     The heating element set  45  is arranged on the substrate  41 . The heating element set  45  is formed on the surface of the insulating layer  43  in the +z direction, as shown in  FIG.  4   . The heating element set  45  is formed of a silver-palladium alloy or the like. The heating element set  45  has a rectangular shape in which the y direction is the longitudinal direction and the x direction is the short direction. 
     As shown in  FIG.  5   , the heating element set  45  includes a first end heating element  45   b   1 , a central heating element  45   a , and a second end heating element  45   b   2  arranged side by side in the y direction. The central heating element  45   a  is disposed in the central portion of the heating element set  45  in the y direction. The central heating element  45   a  may be formed by combining a plurality of small heating elements arranged side by side in the y direction. The first end heating element  45   b   1  is located on the +y direction side of the central heating element  45   a , and is positioned at the end of the heating element set  45  in the +y direction. The second end heating element  45   b   2  is located in the −y direction of the central heating element  45   a  and at the end of heating element set  45  in the −y direction. The boundary line between the central heating element  45   a  and the first end heating element  45   b   1  may be arranged parallel to the x direction, or may be arranged to intersect the x direction. The same applies to the boundary line between the central heating element  45   a  and the second end heating element  45   b   2 . 
     The heating element set  45  generates heat by energization. The electrical resistance value of the central heating element  45   a  is smaller than the electrical resistance value of the first end heating element  45   b   1  and the second end heating element  45   b   2 . 
     The sheet S having a small width in the y direction passes through the center portion in the y direction of the fixing unit  30 . In this case, the controller  6  causes only the central heating element  45   a  to generate heat. On the other hand, in the case of the sheet S having a large width in the y direction, the controller  6  generates heat in the entirety of the heating element set  45 . Therefore, the central heating element  45   a  and the first end heating element  45   b   1  and the second end heating element  45   b   2  are controlled in heat generation independently of each other. Also, the heat generation is controlled in the first end heating element  45   b   1  and the second end heating element  45   b   2 . 
     The wiring set  55  is made of a metal material such as silver. The wiring set  55  includes a central contact  52   a , a central portion wiring  53   a , an end contact  52   b , a first end wiring  53   b   1 , a second end wiring  53   b   2 , a common contact  58 , and a common ring  57 . 
     The central contact  52   a  is arranged on the −y direction side of the heating element set  45 . The central portion wiring  53   a  is arranged on the +x direction side of the heating element set  45 . The central portion wiring  53   a  connects the side in the +x direction of the central heating element  45   a  and the center portion contact  52   a.    
     The end contact  52   b  is arranged on the −y direction side of the center contact  52   a . The first end wiring  53   b   1  extends along the side in the +x direction of the heating element set  45  and on the +x direction side of the central portion wiring  53   a . The first end wiring  53   b   1  connects the end of the first end heating element  45   b   1  in the +x direction and the end of the end contact  52   b  in the +x direction. The second end wiring  53   b   2  extends along the side in the +x direction of the heating element set  45  and on the −x direction side of the central portion wiring  53   a . The second end wiring  53   b   2  connects the end of the second end heating element  45   b   2  in the +x direction and the end of the end contact  52   b  in the −x direction. 
     The common contact  58  is arranged at the end in the +y direction of the heating element set  45 . The common wiring  57  extends along the side in the −x direction of the heating element set  45 . The common ring  57  connects the end sides in the −x direction of the central heating element  45   a , the first end heating element  45   b   1 , and the second end heating element  45   b   2 , and the common contact  58 . 
     In this manner, the second end wiring  53   b   2 , the central portion wiring  53   a  and the first end portion wiring  53   b   1  extend along the side in the +x direction of the heating element set  45 . In contrast, only the common wiring  57  extends along the side in the −x direction of the heating element set  45 . Therefore, the center  45   c  in the x direction of the heating element set  45  is arranged on the −x direction side with respect to the center  41   c  in the x direction of the substrate  41 . 
     As shown in  FIG.  3   , a straight line CL connecting the center pc of the pressing roller  30   p  and the center hc of the film unit  30   h  is defined. The center  41   c  in the x direction of the substrate  41  is arranged in the +x direction from the straight line CL. Thus, the substrate  41  extends in the +x direction of the nip N, so that the sheet S that has passed through the nip N is easily peeled off from the film unit  30   h.    
     The center  45   c  of the heating element set  45  in the x direction is disposed on the straight line CL. The heating element set  45  is contained entirely within the region of the nip N and is located at the center of the nip N. Thus, the heat distribution of the nip N becomes uniform, and the sheet S passing through the nip N is uniformly heated. 
     As shown in  FIG.  4   , a heating element set  45  and a ring set  55  are formed on the surface of the insulating layer  43  in the +z direction. A protective layer  46  is formed of a glass material or the like so as to cover the heating element set  45  and the ring set  55 . The protective layer  46  improves the sliding property between the heater unit  40  and the cylindrical film  35 . 
     As shown in  FIG.  3   , the heater unit  40  is disposed inside the cylindrical film  35 . A lubricant (not shown) is applied to the inner peripheral surface of the cylindrical film  35 . The heater unit  40  is in contact with the inner peripheral surface of the cylindrical film  35  through the lubricant. When the heater unit  40  generates heat, the viscosity of the lubricant decreases. Thus, the sliding property between the heater unit  40  and the cylindrical film  35  is secured. 
     In this manner, the cylindrical film  35  is a band-shape thin film which slides on the surface of the heater unit  40  while making contact with the heater unit  40  on one side. 
     The heat conductor  49  is formed of a metal material having a high thermal conductivity, such as copper. The outer shape of the heat conductor  49  is equivalent to the outer shape of the substrate  41  of the heater unit  40 . The heat conductor  49  is disposed in contact with the surface of the heater unit  40  in the −z direction. 
     The support member  36  is made of a resin material such as a liquid crystal polymer. The support member  36  is disposed so as to cover the side in the −z direction of the heater unit  40  and the both sides in the x direction of the heater unit  40 . The support member  36  supports the heater unit  40  via a heat conductor  49 . Rounded chamfering is formed at both end portions in the x direction of the support member  36 . The support member  36  supports the inner peripheral surface of the cylindrical film  35  at both end portions in the x direction of the heater unit  40 . 
     When the sheet S passing through the fixing unit  30  is heated, a temperature distribution is generated in the heater unit  40  in accordance with the size of the sheet S. When the heater unit  40  becomes locally high temperature, there is a possibility that the heat resistance temperature of the support member  36  made of a resin material exceeds the heat resistance temperature. The heat conductor  49  averages the temperature distribution of the heater unit  40 . As a result, heat resistance of the support member  36  is ensured. 
     The stay  38  is formed of a steel sheet material or the like. A cross section perpendicular to the y direction of the stay  38  is formed in a U shape. The stay  38  is mounted on the surface in the −z direction of the support member  36  so as to block the opening of the U shape by the support member  36 . The stay  38  extends in the y direction. Both ends of the stay  38  in the y direction are fixed to the housing of the image forming apparatus  100 . As a result, the film unit  30   h  is supported by the image forming apparatus  100 . The stay  38  improves the bending rigidity of the film unit  30   h . A flange (not shown) for restricting the movement of the cylindrical film  35  in the y direction is mounted in the vicinity of both end portions in the y direction of the stay  38 . 
     The heater thermometer  62  is arranged in the −z direction of the heater unit  40  with the heat conductor  49  interposed therebetween. For example, the heater thermometer is mounted on and supported by the surface in the −z direction of the support member  36 . The temperature sensitive element of the heater thermometer  62  contacts the heat conductor  49  through a hole passing through the support member  36  in the z direction. The heater thermometer  62  measures the temperature of the heater unit  40  via the heat conductor  49 . 
     The thermostat  68  is arranged similarly to the heater thermometer  62 . The thermostat  68  is incorporated into an electrical circuit, which will be described later. When the temperature of the heater unit  40  detected through the heat conductor  49  exceeds a predetermined temperature, the thermostat  68  cuts off the power supply to the heating element set  45 . 
       FIG.  6    is a top view of the heater thermometer and thermostat (i.e., viewed from the −z direction). In  FIG.  6   , the description of the supporting member  36  is omitted. The following description of the arrangement of the heater thermometer, thermostat and film thermometer is used to describe the arrangement of the respective temperature sensitive elements. 
     A plurality of heater thermometers  62  ( 62   a ,  62   b ) are arranged in the heating element set  45  side by side in the y direction. The plurality of heater thermometers  62  are disposed in the center of the heating element set  45  in the x direction. That is, when viewed from the z direction, the plurality of heater thermometers  62  and the heating element set  45  overlap at least partially. The plurality of thermostats  68  ( 68   a ,  68   b ) are also arranged in the same manner as the plurality of heater thermometers  62  described above. 
     The plurality of heater thermometers  62  includes a center heater thermometer  62   a  and an end heater thermometer  62   b.    
     The center heater thermometer  62   a  measures the temperature of the central heating element  45   a . The center heater thermometer  62   a  is positioned within the central heating element  45   a . That is, when viewed from the z direction, the center heater thermometer  62   a  and the central heating element  45   a  overlap each other. 
     The end heater thermometer  62   b  measures the temperature of the second end heating element  45   b   2 . As described above, the first end heating element  45   b   1  and the second end heating element  45   b   2  are similarly controlled in heat generation. Therefore, the temperature of the first end heating element  45   b   1  and the temperature of the second end heating element  45   b   2  are equal to each other. The end heater thermometer  62   b  is located within a range of second end heating element  45   b   2 . That is, the end heater thermometer  62   b  and the second end heating element  45   b   2  overlap each other when viewed from the direction z. 
     The plurality of thermostats  68  include a central thermostat  68   a  and an end thermostat  68   b.    
     The central thermostat  68   a  shuts off energization to the heating element set  45  when the temperature of the central heating element  45   a  exceeds a predetermined temperature. The central thermostat  68   a  is located within the central heating element  45   a . That is, when viewed from the z direction, the central thermostat  68   a  and the central heating element  45   a  overlap each other. 
     The end thermostat  68   b  cuts off energization to the heating element set  45  when the temperature of the first end heating element  45   b   1  exceeds a predetermined temperature. As described above, the first end heating element  45   b   1  and the second end heating element  45   b   2  are similarly controlled in heat generation. Therefore, the temperature of the first end heating element  45   b   1  and the temperature of the second end heating element  45   b   2  are equal to each other. The end thermostat  68   b  is located within the first end heating element  45   b   1 . That is, when viewed from the z direction, the end thermostat  68   b  and the first end heating element  45   b   1  overlap each other. 
     As described above, the center heater thermometer  62   a  and the central thermostat  68   a  are disposed within the central heating element  45   a  so as to measure the temperature of central heating element  45   a . When the temperature of the central heating element  45   a  exceeds the predetermined temperature, the power supply to the heating element set  45  is interrupted. In addition, the end heater thermometer  62   b  and the end thermostat  68   b  are disposed within the first end heating element  45   b   1  and the second end heating element  45   b   2 . As a result, the temperature of the first end heating element  45   b   1  and the second end heating element  45   b   2  is measured. When the temperature of the first end heating element  45   b   1  and the second end heating element  45   b   2  exceeds the predetermined temperature, the power supply to the heating element set  45  is interrupted. 
     The plurality of heaters  62  and the plurality of thermostats  68  are alternately arranged along the y direction. As described above, the first end heating element  45   b   1  is disposed on the +y direction side of the central heating element  45   a . Within the first end heating element  45   b   1 , the end thermostat  68   b  is located. The center heater thermometer  62   a  is arranged on the +y direction side with respect to the center in the y direction of the central heating element  45   a . The central thermostat  68   a  is arranged on the −y direction side with respect to the center of the central heating element  45   a . As described above, the second end heating element  45   b   2  is disposed on the −y direction side of the central heating element  45   a . Within the second end heating element  45   b   2 , the end heater thermometer  62   b  is located. Thus, the end thermostat  68   b , the center heater thermometer  62   a , the central thermostat  68   a , and the end heater thermometer  62   b  are arranged in this order in the −y direction. 
     Generally, the thermostat  68  utilizes a bimetal curved deformation that is accompanied by a temperature change to connect and disconnect electrical circuits. The thermostat is formed to be elongated in conformity to the shape of the bimetal. Terminals extend outward from both end portions in the longitudinal direction of the thermostat  68 . Each terminal is connected to a connector of external wiring. Therefore, it is necessary to secure a space outside the thermostat  68  in the longitudinal direction. Since there is no space at both ends in the x direction of the fixing unit  30 , the longitudinal direction of the thermostat  68  is arranged along the y direction. In this case, when a plurality of thermostats  68  are arranged adjacent to each other in the y direction, it becomes difficult to secure a connection space of the external wiring. 
     As described above, the plurality of heaters  62  and the plurality of thermostats  68  are alternately arranged along the y direction. Thus, a heater thermometer  62  is disposed adjacent to each thermostat  68  in the y direction. Therefore, it is possible to secure a space for connecting external wiring to the thermostat  68 . In addition, the degree of freedom in the layout in the y direction of the thermostat  68  and the heater thermometer  62  is increased. Thereby, the thermostat  68  and the heater thermometer  62  are arranged at the optimum position to control the temperature of the fixing unit  30 . Further, it is easy to separate the alternating current wiring connected to the plurality of thermostats  68  from the direct current wiring connected to the plurality of heater thermometers  62 . As a result, noise in the electric circuit is suppressed. 
     As shown in  FIG.  3   , the film thermometer  64  is disposed inside the cylindrical film  35  and on the +x direction side of the heater unit  40 . The film thermometer  64  contacts the inner peripheral surface of the cylindrical film  35  to measure the temperature of the cylindrical film  35 . 
       FIG.  7    is an electric circuit diagram of the heating unit according to the first embodiment. In  FIG.  7   , the bottom view of the heater unit  40  shown in  FIG.  5    is located at the top of  FIG.  7   , and the plan view of the substrate  41  shown in  FIG.  6    is arranged at the bottom of  FIG.  7   .  FIG.  7    also shows a plurality of film thermometers  64  along with a cross section of the cylindrical film  35 . 
     The plurality of film thermometers  64  includes a central film thermometer  64   a  and an end film thermometer  64   b.    
     The central film thermometer  64   a  comes into contact with the center portion of the cylindrical film  35  in the y direction. The central film thermometer  64   a  contacts the cylindrical film  35  within the range in the y direction of the central heating element  45   a . The central film thermometer  64   a  measures the temperature of the central portion in the y direction of the cylindrical film  35 . 
     The end film thermometer  64   b  contacts the end of cylindrical film  35  in the −y direction. The end film thermometer  64   b  contacts the cylindrical film  35  within the range in the y-direction of the second end heating element  45   b   2 . The end film thermometer  64   b  measures the temperature at the end in the −y direction of the cylindrical film  35 . As described above, the first end heating element  45   b   1  and the second end heating element  45   b   2  are similarly controlled in heat generation. Therefore, the temperature at the end portion in the −y direction of the cylindrical film  35  and the temperature at the end portion in the +y direction are identical. 
     A power supply  95  is electrically connected to the center contact point  52   a  via a central triac  96   a . The power supply  95  is electrically connected to the end contact  52   b  via an end triac  96   b . The controller  6  controls ON/OFF of the central triac  96   a  and the end triac  96   b  independently of each other. When the controller  6  turns on the central triac  96   a , the power is supplied from the power supply  95  to the central heating element  45   a . As a result, the central heating element  45   a  generate heat. When the controller  6  turns on the end triac  96   b , the power is supplied from the power supply  95  to the first end heating element  45   b   1  and the second end heating element  45   b   2 . Thus, the first end heating element  45   b   1  and the second end heating element  45   b   2  generate heat. As described above, the central heating element  45   a  and the first end heating element  45   b   1  and the second end heating element  45   b   2  are independently controlled in heat generation. The central heating element  45   a , the first end heating element  45   b   1  and the second end heating element  45   b   2  are connected in parallel with respect to the power supply  95 . 
     The power supply  95  is electrically connected to the common contact  58  via the central thermostat  68   a  and the end thermostat  68   b . The central thermostat  68   a  and the end thermostat  68   b  are connected in series. When the temperature of the central heating element  45   a  rises abnormally, the detected temperature of the central thermostat  68   a  exceeds the predetermined temperature. At this time, the central thermostat  68   a  blocks the power supply from the power supply  95  to the entire heating element set  45 . 
     When the temperature of the first end heating element  45   b   1  rises abnormally, the detected temperature of the end thermostat  68   b  exceeds a predetermined temperature. At this time, the end thermostat  68   b  blocks the power supply from the power supply  95  to the heating element set  45 . As described above, the first end heating element  45   b   1  and the second end heating element  45   b   2  are similarly controlled in heat generation. Therefore, when the temperature of the second end heating element  45   b   2  rises abnormally, the temperature of the first end heating element  45   b   1  also increases. Therefore, even when the temperature of the second end heating element  45   b   2  rises abnormally, the end thermostat  68   b  shuts off power supply from the power supply  95  to the entire heating element set  45 . 
     The controller  6  measures the temperature of the central heating element  62   a  by the center heater thermometer  45   a . The controller  6  measures the temperature of the second end heating element  45   b   2  by the end heater thermometer  62   b . The temperature of the second end heating element  45   b   2  is equal to the temperature of the first end heating element  45   b   1 . The controller  6  measures the temperature of the heating element set  45  by the heater thermometer  62  at the time of starting the fixing unit  30 . When the temperature of at least one of the central heating element  45   a  and the second end heating element  45   b   2  is lower than a predetermined temperature, the controller  6  generates heat for a short period of time in the heating element set  45 . Thereafter, the controller  6  starts the rotation of the pressing roller  30   p . The heat generated by the heating element set  45  lowers the viscosity of the lubricant applied to the inner peripheral surface of the cylindrical film  35 . Thus, the sliding property between the heater unit  40  and the cylindrical film  35  at the start of the rotation of the pressing roller  30   p  is ensured. 
     The controller  6  measures the temperature of the central portion of the cylindrical film  35  in the y direction by using the central film thermometer  64   a . The controller  6  measures the temperature of the end portion of the cylindrical film  35  in the −y direction by the end film thermometer  64   b . The temperature of the end of the cylindrical film  35  in the −y direction is equal to the temperature of the end of the cylindrical film  35  in the +y direction. The controller  6  measures the temperature of the center portion and the end portion in the y direction of the cylindrical film  35  during the operation of the fixing unit  30 . The controller  6  performs phase control or wave number control on the power supplied to the heating element set  45  by the central triac  96   a  and the end triac  96   b . The controller  6  controls the energization to the central heating element  45   a  based on the temperature measurement result at the center portion in the y direction of the cylindrical film  35 . The controller  6  controls the energization of the first end heating element  45   b   1  and the second end heating element  45   b   2  based on the temperature measurement result at the end portion in the y direction of the cylindrical film  35 . 
     When a pre-processing execution condition is satisfied during a pre-processing period, the controller  6  determines a method of energization to the heating element set  45  in the pre-processing period based on the temperature measured by the heater thermometer  62  and the film thermometer  64 . Hereinafter, the method in which the heating element set  45  is energized in the pre-processing period is referred to as the pre-processing energization method. The energization of the heating element set  45  means that the central heating element  45   a , the first end heating element  45   b   1 , and the second end heating element  45   b   2  are energized. The pre-processing period is a period from the time when a pre-processing start condition is satisfied to the time when the pre-processing end condition is satisfied. The pre-processing start condition may be any condition, for example, a condition that the image forming apparatus  100  has acquired image information. The preprocessing start condition may be, for example, a condition that an instruction to start the pre-processing is input by the user via the control panel  8  or the communication unit  90 . The pre-processing end condition may be any condition, for example, a condition in which all of the temperatures measured by the heater thermometers  62  are equal to or more than a predetermined temperature (hereinafter referred to as a “first pre-processing end condition”). That is, it may be a condition that the lowest temperature among the temperatures measured by a plurality of heaters  62  is equal to or higher than a predetermined temperature. The pre-processing end condition may be, for example, a condition in which a predetermined time elapses after the energization by the pre-processing energization method is started. The pre-processing end condition may be, for example, a condition that the energization of the controller  6  is terminated by the pre-processing energization method. 
     The pre-processing energization method may be any energization method as long as it satisfies the condition that the first end heating element  45   b   1  and the second end heating element  45   b   2  are energized at an end duty ratio lower than a central duty ratio. The central duty ratio is the duty ratio of the power supplied to the central heating element  45   a . The end duty ratio is a duty ratio of electric power supplied to the first end heating element  45   b   1  and the second end heating element  45   b   2 . The first duty ratio and the second duty ratio may be any duty ratio as long as the second duty ratio is lower than the first duty ratio. For example, the first duty ratio is 50%, and the second duty ratio is 40%. 
     The controller  6  controls the central triac  96   a  and the end triac  96   b  so that the heating element set  45  is energized by the determined pre-processing energization method (hereinafter called “pre-processing”). 
     The pre-processing execution condition may be any condition as long as it includes a condition that at least one of a pre-processing heater condition and a pre-processing film condition is satisfied. For example, the pre-processing heater condition is a condition that at least one of the temperatures measured by a plurality of heater thermometer is lower than a first heater temperature (hereinafter referred to as “the first pre-processing heater condition”). For example, the pre-processing film condition is a condition that at least one of the temperatures measured by the plurality of film thermometers  64  is lower than the film temperature (hereinafter, referred to as “the first film condition”). The first heater temperature may be, for example, 40° C. The film temperature may be, for example, 40° C. 
     In order to simplify the description, it is assumed that the pre-processing end condition is a condition that the energization by the pre-processing energization method is terminated. 
     After the end of the pre-processing period, the controller  6  rotates the pressing roller  30   p . After the end of the pre-processing period, the controller  6  controls the energization of the heating element set  45  based on the temperature measured by the heater thermometer  62  and the film thermometer  64 . Hereinafter, the energization method of the heating element set  45  controlled by the controller  6  after the end of the pre-processing period is referred to as “the post-processing energization method”, and the process of executing the post-processing energization method is referred to as “the post-processing”. Hereinafter, a period from the end of the pre-processing period to the end of the execution of the post-processing is referred to as a post-processing period. In the post-processing, the controller  6  controls the central triac  96   a  and the end triac  96   b  on the basis of the temperature measured by the film thermometer  64 . The controller  6  controls the central triac  96   a  and the end triac  96   b  so that the temperature measured by the film thermometer  64  is maintained at a predetermined temperature. 
     In the following description of  FIGS.  8  and  9   , it is assumed that the pre-processing execution condition is satisfied under the condition that at least one of the first pre-processing heater condition and the first pre-processing film condition is satisfied for the sake of simplicity. 
       FIG.  8    is a flowchart showing processing executed by the controller  6  during a period from the start of the pre-processing period to the execution of the post-processing in the first embodiment. 
     The controller  6  determines whether or not the pre-processing period has been started (ACT  101 ). Specifically, the controller  6  determines whether or not the pre-processing start condition is satisfied. When the pre-processing period is started (ACT  101 , YES), the controller  6  determines whether or not to energize the heating element set  45  in the pre-processing period (ACT  102 ). Specifically, the controller  6  determines whether or not the pre-processing execution condition is satisfied. 
     When the pre-processing execution condition is satisfied in ACT  102  (ACT  102 , YES), the controller  6  determines the pre-processing energization method based on the temperature measured by the heater thermometer  62  and the film thermometer  64  (ACT  103 ). 
     Next to ACT  103 , the controller  6  controls the central triac  96   a  and the end triac  96   b  so that the current is supplied to the heating element set  45  through the pre-processing energization method determined in ACT  103  (ACT  104 ). 
     The controller  6  determines whether or not the pre-processing period has been completed (ACT  105 ). More specifically, the controller  6  determines whether or not the pre-processing end condition is satisfied. When the pre-processing end condition is satisfied (ACT  105 , YES), the controller  6  starts the execution of the post-processing (ACT  106 ). 
     On the other hand, in the process of ACT  105 , when the pre-processing end condition is not satisfied (ACT  105 , NO), the process returns to ACT  105 . 
     On the other hand, in the ACT  102 , when the pre-processing execution condition is not satisfied (ACT  102 , NO), the controller  6  executes the process of ACT  106 . 
     On the other hand, in the process of ACT  101 , when the pre-processing period is not started (ACT  101 , NO), the process returns to ACT  101 . 
       FIG.  9    is a flowchart showing processing for determining the pre-processing energization method by the controller  6  according to the first embodiment. 
     The controller  6  determines whether or not at least one of the temperatures measured by the film thermometer  64  is lower than the film temperature (ACT  201 ). When at least one of the temperatures measured by the film thermometer  64  is lower than the film temperature (ACT  201 , YES), the controller  6  determines whether or not at least one of the temperatures measured by the heater thermometer  62  is lower than a second heater temperature (ACT  202 ). The second heater temperature is lower than the first heater temperature. The second heater temperature may be, for example, ° C. when the first heater temperature is 40° C. When at least one of the temperatures measured by the heater thermometer  62  is lower than the second heater temperature (ACT  202 , YES), the controller  6  determines, as the pre-processing energization method, a first energization method (ACT  203 ). The first energization method is an energization method in which the central duty ratio is a first duty ratio, and the period during which the current is supplied is a first period. For example, the first duty ratio is 70%, and the first period in which the current is supplied is 0.5 ms. In the first energization method, the end duty ratio is lower than the first duty ratio. 
     On the other hand, when all of the temperatures measured by the heater thermometer  62  are equal to or higher than the second heater temperature (ACT  202 , NO), the controller  6  determines whether or not all of the temperatures measured by the heater thermometer  62  are within a first heater temperature range (ACT  204 ). The first heater temperature range is in a range of a temperature equal to or higher than the second heater temperature and lower than a third heater temperature. 
     When all of the temperatures measured by the heater thermometer  62  are within the first heater temperature range (ACT  204 , YES), the controller  6  determines, as the pre-processing energization method, a second energization method (ACT  205 ). The second energization method is an energization method in which the central duty ratio is a second duty ratio, and the period during which the current is supplied is a second period. The electric power supplied to the heating element set  45  by the second energization method during the pre-processing period is less than the electric power supplied to the heating element set  45  by the first energization method during the pre-processing period. For example, the power supplied to the heating element set  45  by the second energization method during the pre-processing period may be 5/7 of the power supplied to the heating element set  45  by the first energization method during the pre-processing period. When the first duty ratio is 70% and the first time period is 0.5 ms, for example, the second duty ratio is 50%, and the second period is 0.5 ms. In the second energization method, the end duty ratio is lower than the second duty ratio. 
     On the other hand, when at least one of the temperatures measured by the heater thermometer  62  is not within the first heater temperature range (ACT  204 , NO), the controller  6  determines, as the pre-processing energization method, a third energization method (ACT  206 ). The third energization method is an energization method in which the central duty ratio is a third duty ratio, and the period during which the current is supplied is a third period. The electric power supplied to the heating element set  45  by the third energization method during the pre-processing period is less than the electric power supplied to the heating element set by the second energization method during the pre-processing period. For example, the power supplied to the heating element set  45  by the third energization method during the pre-processing period is ⅗ of the power supplied to the heating element set  45  by the second energization method during the pre-processing period. When the first duty ratio is 70% and the first time period is 0.5 ms, for example, the third duty ratio is 30%, and the third period is 0.5 ms. In the third energization method, the end duty ratio is lower than the third duty ratio. 
     On the other hand, in ACT  201 , when all of the temperatures measured by the film thermometer  64  are equal to or higher than the film temperature (ACT  201 , NO), the controller  6  determines, as the pre-processing energization method, the third energization method (ACT  206 ). 
       FIG.  10    is a diagram showing a relationship between torque and temperature of the heating element set  45  in the image forming apparatus  100  according to the first embodiment. The horizontal axis in  FIG.  10    represents the temperature of heating element set  45 . The vertical axis in  FIG.  10    represents torque.  FIG.  10    shows that higher temperature of the heating element set  45  results in lower torque. 
     The image forming apparatus  100  of the first embodiment configured as described above includes the controller  6  for controlling the central triac  96   a  and the end triac  96   b  so as to energize the heating element set  45  before rotating the pressing roller  30   p  in accordance with the temperature measured by the heater thermometer  62 , thereby reducing the viscosity of the lubricant applied to the inner peripheral surface of the cylindrical film  35  before the rotation is started. 
     Second Embodiment 
       FIG.  11    is a diagram illustrating a hardware configuration of an image forming apparatus  100   a  according to the second embodiment. The image forming apparatus  100   a  is different from the image forming apparatus  100  in that the controller  6  of the image forming apparatus  100  is replaced by a controller  6   a . In the following description, for the sake of simplicity, the same functions as those of the image forming apparatus  100  will be denoted by the same reference numerals as those in  FIG.  1    to  FIG.  7   , and the description thereof will not be repeated here. 
     The controller  6   a  is different from the controller  6  in that the central triac  96   a  and the end triac  96   b  are controlled so as to energize the heating element set  45  during the pre-processing period regardless of the temperatures measured by the heater thermometer  62  and the film thermometer  64 . 
     Based on the temperature measured by the heater thermometer  62 , the controller  6   a  determines the energization method for the heating element set  45  in the preprocessing period. 
       FIG.  12    is a flowchart showing processing executed by the controller  6   a  in the period from the start of the pre-processing period to the execution of the post-processing in the second embodiment. Hereinafter, for simplicity of description, the same processing as that executed by the controller  6  is denoted by the same reference numerals as those in  FIG.  8    and  FIG.  9   , and description thereof will be omitted. 
     When the pre-processing period is started in ACT  101  (ACT  101 , YES), the controller  6   a  executes the process of ACT  202 . When at least one of the temperatures measured by the heater thermometer  62  is equal to or lower than the second heater temperature (ACT  202 , YES), the controller  6   a  executes the process of ACT  203 , as described in  FIG.  9   . 
     On the other hand, when all of the temperatures measured by the heater thermometer  62  are equal to or higher than the second heater temperature (ACT  202 , NO), the controller  6   a  executes the process of ACT  204 . When all of the temperatures measured by the heater thermometer  62  are within the first heater temperature range (ACT  204 , YES), the controller  6   a  executes the process of ACT  205 . 
     On the other hand, when at least one of the temperatures measured by the heater thermometer  62  is not within the first heater temperature range (ACT  204 , NO), the controller  6   a  executes the process of ACT  206 . 
     Next to the execution of the process of ACT  203 , ACT  205  or ACT  206 , the controller  6   a  executes the process of ACT  104 . Next to the process of ACT  104 , the controller  6   a  executes the process of ACT  105 . Next to the process of ACT  105 , the controller  6   a  executes the process of ACT  106 . 
     The image forming apparatus  100   a  of the second embodiment configured as described above has the controller  6   a  for controlling the central triac  96   a  and the end triac  96   b  to energize the heating element set  45  before rotating the pressing roller  30   p , whereby viscosity of lubricant applied to the inner peripheral surface of the cylindrical film  35  can be reduced before rotation, thereby suppressing occurrence of torque increase. 
     MODIFIED EXAMPLE 
     Hereinafter, the power supplied to the heating element set  45  by the first energization method during the pre-processing period will be referred to as a first power. Hereinafter, the power supplied to the heating element set by the second energization method during the pre-processing period will be referred to as a second power. Hereinafter, the power supplied to the heating element set  45  by the third energization method during the pre-processing period will be referred to as a third power. The first time period, the second time period and the third time period may not necessarily be the same. The ratio of the first period to the second period may be any value which is equal to a second ratio to a first ratio, where the first ratio is the ratio of the first power to the first duty ratio, and the second ratio is the ratio of the second power to the second duty ratio. The ratio of the third period to the third period may be any value which is equal to a third ratio to the first ratio, where the third ratio is the ratio of the third power to the third duty ratio. 
     It should be noted that the pre-processing execution condition does not necessarily depend solely on the temperature measured by the film thermometer  64 . The pre-processing execution condition is, for example, a condition that at least one of the plurality of heater thermometers  62  is equal to or higher than the first heater temperature. 
     In addition, the controller  6  may energize not the first end heating element  45   b   1  and the second end heating element  45   b   2  but the central heating element  45   a  in the pre-processing period. In this case, the viscosity of lubricant located at the end portion of the inner peripheral surface of the cylindrical film  35  is higher than the viscosity of lubricant located at the center portion of the inner peripheral surface of the cylindrical film  35 . Therefore, the lubricant supplied in this way hardly leaks to the outer side of the cylindrical film  35 . 
     The image forming apparatus  100  may further include an ambient thermometer  65  in addition to the heater thermometer  62  and the film thermometer  64 . The ambient thermometer  65  measures ambient temperature of a target object to which the ambient thermometer  65  is attached. When the image forming apparatus  100  includes the ambient thermometer  65 , the controller  6  may determine the energization method based on the temperature measured by the heater thermometer  62 , the film thermometer  64 , and the ambient thermometer  65 . 
     For example, when the temperature measured by the ambient temperature meter  65  is higher than a predetermined value, the controller  6  determines, as the pre-processing energization method, a high power pre-processing energization method. In the high power pre-processing energization method, electric power supplied to the heating element set  45  is higher than the electric power supplied to the heating element set  45  when the temperature measured by the ambient thermometer  65  is lower than the predetermined value. Specifically, in the high power pre-processing energization method, the electric power is supplied to the heating element set  45  for a longer time than the electric power supplied when the temperature measured by the ambient thermometer  65  is lower than the predetermined value. 
       FIG.  13    is a diagram illustrating the ambient thermometer  65  in the modified example. The ambient thermometer  65  may be attached to any position in the vicinity of the fixing unit  30 . The vicinity of the fixing unit  30  is a position where ambient temperature of the fixing unit  30  can be measured by the ambient thermometer  65 . The ambient temperature meter  65  may be attached to the housing  10  located outside the film unit  30   h , for example, as shown in  FIG.  13   . 
     Incidentally, the position of the film thermometer  64  may be any position as long as it is located inside the cylindrical film  35  and on the +x direction side of the heater unit  40 . The position of the film thermometer  64  may be, for example, a position at which an angle θ formed between a line perpendicular to the inner surface of the contact point with the cylindrical film  35  and a line perpendicular to the nip part N is equal to or larger than 45 degrees. 
       FIG.  14    is a diagram showing the angle θ formed in the modified example.  FIG.  14    shows that the angle θ formed by the straight line Lf perpendicular to the inner surface of the contact point with the cylindrical film  35  and the straight line CL perpendicular to the nip N is equal to or larger than 45 degrees. 
       FIG.  15    is a diagram showing a relationship between the temperature measured by the film thermometer  64  and the temperature of the heating element set  45  for each angle θ in the modification example. 
     In  FIG.  15   , the horizontal axis represents time, and the vertical axis represents temperature.  FIG.  15    shows that the longer the angle θ formed by the film thermometer  64  is, the more gradual the temperature of the film thermometer becomes.  FIG.  15    shows that the time change of the temperature measured by the film thermometer  64  having the angle θ of 45 degrees or more is approximately equal to that of the temperature of the heating element set in the range of heating element set  45 . 
     The heating element set  45  includes three heating elements (i.e., the central heating element  45   a , the first end heating element  45   b   1 , and the second end heating element  45   b   2 ). In contrast, the number of heating elements included in the heating element set  45  may be one or two, and may be four or more. 
     The heater thermometer  62  includes two heater thermometers (i.e., the center heater thermometer  62   a , the end heater thermometer  62   b ). In contrast, the number of heater thermometers  62  may be three or more. 
     The plurality of thermostats  68  comprise two thermostats (i.e., the central thermostat  68   a  and the end thermostat  68   b ). In contrast, the number of the plurality of thermostats  68  may be three or more. 
     In the aforementioned embodiments, the heating element included in the heating element set  45  may be a heating element having a positive resistance temperature characteristic. 
     In the aforementioned embodiments, the image forming apparatus  100  or  100   a  includes the fixing unit  30 . In contrast, the image forming apparatus may be a decoloring apparatus, which has a decoloring unit for instead of the fixing unit  30 . The decoloring apparatus performs a process of decoloring (i.e., erasing) an image formed on a sheet by a decolorable toner. The decoloring unit heats the decolorable toner image formed on the sheet passing through the nip to decolorize the toner image. 
     The pre-processing end condition may be, for example, a condition in which at least one of the temperatures measured by the heater thermometers  62  is equal to or greater than a predetermined temperature (hereinafter referred to as a “second pre-processing end condition”). When the pre-processing end condition is the first pre-processing end condition, the occurrence frequency of the situation in which the lubricant is partially fixed is lower than that in the case where the pre-processing end condition is the second pre-processing end condition. Therefore, when the pre-processing end condition is the first pre-processing end condition, the image forming apparatus  100  can suppress the occurrence of torque increase as compared to the case where the pre-processing end condition is the second pre-processing end condition. 
     The pre-processing heater condition does not necessarily need to be the first pre-processing heater condition. The pre-processing heater condition may be a condition that all of the temperatures measured by the heater thermometers  62  are lower than the first heater temperature (hereinafter, referred to as “second pre-processing heater conditions”). The second pre-processing heater condition is a condition included in the first pre-processing heater condition. When the pre-processing heater condition is the first pre-processing heater condition, the occurrence frequency of the situation in which the lubricant is partially fixed is lower than that in the case where the pre-processing heater condition is the second pre-processing heater condition. Therefore, when the pre-processing heater condition is the first pre-processing heater condition, the image forming apparatus  100  can suppress the occurrence of torque increase as compared to the case where the pre-processing heater condition is the second pre-processing heater condition. 
     The pre-processing film condition does not necessarily need to be the first pre-processing film condition. The pre-processing film condition may be a condition in which all of the temperatures measured by the plurality of film thermometers  64  are lower than the film temperature (hereinafter referred to as “second pre-processing film conditions”). The second pre-processing film condition is a condition included in the first pre-processing film condition. When the pre-processing film condition is the first pre-processing film condition, the occurrence frequency of the situation in which the lubricant is partially fixed is lower than that in the case where the pre-processing film condition is the second pre-processing film condition. Therefore, when the pre-processing film condition is the first pre-processing film condition, the image forming apparatus  100  can suppress the occurrence of torque increase as compared with the case where the pre-processing film condition is the second pre-processing film condition. 
     In ACT  201  shown in  FIG.  9    or  FIG.  12   , the controller  6  does not necessarily have to determine whether or not at least one of the temperature measured by the film thermometer  64  is less than the film temperature (hereinafter referred to as “ACT  201  first determination”). In ACT  201 , the controller  6  may determine whether or not all of the temperatures measured by the film thermometer  64  are lower than the film temperature (hereinafter referred to as “ACT  201  second determination”). When the controller  6  executes the ACT  201  in ACT  201 , the occurrence frequency of the situation where the lubricant is partially fixed is lower than that in the case where the ACT  201  second determination is performed in ACT  1 -. Therefore, when the controller  6  executes the ACT  201  first determination in ACT  201 , the image forming apparatus  100  can suppress the occurrence of torque increase compared to the case in which the ACT  201  second determination is performed. 
     In ACT  202  shown in  FIG.  9    or  FIG.  12   , the controller  6  does not necessarily have to determine whether or not at least one of the temperatures measured by the heater thermometer  62  is lower than the second heater temperature (hereinafter referred to as “ACT  202  first determination”). In ACT  202 , the controller  6  may determine whether or not all of the temperatures measured by the heater thermometer  62  are lower than the second heater temperature (hereinafter referred to as “ACT  202  second determination”). When the controller  6  executes the ACT  202  in ACT  202 , the occurrence frequency of the situation where the lubricant is partially fixed is lower than that in the case where the ACT  202  second determination is performed in ACT  1 -. Therefore, when the controller  6  executes the ACT  202  first determination in ACT  202 , the image forming apparatus  100  can suppress the occurrence of torque increase compared to the case in which the ACT  202  second determination is performed. 
     Note that in ACT  204  shown in  FIG.  9    or  FIG.  12   , the controller  6  does not necessarily have to determine whether or not all of the temperatures measured by the heater thermometer  62  are within the first heater temperature range (hereinafter referred to as “ACT  204  first determination”). In ACT  204 , the controller  6  may determine whether or not at least one of the temperatures measured by the heater thermometer  62  is within the first heater temperature range (hereinafter referred to as “ACT  204  second determination”). When the controller  6  executes the ACT  204  first determination in ACT  204 , the occurrence frequency of the situation where the lubricant is partially fixed is lower than that in the case where the ACT  204  second determination is performed. Therefore, when the controller  6  executes the ACT  204  first determination in ACT  204 , the image forming apparatus  100  can suppress the occurrence of torque increase compared to the case in which the ACT  204  second determination is performed. 
     All or part of the functions of the image forming apparatuses  100  and  100   a  may be performed by any hardware, such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). The program may be recorded on a computer-readable recording medium. The computer-readable recording medium is, for example, a flexible disk, a magneto-optical disk, a portable medium such as a ROM, a CD-ROM, or the like, a storage device such as a hard disk incorporated in a computer system, or the like. The program may be transmitted over a telecommunications line. 
     In the above embodiments, the CPU  91  of the controller  6  executes programs for achieving the functions of the image forming apparatus  100  or  100   a , but those functions may be implemented by a circuit such as an LSI. 
     According to at least one embodiment described above, the image forming apparatus  100  and  100   a  can suppress leakage of the lubricant from the application destination by providing controller  6  or  6   a  controlling the central triac  96   a  and the end triac  96   b  to energize the central heating element  45   a  at the first duty ratio and to energize the first end heating element  45   b   1  and the second end heating element  45   b   2  at a duty ratio lower than the first duty ratio before rotating the pressing roller  30   p.    
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms, furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The embodiments and variations thereof are included within the scope and spirit of the invention, and are included within the scope of the appended claims and their equivalents.