Patent Publication Number: US-10310433-B2

Title: Image forming apparatus that controls a temperature of at least one of a rotating member based on a wearing amount of the rotating member and a pressing member based on a hardness change amount of the pressing member

Description:
This application claims the benefit of Japanese Patent Application No. 2017-037191, filed Feb. 28, 2017, and Japanese Patent Application No. 2018-140102, filed on Jan. 30, 2018, which are hereby incorporated by reference herein in their entireties. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming apparatus, such as a copier or a laser beam printer (LBP), that adopts an image forming process, such as an electrophotographic system and an electrostatic recording system, and that includes a heating and fixing unit that heats and fixes an unfixed toner image, formed and borne on a recording material, onto the recording material. 
     Description of the Related Art 
     As a heating and fixing unit (hereinafter called a fixing unit), a film-heating fixing unit excellent in starting up a heater from a sleeping state, as described in, for example, Japanese Patent Application Laid-open No. H04-204980 has been known. In the film-heating fixing unit, a heater serving as a heating source and a pressing roller form a fixing nip portion via a cylindrical fixing film. Since the fixing unit with this configuration easily realizes low heat capacity and a small diameter, the fixing unit can enter a fixable state with relatively less power. 
     In addition, Japanese Patent Application Laid-open No. 2002-148988 has proposed a configuration that further reduces energy supplied to a fixing unit, the configuration using a foam rubber layer made of a sponge material or a balloon material as the rubber layer of a pressing roller. By the adoption of such structures, a heat insulating effect in the thickness direction of the pressing roller is enhanced. Therefore, the amount of heat transferred from a heater to the pressing roller may be reduced, and the heat energy of the heater may be more efficiently used. 
     Meanwhile, there has also been proposed a configuration that further improves energy-saving by enhancing the heat conductivity of the respective constituents of a path through which heat energy from a heater is transferred to toner on a recording material. For example, when the film-heating fixing unit, described above, uses a material having greater heat conductivity in the base layer or the rubber layer of the fixing film, the effect of reducing the control temperature of the heater is obtained. 
     As the low heat capacity of the whole fixing unit and the high heat conductivity of members advance in the manner described above, the degradation of printing quality and reduction in durability due to the wearing of the surface layer of a fixing film and a change in the hardness of a pressing roller have become problems. 
     In order to shorten a thermal starting-up time of a fixing unit, it is necessary to improve its heat conductivity or to reduce its heat capacity. To this end, a heat transferred portion needs to be thinned. In a case in which a heat transferred portion is a portion that rubs and slides like a fixing film, however, heat transfer greatly changes depending on a wearing degree, which may result in a situation in which the fixing unit is not allowed to exert its original performance. 
     The surface layer of the fixing film uses tetrafluoroethylene perfluoroalkylvinylether copolymer (PFA) or polytetrafluoroethylene (PTFE) representing heat resisting resin excellent in mold releasability to prevent the adhesion of toner. The resin hardly transfers heat, however, and the thickness of the surface layer has a major influence on the performance of the fixing unit. 
     When the surface layer of the fixing film wears, heat from the heater easily transfers to toner or a recording material. As a result, a hot offset occurs, or the paper curls after its fixation processing. In addition, the paper may wrinkle depending on a type of the recording material. 
     In addition, when heat is excessively supplied, the recording material shrinks in the longitudinal direction of a fixing nip portion inside the fixing nip portion. Thus, drape-shaped waves occur in a portion of the recording material before the recording material enters the fixing nip portion. When the portion enters the fixing nip portion, a paper wrinkle may occur. 
     On the other hand, the rubber material of the pressing roller degrades and the hardness thereof reduces with repeated stress due to heating and rotation. 
     Problems such as a hot offset and the curling of the recording material occur not only when the surface layer of the fixing film is thinned, but also when the hardness of the pressing roller reduces. When the rigidity of the pressing roller reduces, the width of the fixing nip portion in a recoding-material transporting direction increases and, as a result, a time to heat the recording material increases. Thus, heat is excessively supplied with an increase in the amount of the heat supplied to toner or the paper. In order to deal with the problems, it is assumed to change fixing conditions according to the use situations of an apparatus. For example, a condition such as control temperature is changed based on the number of sheets set in advance. 
     An actual wearing degree is different, however, depending on use conditions. The wearing of the fixing film and the reduction in the hardness of the pressing roller may advance earlier than expected. In this case, the problems described also occur. On the other hand, when the wearing of the fixing film and the reduction in the hardness of the pressing roller do not advance as expected, problems due to the insufficient supply of heat occur. For example, paper jamming resulting from a fixing failure or the accumulation of toner in the fixing unit due to the fixing failure, or the like, occurs. 
     In addition, a difference in the wearing degree of the fixing film or a difference in the degree of the change in the hardness of the pressing roller depending on use conditions has an influence not only on a problem in printing quality, but also on a timing for issuing a life alert. 
     The life of the fixing unit is different depending on the wearing degree of the fixing film or the degree of reduction in the hardness of the pressing roller. According to a method in which the life alert of the fixing unit is issued using information, such as the number of total printed sheets, total driving time, and the number of total rotations, there is a likelihood that the fixing unit comes to the end of the life before the issuance of the alert or the life alert is issued to the fixing unit that has not come to the end of the life. 
     SUMMARY OF THE INVENTION 
     The present invention has an object of providing an image forming apparatus capable of preventing the occurrence of an image failure even when a difference in heat transfer occurs due to the wearing of the surface layer of a fixing member, such as a fixing film, or even when a change in the hardness of a pressing member occurs. 
     In addition, the present invention has an object of providing an image forming apparatus capable of appropriately issuing a life alert. 
     In order to achieve the above object, one aspect of the present invention provides an image forming apparatus that prints an image on a recording material, the image forming apparatus comprising a fixing unit that fixes an image formed on a recording material onto the recording material, the fixing unit having a rotating member with a surface layer, a pressing member that forms a fixing nip portion to sandwich and to transport the recording material with the rotating member, and a heater that heats the rotating member, a temperature sensor that detects a temperature of the rotating member, a wearing amount acquisition portion that acquires a wearing amount of the surface layer of the rotating member, the wearing amount acquisition portion acquiring the wearing amount corresponding to the temperature detected by the temperature sensor, and a control temperature setting portion that sets a control temperature of the heater according to the wearing amount. 
     In order to achieve the above object, another aspect of the present invention provides an image forming apparatus that prints an image on a recording material, the image forming apparatus comprising a fixing unit that fixes an image formed on a recording material onto the recording material, the fixing unit having a rotating member with a surface layer, a pressing member that forms a fixing nip portion to sandwich and to transport the recording material with the rotating member, and a heater that heats the rotating member, a temperature sensor that detects a temperature of the rotating member, a wearing amount acquisition portion that acquires a wearing amount of the surface layer of the rotating member, the wearing amount acquisition portion acquiring the wearing amount corresponding to the temperature detected by the temperature sensor, and a life calculation portion that calculates a life of the rotating member according to the wearing amount. 
     In order to achieve the above object, yet another aspect of the present invention provides an image forming apparatus that prints an image on a recording material, the image forming apparatus comprising a fixing unit that fixes an image formed on a recording material onto the recording material, the fixing unit having a rotating member, a pressing member that forms a fixing nip portion to sandwich and to transport the recording material with the rotating member, and a heater that heats the rotating member, a temperature sensor that detects temperature of the pressing member, a hardness change amount acquisition portion that acquires a hardness change amount of the pressing member, the hardness change amount acquisition portion acquiring the hardness change amount corresponding to the temperature detected by the temperature sensor, and a control temperature setting portion that sets a control temperature of the heater according to the hardness change amount. 
     In order to achieve the above object, still another aspect of the present invention provides an image forming apparatus that prints an image on a recording material, the image forming apparatus comprising a fixing unit that fixes an image formed on a recording material onto the recording material, the fixing unit having a rotating member, a pressing member that forms a fixing nip portion to sandwich and to transport the recording material with the rotating member, and a heater that heats the rotating member, a temperature sensor that detects a temperature of the pressing member, a hardness change amount acquisition portion that acquires a hardness change amount of the pressing member, the hardness change amount acquisition portion acquiring the hardness change amount corresponding to the temperature detected by the temperature sensor, and a life calculation portion that calculates a life of the pressing member according to the hardness change amount. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration view of an image forming apparatus according to a first embodiment of the present invention. 
         FIG. 2  is a schematic configuration view of a fixing unit shown in  FIG. 1 . 
         FIG. 3  is a schematic configuration view of the fixing unit shown in  FIG. 1  and temperature sensors. 
         FIG. 4  is a graph for describing the temperature of members in the case of continuous paper feeding in the first embodiment. 
         FIG. 5  is a graph for describing the temperature of the members in the case of intermittent paper feeding in the first embodiment. 
         FIG. 6  is a graph showing the relationship between the surface temperature and the wearing amount of the fixing film. 
         FIG. 7  is a graph showing the relationship between the thickness of the surface layer of the fixing film and a control temperature. 
         FIG. 8  is a graph showing the relationship between the surface temperature and the amount of a change in the hardness of a pressing roller. 
         FIG. 9  is a graph showing the relationship between the surface temperature and the amount of the change in the hardness of the pressing roller. 
         FIG. 10  is a graph showing the relationship between the use time and the change in the hardness of the pressing roller. 
         FIG. 11  is a schematic configuration view of an image forming apparatus according to a second embodiment. 
         FIG. 12  is a schematic configuration view of a fixing unit shown in  FIG. 11 . 
         FIGS. 13A and 13B  are graphs for describing the temperature of members at paper feeding in the second embodiment. 
         FIG. 14  is a graph showing the relationship between the thickness of the surface layer of a fixing film and a control temperature. 
         FIG. 15  is a graph for describing the temperature of the members in the case of the continuous paper feeding in the first embodiment. 
         FIG. 16  is a graph for describing the temperature of the members in the case of the intermittent paper feeding in the first embodiment. 
         FIG. 17  is a graph for describing the temperature of the members in the case of intermittent paper feeding having a long interval in the first embodiment. 
         FIG. 18  is a graph for describing the temperature of the members in the case of continuous paper feeding in the second embodiment. 
         FIG. 19  is a graph for describing the temperature of the members in the case of intermittent paper feeding in the second embodiment. 
         FIG. 20  is a graph for describing the temperature of the members in the case of the intermittent paper feeding having a long interval in the second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereafter, a description will be given, with reference to the drawings, of embodiments (examples) of the present invention. The sizes, materials, shapes, their relative arrangements, or the like, of constituents described in the embodiments may, however, be appropriately changed according to the configurations, various conditions, or the like, of apparatuses to which the invention is applied. Therefore, the sizes, materials, shapes, their relative arrangements, or the like, of the constituents described in the embodiments do not intend to limit the scope of the invention to the following embodiments. 
     First Embodiment 
       FIG. 1  is a schematic configuration view of an image forming apparatus according to a first embodiment of the present invention. 
     (1) Description of Image Forming Apparatus 
     In  FIG. 1 , reference symbol  1  shows a photosensitive drum in which a photosensitive material, such as an organic photoconductor (OPC), amorphous selenium, and amorphous silicon, is formed on an aluminum cylinder. The photosensitive drum  1  is rotationally driven in an arrow direction, and its surface is uniformly charged by a charging roller  2  serving as a charging device. Next, the photosensitive drum  1  is subjected to scanning exposure by a laser scanner  3 , with laser light L controlled to be turned ON/OFF according to image information, and thus, an electrostatic latent image is formed on the photosensitive drum  1 . The electrostatic latent image is developed and visualized by a developing apparatus  4 . 
     The visualized toner image is transferred onto a recording material P by a voltage applied to a transfer roller  5  serving as a transfer device. Here, in order to align the image forming position of the toner image on the photosensitive drum  1  with the position of the recording material P, a top sensor  8  detects the tip end of the recording material P to adjust the transport timing of the recording material P. The recording material P transported at a prescribed timing is sandwiched and transported between the photosensitive drum  1  and the transfer roller  5 . The recording material P, onto which the toner image has been transferred, is transported to a fixing unit  6 , and thus, the toner image is fixed onto the recording material P as a permanent image. 
     Meanwhile, residual toner on the photosensitive drum  1  is removed from the surface of the photosensitive drum  1  by a cleaning apparatus  7 . A paper discharging sensor  9  provided inside the fixing unit  6  is a sensor that detects paper jamming, or the like, caused by the recording material P between the top sensor  8  and the paper discharging sensor  9 . 
     (2) Description of Fixing Unit  6   
     (2-1) Cross-Sectional Configuration of Fixing Unit  6   
     Next, a description will be given of the fixing unit  6 . 
       FIG. 2  is a schematic view showing the cross-sectional configuration of a central part in the longitudinal direction of the fixing unit  6  representing the first embodiment. 
     The fixing unit  6  includes a heating unit  10  provided with a heater  11  serving as a heating source and a cylindrical fixing film (fixing rotating member)  13 , and a pressing roller  20  serving as a pressing member. A fixing nip portion N is formed by bringing the heater  11  and the pressing roller  20  into contact with each other at a prescribed pressing force via the fixing film  13 . A toner image is fixed onto the recording material P when the recording material P that bears the unfixed toner image is heated while being fed to the fixing nip portion N. 
     The heating unit  10  is mainly constituted of the fixing film  13 , the heater  11 , a heat insulating holder  12  that holds the heater  11 , a metal stay  14 , or the like. The metal stay  14  receives a pressing force from a spring (not shown) and presses the heat insulating holder  12  toward the pressing roller  20 . 
     The heater  11  is a plate-shaped member having low heat capacity, and contacts the inner surface of the fixing film  13 . The heater  11  has an insulative ceramic substrate made of alumina, aluminum nitride, or the like. On the surface of the substrate, a heat generation resistance layer made of silver palladium (Ag/Pd), ruthenium oxide (RuO 2 ), tantalum nitride (Ta 2 N), or the like, is formed by a printing technology, such as screen printing. As shown in  FIG. 2 , on the surface of the heater  11  that contacts the fixing film  13 , a protection layer  11   a , such as a glass layer, that protects the heat generation resistance layer is provided so as not to impair heat efficiency. The heat insulating holder  12  that holds the heater  11  is made of heat resisting resin, such as liquid crystal polymer, phenol resin, polyphenylene sulfide (PPS), and polyether ether ketone (PEEK), and also plays a role in guiding the rotation of the fixing film  13 . 
     The base layer of the fixing film  13  is a heat resisting film having a total thickness of 200 μm or less. By reducing the thickness (reducing the heat capacity) of the base layer, it is possible to increase the temperature of the fixing film  13  to a temperature at which the fixing film  13  is capable of being fixed in a short period of time. The base layer of the fixing film  13  is made of heat resisting resin, such as polyimide, polyamide imide, and PEEK, or a pure metal or an alloy, such as steel use stainless (SUS), aluminum, nickel, copper, and zinc having heat resistance and high heat conductivity. On the other hand, in order to constitute the fixing unit  6  having a long life, it is necessary to make the base layer have substantial strength and excellent in durability. To this end, it is necessary for the base layer of the fixing film  13  to have a thickness of 20 μm or more. Accordingly, the base layer of the fixing film  13  optimally has a thickness of at least 20 μm and not more than 200 μm. 
     In addition, in order to prevent an offset and to assure the separability of the recording material, the surface layer of the fixing film  13  is coated with heat resisting resin excellent in mold releasability, such as fluorocarbon resin, such as polytetrafluoroethylene (PTFE), tetrafluoroethylene perfluoroalkylvinylether copolymer (PFA), tetrafluoroethylene hexafluoropropylene copolymer (FEP), ethylene tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (CTFE), and polyvinylidene fluoride (PVDF), and silicon resin by mixture or singly. 
     Moreover, a silicon rubber layer having a thickness of about 100 μm to 300 μm may be formed between the surface layer and the base layer as an intermediate rubber layer. By forming the intermediate rubber layer, it is possible for the surface of the fixing film  13  to adapt to the unevenness of the surface of the recording material or the unevenness of a toner image and to provide excellent image quality. 
     The pressing roller  20  is an elastic roller having an elastic layer  22  on the outside of a metal cored bar  21  made of SUS, steel use machinability (SUM), and aluminum. In addition, the pressing roller  20  may have a surface layer  23  on the elastic layer  22 , as shown in  FIG. 2 . 
     The elastic layer  22  is an elastic solid rubber layer made of a heat resisting rubber, such as a silicon rubber and a fluorine rubber, a sponge rubber layer in which silicon rubber is foamed to have a greater heat insulating effect, a foam rubber layer in which a hollow filler (such as micro balloons) is dispersed in a silicon rubber layer to make a burned cured object have a gas portion to enhance a heat insulating effect, or the like. 
     In addition, the surface layer  23  is made of PFA, PTFE, or the like. 
     Moreover, the pressing roller  20  receives a driving force to drive in an arrow direction shown in  FIG. 2  via a driving gear (not shown) provided at the end of the cored bar  21 . The driving force is transmitted from a motor (not shown) according to instructions from a central processing unit (CPU)  100  (not shown in  FIG. 2 ) that serves as control means. As the pressing roller  20  is rotationally driven, the fixing film  13  is driven to rotate by the frictional force between the fixing film  13  and the pressing roller  20 . Lubricant, such as fluorine-based or silicon-based heat resisting grease, is interposed between the fixing film  13  and the heater  11 . As a result, the frictional resistance between the fixing film  13  and the heater  11  is kept low, whereby the fixing film  13  may be smoothly rotated. 
     Further, a thermistor  15  is provided on the rear surface of the ceramic substrate as a temperature sensor. According to a signal from the thermistor  15 , the CPU  100  controls the duty ratio of power supplied to the heater  11  so that the heater  11  maintains control temperature. Thus, a temperature inside the fixing nip portion N may be maintained at a desired temperature. The temperature sensor is not limited to a thermistor, and may include various sensors. 
     The fixing unit  6  has a storage medium  33 , such as a non-volatile random access memory (NVRAM), serving as storage means or a storage portion. The storage medium  33  communicates with the CPU  100  and stores the operating statuses of the fixing unit  6 . 
     Note that, in the product specifications of the first embodiment, the image forming apparatus performs printing on A4-size paper in vertical feeding, in which the image forming apparatus is capable of printing 60 sheets per minute. Paper transporting speed inside the apparatus is 400 mm/second. The fixing film  13 , the pressing roller  20 , or the like, that contacts the recording material P also rotates at the same peripheral speed as the paper transporting speed. In addition, the fixing unit  6  is replaceable with respect to the image forming apparatus and is configured to be replaced when the image forming apparatus prints 150,000 (150 k) sheets. 
     Next, a description will be given of the specific examples of the respective constituents of the fixing unit  6 . 
     The fixing film  13  has an outer diameter of 30 mm. The base layer of the fixing film  13  is made of polyimide (PI) and has a thickness of 60 μm, in which carbon fibers are dispersed to improve heat conduction. The fixing film  13  has a silicon rubber layer having a thickness of 250 μm on the base layer as an elastic layer. The surface layer is a PFA layer having a thickness of 20 μm and obtained by coating the silicon rubber layer with a tube made of PFA. 
     The pressing roller  20  has an outer diameter of 30 mm. The cored bar  21  is made of iron and has an outer diameter of 23 mm. The pressing roller  20  has a solid silicon rubber layer having a thickness of about 3.5 mm on the cored bar  21  as the elastic layer  22 . In addition, the elastic layer  22  is coated with a PFA tube having a thickness of 50 μm as the surface layer  23 . 
     The pressing roller  20  is so molded that its hardness as measured using an Asker C hardness meter on a surface (i.e., the surface  23 ) at 1 kg becomes 55°. At this time, the hardness of the silicon rubber layer is 17° under the same measurement condition. 
     In addition, the pressure applied between the pressing roller  20  and the fixing film  13  is 15 kgf. 
     (2-2) Description of Temperature Sensors that Detect Surface Temperature of Fixing Film  13  and Pressing Roller  20   
       FIG. 3  is a schematic configuration view of temperature sensors that measure the surface temperature of the fixing film  13  and the surface temperature of the pressing roller  20  in the fixing unit  6 . 
     Reference symbol  31  shows a non-contact temperature sensor that detects the surface temperature of the fixing film  13 . In addition, reference symbol  32  shows a non-contact temperature sensor that detects the surface temperature of the pressing roller  20 . As the temperature sensors  31  and  32 , non-contact thermistors, such as thermopiles, are preferably used. 
     The temperature sensors  31  and  32  are installed in the image forming apparatus. When the fixing unit  6  is attached to or detached from the image forming apparatus, the temperature sensors  31  and  32  may be structured to retract so as not to interfere with the fixing unit  6 . Alternatively, the temperature sensors  31  and  32  may be included in the attachable/detachable fixing unit  6 . 
     The surface temperature of the fixing film  13  and the surface temperature of the pressing roller  20  measured by the temperature sensors  31  and  32 , respectively, are temporarily stored in the storage medium  33 , such as an NVRAM shown in  FIG. 2 , provided in the fixing unit  6  or the body of the image forming apparatus. The temperature information is loaded into the CPU  100  serving as control means to be reflected on controlling the temperature of the fixing unit  6 . Alternatively, the measured temperature is directly transmitted to the CPU  100  in each case, and then is used for the management of the life of the fixing unit  6 , or the like. A specific control method will be described below. Note that the CPU  100  outputs a temperature control signal and an alert signal to a temperature control circuit  101  and a display panel  102 , respectively. 
     (3) Description of Temperature and Wearing Amount of Surface Layer of Fixing Film  13   
     The surface layer of the fixing film  13  wears as the paper is fed. This is because there is a slight difference in the peripheral speed between the paper material of the recording material P and the fixing film  13 . 
     The fixing film  13  is driven to rotate with the rotation of the pressing roller  20 . The pressing roller  20  transports the recording material P, and the fixing film  13  rotates due to the frictional force between the recording material P and the fixing film  13 . Since the surface of the fixing film  13  is made of a material having a high mold releasability, the fixing film  13  has a low frictional coefficient. In addition, since the inner surface of the fixing film  13  slidingly contacts the heater  11  and the heat insulating holder  12 , the peripheral speed of the fixing film  13  becomes slower than the transporting speed of the recording material P. 
     The recording material P contains minerals, such as calcium carbonate and kaolin, as fillers to make the paper white and opaque. The fillers shave the surface layer of the fixing film  13  as abrasives. 
     The speed at which the surface layer of the fixing film  13  wears depends on the surface temperature and the time during which the temperature is applied. Resin, such as PFA and PTFE used in the surface layer of the fixing film  13 , softens with an increase in the temperature. Therefore, the fillers of the paper more deeply break into the fixing film  13  when the fixing film  13  and the recording material P are pressed at the fixing nip portion N. It appears that the wearing accelerates when a difference in the peripheral speed between the fixing film  13  and the recording material P occurs in this state. 
     Note that the hardness of the resin changes with heating time. The hardness of the resin changes according to elastic deformation due to outer stress and viscosity deformation due to the temperature described above, but the viscosity deformation depends on the heating time. Therefore, there is a likelihood that the resin hardens at the beginning of the heating and softens as the heating time elapses to some extent. 
       FIG. 6  shows the relationship between the surface temperature and the wearing speed of the fixing film  13 . 
     In  FIG. 6 , a horizontal axis shows the temperature of the fixing film  13 , and a vertical axis shows the wearing speed of the surface layer of the fixing film  13 , i.e., the wearing amount of the surface layer for every 1000 sheets. 
     The surface layer is not excessively shaved when the temperature is low, but the wearing amount increases when the temperature becomes high. The wearing amount for every 1000 sheets is 0.07 μm when the surface temperature is about 180° C., but is 0.13 μm when the surface temperature is 200° C. Note that the wearing amounts shown in  FIG. 6  are those considering both the elastic deformation and the viscosity deformation. 
     In the first embodiment, the fixing film  13  starts wearing due to the viscosity deformation when the surface temperature of the fixing film  13  reaches 160° C. In a state in which the fixing unit  6  has been cooled at about room temperature, it takes about 4.8 seconds for the surface temperature of the fixing film  13  to reach 160° C. since the energization of the heater  11  starts. Therefore, the wearing amount is small before 4.8 seconds elapses, and the fixing film  13  almost constantly wears at 0.05 μm for every 1000 sheets. 
     In addition, the time until the viscosity deformation starts changes with the temperature of members. According to the configuration of the first embodiment, the viscosity deformation starts in about 6 seconds when the surface temperature of the fixing film  13  is 160° C. to 175° C., starts in about 3.5 seconds when the surface temperature is 175° C. to 200° C., and starts in about 1 second when the surface temperature is 200° C. or more. 
     The temperature of the heater  11  is controlled based on a signal from the thermistor  15  provided on the rear surface of the heater  11 . In the first embodiment, power supplied to the heater is controlled so that a temperature detected by the thermistor  15  is maintained at the control temperature 200° C. 
     When 60 sheets are printed per minute at the control temperature 200° C. in a state in which the fixing film  13  is brand-new, the temperature of the fixing film becomes 180° C. 
     When the thickness of the surface layer of the fixing film  13  reduces, the surface temperature of the fixing film becomes high even if the control temperature remains the same. In order to prevent this problem, the control temperature is changed according to the thickness of the surface layer of the fixing film  13  in the first embodiment.  FIG. 7  shows the relationship between the thickness of the surface layer and the control temperature. 
     In  FIG. 7 , a horizontal axis shows the thickness of the surface layer, and a vertical axis shows the control temperature at which the surface of the fixing film is maintained at 180° C. when 60 sheets are printed per minute. The control temperature is 200° C. when the thickness of the surface layer is 20 μm, but changes to 196° C. when the thickness is 15 μm, changes to 192° C. when the thickness is 10 μm, and changes to 188° C. when the thickness is 5 μm. As will be described later, the control temperature is used for a life notification timing, or the like. 
     (4) Description of Temperature and Hardness Reduction Amount of Pressing Roller  20   
     When the hardness of the pressing roller  20  is small (when the pressing roller  20  is soft), the time at which the recording material P passes through the fixing nip portion N becomes long and the amount of heat transferred to the recording material P and the toner becomes great. Therefore, the effect of melting the toner is enhanced. 
     On the other hand, when the hardness of the pressing roller  20  is great (when the pressing roller  20  is hard), an inexpensive and small motor that has a small torque to drive the fixing unit  6  and produces a small torque may be used. 
     Since the hardness of the pressing roller  20  is likely to gradually reduce as the pressing roller  20  is used, the hardness is set in consideration of the above circumstances. In the first embodiment, the hardness of the pressing roller  20  is set at 55°. Note that a motor, with specifications under which the pressing roller  20  may be driven so long as the lower limit of the hardness of the pressing roller  20  is above 50°, is used. 
       FIG. 8  is a graph showing the surface temperature and the hardness reduction speed of the pressing roller  20 . In  FIG. 8 , a horizontal axis shows the surface temperature of the pressing roller  20 , and a vertical axis shows the hardness reduction speed for every 100-hour rotating time. 
     A hardness change amount for every 100-hour rotating time is about 1° when the temperature of the pressing roller  20  is about 100° C., and is 2.5° when the temperature is 180° C. 
     Table 1 shows the relationship between the hardness of the pressing roller  20 , a width Ln of the fixing nip portion N, and the control temperature necessary for obtaining constant fixability in the first embodiment. 
     When the hardness of the pressing roller  20  reduces by 2°, the nip width increases by 0.5 mm. In order to obtain the constant fixability, it is necessary to reduce the control temperature by 3°. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Relationship between Rigidity of Pressing roller, 
               
               
                 Nip Width, and Appropriate Control temperature 
               
            
           
           
               
               
               
            
               
                   
                   
                 Appropriate 
               
               
                 Hardness of 
                   
                 temperature change 
               
               
                 pressing roller 
                 Nip width 
                 amount 
               
               
                   
               
               
                 55° 
                  9.5 mm 
                 Reference 
               
               
                 54° 
                  9.8 mm 
                 −2° 
               
               
                 53° 
                 10.0 mm 
                 −3° 
               
               
                 52° 
                 10.1 mm 
                 −4° 
               
               
                   
               
            
           
         
       
     
     (5) Description of Paper Feeding Mode and Temperature of Fixing Film  13   
     A paper feeding sequence based on the fixing unit  6  is constituted of the following four steps. 
     Pre-rotation step: A pre-rotation step includes the preparation step of stabilizing the potential of the photosensitive drum  1  and the rotation of the laser scanner  3 , the transporting step of forming an image on the photosensitive drum  1  and transporting the recording material P, onto which the image on the photosensitive drum  1  has been transferred, to the fixing unit  6 , and a warming-up step of warming up the fixing film  13  and the pressing roller  20 . The warming-up step overlaps with the preparation step and the transport step in terms of time. 
     Paper feeding step: A paper feeding step is the step of causing the recording material P, on which unfixed toner has been transferred, to pass through the fixing nip portion N to be fixed. In the paper feeding step, the recording material P is present in the fixing nip portion N. 
     Inter-paper step: An inter-paper step is the step of placing the interval between the preceding recording material P and the following recording material P when continuous paper feeding is performed. In the inter-paper step, the recording material P is not present in the fixing nip portion N during the continuous paper feeding. 
     Post-rotation step: A post-rotation step refers to a rotating time at which the recording material P is discharged to the outside of the apparatus. 
     A time required to perform each of the steps is set as follows: Pre-rotation step: 6 seconds, Paper feeding step: 0.74 seconds (A4 size: 297 mm), Inter-paper step: 0.25 seconds, and Post-rotation step: 1 second. 
     The temperature of the fixing film  13  and the pressing roller  20  changes according to paper feeding conditions. A description will be given, with reference to  FIGS. 4 and 5 , of a change in the temperature under the above conditions. 
       FIG. 4  shows the temperature transitions of the fixing film  13  and the pressing roller  20  when printing is continuously performed. In  FIG. 4 , a horizontal axis shows time, and a vertical axis shows temperature. A solid line shows the surface temperature of the fixing film  13 , and a broken line shows the surface temperature of the pressing roller  20 . In the time axis, R 1  shows the period of the pre-rotation step, R 2  shows the period of the paper feeding step, and R 3  shows the period of the inter-paper step. 
     When paper feeding is continuously performed, the paper feeding step and the inter-paper step are repeatedly performed until a specified number of sheets are printed after the pre-rotation step (the post-rotation step is not performed). At the paper feeding, the temperature of the fixing unit is controlled according to a temperature detected by the thermistor  15  provided on the rear surface of the heater  11 . The control temperature of the heater  11  is 200° C. 
     In the period of the pre-rotation step R 1 , much power is input to the heater  11  so that temperature on the rear side of the heater  11  becomes a temperature necessary for performing fixation. 
     In the period of R 1 , image formation is performed with an adjusted timing so that the recording material P enters the fixing nip portion N at a point at which the temperature of the thermistor  15  has reached 200° C. 
     In the period R 2  of the paper feeding step and the period R 3  of the inter-paper step, the CPU  100  controls the energization of the heater  11  so that the temperature detected by the thermistor  15  becomes the constant temperature 200° C. 
     Since the heat of the fixing film  13  is taken away by the recording material P when the paper feeding is continuously performed, the surface temperature of the fixing film  13  becomes about 180° C. 
     The temperature of the pressing roller  20  also reduces with the paper feeding and is stabilized at about 80° C. 
     When printing is performed at the fastest throughput of the product, as described above, both the temperature of the fixing film  13  and the temperature of the pressing roller  20  transition to a lower side. 
     When 150,000 sheets equivalent to the life of the fixing unit  6  of the embodiment are printed under the conditions, the surface layer of the fixing film  13  is shaved by 12.6 μm. 
     Meanwhile, when 150,000 sheets are printed at a printing speed of 60 ppm, the pressing roller rotates for about 41 hours. A reduction in the hardness of the pressing roller  20  after the pressing roller rotates for 41 hours is, however, only 0.5° and causes no problem. 
     Next,  FIG. 5  shows the case of intermittent paper feeding in which the temperature of the fixing unit  6  becomes high. In  FIG. 5 , the representation of a horizontal axis, a vertical axis, a solid line, and a broken line is the same as that of  FIG. 4 . 
       FIG. 5  shows the temperature transitions of the fixing film  13  and the pressing roller  20  when one sheet is printed for every 8 seconds. 
     According to the configuration of the first embodiment, the pre-rotation step starts immediately after the post-rotation of the printing of the first paper ends when one sheet is printed for every 8 seconds. In this case, since the fixing unit  6  has been already heated, the control temperature reaches 200° C. immediately after the heater  11  starts warming-up. Since it takes 6 seconds to perform image formation and paper transporting, however, the fixing film  13  and the pressing roller  20  are on standby while the heater  11  maintains a temperature of 200° C. While the heater  11  is on standby, the fixing film  13  and the pressing roller  20  rotate. This is because, if the fixing film  13  and the pressing roller  20  are on standby in a stopped state while the heater  11  maintains the high temperature, local heat history (deformation) is left in the fixing film  13  and the pressing roller  20 , resulting in an influence on an image. 
     Since the heater  11  maintains a temperature of 200° C. for a long time in the period R 1  of the pre-rotation, the temperature of the fixing film  13  reaches 200° C. almost equivalent to the control temperature. Since the pressing roller  20  rotates in a state of contacting the fixing film  13  having the high temperature, the temperature of the pressing roller  20  also increases and becomes about 160° C. In a state in which the temperature of the fixing film  13  and the pressing roller  20  becomes high, as described above, the wearing of the fixing film  13  accelerates and a reduction in the hardness of the pressing roller  20  is promoted, resulting in a likelihood that problems, such as a hot offset, curling, and paper wrinkling, occur before the fixing unit  6  comes to the end the life (150,000 sheets). 
     In addition, in the case of the intermittent paper feeding shown in  FIG. 5 , the temperature of the fixing film  13  may reach 200° C. at a timing at which the tip end of the recording material P enters the fixing nip portion. At this time, it appears from  FIG. 6  that the wearing amount for every 1000 (1 k) sheets is 0.13 μm. When 150,000 sheets are printed, the wearing amount becomes 19.5 nm. As a result, the surface layer is almost shaved. 
     On the other hand, since the pressing roller  20  rotates at a temperature of 160° C., the hardness of the pressing roller  20  reduces by 1.7° for every 100 hours until 100,000 sheets are printed. After that, the hardness of the pressing roller  20  reduces by 0.9° for every 100 hours. When 150,000 sheets are intermittently printed, the pressing roller  20  and the fixing film  13  rotate for 333 hours. Therefore, for the later half 233 hours, the hardness of the pressing roller  20  reduces by a hardness reduction rate (0.9°/100 hours). 
     As a result, the hardness of the pressing roller  20  becomes about 51° at a point at which 150,000 sheets have been printed and reduces to about 50° equivalent to the lower limit of the hardness at which the pressing roller is allowed to be driven. Therefore, the motor does not have room in terms of durability. 
     In a method in which the life alert of the fixing unit  6  is displayed on the operation panel of a product body or a PC monitor depending on the number of counted printed recording materials P, a timing at which the life alert is issued deviates from a timing at which the fixing unit  6  comes to the end of the life. Therefore, there is a possibility that an alert is not appropriately issued to a user and paper jamming due to a stain on the surface of the paper or a driving failure occurs. 
     (6) Control of Embodiment 
     Accordingly, in the first embodiment, the CPU  100  monitors the surface temperature of the fixing film  13  and the surface temperature of the pressing roller  20  with the first temperature sensor  31  and the second temperature sensor  32 , respectively, calculates the wearing amount of the surface layer of the fixing film  13  and a change in the hardness of the pressing roller  20  for every time printing is performed of one sheet corresponding to the surface temperature, and predicts the thickness of the surface layer of the fixing film  13  and the hardness of the pressing roller  20  at the measurement of the temperature. The CPU  100  serves as a wearing amount acquisition portion that acquires the wearing amount of the surface layer of the fixing film  13 , and serves as a hardness change amount acquisition portion that acquires the hardness change amount per unit time of the pressing roller  20 . In addition, the CPU  100  serves as a control temperature setting portion that sets the control temperature of the heater  11  according to the wearing amount of the surface layer of the fixing film  13 , and the hardness change amount of the pressing roller  20  and serves as a life calculation portion that calculates the life of the fixing film  13  and the pressing roller  20  according to the wearing amount of the surface layer of the fixing film  13  and the hardness change amount of the pressing roller  20 . Further, the CPU  100  sets the control temperature according to predicted information on the thickness of the surface layer of the fixing film  13  and the hardness of the pressing roller  20  to prevent the occurrence of an image failure and to issue a life alert at an appropriate timing. 
     Specifically, as for the thickness of the surface layer of the fixing film  13 , a coefficient corresponding to the wearing amount of the surface according to the surface temperature of the fixing film  13  is subtracted from an initial value for every paper printing. Then, based on the calculation result, the control temperature is changed and the life is displayed. 
     As for the pressing roller  20 , a coefficient corresponding to the hardness change amount according to the surface temperature of the pressing roller  20  is subtracted from an initial value for every unit time. Then, based on the calculation result, the control temperature is changed and the life is displayed. 
     The calculation results are stored in the storage medium  33  attached to the fixing unit  6 . 
     In displaying the life, one of the fixing film  13  and the pressing roller  20  that has come more closely to the end of the life is selected. In addition, the display of the life may be performed at a point at which the fixing film  13  and the pressing roller  20  have come to the end of the life, or may be performed in such a way as to display the ratio of the consumption of the fixing film  13  and the pressing roller  20  to the length of the life. 
     Specific Procedure for Estimating (Predicting) Thickness of Surface Layer of Fixing Film  13   
     Next, a description will be given of a procedure for calculating the thickness of the surface layer of the fixing film  13 . First, a thickness coefficient T corresponding to the thickness of the surface layer of the fixing film  13  is calculated. It is assumed that the initial value of the thickness coefficient T is 2,000,000. The initial value may be changed according to the initial thickness of the surface layer of the fixing film  13 . In this case, the initial value is increased or decreased at a rate of 100,000 for every 1 μm thickness. The initial value is stored in the storage medium  33  of the fixing unit  6 . The coefficient is subtracted from the thickness coefficient T such that the thickness coefficient T is updated for every time printing is performed. Any of values shown in table 2 is used as the subtraction coefficient in the first embodiment. The subtraction coefficient is the wearing amount of the surface layer corresponding to the surface temperature of the fixing film  13 . 
     The subtraction coefficient in table 2 is subtracted from the initial value of the thickness coefficient T for every time printing is performed, and the value is stored in the storage medium  33  of the fixing unit  6 . 
     Note that the wearing amount is different depending on how the surface layer of the fixing film  13  receives heat history, and thus, it is necessary to assume a case considering only the elastic deformation and a case considering both the elastic deformation and the viscosity deformation. The subtraction coefficients shown in table 2 refer to the case considering both the elastic deformation and the viscosity deformation. It is assumed that a subtraction coefficient considering only the elastic deformation is “5” in the first embodiment. A period at which the subtraction coefficient considering only the elastic deformation is different according to the surface temperature of the fixing film  13 . Table 3 shows the relationship between the surface temperature and the time at which the subtraction coefficient “5” is applied. 
     When printing is performed at room temperature in a state in which the surface temperature is 40° C. or less, the subtraction coefficient “5” is used for the first two sheets according to the time shown in table 3. 
     Therefore, when three sheets are printed, the fixing unit  6  is left unattended until being cooled, and printing is performed again, the subtraction coefficient “5” is used again. 
     When printing is continuously performed with the fixing unit  6  warmed up as in continuous printing, the printing is greatly influenced by the subtraction coefficients in table 2, but is hardly influenced by the subtraction conditions in table 3 depending on the number of printed sheets. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Subtraction Coefficient of Surface Layer of Fixing Film (Considering 
               
               
                 Elastic Deformation and Viscosity Deformation) 
               
            
           
           
               
               
               
            
               
                   
                 Surface temperature of fixing 
                   
               
               
                   
                 film 
                 Subtraction coefficient 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Less than 180° C. 
                 7 
               
               
                   
                 180° C. or more and less than 190° C. 
                 9 
               
               
                   
                 190° C. or more and less than 200° C. 
                 13 
               
               
                   
                 200° C. or more 
                 18 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Applied Time of Subtraction Coefficient to Surface Layer 
               
               
                 of Fixing Film (Considering Only Elastic Deformation) 
               
            
           
           
               
               
            
               
                 Surface temperature of fixing 
                 Applied time of coefficient 
               
               
                 film 
                 [seconds] 
               
               
                   
               
               
                 Less than 160° C. 
                 No time limit 
               
               
                 160° C. or more and less than 175° C. 
                 6 
               
               
                 175° C. or more and less than 200° C. 
                 3.5 
               
               
                 200° C. or more 
                 1 
               
               
                   
               
            
           
         
       
     
     Next, the control temperature of the heater  11  is determined from the value of the thickness coefficient T corresponding to the thickness of the surface layer (predicted information on the thickness of the surface layer) of the fixing film  13 . The control temperature is determined so that the temperature of the film surface becomes 180° C. at the continuous paper printing. 
     When the relationship between the thickness of the surface layer of the fixing film  13  and the control temperature at which the film surface is set at 180° C. in the first embodiment is calculated by approximation from  FIG. 7 , the following formula 1 is established.
 
Control temperature (° C.)=0.8×thickness (μm) of surface layer of film+184(° C.)   (Formula 1)
 
     When formula 1 is converted into a coefficient formula, the following formula 2 is established.
 
Control temperature (° C.)=0.8×thickness coefficient  T ×100,000+184(° C.)   (Formula 2)
 
     In controlling the temperature, the calculation may be performed. Alternatively, the temperature may be controlled based on a correspondence table, as shown in table 4 below. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Correspondence between Thickness Coefficient 
               
               
                 T and Control temperature 
               
            
           
           
               
               
               
            
               
                   
                   
                 Control temperature 
               
               
                   
                 Thickness coefficient T 
                 [° C.] 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 2,000,000~1600001  
                 200 
               
               
                   
                 1,600,000~1,200,001 
                 197 
               
               
                   
                 1,200,000~800,001     
                 193 
               
               
                   
                 800,000~400,001 
                 190 
               
               
                   
                 400,000 or less 
                 187 
               
               
                   
                   
               
            
           
         
       
     
     Next, a description will be given of displaying the life of the fixing film  13 . In displaying the life, settings are made with some margins. This is because an image failure, such as an image stain, occurs even when the surface layer of the fixing film  13  is slightly shaved. 
     In the first embodiment, it is determined that the fixing film  13  has come to the end of the life at a point at which the remaining surface layer of the fixing film  13  has had a thickness of 4 μm. That is, the remaining thickness is set at 4 μm. The value corresponds to 400,000 in terms of a thickness coefficient. 
     When the life is displayed by a ratio, it is assumed that the initial value of the thickness coefficient T is 100% and the coefficient 400,000 corresponding to the remaining thickness is 0%. For example, when the initial thickness coefficient T is 2,000,000, the display of the life may be reduced by 1% every time the thickness coefficient T is reduced by 16,000. 
     When the ratio display is not performed, the life of the fixing unit  6  may be displayed at a point at which the thickness coefficient has come to 400,000. 
     Next, a description will be given of the pressing roller  20 . First, a hardness coefficient D corresponding to the hardness of the pressing roller  20  is calculated. In the first embodiment, the pressing roller has a hardness of 55° in its brand-new state. In addition, the subtraction coefficient of hardness per 1° is set as 10,000,000. Therefore, the initial value of the hardness coefficient D of the pressing roller  20  is 550,000,000. The value may be changed according to the initial hardness of the pressing roller  20 . In this case, the initial coefficient is increased or decreased at a ratio of 10,000,000 relative to the hardness 1° of the pressing roller and is stored in the storage medium  33  of the fixing unit  6 . 
     In the calculation, the coefficient is subtracted every time printing is performed. The subtraction of the hardness coefficient D of the pressing roller  20  is performed every 1 second when the pressing roller  20  rotates, and the coefficient is changed, as shown in table 5, according to temperature. In table 5, a hardness change amount per rotating time corresponding to the surface temperature of the pressing roller  20  is set in advance. 
     Note that, as shown in  FIG. 10 , a change in the hardness of the pressing roller  20  is great until 100 hours in the early stage of the rotating time, and then the change amount becomes small. 
     This is because a PFA tube coating the surface of the pressing roller  20  expands in the period of the early stage and thus, the hardness reduces as the tension of the surface weakens. Therefore, it is estimated that the hardness change amount in the early stage becomes great. 
     On the other hand, since the hardness reduces only with rubber after the early stage, it is estimated that the hardness change amount becomes relatively small. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Subtraction Coefficient of Hardness of Pressing roller 
               
            
           
           
               
               
               
            
               
                   
                 Subtraction 
                 Subtraction 
               
               
                   
                 coefficient 
                 coefficient 
               
               
                   
                 (100 hours from 
                 (after 100 
               
               
                 Temperature of pressing roller 
                 early stage) 
                 hours) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Less than 100° C. 
                 28 
                 14 
               
               
                 100° C. or more and less than 150° C. 
                 42 
                 21 
               
               
                 150° C. or more and less than 185° C. 
                 70 
                 35 
               
               
                 185° C. or more 
                 100 
                 50 
               
               
                   
               
            
           
         
       
     
     Next, the control temperature is corrected based on the value of the hardness coefficient D (predicted information on the hardness) of the pressing roller  20 . 
     In correcting the control temperature, a correspondence table shown in table 6 is used. For example, the control temperature is 193° C. when the thickness coefficient T of the fixing film  13  is 1,000,000. When the hardness coefficient D of the pressing roller  20  is 535,000,000, however, the control temperature is reduced by 2° C. to be set at 191° C. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Hardness Coefficient D of Pressing roller and 
               
               
                 Correction Amounts of Control temperature 
               
            
           
           
               
               
               
            
               
                   
                   
                 Correction amount of control 
               
               
                   
                   
                 temperature 
               
               
                   
                 Coefficient D 
                 [°] 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 550,000,000~540,000,001 
                 0 
               
               
                   
                 540,000,000~530,000,001 
                 −2 
               
               
                   
                 530,000,000~520,000,001 
                 −3 
               
               
                   
                 520,000,000 or less 
                 −4 
               
               
                   
                   
               
            
           
         
       
     
     Next, a description will be given of displaying the life of the pressing roller  20 . When the hardness of the pressing roller  20  is below 50°, a rotation failure caused when a rotation load torque becomes great occurs in the driving motor adopted in the apparatus of the first embodiment. Before the rotation failure occurs, it is necessary to issue a life alert. It is assumed that the hardness at the use lower limit of the pressing roller  20  with which the motor adopted in the apparatus of the first embodiment normally operates is 51°. The hardness corresponds to 510,000,000 in terms of the hardness coefficient D. 
     When the life is displayed by ratio, it is assumed that the initial value of the hardness coefficient D is 100% and the hardness coefficient 510,000,000 corresponding to the use lower limit is 0%. For example, when the hardness coefficient D in the early stage is 550,000,000, the display of the life may be decreased by 1% every time the hardness coefficient D decreases by 400,000. 
     When the ratio display is not performed, the life of the fixing unit  6  may be displayed at a point at which the thickness coefficient has come to 510,000,000. 
     (7) Effects of Embodiment 
     According to the above procedure, the control temperature may be set at appropriate temperature even when the wearing speed of the surface layer of the fixing film  13  changes, or even when the speed of a change in the hardness of the pressing roller  20  changes depending on use conditions. Thus, the temperature of the fixing nip portion may be appropriately maintained, and excellent printing may be performed without causing problems such as a hot offset and curling. 
     In addition, in terms of the life of the fixing unit  6 , life management may be performed based on the thickness of the surface layer of the fixing film  13  and the hardness of the pressing roller  20 . Therefore, unlike a method in which the life is estimated based only on the number of printed sheets, the fixing unit  6  that has not come to the end of the life may be continuously used without being discarded. In addition, when the fixing unit  6  is used under conditions severe with regard to the wearing of the fixing film  13  and a change in the hardness of the pressing roller  20 , a life alert may be appropriately issued according to the statuses of the components. 
     Hereafter, a description will be given of specific calculation for determining an appropriate control temperature and a specific calculation for performing appropriate life management under different use conditions. In addition, a description will be given of effects under the respective use conditions. 
     First,  FIG. 15  shows the temperature transitions of the fixing film  13  and the pressing roller  20  when 60 sheets are continuously printed per minute (continuous paper feeding) as shown in  FIG. 4 . 
     In  FIG. 15 , a horizontal axis shows time, a vertical axis shows temperature, a solid line shows the temperature of the fixing film  13 , a broken line shows the temperature of the pressing roller  20 , and K shows a timing at which the apparatus temporarily stops for paper feeding. In the graph, an average temperature is shown for every 5 seconds. 
     In the continuous printing, the temperature of the pressing roller  20  gently decreases until about 50 sheets are printed but then gently increases. This is because the temperature of the pressing roller  20  gradually increases with heat supplied from the heater  11  to the pressing roller  20  between the sheets of paper and the pressing roller  20  is heated throughout. 
     An appropriate control temperature corresponding to reduction in the thickness of the surface layer of the fixing film  13  is calculated as follows. Under the paper feeding conditions, the temperature of the fixing film  13  transitions at 180° C. At this temperature, the subtraction coefficient per sheet is 9 based on table 2. 
     For example, the thickness coefficient T at a point at which 10,000 sheets have been printed is calculated as follows: 2,000,000−9×10,000=1,910,000. 
     When the thickness coefficient T is 1,910,000, the control temperature is 200° C. from table 4. 
     In addition, an appropriate control temperature corresponding to reduction in the hardness of the pressing roller  20  is calculated as follows. 
     Under the paper feeding conditions, the temperature of the pressing roller  20  does not reach 150° C. At this temperature, the subtraction coefficient is 42 based on table 5. It takes 10,000 seconds to continuously feed 10,000 sheets. The hardness coefficient D of the pressing roller  20  at this time is as follows: 550,000,000−42×10,000=549,580,000. 
     In this case, the control temperature does not change due to the hardness of the pressing roller  20 . Accordingly, the control temperature at a point at which 10,000 sheets have been printed is 200° C. 
     As described above, the control temperature calculated in terms of the reduction in the thickness of the surface layer of the fixing film  13  is 200° C., and the control temperature calculated in terms of the reduction in the hardness of the pressing roller  20  is also 200° C. Accordingly, it is determined that the appropriate control temperature at a point at which 10,000 sheets have been fed is 200° C. 
     Next, a description will be given of the life of the fixing unit  6 . When calculation is performed using the subtraction coefficient  9  until the thickness coefficient T of the fixing film  13  changes from 2,000,000 to 400,000, the fixing film  13  is allowed to print 178,000 sheets at a point at which 10,000 sheets have been printed as described below: (2,000,000−400,000)/9≈178,000. 
     As for the pressing roller  20 , a time required when the hardness coefficient D changes from 550,000,000 to 510,000,000 and the number of printable sheets at a point at which 10,000 sheets have been printed are calculated as follows: Pressing roller rotating time: (550,000,000−510,000,000)/42=952,380 (seconds)=264 hours. 
     In the first embodiment, 60 sheets are printed per minute, and one sheet is printed per second. That is, the number of printable sheets calculated from the life of the pressing roller  20  is 952,000 at a point at which 10,000 sheets have been printed. 
     When the life of the fixing film  13  is compared with that of the pressing roller  20  based on the number of usable sheets (printable sheets) representing a value converted into the number of used recording materials P (printed sheets), the number of the usable sheets of the fixing film  13  is less than that of the pressing roller  20  in the first embodiment. Therefore, the life of the fixing film  13  is regarded as the life of the fixing unit  6 . 
     Accordingly, the control temperature and the life under the paper feeding condition are as follows: Control temperature at point at which 10,000 sheets have been printed: 200° C., and the number of usable sheets (printable sheets) of the fixing unit  13  until the end of the life at point at which 10,000 sheets have been printed: 178,000 sheets. 
     As described above, the wearing of the fixing film  13  and a change in the hardness of the pressing roller  20  are small in the case of the continuous paper feeding. Therefore, a change amount of the control temperature is small, and the number of the usable sheets of the fixing unit  6  is 178,000 greater than the expected number 150,000. 
     Next,  FIG. 16  shows the temperature transitions of the fixing film  13  and the pressing roller  20  when two sheets are repeatedly continuously printed for every 8 seconds (intermittent printing) in a state in which the fixing unit  6  has been cooled as shown in  FIG. 5 . An increase in the temperature of the pressing roller  20  is greater than that in the case of the continuous paper feeding shown in  FIG. 15 . 
     Under the paper feeding condition, the temperature of the fixing film  13  transitions at 200° C. At the temperature, the subtraction coefficient per sheet is 18 based on table 2. 
     For example, the thickness coefficient T at a point at which 10,000 sheets have been printed is calculated as follows: 2,000,000−18×10,000=1,820,000. 
     When the thickness coefficient T is 1,820,000, it is determined from table 4 that the control temperature is 200° C. 
     In addition, under the paper feeding condition, the temperature of the pressing roller  20  is 175° C. At the temperature, the subtraction coefficient is 70 from table 5. In the case of the intermittent printing, it takes 8 seconds to feed one sheet. Therefore, it takes 80,000 seconds to feed 10,000 sheets. At this time, the hardness coefficient D of the pressing roller  20  is as follows: 550,000,000−70×80,000=544,400,000. 
     In this case, with reference table 6, a correction amount of the control temperature due to a reduction in the hardness of the pressing roller  20  is 0° C. 
     As a result, the control temperature at a point at which 10,000 sheets have been printed is 200° C. 
     Accordingly, under the paper feeding condition of the intermittent printing as well, the control temperature calculated in terms of reduction in the thickness of the surface layer of the fixing film  13  is 200° C., and the control temperature calculated in terms of reduction in the hardness of the pressing roller  20  is 200° C. as described above. Accordingly, it is determined that the control temperature at a point at which 10,000 sheets have been fed is 200° C. 
     When the same calculation is performed at a point at which 20,000 sheets have been fed, the control temperature determined based on reduction in the thickness of the surface layer of the fixing film  13  is 200° C. and the control temperature determined based on reduction in the hardness of the pressing roller  20  is 198° C. In this case, the control temperature determined based on reduction in the hardness of the pressing roller  20  is selected, and thus, is controlled at 198° C. 
     In addition, a description will be given of a method for calculating the life of the fixing unit  6  in the case of the intermittent printing. First, the number of the usable sheets of the fixing film  13  at a point at which 10,000 sheets have been printed is 89,000 as described below when calculated by the subtraction coefficient  18  until the thickness coefficient T of the fixing film changes from 2,000,000 to 400,000.
 
(2,000,000−400,000)/18≈89,000
 
     Next, a description will be given of the number of the usable sheets of the pressing roller  20  at a point at which 10,000 sheets have been printed. A time required when the hardness coefficient D of the pressing roller  20  changes from 550,000,000 to 510,000,000 and the number of printable sheets at a point at which 10,000 sheets have been printed are calculated as follows. 
     Based on table 5, the number of printable sheets is calculated by the subtraction coefficient  70  for the early 100 hours. 
     After that, the number of printable sheets is calculated as follows by the subtraction coefficient  35 : 
     
       
         
           
             
               
                 
                   
                     
                       100 
                       × 
                       60 
                       × 
                       60 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             550 
                             ⁢ 
                             
                               , 
                             
                             ⁢ 
                             000 
                             ⁢ 
                             
                               , 
                             
                             ⁢ 
                             000 
                           
                           - 
                           
                             510 
                             ⁢ 
                             
                               , 
                             
                             ⁢ 
                             000 
                             ⁢ 
                             
                               , 
                             
                             ⁢ 
                             000 
                           
                           - 
                           
                             70 
                             × 
                             100 
                             × 
                             60 
                             × 
                             60 
                           
                         
                         ) 
                       
                       ⁢ 
                       
                         / 
                       
                       ⁢ 
                       35 
                     
                   
                   = 
                   
                     
                       360 
                       ⁢ 
                       
                         , 
                       
                       ⁢ 
                       000 
                     
                     + 
                     
                       422 
                       ⁢ 
                       
                         , 
                       
                       ⁢ 
                       857 
                     
                   
                 
               
             
             
               
                 
                   = 
                   
                     
                       782 
                       ⁢ 
                       
                         , 
                       
                       ⁢ 
                       857 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ( 
                         seconds 
                         ) 
                       
                     
                     = 
                     
                       217 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       hours 
                     
                   
                 
               
             
           
         
       
     
     From here, the number of printable sheets is calculated as follows since it takes 8 seconds to print one sheet: 782,857 (seconds)/8 (second/sheet)=97,857 (sheets). 
     The number of printable sheets at a point at which 10,000 sheets have been printed is 98,000. 
     When the number of the printable sheets of the fixing film  13  and the number of the printable sheets of the pressing roller  20  are taken into consideration, the number of the printable sheets of the fixing film  13  is smaller. Therefore, the life of the fixing film  13  is regarded as the life of the fixing unit  6 . 
     Accordingly, the control temperature and the life of the fixing unit under the paper feeding condition are as follows: Control temperature at point at which 10,000 sheets have been printed: 200° C., Control temperature at point at which 20,000 sheets have been printed: 198° C., 
     Life of fixing unit  6  at point at which 10,000 sheets have been printed: 89,000 sheets 
     Under a severe use condition in which an increase in the temperature of the pressing roller  20  is great, as shown in  FIG. 16 , a timing at which the life of the fixing unit  6  is notified is made earlier than a case in which the first embodiment is not adopted, but the wearing of the surface layer of the fixing film  13  and an image failure due to reduction in the hardness of the pressing roller  20  may be prevented. 
     In addition, an alert may be issued before the apparatus stops due to an image stain or paper jamming due to the wearing of the fixing film  13  and the reduction in the hardness of the pressing roller  20 . As described above, a user or the manager of a network printer may be notified of the life at an accurate timing to urge the replacement of the fixing unit  6 . 
     The above embodiment describes the method for determining the control temperature and the method for calculating the notification of the life and their effects using a use mode in which the continuous printing, as shown in  FIG. 15 , is performed at all times and a use mode in which the intermittent printing, as shown in  FIG. 16 , is performed at all times as examples. A user&#39;s actual use mode is, however, a complicated mode in which the continuous printing and the intermittent printing are alternately mixed together. Accordingly, calculation corresponding to an individual use mode may be performed for every print job, every prescribed time, or every prescribed number of sheets of fed paper. Further, it is possible to perform optimum control and accurate life counting by sequentially accumulating and updating results in the storage medium  33  of the fixing unit  6 . 
     Moreover,  FIG. 17  shows temperature transitions when one sheet is printed for every 10 minutes. Like this, the fixing unit  6  is naturally cooled when the apparatus stops for a long time in operation. Even if printing is continued in this state, the fixing unit  6  is not influenced by the heat of the preceding fed paper. Therefore, both the temperature of the fixing film  13  and the temperature of the pressing roller  20  are constantly low at the start of the printing. 
     A pre-rotating operation is performed for every time printing is performed, but the fixing unit  6  is not rotationally driven until the temperature of the heater  11  becomes 170° C. Therefore, the number of the rotations of the fixing unit  6  is small. When the first embodiment is applied to such a use mode, the rotating time of the pressing roller  20  becomes short. Therefore, the reduction in the hardness of the pressing roller  20  is smaller than that of the case of the continuous paper feeding and does not have an influence on the control temperature. 
     In addition, it takes 8 seconds for the fixing film  13  to perform printing, and the printing ends before PFA of the surface layer causes viscosity deformation in a printing step. Therefore, a wearing amount becomes smaller than that of the case of the continuous printing. 
     The life (number of printable sheets) of the fixing film  13  at a point at which 10,000 sheets have been printed is about 320,000. 
     In view of the circumstances, a change in the control temperature becomes smaller and durability becomes longer in such a use mode as compared with a case in which the apparatus is continuously operated. 
     (8) Other Applied Examples of Embodiment 
     The temperature of the fixing film  13  and the pressing roller  20  is actually measured by thermopiles in the first embodiment, but the temperatures may be predicted based on a paper feeding history, or the like. 
     For example, in the configuration of the first embodiment, the surface temperature of the fixing film  13  is set at 190° C. for the first paper and set at 180° C. for the second and subsequent sheets when the continuous paper feeding as shown in  FIG. 4  is performed. In addition, the surface temperature of the fixing film  13  is set at the same temperature as the control temperature when the intermittent printing as shown in  FIG. 5  is performed. 
     Meanwhile, as for the temperature of the pressing roller  20 , one printing step is divided into temporal segments, such as a pre-rotation period, a paper feeding period, an inter-paper period, and a post-rotation period. Further, in consideration of the amount of heat supplied from the fixing film  13  to the pressing roller  20 , the amount of heat released into air, or the like, in the respective periods, the temperature of the pressing roller  20  may be estimated from the total value of the amount of the heat supplied, the amount of the heat released, or the like. For example, the temperature of the pressing roller  20  is estimated as follows:
         (i) The operation of the body is temporally divided into pre-rotation, inter-paper, paper feeding, and standby segments,   (ii) As shown in table 7, a coefficient is set for each of the segments,   (iii) Every time 100 msec elapse in each of the segments, the coefficient is added,   (iv) The added coefficient is stored as a total count, and   (v) The total count is used as the temperature of each component.       

     An initial value is determined from the temperature of the thermistor  15  provided on the rear surface of the heater  11 . 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Coefficients for Estimating Temperature of Pressing roller 
               
            
           
           
               
               
               
            
               
                   
                 Added coefficient 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 When total 
                 When total 
                 When total 
                   
               
               
                   
                 value of 
                 value of 
                 value of 
               
               
                   
                 coefficient 
                 coefficient 
                 coefficient 
                 Movement of 
               
               
                 Segment 
                 is 0~110 
                 is 110~170 
                 is 170~220 
                 heat 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Pre- 
                 +18 
                 Energization 
               
               
                 rotation 
                   
                 of heater 
               
               
                 Inter-paper 
                 +3 
                 Heater 
               
               
                   
                   
                 suppressing 
               
               
                   
                   
                 energization 
               
            
           
           
               
               
               
               
               
            
               
                 Post- 
                 −1 
                 −5 
                 −10 
                 Heater off 
               
               
                 rotation, 
               
               
                 stop 
               
               
                 Paper 
                 −0.8 
                 −1 
                 −2 
                 Transfer of 
               
               
                 feeding 
                   
                   
                   
                 heat to 
               
               
                   
                   
                   
                   
                 paper 
               
               
                   
               
            
           
         
       
     
     With the setting and the totalization of the coefficients, the temperature of the pressing roller  20  may be estimated. 
     As for the temperature of the fixing film  13 , the estimation of the temperature is made possible with the setting of dedicated coefficients. 
     In addition, the thickness coefficient T of the fixing film  13  and the hardness coefficient D of the pressing roller  20  are stored in the storage medium  33  attached to the fixing unit  6  in the first embodiment, but may be stored in a storage medium on the side of the body of the image forming apparatus. For example, in the case of an image forming apparatus in which a fixing unit is not replaceable, a memory in the body of the image forming apparatus may be used. Further, in a case in which a fixing unit is replaceable and has a plurality of electrodes and individual discrimination means for discriminating the individual type of the fixing unit according to the open/short state between the plurality of electrodes is provided, a memory in the body of an image forming apparatus may be used. 
     The temperature of the fixing film  13  and the temperature of the pressing roller  20  are monitored and used for both the control temperature and the life detection in the first embodiment, but the temperatures may be used for one of the control temperature and the life detection depending on the configuration of the apparatus. In consideration of the durability of the fixing film  13  and the pressing roller  20 , it is possible to determine whether the temperature of the fixing film  13  and the temperature of the pressing roller  20  are used for both or one of the control temperature and the life detection. 
     Second Embodiment 
     Next, a description will be given of a second embodiment of the present invention. In the case of a small image forming apparatus, an engine controller having high processing performance may not be often used to miniaturize electrical components for controlling a body. In addition, a fixing unit is fixed to the body of the image forming apparatus and is not often configured as a replaceable component. An example of performing a configuration proposed in the second embodiment in such an image forming apparatus will be described. 
       FIG. 11  shows an image forming apparatus according to the second embodiment. The same constituents as those of the first embodiment will be shown by the same symbols, and their descriptions will be omitted. The image forming apparatus according to the second embodiment prints 15 sheets of A4-size in vertical feeding per minute. The paper transporting speed of the image forming apparatus is 100 mm/second in image formation. The configuration of a fixing unit  44  is different from that of the first embodiment. 
       FIG. 12  shows a cross-sectional view of the fixing unit  44 . The fixing unit  44  includes a heating unit  34  provided with a heater  35  serving as a heating source and a cylindrical fixing film (fixing rotating member)  38  and a pressing roller  40  serving as a pressing member. A fixing nip portion N is formed by bringing the heater  35  and the pressing roller  40  into contact with each other at a prescribed pressing force via the fixing film  38 . 
     The heating unit  34  is mainly constituted of the fixing film  38 , the heater  35 , a heat insulating holder  36  that holds the heater  35 , a metal stay  37 , or the like. The metal stay  37  receives a pressing force from a spring (not shown) and presses the heat insulating holder  36  toward the pressing roller  40 . Note that reference symbol  39  shows a thermistor attached onto the rear side of the heater  35  and detects the temperature of the heater  35 . 
     The heater  35  is a ceramic heater and has a thickness of 1 mm and a width of 5 mm. The heater  35  is one obtained by forming a heat generating layer on an alumina substrate. 
     The fixing film  38  has an outer diameter of 18 mm, and the base layer of the fixing film  38  has a thickness of 70 μm. The fixing film  38  is made of polyimide (PI) in which carbon fibers are dispersed as a heat conduction filler. On the base layer of the fixing film  38 , PFA having a thickness of 13 μm is coated. 
     The pressing roller  40  has a diameter of 15 mm. The pressing roller  40  is constituted of an aluminum cored bar  41  and an elastic layer  42  and a PFA tube layer having a thickness of 30 μm provided on the aluminum cored bar  41 . The elastic layer  42  is a foam rubber layer. A load having a pressing force of 13 kgf is applied between the heating unit  34  and the pressing roller  40 . Note that the control temperature of the heater  35  at printing (fixing processing) is 200° C. 
     The fixing unit  44  is fixed to the body of the image forming apparatus and is configured to be unreplaceable. Both the body of the image forming apparatus and the fixing unit  44  come to the end of their life when 30,000 sheets are printed. 
     In addition, the fixing unit  44  is miniaturized. Therefore, the temperature of the fixing unit  44  quickly responds to the energization of the heater  35 . 
     The surface layer of the fixing film  38  is coated. Compared with a resin tube, the coating of the surface layer is lower in cost and broadens the range of selecting materials, but tends to reduce durability. Therefore, the fixing film  38  is often used in a small machine having short life. 
     The elastic layer  42  of the pressing roller  40  has a heat insulating structure in which foaming is performed or resin balloons are added to form air bubbles. In addition, the hardness of the surface of the roller may be set at about 40° using foaming rubber. The pressing roller  40  having the configuration may provide a high heat insulating performance and increase the nip width of the fixing nip portion N. The nip width having this configuration is 6 mm. 
     Next,  FIGS. 13A and 13B  show the temperature transitions of the fixing film  38  and the pressing roller  40  when paper feeding is performed with the configuration.  FIG. 13A  shows the case of continuous paper feeding, and  FIG. 13B  shows the case of intermittent paper feeding. In  FIGS. 13A and 13B , a horizontal axis shows time, and a vertical axis shows temperature. In addition, a solid line shows the temperature of the fixing film  38 , and a broken line shows the temperature of the pressing roller  40 . First, a description will be given of the case of the continuous paper feeding in  FIG. 13A . As described above, the horizontal axis and the vertical axis show the time and the temperature, respectively. In addition, the solid line and the broken line show the temperature of the fixing film  38  and the temperature of the pressing roller  40 , respectively. 
     In  FIG. 13A , a period R 5  is a pre-rotation period, and the length of the period R 5  equals 3 seconds. A period R 6  is a paper feeding period, and the length of the period R 6  when A4-size paper (297 mm) is fed equals 3 seconds. A period R 7  is an inter-paper period, and the length of the period R 7  equals 1 second. The control temperature is 200° C. 
     During the paper feeding, the surface temperature of the fixing film  38  is 195° C. when the first paper is fed, but then decreases to 180° C. when the third paper is fed. After that, the temperature of the fixing film  38  is maintained at 180° C. during continuous paper feeding. The temperature of the fixing film  38  is less than the control temperature 200° C. This is because a temperature gradient occurs between the front and rear sides of the surface layer due to the poor heat conduction of a mold releasing layer representing the surface layer of the fixing film  38 . 
     The high temperature of the fixing film  38  in the printing of the first paper results from a heat storage effect obtained when the fixing film  38  is heated in the pre-rotation. 
     The pressing roller  40  has small heat capacity and thus, the pressing roller  40  stores a small amount of heat. Therefore, the pressing roller  40  is maintained at high temperature at non-paper feeding, but the heat of the pressing roller  40  is taken away by a recording material P at the paper feeding and the temperature of the pressing roller  40  immediately decreases. 
       FIG. 13B  shows the temperature of the fixing film  38  and the pressing roller  40  in intermittent printing. In  FIG. 13B , a period R 8  is a post-rotation period and shows an example of the intermittent printing in which a next job is performed immediately after one sheet is fed as a job. In the case of the intermittent printing as well, the temperature transitions of the fixing film  38  and the pressing roller  40  are the same as those of the case of the continuous printing. Since heat is stored in the fixing film  38  at the post-rotation period R 8  and the pre-rotation period R 5 , however, the temperature of the fixing film  38  is as high as 195° C. every time. 
     In a case in which the pressing roller  40  is of a heat insulating type as in the second embodiment, a heat supply with the storage of heat in the pressing roller  40  is not performed. Therefore, the temperature of the fixing film  38  is determined only with heat supply from the heater  35  when the recording material P enters the fixing nip portion N. In addition, since the pressing roller  40  has small heat capacity, the temperature of the pressing roller  40  decreases when the recording material P enters the fixing nip portion N. Due to the small heat capacity, the temperature of the pressing roller  40  slowly increases during continuous printing. 
     Next, a description will be given, with reference to  FIGS. 18 to 20 , of the temperature transitions in cases in which such printing is repeatedly performed.  FIG. 18  shows a case in which 400 sheets are printed in the continuous paper feeding shown in  FIG. 13A .  FIG. 19  shows a case in which the intermittent printing shown in  FIG. 13B  is performed.  FIG. 20  shows a case in which one sheet is printed for every 10 minutes. 
     When the continuous printing is performed as shown in  FIG. 18 , the temperature of the fixing film  38  is stabilized at 180° C. The temperature of the pressing roller  40  gradually increases but does not exceed 100° C. 
     As shown in  FIG. 19 , the temperature of the pressing roller  40  gradually increases when the intermittent printing is repeatedly performed. In anticipation of this, the control temperature is changed. The control temperature at an early stage is 200° C. At this time, the temperature of the fixing film  38  is 195° C. When the printing time exceeds 15 minutes, the control temperature is 195° C. At this time, the temperature of the fixing film  38  is 190° C. In addition, the temperature of the pressing roller  40  exceeds 100° C. in 5 minutes. 
     When one sheet is printed at intervals, as shown in  FIG. 20 , both the temperature of the fixing film  38  and the temperature of the pressing roller  40  increase but do not continuously increase. 
     The wearing amount of the fixing film  38  becomes more disadvantageous as the diameter of the fixing film  38  is smaller. In addition, compared with a film in which a resin is molded into a tube shape and then is coated on a base layer, like a film in which a tube is coated on a base layer, a film in which a resin film is burned to be formed after being coated on a base layer-like coating tends to wear earlier. 
     Table 8 shows the relationship between temperature and wearing amounts according to the second embodiment. The wearing amounts in the table are obtained on the condition that the fixing film  38  is heated for 7 seconds or more and wears due to both elastic deformation and viscosity deformation. 
     Note that time considering the viscosity deformation is the same as that of the case of table 3. A wearing amount due to the elastic deformation is 0.07 μm/1000 sheets (0.7×10 −4  μm/sheet). 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Relationship between Temperature and Wearing Amounts 
               
            
           
           
               
               
               
            
               
                   
                 Wearing amount 
                 Wearing amount 
               
               
                 Temperature 
                 per 1000 sheets [μm] 
                 per sheet [μm] 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Less than 180° C. 
                 0.1 
                 1 × 10 −4   
               
               
                 180° C. or more and 
                 0.2 
                 2 × 10 −4   
               
               
                 less than 190° C. 
               
               
                 190° C. or more and 
                 0.3 
                 3 × 10 −4   
               
               
                 less than 200° C. 
               
               
                 200° C. or more 
                 0.5 
                 5 × 10 −4   
               
               
                   
               
            
           
         
       
     
     The hardness of the pressing roller  40  hardly changes when the pressing roller  40  is used at the temperature between room temperature and 100° C. On the other hand, the hardness of the pressing roller  40  reduces by about 1° in 20 hours when the pressing roller  40  is used at a temperature of 100° C. or more. 
       FIG. 14  shows the relationship between the thickness of the fixing film  38  and appropriate control temperature. The relationship may be expressed by the following formula:
 
Control temperature (° C.)=0.9×thickness (μm) of surface layer of film+188(° C.)  (Formula 3).
 
     Table 9 shows the hardness of the pressing roller  40 , nip widths, and correction amounts of the control temperature. 
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 Relationship between Hardness of Pressing roller, 
               
               
                 Nip Widths, and Appropriate Control temperature 
               
            
           
           
               
               
               
            
               
                   
                   
                 Change amount of 
               
               
                 Hardness of 
                   
                 appropriate 
               
               
                 pressing roller 
                 Nip width 
                 temperature 
               
               
                   
               
               
                 40° 
                 6.0 mm 
                 Reference 
               
               
                 39° 
                 6.2 mm 
                 −2° 
               
               
                 38° 
                 6.4 mm 
                 −3° 
               
               
                 37° 
                 6.6 mm 
                 −4° 
               
               
                 36° or less 
                 6.7 mm 
                 −4.5°     
               
               
                   
               
            
           
         
       
     
     Based on the above conditions, the control temperature and the life are set as follows in the second embodiment. 
     That is, for every paper feeding, inter-paper time representing time information on the interval between paper feeding at previous time and paper feeding at this time and at least one of rotation driving information and information on the number of fed recording materials P are simultaneously acquired. Information on the number of fed recording materials P of the fixing film  38  and information on the rotating time of the pressing roller  40  are simultaneously acquired and stored as heat history information in a storage medium (not shown) provided in the control portion of the image forming apparatus. 
     Then, according to the heat history information stored in the storage medium  33 , a body control portion (not shown) representing control means changes the control temperature of the heater  11 , information on the number of usable sheets, or information on a life alert as the control value of the fixing unit  44 . 
     The calculation is performed based on a change in the thickness of the fixing film  38  and a change in the hardness of the pressing roller  40 , but parameters for the calculation are estimated as follows. 
     Case 1: Case in which one sheet is printed for every 10 minutes or more: Wearing amount of film per sheet: 0.7×10 −4  μm/sheet (assuming wearing due to only elastic deformation), and Change in hardness of pressing roller per 20-hour rotating time: 0.1°/20 hours. 
     Case 2: Case in which 20 or more sheets are continuously printed: Wearing amount of film per sheet: 2×10 −4  μm/sheet, and Change in hardness of pressing roller per 20-hour rotating time: 0.1°/20 hours. 
     Case 3: Cases other than the above cases: Wearing amount of film per sheet: 3×10 −4  μm/sheet, Change in hardness of pressing roller per 20-hour rotating time, Less than 5 minutes after start of apparatus operation: 0.1°/20 hours, 5 minutes or more after start of apparatus operation: 1°/20 hours, and Corresponding one of the cases is determined for every print job to estimate the thickness of the surface layer of the fixing film  38  and the hardness of the pressing roller  40 . 
     As for the thickness of the fixing film  38 , a wearing amount is estimated for every job. Then, the wearing amount is subtracted from an initial thickness. As for the hardness of the pressing roller  40 , the hardness of the pressing roller  40  is estimated according to rotating time for every job. 
     A change in the control temperature and a reflection on the remaining life (the number of printable sheets) based on the information are made as follows. 
     As for the fixing film  38 , the control temperature is calculated using formula 3 from an obtained thickness to be changed. In addition, a life alert based on the fixing film  38  is performed at a point at which the remaining thickness has become 4 μm. 
     Next, as for the estimation of the hardness of the pressing roller  40 , a hardness reduction ratio for each of the cases is estimated according to rotating time for every job. The apparatus of the second embodiment issues an alert indicating the end of the life of the pressing roller  40  at a point at which the hardness of the pressing roller  40  has reduced by 6° from an initial value. Note that a change in load torque hardly occurs in a foaming elastic layer even if hardness changes. In addition, in a case in which a fixing unit  44  is fixed to the body of an image forming apparatus and a driving motor is commonly used between an image forming portion including a photosensitive drum  1  and a fixing unit  44  like the second embodiment, a driving torque has room in many cases and the problem of a driving failure hardly occurs. Due to these reasons, compared with a case in which the pressing roller  40  of the first embodiment that has a solid rubber layer as an elastic layer is used, a life alert may be issued at a point at which a change in the hardness of the pressing roller  40  has become great when the pressing roller  40  of the second embodiment is used. 
     A specific example will be described below. A job in which the continuous paper feeding as shown in  FIG. 13A  is performed corresponds to the case 2. At paper feeding, the temperature of the fixing film  38  does not exceed 180° C. and the temperature of the pressing roller  40  does not exceed 100° C. 
     The control temperature and the life (body life) of the fixing unit  44  at a point at which 10,000 sheets have been printed are as follows: Control temperature at point at which 10,000 sheets have been printed: 198° C., and Life of fixing unit at point at which 10,000 sheets have been printed: 45,000 sheets. 
     A job in which the intermittent paper feeding as shown in  FIG. 13B  is performed corresponds to the case 3. At paper feeding, the temperature of the fixing film  38  is between 190° C. and 195° C. and the temperature of the pressing roller  40  exceeds 100° C. after 5 minutes. 
     The control temperature and the life (body life) of the fixing unit  44  at a point at which 10,000 sheets have been printed are as follows: Control temperature at point at which 10,000 sheets have been printed: 195° C., and Life of fixing unit at point at which 10,000 sheets have been printed: 30,000 sheets. 
     In the case 2, the hardness of the pressing roller  40  changes, but the fixing unit  44  comes to the end of the life due to the thickness of the surface layer of the fixing film  38 . Therefore, the life of the fixing unit  44  is not influenced by the pressing roller  40 . 
     A job in which printing is performed once for every 10 minutes as shown in  FIG. 20  corresponds to the case 1. At paper feeding, the temperature of the fixing film  38  is 195° C. The printing ends, however, before elastic deformation occurs in the surface layer of the fixing film  38 . Accordingly, a wearing amount per sheet is 0.7×10 −4  μm. The temperature of the pressing roller  40  does not exceed 100° C. 
     The control temperature and the life (body life) of the fixing unit  44  at a point at which 10,000 sheets have been printed are as follows: Control temperature at point at which 10,000 sheets have been printed: 199° C., and Life of fixing unit  44  at point at which 10,000 sheets have been printed: 128,000 sheets. 
     As described above, an image failure due to the inappropriate setting of the control temperature does not occur and the life alert may be appropriately issued in any of the applied examples. 
     In actual use situations, it appears that the cases 1, 2, and 3 are mixed together. The setting of the control temperature and the issuance of the life alert may, however, be appropriately performed with the correction of the predicted values of the thickness and the hardness of the pressing roller  40  for every job. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.