Abstract:
A fixing apparatus in which an unfixed image is fixed on a recording medium for bearing the unfixed image by pinching and conveying the recording medium by using a fixing body and a pressure body, wherein in the fixing apparatus in which the pressure body has a core metal, an elastic body layer for covering an outer periphery of the core metal, and a mold releasing layer for covering an outer periphery of a elastic body layer, the controller sets electric energy per predetermined time supplied from the power supply to the pressure body heater to a predetermined quantity or a lower quantity in such a manner that a temperature of the core metal of the pressure body is maintained at a predetermined temperature or a lower temperature.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus using an electrophotographic system. 
     2. Related Background Art 
     Conventionally, in an image forming apparatus adopting an electrophotographic system, a laser printer and the like have been known. Description will be given as to a fixing apparatus provided to a prior art image forming apparatus. 
     In the fixing apparatus provided to the prior art image forming apparatus, a heat fixing apparatus is often used in order perform fixing processing on transferring paper which is a recording medium for bearing a toner image as an unfixed image. That is because the heat fixing apparatus has many merits. For example, the heat fixing apparatus is superior in the storage stability, capable of supporting various devices from a low-speed device to a high-speed device, and has many options of toner. Further, there has been conventionally known a fixing apparatus configured to heat a pressure roller from the inside in order to prevent the pressure roller as a pressure body from being fouled. 
     Here, a heat roller fixing apparatus as an example of such a fixing apparatus will be described in detail based on FIG.  7 . 
     FIG. 7 is a cross-sectional view showing a schematic structure of a heat roller fixing apparatus. 
     The heat roller fixing apparatus includes: a fixing roller  121  as a fixing body; a pressure roller  122  as a pressure body; a heater  135 ( a ) as fixing body heating means which is a halogen lamp as a heating source for the fixing roller  121 ; a heater  135 ( c ) as pressure body heating means which is a halogen lamp as a heating source for the pressure roller  122 ; temperature adjusting thermistors  129  and  130  as temperature detecting means for the fixing roller  121  and the pressure roller  122 ; a release claw  125 ; and a fixing inlet guide  134 . In this example, the both temperature adjusting thermistors  129  and  130  are provided in non-image areas. It is to be noted that the temperature adjusting thermistor  130  is used for a non-printing surface side and it may be hence provided at the center. 
     The fixing roller  121  is configured to have a primer layer (not shown) as an adhesive layer on a hollow core metal  123  made of aluminium, iron or stainless steel and further a fluorocarbon resin layer  124  on the primer layer. As to a thickness of the primer layer (not shown), 5 to 20 μm is appropriate, and 30 to 70 μm is appropriate for a thickness of the fluorocarbon resin layer  124 . This fluorocarbon resin layer  124  may be formed by using a tube consisting of PFA resin or by baking the PFA resin. 
     Since the fixing roller  121  is constituted attaching a high value on the mold releasing property of the surface, the fluorocarbon resin is constituted by pure PFA resin in which a filler is not mixed. A silicon rubber layer may be provided between the fluorocarbon resin layer  124  and the core metal  123  according to needs. 
     The pressure roller  122  has a structure that an elastic body layer  127  consisting of silicon rubber having the heat resistance is bonded on a hollow core metal  126  consisting of aluminium, iron or stainless steel through a primer layer (not shown) and a PFA resin tube  128  is applied on the uppermost layer portion. As this PFA resin tube  128 , there is used a material in which carbon is mixed so that the resistance value becomes 10 4  to 10 12  Ω·cm in order to increase the bias effect from the pressure roller. 
     The heaters  135 ( a ) and  135 ( c ) as heating sources for the fixing roller  121  and the pressure roller  122  are provided inside the fixing roller  121  and the pressure roller  122 , respectively, and the heater  135 ( a ) and the heater  135 ( c ) are configured to be driven by the same power supply circuit as controlling means. By using one drive circuit for the heater  135 ( a ) and the heater  135 ( c ), the heater  135 ( a ) in the fixing roller  121  and the heater  135 ( c ) in the pressure roller  122  can be alternatively energized in accordance with each control unit (in this example 1000 ms) which is a predetermined period in time with a predetermined time length ratio (which will be referred to as a lighting ratio hereinafter), and more power can be supplied to the fixing roller  121  provided on a printing surface side (side carrying the toner  132 ) of the transferring material to be subjected to fixation, thereby avoiding the power loss caused due to a switching time of a switching device. 
     Here, FIG. 8 shows a heater lighting result when an A4-size landscape sheet and an LGL-size sheet are supplied. 
     As shown in FIG. 8, in case of feeding the A4-size landscape sheet, the heater  135 ( a ) is in the on state with the time length of 80% in the control unit in order to maintain the fixing roller  121  at a fixed temperature. Since the heater  135 ( a ) and the heater  135 ( c ) are alternatively turned on, the heater  135 ( c ) is in the on state only 20%. On the contrary, in case of feeding the LGL-size sheet, the heater  135 ( a ) is in the on state only 40% in order to maintain the fixing roller  121  constant, and the heater  135 ( c ) is in the on state 60%. The paper size and the heater lighting ratio of each roller are as shown in Table 1. 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Lighting ratio 
                 Lighting ratio 
               
               
                   
                   
                 (%) of fixing 
                 (%) of pressure 
               
               
                   
                 Paper size x (mm) 
                 heater 
                 heater 
               
               
                   
                   
               
             
             
               
                   
                 A4-size landscape 
                 90 
                 10 
               
               
                   
                 sheet and others 
               
               
                   
                 (x ≧ 297) 
               
               
                   
                 LTR-size landscape 
                 80 
                 20 
               
               
                   
                 sheet and others 
               
               
                   
                 (279 ≦ x &lt; 297) 
               
               
                   
                 B4-size landscape 
                 60 
                 40 
               
               
                   
                 sheet and others 
               
               
                   
                 (257 ≦ x &lt; 279) 
               
               
                   
                 LGL/envelope and 
                 40 
                 60 
               
               
                   
                 others (x &lt; 257) 
               
               
                   
                   
               
             
          
         
       
     
     When the heater  135 ( a ) is not turned on, the high fixing property can be satisfied by heating the pressure roller  122  from the inside. In the above-described structure, both the hardness and the wall thickness of the pressure roller  122  are restricted in order to maintain a nip width to a fixed value or a higher value. 
     However, the above-described conventional structure/controller have the following drawbacks. 
     In the structure mentioned above, since the wall thickness of the pressure roller is large, a considerable difference in temperature is generated between the core metal temperature and the surface temperature of the pressure roller depending on the lighting ratio of the heater for the pressure roller. The heater control is determined based on a temperature of the fixing roller on the printing surface side on which the fixing property greatly depends. Therefore, when the power size is small, excessive power is supplied to the heater for the pressure roller, thereby increasing the temperature of the core metal of the pressure roller. As a result, the primer between the core metal and the rubber is deteriorated, and the pressure roller is damaged. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a fixing apparatus and an image forming apparatus having this fixing apparatus capable of avoiding thermal degradation of a pressure body due to excessive increase in temperature of a core metal of the pressure body. 
     To achieve this aim, according to the present invention, there is provided a fixing apparatus or an image forming apparatus, comprising: a fixing body and a pressure body which are pressed against each other and rotate; fixing body heating means for heating the fixing body upon receiving supply of power from a power supply; pressure body heating means for heating the pressure body upon receiving supply of power from the power supply; temperature detecting means for detecting a temperature of at least one of the fixing body and the pressure body; and controlling means for controlling power supplied to the fixing body heating means and the pressure body heating means from the power supply in such a manner that a temperature detected by the temperature detecting means becomes a predetermined set temperature, an unfixed image being fixed on a recording medium by heating and pressing the recording medium bearing the unfixed image while pinching and conveying the recording medium, the pressure body having a core metal, an elastic body layer covering an outer periphery of the core metal, and a mold releasing layer covering an outer periphery of the elastic body layer, wherein the controlling means sets the electric energy per predetermined time supplied from the power supply to the pressure body heating means to be equal to or lower than a predetermined quantity in such a manner that a temperature of the core metal of the pressure body is maintained at a predetermined temperature or a lower temperature. 
     According to the present invention, since the electric energy per predetermined time supplied from the power supply to the pressure body heating means is set to a predetermined quantity or a lower quantity by the controlling means in such a manner that a temperature of the core metal of the pressure body is maintained at a predetermined temperature or a lower temperature, it is possible to avoid thermal degradation of the pressure body due to excessive increase in temperature of the core metal of the pressure body. 
     Preferably, a power supply circuit for the fixing body heating means sets the electric energy per predetermined time supplied from the power supply to the pressure body heating means to a predetermined quantity or a lower quantity in such a manner that a temperature of the core metal of the pressure body can be maintained at a predetermined temperature or a lower temperature. Since the power supply circuit for the fixing body heating means sets the electric energy per predetermined time supplied from the power supply to the pressure body heating means to a predetermined quantity or a lower quantity in such a manner that a temperature of the core metal of the pressure body is set at a predetermined temperature or a lower temperature, it is possible to prevent thermal degradation of the pressure body caused due to excessive increase in temperature of the core metal of the pressure body. 
     Preferably, by the controlling means, the fixing body heating means and the pressure body heating means are driven by the same power supply (source) circuit. 
     Preferably, with the controlling means, by setting a value of a voltage supplied form the power supply to the pressure body heating means so as to be small as a width of the recording medium to be subjected to fixation in a direction orthogonal to a conveying direction becomes small, the electric energy per predetermined time supplied from the power supply to the pressure body heating means is set to a predetermined quantity or a lower quantity. Since a value of the voltage from the power supply to the pressure body heating means can be set smaller by the controlling means as the width of the recording medium to be subjected to fixation becomes small, it is possible to prevent thermal degradation of the pressure body caused due to excessive increase in temperature of the core metal of the pressure body. 
     Preferably, the temperature detecting means detects a temperature of the pressure body. The temperature detecting means detects a temperature of the pressure body, and the electric energy per predetermined time supplied from the power supply to the pressure body heating means is set to a predetermined quantity or a lower quantity by the controlling means so that a temperature of the core metal of the pressure body can be maintained at a value not more than a predetermine value based on the above-mentioned temperature, thereby avoiding thermal degradation of the pressure body caused due to excessive increase in temperature of the core metal of the pressure body. 
     Preferably, the temperature detecting means detects a temperature of the core metal of the pressure body. The temperature detecting means detects a temperature of the core metal of the pressure body, and the electric energy per predetermined time supplied from the power supply to the pressure body heating means is set to a predetermined quantity or a lower quantity by the controlling means so that the above-described temperature can be maintained at a predetermined temperature or a lower temperature, thereby preventing thermal degradation of the pressure body caused due to excessive increase in temperature of the core metal of the pressure body. 
     Preferably, the fixing body heating means is constituted by two fixing body heating bodies having luminous intensity distributions different from each other, which are provided inside the fixing body and generate heat by power from the power supply. The pressure body heating means is constituted by one pressure body heating body which is provided inside the pressure body and generates heat by power from the power supply. Any one of the two fixing body heating bodies and the pressure body heating body are driven by the same power supply circuit. By using the power supply circuit for one fixing body heating body, the electric energy per predetermined time supplied from the power supply to the pressure body heating body is set to a predetermined quantity or a lower quantity so that a temperature of the core metal of the pressure body can be maintained at a predetermined temperature or a lower temperature, thereby preventing thermal degradation of the pressure body caused due to excessive increase in temperature of the core metal of the pressure body. 
     Preferably, with the controlling means, a set temperature for the pressure body heating means is determined so as to be small as the width of the recording medium to be subjected to fixation in a direction orthogonal to a conveying direction becomes smaller, thus setting the electric energy per predetermined time supplied from the power supply to the heating body heating means to a predetermined quantity or a lower quantity. By the controlling means, a value of the voltage from the power supply to the pressure body heating means is set so as to be small as the width of the recording medium to be subjected to fixation is decreased so that a temperature of the core metal of the pressure body can be maintained at a predetermined temperature or a lower temperature, and it is hence possible to prevent thermal degradation of the pressure body caused due to excessive increase in temperature of the core metal of the pressure body. 
    
    
     These and other objects, features and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view showing a schematic structure of an image forming apparatus according to a first embodiment of the present invention; 
     FIG. 2 is a cross-sectional view showing a schematic structure of a fixing apparatus provided to the image forming apparatus depicted in FIG. 1; 
     FIG. 3 is a block diagram for illustrating drive control of fixing body heating means and pressure body heating means provided on a fixing apparatus shown in FIG. 2; 
     FIG. 4 is a view showing drive timings of the fixing body heating means and the pressure body heating means in the first embodiment according to the present invention; 
     FIG. 5 is a view showing an arrangement position of temperature detecting means in a fixing apparatus according to a third embodiment of the present invention; 
     FIG. 6 is a block diagram for illustrating drive control of the fixing body heating means and the pressure body heating means according to a fourth embodiment of the present invention; 
     FIG. 7 is a cross-sectional view showing a schematic structure of a conventional fixing apparatus; and 
     FIG. 8 is a view showing drive timings of fixing body heating means and pressure body heating means in the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments according to the present invention will now be described with reference to the accompanying drawings. 
     First Embodiment 
     A first embodiment according to the present invention will be first explained. 
     FIG. 1 is a cross-sectional view showing a schematic structure of an image forming apparatus according to this embodiment. 
     This image forming apparatus includes, as shown in FIG. 1, a scanner unit  1  having optical means and scanning means for emitting and scanning a laser beam transmitted in accordance with image information, and a process cartridge  10  including therein main image forming means. 
     The process cartridge  10  is constituted by a photosensitive drum  3  as a latent image bearer, a roller charger  4  consisting of semi-conductive rubber, a developing apparatus  5  for developing toner  6  on the photosensitive drum  3 , a cleaner  8  for removing waste toner from the photosensitive drum  3 , and others. 
     The photosensitive drum  3  in the process cartridge  10  rotates in a direction indicated by an arrow. After the surface of the photosensitive drum  3  is uniformly charged by the roller charger  4 , the photosensitive drum  3  is irradiated with a laser beam emitted by the scanner unit  1  through a mirror  2 , thereby forming an electrostatic latent image on the surface thereof. Further, toner is supplied by the developing apparatus  5 , and this electrostatic latent image is visualized as a toner image. 
     On the other hand, transferring materials (weighting capacity, basis weight: 64 to 128 g) in a sheet feeding cassette  11  are separated one by one and fed by a sheet feeding roller  13 . The fed transferring material  12  is conveyed to a pair of registration rollers  15  along upper and lower guides  14 . The registration rollers  15  are at a halt till the transferring material  12  reaches them. When the end of the transferring material  12  is brought into contact with the registration rollers  15 , skew feeding of the transferring material  12  can be corrected. Subsequently, the registration rollers  15  convey the transferring material  12  to a transferring portion at a position opposed to the photosensitive drum  3  and the transferring roller  7  so as to be synchronized with the end of an image formed on the photosensitive drum  3 . It is to be noted that a sheet feeding sensor (not shown) is provided in the vicinity of the registration rollers  15 , and the state of paper supply, jam, and a length of the transferring material are detected. 
     Electric charges whose polarity is opposite to that of the toner is given by the transferring roller  7  from the back side to the transferring material  12  conveyed to the transferring portion as described above, and a toner image formed on the photosensitive drum  3  is transferred onto the transferring material  12 . The transferring material  12  to which the toner image has been transferred is conveyed to a heat fixing apparatus  18  by a conveying roller  17  and a conveying guide  16 , and the heat fixing apparatus  18  obtains a recording image by fusing and fixing onto the transferring material  12  the unfixed toner image on the transferring material  12  by using heat and pressure. The transferring material  12  after image fixation is ejected to a discharge tray  20  through each conveying roller selected by a flapper. 
     The heat fixing apparatus  18  will now be described in detail with reference to FIG.  2 . 
     FIG. 2 is a cross-sectional view showing a schematic structure of the heat fixing apparatus  18 . 
     The heat fixing apparatus  18  includes a fixing roller  21  as a fixing body, a pressure roller  22  as a pressure body, heaters  35 ( a ) and  35 ( b ) as fixing body heating means which is halogen lamps as heating sources for the fixing roller, a heater  35 ( c ) which is a halogen lamp as pressure body heating means as a heating source for the pressure body, temperature adjusting thermistors  29  and  30  as temperature detecting means for the fixing roller  21  and the pressure roller  22 , a separation claw  25 , and a fixing inlet guide  34 . In this embodiment, the both temperature adjusting thermistors  29  and  30  are provided in non-image areas. It is to be noted that the temperature adjusting thermistor  30  is provided on the non-printing surface side and hence it may be set in the center. 
     The fixing roller  21  is configured to have a primer layer (not shown) as an adhesive layer on a hollow core metal  23  consisting of aluminium, iron or stainless steel and further have a fluorocarbon resin layer  24  on the primer layer. 5 to 20 μm is appropriate for a thickness of the primer layer (not shown), and 30 to 70 μm is appropriate for a thickness of the fluorescent resin layer  24 . This fluorescent resin layer  24  may be formed by using a PFA resin tube or by baking PFA resin. 
     Since the fixing roller  21  is constituted putting a high value on the mold releasing property of the surface thereof, it is constituted by pure PFA resin in which a filler is not mixed in the fluorescent resin. A silicon rubber layer may be provided between the fluorescent resin layer  24  and the core metal  23  according to needs. 
     The pressure roller  22  has a structure that an elastic body layer  27  consisting of silicon rubber with the heat resistance is bonded on a hollow core metal  26  consisting of aluminium, iron or stainless steel through a primer layer (not shown) and a PFA resin tube  28  is applied on an uppermost layer portion. As this PFA resin tube  28 , a material in which carbon is mixed is used so that the resistance value can become 10 4  to 10 12  Ω·cm in order to increase the bias effect from the pressure roller. 
     In this embodiment, the heater  35 ( a ) and the heater  35 ( c ) as heating sources for the fixing roller  21  and the pressure roller  22  are provided inside the fixing roller  21  and the pressure roller  22 , and the heater  35 ( a ) and the heater  35 ( c ) are configured to be driven by the same power supply circuit. A maximum energization heat rate of the heater  35 ( c ) is restricted to a predetermined value or a lower value. 
     The heat fixing apparatus  18  is an example of a central reference that a transferring material whose maximum paper supply size is an A3 (297 mm) width is conveyed with a paper supply center of the apparatus as a reference. 
     The fixing roller  21  is a roller using aluminium for the core metal and having a diameter of 50 mm and a thickness of 3.0 mm. A fluorocarbon resin layer  24  as a PFA mold releasing layer is applied on the surface layer of the fixing roller  21 . In this embodiment, a silicon rubber layer  37  having a thickness of 25 μm is provided between the fluorocarbon resin layer  24  and the core metal  23  in order to improve the fixation property. 
     The pressure roller  22  has a 5-mm elastic body layer  27  consisting of silicon rubber having the heat resistance on a roller of an aluminium core metal  26  having a diameter of 40 mm and a thickness of 4 mm and a PFA resin tube  28  as a mold releasing layer for the surface layer thereof. The pressure roller  22  having a diameter of 40 mm and the product hardness of 600 is used. When the pressure of 100 N is applied, a nip width of 6.0 mm is formed between the fixing roller  21  and the pressure roller  22 . 
     Further, thermistors as temperature detecting means are provided in non-image areas of the fixing roller  21  and the pressure roller  22 . When the fixing roller  21  and the pressure roller  22  are heated from inside by the heater  35 ( a ), the heater  35 ( b ) and the heater  35 ( c ), a surface temperature can be accurately adjusted, thereby assuring the stable fixing property. In this embodiment, printing of 40 sheets of the A4-size landscape/min is enabled in this structure. 
     The heater  35 ( a ), the heater  35 ( b ) and the heater  35 ( c ) as heating sources for the fixing roller  21  and the pressure roller  22  are provided inside the fixing roller  21  and the pressure roller  22 , respectively, and the heater  35 ( a ) and the heater  35 ( c ) are configured to be driven by the same power supply circuit. The heater  35 ( a ) capable of providing a rated output of 700 W upon input of 120 V, the heater  35 ( b ) capable of providing a rated output of 300 W upon input of 120 V and the heater  35 ( c ) capable of providing a rated output of 300 W upon input of 120 V are used, and the luminous intensity distribution of the heaters are symmetrical with respect to the paper supply reference. 
     By dividing the heating source for the fixing rollers  21  into two heaters  35 ( a ) and  35 ( b ), the maximum lighting duty of the heater  35 ( c ) is increased, and a heat quantity for heating the pressure roller  22  is also increased, thereby performing heating with power being effectively distributed to the pressure roller  22  side. Furthermore, a number of the drive circuit is one because the power loss caused during changeover of the switching device can be avoided by alternately energizing one heater  35 ( a ) in the fixing roller  21  and the heater  35 ( c ) in the pressure roller  22 . 
     Specifically, the controlling means according to this embodiment has, as shown in FIG. 3, a circuit  61  for driving the heater  35 ( a ) and a circuit  62  for driving the heater  35 ( b ) and the heater  35 ( c ). Using these independent two drive circuits can control energization of the heaters  35  consisting of three halogen heaters as heating sources from the power supply. 
     Switching devices  63  and  64  are connected to the heater  35 ( b ) and the heater  35 ( c ) and adjust the temperature by selectively switching the heater  35 ( b ) and the heater  35 ( c ). 
     In this embodiment, a low-voltage power supply  65  connected to an outlet  66  supplies an alternating voltage of 90 V to 120 V (or 200 V to 230 V) to the circuits  61  and  62 . 
     FIG. 4 shows the lighting control of each heater when paper sheets of the A4 portrait size are continuously printed. 
     At first, the heater  35 ( a ) as a main heater for warming the fixing roller  21  and the heater  35 ( b ) as a sub heater are controlled in such a manner that a temperature distribution in the longitudinal direction of the fixing roller  21  is maintained constant by setting the lighting ratio of the heater  35 ( a ) and the heater  35 ( b ) in the control unit (one second in FIG. 4) to 50% without variation and that a temperature of the fixing roller  21  is maintained at 190° C. which is a set temperature by changing the lighting duty in the control unit while making reference to a thermistor detected temperature. 
     On the other hand, the pressure heater for warming the pressure roller  22  is controlled in such a manner that the heater  35 ( c ) is turned on if a temperature detected by the temperature adjusting thermistor  30  of the pressure roller  22  is not more than a target temperature which is a set temperature when the heater  35 ( b ) is in the off state and that the heater  35 ( c ) is not turned on if that detected temperature is not less than the target temperature. 
     Here, the control for driving the heater  35 ( c ) for the pressure roller  22  according to this embodiment will now be described. 
     Since the lighting ratio of the heater  35 ( b ) for the fixing roller  21  (electric energy per predetermined time from the power supply) is lowered when the paper size is decreased, a number of times of lighting the heater  35 ( c ) for the pressure roller  22  is increased. As a result, a temperature of a core metal of the pressure roller  22  is increased, and the primer layer of the pressure roller  22  is destroyed. In this embodiment, however, heat degradation of the pressure roller  22  is prevented by lighting the heater  35 ( c ) for the pressure roller  22  with the lighting ratio of 20% irrespective of the paper size, as similar to the paper of the A4-size portrait. Table 2 shows the relationship between the lighting ratio of the heater  35 ( c ) for the pressure roller and the life duration of the pressure roller  22 . 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Lighting ratio 
                 Life duration of 
               
               
                   
                   
                 of pressure 
                 pressure roller 
               
               
                   
                 Paper size x (mm) 
                 heater (%) 
                 (number of sheets) 
               
               
                   
                   
               
             
             
               
                   
                 A4-size landscape 
                 20% 
                 500 K 
               
               
                   
                 sheet and others 
               
               
                   
                 (x ≧ 297) 
               
               
                   
                 LTR-size landscape 
                 50% 
                 300 K 
               
               
                   
                 sheet and others 
               
               
                   
                 (279 ≦  x &lt; 297) 
               
               
                   
                 B4-size landscape 
                 80% 
                 100 K 
               
               
                   
                 sheet and others 
               
               
                   
                 (257 ≦ x &lt; 279) 
               
               
                   
                 LGL/envelope and 
                 98% 
                  50 K 
               
               
                   
                 others (x &lt; 257) 
               
               
                   
                   
               
             
          
         
       
     
     As described above, the life duration of the pressure roller  22  varies by the lighting ratio of the heater  35 ( c ) for the pressure roller  22 . In this embodiment, by setting the lighting ratio of the pressure roller  22  to 20% irrespective of the paper size, the life duration of the pressure roller becomes not less than 500 K irrespective of the paper size. 
     Therefore, according to this embodiment, performing the above-described heater control can prevent heat degradation of the pressure roller from occurring. In this embodiment, although the lighting ratio of the heater for the pressure roller is 20%, the similar effects can be obtained as long as the lighting ratio falls within a range of 0 to 20% taking the fixation property or the structure of the pressure roller and the like into consideration. 
     Incidentally, as similar to the prior art shown in FIG. 7, it is needless to say that the similar effects can be obtained when a number of the heat source for the fixing roller is one. 
     Second Embodiment 
     A second embodiment according to the present invention will now be described. Like reference numerals denote parts similar to the structures in the first embodiment, thereby omitting their explanation. 
     In this embodiment, a set temperature (limit temperature) for the heater  35 ( c ) is determined so as to be small as the width of the paper to be subjected to fixation in a direction orthogonal to the conveying direction is decreased. As a result, the electric energy per predetermined time supplied from the power supply to the heater  35 ( c ) is set to be not more than a predetermined quantity, thereby preventing heat degradation of the pressure roller. 
     That is, in this embodiment, the limit temperature of the pressure roller  22  is different from that in the first embodiment. In the first embodiment, the temperature adjusting thermistor provided to the pressure roller  22  supplies the lighting signal of the heater till a fixed temperature is obtained irrespective of the paper size. In this embodiment, however, the limit temperature of the pressure roller is changed in accordance with the paper size. A temperature detecting device for detecting a surface temperature of the pressure roller  22  is provided to an end portion (non-image area) of the pressure roller  22  in order to control the temperature setting of the pressure roller  22 . 
     In this embodiment, restricting the temperature of the pressure roller can prevent heat degradation of the pressure roller, as shown in Table 3. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Temperature 
                 Temperature 
                   
               
               
                   
                 setting in 
                 setting in 
                 Life duration of 
               
               
                   
                 conventional 
                 this 
                 pressure roller 
               
             
          
           
               
                 Paper size x 
                 art 
                 embodiment 
                 Conventional 
                 This 
               
               
                 (mm) 
                 (°C.) 
                 (°C.) 
                 art 
                 embodiment 
               
               
                   
               
               
                 A4-size 
                 180 
                 180 
                 500 K 
                 500 K 
               
               
                 landscape 
               
               
                 sheet and 
               
               
                 others (x ≧ 
               
               
                 297) 
               
               
                 LTR-size 
                 180 
                 170 
                 300 K 
                 500 K 
               
               
                 landscape 
               
               
                 sheet and 
               
               
                 others (279 ≦ 
               
               
                 x &lt; 297) 
               
               
                 B4-size 
                 180 
                 165 
                 200 K 
                 600 K 
               
               
                 landscape 
               
               
                 sheet and 
               
               
                 others (257 ≦ 
               
               
                 x &lt; 279) 
               
               
                 LGL/envelope 
                 180 
                 160 
                 100 K 
                 650 K 
               
               
                 and others 
               
               
                 (x &lt; 257) 
               
               
                   
               
             
          
         
       
     
     Therefore, according to this embodiment, varying the limit temperature of the pressure temperature in accordance with the paper size can control the heater lighting and prevent heat degradation of the pressure roller. Incidentally, it is needless to say that the similar effects can be obtained when one heater for the fixing roller is provided. 
     Third Embodiment 
     A third embodiment according to the present invention will now be described. It is to be noted that like reference numerals denote parts similar to the structures in the first embodiment, thereby omitting their explanation. 
     In this embodiment, by configuring the temperature detecting means to detect a temperature of the core metal of the pressure body, the temperature detecting means detects a temperature of the core metal of the pressure body, and the electric energy per predetermined time supplied from the power supply to the pressure body heating means is set to be not more than a predetermined quantity in such a manner that the above-mentioned temperature is maintained at a predetermined temperature or a lower temperature, thereby preventing heat degradation of the pressure roller. 
     FIG. 5 shows a structure of a fixing apparatus used in this embodiment. 
     A difference of the fixing apparatus according to this embodiment from the structure of the first embodiment is a position at which the thermistor for the pressure roller is set. Although the thermistor is set on the surface of the pressure roller  22  in the fixing apparatus according to the first embodiment as similar to the temperature adjusting thermistor  30  in FIG. 2, setting the thermistor to the core metal of the pressure roller  22  as similar to the temperature adjusting thermistor  50  in FIG. 5 enables accurate detection of a temperature of the core metal and assuredly prevents heat degradation of the pressure roller caused due to increase in the temperature of the core metal in this embodiment. 
     In this embodiment, the pressure thermistor is set to the core metal, and the temperature adjusting control of the thermistor is effected at 230° C. which is lower than a durability limit temperature of the core metal and the primer layer of the rubber layer by 20° C. As a result, the life duration of the pressure roller is not less than 500 K. 
     Thus, according to this embodiment, in the mechanism for adjusting a temperature of the pressure roller from the inside, setting the pressure roller thermistor to the core metal of the pressure roller can further effectively prevent heat degradation of the pressure roller. Incidentally, it is needless to say that the similar effects can be obtained by directly detecting a temperature of the core metal of the pressure roller irrespective of a number of heaters for the fixing roller. 
     Fourth Embodiment 
     A fourth embodiment according to the present invention will now be described. It is to be noted that like reference numerals denote structures similar to those in the first embodiment, thereby omitting their explanation. 
     This embodiment shows an example in which heat degradation of the pressure roller is prevented by changing an input voltage to the pressure heater in accordance with the paper size. 
     A different point is that the input voltage to the pressure heater is changed. As a result, it is possible to suppress rapid increase in the temperature of the core metal of the pressure roller. The input voltage to the pressure heater is classified into three stages in accordance with the paper size by using a constant voltage difference. 
     Since the same power supply circuit is used, a voltage of 90 V which is similar to that in the first embodiment is inputted to the sub heater of the fixing roller. As the pressure heater and the sub heater for the fixing roller, heaters with a rated voltage of 90V and 300 V are used. FIG. 8 shows the input voltage used in this embodiment. The heaters  35 ( a ) and  35 ( b ) are similar to the first embodiment. 
     The voltage inputted to the heater  35 ( c ) is changed by varying an output voltage of the heater  35 ( c ) by using the circuit  67  shown in FIG.  6 . Table 4 shows the relationship between the input voltage and the life duration of the pressure heater obtained by using the above-mentioned heaters. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                   
                 Life duration of 
               
               
                   
                   
                 Input voltage 
                 pressure roller 
               
               
                   
                 Paper size x (mm) 
                 (V) 
                 (number of sheets) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 A4-size landscape 
                 90 
                 500 K 
               
               
                   
                 sheet and others 
               
               
                   
                 (x ≧ 297) 
               
               
                   
                 B4-size landscape 
                 76.5 
                 550 K 
               
               
                   
                 sheet and others 
               
               
                   
                 (257 ≦ x &lt; 279) 
               
               
                   
                 LGL/envelope and 
                 45 
                 650 K 
               
               
                   
                 others (x &lt; 257) 
               
               
                   
                   
               
             
          
         
       
     
     Thus, according to this embodiment, heat degradation of the pressure roller can be prevented by changing the input voltage when feeding the A4-size landscape paper in accordance with the paper size. It is to be noted that this effect is effective for heat degradation of the pressure roller irrespective of a number of heating sources of the fixing roller. 
     While the invention has been described with reference to the structure disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims.