Patent Publication Number: US-7582344-B2

Title: Heat roller

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation of application Ser. No. 10/739,031 filed Dec. 19, 2003, which is a continuation of PCT/JP02/05442, filed on Jun. 3, 2002. The entire disclosures of the prior applications are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a heat roller. More particularly, the present invention relates to a heat roller suitable to be used, for example, for a fixing device used in an electrophotographic device. 
     BACKGROUND ART 
     An electrophotographic device (copying machine, facsimile device, printer and the like) has an image forming device and a fixing device for fixing an image formed and transferred onto a sheet by the image forming device. The fixing device includes a heat roller. 
     A heat roller is formed of a metallic ring member, rubber covering the metallic ring member and a halogen lamp arranged inside the metallic ring member. However, the halogen lamp is low in thermal efficiency, and moreover, the rubber covering the metallic ring member reduces the thermal efficiency. In addition, it takes several ten seconds to several minutes to reach a predetermined temperature, so that a preheating is required during a stand-by period. 
     Recently, there has been developed a directly-heated heat roller including a sheet-like heating element in which a resistance member is embedded in an insulating member. This heat roller has high thermal efficiency, since the resistance member generates heat when electric current flows through the resistance member and the heat is conducted. The sheet-like heating element is at first formed as a flat heating sheet. The heating sheet is rounded to form a cylindrical sheet-like heating element. The sheet-like heating element cannot keep its cylindrical shape with this state, so that it is attached on an inner surface of a metallic cylindrical tube for use. However, attaching the sheet-like heating element onto the inner surface of the cylindrical tube is difficult work. 
     Therefore, a method for fabricating a heat roller has been proposed wherein a cylindrical sheet-like heating element is sandwiched between an inner tube and an outer tube that constitute a duplex tube. Firstly, the inner tube is arranged at the inner surface side of the cylindrical sheet-like heating element, and then, the outer tube is arranged at the outer surface side of this heating element. Then, pressurized fluid is supplied to the inner tube to expand the inner tube and the sheet-like heating element toward the outer tube, whereby the sheet-like heating element is brought into intimate contact with the inner tube and the outer tube. In this fabrication process, it is unnecessary that the sheet-like heating element is brought into contact with the inner tube and with the outer tube, thereby providing a simple assembling operation. 
     There has been a demand for enhancing thermal efficiency by improving the heat roller including the sheet-like heating element. 
     SUMMARY OF THE INVENTION 
     In view of the problems noted above, the present invention aims to provide a heat roller including a sheet-like heating element and capable of enhancing thermal efficiency. 
     A heat roller according to the present invention includes a cylindrical sheet-like heating element having a resistance member embedded in an insulating member, an inner tube that comes in intimate contact with an inner surface of the sheet-like heating element and an outer tube that comes in intimate contact with an outer surface of the sheet-like heating element, wherein the outer tube is longer than the inner tube. 
     Further, a heat roller according to the present invention includes a cylindrical sheet-like heating element having a resistance member embedded in an insulating member, an inner tube that comes in intimate contact with an inner surface of the sheet-like heating element and an outer tube that comes in intimate contact with an outer surface of the sheet-like heating element, wherein a thermal expansion coefficient of a material of the inner tube is greater than a thermal expansion coefficient of a material of the outer tube. 
     Moreover, a heat roller according to the present invention includes a first cylindrical sheet-like heating element having a resistance member embedded in an insulating member, a first tube that comes in intimate contact with an inner surface of the first sheet-like heating element, a second tube that comes in intimate contact with an outer surface of the first sheet-like heating element, a second cylindrical sheet-like heating element that comes in intimate contact with an outer surface of the second tube, and a third tube that comes in intimate contact with an outer surface of the second sheet-like heating element. 
     Further, a heat roller according to the present invention includes a cylindrical sheet-like heating element having a resistance member embedded in an insulating member, an inner tube that comes in intimate contact with an inner surface of the sheet-like heating element, an outer tube that comes in intimate contact with an outer surface of the sheet-like heating element and a heat-resistant filler layer provided at least between the inner tube and the sheet-like heating element or between the sheet-like heating element and the outer tube. 
     Moreover, a heat roller according to the present invention includes a cylindrical sheet-like heating element having a resistance member embedded in an insulating member, an inner tube that comes in intimate contact with an inner surface of the sheet-like heating element, an outer tube that comes in intimate contact with an outer surface of the sheet-like heating element and an outer layer disposed at an outer surface of the outer tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       Preferred embodiments of the present invention will be described in detail based on the followings, wherein: 
         FIG. 1  is a side view showing one example of a fixing device including a heat roller according to the present invention; 
         FIG. 2  is a sectional view showing a heat roller; 
         FIG. 3  is a sectional view showing a heat roller taken along a line III-III in  FIG. 4 ; 
         FIG. 4  is a plan view showing a pattern of a resistance member in a sheet-like heating element; 
         FIG. 5  is a partial sectional front view showing one example of a heat roller; 
         FIG. 6  is a partial sectional front view showing another example of a heat roller; 
         FIG. 7  is a view showing the heat roller in  FIG. 6  and a support member; 
         FIG. 8  is a sectional view showing one example of a heat roller; 
         FIG. 9  is a sectional view showing another example of a heat roller; 
         FIG. 10  is a view showing an area of a sheet-like heating element of a heat roller used in a test; 
         FIG. 11  is a view showing a pattern of a resistance member in a sheet-like heating element of a heat roller; 
         FIG. 12  is a view showing a temperature distribution in sample  1 ; 
         FIG. 13  is a view showing a temperature distribution in sample  2 ; 
         FIG. 14  is a view showing a temperature distribution in sample  3 ; 
         FIG. 15  is a view showing an example wherein an outer layer is provided at the outer surface of an outer tube of a heat roller; 
         FIG. 16  is a view showing another example wherein an outer layer is provided at the outer surface of an outer tube of a heat roller; 
         FIG. 17  is a view showing an example wherein a heat-resistant filler layer is provided between a cylindrical tube and a sheet-like heating element; 
         FIG. 18  is a view showing another example wherein a heat-resistant filler layer is provided between a cylindrical tube and a sheet-like heating element; 
         FIG. 19  is a view showing an example wherein a fuse and a temperature sensor are provided to a sheet-like heating element; 
         FIG. 20  is a view showing an example wherein a sheet-like heating element is formed of plural resistance members connected in parallel to each other; 
         FIG. 21  is a view showing an arrangement of a temperature sensor; 
         FIG. 22  is a view showing an example of a triple-tube heat roller; 
         FIG. 23  is a view showing an example of a fixing device including a heat roller; 
         FIG. 24  is a view showing an example of a fixing device including a heat roller; 
         FIG. 25  is a view showing an example of a fixing device including a heat roller; 
         FIG. 26  is a view showing an example of a fixing device including a heat roller; 
         FIG. 27  is a view showing an example of a device including a heat roller; 
         FIG. 28  is a view showing an example of a change in power consumption of a fixing device including a heat roller having a sheet-like heating element and a temperature change of the heat roller; and 
         FIG. 29  is a view showing an example of a change in power consumption of a fixing device including a heat roller having a halogen lamp and a temperature change of the heat roller. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
       FIG. 1  is a side view showing a fixing device including a heat roller according to one embodiment of the present invention. A fixing device  10  includes a heat roller  12  and a pressure roller  14  that is pressed into contact with the heat roller  12  and is covered with rubber. A sheet  16  is transported between the heat roller  12  and the pressure roller  14 , whereupon toner carried by the sheet  16  is melted by heat generated by the heat roller  12  and is pressurized between the heat roller  12  and the pressure roller  14 , to thereby be fixed. 
       FIG. 2  is a sectional view showing the heat roller  12  in  FIG. 1 . The heat roller  12  includes a cylindrical sheet-like heating element  26 , an inner tube  28  that comes in intimate contact with the inner surface of the sheet-like heating element  26  and an outer tube  30  that comes in intimate contact with the outer surface of the sheet-like heating element  26 . 
       FIG. 3  is a sectional view showing the heat roller  12  taken along a line III-III in  FIG. 4 . The sheet-like heating element  26  has a heating sheet  26   a  wherein a resistance member  32  is embedded in insulating members  34  and  36 . The resistance member  32  is formed on the insulating member  34  and covered with the insulating member  36 . For example, the insulating members  34  and  36  are made of a polyimide type heat-resistant resin and the resistance member  32  is made of stainless steel. The heating sheet  26   a  is formed as a flat sheet. It is rounded to join both ends of the sheet, to thereby be formed into the cylindrical sheet-like heating element  26 . The inner tube  28  is made of a relatively soft aluminum type material so as to be deformable, while the outer tube  30  is made of a relatively hard aluminum type material such that the heat roller  12  keeps the cylindrical shape. For example, the inner tube  28  is made of pure aluminum (JIS designation 1050, coefficient of linear expansion 23.6), while the outer tube  30  is made of Al—Mg—Si (JIS designation 6063, coefficient of linear expansion 24.4). The outer tube  30  is made of a material having a strength greater than that of the inner tube  28 . 
       FIG. 4  is a plan view showing a pattern of the resistance member  32  on the insulating member  34  of the heating sheet  26   a.  The resistance member  32  is formed on the insulating member  34  so as to meander. The insulating member  36  is laminated on the insulating member  34  having the resistance member  32  formed thereon. Electric current flows through both ends of the resistance member  32 , so that the resistance member  32  generates heat, and the generated heat is transmitted to the sheet  16  via the outer tube  30 . 
     The heat roller  12  having the sheet-like heating element  26 , inner tube  28  and outer tube  30  is fabricated by a tube expansion method utilizing an outer shape die for tube expansion and fluid pressure. At first, the inner tube  28  is arranged at the inside of the cylindrical sheet-like heating element  26 , while the outer tube  30  is arranged at the outside thereof, to thereby form a heat roller assembly. At this time, a gap may be formed between the sheet-like heating element  26  and the inner tube  28  and a gap may be formed between the sheet-like heating element  26  and the outer tube  30 , whereby the heat roller assembly can easily be assembled. Subsequently, the heat roller assembly is inserted into an outer shape die for tube expansion, and pressurized fluid (e.g., water) is supplied into the inner tube  28  at a pressure of 60 Kg/cm 2 . Then, the inner tube  28  is expanded and brought into intimate contact with the sheet-like heating element  26  to thereby expand the sheet-like heating element  26 , whereby the sheet-like heating element  26  is brought into intimate contact with the outer tube  30  to thereby expand the outer tube  30 . The expansion of the outer tube  30  is restricted by the outer shape die for tube expansion. As described above, the inner tube  28  is brought into intimate contact with the sheet-like heating element  26  and the sheet-like heating element  26  is brought into intimate contact with the outer tube  30 . 
       FIG. 5  is a partial sectional front view showing one example of the heat roller  12 . In the heat roller  12  shown in  FIG. 5 , the outer tube  30  is shorter than the inner tube  28 . 
       FIG. 6  is a partial sectional front view showing another example of the heat roller  12 . In the heat roller  12  shown in  FIG. 6 , the outer tube  30  is longer than the inner tube  28 . 
     As a result of considering the relationship between the length of the outer tube  30  and the length of the inner tube  28  in the present invention, it was found that the preferable configuration was such that the outer tube  30  was longer than the inner tube  28 . According to the example shown in  FIG. 6 , the sheet-like heating element  26  is protected by the outer tube  30 , so that it cannot be seen from the outside. The thermal capacity of the inner tube  28  is reduced, while the thermal capacity of the outer tube  30  is increased, whereby it becomes possible to efficiently transmit the thermal capacity required for a fixing operation to the outer tube  30 . The temperature at the end section of the outer tube  30  is likely to lower. Therefore, the thermal capacity at both ends of the outer tube  30  is increased to widen a temperature margin to heat radiation from both ends of the outer tube  30 , thereby improving non-uniform temperature. 
       FIG. 7  is a view showing the heat roller  12  in  FIG. 6  and a support member  38 . The outer tube  30  of the heat roller  12  is supported by the support member  38  having a flange. A terminal section 32T extending from the resistance member  32  of the sheet-like heating element  26  of the heat roller  12  extends outwardly from the end section of the inner tube  28 , and is connected to a power supply member  40 . 
       FIG. 8  is a sectional view showing one example of the heat roller  12 . In the heat roller  28  in  FIG. 8 , the thickness of the outer tube  30  is smaller than the thickness of the inner tube  28 . 
       FIG. 9  is a sectional view showing another example of the heat roller  12 . In the heat roller  28  in  FIG. 9 , the thickness of the outer tube  30  is greater than the thickness of the inner tube  28 . 
     In the relationship between the thickness of the outer tube  30  and the thickness of the inner tube  28  too, the preferable configuration is such that the thickness of the outer tube  30  is greater than that of the inner tube  28  shown in  FIG. 9 . In this case too, the thermal capacity of the inner tube  28  is reduced, while the thermal capacity of the outer tube  30  is increased, whereby it becomes possible to efficiently transmit the thermal capacity required for a fixing operation to the outer tube  30 . However, the temperature at the end section of the outer tube  30  is likely to lower from the temperature at the center of the outer tube  30 , and therefore, the non-uniform temperature at the outer tube  30  is desired to be reduced. 
     Subsequently explained is a test result of a heating temperature distribution of the heat roller  12 .  FIG. 10  shows an area of the sheet-like heating element  26  of the heat roller  12  used for the test, while  FIG. 11  is a view showing a pattern of the resistance member  32  in the sheet-like heating element  26  of the heat roller  12 . In  FIG. 10 , the sheet-like heating element  26  is divided into an area A positioned at both end sections, an area B positioned inside of the area A and an area C positioned at the center. In  FIG. 11 , the pattern of the resistance member  32  of the sheet-like heating element  26  is set such that the heating density in the area A is the highest, the heating density in the area B is the second highest and the heating density in the area C is low. For example, the resistance member  32  is formed to have a width of a line in the area A of 1.46 mm, a width of a line in the area B of 1.46 mm, and a width of a line in the area C of 2.03 mm. The resistance member  32  is made of a stainless steel. 
     In the test, sample 1, sample 2 and sample 3 were prepared for the heat roller  12 . 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Sample 1 
                 Length of outer tube: 
                 380 mm 
               
               
                   
                   
                 Length of inner tube: 
                 340 mm 
               
               
                   
                 Sample 2 
                 Length of outer tube: 
                 340 mm 
               
               
                   
                   
                 Length of inner tube: 
                 380 mm 
               
               
                   
                 Sample 3 
                 Length of outer tube: 
                 340 mm 
               
               
                   
                   
                 Length of inner tube: 
                 380 mm 
               
               
                   
                   
               
            
           
         
       
     
     The inner tube  28  was made of pure aluminum and the outer tube  30  was made of Al—Mg—Si in the samples 1 and 2. The inner tube  28  and the outer tube  30  were made of stainless steel in the sample  3 . The thicknesses of the inner tube  28  and the outer tube  30  were 0.5 mm. 
     Current was made to flow through these samples, and when the temperature of some position of the heat roller  12  reached 160° C., the temperature distribution to the distance in the lengthwise direction of the heat roller  12  was measured. According to the pattern of the resistance member  32  in  FIGS. 10 and 11 , the temperature represented a peak at both ends of the heat roller  12 , but it became low at the center. The peak temperature at both ends and the temperature at the center were as follows (unit: ° C.). 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Peak 
                 Temperature 
                 Temperature 
               
               
                   
                 temperature 
                 at center 
                 difference 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Sample 1 
                 161.6° C. 
                 155.7° C. 
                 5.9° C. 
               
               
                   
                 Sample 2 
                 161.1° C. 
                 151.9° C. 
                 9.2° C. 
               
               
                   
                 Sample 3 
                 163.9° C. 
                 141.3° C. 
                 22.0° C.  
               
               
                   
                   
               
            
           
         
       
     
     From this result, non-uniform temperature is reduced in the heat roller in which the outer tube  30  is longer than the inner tube  28  like the sample 1. It was found that it was preferable that the outer tube  30  was longer than the inner tube  28  in order to improve non-uniform temperature. Further, non-uniformity in temperature was increased in the case of changing the material like the sample 3. The considered reason is that SUS is low in thermal conductivity compared to aluminum. The SUS is advantageous in thermal capacity, but considering a start-up characteristic from when a power switch is turned on, the use of aluminum is advantageous. (The thermal conductivity of the SUS is 14 W/m° C., while that of the aluminum is 210 W/m° C.) 
     The materials for the inner tube  28  and the outer tube  30  are required to be selected by considering its strength and expansion to heat. The outer tube  30  is made of a material having a strength greater than the inner tube  28 . Further, if the thermal expansion coefficient of the material for the inner tube  28  is greater than that of the material for the outer tube  30 , the inner tube  28  whose temperature increases upon the use of the heat roller  12  further expands, thereby providing strong intimate contact between the inner tube  28  and the sheet-like heating element  26 . As a result, a temperature transmission becomes uniform as a fixing device. Therefore, the thermal expansion coefficient of the material used for the inner tube  28  is made equal to or greater than that of the material used for the outer tube  30 . 
       FIG. 15  shows an example wherein an outer layer  42  is provided at the outer surface of the outer tube  30  of the heat roller  12 . The outer layer  42  is formed by coating fluororesin. 
       FIG. 16  shows another example wherein the outer layer  42  is provided at the outer surface of the outer tube  30  of the heat roller  12 . The outer layer  42  is formed by silicon rubber. As shown in  FIGS. 15 and 16 , providing the outer layer  42  at the outer surface of the outer tube  30  can cope with various combinations such as a layout of the heat roller  12  in the fixing device, nip width and toner for use. Further, optimizing the thickness of the silicon rubber causes no problem in irregularities of the pattern of the resistance member  32  that appears on the surface of the outer tube  30  of a duplex-tube heat roller  12  when the outer tube  30  is made thin, whereby the non-uniform temperature is hardly generated and the temperature-rising time can be shortened with the printing quality assured. 
       FIGS. 17 and 18  are views each showing an example wherein a heat-resistant filler layer is provided between the cylindrical tube and the sheet-like heating element  26 . In  FIG. 17 , a heat-resistant filler layer  44  for assisting the intimate contact is provided between the outer tube  30  and the sheet-like heating element  26 , while a heat-resistant filler layer  46  for assisting the intimate contact is provided between the sheet-like heating element  26  and the inner tube  28 . The filler layers  44  and  46  prevent extraordinary increase in temperature due to heat in the case of poor intimate contact, and further make it possible to uniformly and stably transmit heat. 
     In  FIG. 18 , the heat-resistant filler layer  44  for assisting the intimate contact is only provided between the outer tube  30  and the sheet-like heating element  26 . Further, air vent ports can be formed at the inner tube  28  with a suitable size and a space in the configurations shown in  FIGS. 17 and 18 . This is a design for preventing the generation of air bubbles to thereby provide even more satisfactory intimate contact. 
       FIG. 3  shows an example wherein a thickness of the heat-resistant resin film of each insulating member  34 ,  36  in the sheet-like heating element  26  is changed. The use of the heat-resistant resin film as the insulating material enables to select the film thickness. The insulating member  36  on the side of the outer tube  30  that is required to positively transmit heat is made thin, while the insulating member  34  on the side of the inner tube  30  that is loaded upon the fabrication of the duplex tube is made thick, whereby the stability of the product is enhanced and heat transfer coefficient is increased. Therefore, a temperature-rising time can be shortened. The thickness of the heat-resistant resin film is controlled without using a complicated mechanism or control, thereby enabling a further optimum thermal design. 
       FIG. 19  is a view showing an example wherein a fuse  48  and temperature sensor  50  are provided at the sheet-like heating element  26 . The fuse  48  is formed by sectionally reducing a volume of a part of the line of the resistance member  32  for causing a braking of the fuse  48  when current excessively flows. The fuse  48  is formed by reducing the width of the line of the resistance member  32 , not reducing the height of the line, to thereby prevent the pattern of the resistance member  32  from being brought into poor intimate contact after the fabrication of the heat roller  12 . Further, the width of the line is reduced so that secondary processing in the height direction is not required upon forming the pattern of the resistance member  32 , thereby leading to a low cost. A fuse function is conventionally provided at the outside of the heat roller  12 . However, the fuse  48  is formed as a part of the pattern of the resistance member  32  in the present invention, thereby being capable of immediately cutting off the energization to the resistance member  32  with respect to extraordinary heating, whereby safety is also remarkably improved. 
       FIG. 21  is a view showing an arrangement of the temperature sensor  50 . In  FIGS. 19 and 21 , the temperature sensor  50  is formed of a thermistor and provided in the same layer of the resistance member  32  between the insulating members  34  and  36 . Disposing the temperature sensor  50  in the same layer as the pattern of the resistance member  32  provides the heat roller  12  having incorporated therein the temperature sensor after the formation of the duplex tube, so that there is no need to newly use the temperature sensor externally, and therefore, design freedom of the device is remarkably enhanced. Moreover, this configuration can also eliminate a problem of deteriorating coating due to sliding friction between the external temperature sensor and the outer peripheral surface of the heat roller when the external temperature sensor is used. 
     Moreover, the temperature sensor  50  is brought close to the resistance member  32  that is a heating source, thereby being capable of performing efficient temperature control. An external temperature sensor generally used is formed such that a sensor section is attached to an elastic member and its outer periphery is coated with a protecting layer. In the present invention, the elastic member is unnecessary, and the insulating members  34  and  36  sandwiching the resistance member  32  can be used as a sensor protecting layer, thereby being advantageous in view of cost, including assembling performance. 
       FIG. 20  is a view showing an example wherein the sheet-like heating element  26  is formed of plural resistance members  32 A and  32 B connected in parallel to each other. For example, when a rapid increase in temperature is required such as upon turning on or upon a print command, current is made to flow through both heater patterns A and B in this configuration. If the design is such that a fixing temperature can be assured only by the energization to the heater pattern A after reaching a predetermined temperature, power consumption can be reduced. 
       FIG. 22  is a view showing an example of a triple-tube heat roller  12 . The triple-tube heat roller  12  includes a first cylindrical sheet-like heating element  26 X having the resistance member  32  embedded in the insulating members  34  and  36 , a first tube (inner tube)  28 X that is in intimate contact with the inner surface of the first sheet-like heating element  26 X, a second tube  29  (middle tube) that is in intimate contact with the outer surface of the first sheet-like heating element  26 X, a second cylindrical sheet-like heating element  26 Y that is in intimate contact with the outer surface of the second tube  29  and a third tube (outer tube)  30 X that is in intimate contact with the outer surface of the second sheet-like heating element  26 Y. Each of the first and second sheet-like heating elements  26 X and  26 Y has the configuration same as that of the above mentioned sheet-like heating element  2 . 
     The pattern of the resistance member  32  of the first sheet-like heating element  26 X is different from the pattern of the resistance member  32  of the second sheet-like heating element  26 Y. For example, a pattern C of the resistance member  32  of the second sheet-like heating element  26 Y is formed to have a high heating density at its edge section as explained with reference to  FIGS. 10 and 11 , while a pattern D of the resistance member  32  of the first sheet-like heating element  26 X is formed to have a uniform heating density. The pattern C is suitable for normal printing, while the pattern D is utilized for a preheating upon continuous printing. Therefore, only the pattern C is used for printing on a single sheet, while both patterns C and D are used for continuously printing on plural sheets. It becomes possible to hold down the thermal loss upon the continuous printing to the minimum, and further, printing operation is possible immediately after the sheet is inserted. 
     Moreover, in a conventional heat roller using a halogen lamp, it takes much time for a thermal design and a period for trial manufacture of the fixing device including a change in distribution of light of the halogen lamp if there is a change in speed or specification. In the triple-tube heat roller  12  according to the present invention, the sheet-like heating element having several types of heating patterns is prepared in advance, whereby there is no need to newly make a trial product of a heat source because of its combination, which leads to a reduction in the period for trial manufacture and cost. 
       FIG. 23  is a view showing an example of a fixing device including the heat roller  12  having the sheet-like heating element  26 . The fixing device  10  includes the heat roller  12  and the pressure roller  14 . The heat roller  12  is arranged above the pressure roller  14  in  FIG. 1 , but in  FIG. 23 , the heat roller  12  is arranged below the pressure roller  14 . 
       FIG. 24  is a view showing an example of a fixing device including the heat roller  12  having the sheet-like heating element  26 . The fixing device  10  includes the heat roller  12  and a heat roller  18 . The heat roller  18  has a configuration approximately same as that of the heat roller  12 . 
     The fixing devices  10  shown in  FIGS. 1 and 23  are used in a monochrome printer and the like. A fixing device free from waiting time can be provided by heating a printing surface or a back surface of the sheet  16 . Further, the fixing device  10  shown in  FIG. 24  is used in a color printer and a high-speed printer that require an amount of fixing heat. Effective fixing can be executed by simultaneously heating the printing surface and the back surface of the sheet  16 . 
       FIGS. 25 and 26  are views each showing an example wherein the heat roller  12  is used for a belt-type fixing device  10 . In  FIG. 25 , the belt-type fixing device  10  has the heat roller  12 , fixing roller  20 , belt  22  bridged to the heat roller  12  and the fixing roller  20  and a pressure roller  24  that is pressed in contact with the fixing roller  20  via the belt  22 . In this case, heat generated by the heat roller  12  is transmitted to the sheet  16  via the belt  22 , whereby toner carried by the sheet  16  is melted by the heat generated by the heat roller  12 , pressurized, and then, fixed. 
     In  FIG. 26 , a heat roller  25  is used instead of the pressure roller  24  in  FIG. 25 . The heat roller  25  can be configured in the same manner as the heat roller  12 . 
     In the belt-type fixing device  10 , the subject to be heated is the endless belt  22  for fixing operation having low thermal capacity, thereby being capable of shortening a temperature-rising period, and consequently, a temperature-rising period can be further shortened. 
       FIG. 27  is a view showing another device  70  including the heat roller  12  having the sheet-like heating element  26 . The device  70  is, for example, a large-sized electrophotographic printer, wherein the heat roller  12  is used at the position other than the fixing device. In  FIG. 27 , there are a photoreceptor drum  72  and a flash lamp  74  for fixing operation. The heat roller  12  is used for a sheet moisture removing roller  76  arranged at the upstream side with respect to the photoreceptor drum  72 . Further, the heat roller  12  is used for a drum condensation preventing roller  78  arranged in the photoreceptor drum  72 . Moreover, the heat roller  12  is used for a preheat roller  80  arranged between the photoreceptor drum  72  and the flash lamp  74  for fixing operation. Additionally, the heat roller  12  is used for a sheet wrinkle smoothing roller  82  arranged at the downstream side with respect to the flash lamp  74  for fixing operation. 
     As described above, the heat roller  12  can be used for (a) removing moisture on the sheet before the transfer, (b) preventing the generation of dew drops on the photoreceptor drum, (c) executing the preheating before the flash fixing, and (d) smoothing the wrinkle on the medium after the fixing operation. The heat roller  12  is not necessarily be used for all of the above mentioned examples. Further, the application of the heat roller  12  is not limited to the examples shown in  FIG. 27 . The sheet-like heating element  26  can freely and simply set the resistance value, whereby it has high general-purpose properties at the position other than the fixing device. 
       FIG. 28  is a view showing an example of a change of power consumption of the fixing device  10  including the heat roller  12  having the sheet-like heating element  26  and the temperature change of the heat roller  12 . A curve P represents the power consumption and a curve Q represents the temperature of the heat roller  12 . When a print command is inputted, maximum electric power for rising the temperature of the heat roller up to the fixing temperature is supplied (point D), the supplied electric power is controlled at the time when the temperature of the heat roller reaches the fixing temperature (point E), and then, the electric power is stopped to be supplied after the completion of the printing (point F). Symbol G represents a printing period, and symbol H represents a waiting time. When the print command is again inputted, the heat roller is started to be heated (point I). 
       FIG. 29  is a view showing an example of a change of power consumption of the fixing device  10  using a halogen lamp and the surface temperature change of the heat roller  12 . A curve P represents the power consumption and a curve Q represents the temperature of the heat roller  12  having the halogen lamp. When a print command is inputted, maximum electric power for rising the temperature of the heat roller up to the fixing temperature is supplied (point D), the supplied electric power is controlled at the time when the temperature of the heat roller reaches the fixing temperature (point E), and then, the supplied electric power is kept with a small value after the completion of the printing (point F). Symbol G represents a printing period, and symbol H represents a waiting time. When the print command is again inputted, the heat roller is started to be heated (point I). 
     The heat roller having the halogen lamp is low in thermal efficiency compared to the directly-heated heat roller  12 , so that preheating is required after the completion of the printing in order to satisfy the temperature-rising performance. Control for reducing the power consumption is possible in the directly-heated heat roller  12  by taking advantage of excellent temperature-rising time. 
     The features of the above mentioned plural embodiments can suitably be combined to be executed. 
     As explained above, the present invention can provide a heat roller including a sheet-like heating element and excellent in thermal efficiency. A heat roller according to the present invention is always stable even in a high-speed rotation, and further, can supply heat with reduced non-uniform temperature. The speed for increasing the temperature becomes fast, and a degree of freedom in designing the external electrode is enhanced. It has a fuse function prepared for extraordinary heating, whereby the power source input can immediately be cut when the abnormality occurs. The temperature measurement is possible by the temperature sensor incorporated in the sheet-like heating element without newly arranging a component for measuring the temperature. The temperature distribution in the heating area becomes uniform, thereby being capable of holding down the non-uniform temperature to the minimum.