Patent Publication Number: US-2011070006-A1

Title: Temperature equalizing roller and fixing device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from: U.S. provisional application 61/244,728, filed on Sep. 22, 2009; U.S. provisional application 61/244,737, filed on Sep. 22, 2009; the entire contents all of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a technique to improve equalizing temperature performance of a temperature equalizing roller. 
     BACKGROUND 
     Hitherto, a fixing device is used which includes a heat roller to heat toner transferred on a sheet and a pressure roller to pressure bond the toner to the sheet by nipping the sheet in cooperation with the heat roller. The fixing device includes a passing part through which the sheet passes in an axial direction of the heat roller and a non-passing part through which the sheet does not pass. Since the heat of the passing part is absorbed by the passing sheet, and a large temperature difference occurs between the passing part and the non-passing part, and a large temperature unevenness can occur in heating temperature applied to the sheet in the axial direction of the heat roller. The temperature unevenness causes a color unevenness of an image formed on the sheet. Concerning this problem, a technique is known in which a heat pipe is provided in the inside of a heat roller to suppress the large temperature unevenness from occurring in the heat roller, and the heating temperature applied to the sheet is equalized in the axial direction of the heat roller. 
     Besides, as a technique to equalize the heating temperature, a technique is known in which a tension roller including a heat pipe is provided, and a belt is wound around the tension roller and the heat roller. In this technique, the heat roller heats the belt. The heated belt heats the sheet. At that time, the tension roller equalizes the temperature unevenness of the belt generated by heating the sheet. Since the belt the temperature unevenness of which is equalized heats the sheet, the heat applied to the sheet can be equalized in the rotation axis direction of the roller. 
     In recent years, with the improvement of the performance of an image forming apparatus, a temperature equalizing roller capable of equalizing the heat at higher speed and a fixing device including the same are required. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of an image forming apparatus of a first embodiment. 
         FIG. 2  is a perspective view showing a fixing device. 
         FIG. 3  is a sectional view of a tension roller in an axial direction. 
         FIG. 4  is a sectional view of the tension roller perpendicular to the axial direction. 
         FIG. 5  is a sectional view showing a state before joining of a metal pipe and a heat pipe. 
         FIG. 6  is a flowchart for explaining a manufacturing method of the tension roller. 
         FIG. 7  is a view showing a temperature distribution of a belt when an A4-R sheet passes through a fixing device. 
         FIG. 8  is a sectional view showing a metal pipe of a second embodiment. 
         FIG. 9  is a view showing a state where a heat pipe is fitted in the metal pipe. 
         FIG. 10  is a sectional view showing a tension roller. 
         FIG. 11  is a sectional view showing a metal pipe of a third embodiment. 
         FIG. 12  is a sectional view showing a metal pipe of a fourth embodiment. 
         FIG. 13  is a sectional view showing a tension roller. 
         FIG. 14  is a sectional view of a tension roller of a fifth embodiment in an axial direction. 
         FIG. 15  is a sectional view showing a fixing device of a sixth embodiment. 
         FIG. 16  is a perspective view showing a pressure pad. 
         FIG. 17  is a plan view showing a peeling plate. 
         FIG. 18  is a sectional view showing a structure of the peeling plate. 
         FIG. 19  is a plan view showing a peeling member of a modified example. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to an embodiment, a temperature equalizing roller includes a metal pipe, a heat pipe that is located inside the metal pipe and comprises a thermal expansion coefficient higher than that of the metal pipe, and a joining layer that lies between an outer peripheral surface of the heat pipe and an inner peripheral surface of the metal pipe, joins the outer peripheral surface of the heat pipe and the inner peripheral surface of the metal pipe, and comprises a higher thermal conductivity than a thermal conductivity of air. 
     Besides, according to an embodiment, a temperature equalizing roller includes a metal pipe that comprises a tubular shape and comprises an inner peripheral length longer than a peripheral length of a circle comprising a radius which is an average inner radius, and a heat pipe that is in close contact with an inner peripheral surface of the metal pipe and comprises a thermal expansion coefficient higher than that of the metal pipe. 
     Besides, according to an embodiment, a fixing device includes a metal pipe of any of a heating rotator to heat a sheet on which toner is transferred, a rotator to feed a sheet in cooperation with the heating rotator, or a rotator to rotate a belt wound around itself and the heating rotator, a heat pipe that is located inside the metal pipe and has a thermal expansion coefficient higher than that of the metal pipe, and a joining layer that lies between an outer peripheral surface of the heat pipe and an inner peripheral surface of the metal pipe, joins the outer peripheral surface of the heat pipe and the inner peripheral surface of the metal pipe, and has a higher thermal conductivity than a thermal conductivity of air. 
     Hereinafter, embodiments will be described with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a sectional view showing an image forming apparatus  200  of an embodiment. 
     The image forming apparatus  200  includes an image reading section R and an image forming section P. The image reading section R scans and reads an image of a sheet document and a book document. 
     The image forming section P forms a toner image on a sheet based on the image read from the document by the image reading section R or print data transmitted to the image forming apparatus  200  from an external equipment. 
     The image reading section R includes an auto document feeder (ADF)  9  to automatically feed a document to a specified image read position, and reads an image of a document placed on a document tray (specified document placing table) Rt, which is automatically fed by the auto document feeder  9 , or an image of a document placed on a not-shown document table by a scanning optical system  10 . 
     The image forming section P includes toner cartridges  1 Y to  1 K, pickup rollers  51  to  54 , photoreceptors  2 Y to  2 K, development rollers  3 Y to  3 K, mixers  4 Y to  4 K, an intermediate transfer belt  60 , a fixing device  100  and a discharge tray  8 . 
     Besides, the image forming apparatus  200  of this embodiment includes a processor  801 , an ASIC circuit  802 , a memory  803 , an operation display section  805  and a communication section  807 . 
     The processor  801  serves to perform various processes in the image forming apparatus  200 , and serves to realize various functions by executing programs stored in the memory  803 . The memory  803  may be, for example, a RAM (Random Access Memory), a ROM (Read Only Memory), a DRAM (Dynamic Random Access Memory), an SRAM (Static Random Access Memory) or a VRAM (Video RAM), and serves to store various information and programs used in the image forming apparatus  200 . 
     Various settings are displayed on the operation display section  805 . The operation display section  805  may be an LCD (Liquid Crystal Display), an EL (Electronic Luminescence), a PDP (Plasma Display Panel) or a CRT (Cathode Ray Tube). 
     The various settings are changed by operating the operation display section  805 . The operation display section  805  may be of a touch panel type. 
     Hereinafter, the outline of a copy process will be described as an example of a process in the image forming apparatus  200 . 
     First, a sheet picked up by pickup rollers  51  to  54  from a cassette is supplied to a sheet conveyance path. The sheet supplied into the sheet conveyance path is conveyed by plural roller pairs toward a specified conveyance direction. 
     The scanning optical system  10  reads images of plural sheet documents continuously automatically fed by the auto document feeder  9  at the specified image read position. 
     Next, electrostatic latent images are formed on the photosensitive surfaces of the photoreceptors  2 Y,  2 M,  2 C and  2 K for transferring toner images of yellow (Y), magenta (M), cyan (C) and black (K) to the sheet based on the print data of the image read from the document by the image reading section R. 
     Subsequently, toner agitated by the mixers  4 Y to  4 K in the developing device is supplied by the development rollers  3 Y to  3 K to the photoreceptors  2 Y to  2 K on which the electrostatic latent images are formed. By this, the electrostatic latent images formed on the photosensitive surfaces of the photoreceptors are developed. 
     The toner images formed on the photoreceptors in this way are transferred (so-called primary transfer) onto the belt surface of the intermediate transfer belt  60 , and the toner images conveyed by rotation of the intermediate transfer belt are transferred onto the conveyed sheet at a specified secondary transfer position T. 
     The toner images transferred on the sheet are heated and fixed to the sheet in the fixing device  1 . The sheet on which the toner images are heated and fixed is conveyed in the conveyance path by plural conveyance roller pairs, and is sequentially discharged onto the discharge tray  8 . 
       FIG. 2  is a perspective view showing the fixing device  1 . 
     The fixing device  1  heats and pressurizes the toner transferred on the sheet P and fixes the toner to the sheet P. For example, a paper or an OHP sheet can be adopted as the sheet P. 
     The fixing device  1  includes an IH coil  2 , a pressure roller  3 , a fixing roller  4 , a belt  5  and a tension roller  6  as a temperature equalizing roller. 
     The IH coil  2  is adjacent to the fixing roller  4  over the whole area of the fixing roller  4  in the axial direction, and generates a magnetic flux. The IH coil  2  includes an IH coil  21  to heat the center part of the fixing roller  4  in the axial direction, and IH coils  22  to heat both ends of the fixing roller  4  in the axial direction. The IH coils  21  and  22  are independently controlled. The width of the IH coil  21  corresponds to the width of 210 mm of the A4-R size sheet P. 
     When the sheet P of a size of from the minimum size (for example, postcard size) to the A4-R size, which is a fixing process object, is made to pass through the fixing device  1 , current is applied only to the center IH coil  21 , and heats an area of the fixing roller  4  corresponding to the width (size in the rotation axis direction of the tension roller  6 ) of the A4-R size sheet P. When the sheet P of a width larger than the A4-R size sheet P, for example, the Ledger size sheet P of a width of 279 mm is made to pass through the fixing device  1 , the width of the Ledger size sheet P is wider than the heating range of the center IH coil  21 . Thus, in addition to the center IH coil  21 , current is applied also to the IH coils  22  at both sides of the IH coil  21 , and the whole area of the fixing roller  4  in the axial direction is heated. 
     Incidentally, the heating range of the center IH coil  21  has only to be wider than the width of the minimum size sheet P as the fixing process object. When the heating range of the IH coil  21  is narrower than the width of the minimum size sheet P as the fixing process object, it becomes necessary to apply current to all the IH coils  21  and  22 , and it becomes meaningless to divide the IH coil  2 . 
     The pressure roller  3  comes in press contact with the fixing roller  4  through the belt  5 , and forms a nip N. The pressure roller  3  is driven and rotated by the rotating fixing roller  4 . 
     The fixing roller  4  includes a core metal and an elastic layer in sequence from the inside. The fixing roller  4  generates heat by the change of the magnetic flux generated by the IH coil, and heats the belt  5 . Besides, the fixing roller  4  nips the sheet P in cooperation with the pressure roller  3  through the belt  5 , and conveys the sheet P in an arrow direction of  FIG. 2  in cooperation with the pressure roller  3  while rotating. 
     The belt  5  is an endless belt, and is wound around the fixing roller  4  and the tension roller  6 . The belt  5  is heated by the fixing roller  4 . The heated belt  5  heats the sheet P. The fixing device  1  heats the sheet P by the belt  5  at the nip N, and fixes the toner to the sheet P by pressing of the pressure roller  3  and the fixing roller  4 . 
     The tension roller  6  suppresses the temperature unevenness of the belt  5  in the width direction. Besides, the elastic member such as a coil spring or a plate spring urges the tension roller  6 , so that the tension roller  6  applies tension to the belt  5 . 
       FIG. 3  is a sectional view of the tension roller  6  in the axial direction, and  FIG. 4  is a sectional view of the tension roller  6  in a direction perpendicular to the axial direction. 
     The tension roller  6  includes a metal pipe  61 , a heat pipe  62 , a joining layer  63  and shaft members  64 . 
     The metal pipe  61  can be made of, for example, iron for reduction of cost. Besides, when the metal pipe  61  is made of iron, the pipe can be made thin while the strength is kept. The material of the metal pipe  61  may be stainless. The metal pipe made of stainless may produce the same effect as that of the metal pipe made of iron. The material of the metal pipe  61  may be aluminum. When the metal pipe  61  is made of aluminum, as compared with the case where the metal pipe  61  is made of iron or stainless, the thermal conductivity can be improved. 
     The heat pipe  62  is such that volatile hydraulic fluid is sealed in a pipe made of metal such as, for example, copper, and dissipates the absorbed heat from a portion shifted in the axial direction. The heat pipe  62  is located inside the metal pipe  61 , and the outer peripheral surface of the heat pipe  62  is in close contact with the inner peripheral surface of the metal pipe  61 . The heat pipe  62  equalizes the temperature unevenness of the belt  5  in the width direction through the metal pipe  61 . The heat pipe  62  is made of copper, and has a thermal expansion coefficient higher than that of the metal pipe  61 . It is needless to say that as the material of the heat pipe  62 , a metal other than copper can be adopted as long as its thermal expansion coefficient is higher than that of the material of the metal pipe  61 . 
     The joining layer  63  lies between the outer peripheral surface of the heat pipe  62  and the inner peripheral surface of the metal pipe  61 , and joins the outer peripheral surface of the heat pipe  62  and the inner peripheral surface  61  of the metal pipe and has a higher thermal conductivity than that of air. In the pipes  61  and  62 , the joining layer  63  is provided in an area longer than the width of 297 mm of the A4 size as the maximum size of the fixing process object. The joining layer  63  has a melting point lower than a temperature required to join the metal pipe  61  and the heat pipe  62  by using thermal expansion of the heat pipe  62  caused by heating the metal pipe  61  and the heat pipe  62 . The temperature required to join the metal pipe  61  and the heat pipe  62  is lower than a temperature at which the metal pipe  61  on the outer side is ruptured by thermal expansion of the heat pipe  62  on the inner side and is higher than a temperature at which the joining layer  63  fills the gap between the metal pipe  61  on the outer side and the heat pipe  62  on the inner side due to thermal expansion of the heat pipe  62 . 
     As the material of the joining layer  63 , in this embodiment, a solder containing silver having a high thermal conductivity as an additive is adopted as an example. Incidentally, as the material of the joining layer  63 , it is needless to say that another material can be adopted as long as the melting point is lower than the temperature required to join the metal pipe  61  and the heat pipe  62 . 
     The shaft members  64  are coupled to both ends of the metal pipe  61 . The shaft members  64  are supported by bearings or the like, and the tension roller  6  rotates. 
       FIG. 5  is a sectional view showing a state before joining of the metal pipe  61  and the heat pipe  62 . 
     The metal pipe  61  has an inner diameter r 1  of 16.1±0.2 mm and a thickness of 0.3 mm. 
     The heat pipe  62  has an outer diameter r 2  of 15.88 mm. The joining layer  63  is laminated on the outer peripheral surface of the heat pipe  62  in an area longer than the width of 297 mm of the A4 sheet which is the maximum sheet P as the toner fixing process object ( FIG. 3 ). The thickness of the joining layer  63  is desirably 10 to 100 p.m. When the thickness is excessively large, the layer becomes resistance against the heat conduction between the metal pipe  61  and the heat pipe  62 . 
     A manufacturing method of the tension roller  6  including the metal pipe  61  and the heat pipe  62  will be described with reference to a flowchart of  FIG. 6 . 
     First, an operator fits the heat pipe  62  into the inside of the metal pipe  61  (ACT  1 ). At this time, a gap S ( FIG. 5 ) of 0.1 to 0.5 mm is generated between the heat pipe  62  and the inner peripheral surface of the metal pipe  61 . 
     After ACT  1 , the operator heats both the pipes  61  and  62  at 300 to 400° C. for 1 to 4 hours to perform metal joining (ACT  2 ). The heat pipe  62  is made of copper, and has a thermal expansion coefficient higher than that of the metal pipe  61  made of iron. Thus, when both the pipes  61  and  62  are heated at 300 to 400° C. for 1 to 4 hours, the heat pipe  62  is expanded and comes in press contact with the metal pipe  61 , and metal joining is performed. At this time, since the joining layer  63  is made of solder and the melting point is lower than the junction temperature of 300 to 400° C. of both the pipes  61  and  62 , the joining layer becomes liquid. The liquid joining layer  63  expands into the gap S between the metal pipe  61  and the heat pipe  62 . 
     After ACT  2 , the operator cools the metal pipe  61 , the heat pipe  62  and the joining layer  63  by, for example, natural cooling (ACT  3 ). By this, the joining layer  63  becomes a solid in the gap S between the pipes  61  and  62  and fills the gap S. The tension roller  6  can be manufactured by the above process of ACT  1  to ACT  3 . 
     In the tension roller  6  of this embodiment manufactured by the process of ACT  1  to ACT  3 , since the gap S is filled with the joining layer  63 , as compared with the related art tension roller in which air exists in the gap S, the thermal conductivity between the metal pipe  61  and the heat pipe  62  is improved. At the same time, the heat transport efficiency of the tension roller  6  in the axial direction is improved as compared with the related art. Thus, since the fixing device  1  including the tens ion roller  6  suppresses the temperature unevenness of the belt  5  and equalizes the temperature unevenness of the heat temperature applied to the sheet P, the color unevenness generated on an image can be suppressed. 
     Besides, the fixing device  1  can efficiently transport the heat at both ends of the belt  5  as the non-passing area of the sheet P to the center part of the belt  5  as the passing area of the sheet P. Thus, in the fixing device  1  of this embodiment, the output of the IH heater  21  to heat the center part of the fixing roller  4  is reduced and the total electric power can be reduced. Thus, an energy-saving effect can be obtained. Further, in the fixing device  1  of this embodiment, the above remarkable effect can be obtained without decreasing or increasing the related art components (the pressure roller, the fixing roller, the belt and the tension roller) and without significantly increasing the cost for manufacturing the tension roller as compared with the related art. 
       FIG. 7  is a view showing a temperature distribution of the belt  5  when the A4-R sheet P passes through the fixing device  1 . 
     A performance test is performed on a related art tension roller in which a joining layer does not exist in a gap between a metal pipe and a heat pipe and the tension roller  6  of this embodiment in which the joining layer  63  exists in the gap S between the pipes  61  and  62 . 
     First, the related art tension roller is assembled in the fixing device  1 , the surface temperature of the sheet passing area of the belt  5  is kept at 175° C., and the A4-R size sheet P having a width of 210 mm is made to continuously pass through the fixing device  1 . Then, the temperature distribution of the surface of the belt  5  in the width direction is measured. As shown in  FIG. 7 , the temperature of the non-passing area of the sheet P in the belt  5  is 216° C., the temperature of the passing area of the sheet P is 175° C., and the temperature difference between both the areas is 39° C. 
     Next, the tension roller  6  of this embodiment is assembled in the fixing device  1 , the same performance test is performed, and the temperature distribution of the surface of the belt  5  is measured. As shown in  FIG. 7 , the temperature of the non-passing area of the sheet P in the belt  5  is 198° C., the temperature of the passing area of the sheet P is 175° C., and the temperature difference between both the areas is 23° C. 
     From the above test results, it is confirmed that as compared with the related art fixing device, in the tension roller  6  of this embodiment, the heat of the non-passing area of the sheet P in the belt  5  is efficiently transported to the passing area of the sheet P, and the temperature unevenness of the belt  5  can be more equalized. Then, it is confirmed that as compared with the related art tension roller, in the tension roller  6  of this embodiment, the thermal conductivity between the metal pipe  61  and the heat pipe  62  is excellent, and the heat transport efficiency in the axial direction is excellent, that is, the equalizing temperature performance is excellent. 
     Second Embodiment 
     Hereinafter, the same functional part as that of the first embodiment is denoted by the same reference numeral and the explanation of the same functional part will be omitted. 
       FIG. 8  is a sectional view showing a metal pipe  61 A of a tension roller of a second embodiment. 
     In the first embodiment, the inner periphery of the metal pipe  61 A has a circular shape. However, in this embodiment, plural grooves  611  extending in an axial direction of the metal pipe  61 A are formed on the inner peripheral surface of the metal pipe  61 A, and the inner periphery of the metal pipe  61 A is corrugated by these grooves  611 . The depth of the groove  611  is set to be 7% or more of the thickness of the metal pipe  61 A. In the metal pipe  61 A, its inner peripheral length is longer than the peripheral length of a circle C having a radius equal to an average inner radius. Another component of the tension roller of this embodiment is the same as that of the first embodiment. 
     Also in this embodiment, when the tension roller  6 A is manufactured, first, the operator fits a heat pipe  62  into the inside of the metal pipe  61 A ( FIG. 9 ). At this time, a gap S is generated between the heat pipe  62  and the inner peripheral surface of the metal pipe  61 A. Next, the operator heats both the pipes  61 A and  62  to perform metal joining. At this time, a joining layer  63  of solder becomes liquid and expands into the gap S between the pipes  61 A and  62 . Next, the operator cools the pipes  61 A and  62  and the joining layer  63 . By this, as shown in  FIG. 10 , the joining layer  63  is solidified to fill the gap S, and the tension roller  6  is manufactured. 
     Since this embodiment has the same structure as the first embodiment, the same effect as the first embodiment can be obtained. Further, in this embodiment, in the metal pipe  61 A, since the inner peripheral length is longer than the peripheral length of the circle C having the radius equal to the average inner radius, the joining area between the metal pipe  61 A and the heat pipe  62  is increased, and the metal pipe  61 A and the heat pipe  62  can be more firmly joined. By this, the thermal conductivity between the metal pipe  61 A and the heat pipe  62  is improved, and the thermal conductivity of the tension roller  6 A in the axial direction is improved more than the first embodiment. Thus, in this embodiment, the equalizing temperature performance of the tension roller  6 A can be improved more than the first embodiment. 
     Third Embodiment 
       FIG. 11  is a sectional view showing a metal pipe  61 B of a third embodiment. 
     In the second embodiment, the plural grooves  611  on the inner peripheral surface of the metal pipe  61 A extend in the axial direction of the metal pipe  61 A. In this embodiment, a groove  611 B is formed to have a spiral shape while the center axis of the metal pipe  61 B is the center axis of the spiral. 
     In this embodiment, since the groove  611 B has the spiral shape, even when the joint between the metal pipe  61 B and the heat pipe  62  becomes weak, the metal pipe  61 B and the heat pipe  62  does not immediately come off from each other in the axial direction. 
     Fourth Embodiment 
       FIG. 12  is a sectional view showing a metal pipe  61 C of a fourth embodiment. 
     In a tension roller  6 C of this embodiment, grooves  611 C are formed on an inner peripheral surface of the metal pipe  61 C more thickly than the third embodiment, and a joining layer is not provided. Also in the metal pipe  61 C of this embodiment, its inner peripheral length is longer than the peripheral length of a circle C having a radius equal to an average inner radius. 
     When the tension roller  6 C is manufactured, first, the operator fits a heat pipe  62  into the inside of the metal pipe  61 C. Next, the operator heats both the pipes  61 C and  62  to perform metal joining as shown in  FIG. 13 , and then, the metal pipe  61 C and the heat pipe  62  are cooled. 
     In this embodiment, since the inner periphery of the metal pipe  61 C is corrugated, the joining area between the metal pipe  61 C and the heat pipe  62  is increased, and the metal pipe  61 C and the heat pipe  62  can be joined more closely than the related art. Thus, in this embodiment, even if a joining layer does not exist, the thermal conductivity between the metal pipe  61 C and the heat pipe  62  can be improved, the heat transport efficiency of the tension roller  6 C in the axial direction can be improved, and the equalizing temperature performance of the tension roller  6 C can be improved as compared with the related art. Accordingly, the fixing device including the tension roller  6 C equalizes the temperature unevenness of the belt  5 , and can suppress the color unevenness from occurring on an image. Besides, in the fixing device, the tension roller  6 C can efficiently transport the heat at both ends of the belt  5  to the center part, the output of an IH heater  21  to heat the center part is reduced, and the total electric power can be reduced. 
     Fifth Embodiment 
       FIG. 14  is a sectional view of a tension roller  6 D of a fifth embodiment in an axial direction. 
     In the first to the third embodiments, the joining layer  63  is laminated on the whole area of the heat pipe  62  in the axial direction. In this embodiment, joining layers  63 D are separately laminated at positions close to both ends of a heat pipe  62 D in the axial direction, and are positioned. 
     The distance between ends of both areas of the joining layers  63 D, which are positioned to be separate from each other, close to the center of the tension roller  6 D in the axial direction is shorter than a width of 140 mm of the statement (ST-R) paper which is the minimum sheet P as the toner fixing process object. Besides, the distance between the outside ends of both the areas is longer than a width of 297 mm of the A4 sheet which is the maximum sheet P as the toner fixing process object. 
     Besides, in this embodiment, the heat pipe  62 D includes a surface protection member  621 . In the surface protection member  621 , the thermal conductivity of both end sides in the axial direction of the heat pipe  62 D is higher than the thermal conductivity of the center part. 
     Further, in this embodiment, in a metal pipe  61 D, the thickness of the center part in the axial direction is larger than that of both ends. 
     By the above structure, in this embodiment, the thermal conductivity from the metal pipe  61 D to the heat pipe  62 D in the non-passing area of the sheet P can be improved as compared with the related art, and the equalizing temperature performance of the tension roller  6 D can be improved. Thus, the fixing device including the tension roller  6 D can efficiently transport the heat at both ends of the belt  5  to the center part by the tension roller  6 D, equalizes the temperature unevenness of the belt  5 , and can suppress the color unevenness from occurring on an image. 
     Modified Examples of the First to the Fifth Embodiments 
     In the second to the fourth embodiments, the grooves  611  to  6110  are provided on the inner peripheral surfaces of the metal pipes  61 A to  61 C in the axial direction, so that the areas of the inner peripheral surfaces of the metal pipes  61 A to  61 C are increased, and the strength of the metal joining between the heat pipe  62  and the metal pipes  61 A to  61 C is improved. However, the method of providing the grooves on the inner peripheral surface of the metal pipe may not be adopted as the method of increasing the area of the inner peripheral surface of the metal pipe. As the method of increasing the area of the inner peripheral surface of the metal pipe, for example, plural protrusions each having a height lower than the thickness of the metal pipe are provided on the inner peripheral surface of the metal pipe, and concave portions may be provided between the projections. The corner parts of the projections and the concave portions are round. The area of the inner peripheral surface of the metal pipe may be increased by the projections and the concave portions. 
     In the first to the third and the fifth embodiments, although the joining layer  63 ,  63 D is laminated on the heat pipe  62 ,  62 D side, the joining layer may be laminated on the inner peripheral surface side of the metal pipe. 
     In the first to the fifth embodiments, the description is made on the example in which the temperature equalizing roller is applied as the tension roller which is the rotator to rotate the belt wound around itself and the fixing roller. However, the temperature equalizing roller may be provided for the fixing roller as a heating rotator to heat a sheet on which toner is transferred. Besides, the temperature equalizing roller may be provided for the pressure roller as a rotator to feed a sheet in cooperation with the heating rotator. 
     Sixth Embodiment 
     Hitherto, a heating apparatus is known in which a heating rotator and a rotator which is in press contact with the heating rotator nip a sheet on which toner is transferred, so that the sheet is heated and pressed. In this type of heating apparatus, since the toner on the sheet is melted by heat, the sheet becomes liable to be wound around the heating rotator by the melted toner. As a measure against this, a technique is known in which a peeling member whose tip contacts with a surface of a heating rotator is provided in a heating apparatus, and the peeling member peels a sheet from the heating rotator. 
     However, in the related art, since the tip of the peeling member contacts with the heating rotator, there is a fear that the peeling member may damage the surface of the heating rotator. 
     Also in the follow description of this embodiment, the same functional part as that of the first embodiment is denoted by the same reference numeral and the explanation of the same functional part will be omitted. 
       FIG. 15  is a sectional view showing a fixing device  100  of this embodiment. 
     The fixing device  100  includes a pressure roller  104 , a metal belt  102 , an IH coil  110  as a heating unit, a base member  114 , a pressure pad  116  as a pressing member, an opposite member  120 , a peeling member  125 , a coupling member  130  and a housing  106  containing the members  102  to  130 . 
     The pressure roller  104  includes a core metal  105  made of a metal such as aluminum, silicone rubber to cover the core metal  105  and PFA, and has a longish shape. The pressure roller  104  is rotated in an arrow E direction by a drive motor. 
     The IH coil  110  is provided along the outer peripheral surface of the metal belt  102 , and generates a magnetic field H by energization. The IH coil  110  may not be divided or may be divided. Besides, the IH coil  110  may lie in the space inside the metal belt  102 . In this embodiment, although the IH coil  110  is used as a heating unit to heat the belt, a halogen lamp or a ceramic heater may be used as a heating unit. 
     The metal belt  102  includes a heat generation layer. The heat generation layer is formed of a metal material, and generates heat by eddy current generated by electromagnetic induction of the IH coil  110 . The metal belt  102  has an endless shape, and tension is applied by the pressure pad  116  and the base member  114 . The metal belt  102  rotates in an arrow D direction by rotation of the pressure roller  104 . The metal belt  102  nips and conveys the sheet P on which toner T is transferred in cooperation with the pressure roller  104 . A contact portion between the metal belt  102  and the pressure roller  104  is hereinafter referred to as a nip  107 . 
     The base member  114  is positioned in the space inside the metal belt  102 , and supports the pressure pad  116 . The base member  114  is made of aluminum as a non-magnetic substance. The base member  114  has a longish shape extending in the width direction of the metal belt  102  at the inside of the metal belt  102 . The base member  114  includes a main body part  140 , a tension variable mechanism  114 C, a protrusion  114 B and a support member  127 . 
     Both ends of the main body part  140  in the longitudinal direction are fixed to the housing  106 . In the main body part  140 , a surface opposite to the pressure roller  104  has a recess  114 A in the longitudinal direction of the main body part  140 . The pressure pad  116  is fixed to the recess  114 A. 
     The tension variable mechanism  114 C supports apart opposite to a part pressed by the pressure pad  116  on the inner peripheral surface of the metal belt  102 . The tension variable mechanism  114 C includes a support part  141  and an arm part  142 . The support part  141  guides the inner peripheral surface of the metal belt  102 . One end of the arm part  142  is connected to the main body part  140 , and the other end is connected to the support part  141 . The arm part  142  includes an actuator and expands and contracts. 
     The tension variable mechanism  114 C expands and contracts the arm part  142  under the control of the processor  801 , and changes the tension applied to the metal belt  102 . For example, when the fixing device  100  performs a fixing process on a thick sheet P, the tension variable mechanism  114 C shortens the arm part  142  and weakens the tension applied to the metal belt  102 . Then, the width of the nip  107  becomes wide, and the heating and pressing time of the sheet P by the metal belt  102  and the pressure roller  104  becomes long. Thus, even if the sheet P is thick, the fixing device  100  can certainly perform the fixing process of the toner T. On the other hand, when the fixing device  100  performs the fixing process on a thin sheet P, the tension variable mechanism  114 C lengthens the arm part  142 , and intensifies the tension applied to the metal belt  102 . Then, the width of the nip  107  becomes narrow, and the heating and pressing time of the sheet P by the metal belt  102  and the pressure roller  104  becomes short. Thus, the fixing device  100  can prevent excessive heating to the sheet P. 
     The protrusion  114 B is fixed to the side surface of the main body part  140 . The protrusion  114 B supports a temperature sensor  118  through a plate spring  119 . The temperature sensor  118  detects the surface temperature of the metal belt  102  in a non-contact manner. The processor  801  controls the IH coil  110  based on the surface temperature of the metal belt  102  detected by the temperature sensor  118 , and controls the temperature of the metal belt  102 . The temperature sensor to detect the surface temperature of the metal belt  102  may be of a contact type or a thermopile type in addition to the non-contact type. 
     The support member  127  is fixed to the main body part  140  and supports a thermostat  129 . The thermostat  129  contacts with the inner peripheral surface of the metal belt  102 , and turns on an inner switch when the metal belt  102  comes to have a specified temperature (for example, 200° C.). When the inside switch of the thermostat  129  is turned ON, the fixing device  100  interrupts energization to the IH coil  110 , and suppresses the excessive temperature rising of the metal belt  102 . 
     The pressure pad  116  is made of a resin and has elasticity. The pressure pad  116  presses the metal belt  102  to the pressure roller  104  side from the inside of the metal belt  102 . 
       FIG. 16  is a perspective view showing the pressure pad  116 . 
     The pressure pad  116  has a press surface  116 B to press the metal belt  102  to the pressure roller  104  side, and a side surface  116 A perpendicular to the press surface  116 B. A pair of opposite parts  120  ( 120 A,  120 B) protrude from the side surface  116 A. 
     The opposite part  120  has hardness higher than that of the pressure pad  116 . The opposite parts  120  are formed integrally with the pressure pad  116 . The opposite part  120  is opposite to the peeling member  125  through the metal belt  102 , and guides the metal belt  102  from the inside. The respective opposite parts  120  include curved surfaces  122 A and  122 E to guide the metal belt  102 . 
     A pair of the peeling members  125  are provided, and the respective peeling members  125  are opposite to the respective opposite parts  120  through the metal belt  102  on the outlet side (downstream side of the metal belt  102  in the rotation direction) of the nip  107 . The peeling members  125  are separate from the outer peripheral surface of the metal belt  102  through a gap, and peel the sheet P from the metal belt  102 . Each of the peeling members  125  includes a peeling plate  124  and a bracket  126  fixed to the peeling plate  124 . The tip of the peeling plate  124  is separate from the outer peripheral surface of the metal belt  102  through the gap. The peeling plate  124  has a rectangular shape, when viewed in a plane, having a size corresponding to the opposite part  120  ( FIG. 17 ). Besides, the peeling plate  124  includes a base member  124 A and protection layers  124 B laminated on the base member  124 A. The protection layer  124 B may be made of fluorine resin such as PFA or DLC (Diamond-like Carbon). The protection layer  124 B improves the strength of the peeling plate  124 . 
     The coupling member  130  is made of, for example, resin or metal, and a pair of the coupling members are provided correspondingly to the peeling members  125 . One end of the coupling member  130  is coupled to an end of the base member  114  in the longitudinal direction, and the other end is coupled to the bracket  126  of the peeling member  125 . The coupling members  130  are coupled to the respective members  114  and  126  by, for example, screws. Surfaces of the respective ends of the coupling members  130  are struck against positioning members G 1  and G 2  attached to the respective members  114  and  126  and are positioned. The positioning positions of the coupling members  130  by the respective positioning members G 1  and G 2  are set at positions where the gap between the outer peripheral surface of the metal belt  102  and the respective peeling members  125  becomes a specified size (for example, 0.3 mm). The coupling member  130  couples the peeling member  125  and the base member  114 , and supports the peeling member  125 . 
     As stated above, in this embodiment, since the sheet P is peeled from the metal belt  102  by the peeling member  125  separate from the outer peripheral surface of the metal belt  102 , there is no fear that the outer peripheral surface of the metal belt  102  is damaged by the peeling member  125 . Besides, since the peeling member  125  is attached to the base member  114 , which is fixed to the housing  106 , through the coupling member  130 , the gap between the metal belt  102  and the peeling member  125  can be set to the specified size by merely attaching the peeling member  125  to the base member  114 . Here, in this embodiment, although the peeling member  125  is attached to the base member  114 , it is conceivable that the peeling member is attached to the housing. Then, it is conceivable that the attachment position of the peeling member to the housing is adjusted while a gap gage is inserted into and drawn from the gap between the metal belt and the peeling member. As compared with the above case, in this embodiment, since it is not necessary to adjust the attachment position of the peeling member  125 , the peeling member  125  can be easily attached to the fixing device  100 . In addition, since the hardness of the opposite part  120  is higher than the hardness of the pressure pad  116 , the opposite part  120  can guide the metal belt  102  at a specific position, and the gap between the outer peripheral surface of the metal belt  102  and the peeling member  125  can be kept at the specified size for a long period. 
     Incidentally, the fixing device  100  includes, in addition to the respective members, a cleaning roller to remove toner, dirt and dust attached to the outer peripheral surface of the metal belt  102 , and an oil roller to facilitate peeling of the sheet P from the outer peripheral surface of the metal belt  102 . 
     Hereinafter, the control of the fixing device  100  by the processor  801  and the control of the whole image forming apparatus  200  will be described. 
     When the image forming apparatus  200  is turned on, the processor  801  starts warming-up of the image forming apparatus  200 . That is, the processor  801  rotates the pressure roller  104 , and rotates the metal belt  102 . Besides, the processor  801  monitors the surface temperature of the metal belt  102  by the temperature sensor  118 , and heats the metal belt  102  by the IH coil  110  so that the entire area of the metal belt  102  just before the nip  107  in the main scanning direction (longitudinal direction of the metal belt  102 ) comes to have a specified temperature. At this time, the processor  801  changes electric power supplied to the IH coil  110  by changing the amount of voltage (amount of current) supplied to the IH coil  110  or the frequency of voltage (current). 
     When the temperature of the metal belt  102  becomes the specified temperature, the processor  801  terminates the warming-up of the image forming apparatus  200 . The specified temperature is the temperature suitable for fixing the toner T to the sheet P by the metal belt  102 . When the sheet P as the image forming process object is thick, the transfer amount of the toner T onto the sheet P is required to be large, and a large heat amount is required for the fixing process of the toner T. Thus, when the sheet P as the image forming process object is thick, the processor  801  sets the specified temperature to be high. 
     When terminating the warming-up of the image forming apparatus  200 , the processor  801  brings the image forming apparatus into a standby state, and keeps the temperature of the metal belt  102  at the specified temperature. Then, the processor  801  performs, for example, the copy process described in the first embodiment. 
     In the copy process, when the sheet P is conveyed to the fixing device  100 , the processor  801  rotates the pressure roller  104 . Then, the pressure roller  104  and the metal belt  102  having the specified temperature heat and press the sheet P at the nip  107 , and fixes the toner T to the sheet P. At this time, in general, the toner T on the sheet P is peeled from the surface of the metal belt  102  after passing through the nip  107 . However, when the amount of the toner T is large, or the melting temperature of the toner T is high, even if the toner T passes through the nip  107 , the toner remains attached to the surface of the metal belt  102 , and there is a case where the sheet P is wound around the metal belt  102  by the toner T. In this case, the peeling member  125  peels the sheet P from the metal belt  102 . 
     The processor  801  conveys the sheet P, on which the toner T is heated and fixed by the fixing device  100  as described above, by plural conveyance roller pairs in the image forming apparatus  200 , and sequentially discharges it onto the discharge tray  8 . 
     Incidentally, when the temperature of the metal belt  102  does not reach the specified temperature within a specified time from the start of warming-up, the processor  801  determines that abnormality occurs, and stops the warming-up. The processor  801  causes the operation display section  805  to display a screen for urging a serviceman to check the image forming apparatus  200 . 
     Modified Examples of the Sixth Embodiment 
     In the sixth embodiment, the opposite part  120  has the hardness higher than that of the pressure pad  116  and is formed integrally with the pressure pad  116 . However, the opposite part has the hardness higher than that of the pressure pad  116 , and may be formed integrally with the base member (may be supported by the base member). By doing so, the opposite part can more certainly guide the metal belt at the constant position, and can keep the gap between the metal belt and the peeling member at the specified size. 
     In this embodiment, although the peeling member  125  has the rectangular shape when viewed in a plane, the peeling member  125 A may have a comb shape when viewed in a plane as shown in  FIG. 19 . 
     In the sixth embodiment, although the description is made on the example in which the heating apparatus is the fixing device, a color erasing device can also be used as the heating apparatus. When the color erasing device is used as the heating apparatus, the color erasing device erases a toner image by heating and pressing a sheet fixed with color erasable toner which is erased by heating. 
     In the sixth embodiment, although the description is made on the example in which the heating unit heats the belt, the heating unit may heat the pressure roller. 
     According to the sixth embodiment, for example, techniques of the following (1) to (15) can be provided. 
     (1) A heating apparatus includes a rotator, an endless belt that is rotated by rotation of the rotator, and nips and conveys a sheet on which toner is transferred in cooperation with the rotator, a heating unit to heat at least one of the rotator and the belt, a press member to press the belt to the rotator side from the inside of the belt, a base member that is positioned in an inside space of the belt and supports the press member, a peeling member that is separate from an outer peripheral surface of the belt through a gap and peels the sheet from the belt, and a coupling member to couple the peeling member and the base member. 
     (2) In the heating apparatus of (1), an opposite part that is opposite to the peeling member through the belt and guides the belt from the inside is provided. 
     (3) In the heating apparatus of (2), the opposite part has hardness higher than that of the press member and is integral with the base member. 
     (4) In the heating apparatus of (3), the peeling member has a plate shape when viewed in a plane. 
     (5) In the heating apparatus of (4), the peeling member includes a base member and a protection layer laminated on the base member. 
     (6) In the heating apparatus of (5), the base member has a longish shape extending in a width direction of the belt at the inside of the belt, and supports the coupling member at both ends. 
     (7) In the heating apparatus of (6), the heating unit heats the belt. 
     (8) An image forming apparatus includes an image forming section that transfers toner onto a sheet and forms an image, a rotator, an endless belt that is rotated by rotation of the rotator and nips and conveys the sheet, on which the toner is transferred by the image forming section, in cooperation with the rotator, a heating unit to heat at least one of the rotator and the belt, a press member to press the belt to the rotator side from the inside of the belt, a base member that is positioned in an inside space of the belt and supports the press member, a peeling member that is separate from an outer peripheral surface of the belt through a gap and peels the sheet from the belt, and a coupling member to couple the peeling member and the base member. 
     (10) In the image forming apparatus of (9), an opposite part that is opposite to the peeling member through the belt and guides the belt from the inside is provided. 
     (11) In the image forming apparatus of (10), the opposite part has hardness higher than that of the press member and is integral with the base member. 
     (12) In the image forming apparatus of (11), the peeling member has a plate shape when viewed in a plane. 
     (13) In the image forming apparatus of (12), the peeling member includes a base member and a protection layer laminated on the base member. 
     (14) In the image forming apparatus of (13), the base member has a longish shape extending in a width direction of the belt at the inside of the belt, and supports the coupling member at both ends. 
     (15) In the image forming apparatus of (14), the heating unit heats the belt. 
     The invention can be carried out in various forms without departing from the spirit or the principle feature of the invention. Accordingly, the foregoing embodiments are merely examples in any points and should not be interpreted in a limiting sense. The scope of the invention is described by the claims and is not limited by the contents of the specification. Further, all modifications, various improvements, substitutions and alterations belonging to the equivalent range of the claims are contained within the scope of the invention. 
     As described above in detail, according to the technique disclosed in the specification, the technique to improve the equalizing temperature 
     performance in the temperature equalizing roller can be provided. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of invention. Indeed, the novel apparatus described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the apparatus described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.