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A fixing device capable of improving printing speed without increasingly the size of rollers, and making the surface temperature of the heating roller quickly reach a target temperature. The heating roller, which comes into contact with recording sheet having unfixed toner adhering to it, includes a

A fixing device capable of improving printing speed without increasingly the size of rollers, and making the surface temperature of the heating roller quickly reach a target temperature. The heating roller, which comes into contact with recording sheet having unfixed toner adhering to it, includes an elastic body of low heat conductivity and high heat resistance, and has metal foil formed tightly to the external circumferential surface thereof. This meal foil with a small heat capacity, by direct contact on its surface with the heating source, helps the surface of the heating roller quickly rise in temperature.

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1. A fixing device for passing a recording sheet having unfixed toner adhering thereto through a first and a second roller pressed together, and fusing said toner under heat and pressure and fixing said toner to said recording sheet, wherein said first roller heated by a heating source comprises:an

1. A fixing device for passing a recording sheet having unfixed toner adhering thereto through a first and a second roller pressed together, and fusing said toner under heat and pressure and fixing said toner to said recording sheet, wherein said first roller heated by a heating source comprises:an elastic body having a high heat resistance and low heat conductivity coefficient, provided around a mandrel;a heat layer formed to a circumferential surface of said elastic body; anda parting agent layer with water and oil repellency, formed to a circumferential surface of said heat layer; andwherein said second roller comprises an elastic body, and a metal layer formed on a surface of the elastic body of said second roller. 2. A fixing device according to claim 1, wherein said second roller is heated by another heating source and comprises the elastic body which is provided on a mandrel and has a high heat resistance and a low heat conductivity coefficient, and the metal layer which is formed on the circumferential surface of said elastic body and has a small heat capacity. 3. A fixing device according to claim 2, wherein said second roller further comprises a parting agent layer formed on the circumferential surface of said metal layer. 4. A fixing device according to claim 3, wherein said second roller further comprises an elastic body layer of a high heat resistance and a high heat conductivity coefficient formed between said metal layer and said parting agent layer. 5. A fixing device according to claim 2, wherein said another heating source is covered with a casing lined on an internal surface with an adiabatic heat reflecting material and having an opening open to said recording sheet to preheat said recording sheet. 6. A fixing device according to claim 5, wherein said other heating source has a plurality of heat generating units arranged along a rotating axis of said second roller, and wherein said plurality of heat generating units are controlled to selectively generate heat according to the width of said recording sheet. 7. A fixing device according to claim 2, wherein said heating source is arranged on a downstream side of a running direction of said recording sheet to heat said first roller, said another heating source is arranged on an upstream side of the running direction of said recording sheet to heat said second roller, said another heating source having a casing which has an opening facing said recording sheet to preheat said recording sheet, and an internal surface made up of an adiabatic heat reflecting material. 8. A method for manufacturing the second roller according to claim 2, comprising the steps of:immersing a cylindrical electrode having an insulating layer on the outer circumference thereof and a metal material to form the metal layer in an electrolytic solution, connecting said cylindrical electrode to the negative pole and said metal material to the positive pole of a power supply to thereby cause the metal layer to precipitate on the inside surface of the cylindrical electrode by electroplating;supporting said mandrel at the center of the inside of said cylindrical electrode, injecting a liquid elastic material to the inside of the metal layer, and forming an elastic body by crosslinking. 9. A fixing device according to claim 1, wherein said heating source has a plurality of heat generating units arranged along a rotating axis of said first roller, and wherein said plurality of heat generating units are controlled to selectively generate heat according to the width of said recording sheet. 10. A fixing device according to claim 9, wherein said width of said recording sheet is designated by an operation key. 11. A fixing device according to claim 9, wherein said width of said recording sheet is detected by a paper width sensor disposed on the running path of said recording sheet. 12. A fixing device according to claim 1, wherein said first roller comprises a temperature sensor embedded in the vicini ty of said heat layer of said elastic body and in a position not to obstruct a nip deformation of said elastic body. 13. A fixing device according claim 1, wherein said first roller further comprises an elastic body layer disposed between said heat layer and said parting agent layer, and wherein there are provided a plurality of openings in both ends of said heat layer to connect said elastic body and said elastic body layer. 14. A fixing device according to claim 1, wherein a width of said heat layer is greater than a maximum-width of the recording sheet. 15. A method for manufacturing the first roller according to claim 1, comprising the steps of:forming the heat layer from a metal foil formed in a cylindrical shape, and applying a parting agent material to an outer circumference of said metal foil, and forming the parting agent layer by baking for crosslinking; andarranging the mandrel at the center of the inside of said metal foil, injecting a liquid elastic material to the inside of said metal foil, and forming the elastic body by crosslinking. 16. A method for manufacturing said first roller according to claim 1, comprising the steps of:forming the heat layer from a metal foil formed in a cylindrical shape;applying an elastic material with a high heat conductivity coefficient to an outer circumference of said metal foil, and forming an elastic body layer by baking for crosslinking reaction;applying a parting agent layer to an outer circumference of said elastic body layer, forming said parting agent layer by baking for crosslinking; andarranging said mandrel at the center of the inside of said metal foil, injecting a liquid elastic material to the inside of said metal foil, and forming the elastic body by crosslinking. 17. A method for manufacturing said first roller according to claim 1, comprising the steps of:immersing a cylindrical electrode having an insulating layer on an outer circumference thereof and a metal material to form the heat layer in an electrolytic solution, connecting said cylindrical electrode to the negative pole and said metal material to the positive pole of a power supply to thereby cause the heat layer to precipitate on the inside surface of the cylindrical electrode by electroplating;supporting said mandrel at the center of the inside of said cylindrical electrode, injecting a liquid elastic material to the inside of the heat layer, and forming the elastic body by crosslinking; andremoving said cylindrical electrode, applying a parting agent material to the outer circumference of said heat layer, and forming the parting agent layer by baking for crosslinking. 18. A method for manufacturing said first a roller according to claim 17, further comprising the steps of:dispersing fine fluorine powder in said electrolytic solution to cause a metal foil containing fluorine powder to precipitate on the inside surface of said cylindrical electrode to form the heat layer; andsubjecting said metal foil to heat treatment at a predetermined temperature and for a predetermined time. 19. A method for manufacturing said first roller according to claim 1, comprising the steps of:immersing a cylindrical electrode having an insulating layer on an the outer circumference thereof and a metal material to form the heat layer in an electrolytic solution, connecting said cylindrical electrode to the negative pole and said metal material to the positive pole of a power supply to thereby cause a metal foil to precipitate on the inside surface of the cylindrical electrode by electroplating, thereby forming the heat layer;supporting said mandrel at the center of the inside of said cylindrical electrode, injecting a liquid elastic material to the inside of the metal foil, and forming the elastic body by crosslinking; andremoving said cylindrical electrode, applying an elastic material with a high heat conductivity coefficient and a parting agent material to an outer circumference of said metal foil in this order, and forming an elastic body layer and the parting agent layer by baking for crosslinking. 20. A method of manufacturing said first roller according to claim 1, comprising the steps of:holding the mandrel in a mold, and injecting an elastic material to form the elastic body;immersing said elastic body in a palladium chloride solution to cause palladium to precipitate on the surface thereof, and after rinsing said elastic body with water, causing a metal foil to precipitate by electroless plating to form the heat layer; andcutting off both end portions of said metal foil, applying a parting agent material to an outer surface of said metal foil for crosslinking, and baking to form the parting agent layer. 21. A method for manufacturing roller according to claim 20, further comprising the steps of previously dispersing palladium in said elastic material to form said elastic body, and causing said metal foil to precipitate by electroless plating. 22. A fixing device according to claim 1, wherein said heat layer is one of a metal layer with small heat capacity and an organic matter layer containing metal. 23. A fixing device for passing a recording sheet having unfixed toner adhering thereto through a first and a second roller pressed together, and fusing said toner under heat and pressure and fixing said toner to said recording sheet, wherein said first roller heated by a heating source comprises:an elastic body having a high heat resistance and low heat conductivity coefficient, provided around a mandrel;a heat layer formed to a circumferential surface of said elastic body; anda parting agent layer with water and oil repellency, formed to a circumferential surface of said heat layer, wherein said first roller further comprises an elastic body layer which has a high heat resistance and a high heat conductivity coefficient and is formed between said heat layer and said parting agent layer. 24. A fixing device for passing a recording sheet having unfixed toner adhering thereto through a first and a second roller pressed together, and fusing said toner under heat and pressure and fixing said toner to said recording sheet, wherein said first roller heated by a heating source comprises:an elastic body having a high heat resistance and low heat conductivity coefficient, provided around a mandrel;a heat layer formed to a circumferential surface at said elastic body; anda parting agent layer with water and oil repellency, formed to a circumferential surface of said heat layer, wherein both ends of said heat layer are fitted to crenellated sleeve members tightly attached to the circumferential surfaces at the end portions of said elastic body. 25. A fixing device for passing a recording sheet having unfixed toner adhering thereto through a first and a second roller pressed together, and fusing said toner under heat and pressure and fixing said toner to said recording sheet, wherein said first roller heated by a heating source comprises:an elastic body having a high heat resistance and low heat conductivity coefficient, provided around a mandrel;a heat layer formed to a circumferential surface of said elastic body; anda parting agent layer with water and oil repellency, formed to a circumferential surface of said heat layer,wherein said heating source is covered with a casing lined on its internal surface with an adiabatic heat reflecting material, said casing having an opening facing both said recording sheet and said first roller, to simultaneously preheat said recording sheet and heat said first roller. 26. A method for temperature control of a fixing device for passing recording sheet having unfixed toner adhering thereto through a first and a second roller heated by heating sources, and fusing said toner under heat and pressure and fixing said toner to said recording sheet, comprising the steps of:bringing a first temperature sensor into contact with a circumferential surface of an end portion of said first roller not contacting said recording she et;bringing a plurality of second temperature sensors into contact with a center-portion circumferential surface of said second roller contacting said recording sheet, and also into contact with said end-portion circumferential surface of said second roller not contacting said recording sheet;selling a predetermined temperature for the circumferential surface of said first roller to be maintained;determining a temperature difference between the center-portion circumferential surface and the end-portion circumferential surface of said second roller from detected temperatures by said plurality of second temperature sensors;obtaining a temperature of said center-portion circumferential surface of said first roller from said determined temperature difference and a temperature detected by said first temperature sensor; andif said temperature of said center-portion circumferential surface is lower than said predetermined temperature, supplying power to said heating source to heat said first roller. 27. A fixing device having a heating roller heated by a heating source, a fixing belt wound around said heating roller and a first roller, and a second roller pressed to said first roller with interposition of said fixing belt, wherein a recording sheet having unfixed toner adhering thereto is passed between said second roller and a portion of said fixing belt being pressed to said second roller, and said toner is fused under heat and pressure and fixed to said recording sheet, comprising:a temperature sensor arranged adjacent to an inside surface of said fixing belt, and approximately at the center portion of said fixing belt; anda power supply controller for controlling power supply to said heating source by comparing a detected temperature by said temperature sensor with a fixing temperature preset for said fixing belt. 28. A fixing device according to claim 27, wherein said temperature sensor is arranged close to said center portion on the inner surface of said fixing belt. 29. A fixing device according to claim 27, wherein said heating roller is a heat roller including a metal pipe arranged inside said heating source. 30. A fixing device according to claim 27, further comprising another heating source to heat either one of said first roller and said second roller. 31. A fixing device according to claim 30, wherein said another heating source is provided inside the roller. 32. A fixing device according to claim 30, wherein said another heating source generates heat with a larger heat value at the center portion than at a side end portion. 33. A fixing device according to claim 30, wherein said another heating source differs in heat value from said heating source, and wherein generates heat at a larger heat value at the center portion than at a side end portion thereof. 34. A fixing device according to claim 27, wherein said heating source generates heat with a larger heat value at the center portion than at a side end portion thereof. 35. A fixing device according to claim 27, wherein said heating source comprises two heaters different in heat value. 36. A fixing device according to claim 35, wherein said heating source comprises two heaters different in heat value, and one of said heaters generates heat with a larger heat value at the center portion than at a side end portion thereof. 37. A fixing device according to claim 27, further comprising:a temperature sensor for outer side use for detecting a temperature of a side end portion on the outer side of said fixing belt,wherein said power supply controller controls power supply to said heating source by comparing detected temperatures of said temperature sensors and said temperature sensor for outer side use with said preset fixing temperature. 38. A fixing device according to claim 27, further comprising:a temperature sensor for use at a side end portion on the inner side for detecting a temperature at a side end portion on the inner side of said fixing belt; anda temperature senso r for use at the outer side for detecting a temperature at side end portion on the outer side of said fixing belt,wherein said power supply controller controls power supply to said heating source by comparing detected temperatures by said temperature sensors, said inner-side end-portion temperature sensor, and said outer-side temperature sensor with said preset fixing temperature. 39. A fixing device according to claim 27, wherein the center portion of said fixing belt is disposed between two opposing longitudinal edges of said fixing belt. 40. A fixing device having a heating roller heated by a heating source, a fixing belt wound around said heating roller and a first roller, and a second roller pressed to said first roller with interposition of said fixing belt, wherein a recording sheet having unfixed toner adhering thereto is passed between said second roller and a contacting portion of said fixing belt that is pressed to said second roller by said first roller, and said toner is fused under heat and pressure and fixed to said recording sheet, said heating roller being positioned to be directly over said recording sheet prior to the recording sheet passing between said second roller and said contacting portion, comprising:a temperature sensor arranged adjacent to an inside surface of said fixing belt, and approximately at a center portion of said fixing belt; anda power supply controller for controlling a power supply to said heating source by comparing a detected temperature by said temperature sensor with a fixing temperature preset for said fixing belt. 41. A fixing device according to claim 40, wherein said temperature sensor is arranged close to said center portion on the inner surface of said fixing belt. 42. A fixing device according to claim 40, wherein said heating roller is a heat roller including a metal pipe arranged inside said heating source. 43. A fixing device according to claim 40, further comprising another heating source to heat either one of said first roller and said second roller. 44. A fixing device according to claim 43, wherein said another heating source is provided inside the one of said first roller and said second roller. 45. A fixing device according to claim 43, wherein said another heating source generates heat with a larger heat value at the center portion than at a side end portion. 46. A fixing device according to claim 43, wherein said another heating source differs in heat value from said heating source, and generates heat at a larger heat value at the center portion than at a side end portion thereof. 47. A fixing device according to claim 40, wherein said heating source generates heat with a larger heat value at the center portion than at a side end portion thereof. 48. A fixing device according to claim 40, wherein said heating source comprises two heaters different in heat value. 49. A fixing device according to claim 48, wherein said heating source comprises two heaters different in heat value, and one of said heaters generates heat with a larger heat value at the center portion than at a side end portion thereof. 50. A fixing device according to claim 40, further comprising:a temperature sensor for outer side use for detecting a temperature of a side end portion on the outer side of said fixing belt,wherein said power supply controller controls power supply to said heating source by comparing detected temperatures of said temperature sensors and said temperature sensor for outer side use with said preset fixing temperature. 51. A fixing device according to claim 40, further comprising:a temperature sensor for use at a side end portion on the inner side for detecting a temperature at a side end portion on the inner side of said fixing belt; anda temperature sensor for use at the outer side for detecting a temperature at side end portion on the outer side of said fixing belt,wherein said power supply controller controls power supply to said heating source by comparing detected tempe ratures by said temperature sensors, said inner-side end-portion temperature sensor, and said outer-side temperature sensor with said preset fixing temperature.