Image heating device and image forming device using the same

Abstract

An image heating device comprises a cylindrical heating roller with a Curie temperature of 210° C., a magnetization coil for magnetizing the heating roller with an alternating magnetic field, which is arranged inside the heating roller, and a nip portion for heating a recording material that carriers a toner image with heat from the heating roller, while the recording material is being conveyed along said nip portion. The ratio between the amount of heat generated in said heat-generating member at Curie temperature or higher to the amount of heat generated at room temperature in said heat-generating member is not more than ½. With this configuration, the heating roller can regulate its own temperature to stabilize at a temperature that is suitable for fixing, and the problems of partial overheating or underheating, unstable heat generation, or damage of the device can be eliminated.

1. An image heating device comprising:
 a heat-generating member comprising a magnetic layer with a certain Curie temperature;  
 a magnetization member for magnetizing said heat-generating member with an alternating magnetic field, which is arranged in opposition to said heat-generating member;  
 a nip portion for heating a recording material that carries a toner image with heat from said heat-generating member, while the recording material is being conveyed along said nip portion;  
 wherein a ratio between an amount of heat generated in said heat-generating member at Curie temperature or higher to an amount of heat generated at room temperature in said heat-generating member is not more than ½.  
 
     
     
       2. The image heating device of  claim 1 , wherein a thickness of said magnetic layer is at least twice a thickness of a skin depth. 
     
     
       3. The image heating device of  claim 1 , wherein said heat-generating member further comprises a conductive layer with lower resistance than said magnetic layer, which is provided adjacent to said magnetic layer. 
     
     
       4. The image heating device of  claim 3 , wherein
   ρ1/t 1 ≧ρ2/t 2   
 
       wherein ρ1 is an intrinsic resistance of said magnetic layer, t 1  is a thickness of said magnetic layer, ρ2 is an intrinsic resistance of said conductive layer, and t 2  is a thickness of said conductive layer. 
     
     
       5. The image heating device of  claim 3 , wherein the thickness of said magnetic layer is equivalent to or greater than the skin depth. 
     
     
       6. The image heating device of  claim 1 , wherein said nip portion is formed by at least a portion of said heat-generating member, and a pressure member pressed against this portion of said heat-generating member. 
     
     
       7. The image heating device of  claim 6 , wherein at least said magnetic layer of said heat-generating member is a rotatable roller. 
     
     
       8. The image heating device of  claim 6 , wherein at least said magnetic layer of said heat-generating member is a movable film. 
     
     
       9. The image heating device of  claim 8 , wherein said film is loop-shaped. 
     
     
       10. The image heating device of  claim 6 , wherein at least a conductive layer of said heat-generating member is a movable film. 
     
     
       11. The image heating device of  claim 1 , wherein the nip portion is formed by a movable film contacting said heat-generating member, and a pressure member for pressing against said film. 
     
     
       12. The image heating device of  claim 11 , wherein said heat-generating member contacts a rear surface of said film. 
     
     
       13. The image heating device of  claim 11 , wherein said heat-generating member contacts the rear surface of said film from a position upstream of said nip portion to a vicinity of said nip portion, and said magnetization member is provided at the position upstream of said nip portion. 
     
     
       14. The image heating device of  claim 11 , wherein said heat-generating member is provided on the rear side of said film and contacts a portion of said film, and said magnetization member is provided on a surface side of said film. 
     
     
       15. The image heating device of  claim 11 , wherein the pressure member comprises a roller with low thermal conductivity provided on the rear surface side of said film and a pressure roller provided on the front surface side of said film. 
     
     
       16. The image heating device of  claim 11 , wherein said heat-generating member comprises a rotatable roller. 
     
     
       17. An image formation device comprising
 an image formation means for forming an unfixed image onto a recording material; and  
 a thermal fixing device for thermally fixing the unfixed image on the recording material;  
 wherein an image heating device according to  claim 1  is used as the thermal fixing device.  
 
     
     
       18. An image heating device comprising:
 a heat-generating member comprising a magnetic layer with a certain Curie temperature;  
 a magnetization member for magnetizing said heat-generating member with an alternating magnetic field, which is arranged in opposition to said heat-generating member;  
 wherein, when said device is in operation, a temperature at which said heat-generating member stabilizes due to a drop of a relative magnetic permeability of said magnetic layer near said Curie temperature is higher than a temperature where cold offset begins, and  
 wherein said Curie temperature is selected such that, when the temperature of said heat-generating member is stabilized, a temperature of an outgoing portion of a nip portion is lower than a temperature where hot offset of the toner begins.  
 
     
     
       19. The image heating device of  claim 18 , wherein said heat-generating member further comprises a conductive layer with lower resistance than said magnetic layer, which is provided adjacent to said magnetic layer. 
     
     
       20. The image heating device of  claim 19 , wherein
   ρ1/t 1 ≧ρ2/t 2   
 
       wherein ρ1 is an intrinsic resistance of said magnetic layer, t 1  is a thickness of said magnetic layer, ρ2 is an intrinsic resistance of said conductive layer, and t 2  is a thickness of said conductive layer. 
     
     
       21. The image heating device according to  claim 18 , wherein
   Tc≦Tk≦Th+70° C.  
 
       wherein Tc is the temperature where cold offset of the toner begins in said nip portion, Tk is the Curie temperature, and Th is the temperature where hot offset of the toner begins in an outgoing portion of said nip portion. 
     
     
       22. The image heating device according to  claim 18 , wherein
   140° C.≦Tk≦280° C.  
 
       wherein Tk is the Curie temperature. 
     
     
       23. The image heating device of  claim 18 , wherein said nip portion is formed by at least a portion of said heat-generating member, and a pressure member pressed against this portion. 
     
     
       24. The image heating device of  claim 23 , wherein at least said magnetic layer of said heat-generating member is a rotatable roller. 
     
     
       25. The image heating device of  claim 23 , wherein at least said magnetic layer of said heat-generating member is a movable film. 
     
     
       26. The image heating device of  claim 23 , wherein at least said conductive layer of said heat-generating member is a movable film. 
     
     
       27. The image heating device of  claim 18 , wherein the nip portion is formed by a movable film contacting said heat-generating portion, and a pressure member for pressing against said film. 
     
     
       28. The image heating device of  claim 27 , wherein said heat-generating member contacts a rear surface of said film. 
     
     
       29. The image heating device of  claim 27 , wherein said heat-generating member contacts the rear surface of said film from a position upstream of said nip portion to a vicinity of said nip portion, and said magnetization member is provided at the position upstream of said nip portion. 
     
     
       30. The image heating device of  claim 27 , wherein said heat-generating member is provided on the rear side of said film and contacts a portion of said film, and said magnetization member is provided on a surface side of said film. 
     
     
       31. The image heating device of  claim 27 , wherein the pressure member comprises a roller with low thermal conductivity provided on the rear surface side of said film and a pressure roller provided on the front surface side of said film. 
     
     
       32. The image heating device of  claim 27 , wherein said heat-generating member comprises a rotatable roller. 
     
     
       33. The image heating device of  claim 27 , wherein said film is loop-shaped. 
     
     
       34. An image formation device comprising
 an image formation means for forming an unfixed image onto a recording material; and  
 a thermal fixing device for thermally fixing the unfixed image on the recording material;  
 wherein an image heating device according to  claim 18  is used as the thermal fixing device.  
 
     
     
       35. An image heating device comprising:
   a heat - generating member comprising a magnetic layer;        a magnetization member for magnetizing said heat - generating member with an alternating magnetic field, which is arranged in opposition to said heat - generating member;        a nip portion for heating a recording material that carries a toner image with heat from said heat - generating member, while the recording material is being conveyed along said nip portion; and        a movable film that is separated from the heat generating member, the nip portion being formed between a pressure roller and the movable film;        wherein the heat - generating member contacts a contact part of the movable film and is arranged so as to be opposed to the magnetization member at a different position from that of the nip portion, and transmits generated heat to the movable film at the contact part.     
     
     
       36. The image heating device of  claim 35 , wherein said heat- generating member contacts a rear surface of said film.   
     
     
       37. The image heating device of  claim 35 , wherein said heat- generating member contacts the rear surface of said film from a position upstream of said nip portion to a vicinity of said nip portion, and said magnetization member is provided at the position upstream of said nip portion.   
     
     
       38. The image heating device of  claim 35 , wherein said heat- generating member is provided on the rear side of said film and contacts a portion of said film, and said magnetization member is provided on a surface side of said film.   
     
     
       39. The image heating device of  claim 35 , further comprising a pressure member with low thermal conductivity provided on the rear surface side of said film, and wherein the pressure roller is provided on the front surface side of said film. 
     
     
       40. The image heating device of  claim 35 , wherein said heat- generating member comprises a rotatable roller.