Abstract:
An image fixing apparatus includes a pair of rotatable members, the pair comprising a heating rotatable member and a back-up rotatable member press-contacted to the heating rotatable member, wherein the heating rotatable member is settable at a first setting temperature and a second setting temperature which is lower than the first setting temperature, when an image fixing operation is performed; a control system for controlling rotation of the pairs of rotatable members, the control system rotating the pairs of rotatable members when the setting temperature of the heating rotatable member is changed from the first setting temperature to the second setting temperature.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image fixing apparatus usable with an image forming apparatus such as a copying apparatus or a laser beam printer and, more particularly, to an image fixing apparatus wherein an unfixed image is fixed by applying heat thereto by a heating rotatable member. 
     Recently, an image fixing apparatus has been widely used wherein an image supporting material containing an unfixed image is passed through a nip formed between two rotatable members, while heat and pressure are applied to the unfixed image to fix it. Because of the size of the apparatus and the fixing property it is considered that the setting temperature of the heating rotatable member is changed depending on the materials of the image supporting material or the like. 
     Various image forming machines have been put into practice wherein full color image or plural color images are produced. In an the apparatus of this kind, in order to provide a full color or a multi-color image, there is provided a plurality of developing devices for developing latent images formed on a photosensitive drum for the respective colors, and each developing device is exchanged with another one containing different toner, so as to provide full color image or a multi-color image. In this case, the toners having different colors have different proper image fixing temperatures due to the difference in materials constituting the toners. Take for example, an electrophotographic copying apparatus wherein a black color developing device is exchanged with a non-black monochromatic color developing device to develop respective latent images on the photosensitive. The monochromatic toner does not contain magnetic material so as to provide bright color, whereas the black toner is a one component magnetic toner containing magnetic material since the structure of the developing device is simpler and since the maintenance is easier. It is noted that because the black toner contains magnetic material, the proper fixing temperature for the black toner is higher than that of the non-black monochromatic toner. Depending on the material of the resin constituting the toner and on the content of the magnetic material or the like, the temperature difference can be as large as several tens of degrees. 
     Therefore it is considered that the setting temperature of the heating rotatable member is changed in accordance with the material of the toner used. 
     However, a change of the setting temperature involves a problem that a long period of time is required for the temperature to decrease sufficiently when amount of the change is large. Japanese Laid-Open Patent Application No. 115072/1980 proposes provision of a cooling device so as to quickly change the temperature. However, the Japanese Laid-Open Application does not disclose which temperature is intended and does not disclose use with a heating rotatable member. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an image fixing apparatus wherein the setting temperature of the heating rotatable member can be changed quickly. 
     It is another object of the present invention to provide an image fixing apparatus wherein the temperature of the heating rotatable member can be decreased quickly without use of an additional cooling mechanism. 
     It is a further object of the present invention to provide an image fixing apparatus wherein toner images formed of different toners can be fixed without high temperature off-set and low temperature off-set. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of an image fixing apparatus according to an embodiment of the present invention. 
     FIG. 2 is a sectional view of an image fixing apparatus according to another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, there is shown an electrophotographic copying apparatus as an exemplary image forming apparatus using an image fixing apparatus according to an embodiment of the present invention. The image forming apparatus comprises a photosensitive drum 1 and various known means therearound, such as a charging device, an exposure device, a developing device, an image transfer device and a cleaning device. 
     The developing device is replaceable with another developing device containing different toner. By the interchange of the developing device, the image forming operation is possible with different color toners. 
     A predetermined image processing operation is performed by the various devices disposed around the photosensitive drum 1, and the latent image formed on the surface of the photosensitive drum 1 is developed by the developing device 2 disposed close to the photosensitive drum 1. The developing device 2 contains toner T therein and is provided with a lever 3 fixed thereto at a proper position on the top surface thereof, in this embodiment. The position of the lever 3 on the top surface of the developing device 2 is different depending on the material of the toner T contained in the developing device. When a developing device is mounted into the image forming apparatus at a proper position determined for the respective developing devices, the lever 3 of the developing device closes a contact 5 of a microswitch 4 disposed at a proper position in association with the material of the toner. The number of the microswitches 4 disposed at the proper positions in the apparatus may be one or plural depending on the number of toners T usable with the apparatus. 
     By the developing device 2, the latent image formed on the surface of the photosensitive drum 1 is developed into a toner image, which is, in turn, transferred onto an image supporting material such as paper or the like, fed to the photosensitive member. Thereafter, the image is fixed by a heating fixing apparatus 6 provided in the image forming apparatus. 
     The image fixing apparatus 6 comprises an fixing roller 7 contactable with the unfixed toner image and a back-up or pressing roller 8. The fixing roller 7 contains therein a heater 9 and is provided with a thermister 10 for detecting an image fixing temperature. 
     When the developing device 2 is mounted in the developing device accommodating position on the image fixing apparatus, when the lever 3 close the contact 5 of the microswitch 4 disposed at the proper position of the image forming apparatus depending on the material of the toner T contained in the developing device. A signal produced by the close of the contact 5 is transmitted to a control unit 11 so that the material of the toner is discriminated. The control unit 11 is supplied with a temperature detection signal from the thermister 10 of the heating fixing apparatus 6 for detecting the temperature of the fixing roller 7. In response to the temperature detection signal, the control unit 11 produces a control or energization signal to the heater 9 of the fixing roller 7, thus on-off-controlling the heater 9 to properly heat the fixing roller 7. 
     In the control device, a change in the resistance for the temperature detection by the thermister 10 is converted to a voltage change, and the changed voltage is compared with a reference voltage signal corresponding to the setting temperature. The control signal is such that when the temperature detected by the thermister 10 is lower than the setting temperature, the heater 9 is energized, whereas when the temperature detected by the thermister 10 is higher than the setting temperature, the heater 9 is deenergized, thus maintaining the temperature of the fixing roller 9. 
     In this case, the control unit 11 has reference voltages properly predetermined for respective toners, and proper one of the reference voltages is selected corresponding to the material of the toner detected by the lever 3. Thus, by changing the reference voltage, the setting temperature of the fixing roller 7 can be changed properly. The control unit 11 comprises instruction means. 
     When a developing device containing a black magnetic toner is set in the apparatus, the fact that it contains the black toner is detected, and in response to the detection, the control unit 11 selects 180° C. as the setting temperature of the fixing roller 9. Thereafter, the currently set developing device is interchanged with a monochromatic color developing device containing a non-magnetic monochromatic color toner. Then, the fact that the new developing device contains the color toner is detected, and in response to the detection signal, the control unit 11 changes the setting temperature from 180° C. for the black toner to 155° C. lower than it, and it deenergizes the heater 9 and actuates a driving motor 12 for rotating the fixing roller 7. By the rotation of the fixing roller 7, the pressing roller 8 rotates following the fixing roller 7. By the rotation of the pressing roller 8 in contact with the fixing roller 7, the fixing roller 7 is forcedly cooled. 
     When the thermister 10 detects that the surface temperature of the fixing roller 7 reaches the color toner setting temperature, that is, 155° C., the rotation of the fixing roller stops, and the apparatus is prepared for the image forming operation. When an image formation signal is produced, the image is formed with the color toner. 
     The rotations of the fixing roller 7 and the pressing roller 8 for forcedly cooling the fixing roller 7 are preferably performed when the surface temperature of the fixing roller 7 is higher than the new setting temperature. 
     Therefore, if a substantial time is required for the exchange from the black developing device to the color developing device to such an extent that the surface temperature of the image fixing roller 7 decreases to a temperature which is not higher than the color toner setting temperature, the roller rotations for the force cooling are not required. The rotations of the rollers for the force cooling are effected in accordance both with the signal instructing the change of the setting temperature and the current surface temperature of the roller. 
     On the other hand, when the color developing device is exchanged to the black developing device, the setting temperature is increased, and therefore, the roller rotation for the forced cooling is not performed. 
     Experiments have been performed with a copying machine capable of producing 15 copies/min. in a one-to-one magnification for A3 size, provided with a heating roller type fixing apparatus having 900 W heater and a PTFE fixing roller having a diameter of 32 mm and a thickness of 25 mm. In this apparatus, the fixing setting temperature for a black one component magnetic toner is approximately 180° C. to provide good image fixing property without toner offset is 180° C., whereas the proper setting temperature for a non-magnetic monochromatic color toner was approximately 155° C. When the black developing device is exchanged with a color toner developing device, it took approximately 150 sec. when the fixing apparatus was left as it was when the room temperature was approximately 25° C. 
     When the fixing roller and the pressing roller were rotated until the temperature decreases sufficiently, it took only approximately 40 sec. for the surface temperature decreases 180° C. to 155° C. when the room temperature was approximately 25° C. 
     Therefore, the waiting period is reduced to approximately one fourth. 
     Referring to FIG. 2, another embodiment of the present invention will be described. In this embodiment, the image forming apparatus contains a developing device 2a containing a black one component magnetic toner and a developing device 2b containing a red non-magnetic toner. Those developing devices 2a and 2b are urged away from the photosensitive drum 1, and are selectively placed close to or in contact with the photosensitive drum 1 by a cam driving unit 16. By the cooperation among the levers 3a and 3b, microswitches 4a and 4b and contacts thereof 5a and 5b, the discrimination is made as to which developing device is operable, and in response to the discrimination, the heating roller setting temperature is determined. 
     The present invention wherein the heating roller is forced cooled by rotation of the rollers is particularly advantageous when it is used with the image forming apparatus containing plural developing devices, because the developing device can be quickly interchanged. 
     As described in the foregoing, according to the present invention, the temperature of the heating roller can be decreased quickly without necessity of a particular cooling means. In addition, since the rollers are rotated for the cooling, the temperature of the entire surface of the roller is made uniform so that the non-uniform image fixing is produced. 
     In the foregoing description, a roller is taken as an Example of the rotatable member, but the rotatable member may be in the form of a belt or the like. 
     In addition, the change of the setting temperature is not limited to that depending on the material of the toner, but may be depending on the material of the toner image supporting material or another. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.