Abstract:
An image fixing apparatus includes a heater; a film movable together with a recording material in contact with the heater; a pressing member for establishing press-contact between the heater and the film and between the film and the recording medium; and a shutting device for shutting power supply to the heater upon release of the pressure contact.

Description:
This application is a continuation of application Ser. No. 07/915,865 filed Jul. 20, 1992, now abandoned, which in turn is a continuation of application Ser. No. 07/602,776, filed Oct. 24, 1990, also abandoned. 
    
    
     FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image fixing apparatus, usable with an image forming apparatus, for heat-fixing a toner image on a recording material such as a transfer material, more particularly to a heat-fixing apparatus wherein the image is heat-fixed through a thin film. 
     As for an image fixing system for fixing a toner image on a recording material by heat, a heat roller type fixing system is widely used in which a recording medium carrying a toner image is passed through a nip between a heating roller and a pressing or back-up roller press-contacted thereto. This system, however, involves the problem that the warming period until the surface of the heating roller reaches a predetermined operable level, is long. 
     In order to solve the problem, U.S. patent applications Ser. Nos. 206,767, 387,970, 409,341, 416,539, 426,082, 435,247, 440,380, 440,678, 444,802 and 446,449 which have been assigned to the assignee of this application, have proposed a heat-fixing apparatus wherein use is made with a heater having a property of instantaneous temperature rise and a thin film, whereby the warming period is significantly reduced or eliminated. 
     In the usual heat-roller type fixing system, the pressure-contact between the rollers is released for the purpose of jam clearance. The release upon the jam clearance operation is also effective in the apparatus using the thin film and the heater having the property of quick response, in which, however, there is another problem. 
     More particularly, if the heater is energized with the released state, the heater or the film is thermally damaged. This is because, the effective thermal capacity of the heater and the body therearound is small when the pressing member having the large capacity is away from the heater, and the heater and the film have low thermal capacities, and because the heat is not released to the other member. In addition, when the press-contact is released, the overshooting of the temperature upon the start of energization of the heater is larger than upon the press-contact state, and therefore, it is predicted that the thermal damage is produced-upon the start of the energization. The possible damage of the heater includes burn-out of the heater (heat generating layer), damage of the heater substrate (alumina substrate, for example), and fusing of a heater retaining layer (heat-resistive resin such as PPS resin). The damage to the film includes burn-out of the film because the heat thereof does not leak to the pressing member. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a principal object of the present invention to provide an image fixing apparatus wherein the pressure-contact between the heater and the film can be released. 
     It is another object of the present invention to provide an image fixing apparatus wherein the heater and the film can be effectively protected from the thermal damage. 
     It is a further object of the present invention to provide an image fixing apparatus wherein the energization to the heater is stopped upon release of the pressure-contact. 
     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 side view of a pressing mechanism used in the apparatus of FIG. 1 embodiment. 
     FIGS. 3A and 3B are enlarged view illustrating another embodiment. 
     FIG. 4 is a graph showing a temperature rise in the control of the heater. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The description of the preferred embodiments will be made in conjunction with the accompanying drawings wherein like reference numerals are assigned to the elements having the corresponding functions. 
     Referring to FIG. 1, there is shown an image fixing apparatus according to an embodiment of the present invention. FIG. 2 is a side view of a pressing mechanism used therein. 
     The image fixing apparatus includes a low thermal capacity heater 1 fixedly mounted on a main assembly of the fixing apparatus, and a heat generating element 1a extending in a direction perpendicular to a direction of a recording material movement. The heat generating element 1a produces heat by pulse-wise electric power supply thereto. It further includes an image fixing film made of heat resistive material rotatable along an endless path in a direction indicated by an arrow A upon the image fixing operation. It is contacted to the heater 1 and is stretched around a driving roller 3 and a follower roller 4, so that it is tensioned and driven. 
     The thickness of the film 2 is preferably less than 40 microns, in order to avoid large thermal capacity. A pressing roller 5 has a rubber elastic layer, made of silicone, exhibiting good parting property, and is urged to the fixing film 2 toward the heater 1 by a pressing mechanism shown in FIG. 2. It is driven by an unshown driving mechanism. A recording material (not shown) carrying an unfixed toner image is introduced (from the right side) into a nip 6 formed between the pressing roller 5 and the fixing film 2, by which the unfixed toner image is fixed by the heat. 
     The pressing roller 5 is rotatably supported on a pressure lever 7 which is rotatable substantially vertically about a pin 7a planted on the main frame of the apparatus. The pressure roller 5 is press-contacted to the heater 1 by a spring force of a pressing spring 10 stretched between a movable end of the pressing lever 7 and the main frame. 
     Therefore, the pressure is released by the rotation of the pressing lever 7 against the spring force of the pressing spring 10 from a pressing position indicated by broken lines in the Figure in a direction indicated by an arrow B. By further downward urging of the pressing lever 7, it is locked by an unshown locking means at a release position. A microswitch 12 is actuated by the movable end 7b upon the arrival of the pressing lever 7 at the releasing position, in response to which the heater energizing circuit for supplying electric energy to the heat generating element 1a is opened, or in response to which the power source for the main assembly of the image forming apparatus including the fixing apparatus is deactuated, by which the energy supply to the heater 1a is stopped. 
     Accordingly, erroneous power supply to the heater 1 is effectively prevented. In addition, even if the pressure is released during the power supply, the power supply is instantaneously stopped, so that the thermal damage to the heater or the film can be avoided. 
     The following is the possible alternative. Using a sensor 11 such as a photosensor, the pressure release of the pressing roller 5 is detected on the basis of the position of the pressing lever 7. In response to the detection signal thereof, the power source is deactuated, and/or, an error, for example, is displayed in a display means such as unshown display panel or the like, and/or the signal from the start switch is not accepted. 
     FIG. 3 shows another embodiment, wherein an electrode is provided at an end of the pressing lever 7, and a contact 9 is provided at the main assembly side. The power supply to the heat generating element 1a of the heater 1 is started and stopped by the electrode 8 and the contact 9. Thus, the electrode 8 and the contact 9 constitute a switch. When the pressing lever 7 is at the pressing position, the switch is on so that the power supply to the heat generating element 1a is effected (FIG. 3A), and when it is at the release position (FIG. 3B), the switch is off so that the power supply to the heat generating element 1a is prevented. At this time, the image forming apparatus may be stopped in response to a signal indicative of the &#34;off&#34; of the switch. The photosensor 11 may be further provided so that simultaneously with the stoppage of the power supply, an error is displayed, and the operation of the image forming apparatus is stopped. 
     Referring to FIG. 4, the advantageous effects of the invention will be described. Without the present invention, the heater is energized when the pressure-contact is released with the result of temperature rise as indicated by the solid line. It will be understood that the temperature rise is significantly higher than that upon the energization with the press-contact. Such the significant temperature rise can be prevented by the present invention. 
     As described in the foregoing, according to the present invention, even if the pressure is erroneously released, or when the pressure is released for the purpose of jam clearance operation, the thermal damage to the heater and the film can be effectively prevented. 
     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.