1. Field of the Invention
The present invention relates to a toner image fixing device for electrophotographic copying machines, etc., and more particularly to a fixing device by which toner images formed on a sheet are fixed thereto by heating with a heat roller;
2. Description of the Prior Art
As a toner image fixing method for such electrophotographic copying machines, etc., it is known to heat the toner on a sheet with the application of pressure by feeding the sheet to the nip of a heat roller having an internal or external heat source and a press roller in pressing contact with the heat roller. This method has the advantage that the toner can be fixed quickly since the toner is heated directly under pressure, but has objectionable problems, such as offset of toner image and winding of the sheet around the heat roller. For example, if the surface temperature of the heat roller is lower than an appropriate level, the toner image will not be fixed to the sheet effectively but adheres to the surface of the heat roller and is transferred (offset) at the lower temperature to another sheet subsequently fed to the roller. Conversely, if the surface temperature of the heat roller is higher than is appropriate, the toner becomes tacky, and when fixing to the sheet, the toner partly adheres also the heat roller to become transferred (offset) on the next sheet at the higher temperature. Furthermore, the tacky toner frequently causes the sheet to wind around the heat roller. To overcome these problems, heat rollers for the fixing devices of electrophotographic copying machines in recent years are covered with silicone rubber or like non-tacky material over the surface and are used as coated with an offset preventing agent such as silicone oil.
FIG. 1 schematically illustrates an example of a known generally described above fixing device. A heat roller 10 has an internal heat source generally designated H, and a press roller 11 is arranged in pressing contact with heat roller 10 which may be driven by some suitable means (not shown).. These rollers 10 and 11 are covered by a silicone layer 12 over their surfaces and are journalled parallel to one another in a suitable frame 20. A metallic roller 13 typically has a polished surface and is in rubbing contact with a felt element 14 impregnated with silicone oil. A metallic doctor Blade B bears against the surface of roller 13 for controlling the desired thickness of silicone oil applied to roller 13. A rubber roller 15 has its surface in light contact with the surface of metallic roller 13 and heat roller 10, and functions to transmit the rotation of heat roller 10 to metallic roller 13. The silicone oil applied to the surface of rubber roller 15 by the rotation of roller 13 is also applied to the surface of heat roller 10 by rubber roller 15. With such a fixing device, silicone oil is applied to the surface of heat roller 10 almost simultaneously with the start of rotation thereof for preventing offset when a sheet passes between rollers 10 and 11.
However, it has been noted that the aforedescribed arrangement involves difficulties in fully uniformly coating the heat roller surface with silicone oil for the following reasons.
When a large excess of silicone oil is not applied to metallic roller 13 by felt element 14, the surface of roller 13 becomes locally uncoated with silicone oil which produces problems.
Even if an excess of silicone oil applied to the polished surface of the metallic roller 13 is regulated by doctor blade B, any misalignment between the doctor blade and the roller, or any deflection of roller 13 or like causes results in an improper contact between the wiper blade edge and the metallic roller, so that the blade fails to regulate the amount of silicone oil uniformly.
Molten toner particles may thus enter the clearance between blade B and roller 13 and are cooled. When this repeatedly occurs, the initial nuclei grow to enlarge the clearance, thereby leading to an uneven application of silicone oil.
If an increased pressure is applied to blade B to eliminate uneven application, the blade in turn defaces roller 13 which likewise results in an uneven application.
For the causes or reasons given above, the surface of heat roller 10 becomes locally uncoated or coated to excess with silicone oil. Such uneven coating occurs early after the device is placed into use, thereby causing partial offset, leaving oil stains on the sheet during copying and creating other problems. Furthermore, silicone rubber covering 12 of heat roller 10, thus unevenly coated with silicone oil, absorbs the oil and locally swells, with the resulting problems that wrinkles are formed in the sheet passing between rollers 10 and 11.