Electrophotography – Image formation – Transfer
Reexamination Certificate
1999-11-01
2001-04-17
Brase, Sandra (Department: 2852)
Electrophotography
Image formation
Transfer
Reexamination Certificate
active
06219518
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus and more particularly to an image forming apparatus of the type sequentially transferring toner images from an image carrier to a belt or similar intermediate image transfer body one upon the other (primary transfer) and collectively transferring them from the intermediate transfer body to a paper or similar recording medium (secondary transfer), and the intermediate transfer body.
Today, in the electrophotographic imaging art, image transfer using a belt or a roller is replacing traditional corona image transfer from the environmental standpoint. The present invention relates to image transfer using an image transfer roller.
Various kinds of technologies have heretofore been proposed in relation to an image transfer roller. For example, an image transfer roller may be implemented by a sponge roller having a specific hardness for reducing a nip pressure. This transfer roller is directed toward an image free from vermicular defects. The surface of an image transfer roller may have its specific resistance increased in order to prevent an image transfer voltage from varying with the varying environment, e.g., the resistance of a recording medium, as taught in Japanese Patent Laid-Open Publication No. 2-212873.
At the time of secondary transfer following primary transfer, an optimal image transfer current to be applied to the image transfer roller is determined by the transfer characteristic of a sol id portion and that of a halftone portion. That is, a current satisfying both of such characteristics has customarily been selected as an optimal image transfer current. In a low temperature, low humidity environment, it is a common practice to increase a bias to be applied to the roller also because the resistance of a recording medium increases in the above environment.
However, the conventional image transfer roller has the following problems when used in the above conditions. Desirable image transfer is not achievable because the range of optimal currents is extremely narrow or does not exist at all, i.e., the transfer characteristic of a solid portion and that of a halftone portion cannot be implemented at the same time. Further, because the resistance of the roller susceptible to the environment, it is necessary to use a power pack having an extremely great capacity or to heat the roller or otherwise prevent the resistance of the roller from increasing.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-19276, 8-87156, 8-234564, 10-78713, 10-228187, and 11-38782.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an image forming apparatus capable of achieving a sufficient margin as to a transfer current for secondary transfer which is severest in a low temperature environment causing the resistance of a recording medium to rise, and insuring desirable image transfer without resorting to a power pack having an extremely great capacity or the heating of a roller in a low temperature environment conventionally effected to suppress an increase in resistance, and an image transfer roller therefor.
In accordance with the present invention, in an image forming apparatus for sequentially transferring a plurality of toner images formed on an image carrier to an endless intermediate image transfer body one upon the other for thereby effecting primary transfer, and collectively transferring the toner images from the intermediate image transfer body to a recording medium for thereby effecting secondary transfer, an image transfer roller for the secondary transfer includes a metallic core and at least two layers formed on the metallic core and including a base layer formed on the metallic core and a surface layer formed on the base layer. The two layers have a resistance between 1.57×10
6
&OHgr; and 1.57×10
9
&OHgr; for a unit length (1/cm) while the surface layer has a volume resistivity between 1×10
6
&OHgr; cm and 1×10
9
&OHgr; cm.
REFERENCES:
patent: 5010370 (1991-04-01), Araya et al.
patent: 5438398 (1995-08-01), Tanigawa et al.
patent: 5669053 (1997-09-01), Hashizume et al.
patent: 5778281 (1998-07-01), Handa
patent: 5819142 (1998-10-01), Murata et al.
patent: 5857132 (1999-01-01), Mizuishi et al.
patent: 5978639 (1999-11-01), Masuda et al.
patent: 6-19276 (1994-01-01), None
patent: 8-87156 (1996-04-01), None
patent: 8-234564 (1996-09-01), None
patent: 10-78713 (1998-03-01), None
patent: 10-228187 (1998-08-01), None
patent: 11-38782 (1999-02-01), None
Seto Mitsuru
Sugiyama Naoki
Brase Sandra
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Ricoh & Company, Ltd.
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