Electrophotography – Image formation – Charging
Reexamination Certificate
1999-01-29
2001-03-20
Braun, Fred L (Department: 2852)
Electrophotography
Image formation
Charging
Reexamination Certificate
active
06205310
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an electrophotographic recording apparatus including a charging roller for charging the photosensitive drum to a desired potential, and more particularly to an electrophotographic recording apparatus in which the circumferential speed of the charging roller to the circumferential speed of the photosensitive drum is greater than 1.
The conventional electrophotographic recording apparatus, such as a, printer or a duplicator, generally includes a photosensitive drum having a rotation shaft, and a charging roller having a rotation shaft substantially parallel with the rotation shaft of the photosensitive drum and also holding its own external circumferential surface in contact with the external circumferential surface of the photosensitive drum to thereby give a static charge to the external circumferential surface of the photosensitive drum.
The above-mentioned electrophotographic recording apparatus further includes a developing section for supplying a toner to the statically charged external circumferential surface of the photosensitive drum, and a cleaning section for removing the toner remaining on the photosensitive drum which toner was not transferred to a recording paper. Among the techniques for cleaning the photosensitive drum by the cleaning section, there is a recycle system in which the toner recovered from the photosensitive drum by the cleaning section is returned to the photosensitive drum by the cleaning section, and the toner is carried by the rotating photosensitive drum and collected in the developing section.
In the recycle-system electrophotographic recording apparatus, the greater part of the normally charged toner is removed by the cleaning section oppositely charged with respect to the polarity of the charge on the toner. A small amount of the toner which is opposite in polarity to the greater part of the toner remains on the photosensitive drum and passes through the cleaning section.
In the recycle-system electrophotographic recording apparatus, when the small amount of the toner that has passed through the cleaning section enters between the charging roller and the photosensitive drum, the small amount of the toner adheres to the external circumferential surface of the charging roller, thus producing a so-called filming phenomenon. When the filming phenomenon occurs, the resistance value of the charging roller increases due to the toner adhesion, making it difficult for the charging roller to give a sufficient amount of charge to the photosensitive drum to enable stable printing. To prevent the photosensitive drum from being insufficiently charged, the charging roller is rotated faster than the photosensitive drum to thereby reduce chances that the small amount of the toner enters between the charging roller and the photosensitive drum.
In the recycle-system electrophotographic recording apparatus, however, because the charging roller is rotated faster than the photosensitive drum, by triboelectricity produced between the charging roller and the photosensitive drum, the photosensitive drum is charged to a potential so high as to lead to unstable printing during operation for a long period of time. This results in poor quality printing, which has been a problem.
SUMMARY OF THE INVENTION
Therefore, the object of the present invention is to provide an electrophotographic recording apparatus that can suppress changes in the electrical potential of the photosensitive drum to thereby improve printing quality.
To solve the above problem, the present invention has an essential feature as follows.
According to the present invention, an electrophotographic recording apparatus comprises:
a photosensitive drum having a rotation shaft; and
a charging roller for charging the photosensitive drum to a desired potential, the charging roller having a rotation shaft substantially parallel with the rotation shaft of the photosensitive drum and holding an external circumferential surface thereof in contact with the photosensitive drum, the ratio of the circumferential speed of the charging roller to the circumferential speed of the photosensitive drum being greater than 1, wherein the external circumferential surface of the charging roller is defined by a surface layer including a resin and a hardener, and wherein the hardener is included in the surface layer at a proportion of not less than 10 percent and not greater than 30 percent by weight, of the surface layer.
In the electrophotographic recording apparatus according to the present invention, because the surface layer of the charging roller contains 10 percent or more by weight of the above-mentioned hardener, the frictional force produced between the surface layer of the charging roller and the photosensitive drum is reduced sharply, and triboelectricity induced between the charging roller and the photosensitive drum is inhibited.
Because the hardener included in the surface layer of the charging roller is 30 percent or less by weight, this limited content of the hardener inhibits an increase in the amount of the toner adhering to the surface la, which would attend on an increasing surface roughness of the surface layer due to a large amount of the hardener used. Accordingly, an increase in the resistance value of the charging roller due to toner adhesion can be inhibited, thus reducing the insufficiency of the static charge on the photosensitive drum, which would be caused by an increase in the resistance value of the charging roller to inhibit.
In short, in the electrophotographic recording apparatus according to the present invention, it is possible to restrain an excessive static charge on the photosensitive drum by triboelectricity generated between the charging roller and the photosensitive drum. On the other hand, it is also possible to inhibit the insufficiency of the static charge on the photosensitive drum due to an increase in the resistance value of the charging roller attending on this adhesion of the toner to the surface layer. Consequently, the potential of the photosensitive drum is held in a desired adequate range, in other words, the photosensitive drum can be held at a stable potential.
To effectively inhibit the adhesion of the toner to the surface layer, the surface roughness of the surface layer is preferably more than 0 &mgr;m and not greater than 15 &mgr;m. Under this condition, the resistance value of the charging roller can be inhibited from increasing as the result of printing for a long period of time, thus reducing the insufficiency of charge on the photosensitive drum.
The charging roller may be fitted with a generally cylindrical member disposed coaxially with the rotation shaft of the charging roller with its outer surface connected with the surface layer and its inner surface connected with the rotation shaft.
With a charging roller having a higher weight percentage of the hardener included in its surface layer, the fusion bonding force between the surface layer and the photosensitive drum is lower, and there are less chances of the surface layer peeling away from the cylindrical member.
If the surface layer peels off the cylindrical member, a large amount of the toner adheres to the outer surface of the cylindrical member devoid of the surface layer, so that the resistance value of the charging roller increases so much.
According to the present invention, because the surface layer is less liable to separate from the cylindrical member, chances are more effectively reduced that the photosensitive drum is charged insufficiently due to an increase in the resistance value of the charging roller.
To further stabilize the potential of the photosensitive drum, the ratio of the circumferential speed of the charging roller to the circumferential speed of the photosensitive drum is preferably 1.4 or less.
The charging roller is arranged slidably in the direction of the rotation shaft and on a flat plate. The inclination angle at which the charging roller starts to slide when the flat plate is gradually inclined to the horizontal plan
Akin Gump Strauss Hauer & Feld L.L.P.
Braun Fred L
Oki Data Corporation
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