Electrophotography – Image formation – Charging
Reexamination Certificate
2000-06-27
2001-08-07
Chen, Sophia S. (Department: 2852)
Electrophotography
Image formation
Charging
C399S100000
Reexamination Certificate
active
06272303
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-181921, filed Jun. 28, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to an electrophotographic apparatus represented by, for example, a laser printer and, in particular, to a charging device for electrophotographic which electrically charges a recording sheet.
In the electrophotographic apparatus, a reversal development method has been widely adopted as well known in the art. This reversal development method has steps of electrically charging a surface of a photosensitive drum, selectively canceling the charge on the surface of the photosensitive drum by a laser beam to form an electrostatic latent image (negative latent image), and supplying toner particles to the surface of the photosensitive drum to develop the latent image into a toner image. The toner particles are charged with the same polarity as that of the photosensitive drum on a development roller and supplied to the surface of the photosensitive drum. The toner image is formed of toner particles adhered to no charge or less charge area of the charged surface. The development roller is directly contacted with the photosensitive drum to increase the strength of an electric field between the development roller and the no charge or less charge area of the charged surface of the photosensitive drum.
In addition, to electrically charge the surface of the photosensitive drum, a method of bringing a charging brush into contact with the surface of the photosensitive drum is widely used.
A typical electrophotographic apparatus comprises, as shown for example in
FIG. 1
, a photosensitive drum
31
, charging brush
34
, cleaning blade
35
, and developing section
38
. The developing section
38
includes a supply roller
42
and a developing roller
43
. The charging brush
34
is arranged parallel to the shaft of the photosensitive drum
31
and set to be in contact with the surface of the photosensitive drum
31
. The charging brush
34
is formed in a roller-shape by attaching many conductive fibers
52
to one surface of a belt-like cloth
53
and helically winding the cloth
53
around a rotary shaft
51
of a metal such as iron with a double-sided adhesive tape interposed. The fibers
52
are attached as piles to the cloth
53
. In the case where the surface of the photosensitive drum
31
is electrically charged, the charging brush
34
is rotated together with the photosensitive drum
31
, for example, in a direction opposite to the rotational direction of the photosensitive drum
31
at the same circumferential speed as that of the photosensitive drum
31
.
However, the contact charging system using the charging brush
34
poses a problem as will be set out below. That is, since the charging brush
34
is so formed that the belt-like cloth with many fibers
52
attached thereto as shown in
FIG. 2
is helically wound around the rotation shaft
51
, a gap is created between each turn of the cloth
53
. Further, even if the cloth
53
is so wound as to leave no gap between each turn, a helical gap (helical turn) is formed along a boundary of each turn because the fibers
52
are not attached to a full length from end to end in a width direction. When, as shown in
FIG. 1
, charging is made using the charging brush
34
of the above-mentioned structure, the fibers
52
is set in contact with part of the surface of the photosensitive drum
31
and not with the remaining part. Since charging is made in this manner, the amount of charge in the surface of the photosensitive drum
31
is not constant.
In the case where, therefore, a half-tone image is recorded on a recording sheet, there is a tendency that the density becomes higher at an area corresponding to the part not contacted with the fibers of the charging brush
34
and lower at the other area. This uneven density imparts a greater influence to an image quality and causes the deterioration of the image. Further, less area of the fibers
52
of the charging brush
34
is contacted with the surface of the photosensitive drum
31
, thus leaving no contact between the surface of the photosensitive drum
31
and the fibers
52
. This makes it difficult to uniformly charge the surface of the photosensitive drum
31
, causing the surface potential to be irregular.
With the charging section using the conventional charging brush
34
, it is not possible to constantly obtain a high quality image, thus presenting a problem to be solved.
BRIEF SUMMARY OF THE INVENTION
It is accordingly the object of the present invention to provide a charging device for electrophotography which can obtain an excellent image of a uniform density.
According to the present invention, there is provided a charging device for electrophotography, comprising a rotary charging member, having many fibers extending in radial around a rotation shaft and contacted with a surface of a photosensitive image carrier, for rotating to electrically charge the surface of the photosensitive image carrier with the fibers; and a charging control member located on a downstream side of a contact between the rotary charging member and the photosensitive image carrier in a rotation direction of the rotary charging member for temporarily retarding a movement of the fibers of the rotary charging member to increase an effective density of the fibers to be contacted with the surface of the photosensitive image carrier; wherein an end of the charging control member is at a distance of at least 1 mm from the contact between the rotary charging member and the photosensitive image carrier.
In the charging device for electrophotography, the rotation control member retards the movement of the fibers of the rotary charging member to increase the effective density of the fibers to be contacted with the photosensitive image carrier. Further, since the end of the charging control member is at a distance of at least 1 mm from the contact between the rotary charging member and the photosensitive image carrier, it is possible to reliably retard a movement of the fibers of the rotary charging member. For this reason, the limit of the charging characteristic of the rotary charging member is effectively overcome and it is possible to suppress the irregularity in charging to obtain an excellent image of a uniform density.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
REFERENCES:
patent: 5455661 (1995-10-01), Yoshida et al.
patent: 5606401 (1997-02-01), Yano
patent: 5633701 (1997-05-01), Yoshida
patent: 7-128951 (1995-05-01), None
Chen Sophia S.
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Toshiba Tec Kabushiki Kaisha
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