Electrophotography – Image formation – Charging
Reexamination Certificate
2001-02-08
2003-04-08
Brase, Sandra (Department: 2852)
Electrophotography
Image formation
Charging
C399S168000
Reexamination Certificate
active
06546219
ABSTRACT:
CROSS-REFERENCES TO RELATED APPLICATIONS
This application claims priority to Japanese patent application Nos. JPAP2000-030908 (filed Feb. 8, 2000) and JPAP2001-24007, both of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for roller charging, and more particularly to a method and apparatus for performing a charging process relative to an image carrying member in an electrophotographic image forming process.
2. Discussion of the Background
Conventionally, electrophotographic image forming apparatuses such as copying machines, printers, facsimile machines, and so on use a variety of ways for evenly applying a charge to a photoconductive member before generating an electrostatic latent image.
In one exemplary way, a corona discharge is used. A corona charge apparatus includes a charge wire made of tungsten or nickel and which is extended in a metal mesh casing. The charge wire is arranged at a position close to a photoconductive member, and a voltage of a direct current or a direct current overlaid with an alternating current is applied between the charge wire and the photoconductive member so as to produce a corona discharge therebetween. Thereby, the surface of the photoconductive member is charged.
The above corona charge apparatus, however, has a drawback in that various discharge products such as ozone, NOx, etc. are produced due to the relatively high voltage applied. This results in environmental pollution and also causes problems with the image forming process in that the discharge products often produce a coat of nitric acid or nitrate which adversely affects formation of the image.
Therefore, a contact type charge apparatus that produces less ozone and consumes less electricity has been used in place of the corona charge apparatus. Such contact type charge apparatus includes a charge member with a conductive material formed in a roller, a brush, or an elastic blade and which contacts a surface of an image carrying member such as a photoconductive member. The surface of the image carrying member is charged by an application of a voltage between the charge member and the image carrying member.
The roller charge member, for example, includes a metal core and an elastic layer (e.g., conductive rubber) covering the surface of the metal core. When such an elastic layer is left in contact under pressure with the surface of the image carrying member for a relatively long time period, an inclusion such as plastic included in the elastic layer may be exuded to the surface and will be deposited on the surface of the image carrying member. This results in a dirty mark on an image.
Further, in the contact type charge apparatus, the charge process is performed under the condition that the charge member contacts the surface of the image carrying member. Therefore, the charge member may receive the residual toner left on the surface of the image carrying member after an image transfer process. This causes a deterioration of charging performance.
As an attempt to solve the above problems, a roller of the charge roller included in the charge member is provided with spacers, tapes, or films on both ends thereof so that the both ends evenly have a slightly greater diameter. With this charge roller, the surface of the photoconductive member is held distant from the charge roller except for the ends thereof. Thereby, another non-contact type charge apparatus is made. Related techniques for this non-contact type charge apparatus are described in published Japanese unexamined patent application, Nos. 3-240076, No. 4-360167, No. 5-107871, for example.
Further, published Japanese unexamined patent application No. 7-121002, describes an image forming apparatus in which a photoconductive drum is wrapped at both ends with sheet members. A charged plate for applying a charge to the photoconductive drum contacts the sheet members fixed on the ends of the photoconductive drum, thereby charging the photoconductive drum. With such a configuration, an image forming surface of the photoconductive drum preserved between the two sheet members is held apart from the charged plate, while the surface is charged. This is referred to as a non-contact type charging.
With the above non-contact type charge apparatus, portions of the charge roller corresponding to an image forming region do not contact the surface of the photoconductive member. Therefore, it eliminates the drawbacks of the contact type charge apparatus such as the deposition of the material included in the elastic member on the photoconductive member and the transfer of the residual toner deposited on the photoconductive member to the charge member.
However, it is difficult to evenly increase diameters of both ends of the charge roller which is covered by the elastic member, by wrapping a film, for example, around the ends of the wrapping elastic member. This is because the film is needed to perfectly wrap each end surface of the elastic member without a slight gap, and therefore the variations of the film in length are needed to be avoided.
FIG. 31
shows the above-described charge roller in which each film
60
is short in a circumferential direction of the charge roller such that a space S is formed between edges of each film
60
facing each other after a turn. With the charge roller of
FIG. 31
, as illustrated in
FIG. 32
, a gap G held between the surfaces of the film
60
and a photoconductive member
4
loses a distance of &dgr; in the above space S. That is, the gap G is not held constant during a turn of the charge roller.
In the above charge roller, the gap G is typically decreased around the center of the charge roller in its axial direction. This is caused due a deformation of the elastic member around the both ends thereof and so on. Therefore, when using a relatively thin film, the charge roller which is the non-contact type charge roller has a risk at the center portion thereof to contact the surface of the photoconductive member.
Accordingly, the film must be thick enough to avoid the above problem. However, an increase in film thickness will make the gap G greater, in particular, around the positions close to the films, resulting in abnormal discharge. This causes a dirty white mark on an image. In other words, operation of the charge roller is very sensitive to the thickness of the film.
Generally, in the non-contact type charging apparatus, somewhat constant charge voltage can be obtained under the condition that the gap is varied when the charge roller is applied with only the direct current voltage and when the gap between the surfaces of the charge roller and the photoconductive member (e.g., the image carrying member) is smaller than a predetermined value (i.e., 20 &mgr;m).
When the above gap is greater than the predetermined value, however, the charge voltage can no longer be constant and is decreased in accordance with the gap. To compensate such voltage reduction, a DC (direct current) voltage overlaid by an AC (alternating current) voltage is applied to the charge roller. Thereby, a constant charge voltage is produced.
In this case, abnormal discharge may occur when the voltage applied according to the gap is too large. The voltage to be applied is needed to be controlled at a level that does not cause an abnormal discharge. As a result, the gap needs to be smaller than a certain value at which abnormal discharge does not occur. In other words, the thickness of the film is also restricted from this aspect.
On the other hand, as illustrated in
FIG. 33
, the edges of the film
60
may be overlapped when the film has a length slightly longer than a length of a circumference of the elastic member
62
. The overlapped portion has a thickness twice as great as the other portions of the film
60
. Therefore, the gap between the surfaces of the charge roller and the photoconductive member is greater where the overlapped portions contact the photoconductive member in each turn of the charge roller. Thus, the above gap is c
Fujishiro Takatsugu
Hiramatsu Masami
Hirano Hiroko
Ishibashi Hitoshi
Iwasaki Yukiko
Brase Sandra
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Ricoh & Company, Ltd.
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