Electrophotography – Image formation – Charging
Reexamination Certificate
2000-08-04
2001-06-26
Chen, Sophia S. (Department: 2852)
Electrophotography
Image formation
Charging
C361S225000
Reexamination Certificate
active
06253048
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a contact charging device used as a charger in electrophotographic image forming devices, and in particular to charging stability thereof.
BACKGROUND OF THE INVENTION
A charger is used for charging the surface of a photoconductor to a predetermined potential as an initial image forming process in electrophotographic image forming devices such as photocopiers, facsimiles and laser printers. Such a charger used in general is a contact charging device having a charging roller (e.g. a rubber roller) which contacts the surface of the photoconductor while applying a voltage to the photoconductor through the charging roller.
FIG. 8
shows a state of a process for charging the photoconductor by a roller-type contact charging device. As shown in
FIG. 8
, the contact charging device is made up of a charging roller
10
and a direct-current (dc) low voltage power source
2
, the charging roller
10
having a core
10
a
of a cylindrical shape as a center and being covered with an elastic member (charging member)
10
b
which is made of conductive rubber, etc. of a hollow cylinder, and the charging roller
10
coming into contact with a photoreceptor drum
3
at a nip portion (contact portion). The photoreceptor drum
3
is made up of a photoconductor
3
b
formed over a drum body
3
a
which is made of metal of a hollow cylinder.
The dc low voltage power source
2
applies a dc voltage E between the core
10
a
of the charging roller
10
and the drum body
3
a
of the photoreceptor drum
3
which is grounded. Accordingly, an inner peripheral surface (hereinafter referred to as inner surface) of the elastic member
10
b
is set to have a negative potential, and an inner surface of the photoconductor
3
b
a ground potential. When the photoreceptor drum
3
is driven to rotate in a direction of arrow A, the charging roller
10
rotates about the central axis of the core, in a direction of arrow B, following the rotation of the photoreceptor drum
3
. Therefore, the surface of the photoconductor
3
b
which is brought into contact with the surface of the elastic member
10
b
at the entrance of the nip portion is charged while passing the nip portion, thus inducing a potential change.
Referring to
FIG. 8
, a power source is the dc low voltage power source
2
, and the surface of the photoconductor
3
b
is negatively charged by having the drum body
3
a
grounded while making the core
10
a
of the charging roller
10
to have a negative potential. Alternatively, the dc voltage is applied so that a core side of the charging roller is set to have a higher potential with respect to the drum body so as to make a charge potential of the photoconductor a positive polarity. Further alternatively, the power source is an alternating-current (ac) superimposed power source in which an ac component is superimposed on a dc component, and the ac superimposed power source applies a voltage which varies as a function of time.
As shown in
FIG. 8
, the elastic member
10
b
can be regarded as a set of micro-regions whose resistance and electrostatic capacity are equivalent to one another and which are generated by being divided by infinite numbers of division lines in a radial direction linking the inner surface (surface on the side of a rotational center) and the outer surface. Each micro-region is equivalently represented by a parallel circuit made up of resistance R per unit area and electrostatic capacity C per unit area, the resistance R being obtained by multiplying a resistance value measured between predetermined regions of the inner surface and the outer surface of the elastic member
10
b
, respectively, by an area measured on the side of the outer surface, the electrostatic capacity C being obtained by dividing the electrostatic capacity measured between the inner and outer surfaces, by an area of the outer surface. In addition, the photoconductor
3
b
can be regarded as a set of infinite numbers of micro-regions in which a spacing between the inner surface (surface on the side of the rotational center) and the outer surface is equivalently represented by electrostatic capacity C
0
per unit area.
FIG. 9
shows an equivalent circuit when the photoconductor
3
b
is charged by the contact at the nip portion between a surface of the micro-region of the elastic member
10
b
and a surface of the micro-region of the photoconductor
3
b
. In the foregoing arrangement of the contact charging device, a power voltage e(t) shown in
FIG. 9
is equal to a dc voltage E. In this case, it is assumed that the charging roller
10
and the photoreceptor drum
3
are rotating at the same circumferential speed without slipping with each other at the nip portion.
The equivalent circuit shown in
FIG. 9
is formed when the micro-region of the elastic member
10
b
and micro-region of the photoconductor
3
b
shown in
FIG. 8
come into contact with each other upon reaching the entrance of the nip portion, and the dc voltage E is fed to the equivalent circuit, which starts charging the photoconductor
3
b
, i.e. charging the electrostatic capacity C
0
. After that, as the micro-regions move toward an exit of the nip portion, a charge current flows into the electrostatic capacity C
0
in accordance with a time constant C
0
·R (C is small and negligible) which is determined by the resistance R of the elastic member
10
b
and the electrostatic capacity C
0
of the photoconductor
3
b
. This results in increase in a terminal voltage e
c
(t) of the electrostatic capacity C
0
. The charge current from the elastic member
10
b
toward the photoconductor
3
b
is equivalent of injecting negative charge, and is maximum at the entrance of the nip portion, then, decreases toward the exit. Consequently, a potential distribution at the nip portion (surface of the photoconductor
3
b
) takes the form substantially as shown in FIG.
8
. Here, V
0
is an initial potential on the surface of the photoconductor
3
b.
The elastic member
10
b
is required to be of a characteristic which would cause the photoconductor
3
b
to be uniformly charged at the end. It is known that uniformity of charge over the photoconductor
3
b
can be improved by making the time constant sufficiently small by reducing the resistance R of the elastic member
10
b
with respect to a given photoconductor
3
b.
However, adopting only the foregoing method that attempts to improve the charge uniformity by reducing the time constant at the time of charging the photoconductor
3
b
raises a problem. Namely, sufficient charge uniformity cannot be obtained due to restrictions on a setting range of the time constant, which are imposed by adapting to pinhole leakage of the photoconductor
3
b
or by a nip width which can be set.
Japanese Examined Patent Publication No. 92617/1995 (Tokukohei 7-92617 published on Oct. 9, 1995; corresponding to U.S. Pat. No. 5,126,913) discloses another method of obtaining a resistance value of a charging roller for performing uniform charging by means of a charge model of a photoconductor which employed resistance and electrostatic capacity of the photoconductor and the charging roller. In this charge model, however, the electrostatic capacity of the charging roller is used as a constant value. As discussed, since the elastic member
10
b
of the charging roller
10
and the photoconductor
3
b
rotate while keeping contact with each other, their contact face (nip portion) is renewed constantly. Therefore, the surface of each micro-region of the elastic member
10
b
supplies the micro-region of the photoconductor
3
b
with charge whenever it contacts the surface of the micro-region of the photoconductor
3
b
, which results in a potential change substantially as shown in FIG.
8
. In addition, a current does not flow anywhere except at the nip portion during rotation, and therefore, it can be said that the surface of the micro-region of the elastic member
10
b
and the core thereof are at the equivalent potential except at the nip portion.
In this way, a charging operat
Ishii Hiroshi
Kido Eiichi
Chen Sophia S.
Sharp Kabushiki Kaisha
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