Image forming apparatus

Details Image forming apparatus Image forming apparatus

2001-07-02

2002-08-13

Grimley, Arthur T. (Department: 2852)

Electrophotography

Internal machine environment

Particle or contaminant control

C399S050000, C399S174000, C399S176000, C399S175000

Reexamination Certificate

active

06434349

ABSTRACT:

FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an image forming apparatus such as an electrophotographic apparatus, an electrostatic recording apparatus or the like which produces images through an image forming process including a step of electrically charging an image bearing member such as a photosensitive member, a dielectric member or the like.
The image forming apparatus such as an electrophotographic apparatus requires an electric charging step of charging the image bearing member uniformly to a predetermined potential in order to form an electrostatic latent image on the image bearing member. For this purpose, a non-contact type corona charger or the like has been used as a means for the charging. However, the corona charger produces ozone and requires such a high voltage as approx. 10 KV has to be applied between the charging device and the image bearing member.
Recently, a charging means has been proposed to avoid these problems. In such a means, a charge member is directly contacted to the image bearing member and is supplied with a voltage by which the image bearing member is charged uniformly (so-called contact charging device).
A) Charging Roller
A typical contact charging device is a charging roller
2
-X as shown in FIG.
6
.
In the charging roller, the charge member
2
-X-
a
is in the form of a roller (charging roller) having an electroconductive base roller and a surface layer of intermediate resistance layer. The charging roller
2
-X-
a
is contacted to the image bearing member
1
at a predetermined pressure and is rotatably supported on bearings. It is rotated in the direction indicated by arrow b by rotation of the image bearing member
1
which is rotated in the direction indicated by an arrow a. Between the charging roller
2
-X-
a
and the image bearing member
1
, a predetermined charging bias voltage is applied from a voltage source S
1
so that said image bearing member
1
is uniformly charged to a predetermined potential.
Here, the voltage applied to the roller may be (1) a DC voltage only or (2) a DC voltage biased with an AC voltage.
In the case of (1), in order to charge the image bearing member
1
to a potential of −600 V, the applied voltage is approx. −1300 V, and in the case of (2), the applied DC voltage is −600 V and the AC voltage is not less than 1500 Vpp.
The charging mechanism in these cases is based on the Paschen's law, and an electric discharge phenomenon arises in a region satisfying the Paschen's law in which the distance between the charging roller
2
-X-
a
and the image bearing member
1
is within a predetermined range (region H in FIG.
6
).
However, as will be understood from the charging mechanism, the contact charging device of this type creates the discharge which is the same as with the corona charger within a fine space region H, and therefore, the ozone is produced although the amount of ozone production is remarkably smaller than with the corona charger. The ozone produces oxide nitrogen, and if it is deposited on the image bearing member
1
, an image defect is produced due to the low resistance of the deposited matter.
B) Injection Charging Device
This injection charging process system is proposed in Japanese Laid-open Patent Application Hei 6-3921 which is free of such a problem of ozone generation, and therefore, the voltage applied to the charging device can be further reduced.
The feature of the charging process is that surface potential of the charged image bearing member is substantially the same as the voltage applied to the charging device. This system does not use the electric discharge phenomenon, and charge injection occurs into the image bearing member by the transfer of electric charges between the surface of the image bearing member and the charge member contacted thereto.
(1) Magnetic Brush Charging Apparatus
In order to embody the injection charging process, some types of injection charging devices have been proposed.
A typical example is a magnetic brush type charging device
2
-Y as shown in FIG.
7
. The charging device comprises a magnet
2
-Y-
a
, a non-magnetic charging sleeve
2
-Y-
b
containing the magnet
2
-Y-
a
therein, a magnet carrier (magnetic carrier, magnetic powder member)
2
-Y-
c
, an electroconductive regulating blade
2
-Y-
d
and a housing
2
-Y-
e
and so on.
The magnet carrier
2
-Y-
c
is made of magnetic material (particles) which is electroconductive.
The charging sleeve
2
-Y-
b
is disposed in the housing
2
-Y-
e
and is rotatable, and a part of the peripheral surface thereof is exposed to the outside through an opening of the housing. In the charging device
2
-Y, the exposed portions of the charging sleeve
2
-Y-
b
is faced to the image bearing member
1
with a predetermined small gap therebetween. The magnet
2
-Y-
a
is not rotatable The magnet carrier
2
-Y-
c
is retained in the housing
2
-Y-
e
. A regulating blade
2
-Y-
d
is provided in the opening of the housing
2
-Y-
e
and provides a predetermined gap between the regulating blade
2
-Y-
d
and the charging sleeve
2
-Y-
b.
The magnet carrier
2
-Y-
c
in the housing
2
-Y-
e
is magnetically attracted and retained in the form of a magnetic brush on the peripheral surface of the charging sleeve
2
-Y-
b
by the magnetic field generated by the magnet
2
-Y-
a
, and is fed by the rotation of the charging sleeve
2
-Y-
b
. The layer thickness thereof is regulated to a predetermined thickness by the regulating blade
2
-Y-
d
, and the layer is carried to the outside of the opening of the housing
2
-Y-
e
to be brought into contact to the surface of the image bearing member
1
. It rubs the surface of the image bearing member and returns into the housing
2
-Y-
e
with the continuing rotation of the charging sleeve
2
-Y-
b.
The image bearing member
1
is rotated in the direction indicated by an arrow a, and the charging sleeve
2
-Y-
b
is rotated in the direction indicated by an arrow S which is opposite from the rotational direction of the image bearing member
1
at the contact portion (charge portion) between the image bearing member
1
and the magnetic brush of the magnet carrier
2
-Y-
c
. Thus, there is provided a peripheral speed difference between the magnetic brush of the magnet carrier
2
-Y-
c
and the image bearing member
1
so that magnetic brush rubs in the surface of image bearing member
1
with the rotation of the charging sleeve
2
-Y-
b.
In the magnetic brush charging apparatus
2
-Y of this example, the regulating blade
2
-Y-
d
is supplied with a DC voltage of −600 V for example as a charging bias voltage from the voltage source S
1
. Therefore, the portion of the image bearing member
1
to which the magnetic brush of the magnet carrier
2
-Y-
c
is contacted tends to acquire the same potential. This time, if the charge is injected from the magnet carrier
2
-Y-
c
into the image bearing member
1
beyond an energy barrier at the surface of the image bearing member
1
, then the image bearing member
1
is electrically charged. If it cannot be injected beyond the energy barrier or if the charge returns to the magnet carrier
2
-Y-
c
when the magnet carrier
2
-Y-
c
is brought out of contact from the image bearing member
1
, then the image bearing member
1
is not charged. In the phenomenon, the energy barrier at the surface of the image bearing member
1
and a retention performance of the charge are important, and when the phenomenon is taking as a competitive reaction, the frequency of chances of contact between the magnet carrier
2
-Y-
c
and the image bearing member
1
is important.
In order to raise the contact frequency, and the particle size of the magnet carrier
2
-Y-
c
is reduced; the magnetic force provided by the magnet
2
-Y-
a
is made stronger to increase the density of the magnetic brush of the magnet carrier
2
-Y-
c
; and/or the peripheral moving direction of the charging sleeve
2
-Y-
b
is made opposite from the peripheral moving direction of the image bearing member
1
at the charge por

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