Image forming apparatus having variable/controlled timing...

Details Image forming apparatus having variable/controlled timing... Image forming apparatus having variable/controlled timing...

2002-08-06

2004-10-05

Braun, Fred (Department: 2852)

Electrophotography

Image formation

Charging

C399S050000, C399S174000

Reexamination Certificate

active

06801738

ABSTRACT:

FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an image forming apparatus such as an electrophotographic copying machine, a laser beam printer, or the like, the charging means of which for charging an image bearing means such as an electrophotographic photoconductive member, an electrostatically recordable dielectric member, or the like, is such a charging means that employs electrically conductive particles, and in which the electrically conductive particles are supplied from the developing means to the nip portion between the charging member and the image bearing member, by way of the image bearing member.
Conventionally, in an image forming apparatus, for example, an electrophotographic image forming apparatus or an electrostatic recording apparatus, an electrophotographic latent image is formed on an image bearing member such as an electrophotographic photoconductive member, an electrostatically recordable dielectric member, and the like. In order to form the electrophotographic image on the image bearing member, the image bearing member must be uniformly charged. As for a charging apparatus for uniformly charging the image bearing member, a corona type charging apparatus (which is not placed in contact with image bearing member) has been widely used. However, a corona type charging apparatus suffers from a few problems. For example, it generates a large amount of ozone, and in order to charge the image bearing member, it is necessary to apply high voltage, for example, 10 kV, between the charging apparatus and image bearing member, which adds to apparatus cost.
In recent years, the so-called contact charging apparatuses have been devised, and some of them have been put to practical use. In the case of this type of charging apparatus, the charging member of the charging apparatus is placed directly in contact with the image bearing member, and the image bearing member is uniformly charged by applying voltage to the charging member. In principle, however, this type of charging apparatus is the same as a corona type charging apparatus in that it also charges an object based on electrical discharge. Therefore, it also generates ozone, although by a smaller amount. Ozone forms nitric oxides (NOx), which are low in electrical resistance. Therefore, as nitric oxides adheres to the peripheral surface of the image bearing member, the image bearing member fails to be properly charged, resulting in the formation of defective images.
Thus, a charging process which does not suffer from the above described problem, that is, the ozone production, and is lower in the potential level of the voltage to be applied to a charging apparatus, has been proposed in Japanese Laid-open patent Application Hei 6-3921, or the like.
This charging process is characterized in that electrical charge is injected into the image bearing member through the direct exchange of electrical charge between the charging member, and the image bearing member surface placed in contact with the charging member, instead of electrical discharge.
Next, a charging apparatus for carrying out the above described charging process, or charge injection, will be described with reference to a sponge roller type charging apparatus (U.S. Pat. No. 6,128,456 or the like).
Referring to
FIG. 5
, the contact charging member of this type of charging apparatus is made up of a sponge roller
2
-A, which is rotated in the direction b in contact with the image bearing member
1
, and electrically conductive microscopic particles m (relatively low in electrical resistance) adhered to the peripheral surface of the porous portions, that is, the outer layer, of the sponge roller
2
-A. Electrical charge is injected into the image bearing member
1
from the sponge roller
2
-A at the contact area n, as the sponge roller
2
-A is rotated in the direction counter to the rotational direction a of the image bearing member
1
. As a result, the image bearing member
1
is charged to a potential level virtually identical to that of the electrical charge of the sponge roller
2
-A.
The electrically conductive microscopic particles m are particles for enhancing the charging performance of the charging apparatus. As for the material for the electrically conductive microscopic particles m, various substances can be used; for example, microscopic particles of electrically conductive metallic oxide such as zinc oxide, microscopic particles of electrically conductive particles of inorganic substance other than metallic oxides, mixture of microscopic particles of electrically conductive inorganic and organic substances, and the like.
In this system, a DC voltage of −600 V is applied to the sponge roller
2
-A from a power source S
1
. This voltage acts to raise the potential level of the portion of the image bearing member
1
in contact with the sponge roller
2
-A and electrically conductive microscopic particles m to the same potential level as that of this voltage, that is, −600 V. If electrical charge from the sponge roller
2
-A side can break through the barrier, or surface energy, of the peripheral surface of the image bearing member
1
, it is injected into the image bearing member
1
, charging the image bearing member
1
. If electrical charge fails to break through this energetic barrier, the image bearing member
1
is not charged. Further, if electrical charge having been injected into the image bearing member
1
moves back from the image bearing member
1
to the sponge roller
2
-A when the sponge roller
2
-A is separated from the image bearing member
1
, the image bearing member
1
does not remain charged. These phenomena are greatly affected by the energetic barrier of the peripheral surface of the image bearing member
1
, and the charge retaining ability of the image bearing member
1
. On the other hand, if a charging process is viewed as a process comprising a plurality of competing subordinate processes, the frequency at which the sponge roller
2
-A makes contact with the image bearing member
1
is very important.
As for the means for increasing this frequency, it is effective to improve the state of contact between the sponge roller
2
-A and image bearing member
1
. The state of contact between the sponge roller
2
-A and image bearing member
1
can be improved by adhering the electrically conductive microscopic particles m to the porous portion, or the surface layer, of the sponge roller
2
-A, and/or by increasing the relative speed between the peripheral surfaces of the sponge roller
2
-A and image bearing member
1
by making the moving direction of the peripheral surface of the sponge roller
2
-A opposite to that of the peripheral surface of the image bearing member
1
. With the provision of the above described arrangements, the peripheral surface of the image bearing member
1
is charged to a potential level virtually the same as that of the voltage applied to the sponge roller
2
-A, that is, −600 V, uniformly, even in microscopic terms.
FIG. 6
is a schematic drawing of an example of an electrophotographic image forming apparatus which employs, as a means for charging the image bearing member
1
, an injection type charging apparatus
2
which uses the above described electrically conductive microscopic particles m. This apparatus does not have a dedicated cleaning system, and employs a transfer type image formation system.
Designated by a referential code I is a rotational electrophotographic photoconductive member, in the form of a drum, which is rotationally driven at a predetermined peripheral velocity in the clockwise direction indicated by an arrow mark a. Designated by a referential code
2
-A
2
is a sponge roller as a charging member, which is kept in contact with the image bearing member
1
, with the application of a predetermined amount of pressure, forming a contact area n with a predetermined width in terms of the circumferential direction of the sponge roller
2
. A referential code
2
-B stands for a coating device for coating the peripheral surface of th

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