Image forming apparatus with electrically grounded roller

Electrophotography – Image formation – Transfer

Reexamination Certificate

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Details

C347S139000, C399S044000, C399S045000, C399S315000

Reexamination Certificate

active

06226486

ABSTRACT:

FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an image forming apparatus wherein an image is transferred onto a recording material carried on a recording material carrying member.
Heretofore, various image forming apparatuses having a plurality of image forming stations have been proposed in which different color toner images are formed by the image forming stations, and the images are transferred superimposedly onto the same recording material (reporting paper), thus forming a color image.
In one of such apparatuses, a color copying machine of a multi-color electrophotographic type using an endless recording material carrying member is known as a high-speed image forming apparatus.
Referring first to
FIG. 2
, an example of color electrophotographic image forming apparatus will be described. In the apparatus, there are provided first, second, third and fourth image forming stations Pa, Pb, Pc and Pd, by which different color toner images are formed through latent image formation, development and image transfer processes.
Each of said image forming stations is provided with an image bearing member
3
a
,
3
b
,
3
c
or
3
d
, and each color image is formed on the image bearing member which is in the form of an electrophotographic photosensitive drum.
Adjacent to each of the photosensitive drum, a recording material carrying member in the form of a transfer belt of dielectric material
130
is disposed, and the toner image formed on the photosensitive drum is transferred onto the recording material p carried on the transfer belt
130
. The recording material p now having the transferred image is subjected to operation of a separation charger
32
(corona charged) by which the attraction force to the transfer belt
130
is reduced, and it is separated from the transfer belt
130
. Thereafter, the recording material p is fed to a fixing station
9
, where the toner image is fixed by heat and pressure on the recording material, which is then discharged onto an outside tray
63
as a copy or print.
However, this structure involves a problem, when, for example, a recording material having a low resistance resulting by keeping the recording material in a high humidity condition. More particularly, when the recording material may function as a short-circuit between the transfer position and the grounded electroconductive driving roller (separating means)
13
functioning as an opposing electrode, the toner image is not transferred in good order onto the recording material, or the toner image once transferred onto the recording material is transferred back onto the drum at a downstream portion of image forming station (retransfer), with the result of remarkably improper image transfer or formation. Generally, the volume resistivity of the recording material ranges between approximately 10
7
to 10
11
Ohm·cm depending on the material of the recording material or the water content thereof.
This phenomenon will be described in more detail, referring to FIG.
5
. During image formation, the recording material short-circuits between the driving roller
13
and the photosensitive drum in the fourth image forming station. Then, back side positive charge of a part of charge couple at a portion of the transfer belt
130
which are in contact both with the recording material and the driving roller
13
(the hatched portion E in
FIG. 5
) flows to the driving roller
13
, and surface negative charge thereof flows into the transfer charging portion through the recording material. As a result, electric discharge occurs. At this time, the transfer current which is to flow from the transfer charger
24
d
to the photosensitive drum
3
d
(opposite electrode), indicated by the arrows A and B
FIG. 5
, flows to the transfer belt E, as indicated by arrows A and D, with the result that transfer current is not enough.
When the recording material short-circuits between the photosensitive drum
3
d
of the fourth image forming station and the driving roller
13
, the portion of the transfer belt
130
tends to supply the positive charge from the photosensitive drum
3
d
as well as taking all of the transfer current, the resistance of recording material further reduces. As a result, the current flows in the directions A and D and C and D.
Moreover, when the separation discharger
32
is operated in order to separate the recording material and the transfer belt
130
simultaneously with image transfer, the negative charge is supplied from the separation discharged
32
to the photosensitive drum
3
d
through the recording material with the result of improper image transfer.
The direction C is opposite from the direction B which is the direction of proper image formation. By the opposite current, the toner is not properly transferred, or the toner retransfers to the photosensitive drum
3
d.
SUMMARY OF THE INVENTION
Accordingly, it is a principal object of the present invention to provide an image forming apparatus capable of preventing improper image transfer from an image bearing member to the recording material carried on a recording material carrying belt.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.


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patent: 5794110 (1998-08-01), Kasai et al.
patent: 0 713 158 (1996-05-01), None
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patent: 9-106191 (1997-04-01), None

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