Electrophotography – Image formation – Transfer
Reexamination Certificate
2000-03-29
2002-03-19
Chen, Sophia S. (Department: 2852)
Electrophotography
Image formation
Transfer
C399S308000
Reexamination Certificate
active
06360071
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying apparatus, a printer or a facsimile apparatus, and more particularly to an image forming apparatus adapted for transferring an image on an image bearing member onto an intermediate transfer member and then transferring the image on the intermediate transfer member onto a recording material.
2. Related Background Art
FIG. 7
shows, in a lateral views, an image forming apparatus for sequentially superposing toner images of plural colors on a same recording material.
In the main body of the image forming apparatus, there is provided an endless intermediate transfer belt
81
running in a direction indicated by the arrow X. Such intermediate transfer belt is formed with a film of dielectric resin such as polycarbonate, polyethylene terephthalate or polyvinylidene fluoride. A recording material
6
such as paper taken out from a feed cassette
60
is supplied, through registration rollers
13
and a conveyor belt
8
, to a secondary transfer position of the intermediate transfer belt
81
.
Above the intermediate transfer belt
81
, there are provided four image forming units Pa, Pb, Pc and Pd in tandem. These image forming units Pa, Pb, Pc, Pd have substantially same structures are different in forming magenta, cyan, yellow and black toner images respectively.
The structure of the image forming unit will be explained in the following, taking the unit Pa as an example. The image forming unit Pa is provided with a rotatably supported drum-shaped electrophotographic photosensitive member (photosensitive drum)
1
a
, around which provided are process devices such as a primary charger
2
a
, a developing device
3
a
, a cleaner
5
a
etc. Since these units are similar in structure, the photosensitive drums alone are numbered as
1
b
,
1
c
,
1
d
in other units Pb, Pc, Pd. The developing devices of the image forming units Pa, Pb, Pc and Pd respectively contain magenta toner, cyan toner, yellow toner and black toner.
The photosensitive drum
1
a
is irradiated with a laser beam bearing an image signal of the magenta component of the original for example through a polygon mirror (not shown), whereby an electrostatic latent image is formed on the photosensitive drum
1
a.
The latent image is developed with magenta toner supplied from the developing device
3
a
to obtain a visible magenta toner image. When the toner image is brought, by the rotation of the photosensitive drum l
a
, to a primary transfer position where the photosensitive drum
1
a
is in contact with the intermediate transfer belt
81
, the magenta toner image on the photosensitive drum
1
a
is transferred onto the intermediate transfer belt
81
by a primary transfer bias applied to a transfer charging member
4
a
(primary transfer).
By the time a portion of the intermediate transfer belt
81
bearing the magenta toner image moves to the image forming unit Pb, a cyan toner image is formed on the photosensitive drum
1
b
in the image forming unit Pb in the same manner as explained in the foregoing and such cyan toner image is transferred onto the magenta toner image borne on the intermediate transfer belt
81
.
As the intermediate transfer belt
81
moves further, a yellow toner image and a black toner image are similarly transferred in the respective primary transfer portions of the image forming units Pc, Pd onto the aforementioned magenta and cyan toner images. By this time, the recording material
6
supplied from the feed cassette
60
reaches a secondary transfer position through the registration rollers
13
, and the four-color toner images on the intermediate transfer belt
81
are collectively transferred onto the recording material
6
by a secondary transfer bias applied to a secondary transfer member
40
(secondary transfer).
After the secondary transfer process, the intermediate transfer belt
81
is subjected to the surface charge elimination by a residual charge eliminator
12
, and the secondary untransferred toner remaining on the surface is removed by a cleaning member
16
.
The recording material
6
bearing the transferred four-color toner images is separated by a separating portion
10
from the intermediate transfer belt
81
and is conveyed by a conveying belt
8
to a fixing device
11
, in which a pair of rollers apply heat and pressure to the recording material
6
, thereby fixing the toner images. The fixing device
11
is provided a mechanism for coating releasing oil (such as silicone oil) on one of the fixing rollers in order to improve the releasing property between the recording material and the roller, so that such oil is deposited onto the recording material.
In case of one-side image formation, the recording material
6
bearing the fixed toner image is discharged onto a sheet discharge tray of the apparatus after emerging from the fixing device
11
, but, in case of two-side image formation the recording material
6
is contained in a cassette
62
for two-side recording after passing through a surface reversing path
61
provided at the exit of the fixing device
11
and is then fed to the secondary transfer position thereafter for image formation by secondarily transferring the toner image on the other surface (back surface) of the recording material.
The photosensitive drum
1
a
after the primary transfer is subjected to the removal of the toner remaining on the surface thereof after the transfer by a cleaner
51
, and is thus rendered capable of the next image formation. The intermediate transfer belt
81
after the secondary transfer is subjected to the removal of surface charge and of the toner deposited on the surface charge and of the toner deposited on the surface by a residual charge eliminating charger
12
disposed downstream of the secondary transfer position and a belt cleaner
16
disposed downstream of the residual charge eliminating charger
12
.
In the above-described image forming apparatus, when a high voltage is repeatedly applied to the intermediate transfer belt in the respective primary and secondary transfer portions, the surface resistivity of the intermediate transfer belt may gradually decrease to eventually result in an image defect. This phenomenon is presumably attributable to a fact that discharge phenomena appear not negligibly by an electric field formed between the transfer rollers
4
a
,
4
b
,
4
c
,
4
d
and the photosensitive drums
1
a
-
1
d
across the intermediate transfer belt (discharge being presumably generated in gap portions formed at the upstream and downstream sides of an area where the photosensitive drum is in contact with the intermediate transfer belt in each primary transfer portion), thereby deteriorating the surface layer (at the image bearing side) of the intermediate transfer belt or causing deposition of the discharge products onto the surface of the intermediate discharge belt. Such discharge phenomena are presumed to similarly occur also in the secondary transfer portions
In fact, in a durability test by current supply to the primary and secondary transfer chargers in the above-described image forming apparatus, the surface resistivity of the intermediate transfer belt was lowered from an initial value of about 10
13
&OHgr;/□ to about 10
9
&OHgr;/□ with the progress of the durability test (measured under normal temperature and normal humidity conditions, with a voltage of 1 kV applied by an electrode according to JIS-K6911). With such loss of the surface resistivity, the volume resistivity was also lowered from about 10
12
&OHgr;cm to about 10
10
&OHgr;cm (measured under normal temperature and normal humidity conditions, with a voltage of 10 V applied by an electrode according to JIS-K6911). Furthermore, this phenomenon does not necessarily occur uniformly in the longitudinal direction (direction perpendicular to the rotating direction) of the intermediate transfer belt.
Such loss in the surface resistivity of the intermediate transfer belt may result in an image defect s
Canon Kabushiki Kaisha
Chen Sophia S.
Fitzpatrick ,Cella, Harper & Scinto
Tran Hoan
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