Electrophotography – Control of electrophotography process – Control of transfer
Reexamination Certificate
1999-07-09
2001-01-09
Royer, William J. (Department: 2852)
Electrophotography
Control of electrophotography process
Control of transfer
C399S101000, C399S313000
Reexamination Certificate
active
06173135
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic or electrostatic type such as a copying machine, a printer and the like, and more particularly, it relates to an image forming apparatus having a transfer portion urged against an image bearing member.
2. Related Background Art
FIG. 12
shows an example of an electrophotographic image forming apparatus.
In this example, the image forming apparatus utilizes a transfer roller
11
urged against a photosensitive drum
1
as a transfer member for transferring a toner image formed on the photosensitive drum
1
onto a transfer material P.
If toner is adhered to a surface of the transfer roller
11
, smears on a back surface of the sheet will be generated, it is preferable that, when the sheet is not passed through the transfer roller, a cleaning bias having a polarity opposite to that of the toner adhered to the transfer roller is applied to transfer the toner onto the photosensitive drum
1
, and the transferred toner is collected by a cleaning device
5
.
FIG. 10
is a sequence chart showing a relationship between surface potential of the photosensitive drum
1
(photosensitive drum potential) in the above-mentioned image forming apparatus, developing bias, transfer bias and cleaning bias. In this case, a relationship between the surface potential of the photosensitive drum
1
, the developing bias of a developing device
3
and the transfer bias of a transfer device
4
is shown by using a circumferential position of the photosensitive drum
1
as a reference.
The photosensitive drum
1
is uniformly charged with dark potential V
D
of −650V by a charge roller
6
, and the uniformly charged portion is exposed in accordance with image information to form an electrostatic latent image.
On the photosensitive drum
1
, the dark potential V
D
of the surface is maintained to −650V by the charge roller
6
from when pre-rotation in which the charged position reaches a transfer nip portion N is effected in synchronous with a timing for reaching the transfer material P to the transfer nip portion N to when the charged position reaches the transfer nip portion N in synchronous with a timing for passing a trailing end of a final transfer sheet P through the transfer nip portion N. In this case, bright potential V
L
of an image portion of the electrostatic latent image formed by exposure light L becomes −200V.
A DC voltage developing bias of −500V is applied from a developing bias applying power source
10
to a developing roller
9
from when pre-rotation in which the electrostatic latent image on the photosensitive drum
1
is developed and is reached to the transfer nip portion N is effected in synchronous with a timing for reaching the transfer material P to the transfer nip portion N to when a toner image reaches the transfer nip portion N in synchronous with a timing for passing the final transfer sheet P through the transfer nip portion N.
The cleaning bias of −1500V is applied from a transfer roller cleaning bias applying power source
23
to the transfer roller
11
when the surface potential of the photosensitive drum
1
is zero in the pre-rotation and post-rotation. When the sheet is passed, transfer bias V
T
is applied to the transfer roller, and, between the sheets (i.e., in a sheet interval), bias voltage V
TO
smaller than the transfer bias is applied to the transfer roller. In this case, the transfer bias V
T
is a voltage for reserving transfer current required during the passing of the sheet and is varied in accordance with the resistance of the transfer roller
11
. In order to cope with a transfer material P having A
3
size or Letter-size, a voltage capable of reserving current of about 10 &mgr;A is applied to the transfer roller. In the sheet interval, although a voltage for reserving the same transfer current as that in the passing of the sheet is applied, since there is no sheet, the bias voltage V
TO
is made smaller in order to prevent transfer memory to the photosensitive drum
1
.
As methods for cleaning the toner on the transfer roller
11
, as well as a method for applying constant voltage as the cleaning bias as shown in
FIG. 10
, there is a method for applying a negative voltage and positive voltage alternately as shown in FIG.
11
. An example of the latter is shown in FIG.
11
.
The toner adhered to the transfer roller
11
as the smears includes not only the properly charged toner but also toner charged oppositely within the developing device
3
and/or toner which was properly charged and which was then charged with opposite polarity by discharge action generated during the application of the cleaning bias.
If negative toner and positive toner are adhered to the transfer roller
11
in this way, as shown in
FIG. 11
, in pre-rotation and post-rotation, negative bias and positive bias might be applied to the transfer roller
11
as the cleaning bias.
When the cleaning bias is applied to the transfer roller
11
, memory may be generated on the photosensitive drum
1
due to the cleaning bias to affect a bad influence upon next image formation.
The influence of the memory can be prevented by removing AC electricity from the photosensitive drum
1
(charged by the cleaning bias) by means of a charger
2
.
However, since a voltage applied as AC bias for primary charging has a high peak-to-peak voltage such as 1500 to 2000 V
PP
, a discharge phenomenon occurs in the vicinity of a charging nip portion between the charge roller
6
and the photosensitive drum at upstream and downstream sides thereof.
When a photosensitive layer on the surface of the photosensitive drum
1
is formed from OPC (organic photosensitive body), for example, due to a discharge phenomenon at the surface of the layer, molecular chains are broken, thereby tending the OPC layer to be fragile, with the result that the OPC layer will be apt to be worn by the sliding contact with the cleaning blade
13
.
For these reasons, when an AC bias is applied from the charger
2
in order to effect the cleaning of the transfer roller, a service life of the OPC photosensitive layer and therefore a service life of the photosensitive drum
1
will be shortened.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an image forming apparatus which can prevent smears on a transfer roller.
Another object of the present invention is to provide an image forming apparatus which can prevent potential memory on a photosensitive member.
A further object of the present invention is to provide an image forming apparatus in which a service life of the photosensitive member is not shortened.
A still further object of the present invention is to provide an image forming apparatus comprising an image bearing member for bearing a toner image, a transfer member for electrostatically transferring the toner image on the image bearing member, a cleaning bias applying means for electrostatically cleaning the transfer member by applying cleaning bias to the transfer member, and a control means for controlling application of the cleaning bias in accordance with the number of sheets to be image-form ed in one image forming job.
The other objects and features of the present invention will be apparent from the following detailed explanation of the invention.
REFERENCES:
patent: 5132738 (1992-07-01), Nakamura et al.
patent: 5337127 (1994-08-01), Imaue
patent: 5621509 (1997-04-01), Karashima et al.
patent: 5689771 (1997-11-01), Sato et al.
patent: 5692232 (1997-11-01), Okano et al.
patent: 5717981 (1998-02-01), Yamanaka
patent: 5822649 (1998-10-01), Karashima et al.
patent: 5832335 (1998-11-01), Fukasaw
patent: 5857132 (1999-01-01), Mizuishi et al.
patent: 5873019 (1999-02-01), Mizuishi
patent: 5970297 (1999-10-01), Gross
Shimura Masaru
Yuminamochi Takayasu
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Royer William J.
LandOfFree
Image forming apparatus with a bias control feature does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Image forming apparatus with a bias control feature, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Image forming apparatus with a bias control feature will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2467275