Electrophotography – Control of electrophotography process – Control of transfer
Reexamination Certificate
2001-02-27
2002-05-07
Chen, Sophia S. (Department: 2852)
Electrophotography
Control of electrophotography process
Control of transfer
C399S044000, C399S302000
Reexamination Certificate
active
06385409
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an electrophotographic image forming apparatus such as a copying machine or a printer.
FIG. 12
shows a typical conventional image forming apparatus.
A photosensitive drum
101
is rotatively driven. After being uniformly charged to the negative polarity by a primary charging device
102
, the peripheral surface of the photosensitive drum
101
is exposed to a laser beam
103
. As a result, an electrostatic latent image which reflects image data is formed. The electrostatic latent image is developed in reverse into a toner image by a developing device. More specifically, developing devices
104
a
,
104
b
,
104
c
and
104
d
, which contain negatively chargeable yellow, magenta, cyan and black toners, respectively, are mounted in a rotary
104
, which is rotatable about its axis to position one of the developing devices, that is, the developing device for developing the electrostatic latent image currently present on the peripheral surface of the photosensitive drum
101
, at the latent image developing zone where the peripheral surface of the developing device squarely faces the peripheral surface of the photosensitive drum
101
. For example, in order to develop the electrostatic latent image correspondent to the yellow component of the image to be formed, the rotary
104
is rotated to position the yellow color developing device
104
at the latent image developing point so that yellow toner is adhered to the latent image, that is, to develop the latent image into a yellow toner image.
The thus formed yellow toner image is transferred (primary transfer), in a primary transfer station
106
a
, onto an intermediary transfer belt
105
by applying primary transfer bias to a primary transfer roller
109
. The toner which remains on the peripheral surface of the photosensitive drum
101
after the primary transfer process is removed by a cleaning apparatus
107
.
The aforementioned charging process, exposing process, developing process, primary transfer process, and cleaning process are carried out for the rest of the color components, that is, magenta, cyan, and black color components. As a result, four toner images of different color are overlaid on the intermediary transfer belt
105
.
Then, the four color toner images are transferred (secondary transfer) all at once in a secondary transfer station
106
b
by a secondary transfer roller
110
, onto a transfer medium P, which is conveyed from a sheet feeding station (unillustrated).
After the secondary transfer process, the transfer medium P is conveyed to a fixing apparatus (unillustrated), in which the four color toner images are fixed to the surface of the transfer medium P by heat and pressure. Then, the transfer medium P is discharged into a delivery tray (unillustrated).
The toner which remains on the intermediary transfer belt
105
after the secondary transfer process is removed by a cleaner
108
.
Some of the image forming apparatuses are provided with a mechanism which automatically controls the magnitude of the development bias applied to the developing sleeves of the developing devices
104
a
,
104
b
,
104
c
and
104
d
, in order to adjust image density so that image quality is improved. In such an image forming apparatus, charge bias applied to the primary charging device
102
is also varied in magnitude in accordance with the magnitude of the development bias.
However, as the primary charge bias is varied as described above, toner is scattered, detrimentally affecting the final image in terms of color accuracy; degrading the image quality, in particular, in the areas of the image in which the toner images of different color are literally overlaid. This is thought to occur due to the following reason. That is, if the difference between the electrical potential level to which the photosensitive drum
101
has been charged and the voltage level of the primary transfer bias becomes excessive, it becomes impossible for a proper image transfer electric field to be formed; electrical discharge occurs in the non-image portion, detrimentally affecting the image transfer process. On the other hand, if the aforementioned difference is excessively small, not only does a proper transfer electric field fail to be formed, but also it becomes impossible to give electrical charge even to the non-image portion of the intermediary transfer belt
105
during the processes in which the toner images of different color are overlaid on the intermediary transfer belt
105
, and therefore, it becomes impossible to form a barrier composed of electrical potential, to prevent toner from scattering. As a result, images are inaccurately formed in terms of color.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an image forming apparatus capable of preventing toner particles from scattering from the toner images after the toner images are transferred from an image bearing member onto an intermediary transfer member.
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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Miyashiro Toshiaki
Suzuki Takehiko
Tsuruya Takaaki
Canon Kabushiki Kaisha
Chen Sophia S.
Fitzpatrick ,Cella, Harper & Scinto
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