Electrophotography – Having particular structure – Modular or displaceable
Reexamination Certificate
2002-06-25
2004-05-25
Tran, Hoan (Department: 2852)
Electrophotography
Having particular structure
Modular or displaceable
C399S116000
Reexamination Certificate
active
06741821
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a printer or a facsimile, and relates to a process cartridge used for the image forming apparatus. More specifically, the image forming apparatus of the present invention includes a plurality of image formation units. Each image formation unit includes a latent image support for supporting an electrostatic latent image, and a charging unit for charging the latent image support surface. A latent image formation unit forms an electrostatic latent image on the latent image support. A developing unit develops and forms the electrostatic latent image into a toner image. A transfer unit transfers the toner image on the latent image support to a recording medium. Moreover, the present invention relates to a process cartridge used for the image forming apparatus having the plurality of image formation units each equipped with the recording medium surface side the transfer unit to which the recording medium surface side is moved so that the surface of each latent image support of the image formation units is countered one by one.
2. Description of The Related Art
With the electro-photographic equipment, color printers and color copiers performing color image formation are increasing with the demand from the market today.
Among the color electro-photographic equipment, there is a one-drum type image forming apparatus in which the surroundings of one photoconductor are equipped with the developing units of two or more colors, a toner is adhered with those developing units, a synthetic toner image is formed on the photoconductor, the toner image is transferred to a copy sheet, and it is fixed as a printed color image on the copy sheet. On the other hand, there is a tandem type image forming apparatus in which two or more photoconductors are equipped with individual developing units respectively, a monochrome toner image is formed on each photoconductor respectively, those monochrome toner images are transferred one by one, a synthetic color image is printed on a copy sheet.
A description will now be given of comparison between the one-drum type and the tandem type. In the one-drum type, the number of photoconductors is one, and the miniaturization can be easily attained and the one-drum type has the advantage that the cost reduction is possible. However, in order to repeat the image formation two or more times (usually 4 times) using one photoconductor and to form a full color image, the improvement in the speed of image formation is difficult.
On the other hand, in the case of the tandem type, the scale of the equipment becomes large, which causes a problem that the equipment becomes expensive. However, the tandem type image forming apparatus has an advantage that the improvement in the speed of image formation is easy.
There is an increasing demand for a full color image being formed at a high speed that is the same as the speed of monochrome image formation, and the tandem type has attracted attention.
In the case of the tandem type, four photoconductors are arranged in parallel for each color image formation of cyan, magenta, yellow and black, and the image forming apparatus of the tandem type needs to make smaller the diameter of each of the photoconductors and shorten the distances between the four photoconductors, in order to create more space, reduce the equipment size, and achieve cost saving.
As a transfer method to transfer a toner image from a photoconductor to a recording medium, there are an indirect transfer method in which a bias roller is arranged at a downstream portion of the photoconductor, and a direct impression method in which the pressure of an elastic roller as a bias roller on the photoconductor is directly applied to a recording medium between the elastic roller and the photoconductor.
FIG. 7
shows a direct transfer method in a conventional tandem type image forming apparatus. As shown in
FIG. 7
, in the direct transfer method, an image on each of photoconductors
1
is transferred to a recording sheet “s”, which is transported by a sheet transport belt
3
, by using a corresponding one of transfer units
2
, respectively.
In a conventional indirect transfer method, if the diameter of each of the photoconductors and the distances between the photoconductors are reduced in order for the miniaturization and cost saving, it becomes difficult to control the electric current which flows from the bias roller into the photoconductor.
FIG. 6
shows an indirect transfer method in a conventional tandem type image forming apparatus. In
FIG. 6
, reference numeral
40
BK indicates a photoconductor for a black image, reference numeral
40
Y indicates a photoconductor for a yellow image, and reference numeral
10
indicates an intermediate transfer belt which is brought into contact with these photoconductors in common.
As shown in
FIG. 6
, a bias roller
603
BK pushes up the intermediate transfer belt
10
indirectly from the upstream side of the intermediate transfer belt
10
about the photoconductor
40
BK in the conveyance direction. The pressure of the belt
10
is applied to the intermediate transfer belt
10
against the photoconductor
40
BK. Reference numeral
603
BK′ indicates an auxiliary roller, and the auxiliary roller
603
BK′ currently pushes up the intermediate transfer belt
10
from the downstream side of the intermediate transfer belt
10
about the photoconductor
40
BK.
Similarly, reference numeral
603
Y indicates a bias roller for the photoconductor
40
Y, and reference numeral
603
Y′ indicates an auxiliary roller.
The auxiliary rollers
603
BK and
603
Y are grounded respectively, and the bias roller
603
BK and
603
Y are connected to bias power supplies, respectively. For example, if the electric current i
2
which flows to the ground side from the bias roller
603
BK is larger than the electric current i
1
which flows into the photoconductor
40
BK, it becomes difficult to keep constant the electric current i
1
which flows into the photoconductor
40
BK.
The intermediate transfer belt is usually made of a material with middle resistance, and when changes of resistance and electrostatic capacitance according to the environment cause the electric current i
2
to fluctuate, it is very difficult to maintain the transfer conditions of the intermediate transfer belt uniformly by keeping the electric current i
1
constant.
The ratio of the electric current i
2
to the electric current i
1
becomes large as the distance L between the photoconductor
40
BK and the photoconductor
40
Y becomes small. The electric current control will become more difficult in such a case. To avoid this, the bias roller is arranged at the downstream side of the photoconductor in the rotation direction of the intermediate transfer belt
10
. Moreover, to avoid the influences on the other photoconductors as much as possible, it is necessary to take a slightly large distance between the photoconductors and more space than the direct transfer method.
For this reason, in order to attain the miniaturization of equipment, in the tandem type image forming apparatus having the four-drum photoconductors and the intermediate transfer mechanism, the direct transfer method is adopted in many cases. By the direct transfer method, the bias roller serves as a configuration which counters a photoconductor from the front through the intermediate transfer belt, and the electric current which flows into the photoconductor decreases very much, and it becomes possible to keep constant the electric current which flows into the photoconductor.
However, the bias roller is pressurized by the photoconductor from the front through the intermediate transfer belt when adopting the direct transfer method, and the pressure acts on the toner on the photoconductor surface greatly. The toner image tends to be condensed, and the toner at a certain spot of the image on the photoconductor remains without transfer. There is a problem that an undesired image wh
Katoh Kohki
Kawagoe Katsuya
Ogiyama Hiromi
Sawai Yuuji
Sugino Akihiro
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Ricoh & Company, Ltd.
Tran Hoan
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