Electrophotography – Image formation – Charging
Reexamination Certificate
2002-03-15
2004-01-27
Chen, Sophia S. (Department: 2852)
Electrophotography
Image formation
Charging
C361S221000, C399S149000, C399S313000, C492S049000, C492S056000
Reexamination Certificate
active
06684042
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a charging device for use in an image forming apparatus such as a printer, a copying machine or a facsimile machine adopting an electrophotographic system, and an image forming apparatus using the charging device. Particularly, the invention relates to a contact type charging device which comes in contact with the surface of a member to be charged so as to charge the surface of the member to be charged, and an image forming apparatus using the charging device.
2. Description of the Related Art
Of a related-art image forming apparatus of this type such as a printer, a copying machines or a facsimile machine adopting an electrophotographic system, as an apparatus that can form color images at high speed and with high image quality, various so-called full-color tandem machines have been proposed and manufactured practically. In such a tandem machine, four image forming units for forming toner images of respective colors of yellow, magenta, cyan, and black are disposed horizontally in parallel with one another. The toner images of the respective colors of yellow, magenta, cyan and black formed sequentially on the respective image forming units are once primary-transferred in multi-layers onto an intermediate transfer belt. After that, the multi-layer toner images are secondary-transferred from the intermediate transfer belt onto recording paper in a lump, and fixed onto the recording paper. Thus, a color image is formed.
However, in such a full-color tandem machine, four image forming units for forming toner images of respective colors of yellow, magenta, cyan, and black have to be disposed horizontally in parallel with one another, and an intermediate transfer belt has to be disposed under these four image forming units. It is therefore difficult to miniaturize the machine as it is, and the machine is not suitable to be produced as a desktop type printer.
Accordingly, various techniques for making it possible to miniaturize such full-color tandem machines while utilizing the feature that color images can be formed at high speed and with high image quality, that is, various techniques about so-called micro-tandem machines have been proposed, for example, as disclosed in JP-A-Hei.8-36288, JP-A-Hei.8-62920, JP-A-Hei.8-160839, and JP-A-Hei.9-325560 (corresponding to Japanese Patent Application No.Hei.8-166705).
According to such a technique about a micro-tandem machine, four image forming units of yellow, magenta, cyan, and black are disposed vertically, while recording paper is conveyed vertically by a paper conveying belt or directly. Thus, the installation area can be reduced. Further, the four image forming units themselves and photoconductor drums are reduced in diameter for realizing miniaturization so that the machine as a whole can be miniaturized.
When the diameter of each photoconductor drum is reduced thus to miniaturize the full-color tandem machine, it is necessary to reduce the diameter of each charging roll that comes in contact with the surface of the corresponding photoconductor drum so as to charge the surface of the corresponding photoconductor drum. Further, when the diameter of the photoconductor drum is reduced, a blade-like cleaning unit for removing foreign matters such as toner adhering to the surface of the photoconductor drum may be difficult to displace around the photoconductor drum. Because the photoconductor drum has a small diameter, there is also a problem that the abrasion of the drum caused by the friction with the blade becomes so conspicuous that the life of the drum is shortened. In this case, it is inevitable to adopt a cleanerless system using no blade cleaning unit or having a pseudo cleaning unit.
As the charging roll, a roll in which the outer circumference of a metal shaft is coated with a conductive rubber layer with adjusted resistivity is generally used. However, when the diameter of the charging roller is reduced, the hardness of the rubber layer is so high that the contact pressure becomes lower in the center portion than that in the opposite end portions. Thus, there is a problem that it becomes difficult to bring the charging roll into contact with the surface of the photoconductor drum uniformly in the axial direction so that charging becomes ununiform.
Therefore, in order to solve the foregoing problem and to bring the charging roll reduced in diameter into contact with the surface of the photoconductor drum uniformly, it can be considered that the outer shape of the charging roll is formed into a crown shape, or a large amount of plasticizer is added to the rubber layer so as to make the hardness lower.
In this case, however, it is necessary to work the outer shape of the charging roll into a crown shape with precision. Thus, the lowering of the yield results in increase in the cost. On the other hand, when a large amount of plasticizer is added, there arises a new problem that the plasticizer exudes to cause a trouble called “bleed” in which the plasticizer bleeds to have a damaging effect on charging.
Therefore, techniques for solving the foregoing problems while making uniform charging possible even if the diameter of a charging roller is reduced have been proposed, for example, as disclosed in JP-A-Hei.6-175465, JP-A-Hei.8-44142, JP-A-Hei.10-186800, and JP-A-Hei.11-125956.
The contact type charging device according to JP-A-Hei.6-175465 is configured as follows. That is, a metal core supported rotatably is disposed in a flexible tube having at least a conductive layer and a resistive layer outside the conductive layer. The tube is supported from the inside by the conductive flexible layer provided around the metal core. A power supply for applying a charging voltage to the tube to thereby charge a member to be charged in contact with the tube is connected to the metal core. The conductive layer is made shorter than the resistive layer in the same direction as the longitudinal direction of the tube.
On the other hand, the charging device according to JP-A-Hei.8-44142 is a charging device for use in an electrophotographic system as follows. That is, the charging device is constituted by a charging roller having a flexible conductive flexible sheet on the surface. The conductive flexible sheet is a laminate sheet of a low electric resistive layer as an inner layer and a high electric resistive layer as an outer layer. The surface resistivity of the high electric resistive layer is not lower than 10
6
&OHgr;/□ but lower than 10
7
&OHgr;/□, while the volume resistivity of the low electric resistive layer is not higher than 10
5
&OHgr;·cm.
Further, the conductive roller according to JP-A-Hei.10-186800 is a conductive roller constituted by a plurality of layers laminated on one another. The conductive roller is arranged as follows. That is, at least a first layer and a second layer are provided as the laminated layers. The second layer is disposed adjacently to the outer circumferential side of the first layer, and the resistance of the second layer is higher than that of the first layer. The first layer is designed so that the outer circumferential portion of each end surface thereof makes an acute angle with the second layer.
Furthermore, the charging member according to JP-A-Hei.11-125956 is a charging member constituted by a conductive foam material and a conductive resin material applied onto the conductive foam material and for coming in contact with a member to be charged so as to charge the member to be charged. The charging member is arranged as follows. That is, the electric resistance of the conductive foam material is not higher than 9×10
5
&OHgr;. The conductive resin material is made conductive by a conductive material so that the surface electric resistance of the conductive resin material is not lower than 1×10
5
&OHgr;/□ and not higher than 9×10
7
&OHgr;/□. In addition, the product hardness as a conductive member is not higher than 90° in Asker F.
However, the related art has prob
Ishii Yasutomo
Kanai Yutaka
Nagamori Yuki
Takayama Yasuo
Chen Sophia S.
Fuji 'Xerox Co., Ltd.
Oliff & Berridg,e PLC
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