Electrophotography – Image formation – Development
Reexamination Certificate
2001-11-01
2003-03-18
Tran, Hoan (Department: 2852)
Electrophotography
Image formation
Development
C399S302000, C399S308000
Reexamination Certificate
active
06535707
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming system such as a copying machine, a facsimile, or a printer and more particularly to an image forming system wherein an image bearing member and an intermediate transfer member are kept in contact with each other, toner images formed on the image bearing member are primarily transferred onto the intermediate transfer member, and the toner images thus primarily transferred onto the intermediate transfer member are together transferred secondarily onto a transfer medium.
2. Description of the Prior Art
Heretofore, as this type of an image forming system, there is known an image forming system wherein toner images formed on a photoreceptor as an image bearing member are primarily transferred onto an intermediate transfer member by utilizing an electrostatic force and thereafter the toner images on the intermediate transfer member are secondarily transferred onto a transfer paper by utilizing an electrostatic force. The image forming system using such an intermediate transfer member is advantageous in that images can be formed on various kinds of transfer papers, including plain paper and cardboard.
Among image forming systems having an intermediate transfer member, there is known one which is constructed so as to perform image formation in a constantly contacted state of the intermediate transfer member with a photoreceptor. This image forming system does not require the provision of an engaging/disengaging mechanism for the engagement and disengagement of the intermediate transfer medium with respect to the photoreceptor. Thus, the image forming system in question is advantageous in that the components' cost can be reduced by an amount corresponding to the disengaging mechanism and hence space-saving can so much be attained in comparison with an image forming system which has a construction permitting engagement and disengagement of the intermediate transfer member with and from the photoreceptor.
Moreover, in the image forming system having an intermediate transfer member, toner images formed on a photoreceptor can be overlapped onto the intermediate transfer member, thus permitting the image forming system to be widely used as a color image forming system capable of forming a color image on a transfer paper. In the color image forming system there is used a developing apparatus having plural developing units capable of effecting development using different colors of developers, and latent images are developed by corresponding developing units respectively. In such an image forming system, when latent images formed successively on a single photoreceptor are developed by corresponding developing units respectively, there arises the necessity of switching from one to another developing unit in a successive manner. In connection with this developing unit switching operation, there are known a method wherein one and same developing position is used for the developing units and the developing units themselves are moved successively for development to the developing position and a method wherein different developing positions are used for the developing units respectively and the developing units themselves do not move. As examples of the former method are mentioned a revolver method and a slider method. On the other hand, as an example of the latter method there is known a method wherein switching is made from one to another developing unit by bringing only the developer in the developing unit to be used into contact with a photoreceptor with use of an engaging/disengaging mechanism provided in each developing unit.
In a developing apparatus adopting a revolver method or a slide method wherein developing units themselves are moved to a developing position, the developing units are generally arranged so that they can move to the developing position successively in accordance with a development order adopted in forming a full-color image in which image formation is performed using all the developing units. For example, in a revolver developing apparatus adopting a revolver method wherein development is performed in the order of black (“Bk” hereinafter), yellow (“Y”), cyan (“C”), and magenta (“M”) at the time of forming a full-color image, these four-color developing units are arranged side by side in their revolving direction in the order of Bk, Y, C, and M.
In the image forming system provided with such a revolver developing apparatus, the developing unit which is the first to make development must lie in the developing position before the start of development. For moving each developing unit most efficiently at the time of forming a full-color image, the revolver developing apparatus, before the start of the image forming process, is stopped at a home position where the Bk developing unit which is the first to perform development is located nearest to an upstream side in the revolving direction of the revolver developing apparatus. From this home position the developing unit which is used in this image forming process and which is the first to make development is moved to the developing position to effect development.
However, when the image forming process is started and the developing unit which is the first to make development has moved to the developing position, the developer in the developing unit adheres onto a photoreceptor already before the start of development, causing stain of the toner present on the photoreceptor. The first reason for the occurrence of such toner stain is presumed to be as follows. When the developer pressure increases between a developer carrier in a developing unit and a photoreceptor in relation to both a development gap and a developer scoop-up quantity (developer weight per unit area), there increases an impact force between the developer carrier and the photoreceptor with movement of the developing unit, so that it becomes easier for the toner to leave the developer carrier, and the spilt toner adheres onto the photoreceptor with van der Waals' force. The second reason is presumed to be as follows. The potential of a toner layer portion deposited on the developer carrier approaches a surface potential of the photoreceptor rather than the potential of the developer carrier surface, causing an effective bias of development to be changed, with consequent deposition of toner onto the photoreceptor. The toner stain thus generated is conveyed to a primary transfer section as it is adhered to the photoreceptor and adheres onto an intermediate transfer member. If the surface portion of the intermediate transfer member thus stained with the toner stain is an area (a “to-be-transferred area” hereinafter) onto which a toner image on the photoreceptor is to be primarily transferred in this image forming process, the toner stain overlaps a toner image to be subsequently transferred primarily onto the to-be-transferred area. The resulting ground stain causes deterioration of the image quality.
In the case where image formation is to be done using the above image forming system and using, for example, only the three colors of Y, C, and M without using Bk, it is necessary that the Y developing unit which is the first to make development be moved to the developing position after start of the image forming process. In this case, it is necessary that the Y developing unit be moved to the developing position while skipping over the Bk developing unit from the home position. During this movement, the developer in the Bk developing unit comes into contact with the photoreceptor surface at the developing position. Also at the instant of this contact the toner adheres onto the photoreceptor for the same reason as above and the resulting toner stain adheres onto the intermediate transfer member. If this stained portion is the to-be-transferred area, a ground stain results and causes an image quality deterioration like above.
In the case where image formation is to be performed using the above image forming system and using, for example, only the two colors o
Maruta Takayuki
Yamazaki Takahiro
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Ricoh & Company, Ltd.
Tran Hoan
LandOfFree
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