Developing device

Electrophotography – Image formation – Development

Reexamination Certificate

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Reexamination Certificate

active

06507722

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a technical field of a developing device used in image forming apparatus such as electrophotographic copiers and printers and, more particularly, to a technical field of a developing device comprising an image developing roll for carrying developer and conveying it to a latent image carrier.
In conventional image forming apparatus such as electrophotographic copiers and printers, an image is obtained by developing an electrostatic latent image on a latent image carrier such as photoreceptors by means of developer of a developing device, transferring the developed image from the latent image carrier to a recording medium such as paper, and finally fixing the transferred image to the recording medium.
Among conventional developing devices used in such image forming apparatus, there is a type of developing device which employs a contact development system in which an image developing roll carrying a developer and conveying it to a latent image carrier is in contact with the latter. In a developing device employing the contact development system, the image developing roll in contact with the latent image carrier conveys charged monocomponent developer to the latent image carrier, to develop the electrostatic latent image on the latent image carrier with the monocomponent developer. There are other type of conventional developing device which at least comprises an image developing roll for carrying developer and conveying it to a latent image carrier, and a regulating member which is in contact with the image developing roll and regulates the developer being conveyed to the latent image carrier in such a manner that a thin layer of developer is formed on the image developing roll surface. In such a developing device, electrically charged monocomponent developer which is regulated by the regulating member to form a thin layer of the developer on the image developing roll surface is conveyed by the image developing roll to the latent image carrier, thus developing with the thin-layered monocomponent developer an electrostatic latent image on the latent image carrier.
In this respect, a metallic roll made of aluminum or iron is used as image developing roll. Particularly, aluminum roll is commonly used because of its excellent workability and low cost.
The functions required to an image developing roll of a developing device are: (1) property for conveying developer, (2) property for charging developer, and (3) property for preventing the discharging of developing bias.
In order to improve the properties for conveying and charging of developer, a carrier roll (i.e. an image developing roll) is proposed in Japanese Patent Publication No. H6-46331, in which a metallic roll is subjected to a sand blast treatment so that a rough surface morphology is imparted to the roll surface, which is then plated with metal such as nickel. According to the carrier roll disclosed in the publication, the rough surface morphology formed on the carrier roll mechanically increases the developer conveying property of the roll, while also improving the developer charging property of the roll because of increased contact area for developer. By metal plating the rough surface of the metallic roll, the abrasion resistance of the roll is improved.
In order to prevent the discharging of the developing bias, a roll having a specific resistance set at a predetermined value has been proposed. For instance, proposed in Japanese Patent Publication No. H2-26226 is a carrier for a non-magnetic monocomponent toner (that is, an image developing roll) comprising a cylindrical rigid body made of a resin having conductive powder dispersed therein with a specific resistance value in a range of 10
4
to 10
12
W cm in which the cylindrical rigid body has a conductive layer formed on an inner surface thereof or a conductive paint coating with a specific resistance value not exceeding 10
7
W cm formed on the inner surface thereof. Further, proposed in Japanese Patent No. 2705090 is a carrier for a non-magnetic monocomponent toner (that is, an image developing roll) comprising a semiconductive layer formed on its surface, the layer being formed of ceramics such as alumina, having a specific resistance value in a range of 10
4
to 10
12
W cm and a thickness of 100 to 1000 mm. By using the carriers for non-magnetic monocomponent toner disclosed in the above mentioned publications, the discharging of the developing bias can be effectively prevented to prevent the occurrence of image defects, owing to the fact that at least the roll surface has a predetermined specific resistance value.
Meanwhile, as disclosed in above described Japanese Patent Publication No. H2-26226 and Japanese Patent No. 2705090, it is a conventional practice to use a direct current voltage overlapped with an alternate current voltage as a developing bias in a developing device in order to prevent background toning, to obtain an adequate edge effect, and to improve the gradation.
However, in the carrier roll disclosed in Japanese Patent Publication No. H6-46331, a metal plating is applied onto the roll surface which has been roughened by sand blast treatment. The metal plating comes to blur the clear morphology of the roll surface formed by sand blast treatment, in such a manner that sharp ridges bounding the adjacent recesses are rounded by overlaid plating material. Therefore, there is a problem that the effect of the rough surface morphology formed by sand blast treatment to improve the conveying property and charging property of the roll cannot be fully and reliably obtained because of the blurring of the clear surface morphology by the subsequent metal plating, although the metal plating increases the abrasion resistance of the roll surface.
Further, the carrier roll formed of a resin having conductive powder dispersed therein, disclosed in Japanese Patent Publication No. H2-26226, has a problem that it can yield mottling in the developed image because of possible uneven distribution of the conductive powder dispersed in the resin, which leads to uneven specific resistance value over the roll surface.
Furthermore, the toner carrier roll having ceramic semiconductive layer with a thickness of 100 to 1000 mm has a problem of manufacturing difficulty and cost, since the semiconductive layer is formed by spraying ceramic material melted by arc discharging, onto the toner carrier substrate.
It is conceivable to combine the technical items described in foregoing publications so that the roll can be reliably provided with the foregoing three functions. However, such combinations of technical items described in the publications for imparting the foregoing three functions to the roll can bring about following problems.
Namely, a combination of techniques of Japanese Patent Publications No. H6-46331 and No. H2-26226 means that the carrier roll is not a metallic one, but is formed of resin having conductive powder dispersed therein. There is a problem that such a carrier roll is difficult to sand blast to form a rough surface as described in Japanese Patent Publication No. H6-46331, and it is also difficult to improve its abrasion resistance by some treatment. Therefore, the combination of techniques of Japanese Patent Publications No. H6-46331 and No. H2-26226 in such a manner that the image developing roll can perform the foregoing three functions reliably is extremely difficult.
A combination of Japanese Patent Publication No. H6-46331 and Japanese Patent No. 2705090 means that a layer of ceramic semiconductive material melted by arc discharging according to Japanese Patent No. 2705090 is formed on a rough surface formed according to Japanese Patent Publication No. H6-46331. This will lead to a blurred surface morphology in such a manner that sharp ridges bounding adjacent recesses are rounded by overlaid material, as is the case in metal plating of the rough surface described in Japanese Patent Publication No. H6-46331. Therefore, the combination of techniques of Japanese Patent Publications No. H6-46

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