Electrophotography – Image formation – Development
Reexamination Certificate
1999-09-29
2001-02-27
Braun, Fred L. (Department: 2852)
Electrophotography
Image formation
Development
Reexamination Certificate
active
06195522
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The invention relates to a developing device for forming a thin developer layer on a developer carrying member by use of a layer thickness-regulating member.
2. Description of Related Art
In a conventional developing apparatus that uses a nonmagnetic one-component developer (hereinafter referred to as toner), a thickness-regulating blade made of rubber elastically makes contact with the developing roller to regulate the amount of toner passing between the layer thickness-regulating blade and the developing roller, and thereby a thin toner layer is formed.
For example, Japanese Patent Application Publication No. 60-205472 discloses a technique in which a coefficient of kinetic friction between the layer thickness-regulating blade and the toner is set to be greater than a coefficient of internal friction between toner particles, and the coefficient of kinetic friction between the layer thickness-regulating blade and the toner is set to be greater than that between the developing roller and the toner. By the technique disclosed in this publication, a thin toner layer can be formed on the surface of the developing roller due to a mechanical force, that is, a shearing force.
However, in the technique disclosed in this publication, no consideration is given to the action of an electrostatic force generated when the toner is charged due to friction between toner particles and friction between the toner and the developing roller or the layer thickness-regulating blade. Thus, in the above-mentioned technique where the layer thickness-regulating blade made of rubber elastically makes contact with the developing roller, normally charged toner along with uncharged toner and toner weakly charged to opposite polarity pass between the layer thickness-regulating blade and the developing roller, and are supplied to the surface of a photoconductive drum. When such abnormally charged toner is supplied to the photoconductive drum, it adheres to positions other than where it should adhere, resulting in so-called fogging, which denotes a problem that the toner adheres lightly over the background of an image and reduces image quality.
In view of the foregoing, an object of the invention is to provide a developing device that can form a uniform thin toner layer and eliminate fogging.
SUMMARY OF THE INVENTION
To solve the problem associated with the prior art, a developing device according to the invention is structured such that a layer thickness-regulating member is electrically conductive, and that a coefficient of friction produced between the layer thickness-regulating member and toner is greater than that between the surface of a developer carrying member and the toner, and further that a polarized electric field is generated, which applies to normally charged toner a force directed from the regulating member to the developer carrying member.
With this structure, the toner supplied to a pressed portion, where the regulating member is pressed against the developer carrying member, is affected by a friction force produced between the toner and the developer carrying member and that produced between the toner and the regulating member. Since the coefficient of friction produced between the regulating member and the toner is set to be greater than that between the developer carrying member and the toner, a greater friction force is exerted on the toner by the regulating member than by the developer carrying member. Accordingly, when no other forces than the friction forces are exerted on the toner, the toner is stopped from passing between the regulating member and the developer carrying member.
However, since the regulating member is made from an electrically conductive material, a polarized electric field is generated, which applies to normally charged toner a force directed from the regulating member to the developer carrying member. Accordingly, when normally charged toner is sandwiched between the regulating member and the developer carrying member, the force exerted on the normally charged toner by the electric field will counteract the friction force produced between the regulating member and the normally charged toner. Thus, that friction force becomes smaller than that produced between the developer carrying member and the normally charged toner, thereby allowing the normally charged toner to pass smoothly between the regulating member and the developer carrying member.
Consequently, the normally charged toner is supplied to the surface of the developer carrying member and a uniform thin toner layer is formed. On the other hand, since toner weakly charged to opposite polarity or uncharged toner is not affected by the action of the electric field, the friction force produced between the regulating member and the abnormally charged toner becomes greater than that produced between the surface of the developer carrying member and the abnormally charged toner. Thus, the abnormally charged toner is stopped from moving to the developer carrying member and from passing between the developer carrying member and the regulating member.
As described above, according to the invention, only normally charged toner passes between the regulating member and the developer carrying member, and other toner does not pass therebetween. Accordingly, only normally charged toner is supplied to an electrostatic latent image carrying member, and thereby excellent image quality can be maintained without producing fogging.
Further, when the regulating member is arranged to make surface contact with the developer carrying member, the contact area therebetween increases. The toner is frictionally charged over a large area efficiently while sandwiched by the regulating member and the developer carrying member.
Accordingly, uncharged toner or toner charged to opposite polarity is reduced in amount, such toner is less used for the developing process, and, as a result, fogging is more reliably prevented.
Further, when an electrically conductive member having rubber elasticity is used as the regulating member, the toner will be slow in deteriorating even when image forming operations are performed repeatedly.
Specifically, the regulating member having rubber elasticity deforms along the shapes of additives which are fine particles of silica or titanium oxide added to the surface of each toner particle, and no excessive pressure is applied to the additives. Accordingly, the additives can be prevented from being buried in the toner particles. In addition, since the member having rubber elasticity generally has a higher friction coefficient than metal or resins, that member is more likely to apply frictional charge to the toner, and the ratio of normally charged toner can be increased. Thus, the amount of toner charged to opposite polarity or uncharged toner can be reduced, and thereby fogging can be reduced.
In addition, when the regulating member is an electrically conductive member, an electric field can be generated between the regulating member and the conductive member reliably by an electric field generating means, and normally charged toner is allowed to pass smoothly between the regulating member and the developer carrying member.
Further, according to the invention, the developer carrying member is electrically conductive, and a coefficient of friction between at least the surface of the developer carrying member and the toner is set to be greater than a coefficient of friction &mgr;i between the toner particles. At least a contact portion of the regulating member, which makes contact with the developer carrying member is electrically insulative, and a coefficient of friction &mgr;b between the regulating member and the toner is set to be greater than the coefficient of friction &mgr;r between the surface of the developer carrying member and the toner.
With the relationships, the toner supplied to the pressed portion, where the regulating member is pressed against the developer carrying member, is affected by a friction force from the developer carrying member. Also, th
Braun Fred L.
Brother Kogyo Kabushiki Kaisha
Oliff & Berridg,e PLC
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