Electrophotography – Image formation – Development
Reexamination Certificate
2003-01-10
2004-05-11
Grainger, Quana (Department: 2852)
Electrophotography
Image formation
Development
C430S108200
Reexamination Certificate
active
06735409
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for developing, an image-forming apparatus, and an image-forming process cartridge.
2. Description of the Related Art
A process for dry electrophotography is classified into the one-component developing process, where toners are charged by a development sleeve, a blade, or the like; and the two-component developing process, where toners are charged by carriers. The two-component developing process is mainly used for a medium to high-speed machines, because the two-component developing process provides more stably and uniformly charged toners, and provides more toners than the one-component developing process.
A carrier plays a role of charging a toner and of transporting the toner to a developing part. Properties of carrier significantly influences image-forming, and influences creating an image with high quality, accordingly.
Of carrier properties, the electrical resistance of the carrier also has a large influence on developing performance. The low electrical resistance of the carriers leads to a similar result to approaching a development electrode. A carrier having a low electrical resistance is more likely to develop a solid image, compared with a carrier having a high electrical resistance. A carrier having a relatively low electrical resistance is therefore used for a color copier, compared to a monochrome copier, which is required to reproduce letters and fine lines, for the purpose of higher development properties for a solid image.
The electrical resistance of the coated carrier depends not only on the electrical resistances of a material for the coating layer and a carrier core material, but also on a thickness of the coating layer. The thicker a coating layer, the larger an electrical resistance of the carrier. The electrical resistance of a carrier becomes constant, when the layer has more than a certain thickness.
Even among coated carriers whose electrical resistance are adjusted, the electrical resistances of the carriers vary over time due to a stress such as stirring inside a developer, and due to the fact that a coating layer of the carriers are thereby eroded.
The amount of toners to be developed varies over time, and a quality of an image also vary, accordingly.
The changes and differences in development performance cause problems for a quality of an image.
There are methods for stabilizing a developing performance over time, in which a coating film is strengthened, and electric resistances of carriers are lessened. Japanese Patent Application Laid-Open (JP-A) No. 06-110255, No. 2001-117287, No. 2001-117288, No. 2002-229273, and the like disclose a process for strengthening a coating film. Apparatuses have been miniaturized, and photocopying has been speeded up. As a result, the amount of a developing agent is becoming lessened, and a liner velocity at a development sleeve is highly increased. Moreover, a carrier is more stressed, and a coating film is more likely to be eroded. A carrier having a stronger coating film is hence insufficient to stabilize a developing performance over time.
A process for stabilizing a developing performance has been desired, even if an electrical resistance of a carrier varies because of eroding of a coating film, rather than to stabilize a developing performance by strengthening a coating film of a carrier.
SUMMARY OF THE INVENTION
The inventors of the present invention have found out that image properties without an abnormal image can be obtained by supplying a developing agent at the closest part between the latent electrostatic image support and the development sleeve (which may be referred to as a developing part, hereinafter) to a high density, by narrowing a width at a linearly contacting surface of a plurality of magnetic brushes (which may be referred to as a width at a linearly contacting surface), by using carriers having a smaller diameter, and by narrowly distributing carrier particle diameters. Herein, the width at a linearly contacting surface indicates a length of the linearly contacting surface in a direction where the surface of the magnetic brushes rotates.
FIG. 1
shows one example of a process in which the developing agent is supplied to a high density, and
FIG. 2
shows one example of a plurality of magnetic brushes formed by supplying according to a conventional process.
In conventional processes, as compared to processes for supplying at a high density, the magnetic brush, which is formed of a developing agent by magnetism, has a large gap, so the toners to be developed facing the gap extends over a wide region from a direction of development sleeve
1
(valley of a magnetic brush) where the developing electric field is weak, to the tip. Hence, the carriers are easily influenced by electrical resistances. On the other hand, when the developing agent is supplied into the developing part to high density, toners to be developed, which face the gap, are concentrated in a vicinity of a photoconductor
2
as a latent electrostatic image support, where the developing electric field is strong. Even if a low electrical resistance is not given to carriers, toners are more likely to be developed. The difference in a developing performance is less likely to appear.
However, if a developing agent is supplied at a high density, non-uniform concentration caused by a magnetic brush's scraping is more obviously occurs at a half-tone part. This is because a plurality of magnetic brushes strongly contacts a photoconductor as a latent electrostatic image support, and a portion of toners developed on the photoconductor
2
is scraped.
The inventors of the present invention are convinced that, even if a developing agent is supplied at a developing part to a high density, a process for developing and an image-forming apparatus that produces an image without non-uniform image density at a half-tone part can be obtained by narrowing the width at a linearly contacting surface, where toners are also scraped, to 2 mm or less, by forming a fine magnetic brush where toners and carriers are uniformly disposed in which carriers having a smaller diameter are used, and the carrier particle diameters are narrowly distributed.
An object of the present invention is to provide a stable developing performance against the change in an electrical resistance of carriers, and to provide a process for developing and an image-forming apparatus, both of which can produce an image with a good quality.
A process for developing according to the first aspect of the present invention comprises the step of developing a latent electrostatic image on a latent electrostatic image support by a developing agent supplied on a development sleeve, wherein the developing agent is supplied with a density of 1.3 g/cm
3
to 2.0 g/cm
3
at the closest part between the latent electrostatic image support and the development sleeve, the latent electrostatic image support is contacted with a plurality of magnetic brushes formed of the developing agent on the development sleeve, so that a plurality of the magnetic brushes have a width of 2 mm or less at a linearly contacting surface, in a direction where the linearly contacting surface of the magnetic brushes rotates, and the developing agent contains toners, and carriers which comprise magnetic core particles and resin layers to cover a surface of the magnetic core particles the carriers have a weight average particle diameter of 25 &mgr;m to 45 &mgr;m, the carriers contain 60% by weight or more of the carrier particles having a particle diameter of less than 44 &mgr;m, and 7% by weight or less of carrier particles having a particle diameter of less than 22 &mgr;m.
According to the second aspect of the present invention, there is provided the process for developing of the first aspect, wherein a developing gap is 0.4 mm or less, when the developing gap expresses a distance at the closest part between the latent electrostatic image support and the development sleeve.
According to the third aspect of the present inventi
Imahashi Naoki
Kotsugai Akihiro
Takahashi Hiroaki
Yamaguchi Kimitoshi
Grainger Quana
Ricoh & Company, Ltd.
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