Electrophotography – Image formation – Development
Reexamination Certificate
2003-01-27
2004-11-02
Beatty, Robert (Department: 2852)
Electrophotography
Image formation
Development
Reexamination Certificate
active
06813462
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a developing unit and an electro-photographic apparatus such as a printer, a facsimile and a copier for actualizing an image employing colored particles such as toner, and more particularly to a developing unit and an electro-photographic apparatus having a feature in a developing process of forming a toner image on the surface of a photosensitive body.
2. Background Art
Conventionally, the printers of electro-photographic method have a developing process for actualizing an image on the surface of photosensitive body using colored particles, and a transfer process for transferring the actualized image of colored particles onto a recording medium. The colored particles may be a powder called a toner exclusively used for electrophotography.
The photosensitive body is evenly charged over its entire surface, and then partially discharged by radiating light. At this time, a potential contrast with a charged region and a discharged region is formed on the surface of photosensitive body to form an electrostatic latent image.
In the developing process, first of all, the toner particles are charged using a developer. The developer is classified into a two-component type consisting of a mixture powder of the toner and carrier beads as magnetic particles and a one-component type of the toner alone. The developer is received in the developing unit, and agitated.
Specifically, a two-component developer is composed of a mixture of the toner as colored particles of resin powder having a particle diameter of about 10 micrometers, and the carrier of magnetic particles having an average particle diameter of 30 to 100 micrometers such as ferrite, magnetite, or iron powder. A one-component developer contains a magnetic powder in the toner and not using the carrier.
In the two-component type developer, the toner is charged due to the friction with carrier beads. Also, in the developer of one-component type, the toner is charged due to the friction with members.
The developer is conveyed to a developing position opposed to an electrostatic latent image on the surface of photosensitive body by a magnet roller called a developing roller. At this time, a “magnetic brush” in which the developer is arranged like a brush along the magnetic force lines is formed in a region opposed to the photosensitive body.
In the manner, a developing method in which the developer is conveyed to the electrostatic latent image on the photosensitive body by the magnetic roller is called a magnetic brush development.
On one hand, a method for actualizing the electrostatic latent image, called a bias development, is often employed. In the bias development, a bias voltage is applied to the developing roller, and the charged toner particles are separated from the developer on the surface of the developing roller, owing to an electric field generated between a latent image potential on the surface of photosensitive body and the developing roller, and moved to the surface of photosensitive body, thereby performing the image formation.
The charging potential or discharging potential may be employed as the latent image potential, or a potential of an image forming portion of the photosensitive body. Generally, the method of using the charging potential as the latent image potential is called a normal development, or the method of using the discharging potential is called a reversing development.
One of the charging potential and the discharging potential that is not used as the latent image potential is called a background potential. The bias voltage of the developing roller is set at a certain value between the charging potential and the discharging potential, and a difference from the latent image potential is called a developing potential difference.
Similarly, a difference from the background potential is called a background potential difference. Usually, the developing potential difference for governing a development performance itself is set to be greater than the background potential difference. With greater developing potential difference, the formed electric field (developing electric field) is increased, so that the development performance (toner developing amount) is enhanced.
Also, increasing the rotating speeds of the developing roller and the photosensitive body, reducing the distance, and decreasing the electrical resistance of the developer have effect on strengthening the developing electric field, making it possible to increase the toner developing amount.
A developing method of using the magnetic brush development to convey the developer, and the bias development to actualize the electrostatic latent image (hereinafter referred to as a “magnetic brush bias development”) is widely employed.
A relative moving direction of the developing roller and the photosensitive body may be the same or opposite. Also, one developing unit may employ a plurality of developing rollers. In some developing unites, the plurality of developing rollers are rotated in the same direction, but in other developing unites, they are rotated in different directions.
In this case, a developing unit is well-known in which the adjacent developing rollers are rotated in different rotations from the opposite position of developing rollers toward the photosensitive body, and the developer is branched and conveyed like a fountain from the opposite position of developing rollers to the photosensitive body (hereinafter referred to as a “fountain-type developing unit”).
The magnetic brush bias development has a problem that the image edge is difficult to develop in the rotation direction of the developing roller. This problem occurs due to a mechanical factor that the magnetic brush rubs against the surface of photosensitive body, and because the potential of the photosensitive body contacted by the magnetic brush is drastically changed from the background potential of non-image part to the developing potential of image part to change the electrical characteristics of the developer.
Thus, using the fountain-type developing unit, two developing rollers rotated in different directions compensate each other, thereby solving this problem.
The developing roller includes a core having the magnetic poles and a metallic sleeve containing this core, in which the developer is conveyed by rotating the sleeve with the core fixed (hereinafter referred to as a “sleeve rotating method”) or rotating both the core and the sleeve (hereinafter referred to as a “core rotating method”).
In addition, it is called a sleeve rotation developing roll in which the magnetic roll inside the developing roll is fixed. It is called a double rotation developing roll in which the internal magnet roll is also rotated.
The core rotation method is more effective in agitating the developer in a developing area opposed to the photosensitive body than the sleeve rotation method, and has a greater toner developing amount. Therefore, even if the rotating speed of the sleeve is decreased, a sufficient toner developing amount can be assured to reduce the scraped toner image on the photosensitive body caused by mechanical sliding of the developing roller itself, thereby improving the image quality.
The core rotating developing unit was described in JP-A-58-142358.
Further, a developing method using two double rotation developing rolls was described in JP-A-58-142358. As its contents, a first double rotation developing roll rotates the sleeve and the magnet roll in opposite directions, or rotates the sleeve alone, while a second double rotation developing roll rotates the sleeve and the magnet roll in the same direction.
SUMMARY OF THE INVENTION
When the fountain-type developing unit having the feature that no defective image edge is caused due to the electrical factor employs at least one of the developing rollers based on the core rotating method (hereinafter referred to as a “core rotating fountain-type developing unit”), the scraped toner image due to the mechanical factor can be reduced, whereby the very
Ishii Masayoshi
Kawanishi Tsuneaki
Kubota Keisuke
Kuribayashi Natsuki
Matsumoto Hiroyoshi
Beatty Robert
Hitachi Printing Solutions, Ltd.
McGinn & Gibb PLLC
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