Electrophotography – Control of electrophotography process – Control of transfer
Reexamination Certificate
1998-08-04
2001-03-20
Braun, Fred L (Department: 2852)
Electrophotography
Control of electrophotography process
Control of transfer
C399S315000, C399S398000
Reexamination Certificate
active
06205300
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image on a paper by visualizing an electrostatic latent image formed on a photosensitive drum by supplying a developer and by transferring this visualized developer image on the paper.
2. Description of the Related Art
As disclosed in Japanese Patent Publication (Kokoku) No. 61-32667, in a conventional copying machine, at the same time when transfer current is given to the transfer charger, the voltage of the same polarity as the voltage applied to the transfer charger is applied to a transfer guide which leads a paper to the transfer position, thus, the undesired outflow of the charge given by the transfer charger is prevented.
By the way, the transfer current that is given to the transfer charger is given immediately before the leading edge of the paper comes near the transfer position in order to obtain good transfer efficiency. It was found that pits are produced by the excessive transfer in an area near the leading edge of the paper if the transfer guide bias was applied at the same timing as this transfer current.
That is, in the state before the leading edge of the paper being conveyed along the transfer guide, of which electric resistance is dropped, comes near the separation charger positioned at the downstream side of the transfer charger, the electric charge given by the transfer charger is considered to flow out via the transfer guide only. Further, it is also considered that the electric charge given by the transfer charger flows to the transfer guide and also to the separation charger when the leading edge of the paper comes near the separation charger and the outflow amount of the charge increases. Accordingly, if this transfer guide bias is set at a size to give a proper potential at the central part occupying the greater part of the paper, the potential near the leading edge of the transfer paper before it comes near the separation charger becomes large unnecessarily and such transfer faults as pits due to excessive transfer are produced.
As a countermeasure to prevent such transfer faults, the control is considered to change a transfer current value that is given to the transfer charger before and after the leading edge of the paper comes near the separation charger but there is caused a new problem that such transfer faults as pits due to insufficient transfer are produced near the leading edge of the paper if a normal paper of which electric resistance is not dropped is supplied.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above and it is an object to provide an image forming apparatus capable of achieving good transfer efficiency for the overall length of a paper and forming an image of good quality.
According to the present invention an image forming apparatus is provided. The image forming apparatus comprises image forming means for forming a developer image on an image carrier; conveying means for conveying a paper toward the image carrier; transfer means for giving a transfer charge to the paper conveyed by the conveying means and transferring the developer image formed on the image carrier to the paper; a guide member for guiding the paper conveyed by the conveying means toward the transfer means; separation means, arranged at the downstream side of the transfer means along the transfer direction of the paper, for separating the paper having the developer image transferred by the transfer means from the image carrier; applying means for applying a bias voltage of the same polarity as the transfer charge; and control means for setting the bias voltage by the applying means at a first bias voltage before the paper comes to near the separation means when transferring the developer image onto the paper by the transfer means and setting the bias voltage at a second bias voltage that is larger than the first bias voltage at the time when a paper comes to near the separation means.
REFERENCES:
patent: 4408863 (1983-10-01), Ogata et al.
patent: 4412732 (1983-11-01), Ogata et al.
patent: 4491407 (1985-01-01), Mitsuyama et al.
patent: 5125644 (1992-06-01), Masaki et al.
patent: 5541718 (1996-07-01), Oono
patent: 32 20 294 (1982-12-01), None
patent: 59-182482 (1984-10-01), None
patent: 61-32667 (1986-07-01), None
patent: 1-93776 (1989-04-01), None
patent: 5-313518 (1993-11-01), None
patent: 9-44000 (1997-02-01), None
Braun Fred L
Foley & Lardner
Kabushiki Kaisha Toshiba
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