Electrophotography – Image formation – Transfer
Reexamination Certificate
2000-11-30
2003-07-29
Ngo, Hoang (Department: 2852)
Electrophotography
Image formation
Transfer
C399S066000, C399S312000
Reexamination Certificate
active
06600894
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for forming an image by belt transfer, which are used in an electrophotographic copying machine, printer, and the like, and transfer medium chucking method.
In particular, the present invention relates to a method and apparatus for forming an image by belt transfer, which are used in a four-drum tandem color copying machine, printer, and the like, and a transfer medium chucking method of electrostatically chucking a sheet with a transfer belt by charging the surface of the transfer belt by applying charges to it.
As a transfer technique in an electrophotographic image forming apparatus, a method using a corona charger opposing a photosensitive body is known.
This method, however, generates toxic ozone. As an ozone-free transfer technique, one employing a contact method has been proposed.
Japanese Patent Laid-Open No. 6-110343 discloses a technique in which a semiconductive transfer belt and a transfer roller disposed behind the transfer belt are used and transfer is performed by applying a transfer bias to the transfer roller.
As a color image forming apparatus for forming a color image by using a plurality of toners, i.e., Y (yellow), M (magenta), C (cyan), and Bk (black), the following methods are known:
(1) A method of forming four color images on one photosensitive body to superpose them on each other and transferring them at once.
(2) A transfer drum method of holding a transfer medium on a transfer drum and forming a four-color image by rotating the transfer drum four times.
(3) An intermediate transfer body method of forming four color images on an intermediate transfer body and transferring them on a transfer medium at once.
(4) A four-drum method with which four photosensitive bodies are arranged parallel to each other and a four-color image is formed while a transfer medium passes among them.
According to a color image forming apparatus employing the four-drum method, color images formed on four parallel image carriers can be transferred onto a transfer medium in a multiple manner while the transfer medium passes among them, thereby forming a color image. Therefore, an image can be formed with a time ¼ that required by other processes that form a four-color image. This apparatus is suitable for a higher speed process.
In a color image forming apparatus employing the four-drum method that forms Y, M, C, and Bk toner images with four parallel photosensitive drums, a process unit for forming a monochromatic image is provided, so the service life of the process units for forming color images can be prolonged, leading to a cost reduction.
When forming a monochromatic image, a transfer belt and first, second, and third color image forming photosensitive bodies are separated from each other, and only the fourth black image forming photosensitive body is brought into contact with the transfer belt. Then, the operations of the process units for forming color images are stopped. The transfer belt and color image forming photosensitive bodies are separated from each other in order to prevent wear of the transfer belt and color image forming photosensitive bodies.
In the four-drum type image forming apparatus, since four photosensitive bodies are arranged parallel to each other, the distance for which the transfer medium is conveyed by the transfer belt is, e.g., 300 mm or more. When forming a monochromatic image, if the transfer belt and the color image forming photosensitive bodies are separated from each other, after the transfer medium is fed, it must be conveyed on the transfer belt.
A sheet chucking unit is required to stably convey the transfer medium on the transfer belt. In a conventional chucking unit, a transfer belt and transfer medium are sandwiched together, and charges are applied by a charger, thereby chucking the transfer medium.
With this chucking unit, the charges applied to the surface of the transfer belt for the purpose of sheet chucking are discharged in the transfer section to the upper surface side of the photosensitive body. As a result, charges of opposite polarity to that of the image forming charges applied to the photosensitive body are applied. This causes defective charging in the photosensitive body to lead to an image defect.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and apparatus for forming an image, which are capable of preventing an image defect caused by defective charging which occurs when charges applied to the surface of a convey belt for the purpose of sheet chucking are discharged to the surface of a photosensitive body, and a transfer medium chucking method.
An apparatus for forming an image according to the present invention comprises a plurality of image carriers on which a toner image is to be formed, a developing unit for forming a toner image on the image carriers, a convey belt disposed in contact with the image carriers and adapted to convey a transfer medium, a chucking unit for electrostatically chucking the transfer medium onto the convey belt, and a transfer unit for transferring the toner image from the image carriers to the transfer medium in a transfer region where the image carriers are in contact with the convey belt, the chucking unit having a charger for applying charges to a surface of the convey belt, and a countercharge applying unit for applying countercharges necessary to electrostatically chuck the transfer medium onto the convey belt, wherein the charger starts charge application to the surface of the convey belt such that that portion on the surface of the convey belt to which the charges are applied by the charges reaches the countercharge applying unit at substantially the same timing as a timing at which a leading end of the fed transfer medium reaches the countercharge applying unit, and the charger ends charge application to the surface of the convey belt such that that portion on the surface of the convey belt to which the charges have are applied passes through the countercharge applying unit at substantially the same timing as a timing at which a trailing end of the transfer medium passes through the countercharge applying unit.
The apparatus may have a color mode in which a color image is formed by using four color toners including yellow, magenta, cyan, and black, and a monochromatic mode in which a monochromatic image is formed by using black toner, in the monochromatic mode, the convey belt and, of the plurality of image carriers, an image carrier on which a toner image is to be formed with a color toner may be separated from each other, and the transfer medium may be chucked onto the convey belt by the chucking unit only in the monochromatic mode.
An apparatus for forming an image according to the present invention comprises a plurality of image carriers on which a toner image is to be formed, a developing unit for forming a toner image on the image carriers, a convey belt disposed in contact with the image carriers and adapted to convey a transfer medium, a chucking unit for electrostatically chucking the transfer medium onto the convey belt, and a transfer unit to which a bias is applied in a transfer region where the image carriers are in contact with the convey belt, to transfer the toner image from the image carriers to the transfer medium, the transfer unit being electrically grounded while the bias is not applied thereto.
The apparatus may have a color mode in which a color image is formed by using four color toners including yellow, magenta, cyan, and black, and a monochromatic mode in which a monochromatic image is formed by using black toner, in the monochromatic mode, the convey belt and, of the plurality of image carriers, an image carrier on which a toner image is to be formed with a color toner may be separated from each other, and the transfer unit may be electrically grounded only in the monochromatic mode while the bias is not applied thereto.
An image forming apparatus according to the present invention may comprise a plurality of image carriers
Takahashi Masashi
Yoshida Minoru
Kabushiki Kaisha Toshiba
Ngo Hoang
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