Electrophotography – Image formation – Transfer
Reexamination Certificate
2002-10-04
2004-08-24
Chen, Sophia S. (Department: 2852)
Electrophotography
Image formation
Transfer
C399S066000, C399S314000
Reexamination Certificate
active
06782227
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority to Japanese Patent Application No. 2001-308529 filed in the Japanese Patent Office on Oct. 4, 2001, the disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transfer device and an image-forming apparatus including the transfer device, and more particularly to the control of an adsorbing bias applied to a surface of a recording medium so that the recording medium is electrostatically adsorbed to a transfer element.
2. Discussion of the Background
In an image-forming apparatus such as a copying machine, a printer, a facsimile machine, or other similar image-forming apparatus, an electrostatic latent image formed on a photoreceptor functioning as an image carrier is developed with toner to obtain a visual image, i.e., a toner image. Next, the toner image is transferred onto a recording medium such as a recording sheet in a transfer process.
With regard to the transfer process, in the case of single color image formation, a toner image may be directly transferred from a photoreceptor to a recording sheet. In the case of multi-color image formation, toner images of different colors formed on a photoreceptor may be sequentially transferred onto an intermediate transfer element, i.e., a primary transfer, while being superimposed upon each other on the intermediate transfer element. The superimposed multi-color image on the intermediate transfer element may be collectively transferred onto a recording sheet, i.e., a secondary transfer.
An intermediate transfer element may be shaped in the form of drum or a belt. When the intermediate transfer element is shaped in the form of a belt (hereafter referred to as an “intermediate transfer belt”), instead of collectively transferring a superimposed multi-color image onto a recording sheet fed from a sheet feeding device, it has been proposed that a recording sheet is adsorbed to the intermediate transfer belt. Further, toner images of different colors formed on respective photoreceptors are sequentially transferred onto the recording sheet, which moves together with the intermediate transfer belt, while being superimposed upon each other on the recording sheet. This technology is described in, for example, Japanese Laid-open Patent Publication Nos. 63-118780, 5-270686, and 8-152790.
The above-described Japanese Laid-open Patent Publication Nos. 63-118780, 5-270686, and 8-152790 employ a construction in which a plurality of photoreceptors arranged along the intermediate transfer belt in the direction of movement of the intermediate transfer belt, and toner images of different colors formed on the respective photoreceptors are sequentially transferred onto one side of a recording sheet adsorbed to the intermediate transfer belt. That is, the construction allows an image to form on only one side of the recording sheet. Another construction is described, for example, in Japanese Laid-open Patent Publication No. 2001-109325, in which toner images of different colors formed on respective photoreceptors are sequentially transferred onto a first (front) side of a recording sheet, and then subsequent toner images of different colors are sequentially transferred onto a second (rear) side of the recording sheet.
An image-forming apparatus in which a plurality of image-forming units (including photoreceptors) are arranged along an intermediate transfer belt as an intermediate transfer element in the direction of movement of the intermediate transfer belt is called a tandem-type image-forming apparatus. A tandem-type image-forming apparatus often uses toner of four colors, including black toner.
In the above-described background transfer constructions, toner images of different colors are sequentially transferred from photoreceptors onto a recording sheet by applying a transfer bias to the recording sheet via an intermediate transfer belt (hereafter referred to as a “transfer belt”) each time the recording sheet passes transfer positions. As a result, the charge of the recording sheet increases by applying a transfer bias to the recording sheet.
For example, in a multi-color image-forming apparatus that has been widely used recently, an electrostatic latent image formed on a negatively charged photoreceptor is developed with negatively charged toner and formed into a toner image. In the transfer process, the toner image is transferred onto a recording sheet by applying a transfer bias having a positive polarity with respect to the recording sheet. In this image-forming process, each time the recording sheet is separated from the photoreceptor when passing transfer positions, electric discharge is generated between the photoreceptor and the recording sheet. As a result, the negative charge of the recording sheet becomes higher each time the recording sheet passes the transfer positions.
When forming images on both sides of a recording sheet, after the transfer process for the first side of the recording sheet is completed, a subsequent transfer process for the second side of the recording sheet is performed. In this case, a bias for adsorbing a recording sheet to a transfer belt (hereafter referred to as an “adsorbing bias”) to be applied to the second side of the recording sheet must be changed from that applied to the first side of the recording sheet. The adsorbing bias is changed according to the change of electric resistance of the recording sheet caused by the change of humidity of the recording sheet in the fixing process performed after the transfer process for the first side of the recording sheet. In the fixing process, a toner image is fixed onto the recording sheet under the influence of heat and pressure. Thus, by having to change the adsorbing bias applied to the second side of the recording sheet from that applied to the first side of the recording sheet, adsorbing bias control becomes complicated.
In order to improve the above-described negative charging condition of a recording sheet, a background image-forming apparatus may use a discharging AC charger. For example, Japanese Laid-open Patent Publication No. 7-199679 describes an image-forming apparatus in which, after completion of a transfer process for the first side of a recording sheet, a discharging AC charger discharges the recording sheet. Subsequently, a transfer process for the second side of the recording sheet is performed without changing the adsorbing bias to be applied to the second side of the recording sheet from that applied to the first side of the recording sheet.
However, with increasing demands for environmental protection, such as reduction of ozone production, and for cost reduction of an apparatus, an image-forming apparatus tends to have a construction that lacks a discharging device like the discharging AC charger. In a construction without a discharging device, a negative electric charge given to a recording sheet in a transfer process for the first side of the recording sheet must be cancelled by re-charging the recording sheet, before performing a transfer process for the second side of the recording sheet. In this case, an adsorbing bias having an opposite polarity (i.e., a positive polarity) may be required to be applied to the recording sheet so as to cancel the negative electric charge given to the recording sheet in transfer operations which are repeated four times in the transfer process for the first side of the recording sheet.
When forming images on both sides of a recording sheet, the charging conditions of the first and second sides of the recording sheet may be different, due to the difference of humidity between the first and second sides of the recording sheet. When forming an image on the second side of a recording sheet, since the percentage of moisture content of the recording sheet decreases after the fixing process for the first side of the recording sheet, the second side of the recording sheet tends to have a high resistance compared to the first side of the recording
Ishibashi Hitoshi
Takehara Atsushi
Chen Sophia S.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Ricoh & Company, Ltd.
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