1. Field of the invention
The present invention relates to an improved intermediate transfer device which transfers toner on a photoconductor first to an intermediate transfer belt and then onto a paper, and to an improved toner transfer device (hereinafter called the "1st toner transfer device) which transfers the toner on the photoconductor to the intermediate transfer belt in the intermediate transfer device.
2. Description of the prior art
In recent image forming apparatuses for full color image forming, toner images of yellow, magenta, and cyan are separately formed on a photoconductor, and the thus formed toner images are transferred onto an intermediate transfer belt in such a way that one toner image is superimposed on top of the other, the transferred image being further transferred onto copy paper. Once the colored toner images are superimposed one on top of the other on the intermediate transfer belt, the transfer to the copy paper can be accomplished in a single operation, so this method can prevent copy paper from being damaged.
In the conventional 1st transfer device, toner transfer from the photoconductor to the intermediate transfer belt has been accomplished using a transfer charger which generates corona. However, the transfer charger that utilizes corona discharge for transfer of toner has had the following difficulties in terms of toner transfer.
(1) Transfer area is limited, and uneven transfer may be caused because of unevenness in the potential on the reverse surface of the intermediate transfer belt. (Refer to FIG. 9)
(2) When a low-resistivity (10.sup.8 .OMEGA. cm or lower) intermediate transfer belt is used, the transfer belt becomes charged because of the corona, and good quality transfer cannot be obtained for the 2nd transfer (transfer of the toner on the intermediate transfer belt to the copy paper). Therefore, in the conventional 1st transfer device, it has been necessary to restrict the resistivity of the intermediate transfer belt, or to provide a device which removes the charge from the intermediate transfer device prior to the 2nd transfer.
(3) Insufficient pressure between the photoconductor and the intermediate transfer belt has resulted in uneven superposing of the three color image layers, missing portions of the three color image layers, and other defects, that affect the copy quality.