1. Field of the Invention
The present invention relates to an image forming apparatus in which toner images formed on an image bearing member are successively transferred onto an intermediate transfer member, and then the toner images on the intermediate transfer member are transferred onto a transfer material collectively.
2. Related Background Art
There has been proposed an image forming apparatus in which toner images formed on a drum-shaped electrophotographic photosensitive member (referred to as "photosensitive member" hereinafter) as an image bearing member are firstly-transferred onto an intermediate transfer member and then the toner images on the intermediate transfer member are secondary-transferred onto a transfer material collectively, thereby forming an image. Such an image forming apparatus is useful as a color image forming apparatus or a multi-color image forming apparatus in which a plurality of color component images corresponding to color image information or multi-color image information are successively transferred in a superimposed fashion to reproduce a color image or a multi-color image as an imaged print. According to such an image forming apparatus, an image having less deviation in superimposed color components can be obtained.
In the image forming apparatus having the intermediate transfer member, after the toner images are secondary-transferred from the intermediate transfer member to the transfer material such as a paper sheet, there remains residual toner on the intermediate transfer member. Removal of such residual toner is a technical problem which arises during operation of the apparatus. In order to solve such a technical problem, there has been proposed a so-called "first-transferring and simultaneous cleaning" technique in which a charge roller (as toner charging means) is urged against the intermediate transfer member. And, during next firstly-transferring, by applying, to the charge roller, a bias voltage for charging the residual toner to a polarity opposite to the potential on the photosensitive member, the residual toner on the intermediate transfer member is returned to the photosensitive member at the same time when the next toner image is transferred from the photosensitive member onto the intermediate transfer member.
In the conventional image forming apparatus having the intermediate transfer member, the first-transferring and simultaneous cleaning is achieved by simultaneously replacing toners having opposite polarity with each other at a firstly-transfer nip between the photosensitive member and the intermediate transfer member. In the secondary-transferring for transferring the toner images from the intermediate transfer member to the transfer material, although almost all of the toner is transferred to the transfer material, a very little amount of toner remains on the intermediate transfer member as residual toner. Thus, when same images are formed (printed) continuously, since the toner images are successively formed on the same area of the intermediate transfer member, the residual toner on the intermediate transfer member trying to return to the photosensitive member is blocked by the toner being transferred from the photosensitive member to the intermediate transfer member. As the result, the residual toner charged with opposite polarity is gradually accumulated on the intermediate transfer member.
Consequently, when a different image is transferred after the same images were transferred continuously, the accumulated residual toner is transferred onto the photosensitive member while absorbing the toner to be transferred to the intermediate transfer member, thereby generating a ghost image (negative ghost) in the next print image. This is true particularly when mono-color (for example, black) images are printed continuously.