The present invention relates to an electrophotographic apparatus and more particularly to a device for erasing electrostatic charges on a photosensitive member for use in the electrophotographic apparatus.
An electrophotographic apparatus in which a duplicated copy is obtained by forming an electrostatic latent image on a photosensitive member such as photoconductor, visualizing the latent image by toner particles and transferring the visualized toner image on an image receiving paper, has been well known. It is also known to provide in such an electrophotographic apparatus charges that are retained on the portion of the photoconductor other than the latent image forming region are erased by illumination of light, and adhesion of toner particles to an unused portion of the photoconductor, that is, consumption of toner particles is prevented and thus the load of a cleaning device can be decreased. For example, in the Japanese Patent Laid Open No. 50,945/74 and No. 147,336/75, a light shielding means such as a shutter is provided before a light source for erasing electrostatic charges on the photosensitive member, the light shielding means is controlled and driven in accordance with a sequence of a copying device, to thereby erase the unnecessary charges on the needless portion of the photosensitive member, has been disclosed. In the Japanese Patent laid Open No. 11,139/74 means for directing the illuminating light for copy to the surface of the photoconductor is provided near the copy table, so to erase unnecessary charges on the needless portion of the photosensitive member.
In the former technique light is projected on the photosensitive member by mechanical control so that the construction of the light projection device becomes complicated as compared with only electrical control, and results in trouble and thus it is very difficult to apply the former technique to a high speed copying device which needs very high response speed. In the latter technique, it is necessary to movably provide reflecting member which directs copy illuminating light onto the surface of the photoconductor at copy table so that the construction of the directing means becomes complicated and large in size as in the former case.
An electrophotographic apparatus for obtaining a plurality of duplicated copies from electrostatic latent image once formed on the photosensitive member by only one exposure has been provided. In such apparatus, for example, use is made of a photoconductor having good charge retentive characteristics, after once forming the latent image thereon, a developing process for visualizing the latent image to a toner image, and a transfer process for obtaining the duplicated copies by transferring the thus formed toner image to an image receiving paper are repeated with each other to obtain a plurality of duplicated copies. In this case a well known transferring system such as a corona discharging transfer system, biased roller transfer system or the like is utilized for transferring the toner image formed on the photoconductor onto the image receiving system, but in any case, the portion of the photoconductor other than the region on which the image receiving paper is present is subjected to unnecessary charge by corona discharge or the voltage applied to bias roller during transfer is that the toner particles are absorbed on the unnecessarily charged portion in the next developing process. In the corona discharge transfer system, absorption of the toner particles to the portion on which no image receiving paper is present so that the consumption rate of toner particles becomes fast and the life of the cleaning device becomes short. During plural transfer processes the image receiving paper is slightly deviated to place to the unused portion of the photoconductor (other than the region of the latent image to be formed on this portion) is transferred to the periphery of the image receiving paper as a black image so that the duplicated copy becomes inferior. In the biased roller transfer system there is a very small charge which is applied to the unused portion of the photoconductor by one time of transfer, but the toner particles adhered to this unused portion are attached to the bias roller when no image receiving paper is present on the photoconductor so that the back surface of the paper becomes dirty.