1. Field of the Invention
The present invention relates to a multicolor or full-color image forming apparatus for copying machines, electrophotographic printers and the like.
2. Related Background Art
In the past, electrophotographic systems, ink jet recording systems, thermal transfer recording systems and the like have been proposed as recording systems for forming a full-color image. Among them, as disclosed in U.S. Pat. No. 4,483,610, an electrophotographic system utilizing a single photosensitive member can be operated at a high speed and can use a plain paper therefore it has recently been developed more and more. In order to achieve the high speed operation, a technique of increasing the number of revolutions of the photosensitive member has been proposed. However, to further increase the speed of the operation, a system having a plurality of photosensitive members corresponding to the number of colors tends to be esteemed.
More specifically, as disclosed in U.S. Pat. No. 5,014,094, a recording system having four electrophotographic photosensitive members corresponding to cyan, magenta, yellow and black colors for obtaining a full-color image is suitable for the high speed operation, since it can obtain the full-color image merely by feeding a transfer sheet in a straight feeding path.
However, in an image forming apparatus having a plurality of photosensitive members corresponding to the number of colors, when a mono-color image is copied from a black and white original other than a full-color original, the image forming operation including the latent image formation, development, transfer, cleaning and the like is performed only with respect to the photosensitive member associated with the black color. Thus, a surface of this photosensitive member tends to wear faster and greater than the other photosensitive members, with the result that the black color photosensitive member must be replaced more frequently than the other color photosensitive members, thereby reducing the efficiency and increasing maintenance.