As an occasion for making full-color leaflets or posters, there is a demand for full-color printing for a low number of prints unlike that printed by a commercial printing house, but is only in the range of several tens or hundreds, and yet beautiful color reproduction is still required.
In the area where the full-color printing competes with ordinary printing, the resolution higher than that for conventional electrophotographic system, a broader color reproduction area, high speed printing and lower printing cost are required.
As one of methods for forming full-color images, there is known an image forming apparatus wherein respective color toner images are formed on electrophotographic photoreceptors (hereinafter referred to simply as a photoreceptor) for respective colors, which is called a tandem system, and these color images are superposed on an intermediate transfer body or on a recording sheet (in the invention, the intermediate transfer medium and the recording sheet are classified as recording material) (Patent Document 1). This tandem system is fitted to high speed printing, because both monochromatic printing and color printing can be carried out at the same speed. Specifically, based on color-separated image information, namely, on image information corresponding to each color of yellow, magenta, cyan and black, an electrostatic latent image is formed on each photoreceptor separately, and color toner images for respective colors of yellow, magenta, cyan and black corresponding to respective colors are formed on respective electrostatic latent images, thus, these toner images are superposed on the intermediate transfer body or a recording sheet to form a color image. Namely, in the color image forming apparatus of a tandem system, toner images each having a different hue formed by plural image forming units are superposed on the intermediate transfer body or the recording sheet to form a color image, thereby, it is possible to develop an image forming apparatus of an electrophotographic system capable of forming color images at high speed.
On the other hand, digital full-color image forming apparatuses include a top-level machine for reproducing delicate color cast on a high fidelity basis, and in these digital full-color image forming apparatuses, it is necessary that a latent image of digital color pixels (separate color dot pixels) formed on the electrophotographic photoreceptor is reproduced on a high fidelity basis as a toner image, and for that purpose, it is important to select an electrophotographic photoreceptor capable of making each color pixel to be a latent image on a high fidelity basis and to select toner capable of making each dot latent image to be a visual image on a high fidelity basis. Namely, it is necessary to develop an electrophotographic photoreceptor and toner both capable of reproducing delicate color cast on a high fidelity basis. It is further necessary to develop writing of a latent image, namely, to develop an exposure system.
In the case of the electrophotographic photoreceptor used for the aforesaid tandem system, a separate electrophotographic photoreceptor is used for a toner image of each color. Therefore, a mottle of color and color slippage or doubling tend to be caused if a performance of each electrophotographic photoreceptor is not stable. For example, there is disclosed that an amorphous-silicon-based photoreceptor having high surface hardness and high durability makes it possible to obtain a high-definition image that is excellent in halftone reproducibility (Patent Document 2).
On the other hand, there is proposed (Patent Document 3) an image forming apparatus of a tandem system employing polymeric toner wherein particle sizes can be controlled to be uniform, as a method to improve color reproduction of digital full-color.
It is therefore expected that an image forming apparatus of a top-level type that makes it possible to obtain beautiful full-color images may be realized if the aforesaid amorphous-silicon-based photoreceptor and polymeric toner which will be described later are used together in the image forming apparatus of a tandem system stated above. Further, a competitive power against offset printing is increased, because the photoreceptor of an amorphous silicon type is highly durable and it reduces print cost per one sheet.
(Patent Document 1) TOKKAIHEI No. 10-20598
(Patent Document 2) TOKKAI No. 2002-123020
(Patent Document 3) TOKKAI No. 2001-318482
However, it was confirmed that excellent halftone reproduction and color reproduction on a high fidelity basis are not realized even if a full-color image is formed by using the image forming apparatus of a tandem system employing the aforesaid amorphous-silicon-based photoreceptor and polymeric toner, and it is difficult to realize a technology to form full-color images which are required to reproduce halftone color cast required for the top-level machine on a high fidelity basis, even if known technologies are combined simply.
However, when a diameter of a beam for exposure and a diameter of toner are made to be small simply, color slippage is caused and color reproducibility is deteriorated, because dot slippage in exposure is reproduced by toner on a high fidelity basis. Further, indentation of a thin line was also detected, which was a problem.
Namely, an object of the invention is to provide an image forming apparatus capable of forming a full-color image that is required to reproduce, on a high fidelity basis, the high resolution and halftone color cast which are required for a top-level machine.