Electrophotography is a dry copy process whereby copies of documents are made by setting up an electrostatic image of an original document and transferring that image to copy paper. Also known as xerography, electrophotography has become a standard process for creating copies of documents in a host of environments including offices, educational institutions and the like. The fundamental principles of electrophotography are well known to those skilled in the art.
In more recent years, systems for color electrophotography have been created. In many respects, the process of color electrophotography is analogous to standard three-color printing processes used in the more conventional printing arts. Conventional three-color printing component images, commonly referred to as color separations, are created by photographing the original through appropriate filters. Each of the separations is in turn made into a separate printing plate. During the printing process, each plate is inked with an appropriate color determined by the filter used in making the original separation. The printing press is adjusted for proper registration--alignment of the separate color component images with each other. Once the press is properly adjusted, multiple copies of the original color image may be faithfully reproduced.
As is known to those skilled in the art, in monochromatic electrophotography an optical image of an original to be copied is focused onto an electrostatic medium. An electrostatic image of the original is formed on the electrostatic medium, or photoreceptor. Toner materials are then brought into contact with the photoreceptor and held in place on the appropriate portions of the electrostatic image by electrostatic forces. The photoreceptor carrying the electrostatic image is brought into contact with an image receptor which, in the most common applications of electrophotography, is a sheet of paper. Electrostatic charging techniques are used to transfer the toner from the photoreceptor to the image receptor. The toner materials are normally plastics which melt at a predetermined temperature and have appropriate color characteristics once they are melted. The image receptor with these toner particles is passed through a fuser, which is a station in the path of the image receptor. The fuser heats the transferred toner and thereby fixes the image onto the image receptor.
In color electrophotography, three separate color filters are typically used to create three separate color component images in a manner analogous to the creation of color separations in color printing. Each image is developed with a toner having the appropriate color characteristics. Each developed color component image is in turn transferred to the image receptor and overlaid upon the previous image to provide a composite image. The image receptor, typically paper, carrying the composite image is then passed through a fuser in a conventional manner.
It is known in the art of color electrophotography to include an intermediate transfer medium upon which each developed color component image is deposited, between the above-described photoreceptor and the ultimate image receptor or paper. In this way, a composite developed image is built up, one color component image at a time, until an overlaid composite color image, having portions of all three of the color component toners thereon, is created on the transfer medium. Once this is accomplished, the composite image on the transfer medium is transferred to the paper which then passes through the fuser in the normal fashion.
As noted above, color electrophotography or color xerography, is conceptually quite similar to conventional color printing. However, there is a significant difference in the economics of scale. Most importantly, color printing is rarely undertaken for small numbers of copies. In practical color printing environments, the person controlling the color printer normally has ample opportunity to make sure the elements of the press are properly aligned so that proper registration is obtained. In the absence of proper registration, the individual color component images are misaligned and the result is a fuzzy image, with edges of objects being outlined inappropriately with portions of one of the color components.
However, in typical color electrophotography environments, the user is in a hurry. He may need only one or two copies and generally he has neither the training or inclination to adjust for proper registration. Therefore, in prior art color electrophotographic machines, the mechanical elements carrying the photoreceptor medium, the intermediate transfer medium, if used, and the paper have had to be machined to extremely close tolerances in order to maintain proper registration.
In the prior art, this has only been practical by using relatively large drums to carry the photoreceptor and critically machined, and therefore expensive, gearing arrangements by which the entire mechanism is driven from a common prime mover. Naturally, as these mechanical components age and the mechanical elements controlling registration suffer wear, registration, and therefore copy quality, suffers significantly. Therefore, maintenance of critical mechanical alignments in prior art full color electrophotographic systems have been one of the principal factors in keeping the cost of such machines very high with respect to the cost of monochrome copiers.
From the foregoing it will be appreciated that the prior art has not produced a full color print engine which will maintain the critical registration necessary to produce accurate full color copies by overlaying separate color component images without the use of relatively expensive finely machined mechanical parts.