Electrophotographic marking is a well known and commonly used method of copying or printing original documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto the latent image so as to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a typical black and white electrophotographic printing machine. Electrophotographic printing can also produce color images by repeating the above process for each color of toner that is used to make the color image. For example, the charged photoreceptive surface may be exposed to a light image which represents a first color, say black. The resultant electrostatic latent image can then be developed with black toner particles to produce a black toner image which is subsequently transferred and fused onto a substrate. The process can then be repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. If the toner particles are placed in a superimposed registration the desired composite color image is produced. This process is sometimes referred to either as the REaD process (Recharge, Expose, and Develop) or as the IOI process (Image On Image).
While electrophotographic printing has been very successful, the rapid growth of the computer industry has created a tremendous demand for desktop printing machines, particularly color desktop printing machines. Desirable features of desktop color printing machines include high print quality, high speed printing, low cost, and small size. Those desirable characteristics are difficult to simultaneously achieve using prior art electrophotographic printing machine architectures. Therefore, new electrophotographic color printing architectures which might enable high quality, relatively high speed printing at low cost in a desktop printing machine would be highly desirable.
Various approaches have been devised to produce multicolor color copies. The following disclosures may be useful references:
U.S. Pat. No. 3,392,667 PA1 Patentee: Cassel et al. PA1 Issued: Jul. 16, 1968 PA1 U.S. Pat. No. 3,399,611 PA1 Patentee: Lusher PA1 Issued: Sep. 3, 1968 PA1 U.S. Pat. No. 3,955,530 PA1 Patentee: Knechtel PA1 Issued: May 11, 1976 PA1 U.S. Pat. No. 3,957,367 PA1 Patentee: Goel PA1 Issued: May 18, 1976 PA1 U.S. Pat. No. 4,348,098 PA1 Patentee: Koizumi PA1 Issued: Sep. 7, 1982 PA1 U.S. Pat. No. 4,515,460 PA1 Patentee: Knechtel PA1 Issued: May 7, 1985 PA1 U.S. Pat. No. 4,588,279 PA1 Patentee: Fukuchi et al. PA1 Issued: May 13, 1986 PA1 U.S. Pat. No. 4,935,788 PA1 Patentee: Fantuzzo et al PA1 Issued Jun. 19, 1990 PA1 U.S. Pat. No. 5,254,424 PA1 Patentee: Felder PA1 Issued: Oct. 19, 1993 PA1 U.S. Pat. No. 5,352,558 PA1 Patentee: Simms et al PA1 Issued: Oct. 4, 1994 PA1 U.S. Pat. No. 5,355,201 PA1 Patentee: Hwang PA1 Issued: Oct. 11, 1994
The disclosures of the above-identified patents may be briefly summarized as follows:
U.S. Pat. No. 3,392,667 discloses a plurality of print cylinders having gravure engravings on their peripheries. Powder feed hoppers having rotating brushes apply powder to the print cylinders. The powder images from the print cylinders are transferred to an offset roller in superimposed registration with one another. The resultant powder image is then transferred from the offset roller to paper or sheeting.
U.S. Pat. No. 3,399,611 describes four image transfer stations disposed about the periphery of a rotatable cylindrical metal drum. Each image transfer station is basically the same and includes a photoconductive drum charged by a charging wire and then rotated into alignment with an image exposure station to record a latent image thereon. Powder particles are then cascaded across the latent image to develop it. The powder image is then transferred to the surface of the metal drum. The powder particles are of different colors. The completed powder image is transferred from the metal drum to an article to be decorated.
U.S. Pat. No. 3,955,530 discloses a color image forming electrophotographic printing machine. Different color developers are used to develop the latent images recorded on the photoconductive drum. Each developed image is sequentially transferred to an intermediate transfer drum. A cleaning blade is used to clean the photoconductive drum between developing different color developers. The complete image is transferred from the intermediate drum to a copy sheet.
U.S. Pat. No. 3,957,367 describes a color electrophotographic printing machine in which successive different color toner powder images are transferred from a photoconductive drum to an intermediate roller, in superimposed registration with one another, to an intermediary roller. The multi-layered toner powder image is fused on the intermediary roller and transferred to the copy sheet.
U.S. Pat. No. 4,348,098 discloses an electrophotographic copying apparatus which uses a transfix system. In a transfix system, the developed image is transferred from the photoconductive member to an intermediate roller. The intermediate roller defines a nip with a fixing roller through which the copy sheet passes. The developed image is then transferred from the intermediate roller to a copy sheet. The developing unit of the copying apparatus may either be a dry or wet type.
U.S. Pat. No. 4,515,460 describes a color electrophotographic copying machine in which four developer units develop four latent images recorded on a photoconductive drum with different color toner particles. The different color toner powder images are transferred to an endless belt in superimposed registration with one another. The resultant toner powder image is then transferred from the belt to a copy sheet.
U.S. Pat. No. 4,588,279 discloses an intermediate transfer member that has a dry toner image transferred thereto from the surface of a toner image forming member. The toner image is then transferred from the transfer member to a recording paper.
U.S. Pat. No. 4,935,788 discloses a multicolor printing system that uses liquid developing and an intermediate member.
U.S. Pat. No. 5,254,424 discloses a liquid developer material which contains toner particles formed from a urethane modified polyester.
U.S. Pat. No. 5,352,558 discloses a liquid developer system which uses an absorbing belt.
U.S. Pat. No. 5,355,201 discloses an apparatus for developing an electrostatic latent image with liquid toner.
Additionally, copending and commonly assigned U.S. Patent application, "Split Recharge Method and Apparatus for Color Image Formation," Ser. No. 08/347,617 discloses a split recharge configuration wherein a first corona generating device recharges a charge retentive surface having a developed to a higher absolute potential than a predetermined potential, and then a second corona generating device recharges the surface to the predetermined potential.