This invention relates to scavengeless development wherein a plurality of wire electrodes are positioned in a development zone intermediate a photoconductive imaging surface and a donor roll, and more specifically the present invention is directed to a method and apparatus for eliminating or minimizing toner accumulation on electrode wires and the accompanying uncontrolled powder clouding.
The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoreceptor. The photoreceptor comprises a charge retentive surface. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
After the electrostatic latent image is recorded on the photoconductive surface, it is developed by bringing a developer material into contact therewith. Two component and single component developer materials are commonly used.
A typical two component developer material comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single component developer material typically comprises toner particles without the carrier granules. Toner particles are attracted to the latent image thereby forming a toner powder image on the photoconductive surface. The toner powder image is subsequently transferred to a sheet of plain paper. Finally, the toner powder image is subjected to a combination of heat and pressure to permanently fuse it to a final substrate in image configuration.
Single component development systems use a donor roll for transporting charged toner to a development zone or nip defined by the donor roll and photoconductive member. The toner is attracted to the latent image recorded on the photoconductive member by a combination of mechanical and/or electrostatic forces. Scavengeless development is one type of single component development. One type of scavengeless development system uses a donor roll with a plurality of electrode wires closely spaced therefrom in the development zone. An AC voltage is applied to the wires to create a toner cloud in the development zone. The electrostatic fields generated by the latent image attract toner from the toner cloud to develop the latent image. Single component development systems appear to offer advantages in low cost and design simplicity.
Two component development systems have been used extensively in many different types of printing machines. A two component development system usually employs a magnetic brush developer roller for transporting carrier having toner adhering triboelectrically thereto. The electrostatic fields generated by the latent image attract the toner from the carrier so as to develop the latent image. In high speed commercial printing machines, a two component development system may have lower operating costs than a single component development system. Clearly, two component development systems and single component development systems each have their own advantages.
A combination of these systems was described at the 2nd International Congress on Advances in Non-impact Printing held in Washington, D.C. on Nov. 4-8, 1984, sponsored by the Society for Photographic Scientists and Engineers. At that time, Toshiba described a development system using a donor roll and a magnetic roller. The donor roll and magnetic roller were electrically biased. The magnetic roller transported a two component developer material to the nip defined by the donor roll and magnetic roller. Toner is attracted to the donor roll from the magnetic roll. The donor roll is rotated synchronously with the photoconductive drum with the gap between them being about 0.20 millimeters. The large difference in potential between the donor roll and latent image recorded on the photoconductive drum causes the toner to jump across the gap from the donor roll to the latent image so as to develop the latent image.
The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught by Gundlach, the xerographic contrast on the charge retentive surface or photoreceptor is divided into three levels, rather than two levels as is the case in conventional xerography. The photoreceptor is charged, typically to 900 volts. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.cad or V.sub.ddp). The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.dad or V.sub.c (typically 100 volts) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD) and the background areas exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500 volts) and is referred to as V.sub.white or V.sub.w. The CAD developer is typically biased about 100 volts closer to V.sub.cad than V.sub.white (about 600 volts), and the DAD developer system is biased about 100 volts closer to V.sub.dad than V.sub.white (about 400 volts).
The viability of printing system concepts such as tri-level, highlight color xerography requires development systems that do not scavenge or interact with a previously toned image. Since commercial development systems such as conventional magnetic brush development and jumping single component development interact with the image receiver, a previously toned image will be scavenged by subsequent development. Since the present commercial development systems are highly interactive with the image bearing member, there is a need for scavengeless or non-interactive development systems.
U.S. Pat. No. 4,984,019 granted to Jeffery Folkins on Jan. 8, 1991 relates an apparatus in which an contaminants are removed from an electrode positioned between a donor roller and a photoconductive surface. A magnetic roller is adapted to transport developer material to the donor roller. The electrode is vibrated to remove contaminants therefrom.
U.S. Pat. No. 5,134,442 granted to Folkins et al on Jul. 28, 1992 relates to an apparatus in which a developer unit of an electrophotographic printing machine has a plurality of wires which trap contaminants before reaching electrode wires positioned between a donor roller and a photoconductive surface. In addition, a circuit determines when defects and deletions are caused by contaminants spacing the electrode wires from the donor roller.
U.S. Pat. No. 5, 153,642 granted to Folkins et al on Oct. 6, 1992 relates an apparatus in which an electrostatic latent image recorded on a photoconductive member is developed with developer material stored in a developer housing. The developer material advances along a path of travel to a development zone closely adjacent to the latent image. A cleaner, positioned in the path of the developer material and spaced from the photoconductive member, cleans contaminants from the developer material without impeding the flow thereof. The cleaner has a multiplicity of fibers disposed in the path of travel of the developer material.
U.S. Pat. No. 5,212,037 granted to Julien et al on May 18, 1993 relates to a process for avoiding, or minimizing toner contamination of electrodes in a scavengeless electrophotographic imaging apparatus which comprises adding to the donor roll present in said apparatus a toner comprised of resin, pigment,charge additive, and a metal oxide, or a mixture of metal oxides.
Metering/charging blade structures fabricated from EPDM which have been used in wire electrode scavengeless development systems exhibit two severe failure modes. The two failure modes are toner accumulation on electrode wires and the accompanying uncontrolled powder clouding in the nip or development zone. Accumulation of toner on electrode wires used for scavengeless development of prior art devices cause image development failure due to the formation of large agglomerates of toner particles on the wires. These, in turn, can cause imperfections and vacancies in the toner cloud leading to insufficient development on the image bearing member. This shows up as undesirable streaks on the final developed copies.
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