Electrophotography – Image formation – Transfer
Reexamination Certificate
1999-07-23
2001-02-13
Brase, Sandra (Department: 2852)
Electrophotography
Image formation
Transfer
C399S066000, C399S101000
Reexamination Certificate
active
06188863
ABSTRACT:
The present invention relates generally to an electrostatographic printing machine, and, more specifically, concerns an apparatus for assisting the transfer of a toned image from an imaged surface to a copy sheet with the assistance of electrostatic charges.
In a typical electrostatographic copying or printing process using electrophotography, a photoconductive member is charged to a substantially uniform potential and the charged portion of the photoconductive member is subsequently exposed to a light image of a document being reproduced or printed. Exposure of the charged photoconductive member selectively dissipates the charge thereon in the irradiated areas so as to record on the photoconductive member an electrostatic latent image corresponding to the informational areas contained within the original document. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material is made from toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image to form a toner powder image on the photoconductive member. The toner powder image is then transferred from the surface of the photoconductive member to a copy substrate such as a sheet of paper. Thereafter, heat or some other treatment is applied to the toner particles to permanently affix the powder image to the copy substrate.
The electrophotographic printing process described above is well known and is commonly used for light lens copying of an original document and for electrostatographic printing such as, for example, digital printing where the latent image is produced by a modulated laser beam, or ionographic printing and reproduction, where charge is selectively deposited on a charge retentive surface in response to an electronically generated or stored image.
The process of transferring charged toner particles from an image bearing member such as the photoconductive member to an image support substrate such as the copy sheet is enabled by overcoming adhesive forces holding the toner particles to the image bearing member. Typically, transfer of developed toner images in electrostatographic applications is accomplished via electrostatic induction using a corona generating device, wherein the image support substrate is placed in direct contact with the developed toner image on the photoconductive surface while the reverse side of the image support substrate is exposed to a corona discharge for generating ions having a polarity opposite that of the toner particles, to electrostatically attract the toner particles from the photoreceptive member and transfer the toner particles to the image support substrate. An exemplary ion emission corotron transfer system is disclosed in U.S. Pat. No. 2,836,725.
As described, the typical process of transferring development materials in an electrostatographic system involves the physical detachment of charged toner particles from a selectively charged image bearing surface and transfer-over to an image support substrate via electrostatic force fields. A critical aspect of the transfer process involves the application and maintenance of high intensity electrostatic fields in the transfer region for overcoming the adhesive forces acting on the toner particles as they rest on the surface of the selectively charged imaging member. In addition, other forces, such as mechanical pressure or vibratory energy, have been used to support and enhance the transfer process. Careful control of electrostatic fields and other forces is essential for inducing the physical detachment and transfer-over of the charged toner particles without scattering or smearing of the developer material which may result in an unsatisfactory output image.
In addition to careful control of electrostatic fields and other forces when electrostatically transferring a toner powder image to a copy sheet, it is generally necessary for the copy sheet to be in intimate contact with the toner particles on the selectively charged imaging surface. However, the interface between the selectively charged imaging surface and the copy substrate is rarely uniform. In particular, non-flat or uneven image support substrates, such as copy sheets that have been mishandled, paper that has been left exposed to the environment, or substrates that have previously passed through a fixing operation (e.g., heat and/or pressure fusing) often tend to yield imperfect contact with the photoconductive surface. Some printing applications require imaging onto high quality papers having surface textures which prevent intimate contact of the paper with the developed toner images. In duplex printing systems, even initially flat paper can become cockled or wrinkled as a result of paper transport and/or the first side fusing step. Also, color images can contain areas in which intimate contact of toner with paper during the transfer step is prevented due to adjacent areas of high toner pile heights. The lack of uniform intimate contact between the belt and the copy sheet in these situations can result in spaces or air gaps between the developed toner powder image on the selectively charged imaging surface and the copy sheet. When spaces or gaps exist between the developed image and the copy substrate, various problems may result. For example, there is a tendency for toner not to transfer across gaps, causing variable transfer efficiency and, under extreme circumstances, creating areas of low toner transfer or even no transfer, resulting in a phenomenon known as image transfer deletion. Clearly, image transfer deletions are very undesirable in that useful and necessary information and indicia may not be reproduced on the copy sheet.
Transfer deletion has been addressed through various approaches. The following disclosures may be relevant:
U.S. Pat. No. 4,947,214 Patentee: Baxendell, et al. Issued: Aug. 7, 1990
U.S. Pat. No. 5,227,852 Patentee: R. Smith et al. Issued: Jul. 13, 1993
U.S. Pat. No. 5,247,335 Patentee: R. Smith et al. Issued: Sep. 21, 1993
U.S. Pat. No. 5,300,993 Patentee: Vetromile Issued: Apr. 5, 1994
U.S. Pat. No. 5,300,994 Patentee: Gross et al. Issued: Apr. 5, 1994
U.S. Pat. No. 5,539,508 Patentee: Piotrowski et al. Issued: Jul. 23, 1996
U.S. Pat. No. 5,568,238 Patentee: Osbourne et al. Issued: Oct. 22, 1996
U.S. Pat. No. 5,613,179 Patentee: Carter et al. Issued: Mar. 18,1997
U.S. Pat. No. 5,720,094 Patentee: Carter et al. Issued: Feb. 24, 1998
The foregoing disclosures may be briefly summarized as follows: U.S. Pat. No. 4,947,214, U.S. Pat. No. 5,227,852, U.S. Pat. No. 5,300,393, U.S. Pat. No. 5,300,994, U.S. Pat. No. 5,539,508 and U.S. Pat. No. 5,568,238 disclose a system for transferring a developed image from a photoconductive surface to a copy sheet, including a corona generating device and a transfer assist blade. The blade is moved from a nonoperative position spaced from the copy sheet, to an operative position, in contact with the copy sheet for pressing the copy sheet into contact with the developed image on the photoconductive surface to substantially eliminate any spaces therebetween during the transfer process.
U.S. Pat. No. 5,247,335 discloses a transfer blade for ironing a sheet against a photoreceptor belt during transfer, thereby smoothing out deformities that cause deletions. The transfer blade includes a flexible tip to absorb the impact of the blade as it contacts the paper and a spring load to limit and control the force applied to the sheet.
The entire disclosures of the above-referenced patents are hereby incorporated by reference for their relevant teachings.
As taught in the prior art, no portion of the transfer assist blade should contact the imaging surface since such contact will result, in most instances, in the pick up of residual dirt and toner from the imaging surface onto the portion of the transfer assist blade that contacts the imaging surface. In order to ensure that a transfer assist blade contacts
Gross Robert A.
Sass Douglas W.
Brase Sandra
Xerox Corporation
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