A very early approach to making xeroprinting masters is shown in Schaffert, ELECTROPHOTOGRAPHY, page 209, 2nd Edition (1975). According to that approach an insulating toner is fused in imagewise configuration onto a conductive substrate and the substrate is repeatedly charged and toned to create toner images which are transferred to receiving sheets. Unfortunately, this type of master can produce poor image reproduction due to lack of uniformity of charge on the insulating toner. This nonuniformity is believed to be due to the conductive portions of the master affecting the electrical field associated with the insulating portions during the charging process.
Photoconductive elements have been used as masters. The toner image can be formed electrophotographically on photoconductive elements rather than transferred to them as in the Schaffert master. In use, the entire master with the toner image is uniformly charged. It is then blanket exposed to radiation to which the photoconductive surface is sensitive thereby discharging the portions not covered by the image. The remaining electrostatic image is toned and transferred as in previous processes. This process does not create the lack of uniformity found with the original Schaffert xeroprinting masters if the original charging is in the dark, but it requires a blanket exposure step.
U.S. Pat. No. 3,271,146 shows a xeroprinting process using a zinc oxide photoconductive element. Zinc oxide will hold a negative charge in the dark, but is conductive to positive charges. The photoconductiveness to negative charge is used to electrophotographically form the master and its conductivity to positive charge is used in the xeroprinting process.
U.S. Pat. No. 4,804,602, Buettner et al, issued Feb. 14, 1989, and other references note the phenomenon of positive charge injection. Some photoconductor-electrode combinations exhibit the characteristic of positive charge injection at the interface between the electrode and the photoconductor. With these materials, when negative charges are sprayed on the surface opposite the electrode, the positive charges or holes migrate to the surface and neutralize them. The same materials hold positive charges in the dark and may or may not be photoconductive, that is, transport the charges to the electrode when exposed to light.
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