Making negatives and plates for printing by electroerosion: I. Physical principles

Electroerosion printing involves removal of the aluminum overlayer from selected areas of a black-coated paper. “Direct negatives” as well as “direct plates” for use in offset lithographic printing may also be generated by electroerosion if a clear polymer sheet is used as the substrate instead of paper, and the black base layer is omitted. If such a substrate is metallized and written by electroerosion, the desired direct negative is created in principle since the metal stops transmitted light and the polyester does not. The direct plate is simultaneously created in principle since the aluminum is hydrophilic and the polyester is hydrophobic. Practical realization of these concepts required studies of the physical principles of the processes involved, which led to techniques for avoidance of mechanical scratching of the aluminum film during writing. For this purpose a mechanically hard underlayer was applied to the substrate under the aluminum, while a very thin lubricating overlayer having some electrical conductivity was applied over the aluminum. The underlayer consisted of silica particles in an organic binder, while the overlayer consisted of graphite particles in a binder. Although scratching is less for smooth than for rough underlayers, rough underlayers were preferred because they offered better writing reproducibility. In particular, debris created during writing was scoured away from the styli in rough-underlayer samples. For writing, a two-phase driver was used, in which the first phase provided a high current for Joule heating with consequent breaking of direct local aluminum-stylus contacts, while the second phase provided an arc which removed the remainder of the aluminum under the stylus.