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Diazo Coatings

( Originally Published 1963 )

Recently an entirely new technique of platemaking has found wide acceptance in the industry. This process eliminates all of the graining and surface preparation procedures encountered with regular surface plates. The manufacturer supplies a sheet of aluminum which has been grained and treated at the factory ready to be coated and processed. The nature of this protective coating is a very inert material which does not form oxidation contaminants of the surface, but which cannot be reused because the cost of reprocessing is too great. Since the plate is not to be reused, the thickness requirement of the plate is governed solely by the need for strength to process the image and print on the press. Zinc plates which were regrained were 0.020 inch or more in thickness, but a single-use aluminum plate in many cases is 0.012 or less. This is a very substantial saving in metal used for platemaking.

Coating.—The diazo coating can be applied to these aluminum plates in two ways. One method is to apply a pool of coating to the center of the plate and wipe this coating evenly over the plate with a suitable pad, with horizontal and vertical strokes. This method is very successful for a small number of plates but on many plates this procedure, as a continuous task, would be very strenuous. This is especially true of large-size plates. A second method is to apply the coating with rollers similar in arrangement to a washing machine wringer. Two rollers positioned vertically are driven by a variablespeed-drive motor. The lower roller is permitted to turn in a stainless steel trough filled with the diazo coating. The rollers have a means of adjusting the pressure between them so that proper pressure can be obtained to pass the plate between the rollers and place an even coating on the face side of the plate at the same time. It can easily be seen that hundreds of plates of any size can be coated in a matter of hours with very little effort on the part of the operator. The coating dries within a few seconds after it has passed through the rollers. The plate is immediately ready for exposure.

Exposure.—The plate is exposed in the same manner as any surface plate.

Development.—A single lacquer developer solution is applied to the plate and rubbed onto the coating until the image appears to be fully developed. A sensitivity guide is useful to determine correct exposure and development. No further treatment of the image is required.

Final gum.—If the plate is going to press within a few hours, gum is not required, but if the plate is to be held for several hours or days, a final gum should be applied to the plate and wiped down smooth. A very light gum solution of 3° Baume is adequate for protection.

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