A bimetal plate usually has an image-forming copper layer plated onto a sheet of aluminum or steel. The processing of these plates removes the image-forming copper in the nonimage areas. This gives an image in very slight relief which is exactly the opposite condition of a deep-etch plate which recesses the image base slightly below the nonimage areas of the plate.
Cleaning.The copper surface is counteretched with a dilute acid, usually a weak solution of sulphuric acid.
Coating.The cleaned plate is coated with a stencil-resist type of coating (usually deep-etch formula) in the whirler and dried.
Exposure.The plate is exposed through a negative to provide the proper control step on the sensitivity guide.
Development.Deep-etch developer is applied to the plate until the nonimage areas are removed. The sensitivity guide should be observed constantly for proper development.
Copper etching.The nonimage copper must be removed. This is done by the application of a special etch that will not attack the base metal of aluminum or stainless steel. A solution of ferric chloride or ferric nitrate is used. This leaves a stencil-covered copper image on the base sheet.
Removing the stencil.This removal is accomplished by scrubbing with warm water and a fine abrasive household cleanser and a stiff scrub brush. Abrasives which would deeply scratch the copper must be avoided. The copper image is now present on the aluminum or steel base.
Rubbing up.The copper image is made ink receptive by flushing the plate with weak sulphuric acid or cuprasol and rubbing with a pad of stiff rub-up ink. The image is now complete.
Etch and gum.An aluminum etch is applied to the nonimage areas of the plate and a coating of gum is applied and rubbed down smooth. The plate is ready for press.
BIMETAL POSITIVE PLATE
A similar process to produce a bimetal plate from positives leaves a chromium nonimage area over a copper base plate which is slightly raised above the copper base. The procedures in this case remove the chromium from the image areas with the copper base metal used as an image. This image is approximately 0.00005 inch below the non-image chromium surface. This plate provides a chromium metal which is much tougher than aluminum to resist pressure, friction, and wear on the press. The image is protected to a considerable degree because it is below the chromium surface.