Bimetal Plate

A strong, durable printing plate used in offset lithography. Bimetal plates, as their name indicates, utilize a metal base (either stainless steel or aluminum) which is plated with copper. Some bimetal plates also utilize a third metal. In the most common configuration, copper (which is highly oleophilic) is plated to aluminum or stainless steel (which are highly hydrophilic), the copper then being covered with a light-sensitive coating. The plate is brought into contact with a photographic negative of the material to be printed, and the light exposes only the image areas of the negative, hardening the coating. The non-image areas, unexposed and unhardened, are dissolved by chemical treatment. The copper in the non-image areas is removed with an etching solution, baring the water-receptive bottom layer of metal.

Bimetal plates, although the most durable and the most practical for the longest print runs, are also the most expensive. Bimetal plates have many other advantages, as well. In the event of plate blinding or scumming, a simple acid treatment will solve both problems at once, unlike other plates where solving one problem can increase the occurrence of the other. Bimetal plates also allow the reproduction of halftones with a minimum of dot gain. (See Plate: Offset Lithography.)

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