PS Plates, as they are also known as, can be both positive or negative working depending upon the type of light-sensitive coating used on the plates.
Printing plates must have ability to transfer an image to paper, cardboard or other substrates. Printing plates are usually made from metal, plastic, rubber, paper, and other materials. The image is put on the printing plates with photochemical, photomechanical or laser engraving processes.
The offset printing plates used in offset printing are thin (up to about 0.3 mm), and easy to mount on the plate cylinder, and they mostly have a mono-metal (aluminum) or, less often, multimetal, plastic or paper construction.
Aluminum has been gaining ground for a long time among the metal-based plates over zinc and steel. The necessary graining of the aluminum surface is done mechanically either by sand-blasting, ball graining, or by wet or dry brushing. Nowadays, practically all printing plates are grained in an electrolytic process (anodizing), that is, electrochemical graining with subsequent oxidation.