A technical dictionary of printmaking, André Béguin. EN

plastic (engraving on)
It has been som eyhears now that plastic has been used for relief cuts that are done the same way as woodcuts. The plastics used for this purpose are usually of the "plexiglass" type (but there are other materials as well) which can be cut with woodcut tools. Very fine work can be done on such plates and, furthermore, the transparency of this material ensures easy registering when colour printing is done.
I personally have used plastic sheets for engraving and have found that the colour registering in printing is very much simplified by the fact that the colours can be superimposed.