In letterpress printing, a process by which an acid resist containing a photosensitive emulsion is adhered to a sheet of metal, exposed to a film negative, developed, and then the surface is treated with a chemical etchant, which eats through the resist quickly and dissolves the metal in the unexposed, non-image areas, and eats through the resist more slowly in the most highly-exposed image areas, failing to etch the metal in the portions of the image corresponding to solids and shadows. The process of photoengraving is reminiscent of a stencil process; the depth of engraving is not variable as it is in some reist etching systems. This process results in a metal relief plate.
A very similar process as the above is used in the making of molded rubber flexographic plates, the metal relief plate then being impressed into a mold consisting of cellulose fibers and phenolic resins, the rubber then added to the hardened mold to form a raised rubber plate. (See Plate: Flexography: Rubber Plates.)