offset printing

offset printing

 or offset lithography or litho-offset

In commercial printing, a widely used technique in which the inked image on a printing plate is imprinted on a rubber cylinder and then transferred (offset) to paper or other material. The rubber cylinder gives great flexibility, permitting printing on wood, cloth, metal, leather, and rough paper. In offset printing the matter to be printed is neither raised above the surface of the printing plate (as in letterpress printing) nor sunk below it (as in intaglio, or gravure, printing). Offset printing, a development of lithography, is based on the principle that water and grease do not mix, so that a greasy ink can be deposited on grease-treated printing areas of the plate, while nonprinting areas, which hold water, reject the ink. The offset plate is usually of zinc or aluminum or a combination of metals, with the surface treated to render it porous and then coated with a photosensitive material. Exposure to an image hardens the coating on printing areas; the coating on nonprinting areas is washed away, leaving wetted metal that will reject ink. See also xerography.