什么是胶印；平版印刷

一种广泛采用的印刷方法︰图文部分着墨後，先印在橡皮滚筒上，再转印到纸张或其他材料上面。由於橡皮滚筒的挠曲性大，木料、布、金属、皮革、粗糙纸张等都可以印刷。胶印印版上的图文，既不像凸版（参阅letterpress printing）那样凸出於版面，也不像凹版那样低於版面，而是与印版在同一平面上，因此属於平版印刷。胶印由平印发展出来，以水和油不相溶合的原理为基础，图文部分经过感脂处理，能吸收油墨，空白部分因含有水分，而不吸收油墨。胶印版通常用锌、铝或其合金制成，表面经过处理，使其具有渗透性，然後涂上一层感光材料。用原稿曝光後，图文部分的涂层硬化，空白部分的涂层被冲洗，露出润湿的金属，不吸收油墨。

offset printing

orlitho-offsetIn 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.