什麼是膠印；平版印刷

一種廣泛採用的印刷方法︰圖文部分著墨後，先印在橡皮滾筒上，再轉印到紙張或其他材料上面。由於橡皮滾筒的撓曲性大，木料、布、金屬、皮革、粗糙紙張等都可以印刷。膠印印版上的圖文，既不像凸版（參閱letterpress printing）那樣凸出於版面，也不像凹版那樣低於版面，而是與印版在同一平面上，因此屬於平版印刷。膠印由平印發展出來，以水和油不相溶合的原理為基礎，圖文部分經過感脂處理，能吸收油墨，空白部分因含有水分，而不吸收油墨。膠印版通常用鋅、鋁或其合金製成，表面經過處理，使其具有滲透性，然後塗上一層感光材料。用原稿曝光後，圖文部分的塗層硬化，空白部分的塗層被沖洗，露出潤溼的金屬，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.