LITHOGRAPHIC PRINTING

Lithography dominates the U.S. printing industry, accounting for as much as 50 percent of all conventionally printed materials. There are two types of lithographic printing: sheetfed and web. Sheetfed presses run individual sheets of paper through the press, while web presses feed paper continuously from a large roll and can use either heatset or nonheatset inks. Once the image is printed onto the substrate, the paper is either fed back onto a roll or cut and/or trimmed into specific shapes and sizes. Web lithography is designed to print large jobs and is used for newspapers, books, catalogs, periodicals, advertising and business forms. Sheetfed lithography is used mostly for short runs of books, periodicals, posters, advertising flyers, brochures, greeting cards, packaging and fine art reproduction.

The offset lithography process works by first transferring an image photographically to thin metal, paper, or plastic printing plates. Unlike other forms of printing, in offset lithography the image on the printing plate is not recessed or raised. Rollers apply oil-based ink and water to the plates. Since oil and water don't mix, the oil-based ink won't adhere to the non-image areas. Only the inked image portion is then transferred to a rubber blanket (cylinder) that then transfers the image onto the paper as it passes between it and another cylinder beneath the paper. The term offset refers to the fact that the image isn't printed directly to the paper from the plates, but is offset or transferred to another surface that then makes contact with the paper.