Litho - The Principle of Lithography - The Modern Lithographic Process

The Modern Lithographic Process

High-volume lithography is used presently to produce posters, maps, books, newspapers, and packaging—just about any smooth, mass-produced item with print and graphics on it. Most books, indeed all types of high-volume text, are now printed using offset lithography.

For offset lithography, which depends on photographic processes, flexible aluminum, polyester, mylar or paper printing plates are used instead of stone tablets. Modern printing plates have a brushed or roughened texture and are covered with a photosensitive emulsion. A photographic negative of the desired image is placed in contact with the emulsion and the plate is exposed to ultraviolet light. After development, the emulsion shows a reverse of the negative image, which is thus a duplicate of the original (positive) image. The image on the plate emulsion can also be created by direct laser imaging in a CTP (Computer-To-Plate) device known as a platesetter. The positive image is the emulsion that remains after imaging. Non-image portions of the emulsion have traditionally been removed by a chemical process, though in recent times plates have come available which do not require such processing.

The plate is affixed to a cylinder on a printing press. Dampening rollers apply water, which covers the blank portions of the plate but is repelled by the emulsion of the image area. Hydrophobic ink, which is repelled by the water and only adheres to the emulsion of the image area, is then applied by the inking rollers.

If this image were transferred directly to paper, it would create a mirror-type image and the paper would become too wet. Instead, the plate rolls against a cylinder covered with a rubber blanket, which squeezes away the water, picks up the ink and transfers it to the paper with uniform pressure. The paper passes between the blanket cylinder and a counter-pressure or impression cylinder and the image is transferred to the paper. Because the image is first transferred, or offset to the rubber blanket cylinder, this reproduction method is known as offset lithography or offset printing.

Many innovations and technical refinements have been made in printing processes and presses over the years, including the development of presses with multiple units (each containing one printing plate) that can print multi-color images in one pass on both sides of the sheet, and presses that accommodate continuous rolls (webs) of paper, known as web presses. Another innovation was the continuous dampening system first introduced by Dahlgren instead of the old method which is still used on older presses (conventional dampening), which are rollers covered with molleton (cloth) which absorbs the water. This increased control of the water flow to the plate and allowed for better ink and water balance. Current dampening systems include a "delta effect or vario " which slows the roller in contact with the plate, thus creating a sweeping movement over the ink image to clean impurities known as "hickies".

The process of lithography printing is seen here. This simplified version of a lithographic printing press gives you an idea of how the printing process works. This press is also called an ink pyramid due to the fact that the ink is transferred through several layers of rollers with different purposes. The lithographic printing presses are commonly used in newspaper production where fast rolling and jumping rolls are a famous part of the pressing scene in every newspaper production movie.

The advent of desktop publishing made it possible for type and images to be modified easily on personal computers for eventual printing by desktop or commercial presses. The development of digital imagesetters enabled print shops to produce negatives for platemaking directly from digital input, skipping the intermediate step of photographing an actual page layout. The development of the digital platesetter during the late 20th century eliminated film negatives altogether by exposing printing plates directly from digital input, a process known as computer to plate printing.