Lithographic presses account for over 50 percent of U.S. printing
Lithography produces the highest quality printing of any system. Approximately 50 percent of all printed materials produced in the United States are produced on lithographic presses. The term "lithography" comes from the Greek. It means "writing with stone." It was invented in 1798 by Alois Senefelder. The first printing used stones as the medium on a flat press. Today, the stones have been replaced with curved plates mounted on cylinders.
The Lithographic Process
Lithography is based on the principle that oil and water do not mix. It is known as offset printing, because ink from a printing plate is transferred to a rubber blanket, which then transfers the image to paper. Water used in the process would permeate the paper without the use of the intermediate blanket, which squeezes the water out. Offset plates contain image areas, which repel water, and non-image areas, which attract water. On the press, a water-based mixture is applied to the non-image areas first. Then ink is applied to the plate, and is repelled by the water in the non-image areas. Ink remains on the image areas, and is then applied to the intermediate blanket.
Types of Presses
There are two main types of presses; sheet fed and roll fed. On sheet-fed presses, individual sheets of paper of varying sizes are fed into the press. These are usually used for shorter runs of around 10,000 impressions or less. Roll fed, or web presses, use large rolls of paper that run through the press on a continuous basis. These are used for longer runs, such as magazines and newspapers. A perfecting press can print on both sides of the paper on a single pass.
Feeding and Delivery Systems
On a sheet-fed press, the paper is stacked on a tray, and each sheet is picked up with a vacuum device and fed into the press. On a web press, a paper roll is mounted on a roll stand that maintains the proper amount of tension as the paper goes through the press. The delivery system on a sheet-fed press stacks the printed sheets on a tray as they leave the press. The delivery system on a web press either cuts the sheets to a specified size as they are printed or re-rolls the printed material onto a rewinding unit.
Printing, Inking and Fountain Systems
Both press types utilize a fountain, which holds the ink, and a series of rollers which apply the ink to the plate, and a dampening system, which contains the fountain solution and another set of rollers that applies the fountain solution to the plate to keep the non-image areas from getting inked. The plate cylinder contains the plate, usually aluminum, on which the image is right side up. This is transferred to the blanket cylinder where the image is reversed. The blanket cylinder transfers the image to paper, once again right side up.
