A Short Course in Offset Printing

Almost all printed pieces you see are printed utilizing offset printing or lithography. This article will briefly explain how the process works.

A printed piece is often described by the number of ink colors used. Jobs are said to be "one color  or "four color process". One printing plate is necessary for each color which is being printed. Some printing plates are made by exposing the plate through a negative which has been output from a computer. More common today is an automatic plate-setter. The plate is imaged by a laser. The information is fed to the plate-setter from the prepress department computers. This is called "direct to plate".

In either case, the exposure process hardens areas on the plate where ink will be printed on the press sheet. The unexposed areas are then washed away in an developing process.

The result is a plate that has the same image as will be printed on the sheet. The completed plate is mounted on a large plate cylinder on a printing press.

The theory behind offset printing is that oil and water donít mix. Exposed and hardened areas on a printing plate are made of a material that attracts ink. Inks used are oil-based (petroleum or soy) and therefore they coat the exposed areas of a plate. The unexposed area of the plate attracts water so no ink appears in those areas.
As a press starts up, a thin coating of water is applied to the plate through a series of rollers. Next the plate comes into contact with an ink roller. The ink only sticks to the exposed and hardened areas of the plate. The water continues to coat the unexposed areas of the plate. Because the oil based inks and the water donít mix, the image produced is crisp and clear.

The plate then comes into contact with the printing blanket, a hard rubber material mounted on a blanket cylinder. The image is transferred, or offset, onto the blanket. The rubber material is very well suited to hold the ink until it is put onto the paper.

Finally the paper is introduced into the process. The paper passes between the rubber blanket cylinder and the impression cylinder, a hard steel cylinder. Pressure between the impression and blanket cylinders ensure that the image is transferred faithfully to the printed sheet.
Copyright 2005, The Catalog Works, All rights reserved
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