How Does Offset Lithography Work?

Written by laura reynolds
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How Does Offset Lithography Work?
(Wikimedia Commons, DRW & Associates, Inc, Printers National Environmental Assistance Center)

Johannes Gutenberg's invention of the movable-type printing press in the 15th century made reading material available to a wide variety of people, and ignited the growth of literacy and the great universities of Europe. It also transformed printing into a craft as well as an art. Gutenberg's press, revolutionary as it was, could only print one sheet at a time, and had to be re-inked for each impression. The rotary press, consisting of a curved cylinder that could turn and print pages on a continuous sheet of paper, significantly increased the number of sheets that could be printed. But individual print quality varied because ink still had to be deposited directly on the plate to make an impression. Steel sheets replaced the tin type, opening up a wide range of usefulness for graphic and artistic reproduction. An accidental loss of paper led to the discovery that the rubber base on which paper was laid would retain a sharp impression of the plate, and could be used to produce sharper images than could be made with direct plate contact. The new process was called "offset."

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Basics

Lithography uses an aluminium "plate" onto which a photograph of a page or picture is transferred. This plate is attached to a cylinder onto which ink and a water-based solution are jetted or rolled. The water solution cleans the ink paste from the blank areas but the ink clings to the picture details, which have been treated with a waxy substance. The plate cylinder turns against a "blanket" cylinder, covered by a special soft rubber that absorbs the ink from the inked sections. As this cylinder turns, the ink is laid upon paper as it is pushed between the blanket and a third turning cylinder called the "impression" cylinder in exactly the same pattern as appears on the plate. A series of sets of cylinders can create colour prints using different colours of inks, most commonly cyan, magenta, yellow and black (or "key"), known as "CMYK" four-color printing.

Ink (right) and water-based cleaning solutions drop onto the plate cylinder from above.
Ink (right) and water-based cleaning solutions drop onto the plate cylinder from above.

Specifics

Special inks are used for offset lithography that cure or "set" using heat, cold or light. This allows presses to run continuously until a job is finished rather than waiting for ink to "dry" on each lot of paper as it exits each section of the press. Curing machinery is installed between each section and at the end of the press before the paper enters the trimming machine. The water-based, or "fountain" solution contains a surfactant or emulsifier, traditionally isopropyl alcohol, that sweeps the smooth parts of the plate clean of the greasy ink paste. Although fountain solutions are traditionally acidic, more environmentally-friendly alkaline or neutral alternatives are in use in many shops today. Presses may be built to handle either sheets or rolls of paper (sheet-fed or continuous feed)---large-run newspapers and circulars are generally run on continuous feed presses.

The blanket transfers the impression to the paper being fed through the press.
The blanket transfers the impression to the paper being fed through the press.

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