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Offset Lithography

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Liam Isaac

on 10 April 2013

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Transcript of Offset Lithography

OFFSET LITHOGRAPHY Lithography works on the principle that water and oil do not mix "Water rollers" come into contact with the plate cylinder, thoroughly drenching it. However, the image area on the plate cylinder repels the water 1. WETTING: A thin, even coating of oil-based ink is transferred onto the plate cylinder via the "ink rollers". The image area, which had repelled the water earlier, accepts the ink. No ink is retained outside the image area 2. INKING: The actual plate cylinder does not come into direct contact with the paper. A rubber blanket wrapped around a blanket cylinder receives the ink from the plate cylinder and squeezes away the water. This is the stage that gives offset printing its name. 3. OFFSETTING: Paper is fed between the blanket cylinder and an impression cylinder. The ink that was stuck to the rubber blanket is transferred onto the paper 4. PRINTING: Digital image setters allow for the production of negatives for plate making directly from digital images on computers using direct laser imaging

A flexible flat plate is covered with a photosensitive chemical. Light is projected through the negative film onto the plate, producing a positive "image area" once the plate is developed. The image area of the plate is chemically treated so that it attracts ink, but repels water. The non-image area is treated so that the reverse happens: ink is repelled, but water is attracted COMPUTER TO PLATE [CTP]: PLATE CYLINDER BLANKET
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