Offset lithographic printing

The basic principle on which offset litho works is that “oil and water do not mix”. A litho printing plate has non-image areas which absorb water. During printing the plate is kept wet so that the ink, which is inherently greasy, is rejected by the wet areas and adheres to the image areas.

Artwork is produced digitally with graphic design software. An imagesetter is then used to produce films (either positive or negative). When printing with more than one colour there is a separated film for each ink used. Each film is used to make a printing plate by a photochemical process. The plate surface has non-image areas which absorb moisture and repel ink.

The flexible plates, which can be made of a variety of materials, are attached to the plate cylinder. The plate is kept moist throughout so that ink only adheres to image areas.

During every cycle of the press, the ink image is first transferred to a rubber surfaced blanket cylinder and is then transferred from there to the paper. This indirect method is the 'offset' after which the process is named. The blanket cylinder's flexibility both preserves the delicate plate and conforms to the surface of textured papers.

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On-body print/On-disc printing

The term used for the print on the upper surface of any given disc.

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