Offset Lithography

Offset Lithography is by far the most common form of commercial printing.

The basic principle on which it 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 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.

After printing the sheets are taken for finishing — trimming, folding and binding.

The press can either be fed with paper one sheet at a time (Sheet fed) or from a large roll of paper (Web). Web printing is normally reserved for large scale, long run work such as magazines and catalogues.

The basic stages of the modern production process are:

1. Original artwork — photographs, illustrations and text — are scanned and entered into a computer.
2. These elements are combined into a document using page makeup software.
3. Full size films are output using a high-resolution imagesetter. These could be either positives or negatives.
4. Printing plates are made from the films using a photochemical process. The plates are exposed to high-intensity light through the films and then chemically treated so that non-image areas are water absorbent.
5. The flexible plates are attached to the plate cylinders of a litho press and the job is printed.

This diagram shows the production process for a colour job. See our article Four Colour Process Printing for a more detailed explanation.