Print Process Descriptions: Printing Industry Overview: Lithography

Offset Lithographic Printing Process

All offset presses have three printing cylinders, as well as the inking and dampening systems. The plate cylinder, the blanket cylinder and the impression cylinder.

Lithography uses a planographic plate, a type of plate on which the image areas are neither raised nor indented (depressed) in relation to the non-image areas. Instead the image and non-image areas, both on essentially the same plane of the printing plate, are defined by deferring physiochemical properties.

Lithography is based on the principal that oil and water do not mix (hydrophilic and hydrophobic process). Lithographic plates undergo chemical treatment that render the image area of the plate oleophilic (oil-loving) and therefore ink-receptive and the non-image area hydrophilic (water-loving). During printing, fountain (dampening) solution, which consists primarily of water with small quantities of isopropyl alcohol and other additives to lower surface tension and control pH, is first applied in a thin layer to the printing plate and migrates to the hydrophilic non-image areas of the printing plate. Ink is then applied to the plate and migrates to the oleophilic image areas. Since the ink and water essentially do not mix, the fountain solution prevents ink from migrating to the non-image areas of the plate.
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Traditionally, isopropyl alcohol was used to control surface tension in the fountain solution, but in recent years it’s use has been reduced, and in many cases eliminated by using alcohol substitutes. The reason for this shift is due to the VOC emissions attributed to the evaporation of isopropyl alcohol and the level of environmental regulation this lead to. Alcohol substitutes may use glycol ethers such as butyl cellosolve (2-butoxy ethanol) or other glycols to control surface tension.
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