Technology of Offset Printing

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 nonimage 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.

As the plate cylinder rotates, the plate comes in contact with the dampening rollers first. The dampening rollers wet the plate so the non-printing areas repel ink. Then the inking rollers transfer ink to the dampened plate, where ink only adheres to the image areas. The inked image is transferred to the rubber blanket, and the substrate is printed as it passes between the blanket and impression cylinder.

There are three basic lithographic press designs: unit-design, common impression cylinder design, and blanket-to-blanket design. The unit-design press is a self-contained printing station consisting of a plate cylinder, a blanket cylinder, and an impression cylinder. Two or more stations may be joined to perform multi-color printing. A common impression cylinder press consists of two or more sets of plate and blanket cylinders sharing a common impression cylinder. This allows two or more colors to be printed at a single station. A blanket-to-blanket press consists of two sets of plate and blanket cylinders without an impression cylinder. The paper is printed on both sides simultaneously as it passes between the two blanket cylinders.