By far the most important and versatile printing process today is offset lithography. The underlying principles were established at the end of the 18th century. It was discovered that a wet limestone surface would repel an oil-based printing ink but if an image was drawn on the surface with a grease pencil it would repel water and attract ink. Thus, any drawing on the stone surface could be reproduced by bringing a damp sheet of paper into contact with the freshly inked image. This cycle could be repeated several hundred times before the drawing could no longer be faithfully reproduced. Then, in the early part of the 20th century, it was discovered that ink could be transferred from the lithographic surface to an intermediate rubber surface and then to paper. The rubber intermediate, called a blanket, can transfer ink to paper and to a wide variety of materials that cannot be printed directly, including plastics and metals. Because the soft blanket conforms to the texture of the surface to be printed, lithographic image quality is unrivalled.

Today, thin aluminium plates and other materials, such as stainless steel and plastic, are used. The plates are wrapped around the circumference of the printing cylinder and make direct contact with the rubber blanket cylinder. Rubber rollers carry ink and water to the plate surface. The printing plate is treated with an oil-based substance so that water will not adhere to the design. When the dampened printing plate roller is run by the inking rollers, only the design accepts ink. The ink is transferred first to the blanket cylinder and then to the paper, which is why it is called 'offset'.

The Offset Principle

The diagram shows the basic offset litho printing process. Ink is transferred from the lithographic surface to an intermediate rubber surface and then to paper. The rubber transfer cylinder runs in contact with the printing-plate cylinder and receives the inked image from it; this image is then offset onto the paper. The rubber cylinder keeps the delicate printing plate from coming in contact with the printed object. Very long printing runs, or periods when the plates are not changed, are thereby made possible, and the faithfulness of reproduction and sensitive shading, unique to lithography, are preserved. The rubber intermediate, called a blanket, can transfer ink to paper and to a wide variety of materials that cannot be printed directly, including plastics and metals. Because the soft blanket conforms to the texture of the surface to be printed, lithographic image quality is unrivalled.

Offset Lithography Today

Lithographic plates are the least expensive printing surfaces available today, and this fact has contributed greatly to the success of the process. Aluminium plate materials have a thin surface coating of light-sensitive material, such as a photopolymer, that undergoes a solubility change when exposed to an intense source of blue and ultraviolet light. Images are transferred to the surface by exposing the plate through a film positive or negative. Some materials can be exposed directly, as in a graphic-arts camera or by a computer-controlled laser beam, thereby eliminating the expense of film and speeding up the platemaking process.

Modern offset lithographic presses range in size from small sheet-fed duplicators - used for small, single-colour jobs such as brochures and newsletters - to massive web presses capable of printing millions of copies of magazines, catalogues, and packaging materials in full colour. No other process has such a broad range of applications.