|ADAST AD 846
Offset printing makes use of the basic principle of lithography, the mutual repulsive force between greasy printing inks and water. Offset is classified as planographic printing because both the printing and non-printing areas are at the same height (plane).
The ink is applied indirectly in offset printing: the ink from the printing form is applied to a rubber roller, from which it is transferred to the paper; this is an indirect printing method. Since the ink is transferred twice, the image is not inverted sideways from the printing form and the printed material. Unlike letterpress printing, offset printing enables high-quality prints even on paper of lower quality due to the soft surface of the rubber printing roller.
The offset printing process is based on lithography (stone printing). The original stone was replaced by an elastic metal plate, which enabled rotary printing by being attached to a roller. The metal plate was replaced by a rubber surface in the early 20th century. Its adhesiveness has improved print quality even on lower-quality paper.
Based on the printing method, offset printing is divided into two basic categories: conventional (wet) offset and waterless offset (also known as dry offset).
Conventional offset is the most widespread printing technique, used for printing newspapers, magazines, leaflets, brochures and books. The advantage of conventional offset is the ability to print large runs without any special maintenance.
The printing form is an aluminium plate attached to a rotating roller. Two layers with differing properties - hydrophobic and hydrophilic - are applied to the plate surface. The hydrophobic layer repels water and makes it possible to apply greasy ink. The hydrophilic layer admits water and prevents ink from being applied to the printing form.
Historically, the selection of the parts of the plate in which to etch away the hydrophobic layer was made by illuminating film in an exposure unit. Today, the print data are applied using laser. The process of treating the printing form with laser is known as CTP (computer to plate).
Dry offset is a technology that does not use water. The printing form plate is coated with two layers: a top silicone one that defines the no-print area, and a polymer one that defines the printed area.
The silicone layer is removed from the printing form area where the ink is to be transferred. The layer is removed by laser burning (CTP, computer to plate).
The advantage of dry offset is superb-quality print with a very fine raster and accurate reproduction of print pixels.
More demanding maintenance is the disadvantage. Tiny dust particles are generated at the edges of printable and non-printable areas during the printing; they make up blank areas known as "ink failure". The printing plate has to be washed after every few hundred copies to eliminate them.
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