Thursday, 30 September 2010

Design For Print : Offset Lithography


Lithography (or Litho) is a planographic process, as the printing surface is flat rather than raised in letterpress or recessed as in gravure. The area to be printed is treated chemically so that it accepts ink and rejects water, while the non-image, or background, area is treated in the opposite way. The whole surface has ink and water applied to it along with alcohol to aid dispersion. When the plate is pressed against the surface of the paper, only the image area is printed. When lithography was first employed, smooth stone slabs were used to make the printing surface, and this method is still used for limited editions of fine art prints, using flat bed presses. The next development in lithographic printing came with the introduction of grained metal plates. These can be curved around a cylinder to allow the use of a rotary press. Finally the 'offset' principle was developed.
Where lithography is used, it is nearly always as offset lithography. This means that the inked image on the metal plate is 'offset' (printed) onto a rubber blanket wrapped around a rotating metal cylinder and the image is transferred from that blanket onto the paper. One reason for using a blanket is to prevent the delicate lithographic plate from coming into contact with the more abrasive paper surface, which would cause wear and tear on the plate during the print run. Another advantage of the offset principle is that less water comes into contact with paper than in direct lithography. Furthermore, the rubber blanket responds to irregularities of surface so that it is possible to print on a wide variety of surfaces.
Lithography is the predominant process used in printing today, being used for a wide range of items from letterheads to packaging, books and magazines.

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