Text 3 Offset lithography

This is the predominant process used in printing today, being used for a wide range of items from letterheads to books and magazines. The basic process of lithography was invented by Senefelder in Bavaria in 1798, but it was only when the offset principle was applied early this century that lithography started to be used for commercial printing, and only in the 1960s that it started to overtake letterpress as the major printing process. Prior to its commercial development, it was used mainly in the field of art, with prints made from stone printing surfaces.

The development of the process for commercial use was held back by the problems of achieving the right balance between ink and water: the water had a tendency to dilute the ink and this resulted in a rather weak and dull appearance. Now developments in inks, presses and techniques have allowed the process to exploit its obvious advantages.

The lithography process Lithography (generally known simply as 'litho') is a 'planographic¹ process, as the printing surface is flat rather than raised, as in letterpress, or recessed, as m gravure. The area to be printed is treated chemically so that it accepts grease (ink) and rejects water, while the non-image, or background, area is treated to accept water and reject grease. The whole surface has both ink and water applied to it. When the inked and dampened plate is then pressed against paper, only the image area is printed.

The offset lithography principle 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; the image is transferred from the blanket onto the paper, One reason for using the blanket is to prevent the delicate litho plate from coming into contact with the more abrasive paper surface, which would wear the plate more significantly.

An advantage of the offset principle is that less water comes into contact with the paper than in direct lithography. Moreover, the rubber responds to irregularities of surface so that it is possible to print on to a wide variety of surfaces, including metal, for cans and boxes: special inks are used for this, and the printed metal is heat-treated after printing to give a rub- and scratch-resistant surface.

Dry offset printing This version of the offset process is also known as 'indirect letterpress' or 'offset letterpress', There is no dampening, and the zinc or aluminium plate has a slightly raised surface — it is, in effect, a letterpress plate. This prints on to a rubber blanket and the image is then printed on to the paper.

Platemaking costs are high, as plates are made by etching. The process is used for specialist work, such as for colouring books where the colour appears when water is applied or for carton work where very strong, solid colours are required. It is also used for certain kinds of security work, such as banknotes and cheques.

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