Aluminium encyclopaedia

Printing plates (lithographic sheet)

Today, nearly all books, magazines, posters, maps and newspapers are printed using surface or offset printing. The heart of the process is the offset printing plate. It is covered with a light-sensitive coating onto which the image to be printed is applied from a film. The coating is developed and etched so that text and graphics on the plate/sheet appear as microscopic roughening, while unexposed areas remain smooth.

For printing purposes, the plate/lithographic sheet is attached to a cylinder with the image on the outside. A "dampening unit" applies water to the plate continuously but the water only wets those areas where the plate is smooth. After passing through the dampening unit, an "inking unit" applies printing ink to the plate cylinder. As this ink contains oil, it does not adhere to the areas wetted with water (water and oil repel each other), but it does adhere to the etched areas which have remained dry. The plate cylinder then transfers the resultant colour or print image to a rubber-surfaced "blanket cylinder", which then transfers it to the paper.

Printing plates are made from zinc, aluminium, multilayered metal or plastic, whereby the use of aluminium is most widespread. Unalloyed aluminium in the form of strip is used. As the plate/sheet has to be very flat and of uniform thickness, the highest possible demands are placed on the rolling process. The production of printing plates/lithographic.