Printing plates

Aluminum with a thickness of less than about half a mm (usually 0.15 to 0.4 mm) is generally used as the printing plate in offset printing. This plate, after being provided with the printing image, is stretched around a cylinder in the printing press. Modern printing presses today can do this automatically.

The printing plate has a very finely roughened surface (grein) to improve its water-attracting capacity. In the early days of the offset plate, this surface was achieved by placing the plate in a so-called marble tray where a large number of porcelain marbles with a quantity of polishing paste were brought into a shaking motion. This made the surface of the slab very finely roughened.

Today, industrial methods, such as anodizing, are used to give the slab its surface treatment.

After surface treatment, the plate is coated with a photoresist. Through proper exposure, the printing parts harden after which the non-printing parts are washed off. The plate is then basically ready for use on the offset press. There are positive-working offset plates, where the unexposed areas later form the printed image, and negative-working plates, where the exposed areas form the printed image. The latter have lost much of their popularity in recent years. Very widely used in recent years are digital plates, where the image is written directly onto the plate from the computer.