Modern Lithography

The basic principle of lithography, the non mixing of oil and water, has remained unchanged (until only recently with the introduction of waterless plates).

However, the process has continued to advance.

Plate - The printing plate are most commonly made of aluminum coated with a layer of photosensitive coating. With new exposure methods and computer to plate systems, other plate materials have been developed based on polymeric materials.

The plate is exposed to Ultraviolet light through imaged films to produce a surface which has been broken photochemically into 1) an image area which is readily wet by oil (ink) and 2) a nonimage area which is readily wet by water.

When mounted on press a dampened plate will accept ink only in the oil-wet image area.

Blanket - Today the vast majority of commercial lithographic presses use the offset method. Indeed, as printers, we usually treat offset and lithography as synonymous.

This is not strictly true, since electrolithography is used for fabricating integrated circuits and offset electrophotography is used in the Indigo digital press.

In offset lithography, the inked plate doesn’t come in direct contact with the paper.

It is transferred to a (usually rubber) blanket which in turn makes the impression on the paper. Because of the double reversal of the image in offset, the plate image is a direct one.

The soft elastic blanket is easier on the plate than direct contact with paper and the blanket is more effective at pressing the ink into the valleys and pores in the paper surface. We’ll discuss the plate and blanket in more detail.