The plates used in offset printing are slim (up to about 0.3 mm), and easy to attach on the plate cylinder, and they generally have a mono-metal (aluminum) or, less often, multimetal, plastic or paper structure.
Aluminum has been gaining ground for a very long time among the metal-based plates over zinc and steel. The necessary gaining of the aluminum surface is done mechanically either by sand-blasting, ball gaining, or by wet or dry brushing.
Today, almost all printing plates are grained inside an electrolytic technique (anodizing), that is, electrochemical gaining by using subsequent oxidation.
The imaging, ink-accepting covering (light-sensitive coating, thickness around 1micro meter) is applied to the base material. This material is often polymer, or copper for multimetal plates (bi-metal plates).
Light-sensitive, diazo (photo-polymer) pre-coated aluminum printing plates at the moment are the predominant plates in print-shops. The image transfer is produced via the different attributes on the surface from this plates after they are exposed and developed. The remains of the first light-sensitive coating or the light-sensitive coating changed by the effect of light are the ink-accepting elements that create the image.
Chemical transformations occur because of the penetration of photo-effective (actinium) light (light containing UV rays), inducing the light-sensitive coating to react differently based on its type and constitution.
There are two types of photochemical reactions when producing the printing plate:
• hardening of the light-sensitive layer by light (negative plate-making),
• decomposition of the light-sensitive layer by light (positive plate-making).
With positive plate-making and standard printing plate production, a positive film is used as the original, that’s, the non-translucent, blackened sections of the film correspond to the ink-accepting surface elements on the plate.
For negative plate-making with “negative plates” a negative film is applied as an original, that’s, the ink-accepting image areas of the printing plate correspond to the translucent, light areas on the film.
For quality assurance and checking during plate-making, control elements are copied to the plates.
The most practical way for quality checking is to use a color-bar.This is an image that is placed across the offset printing plate, usually on the trailing edge, that contains targets to provide measurement areas for the solid ink densities, dot measurements and other control elements. These color-bars are provided as film and exposed onto the plates along with the image or they may be digitally derived in CTP (computer to plate) systems.
Also for producing and maintenance of plates there is a lot of chemistry we can use.