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The science of offset printing plates
Jan 08, 2019

The science of offset printing plates

The principal operational functioning of all offset presses is the same, whether single colour or multi-colour, sheetfed or web-fed, domestically manufactured or imported. The presses make one impression with each rotation of the cylinders. All offset presses have three printing cylinders (plate, blanket and impression), besides inking and dampening systems. As the plate cylinder rotates, the plate comes in contact with the dampening rollers first and then the inking rollers. The dampeners wet t

Som Nath Sapru

Besides the printing press, the most important aspect of offset printing is the plate – which metal or substance it is manufactured with and what chemical processes it has to go through before it is mounted on the plate cylinder – all these factors matter a lot for the end product — the reproduction.

Printing plates must have the ability to transfer an image to paper, cardboard or any other substrates. Printing plates are usually made from metal, plastic, rubber, paper, and other materials. The image is put on the printing plates with photochemical, photomechanical or laser-engraving processes.

It is well understood that metal printing plates are more expensive but these last longer and have greater accuracy and can be used for larger print runs.

Offset printing plates are thin (up to about 0.3mm), and easy to mount on plate cylinder, and they mostly have a mono-metal (aluminium) or, less often, multi-metal, plastic or paper construction.

Aluminium has been gaining ground for a long time among the metal-based plates over zinc and steel. The necessary graining of the aluminium surface is done mechanically either by sand-blasting, ball graining, or by wet or dry brushing. Nowadays, practically all printing plates are grained in an electrolytic process (anodising), that is, electrochemical graining with subsequent oxidation.

The imaging, ink-accepting coating (light-sensitive coating is applied to the base material. This material is usually a polymer, or copper in the case of multi-metal plates (bi-metal plates). Light sensitive, diazo (photo-polymer) pre-coated aluminium printing plates are now the predominant plates in print shops. The image transfer is produced via different properties on the surface of such plates after they have been exposed and developed. The remains of the original light-sensitive coating or the light-sensitive coating changed by the effect of light are the ink-accepting (oleophilic) elements that create the image.

Chemical changes occur as a result of the penetration of photo-effective (actinic) light (light containing UV rays), causing the light-sensitive coating to react differently depending on its type and structure. There are two types of photochemical reactions when developing the printing plate:

1. Hardening of the light-sensitive layer by light (negative plate-making)

2. Decomposition of the light-sensitive layer by light (positive plate-making)

With positive platemaking and conventional printing plate production, a positive film is used as the original, that is, the non-translucent, blackened sections of the film correspond to the ink-accepting surface elements on the plate.

In the case of negative platemaking with ‘negative plates’, a negative film is used as an original, that is, the ink-accepting image areas of the printing plate correspond to the translucent, light areas on the film. For quality assurance and monitoring during plate-making, control elements are copied onto the plates.

There are several types of lithographic plates in use today.

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