Тексты для дополнительного самостоятельного чтения

на тему «Printing Processes»

Photopolymer plates

Original letterpress printing plates produced from plastic materials first made their appearance in the early post-war years.

There are three main reasons for the present growth of photopolymer plates:

1. The majority use chemicals which are non-polluting; they are time stable, less dangerous and not so costly as those used for zinc, copper and magnesium.

2. Results are constant from plate to plate and therefore more predictable; actual plate processing is fairly simple and lends itself more readily to automation, particularly for newspapers which have installed photosetting.

3. Overall production costs are less than for metal plates.

Two main systems

The present generation of photopolymer plates can be described as photosensitive plastics; in other words they are plastic materials which harden under the action of ultra-violet light and therefore must not be confused with plastic materials for duplicate plates.

There are two main systems for producing these plates; the first system uses a solid plate material, which consists of:

1. A base, backing, or carrier sheet of either flexible steel or aluminium, polyester or rigid aluminium.

2. Non-sensitive bonding layer.

3. The relief layer of photosensitive plastic.

4. A protective covering of clear foil.

The second system uses a photosensitive liquid resin which is purchased in cans, with the addition of backing or base sheets (metal or plastic) and protection foils for the platemaking process. The basic material of the photosensitive plastic, for both systems, can be either polyurethane, polyester polyvinyl alcohol, polyvinyl acetate or nylon. The resin, liquid or solid, can be manufactured with a shore hardness compatible with the type of printing it is intended for, such as magazine and commercial work, book printing, newspaper printing and business forms. Some manufacturers even produce a plate material specifically for production of plates for moulding only (pattern plates). Both systems are also capable of producing plates of varying gauge thicknesses for flatbed letterpress, rotary letterpress, dry offset and flexography.