Plate offset

The plate offset is regarded as a plane printing form (without apparent relief). The quality of reproductability of the plate, must be adapted to the smoothness of the reproductions considered (imitation).

A plate offset lends to the impression has two basically opposite surface qualities of nature:

A printing surface which must accept the fatty Encre and refuse the Eau, (printing zone) ZI.

A surface not-printer covered with a layer of sodium silicate which must be absorbent and refuse ink (nonprinting zone) ZNI.

Larger will be the difference between two surfaces, of better quality will be the plate offset.

The plate offset containing aluminum

Smooth or brushed plate dry

Advisable, when the insolation must be as short as possible, it is not very resistant and does not ensure of good power of inking. Used for small quantity of pulling. Thickness: 15/100.

Plate granulated or brushed wet

This surface, micrograinée with the abrasive or by an electrolytic grainage ensures an excellent adherence of the layer and a good quality Hydrophile. The development of the plate offset must be neat not to leave residues of photosensitive layer in the grainage.

Thickness of the plate: 15/100 or 30/100.

Anodized plate, micrograinée

The surface hardened by anodization resists wear better on machine and oxidizes less quickly, the layer holds more on metal surface. This plate can have received an electrolytic grainage allowing the carryforward of fine screens (imitation). The bombardment of the aluminum plate by the electron S contributes to improve its solidity like its resistance to the Oxydation. Anodic oxidation occurs in an acid medium traversed by a D.C. current.

Thickness of the plate: 15/100 or 30/100.

Plate lithochromée

Anodization is replaced by a galvanic chromium chechmate coating. The Chromium, Absorbent solid and par excellence, has an obvious advantage. Used for important pullings, the humidification of the plate being considerably reduced, it offers excellent returned colors.

The plate without water (waterless)

to see article Press offset: The waterless (offset without damping).

The polymetallic plate offset

It is made up of a base Acier being used as Support,

of a layer of Copper sensitive to the Ink,

of a layer of Chromium sensitive to water.

It has a high cost, used for very high circulation figure.

Thickness of the plate: 0,5 to 1,5 Misters.

Other bases: paperboard - film photographic polyester

The electrographic plate

The electrostatic plate or electrographic, the image is deferred on the layer by differentiation of the positive or negative electric charges (photocopy).

The xerographic plate whose realization is based on the same principle as previously. In xerography, the image is formed on a plate or a drum of selenium, then transferred on the final support of impression.

The plate out of paper or film

The paper plate often known as stereotype by the users of offset of office. To use for weak pulling and of a reduced cost since the film is used as plate.

Treatments of the plate

To each type of plate a treatment recommended corresponds: insolation by lamp with Ultraviolet radiation, Xenon, Halogenous…

Products of revelation adapted to the type of plate used.

Photosensitive layers

The plate offset can be directly insolated by a laser beam of low power without passing by the phase of film, process Computer to Plate, CtP.

The carryforward of the image of film on the plate is carried out by photographic film in direct contact.

The plate must be covered with a photosensitive layer. The majority of the plates offset are précouchées and this layer is often presensitized.

The diazo photosensitive layer

The diazo photosensitive layer is of positive type, it is broken up by the action of the light. The diazo compounds are complex organic compounds whose common character is a molecule containing two Atome S of Azote. They present the property to be broken up by the Lumière.

The diazo offset layer presensitized is used for the plates offset positive. It is photo-soluble.

The photosensitive layer burned by the light breaks up with the revelation of the plate and lets appear metal.

The plate can be preserved several months before use safe from the light.

The photopolymère layer

The photopolymère layer is of negative type, it hardens under the action of the light. The photopolymères are synthetic substances which undergo a molecular transformation (polymerization) when they are exposed to certain luminous wavelengths. The action of the light hardens the layer which becomes insoluble whereas the not insolated parts can break up, which exposes metal.

The presensitized offset layer photo-polymer is used for the negative plate offset. It is photo-hardening.

The treatment of the plate offset lends to employment for a press to print

Before 1969

1 - to clean
2 - to sensitize
3 - to dry
4 - to insolate
5 - to develop
6 - to fix
7 - to dry
8 - to enamel
9 - encrer
10 - to rinse
11 - to strip
12 - to rinse
13 - to strip
14 - to rinse
15 - to prepare
16 - to rinse
17 - to gum

since 1971

1 - to insolate
2 - to reveal
3 - to rinse
4 - to gum

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