Printing Plates

The plates used in offset printing are thin (up to about
0.3 mm), and easy to mount on the plate cylinder, and
they mostly have a monometal (aluminum) or, less often,
multimetal, plastic or paper construction. Aluminum
has been gaining ground for a long time among
the metal-based plates over zinc and steel. The necessary
graining of the aluminum 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 (anodizing),
that is, electrochemical graining with subsequent
oxidation.
The imaging, ink-accepting coating (light-sensitive
coating, thickness around 1μm) is applied to the base
material. This material is usually a polymer, or copper
in the case of multimetal plates (bimetal plates). Lightsensitive,
diazo (photopolymer) pre-coated aluminum
printing plates are now the predominant plates in
printshops. The image transfer is produced via the 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.
The thin coating of aluminum oxide created by the
special treatment of the aluminum base material a particularly
stable water-attracting (hydrophilic) surface
with special retention properties. In processing a precoated
offset printing plate the essential task lies in
achieving surface differentiation using two basic steps,
exposure and developing.
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:
• hardening of the light-sensitive layer by light (negative
platemaking),
• decomposition of the light-sensitive layer by light
(positive platemaking).

If the light-sensitive coating is hardened photochemically,
it becomes insoluble for the developer in the exposed
areas. If, on the other hand, the light-sensitive
coating is decomposed photochemically, the developer
removes the exposed light-sensitive coating from the
base (e.g., aluminum).These two different platemaking
processes (positive and negative platemaking) require
different exposures to create images,that is, different
types of films that are produced in advance.

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. If light falls
on the ink-free areas during exposure, the light-sensitive
layer “decomposes,” which results in the non-image
areas being uncovered (in this case aluminum) during
the developing process. This process has the disadvantage
that film edges and dust, that is, the darker
areas on the film compared to the translucent image areas
on the film copy, are sometimes reproduced on the
printing plate as ink-accepting surface elements.

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.The light-sensitive coating is
hardened on the printing plate by light, so that it stays
in place after the developing process, as opposed to the
unexposed areas, which are removed.
The developed plates are then gummed up to protect
the plate surface and preserve the plate. The finished
printing plate will, of course, be identical in informational
content regardless of whether positive or negative
platemaking is used. Only the kinds of film used
for production are different. Different characteristic
curves between the tone value of the film and that of
the plate must be taken into account in prepress.


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