printing plate systems are generally processed in three basic steps: 1)
pre-exposure processing 2)
washout. A special equipment may be required for each step.
plates, which are ready to be imaged, do not require any pre-exposure
processing. Manufacturers of liquid photopolymers also supply suitable
processing equipment for their plates. These generally include resin
application, exposure and washout steps in one unit. Separate units are also
available for small users, as for the hand stamp market.
typical large platemaking system for liquid photopolymers can make plates with
52” x 80” image area. It can make plates in thickness ranging from 0.017 inch
to 0.280 inch. An in-line processing unit for the imaged plates simplifies
handling of such large plates. All new platemaking equipment is
computer-controlled and is Internet-accessible for remote diagnostics and
first step in the plate- processing is placement of a negative on the glass
plate of the equipment. The negative is covered with a thin transparent plastic
film. The photopolymer is then pumped over the cover film and leveled
automatically to a proper thickness. At the same time, the plate substrate
(polyester film) is rolled over the polymer. The plate is then ready for
exposure from both sides. The top exposure, without a negative, with limited
penetration, provides a hardened base layer, while the bottom exposure, through
the negative, provides the relief image.
In an alternate system, the liquid photopolymer is
metered upon polyester film, aluminum foil or paper placed on a vacuum platen.
Exposure is done off-contact.
Fluorescent light as well as all types of high
intensity UV sources are used for exposure. Exposure times vary from less than
a minute to up to 12 minutes depending upon the type of source, photopolymer and
thickness. Most of the relief plates also require a short "back exposure" for
improved adhesion to the substrate. Litho plates have a very thin coating, so
they can be exposed in ˝ to 2 minutes. Flexographic plates, which are up to 250
mils thick, require up to 30 minutes for black light exposure. The size of
equipment varies from a table top model to a freestanding automatic unit,
processing up to 240 plates/hour.
Laser exposure systems are now
available. These are basically used for computer-to-plate systems. Most
computer-to-plate systems for lithographic plates use thermal plates, which are
not based on photopolymers. Thermal exposure is done at 800-1200 nm. New
violet-light (405-410 nm) imaging systems have now given fresh life to
photopolymer systems. Sometimes also called "blue"
or "violet" laser diodes, these low-cost, solid-state semiconductor devices emit
energy at 405-410 nm in the violet / low-UV wavelength range.
relief-plate-making system is generally sold as a system that includes an
exposure unit and a developing unit. They are often combined in a single
equipment for a fully automatic operation. Development is done in water,
aqueous solutions or solvents.
Some letterpress polyamide relief plates use
tap water. Another uses 85:15 ethanol/water mixture. Flexographic plates are
usually developed in hydrocarbon-based solvent mixtures or aqueous solutions.
In newer flexographic systems the plates after exposure are developed by
removing the unexposed photopolymer by melting it away and absorbing it in a
The wet-processing equipment generally
incorporates spray nozzles and a flatbed or a rotating drum to hold the plates.
Output of the development/ washout units varies from less than 5 plates per hour
to about 240 plates/hour.
After development, the plates are dried in either
the same equipment or separate units. Most solvent-based systems have separate
drawer-type drying ovens.
After drying, plates may require additional
treatment. For example, some flexo plates are dipped for 5 minutes in a
chemical solution to eliminate surface tack.
Some plates are given a post-exposure of UV light for 2-10 minutes in
either the exposure unit or a separate post-exposure unit, to harden the image.
Some companies also offer solvent recovery units.
Lithographic plates can be developed manually
or with automatic processors. The latter can develop and finish plates up to
60" wide and of any length.
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