Offset Lithographic Plate Making
Diazo plates are coated with organic compounds and are developed with a
special solvent. They have a shelf life of about one year. These are used
for print runs of about 150,000 impressions.
Photopolymer plates are coated with organic compounds which are very
inert and abrasion resistant. This makes them last much longer than diazo
plates. They are used for print runs of up to 250,000 impressions
Silver halide plates use photosensitive coatings similar to
photographic film, except that the silver halide emulsions are slower and
for color reproduction are coated on anodized aluminum. The processing
solutions contain silver which must be recovered with the proper equipment
before being discharged to the sewer. Film based silver halide plates are
used for single color printing and metal based silver halide plates are
used in computer-to-plate systems.
Electrostatic plates are based on the principle of the electrostatic
copier. There are two types, inorganic photoconductors on a drum and the
second is organic photo conductor on a substrate. These are used mostly in
quick printing jobs of 100,000 impressions or less.
Bimetal plates use presensitized polymer coatings. There are two types
of bimetal plates; copper plated on stainless steel or aluminum and
chromium plated on copper. These are the most expensive, but rugged plates
and are used for very long print runs. In fact they are capable of print
runs in the millions.
Waterless plates, used on waterless presses only, consist of ink on
aluminum for the printing areas and a silicone rubber for the non-image
areas. These systems require special inks and high grade paper to avoid
debris accumulating on the blanket.
Ablation plates are imaged by digital data and requires no chemical
processing. These plates are digitally imaged by selectively burning tiny
holes in thin coatings of a polyester or metal base. These types of plate
are used on the new computer to plate imaging systems and the brand new
computer to press system. The cost of equipment and materials is high and
the technology is relatively new.
Heat sensitive plates are exposed by infrared diodes in special
imagesetter and processed in water based chemistry. This a relatively new
technology and requires the printer to invest in new equipment that can be
quite costly.
Offset Lithographic Inks:
- Petroleum Based
- Vegetable Oil Based
- UV & EB Curable
- Heatset
There are four common types Lithographic inks, unlike Gravure, Flexo,
and Screen are very viscous to the point they are paste-like. Litho inks
are generally very strong in color value to compensate for the lesser
amount applied. Sheetfed litho inks are similar to oxidizing types of
letterpress inks. To accelerate drying and control ink flow
characteristics litho inks contain solvents (or drying oils) which result
in some VOC emissions from the ink. More
information...
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