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Print Process Descriptions: Printing Industry Overview: Lithography

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.

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