Types of Lithographic Plates

Contact plates. A high percentage of today's commercial printing plates are made photographically using either negative or positive film. These plates are coated with a light-sensitive material that changes its solubility when exposed to light in a contact frame. The contact frame is a device that holds the film and plate together in firm contact under vacuum pressure. A light source that is appropriate for exposing the plate is activated with a timer or light meter for an exacting exposure. Negative-working plates are exposed with high-contrast film with the images in negative form. Positive-working plates are exposed with films in a positive format. In either case, the light that penetrates the transparent areas changes the solubility of the plate coating. After exposure, the plate is developed or processed to remove the coating from nonimage areas.

Negative-working contact plates are the most popular. They have soluble coatings that become insoluble upon exposure to light. The image (clear) areas of the negative allow light to penetrate and form the image areas of the plate. The unexposed coating (in the nonimage areas) remains soluble and is removed during development. See Fig. 2-24.

Positive-working plates are completely covered with an ink-receptive, light-sensitive coating. They are exposed through positives. The unexposed areas (image areas) are insoluble in the developer. The exposed areas (nonimage areas) are solubilized by exposure to light and then removed during development, leaving the water-receptive coating as the nonimage areas.

Surface plates. Whether negative or positive, there are tow general categories of contact plates: (1)surface and (2) multimetal. Surface plates may be either presensitized or wipe-on. Wipe-on plates are still sometimes used in the newspaper industry but are becoming more rare in recent years. Presensitized plates are coated with a sensitizing agent by the manufacturer; wipe-on plates are coated by the platemaker before imaging the plates. A surface plate after processing is shown in Fig. 2-25.

Presensitized plates consist of a thin film of light-sensitive material -- usually a diazo compound or a photopolymer -- that is coated on the sheet metal. Photopolymer coatings consist of polymers and photosensitizers that react (crosslink) during exposure to light to produce a tough, long-wearing image area. Diazo coatings also react with light to produce a tough, long-wearing image area. After exposure, the plates require special organic or aqueous solvents for processing. Both negative-working and positive-working plates are available with diazo or photopolymer coatings.

Multimetal plates. Very durable positive-and negative-working surface plates that last for more than 1000000 impressions under normal conditions are available. For even longer pressruns, multimetal plates are available. Multimetal plates include both bimetal and trimetal types, which are made of two and three laminated metals, respectively. A multimetal plate after processing is shown in Fig. 2-26.

Bimetal plates consist of a base metal, usually aluminum or stainless steel, on which a thin layer of copper has been electroplated. Copper, which is naturally oleophilic, forms the ink-receptive image areas and is removed from nonimage areas. The bare base metal, the nonimage area, is easily desensitized to become water-receptive. Bimetal plates are supplied in smooth and grained form.

A special type of bimetal plate is made by electroplating a thin layer of chromium over a base metal of copper or brass. The chromium is etched away during processing, exposing the base metal in the image areas. Chromium remains on the plate in the nonimage areas.

Trimetal plates are made by electroplating two metals on a third base metal. The base metal is usually aluminum, mild steel, or stainless steel. A layer of copper is first electrodeposited on the base metal, followed by a thin layer of chromium. Chromium forms the nonimage areas and is etched away to bare the copper in the image areas. If grained plates are desired, the base metal sheet is grained before the other metals are plated on it. The films of these metals are so thin that they have little effect on the grain.

Digital-direct plates. The digital-direct plate workflow is usually referred to as direct-to-plate or computer-to-plate(CTP) technology. The prepress workflow requires that imposition and proofing are done digitally, eliminating the need for film, stripping, and filmbased proofing.