Types of Lithographic Plates

Types of Lithographic Plates

Contact plates represent a significant portion of commercial printing plates produced photographically using negative or positive film. These plates are coated with a light-sensitive material that alters its solubility upon exposure to light within a contact frame. This device holds the film and plate together under firm vacuum pressure. An appropriate light source activates for a precise duration, controlled by a timer or light meter. Negative-working plates are exposed using high-contrast film in negative form, while positive-working plates use positive film. In both cases, light penetrating the transparent areas modifies the coating's solubility. The plate is subsequently developed or processed to remove the coating from nonimage areas.

Negative-working contact plates are the most prevalent type. Their soluble coatings become insoluble when exposed to light. The clear image areas of the negative film allow light to pass through, forming the plate's image areas. The unexposed coating in the nonimage areas remains soluble and is removed during development, as illustrated in Fig. 2-24.

Positive-working plates feature a complete coating of ink-receptive, light-sensitive material. They are exposed through positive films. The unexposed areas (image areas) remain insoluble in the developer. The exposed areas (nonimage areas) are solubilized by light and are removed during development, leaving the water-receptive coating as the nonimage areas.

Surface plates. Contact plates fall into two general categories: (1) surface and (2) multimetal, regardless of being negative or positive. Surface plates may be either presensitized or wipe-on. Wipe-on plates, while still occasionally used in the newspaper industry, have become increasingly rare. Presensitized plates are coated with a sensitizing agent by the manufacturer, whereas wipe-on plates are coated by the platemaker just before imaging. A processed surface plate is shown in Fig. 2-25.

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

Multimetal plates. Highly durable positive- and negative-working surface plates capable of lasting for more than 1,000,000 impressions under normal conditions are available. For even longer pressruns, multimetal plates are used. These include both bimetal and trimetal types, constructed from two and three laminated metals, respectively. A processed multimetal plate is shown in Fig. 2-26.

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

A specialized bimetal plate is created by electroplating a thin layer of chromium over a base metal of copper or brass. During processing, the chromium is etched away to expose the base metal in the image areas, while chromium remains on the plate in the nonimage areas.

Trimetal plates are manufactured by electroplating two metals onto a third base metal, typically aluminum, mild steel, or stainless steel. A layer of copper is first electrodeposited on the base metal, followed by a thin layer of chromium. The chromium forms the nonimage areas and is etched away to bare the copper in the image areas. For grained plates, the base metal sheet is grained before the other metals are plated. The thin metal films have minimal effect on the underlying grain.

Digital-direct plates. The digital-direct plate workflow, commonly known as direct-to-plate or computer-to-plate (CTP) technology, eliminates the need for film, stripping, and film-based proofing. This prepress workflow requires that imposition and proofing are completed digitally.

Looking ahead to 2026, the lithographic plate sector continues to integrate with broader digital transformation trends. Automation in CTP workflows and a focus on sustainable chemistry for plate processing are key industry priorities. These developments aim to enhance efficiency while reducing environmental impact across the printing lifecycle.