Lithographic Plate Composition

Lithographic Plate Composition

Conventional lithographic plates operate on the principle of oil-water repulsion, featuring two distinct surface areas. Oleophilic image areas attract oily ink, while hydrophilic nonimage areas attract water. As the press's dampening and inking systems distribute both water and ink across the entire plate, these chemically opposed areas remain separate.

Plate base. The majority of modern lithographic plates utilize a thin metal base, with aluminum being the predominant choice. Its popularity stems from a combination of light weight, flexibility for wrapping around the cylinder, and sufficient hardness to withstand surface compression. Alternatives include stainless steel, mild steel, and brass.

Gauge tolerance. Strict gauge tolerance in the sheet metal is critical for plate manufacturing. For plates up to 22 x 34 in. (559 x 864 mm), thickness must not vary by more than 0.001 in. (0.025 mm). A 0.012 in. (0.305 mm) plate, for instance, must measure between 0.0115 in. (0.292 mm) and 0.0125 in. (0.318 mm) across its entire surface—a tolerance of +/-0.0005 in. (+/-0.013 mm). Excessive variation causes pressure inconsistencies during printing, leading to uneven tints and solids.

Plate gauge. The required plate gauge depends on the cylinder undercut and press size. Standard thicknesses range from 0.0055 in. (0.14 mm) to 0.020 in. (0.51 mm), with plate sizes reaching up to 59 x 78 in. (1499 x 1981 mm). Larger-diameter press cylinders necessitate thicker plates to minimize stretching and cracking. Matching plate thickness to the cylinder undercut reduces the need for packing; a single sheet is less prone to creep and compression than multiple layers.

Plate graining. A smooth aluminum surface causes water to bead, which is incompatible with lithography. To ensure a plate accepts a uniform film of water, its surface is grained through mechanical, chemical, or electrolytic processes. This preparation enhances press latitude, reduces vacuum frame drawdown time, and helps eliminate halation. Figure 2-30 shows a grained, anodized aluminum plate at two magnifications.

In the United States, slurry brush graining is a common method. A continuous web of aluminum passes under rotating nylon brushes with a mixture of abrasives and water, producing a uniform, dark-colored, fine grain suitable for presensitized and wipe-on plates. A subsequent chemical etch yields a lighter, cleaner, and slightly rougher grain.

Chemical graining offers an alternative to mechanical brushing. Primarily used for treating smooth, short-run plates before coating—often double-sided plates for small presses—this method cleans and slightly roughens the surface. Bimetal aluminum plates are chemically grained to a much rougher texture, while some premium long-run plates undergo electrochemical graining for a uniform, rough grain. Stainless-steel bimetal plates are an exception; they remain smooth and ungrain ed because stainless steel is naturally hydrophilic.

Rougher-grained plates provide significant advantages: improved ink/water balance latitude, faster vacuum frame drawdown, reduced susceptibility to dirt and hickeys, greater on-press durability, and less dot slur. However, they are less capable of holding fine highlight dots compared to smooth-grained plates.

Silicating. Beyond graining, specific chemical treatments are essential for certain plate types, particularly wipe-on and negative-working diazo presensitized plates. Diazo compounds can react with untreated metals, so aluminum is often treated in a hot sodium silicate solution. This creates a barrier layer to prevent reactions, desensitizes the plate for better water receptivity, and improves the surface for bonding with the diazo coating.

As lithography advances toward 2026, the core principles of plate composition remain foundational. The industry continues to prioritize precision manufacturing and consistent material performance. Ongoing developments in automation and sustainable plate processing aim to enhance reproducibility while reducing environmental impact, ensuring this traditional technology adapts to modern manufacturing demands.