Types of Offset Lithographic Inks

Film negative Photopolymer coating Aluminum plate 

Negative lithographic plate Positive lithographic plate 

In fabricating positive-working plates (Fig. 10.3), film positives are used as masks to image the unprocessed plates coated with UV-photosensitive polymers. The exposed parts of photopolymers (i.e., the areas under the clear areas of the mask) become more soluble in the developing solvent and are washed away, while the unexposed areas of the photopolymers remain unaltered and are left on the mask. Following processing, a thin layer of gum is applied over the plate to protect the nonimage areas from ink. 

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