Means of Lowering the Contrast of Association Product Printing Plates

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Wang, Ming-Hwa
Association product; Baking; Continuous-tone printing; Contrast; Halftone process; Lithography; Photosensitive coating; Printing plate
thesis / dissertation description
The Association Product is a photosensitive coating for continuous-tone litho plates (It can also be used as a plate coating for halftone printing). It consists of phenolic resin and poly(ethylene oxide) and is photosensitized by a water soluble sensitizer. Continuous-tone lithography has better quality potential than the halftone process. Because the halftone process converts the continuous-tone image of the original into small dots of varying sizes, it thus inherently degrades the image quality and adds time and material costs in screening. The water soluble sensitizer converts the hydrophilic Association Product to an oleophilic state in direct proportion to exposing energy, which makes the Association Product become a single-phase plate, and thus simplifies the processing procedure. The curing property of phenolic resin makes the Association Product hardened simply by applying heat, and thus gives a long-run plate. A significant problem of the Association Product printing plate is that the contrast is too high (gamma equal to 3.0 or higher). This study investigated possibilities of lowering the contrast of Association Product printing plates by physical ways such as varying the baking time and the thickness of the photosensitive layer, and a factorial experiment was designed. It was found that the thinner the coating, the lower the contrast; the less the baking time, the lower the contrast. In the best combination of conditions in this experiment (proofs printed by an Association Product plate which was coated by a coating solution containing 1000 ml of DMF and without baking), the gamma value was 1.4, which was much better than it is usually. Baking hardened the Association Product printing plate, so after-process baking was used to get a long-run plate. Although after-process baking lowered the density of non-image area, the contrast was increased from 1.4 to 2.7.