Preparation of the printing plate for an offset litho printing press applies a sprayed surface coating of a thermal reactive chemical and a second activator chemical for use within a month for direct exposure and use without developing

摘要

Preparation of the printing plate involves applying a sprayed surface coating of a thermal reactive chemical and a second activator chemical for use within a month for direct exposure and use without developing. For on-site preparation of lithography printing plates, a coating (3) is applied to the surface (1) of the printing side with a storage life of one month or less. The coating is composed of a thermal reactive chemical which, when exposed to infra red rays, alters its affinity at least either to ink or water. It includes a second chemical which, when mixed with the first chemical, increases the infra red sensitivity of the substance. The image is exposed by infra red rays on to the printing side within one month of the coating application. The two chemicals (4,5) are mixed together, less than a month before the plate is to be coated. The surface coating forms a process-free thermal printing plate, which requires no chemical developing and, after the image exposure, is used directly on the printing press. The two chemical components are sprayed (6,7) on to the plate surface by separate jets, in an overlapping coating action. Or they are mixed together at the jet directly before spraying on to the plate surface. The printing surface is the plate cylinder (2) in an offset litho press. After use, the coating is removed for a fresh coating to be applied when required. The first chemical, with a thermal reaction, is a polymer with a reaction to water which is either hydrophilic or hydrophobic, which is reversed on exposure to infra red rays. The second chemical is an activator which increases the polymerization of the coating substance. The activator is an organic acid and preferably a polysulfonic acid. The second chemical can also be to increase the image resolution. The polysulfonic acid is used in a proportion of 1-20 wt% of the polymer in the first thermal reactive chemical component.