Offset printing plate based on chromium plated steel - has printing elements treated with higher unsatd. fatty acid and hydrophilised non-printing elements

摘要

Offset printing plate consists of a Cr-plated steel plate with coplanar printing elements of a chemical cpd. of Cr with a higher monobasic unsatd. fatty acid or its water-soluble salt and nonprinting elements of hydrophilised Cr. The metal plate is given a copying coat based on PVA or a photopolymerisable compsn. After drying, this is provided with an image, which is developed, and the nonprinting elements are hydrophilised. A diapositive is used as original, so that the image areas are not hardened. These areas are removed by development at 18-50 deg.C, using water in the case of PVA and a 0.2-1% aq. soln. of an alkali metal salt or hydroxide for a photopolymerisable compsn. The printing elements are hydrophobised at 18-50 deg.C by treatment with a 2-10% soln. of a higher monobasic unsatd. fatty acid in vaseline oil, a vegetable oil or a lower aliphatic alcohol or with a 2-10% aq. soln. of a salt of this acid. The hardened areas are then removed from the nonprinting areas at 18-50 deg.C, using a 0.1-0.5% aq. soln. of an oxidising agent and then an aq. soln. contg. 1-3% reducing agent and 3-5% mineral acid in the case of PVA and a 1-3% aq. soln. of alkali metal salt or hydroxide for a photopolymerisable compsn. These elements are then hydrophilised at 18-35 deg.C by treatment with a 5-10% aq. soln. of an alkali metal salt of a hydroxy-acid or a 1-2% aq. soln. of a hydrophilic colloid. The plate has high mechanical strength, can give large editions and can be produced easily.