GB640470A - Improvements in the manufacture of lithographic printing plates - Google Patents

Abstract

640,470. Printing-surfaces. COATES BROS. & CO., Ltd., and COATES, L. Sept. 4, 1947, No. 24434. [Class 100 (ii)] A bi-metallic planographic printing-plate is formed with a non-printing water-receptive surface of a metal such as chromium having its surface in slight relief relatively to the printing- surface of smooth copper. The plate is formed by coating a copper, or a copper-coated zinc, iron or aluminium plate with chromium by electrolytic or other means. A design is then transferred to the chromium by printing or photography. In the first case the plate is treated with a 2 per cent solution of nitric acid to make the chromium surface receptive to transfer ink, and the plate with a positive transfer image is flooded with an etch-resisting lacquer, and when dry the transfer ink is washed out by turpentine or white spirit leaving the etch resist on the uninked parts. The plate is etched with a solution of equal parts glycerine and hydrochloric acid. The resist is dissolved out in alcoholic caustic soda, and degreased with an abrasive powder moistened in 10 per cent caustic soda, and finally passivated by a 2 per cent solution of sulphuric acid, or a solution of phosphoric, or lactic acid. The etch-resisting lacquer may be composed of shellac 10 parts; methylated spirit 40 parts; ammonia 9 parts; water 39.7 parts; ammonium dichromate 1.0 parts; urea 0.3 parts. An alternative composition contains phenol formaldehyde resin, cellulose nitrate, alcohol, amyl acetate, ethyl lactate, butyl alcohol, methyl violet. To prepare the design photographically the chromium is coated with a solution of glue and ammonium bichromate and exposed through a line or screen photo-positive. The developed negative is dried and dyed with methyl violet, and the etching is carried out until the copper image is seen. After the negative is dried and hardened it is etched and finished as in the prior process.