GB986603A - Lithographic printing plates - Google Patents

Abstract

In the production of lithographic printing plates, as a preliminary step, sheets or plates of chromium, steel or aluminium are cleaned and etched and then brought into contact with an aqueous solution containing at least 0.5% by weight of gluconate ions. The source of gluconate ions may be gluconic acid or an aqueous solution of iron, nickel, zinc, calcium, magnesium, or potassium gluconate. The plates may be cleaned and etched by chemical or mechanical means. The concentration of gluconate ions is preferably from 2-4% by weight of the solution, the pH of which is preferably from 0.1-3.0. The solution containing the gluconate ions advantageously also contains one of the ions of tartrate, acetate or carbonate but contains no phosphate, nitrate or oxalate ions. In one example an aluminium plate is immersed in a bath of trichloroethylene to remove grease and is then grained by passing it on an endless belt under a set of revolving brass bristle brushes located on the perimeter of a rotating circular support. During this operation the plate contacts for one minute a slurry comprising 4800 grams of pumice, 4000 ml. of deionized water, 4 grams of sodium hydroxide and 4 grams of sodium gluconate. The plate is then flushed with water and subsequently with an acetic acid solution. After a further flushing with water the plate is dried by gas burners. Formic, lactic or gluconic acids may be used in place of the acetic acid rinse. A strongly acid gluconate treatment solution is preferred with chemically etched plates. The treated plate may be coated with a nylon diazo resin solution and imaged by photographic techniques. Etching. In one example a suitable composition for a cleaning and etching bath comprises 3785 ml. of water, 28.35 gms. of trisodium phosphate, 141.75 gms. of sodium hydroxide, and 0.40 gms. of sodium lauryl sulphate, and the treated plate is subsequently immersed in a desmutting bath containing an aqueous solution of nitric acid. In a further example an aluminium plate is treated with trichloroethylene, grained with bristles and flushed with an aqueous solution of pumice. The metal exposed during imaging by a photographic process is etched with a ferric chloride solution comprising 2630 ml. of calcium chloride solution, 1000 gms. zinc chloride, 750 ml. iron perchloride solution, 37 ml. hydrochloric acid and 70 gms. of cupric chloride.