GB547568A - Method of preparing photocollographic printing plates - Google Patents

Abstract

547,568. Photo-mechanical printing surfaces. MACBRIDE, J. W. March 10, 1941, No. 3230. Convention date, Sept. 14, 1940. [Class 98 (ii)] An improved collotype printing-plate is produced by coating a plated metal base with hardened colloid, drying, bathing it in an alkali bichromate sensitizing solution, drying it, e.g. in a current of air at a controlled rate to control the printing contrasts of the plate, exposing it to light under a negative to produce imagewise hardening of the colloid, immersing it in water bath at a predetermined temperature, which bath swells the unexposed colloid faster than the hardened colloid above it so that the latter becomes reticulated and presents a grainy surface having a degree of fineness dependent on the temperature, and finally washing out the residual sensitizing agent. The plate may then be made ready for printing by treatment with a hygroscopic fluid, having a non-glycerine base such as ethylene glycol or di-ethylene glycol, which will not attack the colloid, but will be merely absorbed at the unhardened parts to maintain them moist. The base is preferably of copper, mechanically or chemically roughened, and plated with nickel or silver. Plated zinc or iron may also be used. The colloid may be gelatin hardened with chrome alum. The printing contrasts of the plate may be varied by choice of the sensitizing agent (potassium bichromate gives the strongest contrast), or of the strength of the sensitizing bath, as well as by the rate of drying of the sensitized coating. A known collotype process is described in detail.