GB202601A - Improvements in and relating to photo-mechanical printing - Google Patents

Abstract

202,601. John, R. Aug. 16, 1922, [Convention date]. Photo-mechanical printing surfaces.-In making a gelatin or colloid printing-surface, more especially for litho or offset work, the colloid film. is sensitized with a chromic salt, exposed and immediately subjected to the action of a selective hardening agent consisting of alcohol and/or ether. The colloid film may also be hardened after removal of the chromic acid salt by alcohol or by heating, and the colloid film may also be hardened, as by formaldehyde, before sensitizing- the film. The printing-plate can be damped with water only, no glycerin or other agent being necessary, and the necessity for periodic gumming is avoided. The plate can be sensitized after coating it with colloid, or :he sensitizing salt can be added to the gelatin coiting solution provided the temperature is kept below 130‹ F. A thin sheet of metal, celluloid or other material may be used. The metal plate is grained or roughened to facilitate, adhesion of the coating, but not for reticulating the printing-surface. A gelatin coating consists of 2250 grains of hard gelatin in 32 ounces of water, the gelatin being soaked in cold water and brought into solution by heating to 120‹ F. The plate is coated at this temperature to a thickness of .088 to .0012 inches, and dried. The gelatin is next hardened in a bath of alcohol (32 ounces) and 40 per cent formaldehyde (1 ounce), dried, washed in running water, and again dried. The sensitizing solution contains ammonium bichromate (450 grains), alcohol (32 ounces), and water (16 ounces), to which a light-restraining agent (e.g. 90 grains of erythrosine) may be added. Exposure, is effected as soon as possible. A half-tone, negative with a very fine screen or grain may be used. After exposure, the plate is treated before washing out the bichromate in a bath of grain alcohol of high proof (95 per cent or higher), or of ether, or ether and alcohol, in order to increase selectively the hardening action in the exposed parts without hardening the unexposed parts, and leave the film in a dry state. After washing to remove the soluble bichromate remaining, the plate may again be treated with alcohol to further harden the hardened portions. The plate is then heated in an oven to 400‹ F which completes the hardening of the exposed parts, gives a permanent printing-surface., toughens the unhardened parts without affecting their water receptivity, and increases the adhesion of the gelatin and the grained plate.