GB664353A - - Google Patents

Abstract

664,353. Photo mechanical printing surfaces. TILLOTSON & SON, Ltd., WILD, L., and SMITH, J. Feb. 24, 1949 [Feb. 28, 1948], No. 6310/48. Drawings to Specification. Class 98 (ii). Before the preparation of a photomechanical printing surface the surface is contoured, approximately in accordance with the density variation over the subject, by a preliminary etching after coating with a first photo-sensitive resist and exposing the resist under a diffuse negative of the subject. A printing plate of bronze, brass, zinc or copper is cleaned and coated with the first resist, preferably of bichromated gelatine containing also ammonia, Turkey red oil, and the opaque paste containing iron oxide and known under the Registered Trade Mark " Photopake ". This is exposed under the diffuse negative developed in water and etched, for example with ferric chloride. A further resist, containing bichromated fish glue or albumen, is then applied and exposed under a half-tone negative registered with the position of the diffused negative, developed in water, and fixed, for example by burning in. The plate is then etched. The two negatives are made successively with the same camera setting, the diffusion of one being produced by displacement of the half tone screen away from the plate, and being assisted by exposing at full lens aperture. With a line subject an ordinary negative is used instead of a halftone negative but a plain glass plate is inserted in place of the half tone screen, so that the same size image is produced as when using the half tone screen to diffuse the other negative. A lateral inversion is produced with a prism when photographing to enable the negatives to be printed emulsion side downwards on the printing plate. Preferably, the half-tone negative, at least, has a film base to enable it to follow the contours of the plate produced by the first etching. Alternatively, the image layer may be stripped and applied directly to the plate.