Patent GB-573798-A (GB)
Title: Improvements in the production of photographic stencils
IPC: G03F1/54
Filed: 1943-12-09
Abstract: 573,798. Photographic wash-out reliefs. KODAK, Ltd., GRESHAM, D. C., and LOENING, E. E. Dec. 9, 1943, No. 20605. Drawings to Specification. [Class 98 (ii)] [Also in Group XXXVI] A stencil is produced from material comprising a light-sensitive layer comprising a hardenable colloid superimposed on another (preferably hydrophobic) layer more easily etchable than the former (when hardened) by exposure, dissolving away the image layer imagewise, and dissolving away the other layer in the areas laid bare with a bath which simultaneously etches the unhardened and less hardened parts of the upper layer but not the hardest parts thereof. The light-sensitive layer may be a gum-bichromate or silver halide emulsion layer. The lower layer may be soluble in acid baths, and the viscosity of the bath may be adjusted to obtain the best balance between the rate of the effect on the lower layer and that on the upper layer. If the light-sensitive layer is of gelatine emulsion, the imagewise solution may be performed with lactic acid, calcium chloride or magnesium chloride solution, or a mixture of glycerine, water, and sulphuric acid. The lower layer may be of shellac or " Bakelite " (Registered Trade Mark) and may be rendered insoluble after imagewise removal by baking. If the imagewise removal of the light-sensitive layer is performed in an acid bath, the other layer may be dissolved imagewise in an organic solvent. If the other layer is dissolved imagewise in an acid bath, the light-sensitive layer may be dissolved imagewise in warm water. Both layers may be removed imagewise in an acid bath, e.g. lactic acid to. which may be added ethyl lactate. Loading agents such as sodium sulphate may be added. The stencil may be produced on an electrically-conducting support which can be plated in the unprotected areas and the plated metal stripped from the support to produce a very fine grid, e.g. an electrical resistance. A thin sheet of copper may be nickel-plated and highly polished, coated with a thin layer of " Bakelite " soluble in lactic acid, coated with a layer of gum sensitized with potassium dichromate, exposed to a line positive of the desired grid, the unexposed portions washed away, the layer dried and treated with a mixture of lactic acid and ethyl acetate, the support made the anode in a copper-plating bath, the plate rinsed in water and made the cathode in a copper-plating bath, the gum and " Bakelite " removed by a solvent, and the grid stripped off. A lead design for use with X-ray exposures may be produced by coating a thin brass sheet on both sides with a layer of " Bakelite " soluble in lactic acid, coating both sides with bichromated gum, exposing both sides to the design in register, washing away the unexposed portions, dissolving the unprotected " Bakelite " in a lactic acid bath, drying, treating with aqueous lactic acid and immersing in a lead perchlorate plating bath between two lead anodes. The " Bakelite " layer may contain a colouring matter. A photolithographic printing surface may be produced by coating a zinc or aluminium plate with a solution comprising Bakelite ", Sudan Red, acetone, and amyl acetate (and, if necessary, methylated spirit), overcoating with a gelatino-silver halide emulsion, exposing to a half-tone positive, developing in a tanning developer, removing the unhardened gelatine, drying, removing the unprotected " Bakelite " with lactic acid, and treating in a bath of nitric acid or ferric chloride to etch the bared metal. The exposed material may be developed in a non-tanning developer and the gelatine subsequently imagewise hardened. The process may be applied to line work. The plate may be processed by reversal by first developing in a non-tanning developer, washing, and developing in a tanning developer in the light, or a positive image may be obtained using an etch-bleach bath containing hydrogen peroxide. A graticule may be obtained by coating glass with a " Bakelite " layer, drying and warming to harden, overcoating with bichromated gum, exposing and removing unhardened gum and the " Bakelite " with a lactic acid bath (or first removing the unhardened gum. in a glycerine bath containing sulphuric acid and then the " Bakelite " in the lactic acid bath), and etching with hydrofluoric acid. Specifications 28415/07, 18775/08, [both in Class 98], and 473,446 are referred to.