Patent GB-737651-A (GB)
Title: Improvements in and relating to methods and materials for making overlay masks adapted for use in the production of photomechanical printing surfaces
IPC: G03F1/00
Filed: 1950-03-07
Abstract: 737,651. Producing photo-mechanical printing surfaces. PRINTING ARTS RESEARCH LABORATORIES, Inc. March 7, 1950 [March 16, 1949], No. 5768/50. Class 98 (2). A mask for use in the production of photomechanical printing surfaces having highlight, line and solid areas free of half-tone screening from an original having such areas and also areas to be reproduced in half-tone is produced by applying a layer of colour-changeable substance to a support, including in either the halftone or the other areas of the original a colouraltering substance reactive with the colourchangeable substance to effect colour change thereof, and contacting the moistened layer of colour-changeable substance with the original. Alternatively, an intermediate sheet having an absorbent surface may be contacted first with the original and then with the layer of colourchangeable substance. The surfaces to be contacted may be passed between geared rollers covered-with soft rubber or similar material. As shown, an ultraviolet absorbent, e.g. quinine bisulphate and sodium sulphite, are included in the half-tone areas 20a and 20b of the original, Fig. 1, but not in the line-and-type area 22 or the background areas 21. A transparent sheet carrying a colloid layer containing Fuchsine is moistened and contacted with the original, whereby the sodium sulphite in the half-tone areas bleaches the dye in the area 23 of the masking sheet, Fig. 2. A negative, Fig. 4, made from the original masked by the mask of Fig. 2 and through a yellow filter bears an image 20a<SP>1</SP>, 20b<SP>1</SP> of the half-tone areas on a clear background, while a line negative, Fig. 5, made from the masked original through a visually opaque but ultraviolet transparent filter has negative areas 21b, 22b with transparent areas 20c and 20d corresponding to the half-tone areas. The negatives of Figs. 4 and 5 may be produced as a single negative. To avoid risk of transfer of dye to the original, an intermediate gelatin-coated sheet may be moistened and contacted with the original and may then be contacted with the dyed colloid layer used to form the mask of Fig. 2. The intermediate sheet may be transparent and may absorb dye to form the mask itself or to form a supplementary mask used in register with the mask proper. The colloid layer may be hydrophobic and the colour-altering compound may be soluble in an organic solvent, e.g. acetone. The rear of the masking sheet may be coated with an anti-curling layer. The colour may be incorporated in the original and the colour-altering substance in the masking sheet, e.g. the half-tone areas of the original, may contain Brom Cresol Purple in its acid yellow form, and an intermediate sheet may be used to transfer dye to a masking sheet of pH suitable to convert the transferred dye to its blue-violet basic condition. Alternatively, the half-tone areas may contain citric acid and the masking sheet Brom Cresol Purple in its basic blue-violet condition, a yellow filter being used for recording the half-tone areas of the masked original and a blue filter for recording the other areas of the masked original. The paper for the original drawing may be coated with an acid salt and the tone areas of the drawing made with materials containing a basic compound sufficiently alkaline at least to neutralize the acidified paper surface. The masking sheet contains Heptamethoxy Red or other indicator dye, and minus-red and red-transmitting filters are used in the copying. A gelatin-coated layer may be contacted with an original containing a soluble reducing agent in the tone areas, and then dye may be applied to the gelatin by painting, spraying or imbibition. The tone areas of the original may contain sodium carbonate or disodium phosphate, and a moistened intermediate layer may transfer such basic compound to a masking layer containing p-nitrophenol in its colourless acid condition, the transferred basic compound converting it to its yellow basic condition. Half-tone and line negatives are made .from the masked original using yellow and blue filters respectively. Specification 525,300 is referred to.