Patent GB-278460-A (GB)
Title: Improvements in and connected with the production of printing surfaces
IPC: C08J5/24
Filed: 1926-07-16
Abstract: 278,460. Hole, E. S. July 16, 1926. Printing-surfaces; stercotype. - Printing-surfaces or matrices for the production thereof are prepared from a base of paper, cellulose, paper pulp, or the like impregnated with a material of the type produced by the action of phenol and formaldehyde or the like, the material being still in the water-soluble state when applied to the base and when placed in a press for moulding against an original. The impregnant consists of a mixture of an aldehyde, a coal-tar intermediate, and an alkaline condensing-agent, preferably formaldehyde and phenol in approximately equal proportions. and a small amount of potassium hydroxide, these being boiled together until no more formaldehyde is evolved. A strip of aluminium or a small amount of boric acid may be added to increase the seething and indicate more clearly the end of the reaction A quantity of cold water is then added, the mixture again boiled, and a solution of ammonium chromate, or ammonium dichromate and ammonia, in water, together with a small amount of copper hydroxide, added. Other chromates or chromic acid may be used. or, if desired, the alkali can first be neutralized and a smaller amount of ammonium dichromate employed. Methylated spirits can also be added, and glycerine, or calcium chromate or borata may be included according as it is desired that the material should dry slowly or rapidly. Sheets of filter paper, cellulose, or paper pulp are impregnated with the material and then allowed to dry. Matrices are prepared from the impregnated sheets by pressing them, at a temperature of about 300 F., on to the originals, which are! preferably heated before the sheets are imposed. The platen bearing against the impregnated sheets is preferably heated 40-60 hotter than the other platen. The platen may be greased to prevent the sheet from sticking. The matrix so prepared can then be used in the preparation of other printing-surfaces from impregnated sheets, the effect of heat and pressure on the impregnant being first to render it temporarily fluid and then to harden it permanently. The sheets intended to be used as matrices may be immersed, after drying, in a weak solution of ammonium or other chromate or dichromate. preferably with an excess of ammonia, in order to prevent sticking. In making printing-surfaces, the cold matrix is preferably placed on a cold metal plate so that the heat of the platen applied to the impregnated sheets affects them before the heat of the other platen which has to traverse the cold plate and matrix. A partly metallic surface for the printing-surface can be obtained by dusting the sheet before heating with the desired metal powder. In order to enable exact reproductions of originals having deep recesses to be prepared, the recesses are first, filled with a material which will unite with the impregnating mixture of the sheets. This material consists of powdered cellulose (such as wood flour), chromic acid solution, and the original sheet-impregnating mixture before the addition of the dichromate, this material being then dried and ground to a fine powder. The amount of chromic acid depends on the amount of potash in the original mixture, and zinc, copper, or aluminium chromate may be dissolved in the chromic acid, or the chromic acid can be replaced or supplemented by calcium chromate. Other metal salts or metals in powder form can be added to the wood flour to produce printing-surfaces partly of metal, and the wood flour can be substituted or supplemented by charcoal. lampblack. powdered cork. or asbestos. In making a matrix from type set by hand or machine, the interstices are filled by a paste made of talcum powder or plumbago and water. The paste is spread over the surface of the type, a number of sheets of blotting-paper being then superposed and the whole subjected to heat and pressure. The surface of the type is then brushed clean, leaving the crevices below the printing- surface of the type filled with the powder which was forced in by the blotting-paper. Also, the sheets used in the preparation of the matrix are preferably interleaved with one or more dry sheets of filter paper. To obtain surfaces of uniform thickness, the backs can be sandpapered or planed, and the surfaces can be strengthened by a sheet of textile at the back of or among the impregnated sheets. The matrices can be used either alone or in conjunction with an ordinary flong matrix for casting metal stereotypes. According to the Provisional Specification, the filling-material for use with originals having deep recesses consists of wood flour, zinc or copper dust and chromate, calcium borate, and the material for impregnating the sheets. The filler for use when preparing matrices from type consists of plaster of Paris. The sheets can be provided with a backing of veneer or other material capableof being sandpapered or planed to enable printing- surfaces of any desired thickness to be obtained.