GB704148A - Improvements in and relating to the manufacture of printing screens - Google Patents

Abstract

704,148. Stencils. HLUBUCEK, J. Aug. 9, 1951 [Aug. 18, 1950], No. 20534/50. Class 100 (2) The manufacture of a printing screen comprises applying plasticized polyvinyl chloride in paste form in one or more coatings to the base material of the screen to be made, so as to form thereby those areas which are to be impermeable to the printing medium, and heating the applied coating or coatings to solidify the same to an elastomeric state. In one form, a temporary screen is prepared having a negative of the design, and the paste is applied through the permeable portions of the negative design on to the base material. The temporary screen is prepared photographically, using gelatine or polyvinyl alcohol, or by placing over one side of it a cut-out paper or like stencil of the design prepared, and applying by a squeegee a coating of the paste to the other side, whereupon the stencil temporarily adheres to the screen. The screen is stretched on a frame which is placed within a larger frame on which is stretched the material to form the final screen, the latter laid on a support such as a polished aluminium or other metal, or glass sheet, the paste applied through the temporary screen, and the coating heated. In another form, a cut-out or photographic positive of the design, such as a greaseproof paper stencil, is prepared and laid on a support, and over it a stretched silk-screen is placed and the paste applied. The paste passes on to the support in the permeable areas, while in the impermeable areas it causes the positive to adhere to the screen and remains in and on the screen. After heating, the positive is manually stripped off. The paste may comprise 40-60 parts by weight polyvinyl chloride polymer as a powder or in fine particles, 60-40 parts tricresyl phosphate or other plasticizer, the parts of polyvinyl chloride and plasticizer making a total of 100, and up to 30 parts of an inert filler, or 50 parts polyvinyl chloride, 50 parts tricresyl phosphate or other plasticizer, 1 part white lead, and 20 parts whiting, and is evenly spread with a rubber squeegee. Heat stabilizers, dyes, or pigments may be added. The coating is hardened by heating in an electric oven for two or three minutes to 155‹ C. Before heating, substances such as natural or synthetic resins, which are compatible with the printing uses to which the screen is to be put, may be added for toughening purposes. The screen material may be Indian silk of about 30 mesh, bolting silk, organdie, or metal gauze. If a thicker coating is required, a first coating is pre-gelled by heating to 100‹ C., after which a further coating can be applied, the whole then being cured at 155‹ C. The temporary screen can be washed out and used for a new design, or used repeatedly to prepare identical screens. If the final screen is to have a repeat design, a temporary screen of a single design unit may be used and the final screen built up by accurately registered steps, each step being pre-gelled at 100‹ C.