Patent GB-711209-A (GB)
Title: Improvements relating to screen printing apparatus
IPC: B41C1/14
Filed: 1949-11-26
Abstract: 711,209. Stencil sheets. GMACH, L. T., and GMACH, J. L. T. B. Feb. 22, 1951 [Nov. 26, 1949], No. 27516/49. Class 100 (2) A screen printing element comprises a metallic foil stencil formed on or secured to a metallic mesh screen, which is flattened at its points of contact with the stencil, or a screen in the form of a metal deposited in the form of a grid and flat where it is in contact with the stencil. The screen may be a wire mesh whose flattened surface is formed by grinding, pressing between platens or rollers, or selective plating or deposition of amalgam or a solder on the wires. The holes may be filled with a temporary filler such as bitumen or varnish and the surface ground to expose the wires and flatten them, the filler being dissolved out after securing or forming the stencil. In one process, the screen so formed is welded, soldered, brazed or cemented to a metal foil of a different metal or alloy from the screen, the resist pattern then being imposed on the foil surface, and the uncoated portion dissolved by chemical or electrolytic etching. If the stencil is to be very thin, the screen and stencil can be of the same metal, etching being employed until the unprotected parts of the foil are dissolved without destroying the wire mesh structure. Alternatively, the metals may be the same and the screen be protected by resist or lacquer coatings. In another process, the screen is formed by etching or cutting a metal foil, and the preformed stencil secured to the screen. Alternatively both the screen and stencil may be formed by selective deposition of metals or alloys on temporary carrier plates. In another process, the stencil is deposited on the screen, which is first formed and treated with a temporary non- conductive filling the surface of the wire being flattened and exposed by grinding, and the ground surface then rendered electrically conducting by treatment with mercury amalgam, reduction of a silver salt on the surface, or vacuum deposition of a metal coating, and the foil electrolytically deposited on that surface, the combined screen and foil then being subjected to selective etching as before. Alternatively, the conducting surface of the treated screen may be printed with the resist pattern and the stencil formed directly thereon by electrodeposition or other chemical deposition of metal to the desired thickness. In another process, the mesh may be filled, ground printed with resist and the unprinted parts have metal deposited thereon by chemical means, such as the reduction of silver salts, to form the stencil. For example, the mesh may have from 80 to 200 apertures per linear inch and the thickness of the stencil be less than 0.005 inches. The screen may be composed of 20-50 per cent nickel chromium alloy, bronze, stainless steel, or other metals or alloys, and, if necessary, protected against etching by a lacquer or other organic coating. If the stencil is formed on the screen by electrolytic deposition, it may be of silver reinforced by copper or nickel coating, the temporary filler being shellac, asphalt, or a natural or synthetic resin. The completed stencil may be strengthened and hardened by electrolytic deposition of harder metals thereon. According to the Provisional Specification, the foil may be attached to the mesh by an adhesive. The screen may be pre-formed by masking a flat metal carrier plate against electrolytic deposition so as to leave unmasked the mesh design, and electro-depositing metal on it. The stencil may be 0.002 to 0.006 inch thick. A metal screen may be impregnated with a photosensitized material, such as a bichromated colloid, a shellac, or a vinyl compound, a negative design of the desired stencil photographically imposed, the interstices to be masked cleared, a softened mercury amalgam wiped over the screen, and the remainder of the sensitized material removed in warm alkaline solution. If the stencil is to be reversed, it is placed on an intermediate carrier plate before transfer to the screen. A pre-formed stencil may be formed by electrodeposition on to a wax layer, rendered conductive, on a carrier plate. The carrier plate may be a plastic sheet with a conducting surface of graphite, chemically deposited silver or formed by vacuum vaporization of metal in an electric arc.