Friday, August 1, 2008

Molds and Mandrels for Electroforming

Metallic and nonmetallic materials are used as molds or mandrels for electroforming. Surface passivity characteristics make chromium plated metals useful as metallic mandrels that require neither a surface treatment for providing conductivity nor a separating film for providing ready removal of the electroform. Surface passive metallic mandrels can be used repeatedly without intermediate surface treatments, although a thin film of graphite is sometimes applied before each use to facilitate the final separation of the deposit. A mixture of graphite and wax has been utilized as a parting medium in the electroforming of tubing that was subsequently drawn into trombone slides. A thin suspension of graphite in kerosene is sometimes used.

Temporary mandrels are used only once and are then deformed or destroyed when deposition is completed. Low melting alloys containing lead, tin, bismuth, cadmium, or mercury, or combinations of these, are melted out and recast. In electroforming Pitot-static tube, low melting mandrels were employed. Aluminum and zinc can be cast or machined into mandrels and subsequently chemically dissolved.

Nonmetallic materials used for mandrels are asbestos, asphalt, cloth, gelatin, glass, paper pulp, plastics, plaster, pottery and other ceramics, rubber, wax, and wood. Each of these must be made surface conductive by one of the procedures discussed, after molding is completed. Plaster, woods and ceramics require sealing to fill pores near the surface before the application of the conductive film.

Thermoplastic sheet, such as vinyl acetate and vinyl chloride copolymer, has found wide acceptance in the electrotyping industry as a molding medium. Impression of printing surfaces are made in the heated plastic sheet, which is then cleaned, sensitized with a stannous chloride solution, and silvered by simultaneously spraying an ammoniac silver nitrate, formaldehyde, or hydroxylamine hydrochloride. Such molds largely replaced the graphitized wax and lead molds used formerly. Copper is deposited on the silver (using a low current density for a few minutes) until the metal film is thick enough to carry a higher current. Then the current density is raised to about 22 A/dm2 or more for another 40 to 70 minutes until a shell 180 to 380 mm thick is deposited.

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