Wednesday, April 9, 2008

Sulfamate Baths

Sulfamate baths are increasingly used for electroforms because their deposits have low tensile stress. The Watts baths require the use of a stress reduces for low internal stress, and because of the sulfur content of such deposits, it is necessary to avoid temperatures above about 370 oC for such electroforms. At about this temperature and higher, the sulfide-containing nickel suffers severe embrittlement. In general, for electroforming operations, the stress reducers should be used at the lowest concetrations to accomplish minimum stress.

Electrotypes are the oldest kind of electroformed article. Electrotyping is a process for producing duplicate printing plates to eliminate the need to reuse the expensive original, usually a composite of photoengraving and type. In modern methods a plastic mold, usually vinyl, is made from the original and made conductive with a reduced silver film and then plated 150 to 200 μm thick which is the required nickel shell thickness. A low stress bath is necessary to start the plating on the silver film without preliminary copper plating. Nickel sulfamate baths are generally used on both lead and plastic molds. After an initial plate of about 7.5 μm is deposited at medium current densities on the vinyl mold, the rate of plating can be increased to 20 to 30 A/dm2 to produce an all nickel electrotype shell in 30 to 40 min. Small concentrations of nickel fluoborate can be added to the sulfamate to prevent burning at the edge of the mold. In stereotypes, a soft low-melting lead-based metal is used for printing and is protected against wear by nickel electroplate about 13 μm thick, usually from the sulfamate bath.

A large new application for electroformed nickel is the cylindrical screens that are used for textile printing. These screens are about 125 μm thick, 12.5 to 30 cm in diameter, and 2.5 to 6 m long. They are used in tandem with each roll applying a pattern of single color to the fabric.
Important applications of nickel electroforming include Pilot-static tubes for air-speed indicators, precision tubing of rectangular Eros section, and radar fittings, waveguides, computing cams, large searchlight reflectors, constant mesh screens, and Venturi tubes used in the jet propulsion program. Electroforming plastic dies and casting dies has an advantage owing to the high cost of bobbing them out of solid metal. Electroformed molds are used for nylon gears, ratchets, cams, and plastic toys and dolls.

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