Metal Finishing Guide Book

2013

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parts directly into the chromium tank. Since the parts are not held by a rack, poor or no contact with cathode leads is common. Contact must depend upon gravity, while being dampened by the solution, to hold the part against the cathode contact points. To increase the likelihood of good contact and satisfactory plating, platers use low current density, low temperatures, and specially designed barrels and trays. Cathode contact points in barrels are constructed from wire mesh, solid steel liners, button contact points less than an inch apart, or steel bars that tumble the parts as they move. Large barrels with small loads also help to increase the frequency of contact and reduce temperature build up inside the confined space of the barrel. In general, the highest practical current density possible, without burning, should be used. Screw and spiral design plating equipment are also used. The parts are put into the barrel at one end and transferred through the barrel for plating inside the threads of the screw. This permits a continuous flow of plated parts. Vibratory agitation and centrifugal force barrels are also available. In all cases, the barrels must be constructed so that the hydrogen gas generated during plating can escape from the barrel rather than being trapped and possibly exploding. Tray plating requires that parts be layered onto a metallic screen and vibrated or tumbled during plating. Trivalent chromium solutions have recently been tried for barrel and tray plating of chromium. Since current interruptions do not hurt the deposit in trivalent chromium processes, and burning is not a problem, this technology will probably become more popular in the future. 260

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