Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 334 of 707

Detail Components There are important equipment fea- tures that substantially affect plating system performance and serviceability. It is very important to consider these items and their benefits when selecting barrel-plating equipment. Horizontal barrel assemblies Fig. 10. Dangler-style interior barrel cathode contacts. equipped with an idler gear will result in fully submerged operation of the barrel, ensuring maximum current and solu- tion access to the work. Fully sub- merged barrel plating also minimizes any potential for problems with accumulated or trapped hydrogen. Barrel rotation causes a cascading action of the workload inside the barrel. Because of this, the center of gravity of the workload is shifted to one side of the barrel assembly. Tank-driven, horizontal barrel assemblies equipped with an idler gear offset the center of gravity of the cascading workload to the proper side to best resist the tendency of the rotating tank drive gear to lift the barrel contacts from the tank contact points; therefore, use of an idler gear on the barrel assem- bly helps maintain, and optimizes, good electrical contact between the barrel assembly contacts and the cathode contact saddles of the tank. Conversely, a bar- rel assembly without an idler gear promotes poor electrical contact because the cen- ter of gravity of the workload is shifted to the opposite side and works against main- taining good, positive, constant contact. Another positive feature is hanger arms made of non-conducting materials such as plastic. Non-conducting hanger arms eliminate "treeing", stray currents, and possible loss of plating-current efficiency. (Treeing is the accumulation of deposit- ed metal on the plating barrel or any components of the barrel assembly because of stray currents.) Design simplicity and efficiency of barrel equipment are important for ease of maintenance, particularly for components operating below the solution level. The use of alloy fasteners that are nonreactive to the chemical system in use is especially important for acid-based plating systems such as chloride zinc. HOIST SYSTEMS, TANKS, AND ANCILLARY EQUIPMENT It is important to the performance capabilities of a barrel hoist and tank system to review the following items and include the advantageous features where possible. Most barrel-plating tanks are designed to maintain the solution level approx- imately 5 in. below the top rim of each tank. At this level, the plating barrels should run fully submerged, eliminating the potential for excess hydrogen accumulation. Operating with a solution level higher than 5 in. below the top rim of a tank can cause the solution to be splashed out during barrel entry or exit, resulting in wast- ed solution, treatment issues, and, possibly, environmental problems. Solution loss and adjacent tank drag-out contamination can also be minimized by equipping the barrel hoist system with "up-barrel rotation". A drive mechanism on the hoist rotates the barrel and load in the overhead, above-tank position, facil- itating better drainage before moving to the next process station. This is especially helpful when finishing cupped, or complex-shaped, parts. Locating the plating-tank anodes (including anode baskets or holders) in the 333

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