Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 329 of 707

cessfully barrel plated. Generally, parts that weigh less than 1 lb each and are each less than 25 cubic inches in volume can be barrel plated successfully. A simple shape is obviously easiest to barrel plate. Barrel plating is usually the most successful, cost- effective way to plate threaded parts and fasteners properly. The tumbling action of the barrel makes and breaks the electrical contact throughout the workload, yielding the most even coverage on the root, mean diameter, and crest of the threads. Part material must not be adversely affected by any baths required in the total plating-process cycle. A trial load is a useful tool for evaluating which type of bar- rel equipment and technique can be utilized for plating a particular part. Long workpieces and entangling parts, such as rods, bars, or tubes, can be suc- cessfully barrel plated. Methods used to plate these parts include long barrels; lon- gitudinal and radial compartments; rocking motion; and various, special sta- tionary contacts (see Fig. 4). Special extra-length barrels allow long parts to fit, whereas compartmented barrels confine movement of long parts and entan- gling parts, helping to eliminate bridging or entanglement. Limited barrel oscil- lation or rocking motion (usually 180O of rotation or less) accomplishes the same task by minimizing part movement. To do this, a reversing switch, or contactor, along with an adjustable control timer can be installed on the barrel drive to rotate the cylinder alternately in each direction. The barrel interior can be equipped with stationary cathode contacts to plate small, delicate, or nesting parts (for example, small electronic components with projecting fingers). Stationary contacts rotate with the cylinder so that there is lit- tle relative movement between the workpieces and the contacts. As a result, the work cascades over or around the stationary contacts, and less abrasion or edge contact takes place, minimizing the potential for damage to the work (see Fig. 7). Disk, center-bar, cup, strip, button, hairpin, and chain are some types of sta- tionary contact. Certain types of stationary contacts, such as strip contacts, assist tumbling of the work. Parts that are flat or lightweight should be plated in barrels with uneven inte- rior surfaces that are not flat and smooth. A convoluted or uneven barrel interi- or surface, such as grooved, ribbed, or dimpled, promotes tumbling and eliminates much of the sticking of flat workpieces. When finishing recessed or cupped parts, other smaller parts, which are to be plated to the same specification, may be mixed in with the load to provide contact into recessed areas; however, the cost of the time spent to separate the smaller parts from the others after plating/processing must be acceptable. BARREL EQUIPMENT DESIGN All designs of barrel equipment, including horizontal and oblique, should include features to optimize productivity. Reduction of labor requirements and improved ease-of-maintenance are important factors for well-designed compo- nents and systems. Some of these important features are discussed in the following sections. Barrel Construction Barrels should be made of materials that are chemically and physically inert to use in each bath or piece of equipment in the plating line. It is important that the barrels be capable of operation in excess of maximum bath temperatures in the entire system. 328

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