Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 651 of 707

finishing plant engineering FUNDAMENTALS OF PLATING RACK DESIGN BY STEEN HEIMKE BELKE MFG. CO., CHICAGO The primary purpose of a plating rack is to hold a part in the most advantageous position for exposure to a plating current, which flows from an anode. Plating pro- vides protective finishes to parts fabricated prior to plating so that the metal finish will not be damaged or ruptured during the fabrication process. Parts requiring a fin- ish have an infinite variety of shapes and sizes, resulting in the need for fabrication of a custom plating rack. Before a plating rack can be fabricated, certain questions must be asked. What kind of plating will be done? What solutions will the rack be exposed to? What rate of production is necessary to be cost effective? Will the tips be stripped with a proprietary solution? What portion of the part is to be plated? How should the piece be held for proper density of the plating finish? What sort of tip must be designed for proper positioning? Will this design provide quick and easy racking and unracking? For determining the answers to some of these questions, the basis for a good rack design will be developed. Proper rack design should be started with a description of the part, detailing any special surface problems, shading, and contact tip marking. Where can the piece be held? The number of pieces per rack will be determined by current per rack, weight of each part relating to total weight of the rack, and, most importantly, by the design of the rack. RACK DIMENSIONS The most important fundamental of plating rack design is determining the proper dimensions, making sure that each rack will fit with parts affixed into the smallest process tank in the plating line. Dimension A—Overall Length This is the distance from the cathode bar to the bottom of the rack, keeping in mind that each process tank has different space requirements relating to anodes, steam coils or immersion heaters, air agitation pipes, filters, overflow dams, and mechan- ical agitation. The rack should be several inches off the bottom of the tank, allow- ing for some accumulation of sludge. Also, improper anode length could result in a very uneven deposit. Most parts should be positioned a minimum of 2 in. under the solution sur- face. It is important to check solution levels in all process tanks. Determine the dimension from the lowest level tank, thereby assuring complete immersion throughout the plating cycle. Overall length is determined by Figure 1, dimension A, which is the distance between the cathode hook and the bottom of the rack. 650

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