Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 231 of 331

Fig. 2. Removable side bars protected inside the crossbar. centers, assuming a 30° maximum incline/decline. Proper Grounding for Electrostatic Applications Nearly all part hooks eventually lose their ground between the part and the hook as they are used through the finishing system. The ground may be lost within as little as 2–3 cycles or as many as10–12 cycles, depending on the amount of paint being applied and whether the same kind of part is hung. A loss of ground causes loss of paint wrap, uneven paint distribution, blemishes, rejects, scrap, and wasted paint, particularly if there is no recovery system. When loss of ground occurs, it is necessary to either clean the hook or, if pos- sible, remove it and replace it with a clean one. In most cases cleaning requires ei- ther burning or chemically stripping the entire rack or burning or grinding just the hook end. The other option,replacing the hook, may be faster depending on the de- sign of the rack and hook connection. There is a rack available that uses spring steel hooks that can be quickly and easily removed even after many cycles (see Fig. 3). The contact to the rack is shielded from paint and always remains clean for grounding. Rack Maintenance Optimizing rack density automatically reduces rack maintenance. Fewer cycles automatically means less cleaning. A common problem with welded racks is the hook breakage that occurs due to the annealing at the weld point of the hook to the rack. This problem causes less product to be painted and gives an inaccurate count of finished parts. Hooks that can be removed and easily replaced are de- sirable. Part hooks made of stainless steel will retain strength for a longer period of time, particularly if cleaned in a burn-off oven. Stainless steel is also a requirement if spring steel is used to avoid annealing. Most burn-off ovens operate at approxi- mately 800°F, whereas the annealing temperature of stainless steel is approximately 1,900°F. Flexibility Flexibility in racking is very important in any finishing job, large or small. The flexibility achieved with modular racking gives one the ability to concentrate on density and obtain it quickly. Moving part hooks around is not the important part of flexibility. It is having the ability to put a rack together quickly, without 230 be repositioned as close as possible to eliminate wast- ed horizontal space between parts (see Fig. 2). The overall width of a rack should be determined by maximizing the number of pieces on the rack in re- lation to the distance be- tween conveyor pendants and the degree of incline re- lated to any hills in the sys- tem. Most racks are de- signed to be 20 in. wide and to be center hung on 24-in.

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