Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 198 of 331

markably even temperatures. The air movement is induced by high-velocity nozzles that also prevent direct impingement of heated air on the work surface, a notable cause of "skin-drying," resultant bubbles and blemishes, and powder blow-off. Burner size:When selecting burners for an oven concentrate on the total needs of the job at hand; then allow for a reserve capacity to handle higher work loads with- out expensive alterations. This provides the greatest overall economy and equipment flexibility because the burners are efficient throughout their operating ranges. Combustion chambers: Ovens are usually designed with the combustion chamber as an integral part of the unit, streamlining the appearance and eliminating the need for exterior ductwork. This compact arrangement also reduces power requirements connected with excessive ductwork, saving floor space as well. Disposable filters: Intake air filters can minimize the intake of dust, which can cause finish flaws. The filters should be arranged for quick and easy changing without use of tools. The best disposable filters to use are frameless, eliminating the usual card- board frame as a possible fire hazard. Simple erection:Ovens should be constructed of prefabricated panels for easy erec- tion. All components—panels, structurals, ductwork, wiring, and piping—should be keyed to assembly drawings and manufactured to fit easily, quickly, and precisely in the field. The purchaser can in many cases erect the oven with existing plant person- Fig. 9. Layout for cure/bake oven sample design. nel under supervision for significant savings. For more complex ovens suppliers pro- vide a complete service ranging from basic erection to complete turn-key jobs. Dry-Off Ovens The dry-off oven is usually a continuation of the washer. Drying metal pans with no "puddling" of water requires three to five minutes, depending on temperature. At 4 fpm you will need 12 to 20 ft of dry-off oven, excluding air seals. The next unit requiring the largest amount of floor space is the cure oven. Assume for design purposes that the paint you are using requires a 20-minute cure at 350°F. Again, conveyor speed is 4 fpm so you have to be in the oven at temperature for 80 ft of conveyor travel. The oven can be fabricated in practically any configuration, depending on the floor space available. It could be a single-pass oven (80 ft long) or a two-pass oven 197

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