Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 156 of 331

Since the autodeposition reaction is diffusion controlled, the supernatant film on freshly coated pieces lifted from the bath continues to deposit paint solids. As a result, there is minimal loss (i.e., transfer efficiency averaging 95%) of solids to the water rinse following the coating bath. This effect is further enhanced by the ab- sence of any external force (e.g., electric current), which would increase the con- centration of solids at the immersed surface of the work. Since bath solids are maintained at only 5% to 7% by weight, any dragout effects are minimal. Autodeposition is a versatile means of coating complex parts and assemblies. For example, in the automotive industry, the following parts are coated by au- todeposition: jacks, brake backing plates, fan blower housings, suspension com- ponents, headlamp mounting assemblies, intermediate steering shafts, leaf springs (individual and as assemblies), power brake booster housings, seat frames, seat tracks, and miscellaneous brackets, connectors, and fasteners. Some nonauto- motive examples include office furniture components (e.g., drawer slides, file frames), appliance accessories, hand tools, exercise equipment, and patio furniture. EQUIPMENT CONSIDERATIONS All of the stages for an autodeposition process, with the exception of the coat- ing tank, are identical to those employed in other finishing processes and are not discussed here. For this reason, retrofit of an existing coating system to au- todeposition can be relatively simple. The following comments pertain to the coat- ing tank alone: Materials of Construction: The coating tank is a mild steel tank lined with an acid- resistant material. To avoid damage to the liner by parts or racks falling into the tank, it is advisable to use materials with high impact resistance. Tradition- ally, three-ply (soft, hard, soft) rubbers have been employed. Agitation: A gentle agitation of the coating bath is provided by properly spaced mix- ers with AC variable frequency drive (0.5-hp motors typical). Agitation is required only when workpieces are in the paint tank and may be decreased or stopped dur- ing downtime since no paint settling will occur in short periods (e.g., 2–3 weeks). Cooling Equipment:The temperature of the bath should be controlled in the range of 68 to 72°F. Heating/cooling coils in the bath are required (1) as a safeguard against accidental heat/cold carry-in; or (2) if ambient plant conditions warrant (heating or cooling). Even when a bath is used to process metal at a high rate, there is no measurable temperature increase due to exothermic chemical reactions. 155

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