Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 109 of 331

lent with waterborne paints. Sticking: Small objects, such as fasteners processed in baskets or trays, can fuse together during cure. Processes, such as autodeposition or electrodeposition, that utilize water rinsing following the coating tank generally do not produce this ef- fect. Viscosity: Control is critical. High viscosity gives thick films and excessive con- sumption. Low viscosity produces thin films. TYPICAL COATINGS USED Selection of a coating system (i.e., resin type, pigment color) is directly related to the performance intended for the finished parts. Although any formulation with the appropriate viscosity for acceptable transfer efficiency (i.e., greater than 90%) can be used, properties of appearance, quality, cost, and other factors must be taken into account. There is a trend toward the use of waterborne formulations because these are both fire resistant and ecologically desirable. Defoamers are often required to con- trol foam in waterborne systems; however, silicone-containing materials must be avoided. Waterborne paints are often more aggressive toward equipment than sol- vent-borne formulations. EQUIPMENT REQUIREMENTS If high-volume throughputs are desired, a continuous conveyor for work trans- fer is usually employed in contrast to a manual or programmed hoist. Circulat- ing pumps are required to maintain uniform viscosity and constant paint com- position. The "bead" that characteristically forms on the lower most edge of a draining part is sometimes removed by ultrasound, by electrostatic detearing, or by air jets; the latter is generally the least expensive technique. A controlled withdrawal rate is useful in controlling coating thickness. This is more readily varied with a hoist system. Tank/Agitation System A tank lining chemically resistant to the coating employed is required. Epoxy-type coatings with a 15-mil minimum thickness are commonly employed. The tank and piping for circulation can be mild steel for solvent-borne coatings but should be stainless steel for waterborne systems. Tank volume is dependent on work package size and the transport system in use, with conveyorized systems requiring longer tanks than a hoist system. The work package should be submerged at least 3 in. below the surface and at least 6 in. above any circulation piping at the bottom of the tank. Overflow weirs, to remove floating debris, are usually located at the entrance end of the tank and/or along the sides. Agitation can be either from an eductor positioned on the bottom of the tank bi- secting the longest dimension, which tends to minimize parts falling from the rack, or by eductors directing flow along the bottom of the tank, which results in a cir- cular motion. Centrifugal- or turbine-type pumps are normally used for solvent systems; however, where shear sensitivity and/or foam are potential problems (as with waterborne paints), double-diaphragm pumps, with an accompanying surge sup- pressor, are recommended. As with other metal components, stainless steel is rec- ommended for wetted pump surfaces when waterborne systems are used. 108

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