Metal Finishing Guide Book


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coating materials and application methods DIP COATING BY THOMAS C. JONES HENKEL SURFACE TECHNOLOGIES, MADISON HEIGHTS, MICH. Dip application of a protective coating involves simply immersing a workpiece into a suitable tank containing the coating material, allowing the part to drain after withdrawal, and force drying or baking the wet coating to achieve the finish. Dip coatings are used in many industries for both primer and one-coat finishes. Thorough cleaning of parts is essential prior to dipping. For optimum quality, a phosphate conversion coating is also recommended. BENEFITS Simplicity: Manpower and equipment requirements are minimal. The process is easily automated. Low Cost: Paint utilization should be relatively high (e.g., greater than 90% transfer efficiency) on properly operated systems, since nonused paint (drainage) is mostly recovered and returned to the system. Ease of Control: Minimally skilled operators can maintain solids, viscosity, and other factors for acceptable application properties. Good Coverage: Except for air bubbles or pockets, all contact areas are coated. Close racking of parts is possible. Consistency: Similar parts receive coatings similar in appearance and film thickness (i.e., the process is independent of the operator). LIMITATIONS Nonuniform Coatings: "Wedges" (thin films on upper surfaces, thicker on lower surfaces) tend to form on vertical surfaces. Flow lines around holes or openings can also occur. "Beads" on bottom edges are inherent defects, although proper viscosity control can minimize this effect. Part Design and Hanging: Improperly racked parts can bucket paint, leading to waste and potential blistering in the puddled areas. Entrapped air pockets can prevent access of paint, with resultant bare areas. It may be necessary to design drain/access holes into some workpieces to allow for immersion application. An attempt should be made to rack a part so that drainage occurs from a single point. Oscillation during immersion can sometimes remove air pockets. Solvent Washing: Entrapped solvent during the curing process can resolubilize an already dried film, resulting in bare areas. Product Change: A change from one formulation to another requires either extensive cleaning and recharging of a single tank or the availability of multiple dip tanks. Thoroughness of clean-out is especially important when switching incompatible materials (e.g., replacing a solvent-borne system with a waterborne system). Flammability: The potential for fire is always present when solvent-borne dip primers are used. With waterborne systems, this problem is greatly reduced. Foam: Undesirable foam, which usually originates in the paint recirculation system, can produce voids or craters in the final finish.This problem is more prevalent with waterborne paints. Sticking: Small objects, such as fasteners processed in baskets or trays, can 165

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