Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 390 of 707

A tumbler is a rotating drum, which rolls the parts against each other like a cement mixer. The parts can be burnished either wet or dry using a plastic or ceramic media with an abrasive or a polishing compound. Selecting the desired com- bination of these effects will produce a variety of different burnishing possibilities. The parts can generally be taken right off the process line, without drying, and loaded directly into the tumbler. Vibratory finishers operate in a similar manner but use a vibrating bowl rather than a rotating drum. As mentioned the vibratory bowls can also be charged with differ- ent types of media and compounds to achieve the type of contrast desired. Both the tumbler and vibratory mill will produce a nondirectional pattern on the part surface and cannot really reproduce the effect achieved by a hand-buffing operation; however, they operate at much lower cost and can be preprogrammed to produce the identi- cal result batch after batch. Consequently, they are less dependent on the human fac- tor for consistent quality. For certain parts compromising on quality a bit in order to control the cost allows the manufacturer to sell the finished piece at the desired price point and still make a profit. PROTECTIVE TOPCOATS After coloring and highlighting are completed the part is ready to be topcoated to protect it from corrosion. Even though the parts may look completely finished the decorative antique finish is quite susceptible to corrosion or tarnish unless pro- tected. The products most often used to accomplish this are clear lacquers. As in all the previous operations there can be many options open to the finisher, depending on the durability required of the final finish, operating cost, equipment cost, environmental concerns, etc. In actual practice there are a few options that provide the most benefits. These products can be water-based or solvent-based and commonly utilize acrylic or urethane polymers to form a protective film. The acrylics are the lower cost option and can provide an effective topcoat for many parts used indoors only, such as light fixtures, wall sconces, etc., that do not see heavy wear. Generally, solvent-based lac- quers are more protective than water-based products but also present a potential solvent fume problem in terms of discharge into the atmosphere. Air-Dry Lacquers Baking or Cross-Linkable Resins These products are widely used on parts that require high wear resistance and/or outdoor exposure and include polyurethanes, epoxies, and nitrocellulose lac- quers—all of which can cross link during drying to form a very dense and tenacious film. Very often they are cured in an oven at 250 to 350° F for 10 to 20 minutes to speed drying. These products are suitable for high-value parts or surfaces that must be exposed to outdoor weathering elements. It is also possible to use lacquers containing corrosion inhibitors that specif- ically protect copper alloys. The most widely used is benzotriazole and its relat- ed compounds. These materials can be blended into many types of lacquers in small concentrations and provide an extra measure of corrosion resistance, mak- ing them particularly well suited for use on items such as marine hardware, building components, etc. Clear Powder Coats Relatively new on the scene these topcoats produce coating thicknesses of 2 to 4 mils and offer extremely high protection levels. They are applied like any other powder 389

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