Powder Coating

Aug2016

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Learning how to properly powder coat aluminum components Q I'm an engineer for a com- pany that makes electric ther- mal cooling systems for the transit bus market. In this market, we have to deal with very harsh environ- ments—hot, dirty, laden with misc. chemicals, high vibration, and more. Our cooling systems typically consist of an outer fabricated steel shroud paired with a variety of aluminum heat ex- changers (radiators, CACs, oil coolers, or condensers), both of which are out- sourced for fabrication and coating; we do not coat in-house. Currently, the steel is powder coated with an epoxy and the heat exchangers are painted with an acrylic. In the past, we had a weak coating specification which has led to some field failures and learning opportunities. I have been tasked with writing two coating specifi- cations—one for steel (primarily outer shrouds) and one for aluminum (pri- marily heat exchangers). We've learned that proper pretreatment is necessary for steel components. Re- moving the laser scale and applying an iron phosphate conversion coat over clean steel seem to be moving in the right direction, although I think we will be adjusting our powder in the future. We'd like to move our heat exchangers to powder coat as well, but I've had varied luck in determining the proper steps in coating aluminum components. I've heard that iron phosphate can work, that it can't work, that the acid etching is nec- essary but de-smutting is not, that hex- chromate is the only way to go, and so forth. I'm lost as there seems to be no consistent direction for aluminum as there was with steel. Could you please guide me as to what should be done for proper powder coating of aluminum? We haven't defined a mininum perfor- mance criteria yet, but it will be in the 2,500 hour salt-fog range. My end goal is to create an easy-to-read robust coating specification that will ensure a quality product. Thanks. S.M., Escanaba, Mich. A I have a philosophical differ- ence with your technical ap- proach to your coating perfor- mance issues. I prefer to specify the de- sired appearance, environmental per- formance, and mechanical properties that I want the coating to achieve on my part and let the reputable custom coater or fabricator select the process to achieve these goals. Specifying the process, as you are doing, does not en- sure success or allow you to measure if the desired results were attained. Just because the baker has all the right ingre- dients, does not mean that the cake he bakes will taste good! The approach I take requires clearly specifying the performance of the coat- ing, based upon the design goals of the product, the conditions it will be fielded in, and the length of time it is expected to perform. This way you get the Devil's Food cake you wanted, not the Angel Food cake the baker made. However, if y o u w a n t t o p r o c e e d u s i n g y o u r method, you need to evaluate conver- sion coatings that can provide the 2,500-hour corrosion performance on the substrates you are using. This will be more difficult using steel substrates than when using aluminum substrates. To get this performance with steel you will need to use galvanized steel, zinc phosphate, zinc rich primers, and poly- ester topcoats. Aluminum can attain this performance using chromate con- version coatings or zirconium conver- sion coatings and a single topcoat of polyester powder. Both the zinc phos- phate and the chromate conversion coatings (or the zirconium conversion coating) require multiple steps to re- move organic and inorganic soils before the substrate conversion can occur. These steps include using alkaline and acid cleaners and conditioners along with fresh water rinses. Final DI/RO rinses wouldn't hurt either. The best place to start is with discussing your design with pretreatment chemical suppliers and powder formulators to de- termine the entire package necessary to meet your target performance in the con- ditions the product will be subjected. For additional information, check this maga- zine's online Article Index at www.pcoat ing.com for articles that I wrote in the past on corrosion performance and coating specifications. —N.L. Searching for an alternative to tape for masking Q What kind of alternatives exist for using tape for mask- ing? Plugs at load didn't work, and tape ahead of pretreatment didn't hold up in the washer. I've thought of permanent masks on the hooks but worry about part location when hanging and mask distortion when burning off the hooks. This also would make my hooks VERY product specific and com- 24 POWDER COATING, August 2016 Questions & Answers George Trigg GRT Engineering Nick Liberto Powder Coating Consultants

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