Special pretreatment process enhances powder coating adhesion on magnesium...
More electronics equipment manufacturers are moving beyond plastic to magnesium to make cell phones and laptop computers lighter and more durable, and they are choosing powder coating as the high-end external finish to satisfy consumer demand for product durability.
"Magnesium has been a big area of growth for us," said Mike Meagher, vice president of manufacturing and engineering for Applied Coating Technology, a custom coater located in Minneapolis, MN. Since ACT took its first magnesium job 3 years ago, the orders have continued coming in. Bar code scanners for use in parcel delivery have already moved from plastic to magnesium. Cell phone manufacturers are finding that magnesium offers a lighter phone with a more stable frame.
Seattle-based Itonics Inc. was ACT's first partner in magnesium. Itonics makes laptop computers for clients who use them in service on the road, including Sears' service technicians and AT&T's diagnostic teams. "Itonics makes a water-tight laptop that the technician can throw on the front seat of his van and not worry that it may fall off the seat onto the floor," said Mr. Meagher. The laptop, fitted with a rubber armor boot to absorb the daily impact of life on the road, relies on powder coating for abrasion resistance as it jostles and bounces from job to job.
The magnesium is a critical component for the road-hardy laptops, making them lightweight and durable. However, ACT faced a few significant hurdles in the preparation of the magnesium surface prior to powder coating. It knew from previous experience that a process that only cleans the surface would result in powder adhesion problems with the magnesium substrate. Industry standards for the treatment of magnesium prior to painting called for an alkaline chromate process. However, that process proved to be less than desirable because of waste treatment and disposal concerns. An alternative chromate process proved ineffective for ACT. The alternative chromate process left too many by-products, or smut, on the metal, resulting in a number of quality issues from appearance to performance of the powder topcoat.
"Magnesium presents a lot of problems in getting cleaned and prepped," Mr. Meagher stated. One of those problems is degassing the porous magnesium to prevent the coating from trapping pockets of air and driving them out during the curing process, creating imperfections in the finish. "You can't just clean the magnesium. It doesn't lend itself to traditional prepping."
ACT called in its pretreatment supplier to solve the pretreatment problem and devise a process that provided the quality, performance and environmental impact that ACT and its customer required. The company developed a novel pretreatment process for magnesium that did not produce smut on the surface of the magnesium. The process involved four steps: alkaline clean; water rinse; organometallic composite coating; and water rinse.
The selection of a specially formulated conversion coating was the key. The product was designed for use as a reactive (rinseable) coating for reactive substrates. "The critical step was eliminating the smut and getting the powder coating to stick to the substrate," said Mr. Meagher. "With that problem solved, powder coatings fit another unique application."
ACT began powder coating in 1976 for Litton's microwave ovens. "Over the years we've increased the amount of powder work we do, and now about 60% of our painting is with powder," Mr. Meagher said. "It's become a requested finish for quality, durability and environmental reasons."