Products Finishing

OCT 2013

Products Finishing magazine is the No. 1 industrial finishing publication in the world. We keep our readers informed about the latest news and trends in plating, painting, powder coating, anodizing, electrocoating, parts cleaning, and pretreatment.

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Page 93 of 115

CLINIC POWDER COATING Applying Metallic Powder Q. We apply a metallic, silver powder to 20-50 parts at a time. The parts are sandblasted, and many of them are also processed through a five-stage iron-phosphate wash system. We have a lot of problems with the pigment "bunching" and standing up on the edges. We have tried many combinations of gun settings, with and without ground wires, recoat modes, manual modes with kV turned way down, etc. We cannot seem to eliminate the problem. —K.S. A. Metallic powders have unique charging characteristics. The metallic component is highly conductive, while other materials in the formula are less conductive. This can cause the metal-flake to react differently than the organic resin and other pigments and align in undesirable patterns. Some silver products are particularly difficult to apply, and it is hard to overcome the electrostatic behavior of the material. Bonding source and technique can have a profound impact on the electrostatic behavior of a powder material. Before you do anything else you should check your gun with a voltage meter to make sure you are getting the right amount of voltage. If the output voltage is below the set point, you will lose your charge completely when you reduce the kV and the gun will not work. If the voltage is not right, try a different gun or get yours fixed. After you have confirmed that the voltage output is properly calibrated, try a reduction in voltage to around 55 to 65 kV, micro-amps very low (10 to 25), and very low velocity. Hold the gun farther from the part than normal, and spread the parts out to leave more space between them than you would with another powder. If you are using a box-feed device, replace it with a feed hopper. If this does not work well, gradually increase the voltage and amperage to see if it has any impact at all. Make sure the fluid bed hopper is kept at a constant level and fluid pressure is moderate so that the surface appears to have a gentle roll, like water at a simmer on a burner. If none of this works you may want a different try a different silver material. And remember that, while most metallic powders can be effectively reclaimed, some are difficult. Rust Prevention Q. Below is the specification given for pretreatment and powder coating of the outdoor enclosures that I have to coat: • Zinc phosphate (as basic coating) or blast cleaning (SA 2.5 with a profile of 40-80 micron) • Coating thickness of electroplating should be 4-5 microns. • Curing time should be maintained as per manufacturer recommendation. • Color: RAL7035 • Finish: semi-matt • Polyester TGIC-free powder coating • Coating thickness should be 80-100 microns. 92 OCTOBER 2013 — Is this good enough to prevent rust from appearing, and, if so, how long would it last? Ten or 20 years? Also, which will be better: zinc phospate or blast cleaning? —D.F. A. Your questions cannot be answered in precise terms. Rust is impacted by many things besides the coating and method of preparation. The design of the part also is important. If there are areas that are hard to cover with an adequate thickness they may fail faster than the rest of the part. Seams, RODGER TALBERT inside corners, holes, Consultant edges and other areas may be more vulnerable to rust than flat surfaces. The installation also is important. If holes are drilled or parts are scratched, the coating may fail more rapidly where it is damaged. The installation site where the part is located is a big issue. Miami, Fla., is tougher on a coating than Spokane, Wash., for example. The bottom line is that there is no sure way to say that rust will not appear. In most cases some rust will occur at some time. A steel part that is properly prepared, coated and installed could last a long time with limited rust. As outlined above, precise measurement is not possible without some particulars on design and location. Five to 10 years is reasonable if the job is done correctly. If the part is designed to make it easy to coat and you prepare the surface right, use a good primer coat and apply a durable topcoat, the product could last even longer with minimal wear. The question of whether zinc phosphate or blasting is better is tough to answer, too. Zinc phosphate provides a barrier coating, and it can provide substantial corrosion resistance. Blasting provides an excellent anchor pattern and adhesion over time. What is important is that the coating and preparation are working together towards the goal of corrosion resistance. Personally, if I was going outside with steel, I would want to blast, powder prime and powder topcoat. The specification is somewhat confusing. Electroplating? This is not clear at all. Why electroplate and what type of plating? Chrome, zinc or nickel? I would much prefer powder primer at 75 to 100 micron. If the surface is blasted and you use a powder primer with zinc, you will have a great corrosion barrier. In general, I would recommend a blasted surface with

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