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CLINIC POWDER COATING Cure Cycle for Brass Q. I am getting ready to powder coat some brass parts. What temperature should I use, and how long should I keep the parts in the oven? —T.D. A. The length of time in the oven and the cure temperature depend on the powder material, the mass of the part and the substrate material. You should refer to the Technical Data Sheet (TDS) supplied from the manufacturer of the powder material to find out what the cure cycle for the powder should be. It should have some flexibility so that you can use a lower temperature if you have more time. A typical cure curve might be 400°F for 12 min. If you lower the temperature to 375°F, the time must be extended to 15 min., or if you lower the temperature to 350°F the time might go to 18 min. Keep in mind that the time listed means peak metal temperature. This is where the mass of the part affects overall time in the oven. If it takes 5 min. to raise the metal to the set point, the cycle time will include 5 min. of bring-up time plus the intended cure time. A heavier part will take longer to elevate to the intended cure temperature. The substrate material may also be a factor. Steel is more tolerant of high temperature, so the shorter time and higher temperature may work well for a steel part. Castings and some other materials are less tolerant of higher temperature, so a lower temperature and longer cycle may be better. Brass is sometimes sensitive to temperature and may develop some pinholes due to release of trapped air or contamination in the porous surface. Find out how long it takes to bring the brass up to your cure temperature. Use a lower temperature if possible. You can consult the TDS to see how low you can go and still get a good cure. Add the bring-up time to the cure time and you have your cycle. Keep in mind that brass, like any surface, must be clean before powder coating. Ultrasonic cleaning is a good way to clean brass if it is available. Breathing Protection for Powder Coating Q. Our air tests show that our powder coaters do not need air supply hoods at the area of application. We require them to wear a paper mask for protection. What is the best respiratory practice for this process?—J.C. A. Powder is characterized as a nuisance dust and should not be inhaled. Typically, a dust mask or particle mask with a Class N95 filter approved by the National Institute for Occupational Safety & Health (NIOSH) is acceptable when handling powder, cleaning the powder booth or spraying through a window in the booth wall. When the operator is spraying powder inside the booth, he is supposed to have a fresh air supply because the concentration of dust is much higher. He should have a NIOSH-approved hood and suit that is fed with filtered air. You can find the filtering systems at a good paint 60 SEPTEMBER 2013 — pfonline.com pfonline.com/experts supply distribution company or online. Cartridge-style filtered respirators are designed for vapors and are not as good an option for powder coating. Controlling Film Thickness Q. We are facing problems with uneven film thickness when we apply powder coating to our parts. How can we control the thickness of the film?— V.S. A. Film-build control is related to all of the variables of applicaRODGER TALBERT tion and requires some Consultant understanding of the powdercoating@pfonline.com adjustments that affect the spray operation. The starting point is earth ground. You must be certain that the parts have a good path to earth ground with no appreciable resistance. The National Fire Protection Association (NFPA) recommends that the resistance should be less than 1 megohm for fire protection, and the Powder Coating Institute (PCI) agrees with this recommendation for efficient application. Actually, resistance should be zero for the best possible results. Poor earth ground will cause inconsistent film build and light coating in critical areas. Clean hooks and hanger points are critical. The part has to make metal to metal contact from the part to the hook and all the way to the ground source. Part geometry is also a factor. More complex shapes will impact the electrostatic behavior of the powder and apply heavier film in areas that have lower resistance. Deeper cavities or bends are harder to cover due to Faraday cage effect. Faraday is a reference to electrical resistance in recesses. A key to success in Faraday areas is limiting the micro-amperage. The micro-amps will increase when the gun is moved closer to the target. Excess micro-amps will cause the resistance to increase and contribute to inconsistent film build. The powder will build heaviest where the resistance is low and lower where resistance is high. Keep the current low (10 to 25 µA) to help offset the natural resistance in the Faraday areas. Gun flow rate is another major factor. You need to control the powder flow rate to an even pattern with a reasonable amount of powder and low velocity. Higher velocity or high powder volume will make it hard to apply and control film build. Low powder output with high velocity is the worst possible situation. Gun-to-target