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 application and requires some understanding of the 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 distance is another factor. When you are too close to the part, you will produce heavier films with more orange peel. If you are too far away, you may have light coat. Normal distance for automatic guns is 8 to 12 inches and normal distance for manual guns is 4 to 8 inches. Manual guns may be moved closer to reach inside of cavities, but it is critical to avoid excess current draw when the guns is moved in and avoid extended dwell time in one location.
Training is a critical factor, especially for manual application. Good technical training will have a profound impact on manual application and film build control. An outside source can provide insights on gun settings, patterns and spray technique. Bring in a professional trainer or send your operators to a training program so they can optimize the gun settings.
Make sure you have ground, set up a good flow rate, maintain a reasonable distance away from the part and keep your gun in good repair. For further information on control of film build, search pfonline.com for additional articles. Consider purchasing the PCI’s handbook at powdercoating.org or the Chemical Coaters Association International’s powder coating manual at ccaiweb.com. You should also seek some training if you can. PCI offers regular workshops on application and spray control, and you can find information about these workshops on the calendar at the PCI website.
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