Measuring and Controlling Transfer Efficiency

Q. Our automatic powder guns are nine years old and doing okay, but we want to enhance our transfer efficiency and reduce material cost. We keep the guns in good working order, but results are inconsistent and I'm not sure how to get accurate measurements. What is the right way to measure powder gun transfer efficiency and how can it be improved?

A. Measuring transfer efficiency (TE) is about determining ratio of powder sprayed versus the amount on the parts. The easiest way to measure accurately is by weight. Weigh a given number of parts before coating. Weigh your hopper or box of powder to determine how much powder is in the initial supply. Coat the parts that you weighed and then weigh them again. Weigh the hopper or box after coating. The difference in the hopper weight is the amount of powder sprayed. The difference in part weight is the amount of powder applied. Calculate the amount of the parts as a percent of the total amount sprayed and you have your TE value. Keep in mind that the type of part coated affects TE. More surface area and less air yields higher TE, so coating sheet metal offers better efficiency than a wire basket.

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Higher TE lowers overall costs, reduces equipment wear (less spare parts), lowers maintenance, improves finish quality and increases throughput. Below are factors for improved TE:

1. Gun voltage/amperage settings. Typical voltage range is from 50 to 100 kV. Higher voltages generally produce heavier powder deposition. 10 to 20 microamps is the optimum current draw for good deposition and penetration into Faraday areas. Test to determine the optimum settings for your parts and powder.
2. Gun air adjustments. Typically, the lower the pressure, the higher the TE. This allows greater exposure to the corona field, more consistent film thickness, less abrasion and less likelihood of orange peel. Test to determine the right amount of powder and then keep the pressure as low as possible.
3. Gun position. As the gun gets closer to the part, voltage decreases while current draw increases. As the gun current exceeds optimum levels, more ions are created and flow faster to the part, resulting in back ionization. Be consistent and observe the results. Typical gun-to-part distance is 8"-12" for automatic guns and 4"-10" for manual guns. Distance reduces transfer efficiency and close range increase back ionization and orange peel.
4. Part profile. Is your part simple or complex, and how large is its surface? These questions will determine the dwell time needed to effectively coat your parts. Try to avoid higher delivery pressure to coat quickly. Work on a ratio for the number of guns, amount of part surface and time needed to coat at reasonable air pressures.
5. Part presentation. The right part presentation will affect TE. Your primary coating surface should be within five degrees of vertical, multiple parts should be on the same plane, and where necessary, the same level. Be consistent with your racking setup.
6. Line density. Loading the conveyor with as many hangers as possible and as close as possible increases TE. If done well, there is less powder sprayed into the air missing the part. Don't overload the line with too much metal to coat in the time you have based on line speed and make sure the surfaces are accessible to the guns.
7. Booth air velocity. Booth airflow must eliminate powder migration from the booth but not interfere with the coating process. Too little airflow, the powder migrates out of the booth; too much, and the powder never makes it to the parts.
8. Nozzle type. Different nozzles have different effects. A fan spray nozzle has a fairly large pattern with more forward velocity. Conical nozzles have a softer forward velocity with different pattern sizes dependent on the diameter of the nozzle. Select the nozzle that best fits the part size and geometry and experiment with different ones. Match the pattern to the target.
9. Operator technique. This one is simple: An untrained operator always deposits more powder than required—it is human nature. Train them and reward them for good parts that are within your target film build.
10. Gun movement/triggering. Gun movement and triggering will increase TE by depositing powder when and where it is needed. Avoid spraying air. Gun movement enhances film thickness consistency, allowing repeatable results and optimizing gun settings. Gun triggering will reduce overspray and edge film build.
11. Humidity and temp. Too much heat can start physical/chemical change in powder. Too much humidity causes powder to clump and spit. A powder room isn't mandatory but can reduce cost and improve consistency.
12. Grounding. One of the most critical aspects of efficient powder coating is a proper ground. All substrate components must be grounded with a resistance to ground not exceeding 1 megaohm. Ensure all contact points are free of cured powder so ions have a clear path to ground. Bad ground creates more rejects due to light coating and inconsistency in coating thickness.
13. Powder particle size distribution. Smaller particles can be difficult to fluidize and may not have enough charge to attract to the part. Larger particles can lead to thicker film builds and more orange peel. Control particle size by consuming reclaim as fast as you generate it and work with your powder vendor to ensure consistent particle size distribution.

There are numerous variables that affect transfer efficiency. Accurate and consistent adjustment of these variables can provide high efficiency and consistent measurement. If you have standard settings that work and you see them drifting to higher flows and reduced transfer efficiency, check the system closely for issues.

Originally published in the March 2017 issue.