What Issues Affect Transfer Efficiency?

Q. My boss has asked me to develop a plan to improve our transfer efficiency on our powder line. Our 12 automatic guns are over six years old and we have pretty good success with them. We have performed the necessary maintenance on them as recommended by the supplier. According to what I hear and read we should be getting better results. How do I enhance my transfer efficiency?—J.K.
 

A. This is a good idea for you to work on for continuous improvement. Higher efficiencies can lower your overall costs, reduce equipment wear (fewer spare parts), lower maintenance costs, improve finish quality and produce greater product throughput. 
Transfer efficiency is defined as the ratio between the amount of powder deposited on the part to be coated and the amount of powder sprayed. This efficiency is usually expressed as a percentage, and the amounts of powder are usually determined by weight. First, a quick review of the items that affect transfer efficiency:
Gun voltage/amperage settings – Typical voltage range is from 20 to 100 kV. Higher voltages will generally produce heavier powder deposition. Ten to 20 microamps is the optimum current draw for good deposition and penetration into Faraday areas. Higher amperage is fine for flatter surfaces. Be aware that the target distance affects the optimum gun settings. For distances of eight to 12 inches the gun settings can be fairly high. When the gun is closer the voltage and current should be reduced. 
Gun air adjustments – Typically the lower the pressure, the higher the transfer efficiency. This enables greater exposure to the corona field, more consistent film thickness, less abrasion to wear parts and less likelihood of orange peel. The powder flow rate should be set to a level that covers the parts and generates limited overspray. The velocity should be as low as possible.
Gun positioning – When the gun gets closer to the part, voltage will decrease and current draw will increase. As the gun current exceeds optimum levels, more ions are created and they flow faster to the part, resulting in back ionization. Keep the amperage low when the gun is close.
Part profile – Complex shapes may require manual touch-up. Automatic guns should not be set excessively high to fill inside corners because it will waste powder and result in more rejects. 
Part presentation – Parts should be hung for easy access to the surfaces. Multiple parts should be on the same plane and in a consistent pattern and height. 
Line density – Loading the conveyor with as many hangers as possible and hanging them as close together as possible will increase transfer efficiency. If this is done, it will reduce overspray. Avoid excess load, though. Too much metal may require too much powder flow or take a long time to cover.
Nozzle type – 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. Experiment with different nozzles to see which one is best for your application.
Operator technique – The more your operators know, the more effectively they will deliver higher transfer efficiency. An untrained operator will always deposit more powder than is required. Make sure they are trained. 
Gun movement/triggering – Proper gun movement can enhance your film thickness consistency, allowing more repeatable results and optimization of your gun setting. Gun triggering will reduce overspray and edge film build and result in higher transfer efficiencies.
Humidity and temperature – Too much heat can start the physical/chemical change in powder. Too much humidity can cause powder to clump and spit. Very low humidity will cause problems with charging efficiency. Control temperature between 60°F and 80°F and humidity between 40 and 60 percent RH.
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 one mega-ohm. Make sure all contact points are free of cured powder.
Powder particle size– Proper particle size distribution is important for effective powder deposition. Smaller particles can be difficult to fluidize and may not have enough charge to be strongly attracted to the part. Larger particles can lead to thicker film builds. Control particle size by consuming reclaim as fast as you generate it and work with your powder vendor. 
If you can work on the list of issues above you should be equipped to reduce your powder use and your operating cost. Best of luck!