By: Nick Liberto, P.E.
First, let’s review the science.
Charging Theory: Powder particles are attracted to grounded part surfaces by the electrostatic charge imparted on them by the gun generator – the higher the charge on the particle, the better the transfer efficiency. In corona charging systems, the amount of charge on the particle is explained by the following formula:
Notice that some factors are more important than others. For instance, electrostatic field strength is directly proportional to applicator electrode voltage. Also, the distance between the part and the applicator (called the target distance) will directly affect electrostatic field strength.
The charge on the powder particle (which causes the attraction) is most affected by the amount of time the particle is in the field (by its square). The time and field strength will determine how much powder is attracted to the part (i.e., first pass transfer efficiency). The time the powder particle is within the electrostatic field is most easily controlled by adjusting the velocity of the powder pumped through the gun and reducing the speed of the applicator motion. It is a known fact that systems that use reduced powder velocity and slow gun motion will provide the best coating efficiency with the least effort.
Now that you understand the science behind how the corona electrostatic gun works, we can explore how to perform practical, everyday techniques to improve first pas transfer efficiency. These techniques apply to both automatic and manual applications.
1. Gun Voltage
Always start with the highest voltage settings your gun can accommodate to obtain the best first pass transfer efficiency. You may have to back off this “maximum” voltage setting if you have difficulties with Faraday Cage areas or experience electrostatic rejection issues (“starring”) with thicker film builds. Some equipment may allow you to control the current output to reduce the cause of these problems as well.
2. Part Ground
Verify that your part ground is less-than-or-equal-to 1 megohm resistance to electrical ground. This not only ensures you are operating safely in accordance with NFPA #33 guidelines, but it will reduce the problems with Faraday Cage areas an electrostatic rejection, discussed above, and allow you to operate the gun at the highest possible voltage setting.
3. Gun Target Distance
The spray gun should be between 6″ and 12″ to provide the best transfer efficiency. Theoretically, you can improve transfer efficiency by using the gun closer than 6″, but often the aerodynamics of the powder around the gun spray pattern device will disrupt the electrostatic attraction forces and cause the opposites effect.
4. Powder Particle Speed
The faster the particle travels, the less time it is within the electrostatic field and the lower the first pass transfer efficiency. Reducing powder output velocity, using gentle gun pattern devices, and slowing gun motion all play an important role to reducing the powder particle speed. Remember that the time the powder particle is within the electrostatic field is the single most important parameter to improving transfer efficiency.
5. Part Orientation
Position the primary coating surfaces toward the automatic guns or the manual sprayers to ensure optimum coating results. Proper part orientation will ensure that the spray gun can more easily coat the surface without wasted effort. Wasted effort always results in poor transfer efficiency as you cannot adequately apply the previously mentioned principles while coating the part.
These five simple keys to coating success will result in higher first pass coating efficiency. If you get the powder on the part efficiently, you will generate less overspray. This will mean you ill scrap less powder in spray-to-waste operations and keep virgin-to-reclaim ratios within the ideal 70:30 percentage range in reclaim systems. Your checkbook will thank you for implementing these five easy improvements.
Nick Liberto, P.E. is president of Powder Coating Consultants, division of Ninan, Inc., an independent technical consulting firm in Bridgeport, CT. He can be reached at firstname.lastname@example.org.