Q. We apply a metallic, silver powder to 20-50 parts at a time. The parts are sandblasted, and many of them are also processed through a five-stage iron-phosphate wash system. We have a lot of problems with the pigment “bunching” and standing up on the edges. We have tried many combinations of gun settings, with and without ground wires, recoat modes, manual modes with kV turned way down, etc. We cannot seem to eliminate the problem. —K.S.
A. Metallic powders have unique charging characteristics. The metallic component is highly conductive, while other materials in the formula are less conductive. This can cause the metal-flake to react differently than the organic resin and other pigments and align in undesirable patterns. Some silver products are particularly difficult to apply, and it is hard to overcome the electrostatic behavior of the material. Bonding source and technique can have a profound impact on the electrostatic behavior of a powder material.
Before you do anything else you should check your gun with a voltage meter to make sure you are getting the right amount of voltage. If the output voltage is below the set point, you will lose your charge completely when you reduce the kV and the gun will not work. If the voltage is not right, try a different gun or get yours fixed.
After you have confirmed that the voltage output is properly calibrated, try a reduction in voltage to around 55 to 65 kV, micro-amps very low (10 to 25), and very low velocity. Hold the gun farther from the part than normal, and spread the parts out to leave more space between them than you would with another powder. If you are using a box-feed device, replace it with a feed hopper. If this does not work well, gradually increase the voltage and amperage to see if it has any impact at all. Make sure the fluid bed hopper is kept at a constant level and fluid pressure is moderate so that the surface appears to have a gentle roll, like water at a simmer on a burner.
If none of this works, you may want a different try a different silver material. And remember that, while most metallic powders can be effectively reclaimed, some are difficult.