I am trying to put a second coat onto a part that is already powder coated, and the second coat will not stick to the part. Do you have any suggestions?
Powder AdhesionI am trying to put a second coat onto a part that is already powder coated, and the second coat will not stick to the part. Do you have any suggestions?
Charged powder is attracted to a grounded, conductive surface when powder is applied. Once the powder comes in contact with the bare metal, the electrical charge will begin to dissipate from the powder to ground. However, all of the charge does not immediately go to ground. If it did, the powder would fall off of the part. Instead, the charge slowly bleeds off through the grounded part, hanger and conveyor, and eventually to earth. Once the powder has been cured, the surface is insulated, and the application efficiency during a recoat is lower due to resistance from the first coating. The surface of the part will become charged quickly and will reach a point where the charged powder on the part is at, or near, the same potential as the powder being applied, thereby creating back ionization or rejection.
To prevent this problem, the application setup must be modified to compensate for the resistance of the first coat. The electrostatic charge should be reduced (either by lowering voltage or limiting current) and/or the gun-to-part distance should be increased. You might also need to adjust the powder delivery settings to increase the output by 10 to 20 percent. Be aware that the application will require more time and the transfer efficiency will decrease when doing recoats, but application is possible. The reduction in charge, combined with a slight increase in flow rate and the increased distance of the gun from the target should provide the coverage needed. Be sure to cover the entire part to avoid a dry-spray appearance.
Automated Powder TransferWe currently feed fresh powder to our hopper by hand with a scoop and would like to automate the process. Can you tell us what options are available for the transfer of the powder from the box to the feed hopper?
A fresh powder feed system can be used to supply virgin material to the feed hopper or collector module. The feed system uses a vibratory table to agitate the powder and a powder pump that uses minimal air to move the powder and keep the dust to a minimum. Depending on the amount of powder you wish to move, other options are also available for moving powder. For example, 300-lb drum unloaders or 1200- to 1500-lb palletized box feeder units enable the operator to have automatic or manual control of the powder replenishment process. These two units incorporate a venture-style pump system that uses compressed air to transport the powder.
Once you have decided on the type of unit you wish to use, you then need to decide if you are going to manually or automatically activate the transfer process. Manual activation requires a person to "flip a switch" or "turn a valve" to begin powder transport. Once this occurs, the unit will transfer powder until the container is empty or the operator turns off the unit. Typical problems are forgetting to turn the unit on and running the hopper empty. The opposite situation can occur when the operator forgets to turn the unit off and overfills the hopper. Conversely, automatic replenishment uses level sensors in the hopper to determine the level of powder in the fluidized hopper, as well as to activate and deactivate the fresh feed unit as needed.