We have an automatic powder system with 16 corona guns that are used to coat luminaries and fluorescent lights. Because of some Faraday problems in our parts, we decided to place a manual tribo gun after the corona guns. Now, when the operator uses the tribo, all the powder is repelling and removing from the edges of the part. This is totally the opposite of what we wanted. We believe it might be caused by the effect of the corona guns. What are we doing wrong?
Lower Voltage for Faraday Cage Effect
We have an automatic powder system with 16 corona guns that are used to coat luminaries and fluorescent lights. Because of some Faraday problems in our parts, we decided to place a manual tribo gun after the corona guns. Now, when the operator uses the tribo, all the powder is repelling and removing from the edges of the part. This is totally the opposite of what we wanted. We believe it might be caused by the effect of the corona guns. What are we doing wrong?The corona gun charges by adding electrons to the powder to produce a negative charge. The tribo gun charges by removing electrons from the powder to produce a positive charge. When you spray powder with a positive charge onto a surface that has a high negative potential (from the corona charging process) you neutralize the charge effect of both and that causes powder to fall off the surface.
For Faraday cage effect, you should work on lower voltage and the correct spray pattern. Pre-touchup is also a possible way to help with the Faraday areas because the resistance is lower in the Faraday area prior to application with the 16 automatic guns. Excess fine particles in your powder may be contributing to your difficulty with the Faraday areas also. If you reclaim powder you may be accumulating fine particles in the reclaimed powder. Try using a manual corona gun with virgin powder (no reclaim in the touch-up material) and use lower voltage.
Abrading Aluminum
If platinum powdercoat is abraded with a CS10 taber wheel or even a pencil eraser it darkens significantly very quickly. Our supplier blamed unbonded leafing aluminum, but when they supplied fully bonded leafing aluminum it did the same thing. It seems that only certain types of friction create this effect, seemingly those that perhaps have a higher coefficient of friction and are able to remove the aluminum flake from the surface, perhaps heat is involved? Please explain this phenomenon and what it is called and why do only some abrasives cause this effect? What is the solution? Other types of paint? What properties do we need to prevent this with platinum short of a clear topcoat?I am not aware of any particular name for this. What is happening is that the abrasive is removing any resin that may be laying over the aluminum flake and abrading the surface of the aluminum so that it does not reflect light. Some abrasives will polish the surface and not cause the darkened look. A vulcanized rubber eraser often contains some high-silica pumice or similar abrasive that scores the soft aluminum surface and creates a smudge. The Taber abrader cuts the aluminum surface. A soft substance like leather will polish the aluminum.
The answer lies in having enough resin covering the aluminum flake to resist mild abrasives. Controlling this is hard since there is no traffic cop in the powder formula to direct the locations of each flake of aluminum and it is likely that some will be near the surface. The only sure way to provide the necessary abrasion resistance is to add a clear coat over the metallic coating.
