Setting Powder Guns for Raw Parts
Powder expert Rodger Talbert says for metallic powders, you should concentrate on consistent gun-to-target distance and stroke patterns.
Q. What kV setting would you suggest for spraying raw/unpainted parts with platinum metallic powder? We usually run textures at 70 kV and set the guns at 60 kV for repaints. Do these setting sound right?
A. We have often discussed how electrostatic charging works and why it is helpful to control micro-amps. Basically, when you cannot control micro-amps, your system becomes much more vulnerable to all other application variables. Humidity levels, gun-to-target distance, Faraday areas and certain types of powder (textures and metallic, for example) can make application inconsistent and generate more rejects. The lack of control over micro-amps limits your options from the start. Your movement to lower kV is the right approach, but it is a limited way to control the flow of current (micro-amps).
For metallic powders, you should concentrate on consistent gun-to-target distance and stroke patterns. Your kV setting is probably OK at 70 for raw parts and slightly lower for recoats. On the recoats, I would increase flow rates slightly to help limit the dry spray and work with the gun farther away, if practical. It would be better if you could reduce micro-amps, of course.
Dry humidity is a problem because that makes it harder to develop a proper corona field and provide sufficient charging electrons to the powder. Effective electrostatic depends on a discharge of negative ions to break down air molecules and create the high number of electrons needed for efficient charging. The part is earth-grounded, and that sets up a fairly reliable path of energy and powder from the gun to the part. When the air is very dry, it upsets that balance of flow from the gun to the part because of unwanted static charge present in the atmosphere. The addition of humidity reduces the voltage that is held by surrounding objects.
I would consider adding humidity in the winter. This can be used for consistent gun settings and more effective electrostatic charging. Also, you really need a gun update to control the micro-amps for optimum application of any type of powder. Finally, you must work harder to ensure proper earth grounding at all times. These three issues will inhibit your success and cause much higher reject rates. You are wasting a lot of powder and generating a lot of unnecessary rejects running without these controls.
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