Q: Over a three-month period, we have had a condition with our powder coating system where our part mil readings decrease. We have tried to increase the automatic guns’ powder output, in hopes the part mil thickness would increase.
Increasing the automatic guns’ powder output does not increase the mil thickness. This problem does not happen all the time. I have listed items we have tried to resolve the problem:
- Checked and repaired all automatic guns (pumps, inserts, automatic gun function test and system grounding test). No problems were found.
- Sent powder samples to be analyzed by the powder paint manufacturer. The powder paint manufacturer concluded there were no problems found with powder paint samples sent to them. The test performed were particle sizes analysis and dry flow powder paint testing.
- Part hangers and hooks inspected; no large amounts of powder buildup were present.
Our part thickness has decreased from the acceptable powder thickness of range of 2-4 mils to the unacceptable thickness range of 1-3 mils. Any help resolving this problem, I would greatly appreciate. Thank you for your time.
A: Sounds like a very perplexing problem. You mentioned that you checked the automatic guns. Did you check the voltage/current output of each gun? Have you also checked your line speed? Does it fluctuate?
Another test is to measure the actual powder output of your guns. Secure a vacuum cleaner bag to each gun (a rubber band will suffice) and trigger the gun for 30 seconds. Weigh the powder collected for each gun. The difference should be within 5% of each gun.
Another place to look is the ratio of reclaim to virgin powder being introduced to the guns. I recommend that you keep the reclaim to virgin ration below 25% reclaim.
I would also be inquisitive with the conclusions drawn by your powder supplier regarding particle size. What type of data did they give you? Was it for a number of boxes? Was it an average of many measurements? Was it for more than one batch? It is helpful to see the entire range of particle size data for a number of boxes (or drums). This includes average (or median) particle size, percentage below 10 microns and percentage above 90 microns. A high level of fines (> 7% below 10 microns) causes poor transfer efficiency. Have you measured the particle size of your reclaim/virgin mix of powder? Is there a high level of fines in that mixture?