Is Aluminum Harder to Paint?

Q. We powder coat primarily aluminum and we have approximately 6,000 different parts we coat on a daily basis in our plant and coat them approximately 18 different colors. Our line takes about 90 minutes to do a cycle and runs at about 17 FPM. Currently, we do reclaim on our larger colors and are only getting about 50–60% transfer efficiency on most of our colors using the electrostatic powder spray coating. I am relatively new to this, but I am told with aluminum this is expected because it is harder to paint.

I don’t completely agree with this statement and I was wondering if you could give me some ideas on what to check for to improve our line. How can I check to make sure we have a quality ground? Can I improve our ground, and how? Are there other things that we should do to improve our TE? A.M.

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A. I’m happy to help but you ask a big question. Ground can be checked with an ohmmeter. Connect one lead to a part on the line and the other to the rail. Resistance should be below 1 megohm. Keep clean hooks at all times to avoid loss of ground. The rack contact must always make metal contact with the part.

Aluminum is not harder to coat than steel. Transfer efficiency (TE) is related to many factors and I cannot give you a quick email explanation of how to get the best results. Some of the important issues are: Rack/part density: more metal, less air. As long as you can get coverage you should load up. When coverage suffers you may have parts too close together.

Flow rate: many companies pump too much powder. TE is best at lower flows (20 to 30 lbs per hour per gun). Add guns if necessary but keep flow rates as low as possible as long as you can get good coverage. Excess flow rate not only reduces initial TE, it also wears out gun parts faster, consumes more compressed air and breaks down the powder particles size, further reducing transfer efficiency.

Correct setup: position and distance are important. Experiment with accurate positioning and document the correct setup.

Application technique: many manual operators have had little formal training and they could use a good workshop to learn more about the gun and how to use it.

Atmospheric control: heat and humidity affect TE. Control to 60–80°F and 40–60% RH. Excess humidity will cause problems with flow. Excessively dry air will cause application problems due to static in the air.

There are other important issues. I suggest you attend a program or training session sponsored by PCI or another qualified institution to learn more about application efficiency. Go to one of our workshops or buy the Powder Coaters Handbook.