Clumping of Powder
What causes some of our powders to clump up and resist fluidization and flow?
Q. We have noticed that some of our powders tend to clump up and resist fluidization and flow. This causes rejects due to inconsistent film build, gun spits and other issues. What causes the clumping and what can we do about it? R.B.
A. Oliver Cromwell said, “Keep your faith in God, but keep your powder dry.” Powder coating materials are hydroscopic and they can attract and hold moisture. Moist powder will clump up and cause the problems you describe. To avoid these problems, many powder coaters store their powder in a controlled room to keep it dry. A controlled application area is also helpful.
In addition to moisture, powder is sensitive to the distribution of the particle size. Finer particles (smaller than 20 micron) do not have a strong attraction to the grounded part. Finer particles also cause problems with agglomeration and poor flow characteristics. The result is that a blend of powder with a lot of fines will clump together more readily and cause the kind of problems you describe. Have your supplier check the fresh powder for particle size, and then compare it to the powder in your hopper and reclaim module. You may find that you have accumulated too many fines or the fresh powder is running too fine. Control the reclaim to avoid a buildup of excess fines. That means that you have to consume the overspray as fast as you generate it or you will reduce the effectiveness of your material. Also check all of your pumps, especially your transfer pumps. They can get clogged and require more pressure to move the powder. More pressure means more fines (the powder is fractured by the higher pressure) and more impact fusion. Speaking of pressure, take a look at your air pressure in general. Excess flow generates more overspray and breaks down the powder more rapidly.
As with any powder application problem, check your ground. Poor ground makes it harder to build film, especially in Faraday areas. Poor application often leads to (you guessed it) higher pressures in order to get the needed coverage. The downward spiral continues.
Keep your powder dry, keep your pressures controlled, use up your reclaim and keep your pumps clean. That should take care of the clumping unless you have a bad powder. For every 100 times a coater thinks he has bad powder, it usually turns out to be one or fewer times that the problem is with the fresh powder.
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