Star Bursts in the Coating

Q. We are using a wrinkle dark gray powder and often have a problem with “star bursts” all over the product. The stars are visible in the dry film and they do not flow out in the cure oven. We also spray a wrinkle black and get this problem occasionally, but it is not so pronounced. We do not have this problem with our smooth powders. We have cleaned the powder booth and the fee hopper, changed filters and everything else we can think of, but we cannot make the problem go away. Do you have any suggestions? R.B.

A.There are a few possible causes of contamination, but it is unlikely that a contaminant would affect the one powder but not others. One common problem that causes the “star burst” defect that you describe is not contamination but an electrostatic issue called ‘back ionization’ that can occur when buildup of free ions at the part surface is too high. It could be that the current setting is too high, the grounding of the part is poor, the film build is too thick or the spray technique is incorrect. Location of the gun to the part (target distance) also affects how quickly the back ionization will occur. The closer a gun gets to a part, the faster it occurs.

Back ionization occurs when charged powder particles and free ions build on the surface and the part cannot accept the level of current. As the powder builds, it insulates the grounded surface, resulting in a rough surface texture (in the uncured state). Resistance grows at the surface due to the insulating effect of the film, and it becomes harder for the surface to attract additional powder build, especially finer particles. Larger particles have more cumulative charge, and they will continue to build and create a film with more orange peel. If the resistance gets too great, the free ions at the surface will break down air molecules and the positive ion in the air molecule will be strongly attracted to the gun electrode. When it breaks out of the film towards the electrode it will cause a star burst pattern to appear in the film. A powder that is very receptive to the collection of electrons may accelerate the problem.

To avoid back ionization, try the following:

• Make sure the part is grounded and there is not resistance to the flow of current. Keep the rack contact points and the conveyor clean.

• If your gun has a current limiter, you should cut the current to a maximum of around 20 microamps when applying this powder.

• If you do not have a current limiter, you should turn the voltage down to around 60 kV.

• Optimize the target distance to the part and attempt to maintain that distance. The closer you get to the part, the more likely the problem is to occur.

• Practice good application technique: coat Faraday areas (inside corners) first, keep the flow rate at a moderate level and maintain steady gun motion.