Craters on Powder Coated Aluminium Sheets/Extrusions

Letter 1021

Craters on Powder Coated Aluminium Sheets/Extrusions

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We have a contamination problem with our combined powder and wet PVF2 (fluorocarbon) line.

Little craters are forming on the coated aluminium eets/extrusions surfaces. We haven't been able to pinpoint the source(s), but we believe that the contamination is from as many as three sources - oil in compressed air, silicone from oven, and airborne particles. We have carried out numerous trials to isolate different parts of the line and different powders, but we get conflicting results. The only sure source is the compressed air.

The craters are not forming on parts coated with 'rippled'-finish powder, nor wet PVF2 (fluorocarbon).

Anybody ever had a similar problem?

Johnny Foo
- architectural components

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Try running unpainted parts through your cure oven, then painting and curing to see if you have outgassing from the metal itself, it's possible the powder giving you the trouble could be adjusted to flow longer and get rid of a slight outgassing problem. Also try contaminating a part with some of your PVF2 both wet and dry, and try contaminating some of your powder with that paint, to see if airborne contamination from that process is causing your craters. Just the first couple of things I thought of...Hope its helpful.

Jeff Watson
- Pearland, TX

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In a situation such as you described it is important to document the types and number of each type of defect that you have, then go back in the process and search for the contaminant causing each problem. Examples of defect causers are as you mentioned silicones, and oils, but also fibers and particles that are floating around in ovens. Jeff's discussion of the powder problem was on the money as well.

To help further, I would need to know something about your pretreatment and paint systems. What type of pretreatment and how applied (how many stages)?

Nothing short of an excellent microscopic analysis of the surface will help you determine the cause of the defect. Also the defects themselves (if they are particles or fibers) sometimes require elemental analysis (SEM with EDAX) to determine their composition.

One way to determine if it is the paint or pretreatment system that is causing the defects is to take parts and solvent wipe them (completely clean) and skip the pretreatment system altogether. Put those parts right into the paint booth and oven. If the same defects appear, then the paint system is where the defects are occurring. The only exception to that would be if they were caused by the rag that you wiped the parts with. If there are no defects then the pretreatment system is suspect. Maybe the parts are not being cleaned effectively or one or more of the baths have high levels of contamination.

Anyway, good luck finding the problem. Email me if I can be of additional assistance.

Craig Burkart
- Naperville, Illinois

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