Powder coated steel is blistering after baking

A. Zinc coated steel is difficult to powdercoat. A competent pretreatment is required. Without it, success is impossible consistently. You didn't mention your pretreatment system?

Applying PU paint over powdercoating seems to be aiming for a problem. Why do that? you could use PU powder if the coating chemistry is required to be PU.

MEK solvent rub test "poor". That suggest the powder isn't cured properly. Either too low temperature or not enough time at temp to cure the powder properly.

A. Tabatha,

This sounds like the metal is contaminated with oil or grease. However there may be another explanation if you are absolutely sure that the metal is clean. Check the compressed air supply and make sure there is no oil contamination. This is quite common also.

Hope this helps,

A. Drew makes some good points. I would also suggest checking your rinses. I had a customer who made farm implements. They suddenly started having problems like yours. Turns out the environmental guy turned down the overflow rate on the rinses to "save" on things. The build up of oil and soils was being redeposited right back onto the parts.

Steel can also be a culprit, particularly HRS. Any etching on it can bring all types of smut to the surface which can cause problems. I've seen people have lots of problems since they started buying the cheapest steel they could get.

A. By dictionary meaning of blisters -THIN BUBBLE ON SKIN-in our case thin bubble on paint skin/film/finish.
1) If oil/grease is present on the component due to poor degreasing, water break test will fail.As a result patchy phosphate coating can be seen.Painting on such oily component will result in microcissing or CISSING PATTERN on the film.The same will be the result if compressed air supply to the gun is contaminated with oil.
2)There is always possibility of PT chemicals oozing out from tubular parts or small holes or welded nuts.This oozing takes place when baking process is on.These oozed out traces of chemicals can cause formation of BLISTERS in the paint film immediately or within few days of coating.If you have drying oven for drying pretreated parts please check for any such oozing.Temp.should not exceed 110 degree C. If you find oozing to even very small amount then avoid painting such parts.
3)You have stated that steel is clean.Many times we feel that steel is clean but black scale/rust/micro pit marks does exist.If there is no de-rusting stage in PT process or complete de-rusting is not insured, this black rust remains as it is.WE KNOW THAT "COEFFICIENT OF EXPANSION OF RUST IS DIRECTLY PROPORTIONAL TO THE RISE IN TEMP."So when component, having black scale/rust even in micro-form, is painted and baked, and as temps.are 180 to 220 °C for powder baking, this rust TENDS TO GET EXPAND,TRIES TO BREAK THE PAINT FILM CAUSING BLISTER FORMATION.
I have personally experienced these facts mentioned above and worked on the same and sorted out the problem.
Hope the same will help you. All the best.
Regards,/P> SATYAJIT VASANT PANDIT
- PUNE, MAHARASHTRA, INDIA

? What do you mean by the word "cissing" ?

? Please explain the term "cissing"
Regards,
Bill

A. I heard the term "cissing" again today, and assume it means a small crater in the coating, circular, but not full depth to the substrate.

Usual cause is contamination of the substrate, water, oil, dirt, dust.

A. As you manufacture powder I would guess that the normal powder application problems do not relate to this cissing defect. As a powder mfgr. you must manufacture other gloss levels for which do not appear to have this problem with.
Questions.
A) What is the resin system?
B) Is the problem only on the works extruder or does it occur using the lab extruder as well?
C) Is it a new batch of resin or has the problem always been with you even with another mfgr. of resins?
D) What energy source do you use to cure the powder gas/electric.

First thoughts are that cissing and lack of gloss sounds like a raw material problem or you are adding too much/little or leaving out an essential or adding an incompatible ingredient.

A. Hi cousin. In January you asked how to rework such parts, and I said (topic 1152) I'd need more detail and hopefully some pictures to understand what you are trying to rework. Are you saying you just want to touch up the defective area? That doesn't sound promising.

Although metal finishing is complicated, such that identifying a 'root cause' of a defect isn't always easy, I think you are on the right track that cissing is usually caused by contamination -- sometimes water, usually oil, and always silicone if you have not yet proven to yourself that no silicone is used in your factory, and the part is not exposed to silicone at any time. Best of luck.

Regards,