What are some possible causes for bubbling of powder over e-coat? I am a custom coater who occasionally powder coats an e-coated carbon steel (hot-rolled pickled and oiled) part for a particular customer.
Dear Joe,What are some possible causes for bubbling of powder over e-coat? I am a custom coater who occasionally powder coats an e-coated carbon steel (hot-rolled pickled and oiled) part for a particular customer. The powder is a urethane polyester and coats fine over non-e-coated, pretreated substrates. Millage is fine on the e-coat and on direct applications. The powder vendor is leaning toward the e-coat as a possible problem, but the e-coat supplier is not saying anything. Can you help?
Hello Jeff,Your problem is not that unusual. I suspect that the e-coat may not be completely cured. The first thing I would check is the cure of the e-coat. Perhaps a solvent rub test will shed some light. I would also take an e-coated part and bake it twice, then powder coat and cure the powder. If this eliminates the problem, then the e-coat was emitting volatiles through the powder as it was curing.
These are the first things to test. Please let me know if this solves your problem.
Dear Joe,We are a leading manufacturer of automobile truck wheels in southern India. We are planning to set up a powder coating plant that will topcoat both solid and aluminum metallic powders over cathodic electrodeposition (ED) black primer (epoxy-based). We require salt spray testing of 2000 hrs and ultraviolet testing of 2000 hrs. What is your advice on the selection of powder? We have a restriction on the dry film thickness on the nave area, where the wheel is mounted on the truck (max. 50 microns). Can a powder system be used at this thickness without compromising quality?
Dear Mr. Srinivasan,It is possible to get 2000 hrs salt spray and 2000 hrs UV with a powder/e-coat system if you do everything right. The powder coating should be a superdurable polyester. This system will work at 50 microns powder thickness. The e-coat must be a highly corrosion resistant, cathodic epoxy type.
As important to the coating qualities is the cleaning/pretreatment of the steel wheels. The wheels must be completely clean and then receive a very high-quality zinc phosphate pretreatment.
The testing protocols will also affect your results. Scribed vs. unscribed coatings vary greatly in salt spray performance. Obviously, unscribed coatings perform better. The type of UV test used will also influence performance. A superdurable polyester powder will pass 2000 hrs UVA exposure; however, it will not pass the same duration of UVB exposure. The best approach is to test the coating systems before you make the commitment in your production facility. If the superdurable powder doesn’t quite meet the UV exposure test, then you will have to evaluate an acrylic powder. These make fantastic clearcoats but are difficult to pigment.
Dear Joe,We are having some issues trying to powder coat over a magnesium substrate. We apply a conversion coating to the part prior to painting. The customer says that there are no mold release issues. However, we continue to see what appears to be residual under the surface that cannot be painted over. Your thoughts would be appreciated.
Larry Dietz, Professional Plating Inc.
Hello Larry,It sounds like you are trying to coat a cast part. If you believe that residual release agents are present on the surface, I suggest that you perform an extra rigorous cleaning process on some of the parts. I would abrade the parts with 400 grit sandpaper, followed by cleaning with a strong solvent, such as MEK (methyl ethyl ketone). Preheat the parts to drive off any residual solvent, then powder coat and bake. If this eliminates the problem, then indeed a residue is present that your current cleaning is not removing. If this is the case, then you may have to alter your cleaning-pretreatment scheme.
Good luck. Let me know how everything turns out.
Ask Joe Powder is a regular feature of Finishing Today magazine. Please send your questions to email@example.com.