Products Finishing magazine is the No. 1 industrial finishing publication in the world. We keep our readers informed about the latest news and trends in plating, painting, powder coating, anodizing, electrocoating, parts cleaning, and pretreatment.
Issue link: https://pf.epubxp.com/i/213540
POWDER COATING oil in the steel. Manufacturing processes associated with making the part can produce heat that transmits to the part's material. Furthermore, the drying and curing processes apply more heat with higher dwell times, possibly pressurizing the oil and reducing its viscosity at the same time. The low-viscosity pressurized oil would potentially seek an area of least resistance for pressure relief, and the surface of the part would be the most conducive area for the liquid to travel to. The pressure exerted by the liquid may not be enough to permeate the jelled skin portion of the uncured powder, but the center and lower portions of the powder layer may not yet be fully cured. The potential elasticity of the semi-cure portion of the coating could allow the oil solution to move it until the pressure was diminished. The end-product of this theory would be a crater-like defect in the part. However, the change in the color shade (especially yellow colors) may indicate the presence of contamination. Then again, it may be as simple as an area of thinner coating on the surface.—D.K. A: I have seen similar defects before where trapped moisture, usually oil, creates a blister in the film. I think you have some residual oil on the surface before you apply the coating. As the part heats up in the cure oven, it releases trapped air with residual oil under the film. The coating is actually curing from the inside to the outside, so the film is too firm at that point to allow the evolving material to release out of the film. The film raises and the material gradually dries CLINIC out, and then the film collapses back onto the substrate. You may have good initial adhesion, but that area of the part is probably vulnerable to moisture penetration and coating failure in the future. Given the preparation process, it is hard to imagine that anything is left on the surface. However, that is almost certainly moisture bubbling up in the coating, based on the appearance. It is also informative that you do not have this problem with CRS or with heavier-gauge steel. That would indicate that the pickle and oil process associated with the lighter-gauge material leaves porosity and trapped oil that is hard to remove with chemicals. You may be able to relieve it by using a hotter dry-off oven. It is releasing in the cure cycle, so heat does cause it to evolve from the substrate. A hotter dry cycle may relieve it before coating. Try some parts with a 400°F dry cycle to see if the problem can be solved with a thermal cleaning. You may also want to eliminate the degreasing/sanding process and just have some parts blasted to remove laser and most of the oil. It is possible that the solvent is wetting the surface but not removing all of the oil, and the sanding operation cannot dig oil out of pores in the steel. You could have the defect inspected by a good lab to find some elemental evidence of what is on the surface. The elemental nature of the surface of the steel and the back of the coating defect could be the missing clue to the oil that is bubbling during cure. When equipment is down for repair, your mission is compromised. More MRO and OEM facilities depend on SIFCO ASC than any other plating specialist. By creating a unique plating process, SIFCO ASC offers a method to electroplate deposits without the use of an immersion tank. This enables you to enhance and repair critical components on-site, saving money and signifcantly reducing down time. Find out how our portable Cadmium and Zinc-Nickel kits can get you airborne in no time. They're designed for AOG operations where repair times are critical. SIFCO ASC Portable Plating Kit Learn more about how we can help you. Call us today on 877.650.6488 or email us at sales@sifcoasc.com PRODUCTS FINISHING — pfonline.com 33