Voids In E-Coat Film

We manufacture wheels in India. Our component is an automobile wheel consisting of an outer circular rim and an inner disc. The problem is that a small zone nearer the interface (rim-disc) is not getting E-coated on a few of the components (approximately 10%). Our E-coat preparation process includes alkaline clean and phosphate. Can you help me with the causes of this problem and counter measures?

Q. We manufacture wheels in India. Our component is an automobile wheel consisting of an outer circular rim and an inner disc. Both are welded together through a round fillet weld on one side. The welded part is then painted by the electrocoating process. The problem is that a small zone nearer the interface (rim-disc) is not getting E-coated on a few of the components (approximately10%). Our E-coat preparation process includes alkaline clean and phosphate. Can you help me with the causes of this problem and counter measures? K.R.

A. Because you are only having the problem on a fraction of your production parts, it would seem to be a problem with surface contamination that’s not being removed with your cleaning process. You might perform an incoming inspection on the next lot of parts to see if the wheels are being presented for coating with occasional high concentrations of certain soils in this area. If so, you will need to take a survey of your manufacturing process to see if there is something going on that is causing this condition.

If you don’t find components with unusual areas of contamination with soils that could be very difficult to process, then you might have a situation where air pockets are forming during the immersion of the wheels in various baths and especially the electrocoat bath. Air entrapment can prevent the electrocoating paint from being deposited. If your parts are just seeing thin or voids in the coating in the very bottom of the crevice formed at the interface of the rim and disc, then you may be having a problem with “throw power” in your tank. For that you might consider increasing the voltage or allowing the wheels to remain in the bath a little longer, or both.

Other things you might check include circulation in the electrocoat tank and the design of your racks. In some cases the parts can enter and exit the electrocoating tank in a manner that creates an air pocket. It could be because of the part configuration. And it could be the result of air escaping from wheels that are racked beneath and rising to be trapped in the wheels in question.

If any of you readers have had this experience, please e-mail your suggestions.