Pinhole size air pockets

 Q.We make a small motor housing—about 4 inches in diameter and 4 inches high. There are several blind knockouts on the exterior surface. They are like sharp depressions in the surface about 1/8– 3/16 inch wide or long. The shallow depressed area is about 1/16 inch deep. The bottom corners are sharp 90° angles.

E-coat is specified for the part, and no voids of any size are allowed. We have been working with a coating company that is very good at E-coating small parts, and they have solved air pocket issues for us before—primarily using innovative rack designs. Together, we have tried everything we can think of, from pre-treatment experiments to fancy rotating racks, changes in pre-treatment and changes in E-coat bath immersion time and voltage settings. We still get a smattering of those pinhole size air pockets in the coating. Do you have any ideas? D.S.H.

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A. I know the part design that you described very well and I know exactly what you are talking about. One of the E-coat applicators our company represents has recently gone through the very same frustrating exercise over a period of several months looking for a solution. After involving application specialists from both the pretreatment chemical suppliers and the E-coat supplier, we finally came to the conclusion that this design feature was not compatible with the coating process. The sharp angle in the bottom of each knockout or recessed feature causes a serious problem for adequate capillary action to allow wetting of that surface. Some folks thought it might be due to a phenomenon like the Faraday cage effect seen in powder coating. Others thought the liquid molecules simply could not penetrate into that tiny crevice. A design change has been suggested, but we are working with a production part and any change will take time to go through the change process.

Although it’s not the desirable outcome, we finally concluded the only way to keep the project in motion is to add a touch-up operation and an inspection operation after E-coat. If any of our readers have found another way to solve this problem within the standard coating process, please drop me an e-mail.