These remarks relate to the problem question in the July 2004 issue. This problem reminded me of one we ran into on a painted plastic part. What we found out was that the grade of ABS we were using was formulated with a mold lubricant to improve mold life. (These items are often found in plastics but never really thought about when being painted.) The manufacturer had a wide specification on the material and whenever the concentration got close to the upper limit, an invisible waxy skin would form on the part. When we painted the part and performed the crosshatch test, the paint would peel off causing a reject. Actually what we found is the paint was adhering well to the waxy skin but the waxy skin did not adhere to the ABS. The way we found out about this issue is we did a crosshatch test on the non-painted parts and saw the waxy skin on the tape.
Resolving the problem was very easy once we asked the plastic manufacturer about the additives they use and their manufacturing process. We did that by specifying a lower additive concentration in the plastic we used. Once we addressed the additive concentration issue, we never had an adhesion problem again. M.R.
In the July Painting Clinic, M. Y. was able to get paint to adhere to natural ABS, but had problems adhering to black ABS parts. The difference in concentration of mold lubricant wax in their formulations could account for the difference in paint adhesion to the natural and black ABS surfaces. These are the kinds of problems that keep you busy, as they did me, when I had a real job. Otherwise, “they” wouldn’t need us.