Powder Priming of SMC. Part I: Assessment of the Current Technologies

Abstract

A cosmetic defect called ‘popping’ on the surface of sheet molding compound (SMC) body panels that are coated with powder primers has become a major hurdle in expanding the usage of SMC in automotive applications. General Motors R&D, therefore, initiated a project to study the mechanism of popping and to explore different solutions to this cosmetic problem. The current report covers the benchmarking phase of this project, in which a variety of SMC materials and conductive primers/sealers were evaluated for their ability to produce a pop free surface. In this work, among other factors, the effects of molding pressure and the panel moisture content on the degree of popping have been studied. The experimental results showed that popping increased with the increase in moisture content and with the decrease in molding pressure. The extent of popping, however, varied with the type of SMC and the conductive primer. We found that none of the SMC materials or conductive primers were able to eliminate the pops completely at high moisture levels. More importantly, it was concluded that moisture is not the only cause of the popping and there are other factors that contribute. The results of the current work will be used in the next phase of the investigation, which is focused on identifying solutions to this cosmetic problem.