Reducing Defects to Improve the Competitiveness of SMC in Exterior Parts

Despite the wide use of sheet molding compound (SMC) parts in many exterior applications, it is no longer the sole competing material for replacing sheet steel. Sharper competition from thermoplastics and polyureas is forcing designers to look at the total system cost of any replacement for steel. To succeed in a more crowded arena, class-A SMC parts must distinguish themselves in both cost and performance.

The delivered cost of a finished SMC part contains contributions from both rejected and refinished parts. The defects that cause some of the rejects and many of the near rejects are porosity and blisters. Porosity is responsible for the defects that show up after painting a part. Pinholes in the paint film are repairable, but only at great expense. Blisters, on the other hand, are more difficult to repair and often result in a complete reject.

Many factors influence porosity and blistering. We have examined the effect of molding conditions, mold design, and materials. With an understanding of these defects some molders have reduced the cost penalties from scrapping and refinishing SMC parts.