SMC - Sink Mechanisms and Techniques of Minimizing Sink

Two major causes of sink marks above ribs and bosses in compression molded SMC (sheet molding compound) parts are discussed-polymerization shrinkage and thermal shrinkage. Polymerization shrinkage has been minimized through the use of low profile resin systems. Differential thermal shrinkage is the result of fiber orientation and resin-glass variation that may occur as SMC material flows into the ribs and bosses.

A design investigation has shown that ribs with minimum lead-in radii have significantly less sink than ribs with more generous 1/8 in lead-in radii. The effects of various rib geometries on sink have been documented.

A materials study has shown that the use of short fibers and increased glass loading generally reduces the level of sink. More importantly, the use of “composite” molding techniques has permitted no sink molding on a lab basis. The term composite here refers to the simultaneous molding of two different SMC materials. The best results have been obtained with the use of continuous strand mat reinforcement SMC near the flat surface in combination with 1/2 in fiber reinforced SMC in the rib area.