Laminar Flow and Backrind Formation in Molding of Viscoelastic Silicone Rubber

Abstract

When a thermoset polymer is cured at elevated temperature in a closed mold, thermal expansion can produce flaws in the finished product. Those flaws occur when rising internal pressure pushes the mold open and cured polymer flows out through gaps at the parting lines. Known as backrind, such defects are particularly common in compression molding, where the increasing pressure of a trapped, incompressible polymer can overwhelm the clamping pressure on the mold and expel polymer from the mold pocket. If that ejected material has already cured, it leaves behind structural damage and consequently a flaw in the finished product. Backrind usually appears as a ragged seam line near the gap where cured polymer exited the mold. Its appearance is typically irregular and fragmented, suggesting no particular pattern or uniformity to the process that produced it. In such cases, the cured polymer acts predominantly as a viscoelastic solid as it is driven toward and through the parting line. The backrind's ragged character results from tearing and fragmentation of that solid. It is possible, however, for the cured polymer to act predominantly as a viscoelastic liquid as it flows toward and through the parting line. Since the Reynolds number is low, the flow is laminar and the backrind bears witness to that laminar flow. More specifically, the backrind's observed shaped corresponds to isochronous contours in the laminar flow toward the parting line, contours that can be predicted using computational fluid dynamics.