Indentation test as a tool for monitoring the solidification process during injection molding

Abstract

An inline method for monitoring the solidi-fication process during the injection molding of semicrystalline polymers is demonstrated. The method has been applied to various poly(ethylene terephthalate) (PET) and poly(buthylene terephthalate) (PBT) samples. The technique is based on a simple device by which an additional ejector pin is pushed onto the injection-molded part with a fixed force at different times during the solidification phase while the mold remains closed. The residual deformation (the so-called indentation depth) due to the applied load is measured offline after ejection. By the performance of indentation at different times during the cooling phase, an indentation depth profile, that is, the residual deformation as a function of time, is obtained. With a simplified solid/liquid two-phase model, the evolution of the solidification front in the mold as a function of the cooling time can be determined from the indentation curve. The obtained experimental results agree well with calculations based on the classical theory of heat penetration. Descriptions of several materials (including PET and PBT) with variations in the molecular weight (PET) have been obtained under different operating conditions (various mold temperatures, holding pressures, and indentation pressures). The results show that the indentation test may be regarded as a powerful tool for monitoring the solidification process during injection molding and, therefore, for optimizing injection-molding processing conditions according to material characteristics. © 2003 Wiley Periodicals, Inc.