Assessing the suitability of various grades of polypropylene for injection molding through flow-length measurements

Abstract

The capability of a polymer to be molded by the injection molding process is referred to as its moldability. Injection molding technicians typically estimate moldability using a parameter known as the Melt Flow Index (MFI), but this metric can be misleading as it reflects the polymer’s ability to flow under specific conditions that may differ considerably from those encountered during injection molding. A more relevant parameter is the flow length, which represents the distance a polymer travels in a thin, cold mold cavity during the injection molding process. In this study, three types of commercial polypropylene were injection molded in cavity and the flow lengths were measured based on injection pressure and temperature. The outcomes were then compared to the polymer’s rheological properties. The findings indicate that the MFI is not a reliable indicator of polypropylene’s moldability. An empirical equation is suggested to predict polypropylene’s flow length as a function of injection pressure.