Volume 41, Issue 1 (January 2013)
Understanding Melt Index and ASTM D1238
In plastics manufacturing, the melt flow index (MFI) is used as a routine indicator of rheological behavior when more expensive and laborious determinations of well-defined material functions are impractical. The MFI is the mass flow rate in a pressure driven flow through a standardized abrupt cylindrical contraction into a short tube performed under a standardized combination of pressure drop and temperature. In this paper, we use a finite element model to explore the connections between rheological properties and melt index. We explore the role of shear thinning by modeling the flow through the melt indexer using the Bird-Carreau model. We then explore the role of melt viscoelasticity in the MFI using the corotational Maxwell model. We present our results in dimensionless charts designed to help plastics engineers specify the MFI of a plastic for an industrial manufacturing process of known material functions. Worked examples are included to show how to use the results.