Flow Behavior and Exit Pressures of Corn Meal Under High-Shear—High-Temperature Extrusion Conditions Using a Slit Die

Corn meal and commercial low density polyethylene (LDPE) were extruded through a slit die using a laboratory model single-screw extruder at various extrusion conditions. Flow curves for the corn meal showed severe shear thinning behavior (n≤0) at lower moisture contents; molecular degradation, pressure dependence of viscosity, viscous dissipation, slip, and yield stress were identified as the possible factors. Survey of the available literature and the behavior observed in this study indicates that molecular degradation is the major factor responsible for n ≤0. A modified viscosity model that includes the die entrance pressure (barrel pressure) for molecular degradation effects was developed; such a model also accounts for the pressure dependence of viscosity. Negative exit pressures for LDPE and both positive and negative exit pressures for corn meal were obtained. Statistical analysis indicated large errors in the exit pressures predicted by the linear extrapolation. These errors originate from several sources, thereby preventing the application of the exit pressure method as a simple method for estimating N₁ of complex materials such as corn meal.