Investigating Rheological Techniques to Model and Predict Operating Conditions of a Single Screw Extruder with Internal Restrictions

Amudhan Ponrajan, Purdue University


Understanding rheology of raw materials and the numerous transformations they undergo is an essential aspect of modeling and predicting extrusion conditions. Both off-line capillary rheometry and in-line extruder fed rheometry techniques have been used to model rheology of materials during extrusion. Investigations on use of an off-line capillary rheometer to model apparent viscosity of full fat soy flours revealed inability of the capillary rheometer to handle food/biological materials high in oil content (20%) at high temperature (80°C) and low moisture content (15% wet basis). The lack shear degradation in the capillary rheometer also resulted in over-prediction of corn flour viscosities at high temperatures (80 to 120°C), shear rates (1 to 100 s-1) and low moisture contents (35 to 40%). A novel two-opening die attached to a small scale extruder operating at 100 to 300 rpm was used to measure in-line viscosities of cornmeal at 32.5 to 37.5% moisture during extrusion. Comparison of off-line versus in-line cornmeal viscosities at similar conditions consistently indicated that for food/biological materials sensitive to shear degradation, in-line viscosities were lower. Pasting property measurement revealed differences in material transformation which could be used to explain differences between techniques. Mechanistic model for the small-scale extruder with screw restrictions was built by considering a modified plane Couette flow setup with varying gaps and heat fluxes leaving at both plates, for one screw, one restriction and a die setup. The model successfully predicts the effect of screw restriction gap, extrusion pressure, viscous dissipation and heat fluxes.




Okos, Purdue University.

Subject Area

Food Science|Agricultural engineering

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