AVERAGE SHEAR RATE IN A TWIN-SCREW EXTRUDER AS A FUNCTION OF DEGREE OF FILL, FLOW BEHAVIOR INDEX, SCREW SPEED AND SCREW CONFIGURATION

ABSTRACT Mixer viscometry assumptions were used to model average shear rate in a co‐rotating twin‐screw extruder. Screw speed was varied at 50, 100, 200, 300 and 400 rpm in an MPF‐19APV twin‐screw extruder for Newtonian and non‐Newtonian fluids with different flow behavior indices (n). A composite screw configuration was used. Degree of fill was varied from 0.4 to 1.0. Also, for three separate all‐one‐type screw configurations, screw speed was varied from 50 to 400 rpm to estimate the average shear rate for 30° forwarding paddles, single‐lead, and twin‐lead screws individually for 1.0° fill. A modified matching viscosity technique was used to estimate average shear rate for all conditions. Average shear rate was modeled as a function of extruder constant (k′), screw speed and an empirical parameter α. The k′ increased as a power‐law and α decreased linearly with degree of fill. The k′ ranged from 5.7 to 45.1 sα−1 rev−α and was independent of screw speed and flow behavior index. Alpha increased with n. Average shear rates for the composite screw configuration ranged from 10.5 to 404 s−1 for fluids with flow behavior indexes of 0.24, 0.60 and 0.67, respectively. For the separate screw configurations at fill = 1.0, average shear rate at a constant screw speed was highest for 30 forwarding paddles, followed by single‐lead and twin‐lead screws. PRACTICAL APPLICATIONS Practical applications include providing a convenient method to estimate average shear rate for composite screw configurations under partial‐fill conditions. Also, the study provides the basis for follow‐up studies where the influence of shear on food product quality and nutritional value can be examined.