Effects of Extruder Die Nozzle Dimensions on Expansion and Micrographic Characterization During Extrusion of Acetylated Starch

Starch acetate (DS 2) with ethanol content of 16% (d. b.) was extruded in a twin‐screw extruder with nozzle length to diameter (L/D) ratios varying from 2.9 to 13.1, constant screw speed of 140 rpm and barrel temperature of 160°C. Effects of variation in L/D ratio and the individual effects of variations in L and D on expansion and micrographic properties were studied. With smaller D and larger L nozzles, back pressure developed in the die was immense, which resulted in high initial expansion and then collapse of extrudates, after a brief time, leading to smaller diameter extrudates. It was concluded that the pressure drop exerted tensile forces in the cell material in excess of its elastic limits. SEM results of the samples showed the effects. It is important to consider the elastic properties of the material in predicting ultimate radial expansion. The expansion phenomena have been explained in detail.