Visualization experiments on extrudate swell behavior at the microscale

Abstract

Extrudate swell exerts a significant impact on the shape and dimensional accuracy of extruded products. It is one of the important factors to be considered when designing the extrusion die and controlling the quality of products. The die swell ratio (B) is used to evaluate the degree of extrudate swell. In this article, comparative experiments were performed in order to simulate and observe in a precise fashion the extrudate swell behavior of carboxymethyl cellulose solution at a microscale by means of a visualization device. We chose micro dies capillaries where the inner diameter was in the range of 0.1–0.5 mm. The effects of capillary geometry, entrance velocity, and shear rate (shear stress) on extrudate swell were investigated. The results indicated that shear rate (shear stress) increases and the value of B is enhanced with decreasing capillary diameter under the same capillary entrance velocity. At different entrance velocities, the die swell behavior from 0.3- to 0.5-mm-diameter capillaries was similar to that of conventional ones, and the value of B increased with increasing shear rate. Additionally, the extrudate swell from the 0.2-mm-diameter capillaries revealed a decline, whereas that from the 0.1-mm-diameter capillaries first diminished and then increased concomitantly with rising shear rate. We expect that this was due to wall slip that occurred analytically in our micro channel.