Can die swell be predicted?

Abstract

The term “die swell” is commonly used to describe the increase of diameter of polymeric extrudate upon emerging from the extrusion die. Understanding the phenomenon is very important in the extrusion of thermoplastics and elastomers. Some progress has been made in interpreting the phenomenon (1). However, the practical requirement as well as the critical test for any interpretation are quantitative prediction of the die swell at a given condition of extrusion. Presently available theories have not been adequately demonstrated to fulfill this need (2 to 4). This is particularly true with the shorter dies, where the die swell depends on the memory of the deformation at the die entrance. This paper describes a technique, by which die swell can be quantitatively calculated within the precision required in practice. The method is based on the theoretical interpretations developed previously (5, 6). Several samples of linear polyethylenes are used to illustrate the method.