Control of extrudate swell and instabilities using a rotating roller die

Abstract

• The moving walls (rotating rollers) provide a flexible mean to control die swell and instabilities • The smooth convergent geometry of the rotating die prevents volume instabilities normally related to entrance effect. However very large stresses on the melt are generated in the stationary mode so much so that surface roughness instabilities, not usually reported with polystyrene, appear on the extrudate surface. • This surface roughness resembles sharkskin but the mechanism leading to it is different to that leading to sharkskin. • This observation is a first as PS is reported to never exhibit surface roughness. • The rotating die is a versatile tool to produce sheet of various thicknesses as the gap can be changed by moving the rollers up and down. • The experimental data presented are underpinned by simple theoretical considerations, showing the importance of the wall shear stress and the changes in extension at the die exit. Thermoplastics extruded from dies will always swell and above a critical flow rate display instabilities (sharkskin, stick-spurt or gross melt fracture). Prior research has shown that the best way to suppress these instabilities is to reduce the entry converging angle using a smooth convergence and induce permanent wall slip. In this research we go a step further by allowing the walls to move using a rotating roller die. Thus both extrudate swell and flow instabilities become controllable. This paper presents data to support this claim. The practical benefits are important as stable operation at higher flow rates become permissible. Also, by providing extra control variables, this device becomes a useful tool to help unravel the causes of the various instabilities that arise in polymer melt die extrusion. A first from this research, using this roller die geometry we were able to tease out surface roughness instability with polystyrene hitherto not reported.