An Experimental Apparatus for Measurement of Elongational Flow Properties for LDPE Melt and the Effect of Testing Conditions

Abstract

An experimental rig was designed and constructed for melt strength measurement and was assembled at the end of a single screw extruder used for the production of low-density polyethylene (LDPE) melt. The experimental rig was coupled with a high speed data logging system and a personal computer for the real-time measurement of melt strength. The molten LDPE was extruded through a capillary die, forming a continuous filament before being pulled down by speed-adjustable mechanical rollers until the filament failed. A digital camera was used for measuring the actual extrudate size at failure point. The drawdown forces as a function of volumetric flow rate from the extruder, roller speed, die temperature and take-up style were of interest in this study. It was found that the experimental rig could be used for accurate measurement of the mechanical strength for the LDPE melt. The experimental results suggested that the melt strength of LDPE was dependent upon the volumetric flow rate through the die from the screw extruder, roller speed, and the take-up style. For ladder-step take-up, increasing roller speed resulted in non-linear increases in the drawdown forces, the drawdown force changes being associated with the molecular disentanglement and elastic resistances of the branched LDPE melt. The drawdown forces of the LDPE melt measured under the rapid speed take-up method were 40–60% greater than those tested under the ladder-step speed take-up method, depending on the volumetric flow rate used in the screw extruder. The tensile viscosity of the LDPE melt was found to decrease slightly with strain rate and die temperature in the testing conditions used in this work.