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Rheology of short glass fiber‐reinforced thermoplastics and its application to injection molding. III. Use of a high shear rate capillary rheometer in the injection molding shear rate range
Author(s) -
Crowson Roger J.,
Scott Anthony J.,
Saunders Derek W.
Publication year - 1981
Publication title -
polymer engineering and science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.503
H-Index - 111
eISSN - 1548-2634
pISSN - 0032-3888
DOI - 10.1002/pen.760211203
Subject(s) - materials science , rheometer , die swell , shear rate , composite material , polypropylene , rheology , molding (decorative) , shear (geology) , viscosity , capillary action , extrusion
A capillary rheometer has been designed and built which is capable of making measurements in the shear rate range: 100 s −1 < \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm \dot \gamma } $\end{document} < 3 × 10 6 s −1 . The rheometer uses the injection unit of an injection molding machine. A modification of this rheometer has been built which consists of two dies in series. Both dies may be used for rheometric purposes, but the second die is intended primarily to act as a valve to allow the mean pressure to be varied in the first die, while keeping the pressure drop constant. In this way the pressure dependence of viscosity may be measured in the injection molding shear rate range. Measurements have been made on unfilled and short glass fiberreinforced polypropylene melts and flow curves covering seven decades of shear rate are presented for the unfilled material. The temperature of the extrudate may be as much as 70°C higher than the barrel temperature. Two effects, shear heating and compression heating, contribute towards this temperature rise, and calculations of the extrudate temperature are in good agreement with measured values. At high shear rates the viscosity is affected by the pressure needed to drive the flow. The effect of pressure on the viscosity outweighs the effect of temperature on the viscosity at high shear rates. A value for the pressure coefficient of viscosity of unfilled polypropylene β = 0.0058 m 2 MN −1 has been obtained.