Positive hole pressures and negative exit pressures generated by molten polyethylene flowing through a slit die

TLDR: In this article, a slit die rheometer was constructed to measure elastic and viscous properties of molten polymers at low shear rates, and the wall shear stress σ and the extrapolated exit pressurePx were determined by means of two pressure transducers mounted flush with a die wall.

Abstract: A new slit die rheometer has been constructed to measure elastic and viscous properties of molten polymers at low shear rates. The wall shear stress σ and the extrapolated exit pressurePx are determined by means of two pressure transducers mounted flush with a die wall. The hole pressureP* is obtained from one of the flush-mounted transducers and a third transducer mounted in a transverse slot opposite the flush-mounted transducer. The wall shear rate\(\dot s\) is obtained from a metering pump. Electrical heaters give melt temperatures that are uniform to within ±0.1 °C at 150°C. For two low-density polyethylene samples of Melt Index 2 and 50, at shear rates in the range 0.1 to 8 s−1, it is found that (a) viscosity values agree with those obtained using two Weissenberg Rheogoniometers (WRGs); (b) hole pressure data agree with the predictions of the “HPB” transverse-slot equationN1 = 2σdP*/dσ when WRG data are used for the first normal stress differenceN1; and (c) exit pressures are large and negative, in marked disagreement with certain published equations relatingPx,N1 andσ. An error analysis shows thatPx values can contain significant negative contributions arising from the pressure dependence of viscosity, even at low shear rates. As a means for in-line measurement of melt elasticity at low shear rates, the results favor the use of the hole pressure, but raise serious questions about the use of the exit pressure.