Effect of Capillary Dimensions on Extrudate Swell of Molten Polymers

Abstract

<p>The effect of capillary dimensions on the extrudate swell of molten polymers is studied with three branched low-density polyethylenes (LOPE) and a broad distribution polystyrene (PS) samples. The polystyrene and one of the low-density polyethylene samples (DHDY-6873) are tested in detail at three extrusion temperatures ranging from 160°C - 220°C, while the other two low-density polyethylene samples are run at one temperature only.</p> <p>For all polymers, extrudate swell decreases exponentially with increasing length-to-diameter (L/D) ration of capillaries. For a die-orifice (L/D → 0), extrudate swell increases with entry angle up to 150° and decreases thereafter. At constant reservoir diameter DR, extrudate swell also increases with DR/D ratio up to 18 and then drops.</p> <p>An empirical decay equation is found to correlate well with the extrudate swell data for low-density polyethylenes. However, there is no direct correlation for polystyrene.</p> <p>Huang and White's new expression predicting extrudate swell with long capillaries does not correlate reasonably well with the experimental data for polystyrene. However, the model as previously suggested by Graessley gives better approximation. Tanner's new inelastic theory on extrudate swell plays no significant part in the present study. The effect of "frozen-in-stresses" on extrudate swell is found to be small.</p> <p>Huang and White's new model predicting extrudate swell for an orifice-die correlates well with the polystyrene data at low shear rate region (<30 1/sec), but indicates considerable discrepancy at high shear rate region. At present, there is no reilable predictive theory for extrudate swell at higher shear rates both with short and long capillary dies.</p>