Orifice die entry pressure drops are plotted against processing rate for each of the three grades and temperatures studied (Fig. 8). Orifice die entrance pressure drop (PO ), sometimes termed ‘entry pressure’ is created due to sudden contraction of the fluid at the die entry region and is therefore a measure of the materials resis- tance to tensile or stretching flow. Two techniques are commonly used for the determination of PO . Bagley observed that a plot of pressure drop versus die length to diameter ratio (subsequently called a Bagley plot) gave a straight line with positive intercept [26]. This implied that through using several different length dies it is possible to extrapolate back to obtain a zero length pressure drop. Although this extrapolation can provide a reasonable prediction of