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 resistance
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