Surface roughness and in some cases foaming was
observed for extrudate exiting the capillary dies at high shear rates
as shown in images obtained from scanning electron microscopic
(SEM) images shown in Fig. 5, which suggests that flowinstabilities
and/or slip occurred. The onset of surface instabilities correlated
with the discontinuities observed in Figs. 3 and 4, and therefore
occurred at lower rates for higher molecular weight grades. Generation
of instabilities during melt extrusion of polymers has been
reported for many polymers used in the plastic industry [22,23]
and have been observed to occur above a critical value ofwall shear
stress. Deviations from the power lawmodel have been reported to
correlate with flowinstabilities [24] and fromthe zerowall velocity
assumption [25]. The onset of foamingwas found to be independent
of rate by reversing the order of test stages shown in Table 1. Instabilities
were formed at high strain rates as with tests carried out
in the original order, but foaming occurred in the low rate stages
at the end of the test, suggesting a dependence upon time and/or
pressure rather than rate.