Deviation from the power
law is likely to result from a breakdown of the assumptions on
which the model is based, namely (i) zero fluid velocity at the wall,
(ii) fluid streamlines are parallel to thewall, (iii) uniformhydrostatic
pressure across any radial section of the capillary and (iv) flowis viscometric.
Themost likely causes of deviation frompower lawat high
rates are melt flowinstabilities and non-zero velocity at the diewall
(i.e. wall slip).