Measurements on seven rigid PVC compounds were carried out with a slit rheometer working in combination with an injection moulding machine. Plastication of the compounds occurred in the screw of the plastication unit, which also forced the melt through the die with a controlled forward velocity. The rectangular slit had a length of 90 mm and a width
B of 20 mm. The height
H could be varied between 0.8 and 3.3 mm. Pressures and temperatures were recorded at several positions in and before the die. Measurements were carried out at shear rates from 10 to 2000 s
–1.
When the reduced volume output
[(V)\dot]red = [(V)\dot]/BH2\dot V_{red} = \dot V/BH^2
was plotted against the wall shear stress
[(V)\dot]/BH\dot V/BH
againstvG » [(V)\dot]/BHv_G \approx \dot V/BH
When, in the case of wall slip, the smooth inner surfaces of the die were replaced by surfaces with grooves perpendicular to the direction of flow, slip flow was prevented and the flow curves
[(V)\dot] = f(Dp)\dot V = f(\Delta p)
were shifted to much higher values of
[(g)\dot]\dot \gamma
relation for the shear flow could be derived, which was used to calculate the superimposed shear flow
[(V)\dot]S = f(tW )\dot V_S = f(\tau _W )
. Exact values of the slip velocity were then given by
vG = ([(V)\dot] - [(V)\dot]S )/BHv_G = (\dot V - \dot V_S )/BH
. Wall slip only occurred for compounds with a high shear viscosity, which corresponds to a high molecular weight (
K-value).
Key words Slip flow - PVC compound - injection moulding - grooved die surface - master curve
Dedicated to Professor H. Janeschitz-Kriegl on the occasion of his 60th birthday.