ISSN:
1435-1528
Keywords:
Slip flow
;
PVC compound
;
injection moulding
;
grooved die surface
;
master curve
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
Abstract 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 widthB of 20 mm. The heightH 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 $$\dot V_{red} = \dot V/BH^2 $$ was plotted against the wall shear stressτ W , only four compounds showed master curves independent ofH, which is indicative of wall adhesion. In the other cases this plot did not produce such a master curve, but the plot of the mean velocity $$\dot V/BH$$ againstτ W was independent ofH (slip curve). This indicated that slip flow prevailed with a slip velocityv G ≈ $$v_G \approx \dot V/BH$$ 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 $$\dot V = f(\Delta p)$$ were shifted to much higher values of τ Wc Above a critical value of the wall shear stress (τ Wc ) at which slip flow began, the output became nearly independent ofτ W . From the measurements made belowτ Wc a τ vs. $$\dot \gamma $$ relation for the shear flow could be derived, which was used to calculate the superimposed shear flow $$\dot V_S = f(\tau _W )$$ . Exact values of the slip velocity were then given by $$v_G = (\dot V - \dot V_S )/BH$$ . Wall slip only occurred for compounds with a high shear viscosity, which corresponds to a high molecular weight (K-value).
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01357956
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