where vp denotes the velocity perpendicular to the screw axis, x, y and z are the horizontal, vertical and screw axis directions, respectively, 6 is the helix angle, D is the inner diameter of the barrel, N is the screw speed, H is the height of the flight. In the authors' extruder rotating at 20 rpm (4 cm/s) with a closed die dictating pressure flow, theory predicts the point of maximum velocity (5 cm/s at 1/3 H), exactly as found from the velocity profiles.

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