| A new constant-shear manifold for film and sheet balances the flow of virtually any polymer or blend, regardless of viscosity, for dies up to 36 in. wide, according to Premier Dies Corp., Chippewa Falls, Wis. The constant-shear principle was developed in the early 1980s to balance flow using a rounded “horseshoe” manifold shape instead of a “coat-hanger” shape. The horseshoe causes polymer to reach full width across the die faster, providing more uniform residence time than a coat-hanger die, in which polymer fl ows from the center out with longer residence at the die ends.
Early constant-shear manifolds were largely for R&D, since they could only be used for narrow dies of 8 to 14 in. Two years ago, Premier contracted with software consultant Prof. Mohamad Elgindi of the Univ. of Wisconsin, Eau Claire, to optimize a horseshoe for wider dies. The result is a new die with a distribution channel following the horseshoe curve, which gets deeper as the die widens. Premier has built five of the new dies, which cost 10% to 20% more than conventional coat-hanger dies but can run polyolefins as efficiently as polyesters, Premier says. In one application for rigid PVC, the die reportedly reduced color-change time by as much as 75%. The principle is also applicable to pelletizing and fibers.
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