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2/1/2005

Barrier Screw Has a New Angle

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A new barrier screw design alters the profile of the barrier flight, which holds back unmelted plastic and allows melt over the flight into the melt channel.

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A new barrier screw design alters the profile of the barrier flight, which holds back unmelted plastic and allows melt over the flight into the melt channel. Typically the crest of a barrier flight is flat, which shears the melt as it flows over the crest. In the new CRD barrier section (U.S. Patents 5,932,159 and 6,136,246) from Rauwendaal Extrusion Engineering Inc., Auburn, Calif., the crest of the barrier flight is at an angle slanted toward the solids bed. This wedge-shaped region allows the melt to accelerate and stretch, causing elongational flow, with better mixing and less shear heating.


The barrier section is relatively short and located toward the end of the melting zone, which Rauwendaal says eliminates plugging and surging. The first screw with the new barrier flight was built three years ago, and there are close to 100 now in production of sheet, film, profiles, and tubing. Rauwendaal notes that the CRD barrier screw works well with smooth-bore and grooved-feed extruders over 50-mm diam. and with high-viscosity materials.

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