Screw design firm Barr Inc. introduced a new VBET-brand screw, which increases the conductive melting capacity and achieves greater melting rates with improved extrudate qualities, with low shear. VBET is designed for single-screw extrusion.
A blown film processor approached Barr for help blending a metallocene linear low density polyethylene, but experienced a considerable melt fracture in the film, according to Jeff Myers, Barr's vice president of sales. The customer thought that was caused by residual metallocene solids, the result of lubrication of the metallocene pellets by the lower-viscosity LLDPE, so that the metallocene resins could not be melted by shear. The customer did not want to add processing aids to reduce melt fracture, Myers said.
Barr recognized the limitations of barrier screws, since with their typical design, about 90 percent of the polymer is melted by high shear in the barrier section. Temperatures are higher, which is undesirable in many applications.
The solution was VBET, which uses an elongated section like that used on Barr's ET screw. VBET uses more channel peaks than the older ET. Barr eliminated the barrier section and intentionally broke up the solid bed sooner, making a screw with very low shear.
The result, tested on blown film, was a significantly higher output rate before melt fracture, said the Virginia Beach, Va., firm.
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