Defect Reduction in Sheet Extrusion
A manufacturer suffered low process yields due to the
presence of "spots" or "gels" in a plastic sheet product
made from a blend of polyethylene with a ethylene-propylene
copolymer. Gel permeation chromatography of these defects
revealed that they consisted of a poorly dispersed high molecular weight fraction of the copolymer.
A novel laboratory process simulator was designed and built
to reproduce the material stress and temperature history
as it advanced through the extrusion operation from the
hopper to the sheet die. The primary dispersion mechanism
in the process was identified and a screw profile
modification dramatically reduced the frequency of "gels"
in the final product.