Abstract
Variances in polymers processed by single-screw extrusion are investigated. While vortical flows are well known in the fluids community and fountain flows are well known to be caused by the frozen layers in injection molding, our empirical evidence and process modeling suggests the presence of vortical fountain flows in the melt channels of plasticating screws adjacent to a slower-moving solids bed. The empirical evidence includes screw freezing experiments with cross-sections of processed high-impact polystyrene (HIPS) blended with varying colorants. Non-isothermal, non-Newtonian process simulations indicate that the underlying causality is increased flow conductance in the melt pool caused by higher temperatures and shear rates in the recirculating melt pool. The results indicate the development of persistent, coiled sheet morphologies in both general purpose and barrier screw designs. The behavior differs significantly from prior melting and plastication models with the net effect of broader residence time distributions. The process models guide potential strategies for the remediation of the processing variances as well as potential opportunities to achieve improved dispersion as well as complex micro and nanostructures in polymer processing.
Highlights
The global plastics industry exceeds one billion kg in production output per day, with practically all of this material processed by one or more plasticating screws
Maddock [1] performed screw freezing experiments with single-screw extruders to investigate the melting mechanism of polymer feedstocks, observing that the polymer melt first develops as a film on the surface of the barrel; this behavior was modeled by Tadmor et al [2,3]
As depicted in the channel cross-section of Figure 1a, the melt film first develops at the end of the feed section of the screw where the significant compression of the polymer provides for improved heat conduction between the polymer and the barrel
Summary
The global plastics industry exceeds one billion kg in production output per day, with practically all of this material processed by one or more plasticating screws. Our research is guided by modeling and simulation, which support the understanding of the interaction between the machine design, material properties, and processing condition. Maddock [1] performed screw freezing experiments with single-screw extruders to investigate the melting mechanism of polymer feedstocks, observing that the polymer melt first develops as a film on the surface of the barrel; this behavior was modeled by Tadmor et al [2,3]. The polymer film is wiped from the barrel by the active flank that is pushing the polymer feedstock down the length of the screw. As the flank flight wipes off the polymer melt film, a melt pool develops adjacent to the active flank. The melt pool is typically the full thickness of the channel adjacent the active flank, with some recirculatory flow.
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