Improving the throughput of plasticating extruders : manufacture of non-circular pellets, measurement of their interparticulate friction coefficient, and implications for screw design

This dissertation addresses the hypothesis that the interparticulate friction coefficient of a bed of polymeric particulates is a function of the cross-section of the pellets. Most pellets have a circular cross-section especially those that are strand cut. But when such pellets constitute a particulate bed there are few restrictions to adjacent pellets turning with respect to each other, or of the beds rupturing under shear load. This rupture can occur in plasticating extrusion, and it results in a poorly mixed melt with an inhomogeneous temperature distribution. The temperature fluctuations cause viscosity fluctuations which in turn cause pressure fluctuations. The pressure fluctuations cause throughput variations which lead to thickness variations in the product. That results in poor quality product. This sequence of events is termed surging. If the rupture of the solid bed can be prevented then surging can be curtailed. An increase in the interparticulate friction coefficient is expected to reduce solid bed rupturing. And the interparticulate friction coefficient can be increased if pellets with non-circular cross-section constitute it. Due to the fact that trilobal fibers are used to provide interlocking in the textile industry, the trilobal cross-section was first tested. This led to tests on bilobal and quadrilobal cross-sections as well. However, prior to testing the pellets required manufacture. Since bilobal, trilobal and quadrilobal cross-sections are different profile cross-sections, it was first necessary to design and build a series of profile extrusion dies. Each die was capable of producing a variety of pellet cross-sections if the material was changed, or if the throughput rate was varied. So many pellet geometries could be readily produced. Each variation of material and geometry was tested independently in a direct shear cell to measure the interparticulate friction coefficient. It was clearly found that the highest interparticulate friction coefficients occurred with pellets of bilobal cross-section, a 31% to 81% increase over pellets with circular cross-section. In addition pellets with trilobal and quadrilobal cross-section exhibited an improved interparticulate friction coefficient, a 20% to 34% increase compared to pellets with circular cross-sections. Consolidation pressure and time had minimal effects whereas agitation caused interparticulate friction coefficient increases from 51% to 89%. The effect of additives in the particulate continuum on the interparticulate friction coefficient was also studied. Since production runs could not be made to test if surging could in fact be reduced, due to the vast amounts of feedstock required, resort was made to numerical experimentation. An existing software package was modified to allow study of the transport of a portion of the solid bed. In this way rupture could be predicted based on the value of the interparticulate friction coefficient. Thus it could be inferred that surging would be reduced