Modeling Study Of Transition From Solid Phase To Solid-melt In The Process Of Vane Extrusion

Most of the modern polymer processing machines are based on screws, such assingle-screw extruders, twin-screw extruders and so on. In screw extruders, the solid polymerare conveyed and compressed by a drag-induced mechanism, then melted by a drag-inducedmelt removal mechanism[1], which depends on the material properties and needs a longthermo-mechanical history. To overcome these disadvantages, Qu designed novel screwlessmachines known as vane extruders, which were composed of a number of vane plasticizingand conveying units (VPCUs). Vane extruders operate under a positive displacementconveying mechanism and exhibit very narrow residence time distributions, which can largelyshorten the thermo-mechanical history and improve the suitability of materials[10].Melting is a very important elementary step in polymer processing. In plasticizingextruders, the melting and plasticizing process often consumes maximum energy andfrequently determines the production capacity and quality. This article examined the meltingphenomena taking place in the vane extrusion by cooling experiments and visualizationexperiments and developed two melting models.With the vane extruder and the visualization VPCU device, the melting phenomenon ofHDPE particles under different operational conditions was investigated. It is shown that thepositive displacement conveying mechanism and mandatory deformation of particles madethe melting evolution in vane extruders totally different from that in single-screw extruders.The length of solid conveying and melting zone in vane extruders was much shorter andisland-sea melting model was formed instead of Tadmor’s melting model. Plastic energydissipation (PED), friction energy dissipation (FED) and viscous energy dissipation (VED)were very important sources of heat energy for polymer particles melting in vane extruders.Eccentricity of VPCU, which led to the compression and deformation of particles, was adecisive factor in polymer melting in vane extrusion. Due to the eccentricity, mechanicalenergy was dissipated into heat via the deformation of HDPE particles and thethermo-mechanical history was shortened remarkably. Thermal conduction is the dominantsource of energy at low rotating speeds, while the effect of thermal conduction is not obviousat higher rotating speeds.Based on the melting phenomena, solid-bed melting model and island-sea melting modelwere developed and verified. It was shown that the island-sea melting model was animportant mechanism to realize high efficiency melting progression. The melting length ofisland-sea model with PED was consistent with that of the cooling experiments.