Effect of process parameters and die design on the orientation of reinforcement fibers in extruded polymer composite
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Date
2017
Authors
Khorsand, Amirreza
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Abstract
Three micro-structural parameters that influence the mechanical properties of discontinuous fiber thermoplastic composites are Fiber Orientation Distribution (FOD), Fiber Length Distribution (FLD) and fiber volume fraction (Vf). During manufacturing FOD in a composite would change due to fiber rotation caused by shear flow, extensional flow or a combination of both. Previous studies on flow-induced orientation is limited only to injection molding, capillary and parallel plate rheometers. Knowledge on the combined effect of die design and process variables on fiber orientation during extrusion of composites using a twin screw extruder is currently not available Developing this knowledge is the goal of this thesis. HDPE (High Density Polyethylene)fibers and HDPE composite fibers consisting of the each of the following reinforcements (micron-sized glass fibers (aspect ratio of 27), glass spheres (aspect ratio of 1), and carbon nano tubes (CNT) were extruded by varying die design (9 dies),screw speed, and reinforcement volume fraction in the feed. The FOD) and Vf in the composite melt, determined using X-ray tomography of the frozen samples, was found to vary along the extrusion line as well as with the variables identified above. Hence, the viscosity of the melt, determined using Cogswell approach and verified independently using rheometer measurements and modeled using modified Ausias’ model, also varied. The change in FOD in the melt within the die due to imposed extensional and shear strains was successfully modeled using Dihn-Armstrong model. The change in the viscosity of the melt within the die due to changes in FOD together with pressure gradient across the die, which develops concurrently, determined the mass flow rate (i.e volumetric flow rate) of composite melt out of the die. This demand on mass flow rate imposed by the die was met by the extruder by varying the amount of melt coming out of the reservoir (at the end of the screws).Simulation suggests that shear strain results in better orientation than extensional strain.. The die with least cone angle (30o) resulted in maximum fiber orientation and output mass flow rate at least die pressure, when compared to other dies for the same feed rate and screw speed.
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Keywords
Fiber Orientation Distribution (FOD), Fiber Length Distribution (FLD), fiber volume fraction (Vf), twin screw extruder