Fiber orientation in injection-molded composites: A comparison of theory and experiment
Bay, Randy Scott
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Permalink
https://hdl.handle.net/2142/22217
Description
Title
Fiber orientation in injection-molded composites: A comparison of theory and experiment
Author(s)
Bay, Randy Scott
Issue Date
1991
Doctoral Committee Chair(s)
Tucker, Charles L., III
Department of Study
Mechanical Science and Engineering
Discipline
Mechanical Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Mechanical
Engineering, Materials Science
Plastics Technology
Language
eng
Abstract
Experimental measurements of fiber orientation are reported for two parts injection molded from nylon 6/6 reinforced with 43 weight percent of glass fibers. The parts are a center-gated disk and a film-gated strip. Orientation is measured from polished cross-sections and reported as a function of position, both across the thickness and in the flow direction. Both parts have a layered structure, with outer shell layers of flow-aligned fibers surrounding a central core of either random-in-plane (strip) or transversely aligned fibers (disk). The experiments are compared to predictions of a finite difference simulation of the mold filling problem. The simulation predicts the presence, nature and location of the layers very well. However, it overpredicts the small out-of-plane fiber orientation and places the core-shell transition too close to the midplane. A comparison with selected experimental results suggests that the major source of error is the closure approximation used by the fiber orientation equation. The simulation is exercised for a variety of cases to show the importance of material and process parameters. Injection time is an important parameter, but injection temperature and mold temperature have little effect on fiber orientation. A method is presented to predict the thermo-mechanical properties of injection molded composites. The property predictions suggest that current accuracy of orientation predictions in injection molding is not adequate to accurately predict these properties. To improve the accuracy of the orientation predictions a new closure approximation is developed. When used to predict orientation in injection moldings, this new closure approximation predicts correctly the small out-of-plane fiber orientation and places the core-shell transition further from the midplane, but the core layer is still predicted to be thinner than in the experiments. It appears that the transient behavior of the theory is limiting how accurately the location of the transition is predicted. Experimental results from the shell region of injection molded strips are compared with distribution function results; the comparison demonstrates clearly that the interaction coefficient decreases as the volume fraction of the fibers increases. An empirical model based on these results relates the interaction coefficient to the fiber concentration and aspect ratio.
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