| A common practice in injection molding is to reinforce the polymer matrix with short fibers, the purpose being to improve the mechanical properties of the material. Properties affected by the presence of the fibers include the elastic modulus, the tensile strength, and the thermal and electrical conductivities. A typical short- fiber composite consists of roughly 10,000 fibers per cubic millimeter, oriented in various directions.; Existing models for predicting fiber orientation are usually incorporated into numerical simulations that are based upon the Hele-Shaw approximation. In molded features where the thickness changes suddenly, there exist significant in-plane deformation rates and out-of-plane velocity components, which are neglected by the Hele-Shaw approximation. The incorrect calculation of the flow field in these regions has an adverse effect on the fiber orientation prediction. Because these features are typically locations of high stress in the solidified composite, an accurate prediction of the fiber orientation is needed.; In this study, numerical methods for predicting fiber orientation in arbitrary three-dimensional geometries were developed. The formulation includes non-isothermal and non-Newtonian effects. The equations were discretized using the finite element method and solved using the Newton-Raphson method. Mold filling was treated using the volume of fluid method. Fiber orientation predictions are shown for several different geometries, including two inherently three-dimensional features, and compared to experimental data. Results show that the fiber orientation is accurately predicted by the model. These are the first predictions of fiber orientation in three-dimensional molded features, and the first comparison of predictions to experiments, that have been reported.; The effects of coupling the rheology and fiber orientation for a fiber suspension through a orientation-dependent viscosity were also examined. Calculations were performed for axisymmetric contraction flow, axisymmetric expansion flow, and radially diverging flow in a thin cavity. For the axisymmetric contraction and expansion, the flow patterns for the fiber suspension and for the suspending fluid alone were distinctly different. Results from numerical simulation of the contraction were found to quantitatively agree with available experimental data. For the radially diverging flow, the effects of coupling the rheology and fiber orientation were negligible. |
