Characterization of short glass fiber filled polystyrene by fiber orientation and mechanical properties

A quasi‐steady state, non‐isothermal, compressible, inelastic, and creeping flow of polymer melt into a thin cavity is analyzed to predict fiber orientation states. Modified Cross model and Tait's state equation are adopted to consider shear‐thinning behavior and compressibility of the polymer melt. Second order tensors are introduced to describe 3‐dimensional fiber orientation. Flow‐induced fiber orientation can be predicted by solving the equations of change for the orientation tensor with a suitable closure approximation. The orthotropic closure is applied except for the case of low interaction coefficient. Fiber orientation develops mainly due to shear flow in the skin layer and due to stretching effect in the core layer. It turns out that the compressibility, which induces additional velocity gradients during packing, reduces development of the fiber orientation. Results are dependent upon the magnitude of the interaction coefficient. The larger the interaction coefficient, the smaller the orientation development and the effect of compressibility. To predict orientation dependent mechanical properties, the orientation averaging for an arbitrary orientation is carried out from the properties of a transversely isotropic unit cell. The compressibility reduces the axial modulus and increases the transverse modulus. Opposite trends are observed for thermal expansion coefficients. It is also observed that the consideration of compressibility reduces the overall anisotropy of the molded product. Effects of compressibility on mechanical properties of the parts are reduced as the interaction coefficient becomes larger.