Description and modeling of fiber orientation in injection molding of fiber reinforced thermoplastics

As mechanical properties of short fiber reinforced thermoplastic injected components depend on flow induced fiber orientation, there is considerable interest in validating and improving models which link the flow field and fiber orientations to mechanical properties. The present paper concerns firstly the observation and quantification of fiber orientation in a rectangular plaque with adjustable thickness and molded with 30 and 50 wt% short fiber reinforced polyarylamide. An automated 2D optical technique has been used to determine fiber orientations. A classical skin (with orientation parallel to the flow)-core (with orientation perpendicular) structure is observed for thick plaques (thickness greater than 3 mm) but the core region is fragmentary for thickness less than 1.7 mm. It is shown that the gate design and different levels of fiber interactions, due to different fiber concentrations, are responsible for these observations. Secondly, computer simulations of flow and fiber orientation are shown. The agreement with the actual data is good, except in the case of the core for thin plaques. The limitations that have to be resolved come not only from the standard fiber orientation equations, but also from the flow kinematics computation.