Process-induced skewness of flow fronts and fiber orientations in LFT-D compression molding considering processing, characterization, and simulation

Mechanical properties of long fiber-reinforced thermoplastic (LFT) materials are defined by the fiber microstructure, including fiber orientation (FO). Compression molding of directly compounded LFT-D semi-finished materials results in pronounced anisotropy caused by fiber orientation mechanisms during material flow, with fibers aligning in flow direction. However, various authors noticed a deviation of the FO from the anticipated flow direction, which affected the mechanical properties and possibly blurred experimental conclusions. This study investigates possible reasons for this phenomenon, considering the process chain from plastificate and molding to final FO. For this purpose, we conduct a comprehensive mold-filling study comprising short shots and plates for mechanical characterization. A method is presented to characterize the skewness of the flow front of short shots. In addition, a method for deriving FO from tensile discs is applied. Both results are compared to a state-of-the-art simulation in which results from the characterization of the semi-finished LFT material, the plastificate, are considered. The results indicate that the density distribution of the LFT-D plastificate is not homogeneous; the influence of extrusion time causes a density gradient of around 10 %. This can be traced across the mold filling, where the flow fronts as well as the FOs are skewed towards the most recently extruded portion of the LFT, as well as to the FO. An FO deviation from the flow direction between 10° and 15° is found.