Mechanical characterization of frontal and flowing weld lines in injection-molded short fiber-reinforced thermoplastics

Occurrence of weld lines (WLs) in injection-molded thermoplastic parts is often unavoidable and induces a significant reduction in the mechanical properties. In this work we present a mechanical characterization of the WLs induced in injection-molded PBT (Polybutylene terephthalate) wt. 30 % glass fiber-reinforced which constitutes the experimental reference for validating the respective prediction models. We considered frontal and flowing WLs by using different insert shapes, two flow types (parallel and divergent) and two thicknesses. We found that the influence of the process parameters and the insert shape (in the case of flowing WLs) on the quasi-static tensile properties of the WL is relatively low. Also we found that frontal WLs induce a more pronounced reduction of the tensile strength in comparison with the flowing WLs. Based on rheological tests performed on the unreinforced PBT matrix, we determined that the healing ability of the matrix at the WL is high. On the other hand, we observed that the unfavorable induced fiber orientation (FO) along the flowing WL does not vanish with the distance from the insert. The last results suggest that the FO is the dominant factor of the WL strength reduction, while matrix inter-diffusion does not play a significant role for this particular material system.