The aim of this work is an extensive experimental mechanical and microstructural characterization of flowing weld lines (WLs) in injection-molded short glass fiber-reinforced polybutylenterephthalate (PBT) using X-ray computed tomography and digital image correlation (DIC). It was found that the induced fiber orientation (FO) in a flowing WL is similar to that induced by flow along a wall. In this test case, the impact of the flowing WL on the FO did not vanish after a flow length of 70 mm. The shape of the inserts, which originated the flowing WLs, only had a marginal effect on the induced FO gradient. By reducing part thickness, the erasing of the FO gradient induced by the WL is reached at shorter flow distances. At the WLs, there is a reduction of the fiber volume fraction in comparison to the regions far from the WL plane. DIC results show a pronounced strain localization at the WL, which can be explained by the FO gradient induced by the WL.

Different factors contribute to the weakness of weld lines (WLs) induced by injection molding such as unsuitable fiber orientation (FO), incomplete polymer matrix diffusion, voids and V‐notches. This study aims to characterize the contribution of each factor on the weakness of frontal WLs in a short glass fiber‐reinforced polybutylene‐terephthalate characterized by extensive X‐ray computed tomography and mechanical tensile testing assisted with digital image correlation. A reduction of 50% of the stress at break and almost 40% of the strain at break is observed despite the complete matrix healing at the WL interface and the absence of V‐notches. Frontal WLs induce a FO gradient starting 2 to 3 mm before the WL plane. The fibers in the WL region mainly orient in transverse‐to‐flow and thickness direction. This FO gradient localizes the deformations, which leads to failure at a strength near to the one of the unreinforced variant. Voids formation in frontal WLs seems to be driven by large gradients of FO and subsequent anisotropic shrinkage. In addition, this shrinkage behavior at the WL causes an increase of thickness. By applying higher packing pressures, the fibers orient more in flow direction at the core of the WL, leading to a higher tensile strength and a lower content of voids. Finally, we can conclude that the FO is the dominant factor controlling the mechanical performance in frontal WLs.