Large-scale X-ray microcomputed tomography (mu-CT) is used to investigate microstructural features of weld lines in a short glass fiber reinforced polymer. The main originality of this work is related to the evaluation of local structural and mechanical characteristics in zones of unmastered heterogeneities. Complete and incomplete injection molded plates are considered to investigate weld lines with and without process-induced porosity. Using a helical scanning trajectory, the full 3D microstructure is obtained at centimetric scale to cover a large zone including first contact between colliding fronts at a voxel size of 3 mu m. Microstructure analyses show that weld line area is ill-ordered at the fiber and structure length scales. Near the first contact point, fiber orientations show the signature of vortexes, which mark the presence of repulsive forces. The presence of micrometric porosity is only confirmed in incomplete plates. mu-CT images are used as inputs to create a full-scale finite element model for assessing strain localization. Predicted principal strains are compared to digital image correlation measurements performed during uniaxial tensile tests. Full-scale modeling combined to 3D high-resolution imaging proves high potential to correlate local fiber heterogeneities with strain localization effects.