Terahertz (THz) reflective imaging and scanning acoustic microscopy (SAM) were utilized to characterize the weld lines in an injection-molded thermoplastic electrical receptacle plate. V-notched weld lines are found in both THz and SAM results. Enhanced THz birefringence is observed close to the weld lines and drops moving away from them, indicating that the degree of anisotropy decreases and the state eventually becomes isotropic over a certain length scale. These results demonstrate that THz techniques are capable of quality control for injetion-molded thermoplastic components in a nondestructive fashion.