During injection moulding, thermoplastic materials are heated up to the molten state and then forced into a mould cavity at high pressures. For complex shaped components it is often necessary to use multiple sprues or to let the melt run around obstacles, which leads to the formation of weld lines. If two streams coming from opposite sides meet in the mould, the molecules of the two melt streams have different orientations and thus suppress effective crosslinking between the two melt fronts. Apart from being visible to the naked eye, such weld lines do reduce the strength of the part locally. This effect is even stronger when short fibre reinforced plastics are used. The fibres are aligned parallel to the weld line, while far from the weld line they are aligned parallel to the flow direction of the melt. This causes ad-ditional weakening of the part. Tensile testing specimens with and without weld line were pro-duced having glass fibre content ranging from 30 % to 50 % mass fraction. To characterize weld lines in short glass fibre re-inforced polymer, various ultrasonic methods were employed. Contact coupled ultrasound with longitudinal waves in through thickness direction was used to find the stiffness increase in-duced by the fibres being oriented parallel to the weld line. Addi-tionally, shear waves in through thickness direction were used to quantify the angular anisotropy of shear stiffness in the weld line as compared to regions further apart from it. The velocity of guided waves excited by air-coupled ultrasound transducers was measured perpendicular to the weld line and in pristine material. Air-coupled ultrasound in transmission visualized the weld lines in c-scan images. Results obtained from ultrasonic measurements were correlated to tensile modulus and strength measured destructively to understand, which ultrasonic method is able to substitute destructive testing.