In this work, a theoretical approach based on diffusion and the Flory-Huggins theory of the free energy of mixing is considered to predict the weld-line strength in injection-moulded polycarbonate (PC) and its blend with high-density polyethylene (PE) as a function of melt temperature and contact time. A comparison between the model predictions and experimental results was also performed for pure PC as well as for its blend. Different melt temperatures ranging from 220 to 300 degrees C and a constant total cooling time of 15s were used. Two types of specimen were produced, one with a dual-gate mould cavity and another with a single gate. In the case of PC, the model predicts satisfactorily the weld-line strength as a function of injection temperature. High-density PE was also moulded in the same conditions but in a lower temperature range (145 to 220 degrees C). The results showed that the tensile strength of PE remains unchanged in the presence of weld lines. For the PC/PE blend, the results showed a good agreement between the model predictions and the experimental results. It was also possible to predict the weld-line strength of the blend as a function of its composition and temperature.