Weld line strength has been measured in five engineering thermoplastics. The effect of processing parameters was studied using experimental design. Four parameters-holding pressure, injection velocity, melt temperature, and mold temperature-were varied in two levels. For each parameter setting the weld line strength was measured and compared with the bulk strength via a weld line factor (WL-factor), defined as : strength of specimens with weld line/strength of specimens without weld line. Results from flexural tests on both filled and unfilled materials showed that the weld line factors for cold weld lines varied between 0.25 and 0.98 depending on material and parameter setting. Highest WL-factors were obtained for unfilled materials and using high melt temperature, high holding pressure, and low mold temperature. Results from instrumented falling weight impact tests on weld lines sometimes showed a WL-factor higher than 1.0. There was generally a poor correlation between results from impact tests and results from flexural tests.