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In the continual drive toward better quality and long-term reliability of products, an examination of all factors that affect performance is necessary. For example, in plastic products such as injection-molded headlamp reflectors, the source and distribution of internal stresses plays a crucial role. Specifically, in polycarbonate reflectors, a detailed examination of internal stresses has resulted in considerably improved parts with a lower failure rate during manufacture and testing and hence a higher yield in production. Experimental analysis of a typical polycarbonate headlamp reflector is described throughout the course of this paper. The distribution of internal stresses is determined via a thermomechanical procedure developed for the purpose of this investigation. The same technique has also been used to identify one of the main non-thermal sources of such stress, viz. solvent-induced stresses during the coating of the reflector with MIBK (methylisobutylketone)-based coatings.