Thermomechanical Analysis for the Investigation Solvent Induced and Residual Stresses in an Injection Molded Automotive Headlamp Reflector

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. Systematic studies have established the role of the MIBK in creating internal microdamage within the material. These “damage” sites then act as crack initiation sites within the polymer. While the solvent has a damaging effect on the polymer under normal circumstances, its ejection from the matrix by heating to temperatures above its boiling point has catastrophic effects. Oftentimes the cure temperature for the coating is above the boiling point of the solvent as it is assumed that nonvolatilized solvent on the surface is minimal. An experimental technique has been developed and applied to a typical headlamp reflector in the determination of the residual stress/strain distribution in a molded part. This technique offers potential as a diagnostic technique for determining internal stresses in molded parts and predicting and resolving long-term reliability issues.