Relaxation of residual stresses in plastic cover lenses with applications in the injection molding process

The quality of molded parts made of clear thermoplastic polymers is determined by the absence or presence of residual stresses. In this work we analyzed the flow-induced residual stresses, which cause serious damage to plastic parts used in automotive and optical applications. Plastic cover lenses (PCLs) are important components of LED and bulb headlights that are produced by the injection molding process (IMP). However due to multiple characteristics of PCLs as, complex geometry, size and wall thickness residual stresses are difficult to keep under control during the IMP, causing cracking under chemical attack, ultraviolet radiation, abrasion and other environmental conditions. These residual stresses in PCLs were characterized by a chemical attack and photoelasticity method in the present work, and the methods were compared and validated. A stress-relieving technique was then successfully applied, implementing a thermal treatment for prototypes, and after its validation in the laboratory it was applied in a series production. These experiments were based on a design of experiments (DOE) for the annealing process considering the glass transition temperature (Tg) of the polycarbonate and allowing the relaxation of the internal microstructure of PCLs without causing degradation, helping us to increase the production efficiency and improve the component performance in the plant.