Introduction 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 in- jection molding process (IMP)。 However due to multiple characteristics of PCLs as, complex geom- etry, 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 in- ternal microstructure of PCLs without causing degradation, helping us to increase the production efficiency and improve the component performance in the plant。73901

Nowadays plastic cover lenses (PCLs) are widely used in headlights due to their advantages in terms of physical properties and performance in comparison with glass panes。 The lower cost of production, the influence on fuel consumption, and in consequence the CO2 emissions by automobiles are among the most important considerations in replacing the use of glass panes with polycarbon- ate PCLs。 PCLs are manufactured by an injection molding process (IMP), which is a high-speed automated process that is used to pro- duce plastic parts with very complex geometries at relatively low cycle times。 This process involves a series of sequential process steps: mold filling, packaging, holding, cooling, and part ejection [1]。 In this complex process, a real challenge is keeping the quality of the raw material's properties when it has undergone the physical change from solid to melted material and finally transformed into a stable PCL。 A major problem frequently encountered occurs during surface finish process of PCLs。 This process consisted in the application of a hard coating layer on inner and outer lens surface, a process which is necessary to protect the PCL's surface from media that cause degradation of polycarbonate properties due to ultraviolet radiation (UV), abrasion, and different environmen- tal conditions, and this  finish is  decisive because it  provides  an effective  diffusion  barrier against these physical   phenomenon;

however in some cases the hard coating layer application reveals where the residual stresses are located in the microstructural order- ing of PCL and causes stress-cracking。

Residual stress is a major problem in this process。 It is clear that a right understanding of residual stresses is essential to predict the dimensional and shape stability of a product under different environmental conditions [2]。 The origin of the residual stresses is asso- ciated with two sources。 Firstly, the normal stress in the molding process is originated by the viscoelastic nature of the polymeric melt that develops during the filling, packing, and holding states。 This normal force is relatively small but is important for large molecular orientations that occur in the PCL injection process。 Secondly, thermally induced stresses are originated by non-uniform cooling in the molding part。 This last phenomenon takes place due to a rapid increase in the rigidity of the material as it passes through the glass transition temperature, Tg; as a consequence, a highly non-uniform temperature distribution exists across the product wall [2–5]。 In- vestigations by Zhang et al。 [6] predicted thermally induced residual stresses that form during the injection molding using an approx- imate model and found how the residual stress in an IMP is affected by process conditions and material properties based on a model of solidification  (mold–solid-layer–fluid)。