Residual stresses have already been studied by different authors: Postawa [7] studied the residual stress distribution in injection molding parts made of polystyrene (PS), using the monochromatic photoelasticity technique for different processing parameters for the qualitative estimation of internal stresses。 It was found that the most important variables were a low hold pressure and a high injection temperature。 On the other hand, Can Weng [8] studied the presence of residual tresses in micro-lens arrays using the birefringence method to evaluate the residual stresses and finite element simulations to estimate their distribution。 Comparable results from both experiments are correlated and the most important process parameter is the mold temperature。 These studies re- vealed that the molding process parameters are highly correlated with the origin of residual stresses and the most important variables are the melt temperature, Tf, mold temperature, Tw, packaging pressure, Pp, holding pressure, Ph, filling time, Ft, cooling time, tch, and velocity of the flow in cavity, Vi, or in some cases the tool design,   TD。

These residual stresses cause serious damage to PCLs when they are exposed to different environmental conditions。 Jacques [9]

investigated the susceptibility of glassy polymers to cracking when exposed to gasoline with different percentages of aromatic com- position, where polycarbonate PC, polymethyl methacrylate PMMA, and cellulose acetate butyrate CAB have different threshold reac- tions, influenced by geometrical factors and reduced critical strain; however the use of a specific test medium can be useful to determine the concentration of the residual stresses in PCLs。

The use of non-destructive methods is one of the most important techniques for actually determining the residual stress in trans- parent plastics and glass。 Yeager [10] analyzed specific areas of plastic parts to determine the residual stresses, using a compensator to find the retardation value for a PC involving the Brewster's constant and other physical properties of material for a commercial poly- carbonate, Makrolon。 Bayer Material Science [11] published studies about stress crack tests for commercial PC-Makrolon, which has been tested with different media regarding the use of the part subjected to mechanical  stresses。

The main objective of this paper is the analysis of the flow-induced residual caused by IMP。 The use of chemical attack and photoelasticity methods shows comparable results, which were correlated with a previous qualitative analysis by photoelasticity car- ried out to detect areas of PCLs subject to failure due to residual stresses。 Finally a stress-relieving technique was successfully applied, implementing a thermal treatment for prototypes samples, and after its validation it was applied in series production as quality con- trol in the plant。

Fig。 1。 3D solid model of PCL's injection molding tool cavity。

Fig。 2。 Plastic cover lens subject to analysis。

2。 Experimental methods and instruments

2。1。 Material used

Investigations of residual stresses were carried out on PCLs made of an amorphous commercial clear polycarbonate Makrolon AL 2447 crystal, with Tg = 144 °C, a physical property of polymers which plays an important role when we are talking about dimensional stability  and stress-relieving techniques。

Samples under analysis were produced from polycarbonate pellets by a multi-stage injection molding process (IMP)。 Since the PCL surface is very sensitive to media that cause cracks, the outer surface of the PCL is covered in a hard coating layer which enhances the resistance to critical environmental conditions like abrasion, chemical attack, and ultraviolet radiation。 On the other hand the appli- cation of a hard coating layer over the inner surface of the PCL reduces the surface tension, avoiding the creation of droplets where environmental conditions of high humidity  prevail。