Once the section of PCL to be analyzed has been detected, as illustrated in Fig。 3, samples were partially immersed in the test media in a room with a controlled temperature at 23 ± 2°C, controlling the exposure time in order to evaluate the presence of cracks and their evolution over time to obtain an exact correlation of the residual stress   magnitude。

Table 1

Test fluids used for destructive analysis by chemical attack。

Test medium Volume ratio Exposure time (minutes) Reaction threshold (MPa)

Ethyl Acetate and n-propanol 1:1 0 to 15 N 4 and 6 b

Propylene carbonate – 0 to 3 N 2 and 4 b

Fig。 7。 Determination of fringe orders during qualitative    analysis。

3。 Results and discussion

The stress cracking characteristic of polycarbonate was evaluated by measuring the minimum strain required to produce serious damage to PCLs, causing cracks, and those samples were exposed to propylene carbonate, ethyl acetate, and n-propanol, where a non- homogenous region of an isochromatic fringe pattern was identified on the PCLs by qualitative analysis, as shown in Figs。 6 and 7。 Residual stresses are found to be concentrated in the region where fluid follows a complex path in the mold cavity close to the boss, where a screw is located during the assembly process with other parts。 This can be explained by the section change, slider area, and geometrical characteristics of the injection molding  tool。

The chemical attack technique was used to determine the level of internal stress in the microstructural ordering of the polycarbon- ate, defining those variables involved in the phenomenon of cracking shown in Fig。 8, where the failure mode of cracking is a variable that is dependent on exposure time and residual stress level。 Based on that, nine points were defined (1 to 9) in Fig。 9 in order to de- termine the exact period of time when cracks occur at each point after applying propylene carbonate, ethyl acetate, and n-propanol。 The results of the chemical attack test were consistent with those identified by photoelasticity, as shown in Fig。   10。

In this work a design of experiments (DOE) was carried out in order to achieve the relaxation of internal residual stresses on PCLs by two different methods of heat treatment。 The first method, characterized by batch force hot air (BFHA), carried out and validated in the laboratory。 The second method by conveyorized force hot air (CFHA), implemented during series production in the plant once the factors and levels of DOE used by BFHA have been determined。

BFHA consisted of placing PCLs on racks inside an oven in which forced hot air was circulating with the aim to achieve the relax- ation of the internal microstructure of PCLs。 The start-up of heat treatment by BFHA was conducted according to the DOE 32, shown in Table 2。 A Memmert brand oven, model 800 D06064, was used in the experiments。 Temperature values for DOE were defined based on the Tg = 144 °C for this commercial PC, and periods of time for the annealing process were established according to values found in the literature。

Fig。 8。 Sample subject to chemical attack by propylene carbonates which cause stress-cracking。

Fig。 9。 Residual stress magnitude determined as a function of time using chemical attack which causes crack。

After the completion of experiments for BFHA, four important aspects were evaluated to take the decision about the optimum combination of factors and levels for this  DOE。