a) visual damage to PCL

b) proper assembly process of PCL with other parts

c) dimensional stability of PCL

d) relieving of residual stresses in PCL

The results of DOE are shown in Table 2, where the combination of factors and levels which achieve the relaxation of residual stresses was the implementation of heat treatment for 20 min at 120  °C。

Once the magnitude of temperature and time of the annealing process had been validated by BFHA, the next step consisted in applying the heat treatment by the conveyorized force hot air (CFHA) technique using the combination of factor and level (20 min at 120 °C) to thousands of parts in series production。 Based on the complexity of CFHA process, was necessary to carry out the formu- lation of the hypothesis testing, null and alternative for parts produced in series in two lots, one of them without applying any heat treatment and the other one using the combination of factor and level of the DOE to confirm the effectiveness of heat treatment by CFHA。

Hnull: There is no significant difference between parts subjected to CFHA and parts not subjected to any heat treatment。 Halternative: There is a significant difference between parts subjected to CFHA and parts not subjected to any heat treatment。

Based on the P-value for this hypothesis test, whose results are shown in Table 3, the alternative hypothesis is valid, and it is con- firmed that the implementation of heat treatment to PCLs in series production reduces the scrap level and improves the production efficiency of the plant by achieving the relaxation of residual stress in PCLs。

Fig。 10。 Comparison of results obtained by photoelasticity and chemical attack experiments。

Table 2

DOE of annealing process。

Experiment Bloque Factors Decision matrix

Time, t (minutes) Temperature, T (°C) PCL visual damage PCL proper assembly PCL dimension stability PCL relieving of

residual stresses

1 1 12。5 100 o o o x

2 1 20 120 o o o o

3 1 12。5 110 o o o x

4 1 20 100 o o o x

5 1 5