It is always desirable to fix acceptance criteria for porosity at the beginning of a product life cycle. But most of the time it is difficult to implement, as it is not easy to estimate and fix the extent and locations of the defects. Here is one such case where the simulation results in correlation with the actual defects were used to fix the acceptance criteria with the customer. The part is a thermostat housing for an automobile application.
Once the bulk production started, the porosity levels were found to be very high, especially in two areas. First is the area were a 3mm depth groove is machined on an internal diameter surface. Second is the area where an angular solid boss is machined by drilling and tapping.
The 3mm groove area porosity was due to flow defect concentrations as revealed by the flow simulations (Figure 6a). It was also easy for the customer to see that this porosity will not affect the product requirement at it is only a recess for a circlip.
Figure 6a - Flow Simulation results showing defects concentration in the Groove machining area.
Figure 6b - Solidification Simulation results showing hot metal at the angular solid boss area.
The angular solid boss area porosity was due to shrinkage, as revealed by the solidification simulation (Figure 6b). It was also clear that this porosity will not result in any leakage or breakage.
In both the areas, the correlation of the results and the actual defects were spot on. Part design and die design options are ruled out and hence mutual acceptance levels has been arrived at and agreed upon with the customer.