Case Study 1103: Cause and Effect

This case is also a continuation of the previous two, in terms of cause and effect correlation and acceptance criteria.

Porosity was always an issue since the beginning of the development of this part, as it is needed to be pressure tight. But the intricacy of the part and the porosity requirements coupled with the design issues discussed in the previous two cases were already causing heavy rejections.

One day during the course of the above improvements , we received a SOS quality alert from the customer, claim­ing that there was a sudden spurt of rejection levels to more than 65% due porosity from the earlier level of about 20% in the previous batch.

The process audit found the surge of defects was attributed to a particular cored hole in the casting that was too deep in a particular depth and in a particular direction. The die history revealed that there was a request from the customer to change the cored hole from the existing 4.5 Dia to 3.5 Dia, to solve a position issue in the machining of the part.

Figure 3a - Correlation of Part model (left) and simulation result (right).

As we suspected, the issue may be pertaining to shrink­age porosity in the hole. A solidification simulation was carried out.

The simulation categorically revealed that there is a potential shrinkage porosity issue at the location (Figure 3a) and addition of machining stock of 1 mm over the diameter is not at all recommended.

Even though the customer refused to buy our idea ini­tially, they were forced to revert back to the original core diameter of 3.5 mm upon seeing the simulation results. The batch produced immediately after the core pin change had a porosity level of around 15% which clearly vindicated our stand and resolved the issue and cleared our name.

This has also resulted in the customer having a more pragmatic look at acceptance levels and relaxed the accep­tance criteria in terms of porosity without compromising on other requirements, including the leak. And the new level of acceptance reduced the defects to less than 5%, making the part more viable and interesting to produce.

Figure 3b - Suggested Part Design Improvements.

To mitigate the porosity levels and flow related issues CRP has suggested more part design improvements (Figure 3b) focusing on further radius improvements at specific areas and avoiding uneven wall thicknesses.

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