The die casting process is generally associated with dimensionally critical parts and focuses on the specifications. The aesthetical requirements are not exclusively defined and monitored. This results in unpredictable effect on the product quality and acceptance when it comes to the parts associated with good outward appearance, look and appeal.
This case study is an example of one such die for a part a light dome of a luminary application. This part must be absolutely free from surface defects, die erosion, etc.
Figure 11 shows the comparison of a good and a bad part. The heat checks starts appearing in about 5,000 shots, and the die surface deteriorates very fast, resulting in lots of effort to finish the part by polishing, relishing etc. The die also needs a special attention every 2,000 shots, resulting in production stoppages.
Several measures are taken to overcome this issue by change of:
Figure 11 - Comparison of good (left) and bad (right) parts.
And, the problem still persisted. So, it was decided to study the process more closely to find out the root cause of the problem. As one of the measures, die temperature was monitored using an infrared thermometer. The thermal profile indicated a hot spot at the location which was prone to frequent failure.
Analysis of Results
A thermal die cycling simulation analysis was done to ascertain the reason for the hot spot. The results did not indicate any heat build-up due to continuous thermal cycling of the die even without cooling channels, effectively ruling out this as a cause of failure (Figure 12a).
While analyzing the velocity profile, the author tumbled upon the root cause of the failure. It was observed that the entire volume of the shot flows only through the failure area, leaving all the heat at that location. This explains the reason for the hot spot and failure (Figure 12b).
The heavy die erosion opposite to the gate area was also a worrying factor of this design. Hence, alternate shot designs were developed and flow analyzed to choose the optimum design.
Figure 12 - a) Thermal die cycling did not indicate any hot spot at the heat check area; b) velocity profile indicates complete volume of shot flowing through the heat check area.