The content for this article was contributed by Mark Littler of Littler Diecast Corporation.
Littler Diecast Corporation, a producer of high pressure die castings, was recently able to redesign and die cast an electrical switch frame for an aerospace application. Formerly produced by a different manufacturer, there were defect problems in a high number of the castings and a new design was needed to achieve a lower scrap rate. Littler Diecast was able to demonstrate that they could pinpoint the defects through simulation without previous knowledge of the problems. This impressed the client enough to land them the job.
Identifying the Problem
The switch is cast from A380 aluminum and is approximately 1 ¼” x 1” x 1/2” in size. Littler Diecast found that porosity problems were plaguing the part in two locations: the plate and the chimney. This was confirmed by the customer. Holes were forming in each of the locations because of the way the part filled. The flow would enter through a single gate as shown in Figure 1, jet to the far side of the plate and then backfill, trapping air pockets that do not always close due to early solidification. The same problem was found in the chimney: fluid would jet to its furthest extent and then backfill, creating trapped air that could not vent through the parting line.
The Original Part Design
There were other problems with the original design of the part. There was a lot of die erosion around the slot for the lock washer and the sealing surfaces on the bottom of the plate. The overflows located at the corners of the part were not large enough to allow defects to flow out.
Using FLOW-3D, Littler Diecast was able to analyze the flow behavior and visually determine what was occurring. With such a small part, early solidification is a problem due to the rapid cooling in thin sections. If flow jets across the part and back, the fluid has more time to cool and create entrapped air. It is best to have the hottest liquid coming in last. With this in mind, Littler Diecast was able to test a number of ideas and achieved a design that minimized the potential for problems and maximized the process window.