Dear experts and friends! Help me figure out what's going wrong here! It's dented in the middle.

How to Fix Sinking Defects in Large Flat Investment Castings: Shell-Building Process & Delamination Analysis

This is a question raised by a friend in a technical exchange group on TikTok. Below are several photos he posted. From the images provided by this friend, the issue is not very clear—only slight clues can be observed. His problem is that the casting has sunken in. Someone in the group asked whether there was an issue with the wax pattern, and whether reverse deformation allowance was added in that area during mold design. It was suggested to first investigate the mold’s wax pattern.

The friend also posted a picture of the wax pattern. The wax pattern showed no deformation. Another friend mentioned that they had encountered a similar issue, which turned out to be caused by the shell-building supervisor failing to control humidity. The problem disappeared when the second layer was applied five hours earlier, especially for flat components. In another instance, a large flange had a depression, which was traced back to the mold shell being moved before it cooled after pouring. Leaving it untouched for half an hour resolved the issue entirely.

In fact, this is a common problem in investment casting, specifically related to shell-building for large flat castings. It can be complex yet simple to explain. First and foremost, it is crucial to acknowledge the challenges of large flat castings and design the shell-building process differently from other castings. This is very important. What we often see is that the shell-building processes for most castings are largely similar, with only differences in the number of layers applied.

Actually, this is a delamination defect—the separation of the face coat from the transition layer. A possible reason is excessive drying of the face coat, as mentioned by the friend above. It is essential to monitor the humidity of the face coat and the drying time. Of course, for large flat castings, cleaning the wax pattern is equally important. Considering these factors, the aforementioned issues can generally be well-controlled.