An undercut refers to part features that prevent straight ejection at the parting line. Undercuts cause mold complexity and therefore lead to higher mold construction and maintenance costs. If possible, it’s preferred to redesign the part to avoid undercuts. Shown in Figure 1, to form the rectangular opening to the left, a retractable core is required. However, by extending the opening all the way through the top surface, it can be formed by a static core moving in the draw direction of the tool.
Figure 1 - Gradual transition of wall thicknesses
Undercuts can't always be avoided. They can even be essential to give the part a certain functionality. There are several ways to allow for or specifically create an undercut.
When applying a bump-off feature, the part should only temporarily be deformed, and after ejection, be returned to its original shape. This becomes easier when the material used has an intrinsic flexibility.
DSM’s Arnitel (TPC) can be used as an alternative to conventional rubbers. This material is available in a broad range of hardnesses. Especially the soft Arnitel grades show excellent elasticity and allow for bump-off features.