The Undercut Problem

An undercut is any feature on a part that prevents it from being pulled straight off the mold. Imagine trying to pull a solid ring off an hourglass—the wider top prevents the ring from sliding up. In standard vacuum forming, undercuts are impossible to demold.

Solution: Pneumatic Sliding Cores

To achieve undercuts (such as mounting tabs or snap-fit lips that curve inward), we design complex, multi-part aluminum molds featuring pneumatic sliding cores.

During the forming process, a pneumatic cylinder pushes a side-piece of the mold outward to create the undercut geometry. The plastic is formed over it. Once the part cools, the cylinder retracts the side-piece inward, freeing the undercut and allowing the part to be ejected vertically.

Cost vs. Design Intent

Articulated molds are significantly more expensive to manufacture than static molds. At M/S RV Chumble, our engineering team works with clients to analyze whether an undercut is strictly necessary. Often, we can redesign the part to eliminate the undercut entirely, saving thousands in tooling costs while maintaining the original design intent.