Undercut (side actions)

A simple two-part mold opens in one direction. Features that would be trapped (a hole in a side wall, a snap-fit hook, a groove, an inward lip) are undercuts. They can be produced with side actions (slides), lifters, collapsible cores or hand-loaded inserts, each of which adds tooling cost, maintenance, cycle time and sometimes visible witness lines.

Many undercuts can be designed out: forming a snap hook through a window in the opposite wall (a pass-through shutoff), moving a hole so it lines up with the mold opening direction, or using shallow flexible features that can strip off the core.