Undercut

Why pull direction matters#

An undercut is defined relative to a direction. A hook may release cleanly when the mold opens along one axis and trap the tool when the direction is reversed. Autodesk Fusion Design Advice asks for a pull direction, then reports regions that would interfere with ejection along that direction.

Insufficient draft and an undercut are related but different. More draft can help a nearly parallel wall release. It cannot make mold steel pass through a hook, reverse-facing recess, or other mechanically trapped feature.

Common dispositions#

The simplest response is to remove, relocate, or reorient the feature so it clears the main pull. When the function requires the geometry, the tool may need a side action, lifter, pickout, sliding shutoff, bumpoff, or collapsible core.

Protolabs lists side actions, pickouts, sliding shutoffs, and design changes among the ways an injection-molded undercut can be handled. Each option has limits tied to material, geometry, production volume, maintenance, and cost, so the molder should review the decision before tooling release.

What analysis does not prove#

A CAD accessibility or draft check can identify geometry hidden from one direction. It does not select a reliable mold mechanism, prove that a flexible part can strip over a feature, or show that a shutoff will resist wear and flash in production.

Record the accepted undercut, its planned action, and the relevant parting line with the released revision. A red region dismissed without that decision is not resolved; it is merely hidden.