What Is an Undercut?

An undercut is any feature on a molded part that prevents it from being pulled straight out of the mold in the direction the mold opens, such as a side hole, snap-fit hook, internal groove or thread. Undercuts need extra mold mechanisms—usually side actions or lifters—or a design change so the part can release.

What Does Undercut Mean in Injection Molding?

A basic two-plate mold opens in one straight line, the line of draw. Every surface of the part must be able to slide away from the steel along that line. A feature that sits "under" another surface when viewed from the draw direction, such as a hole through a side wall or a latch that hooks sideways, would be trapped by the mold and torn off or jammed on ejection.

Undercuts are not a design mistake in themselves. Snap fits, side ports, threads and clips are often exactly what a part needs. The question is how the mold will release them.

How Are Undercuts Molded?

There are five common ways to mold undercuts, each with different tooling impacts:

• Redesign with a shut-off: Reshape the feature so the core and cavity meet through it, often with a slot or opening in the opposite wall. Best for simple latches and snap hooks; no added tooling cost.
• Side action (slide): A core pulls sideways out of the feature before the part is ejected, driven by angled cam pins or its own actuator. Best for holes and features on outside walls; adds a moving component and cost.
• Lifter: An angled core moves with the ejector system, sliding sideways off the feature as the part is pushed out. Best for internal clips and undercuts inside a wall; adds cost and needs room behind the feature.
• Hand-loaded insert: A loose insert is molded in, removed with the part and pulled out by hand. Best for prototype and low-volume tools; low tool cost but slower cycle.
• Stripping (bump-off): The part flexes over a shallow undercut during ejection. Only works with forgiving materials and small depths.

How to Design Around Undercuts

• Check every feature against the draw direction early; many undercuts disappear if the parting line or pull direction is chosen differently.
• Ask whether the feature can face the draw direction.
• Group side features on the same face so one side action can form several of them.
• Allow room for side actions and lifters, including a clear travel path.
• Keep mechanisms only for features that earn them; each side action adds cost, maintenance and sometimes cycle time.

Undercuts are one of the items a DFM review is designed to catch before tooling starts.