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.
In short: if a feature would hook on the steel as the mold opens, it is an undercut, and it will cost either tooling complexity or a redesign.
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.
The word has other meanings too, in welding, machining and even hairdressing. In molding it always refers to this one problem: a shape the mold cannot release by opening straight.
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?
| Method | How it works | Best for | Tooling impact |
|---|---|---|---|
| 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 | Simple latches and snap hooks | None, the lowest-cost option |
| 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 | 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 | Internal clips and undercuts inside a wall | Adds cost; needs room behind the feature |
| Hand-loaded insert | A loose insert is molded in, removed with the part and pulled out by hand | Prototype and low-volume tools | Low tool cost, slower cycle |
| Stripping (bump-off) | The part flexes over a shallow undercut during ejection | Shallow undercuts in flexible plastics | 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. A hole through the top face needs no mechanism; the same hole through a side wall does.
- Group side features on the same face, so one side action can form several of them.
- Allow room. Side actions and lifters need space in the mold and a clear travel path, which can limit wall thickness and feature spacing nearby.
- Keep mechanisms 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. At NICE Rapid that review is the first step of every plastic injection molding project, and on LSR parts undercut and lifter requirements are evaluated before the tool design is confirmed.
Frequently Asked Questions
What is an undercut in injection molding?
It is a feature that stops a molded part from being ejected straight out of the mold along the direction the mold opens. Side holes, snap-fit hooks, internal grooves and threads are common examples.
How do you mold a part with an undercut?
Either redesign the feature so the mold can release it by opening straight, or add a moving mechanism to the mold. Side actions pull sideways out of external features before ejection, and lifters slide off internal features as the part is pushed out.
Do undercuts increase tooling cost?
Usually, yes. Every side action or lifter is an extra moving component to machine, fit and maintain, and some add cycle time. A simple redesign, such as a shut-off, often removes the need for a mechanism entirely.
Related Services at NICE Rapid
- Plastic Injection Molding — a DFM review of the production design files comes before tooling, when undercuts are cheapest to fix.
- LSR Molding — undercuts and lifter requirements are evaluated during DFM, before the tool design is confirmed.
- Request a Quote — send a STEP file and quantities; note any snap fits or side features the part must keep.
Sources
- Texas A&M University (Richland College injection molding curriculum), Part 4: Injection Mold Design — molds with "moving cores and side-action cams" are needed "if the geometry of the part has undercuts"; shut-off hole and latch examples with "no side action required" versus an angled latch where a "side action cam [is] required" — people.tamu.edu
- US Patent 6,116,891, Background — side-action coring elements form "holes or apertures or undercuts through sides" of molded pieces and "must be retracted from the cavity as the mold is opened and before ejection of the part"; conventionally driven by angled guide pins or cams — patents.google.com
- US Patent 5,137,442, Background — "Internal core lifters are used within a mold device to mold and eject undercuts"; the core blade is attached to the ejector system and "is required to move horizontally away from the undercut during a vertical ejection of the plastic part" — patents.google.com
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