Designing for Undercuts: When They Work and When They Don’t
What Counts as an Undercut
An undercut is any geometry that would tear, distort, or lock the part in the mold if you tried to eject it straight out. Common examples include a slot for a power switch on the side of a housing, a hole running through a boss perpendicular to the pull direction, external threads on a fastener, and locking tabs on a snap-fit closure. None of these are unusual design intents. They’re just geometry that a simple two-plate mold can’t produce without help.
Redesign First: Parting Line and Draft
Before reaching for mechanical tooling, the cheapest fix is often a redesign. Moving the mold’s parting line so it intersects the feature can eliminate an undercut entirely on many external geometries, effectively letting the two halves of the mold separate right through the feature instead of around it. This works especially well on standoffs, bosses, and other features near the part’s natural draft lines. Engineering guidance on avoiding undercuts lays out this approach alongside other no-extra-tooling fixes, including consolidating features so a zigzagging parting line can clear several undercuts at once. If a redesign genuinely can’t eliminate the feature, that’s the point where mechanical tooling becomes the right call, not the default first move.
When You Need Mechanical Tooling
Two mechanisms handle the undercuts a redesign can’t. Side actions, also called sliders or cam-pin slides, retract sideways as the mold opens to clear an external undercut before ejection. Lifters do the same job for internal undercuts, moving at an angle to pull clear of a feature that would otherwise trap the part inside the core. Both require additional space in the mold base, additional maintenance over the tool’s life, and additional cost, and both need enough draft on the sliding surfaces to avoid dragging or damaging the part on every cycle. This is exactly why undercuts function as a real cost driver rather than a cosmetic afterthought. Every side action or lifter is a moving mechanical component built into a production tool, and it behaves like one, with wear, tolerance stack-up, and maintenance implications that a straight-pull feature never has.
Draft Angle Rules That Apply Either Way
Draft angle matters on every vertical surface, undercut or not, and it’s worth getting right before an undercut ever enters the conversation. Insufficient draft causes ejection drag, cosmetic marks, and accelerated tool wear even on features that technically aren’t undercuts at all. Recent peer-reviewed research on draft angle design confirms this isn’t just a rule of thumb: the interaction between draft angle, surface texture, and dimensional accuracy is measurable and material-dependent, with a 2025 study in the journal Polymers documenting how draft angle choices affect both surface replication and part accuracy differently across semi-crystalline and amorphous resins. Textured surfaces need more draft than smooth ones to release cleanly, and that requirement compounds fast on a feature that’s already an undercut.
Fiber-Reinforced Materials Change the Math
Material choice affects which undercut strategies are even available. Flexible, unreinforced resins can sometimes tolerate a bump-off, where the part deforms slightly to strip past a shallow undercut without mechanical tooling. Fiber-reinforced plastics generally can’t take that approach. The reinforcement reduces the material’s ability to flex without damage, which means an undercut that would be a low-cost bump-off in an unfilled resin often requires a full side action or lifter in a glass-filled or carbon-filled equivalent. That’s a material and tooling decision that needs to happen together, not sequentially.
Why This Is a Conversation, Not a Quote
An automated quoting engine can flag an undercut. What it generally can’t do is tell you whether moving your parting line two millimeters eliminates the need for a slider entirely, or whether your specific glass-filled nylon rules out the bump-off that would have worked in an unfilled version of the same part. Those are engineering calls that depend on the whole part, not just the flagged feature, and they’re exactly the kind of thing a manual DFM review catches before tooling gets cut instead of after.
Send us the drawings and our team will tell you plainly whether that undercut is worth the tooling it requires, or whether a redesign gets you the same function for less.
