An undercut in injection molding is any feature that stops a part from moving straight out of a simple two-part mold, such as a slot, a thread tooth, or a locking tab. When an undercut cannot be designed away, the mold needs moving tooling such as a slide, a lifter, or a collapsible core, and that tooling adds cost and time. Settling which injection molding undercuts can be designed out, and which have to stay, before the geometry is final decides how complicated the mold gets.
Our injection molding DFM (design for manufacturability) guide covers the wider rules on draft and wall thickness, and design for manufacturing takes the process-neutral view. Kemperle’s design and engineering work runs from CAD (computer-aided design) through manufacturing handoff, so undercuts can be dealt with in the model before it goes to a molder for quoting.
What Are Undercuts in Injection Molding?
A two-part mold opens along one line, and the part has to slide out in that same direction. An undercut is a feature that blocks that movement, because material is in the way while the mold opens or during ejection. Protolabs Network’s examples are power switch slots, the teeth of a thread, and locking tabs.
Each of those is a working feature of the finished part, so the useful question is how the mold will open around it. We would walk the CAD model along the pull direction before anything else and mark every feature the mold cannot clear in a straight pull. Those marks become the list to design out, soften, or pay for.
How Do Side Actions, Lifters, and Bumpoffs Release Undercuts?
A side action, or cam, is a mold piece that acts from outside the part, moving perpendicular to the direction the mold opens. It forms a side-facing feature while the mold is closed and then withdraws so the part can eject. Xometry calls this kind of side action a slide, and Protolabs adds a detail that catches designers: the motion is perpendicular to the mold opening, not necessarily to the wall the feature sits on. Check the pull direction rather than assuming the wall sets it.
Lifters do the same job inside the part: Xometry describes them as creating and releasing internal undercuts, such as an internal tab or an overhang. A collapsible core folds inward to clear a circular undercut, such as a large internal thread, so the part can come off. DuPont and Komaspec list both among the moving tooling molders use. Where a cam cannot reach an interior surface, Protolabs uses pickouts: inserts that sit in the mold during injection and eject with the part, then come out of it, leaving the undercut behind.
Draft follows the motion. Because a side action does not move with the main tool, Xometry notes that an undercut it forms needs draft angles specific to the action’s movement.
A bumpoff skips the mechanisms. It is a small undercut the part is forced over during ejection, so the plastic flexes and the mold stays put. The leading edge needs a ramp or radius rather than a hook, and Protolabs lists TPE (thermoplastic elastomer), unfilled polyethylene, and polypropylene as flexible enough, with glass-filled nylon and polycarbonate too rigid to work well. Protolabs adds that the undercut should sit away from corners and ribs that stiffen the wall.
| Feature | Release options |
|---|---|
| Slot or hole in a side wall | Side action, or a shutoff where the geometry allows |
| Groove or recess on an outer surface | Slide, or move the parting line across it |
| Snap-fit hook | A window beneath the hook so a shutoff forms its underside |
| Internal tab or overhang | Lifter, or a pickout where a cam cannot reach |
| Circular undercut or large internal thread | Collapsible core |
| Shallow, rounded lip in a flexible resin | Bumpoff, with no moving tooling |
Can You Design Out Undercuts in Injection Molding?
Sometimes the design can drop the undercut, and that is the cheapest outcome. Protolabs Network lists four routes: shutoffs, moving the parting line, bumpoffs, and sliding side actions and cores, and it reserves the last for parts that cannot be redesigned. Xometry gives the same advice: ask whether a side action is needed at all, or whether a redesigned slot feature avoids it.
A shutoff (sometimes written shut-off) removes material under the undercut so the two mold halves form the feature by closing against each other. Under a snap-fit hook, that means a window through the floor: a pad on one half comes up through the window to form the underside of the hook, and no side action is needed.
Where the two mold faces meet they must be drafted, and Protolabs sets the minimum at 3 degrees. That gives a tight seal when the mold clamps shut and keeps the surfaces from wearing as it opens, since a worn shutoff leaks resin between the faces and leaves flash.
For a feature on an outer surface, the simplest fix is moving the parting line across it and adjusting the draft to match, according to Protolabs Network. We would settle the parting line with the molder before the geometry is frozen, since it decides which features become undercuts in the first place.
Casting in silicone treats undercuts differently. Smooth-On’s tutorial for a plaster original with deep undercuts calls for a strong, flexible platinum silicone, and the finished mold is opened with a razor cut down one side, leaving a zig-zag seam that closes for casting. Because the rubber flexes open, the undercut needs no moving mechanism in the mold. We make silicone molds too, and our comparison of silicone molds against 3D printing shows where each one wins.
How Do Injection Molding Undercuts Affect Tooling Cost?
Every undercut mechanism is extra tooling. Protolabs Network says it adds cost and overall time, and Xometry says side actions can raise molding costs dramatically. Side actions also need room to move in and out, so the feature has to sit where a mechanism can reach it.
Moving parts wear as well. Protolabs warns that a worn sliding shutoff leaks resin between the mold faces, and the flash that results can interfere with how the part works.
For a prototype run or a small batch, a cheaper mechanism exists. Xometry describes hand-loaded cores, which are placed by hand, molded around, and removed from the part each cycle, as a lower-cost alternative to automated slides and lifters for prototype and low-volume tooling. The trade-off is manual work every cycle, so we would ask the molder whether a hand load fits the quantity.
We would not quote a single figure for what an undercut adds, because it varies with the part, the tool, and which mechanism it needs. The molder’s quote is where that number comes from. For the volume math behind tooling decisions, read our comparison of 3D printing versus injection molding for small runs.
What Should You Send a Molder Before Requesting a Quote?
A complete package keeps the molder from guessing. We would send a 3D CAD model in a standard exchange format, a drawing that marks critical dimensions and tolerances, the resin and color, the expected annual volume, and the surfaces that must stay cosmetic or textured.
Flag every undercut you know about and the release method you expect. That note heads off a quote with mechanisms nobody planned for, and it gives the molder a chance to propose a shutoff or a parting-line move first.
Before the geometry is frozen, send the model through the contact form with the number of parts you need, and we will talk through how the mold opens around each undercut. If you would rather speak first, call 718-557-9578.
