An undercut in injection molding is any feature that keeps a part from pulling straight out of a two-part mold, such as a side hole, a groove, or a snap-fit hook (a small hook that clicks parts together). Molders (the companies that run production molds) release undercuts with side actions, lifters, collapsible cores, or by flexing the part out, and each option adds moving components to the tool (the steel mold) that raise its cost, so a design that removes the undercut is the cheapest fix. Understanding injection molding undercuts before the geometry is final is the difference between a simple two-plate mold and one full of mechanisms.
Kemperle does not run production injection molds. We work at the design stage, preparing geometry that a molder can quote and tool, and this guide covers what we look for: the types of undercuts, how each is released, how to design around them, and how they change tooling cost. For the broader rules on draft (the small angle on walls that helps a part come free of the mold), walls, and gates (the openings that let molten plastic in), see our injection molding DFM guide (DFM is short for design for manufacturability), and for the process-neutral view see design for manufacturing.
What Are Undercuts in Injection Molding?
A two-part mold opens along a single line, called the pull direction, and the part must slide out along that same line. Any feature that hooks around the steel, so the part cannot move in the pull direction without breaking, is an undercut. External undercuts sit on the outside of the part, like a side hole or an outer groove, and internal undercuts sit inside it, like a clip or an internal thread.
Undercuts are easy to create by accident, because many useful features produce them: hooks, threads, side ports for cables or vents, recessed labels, and internal clips. The feature works in the assembled product, so the problem only appears when the mold is designed around it and the steel cannot open.
How Do Side Actions and Lifters Release Undercuts?
A side action, also called a slide, is a block of steel that moves sideways, perpendicular to the pull direction, to form the feature and then retracts before the part is ejected. A lifter is an angled rod that rides with the ejector system (the plate of pins that pushes the part out) and moves inward as the part moves out, which frees an internal hook or clip. Both are moving components that need room to travel, their own cooling, and precise fits.
A collapsible core is a segmented steel core that shrinks inward after molding so an internal undercut, such as a thread or groove, can pass over it. Unscrewing mechanisms rotate a threaded core out of the part for full threads, and hand-loaded inserts move the feature into a separate piece that is placed in the mold each cycle (one round of injection and ejection), which adds labor and slows production. Forced ejection, also called bump-off, skips the mechanisms entirely: a shallow, rounded undercut in a flexible resin (the plastic material) flexes over the steel as the part is pushed out.
Match the method to the feature. Side actions suit external undercuts, lifters suit internal hooks and clips, collapsible cores suit internal threads and grooves, and forced ejection suits shallow rounded lips. Hand-loaded inserts fit low volumes where the extra labor stays affordable.
How Can You Design Around Injection Molding Undercuts?
The cheapest undercut is the one the design removes. A side hole can be formed with a shut-off, a spot where the two halves of the mold press together to form the opening, when the hole passes fully through the wall and its surfaces carry a slight angle, at least about 3°. A snap-fit hook can be molded with a window (a rectangular opening) cut through the wall beneath it, so the steel that forms the hook comes straight up through the window and no lifter is needed.
Other changes work just as well. Move the parting line so it crosses the feature, turn an internal thread into an external one or replace it with a thread-forming screw (a screw that cuts its own thread in a plain hole), split one part into two pieces that are assembled after molding, or round a shallow lip so it can flex free with forced ejection. Each change trades a small design compromise for a simpler, cheaper tool.
We see the difference between casting and injection molding in our own work. For a private residence in New York, we scanned hand-sculpted ornamental plaster elements, whose fine undercuts are part of what makes them worth reproducing, and corrected draft angles, wall-thickness allowances (the minimum wall a cast piece needs), and panel breaks (the points where a large element is divided into pieces) in CAD before 3D-printed master patterns (the physical originals a mold is made from) went to mold-making. Flexible molds, such as silicone, can release detail like that with no moving parts, while a steel injection mold needs a mechanism for each undercut, which is why injection molding undercuts cost more than the same detail in a cast part.
| Feature | Undercut type | Release method |
|---|---|---|
| Side hole through the wall | External | Side action, or a shut-off if the hole passes through the wall |
| Groove or recess on an outer wall | External | Side action, or move the parting line across it |
| Snap-fit hook | Internal or external | Lifter, or a window cut beneath the hook |
| Internal thread | Internal | Collapsible core or unscrewing core |
| Internal clip or recess | Internal | Lifter or collapsible core |
| Shallow rounded lip in a flexible resin | Either | Forced ejection with no moving parts |
When an undercut is functionally necessary, document it on the drawing, note the release method you expect, and confirm it with the molder before the geometry is frozen. That note prevents the surprise of a quote that includes mechanisms nobody planned for.
How Do Injection Molding Undercuts Affect Tooling Cost?
Every undercut mechanism is extra steel that has to be designed, cut, fitted, and maintained, which is why injection molding undercuts are one of the main drivers of tooling cost. Slides and lifters wear over time, need room to travel, and can leave flash (thin excess plastic) where they seal against the mold, so they also add service cost across the life of the tool.
| Cost driver | How undercuts add to it |
|---|---|
| Moving components | Each slide, lifter, or core is a separate mechanism with wear surfaces |
| Mold size | Slides need room to travel, which calls for larger steel blocks and larger production equipment |
| Fitting and test runs | Mechanisms need precise fits and seals before the tool runs clean |
| Maintenance | Moving steel wears, and worn seals let flash through |
| Cycle time | Mechanisms must move before ejection, which can slow each cycle |
The table shows the direction of each cost, not the amount, because prices vary with part size, the number of parts made per cycle, and steel choice. For the volume math that decides whether tooling pays off, and for lower-cost interim options, read our comparison of 3D printing versus injection molding for small runs. For how molding and casting compare with printing more broadly, see this comparison of molding and casting with 3D printing.
What Should You Send a Molder Before Requesting a Quote?
A complete package gets a faster and more accurate quote. Send the 3D computer-aided design (CAD) model in STEP format (a neutral 3D file type that every CAD system can open), a drawing that marks the critical dimensions and tolerances (the allowable variation in each dimension), the resin and color, the expected annual volume, and the surfaces that must stay cosmetic (visible and blemish-free) or textured. Flag every undercut you know about and the release method you expect, so the molder prices the mechanisms you planned for.
This is where our design and engineering services fit: we take a concept or prototype, review it against the process, and deliver a production-ready CAD model with undercuts removed or documented, so a mold maker can move straight into tooling design. If your part is headed for injection molding, phone 718-557-9578 or request a design review before the geometry is frozen.
