Concept renderAn undercut is any feature that stops a part from being pulled straight out of the mold. You can design it out with a moved parting line, a shutoff or a window, or form it with a side action, lifter, hand-loaded insert or bump-off. Each mechanism adds tooling cost, cycle time or design limits.
Small device housings collect undercuts fast. A USB-C port in the side wall, two buttons on the edge, a microphone hole and four snap hooks can each need a moving part in the mold. This guide covers where undercuts appear in a wearable or tracker shell, the tooling options, and how to design them out before a quote. Every number here comes from a published design guide linked below.
What is an undercut in injection molding, and why does it matter?
A mold opens in one direction, called the pull direction. Protolabs defines undercuts as features that prevent an injection-molded part from being ejected from the mold (Protolabs, 6 ways, as of November 2026). Anything that points sideways, hooks over steel or wraps around a core is an undercut.
Each undercut needs a fix: change the part so it pulls straight, or add a tool mechanism that forms and releases the feature. Protolabs says side actions add tooling cost but are faster in production, and pickouts are simpler but slower (Protolabs injection molding guide, as of November 2026). So the undercut count drives tool cost, cycle time and wear. Our guide to injection mold tooling explains how the tool is built and what adds to its cost. Protolabs also warns that faces with insufficient or wrongly oriented draft can act as an undercut, and it recommends at least 0.5 degrees on all vertical faces in its process (Protolabs, side-actions and inserts, as of November 2026). For draft values by face type, read our injection molding draft angle guide.
Where do undercuts show up in a device housing?
Device shells repeat the same few features. The fixes column is our engineering reasoning from the mechanisms in the sources below. Check it against your own part with the tool shop.
| Feature | Why it is an undercut | Usual fixes |
|---|---|---|
| USB-C or charging port in a side wall | The opening points sideways, so a core that forms it has to move sideways too. | A side action, or a parting line that passes through the opening. A split shell along the port line can avoid the slide. |
| Button holes and button caps on the edge | Same as the port: the hole axis is perpendicular to the pull. | Group side openings on one face so one slide forms them, or drill small holes after molding at low volume. |
| Microphone and speaker holes on the side | Small holes that face sideways need a core pin that moves sideways. | Move the acoustic port to a face that opens with the mold, or form it with the same slide as the port. |
| Snap hooks on the outside of a shell | The hook lip faces sideways and locks the part in the tool. | A sliding shutoff with a window under the hook, a side action, or a bump-off if the resin can flex. |
| Snap hooks and clips on the inside of a shell | An external slide cannot reach an interior face. | A lifter, a hand-loaded insert, or a redesign that opens a window under the hook. |
| Internal grooves, threads and bosses with lips | The feature wraps around a core and cannot slide off it. | A bump-off for a small lip in a flexible resin, or a collapsible core or unscrewing action for threads. |
Ports and buttons on the edge
Thin wearables and trackers often put the charging port and buttons on the edge. If the shell splits into a top and bottom half along the edge, the parting line can sometimes run through the port and button holes, and the two halves form them with no slide. If the shell is one deep cup, each edge face with an opening usually needs its own slide.
Snap fits that close the shell
Snap hooks replace screws in many small housings. Protolabs notes that clips and snap fits can reduce screws, adhesives and secondary hardware, with the trade-off of added stress and flex (Protolabs injection molding guide, as of November 2026). Inside hooks are the costly ones, as the table shows.
How does side action injection molding form an undercut?
When you cannot design an undercut out, the tool has to form it, with a side action or one of the other mechanisms below. Limits marked Protolabs apply to its own process. Other shops set their own, so ask yours.
| Mechanism | What it forms | Trade-offs and stated rules | Source |
|---|---|---|---|
| Moving the parting line | External features the split can pass through. | No moving parts in the tool. Parting line placement may be limited by part geometry and resin flow. | Protolabs, 6 ways, as of November 2026 |
| Sliding (telescoping) shutoff | Clip and hook features, and through-holes perpendicular to the pull. | Shutoff faces need at least 3 degrees of draft. Shutoff wear can let resin leak between the faces and leave flash. | Protolabs, complex features, Protolabs, 6 ways, as of November 2026 |
| Side action (slide or cam) | External undercuts such as side ports and holes. | Moves perpendicular to mold opening and slides in and out automatically each cycle. Adds tooling cost but is faster in production. Protolabs lists maximum side core dimensions of under 213.84 mm wide, under 60.38 mm high and under 73.66 mm of pull for its process. | Protolabs, complex features, Protolabs guide, Protolabs, 6 ways, Protolabs design guidelines, as of November 2026 |
| Lifter | Internal undercuts such as inside tabs and overhangs. | Moves into place as the tool closes and away as it opens, like a slide. Moves sideways off the undercut as the part is ejected, usually on an angled path. | Xometry, Hubs, as of November 2026 |
| Hand-loaded insert (pickout) | Interior undercuts and shapes a slide cannot reach. | Loaded by an operator, ejected with the part, then removed by hand. Simpler but slower than a side action. Protolabs applies a small per-piece handling upcharge. | Protolabs, 6 ways, Protolabs guide, Protolabs, side-actions and inserts, as of November 2026 |
| Bump-off (stripped undercut) | Smooth, well-radiused lips and ridges. | The part flexes over the steel during ejection. Needs a flexible resin. The more rigid the resin, the harder the part is to eject. | Hubs, Protolabs, 6 ways, as of November 2026 |
| Collapsible core or unscrewing action | Circular undercuts, large internal threads, threads. | The core collapses inward, or the threads are unwound between cycles. For low volumes, a hand-loaded core may cost less than an unscrewing action. | Xometry, as of November 2026 |
| Secondary operation | A few small holes at prototype or low volume. | Drill or mill after molding. This can be more cost-effective for prototypes and low-volume production. | Protolabs, 6 ways, as of November 2026 |
Protolabs states that it does not currently offer lifters, so check which mechanisms your own shop supports before you design around one (Protolabs, side-actions and inserts, as of November 2026). Protolabs also states that in its process, cams cannot form undercuts on non-exterior surfaces, so interior undercuts use pickouts (Protolabs, complex features, as of November 2026). For mold inserts, it asks for a size of at least 0.5 in. (12.7 mm) across and no larger than about 6 in. (152.4 mm), placed on the B-side of the mold or with a footprint that connects to the parting line (Protolabs, side-actions and inserts, as of November 2026).
Material matters too. Protolabs says side actions work best with rigid materials such as nylon, polycarbonate and acetal, and names LDPE, TPE and TPU for bump-offs (Protolabs, 6 ways, as of November 2026). Extreme Molding says reinforced plastics do not suit bump-offs (Extreme Molding, as of November 2026). A rigid or filled housing resin usually rules out a bump-off on the shell.
How do you design an undercut out of the part?
The cheapest undercut is the one that is gone. Work through these moves in order before you accept a slide.
- Move the parting line. Moving the parting line to intersect the undercut is sometimes the easiest fix (Protolabs, 6 ways, as of November 2026).
- Reorient the part in the tool. Turning the part so a feature runs parallel to the mold opening can remove the undercut (Protolabs, side-actions and inserts, as of November 2026).
- Open a window under hooks. A through-hole under a snap hook lets a sliding shutoff form the hook from both halves. The shutoff faces need at least 3 degrees of draft, and the part wall itself must be drafted by more than 3 degrees for windows (Protolabs, complex features, as of November 2026).
- Add draft. Protolabs says that if you increase the draft enough, the part can slide away from the core and a side action may not be needed (Protolabs, side-actions and inserts, as of November 2026).
- Group side features. Put the port, buttons and side holes on one face so one slide forms all of them. This is our engineering reasoning, and the tool shop has the final say.
- Move a feature to another part. A button cap, port trim or clip can live on a separate small part so the main housing pulls straight.
- Use a secondary operation at low volume. Drilling or milling a hole after molding can be more cost-effective for prototypes and low-volume production (Protolabs, 6 ways, as of November 2026).
Sealed devices add a constraint. Extreme Molding advises against a parting line on a sealing surface when molding liquid silicone rubber (Extreme Molding, as of November 2026). We apply the same caution to gasket faces on rigid shells, as engineering reasoning. For a sealed tracker housing, read our guide to IP67 and IP68 enclosure design before you place splits.
Which snap-hook details keep the mold simple?
A snap hook must release from the tool as well as hold the shell closed. These published rules cover the tool-facing details. Treat them as starting points and confirm them with your tool shop.
| Topic | Stated rule | Source |
|---|---|---|
| Draft on the arm | Include 1 to 2 degrees of draft on both sides of snap arms and mating features. | Fictiv, as of November 2026 |
| Gate location | Gate near the base of the snap for even filling and fewer weld lines. | Fictiv, as of November 2026 |
| Bump-off lead angle | For a stripped undercut, use a 30 to 45 degree lead angle and keep it away from ribs and corners. | Extreme Molding, as of November 2026 |
| Window or shutoff | A hole in the floor under a clip removes the undercut. Shutoff faces need at least 3 degrees of draft, and a window wall needs more than 3 degrees. | Protolabs, complex features, as of November 2026 |
For arm sizing from permissible strain, root radius, return angle and resin choice, see our guide to snap fit joint design.
What should you ask your hardware team before tooling?
Run these checks on the CAD before it goes to a tool shop. In our gates, EVT includes a first enclosure and tooling comes at DVT / PVT, so EVT is the right time for these checks. Our guide to EVT, DVT and PVT explains where tooling falls.
| Check | Why it matters |
|---|---|
| Pull direction is set and written on the drawing | Undercuts are defined relative to the pull, so nobody can count them without it. |
| Draft analysis run on every face | Faces with zero or reversed draft act like undercuts and surprise you in the tool review. |
| Each undercut is listed with its mechanism | One row per feature. The count drives tool cost and cycle time. |
| Side openings grouped by face | Ports, buttons and acoustic holes on one face can share one slide. |
| Snap hooks have a window or shutoff plan | A hook with solid wall under it needs a slide or lifter. A window under it can let the two mold halves form it. |
| Resin matches the mechanism | Bump-offs need a flexible resin. A glass-filled or very rigid resin rules them out. |
| Sealing surfaces are clear of parting and insert lines | Keep splits off the seal path. |
Then take these questions to your team and to the tool shop.
- What is the pull direction, and who signed off on it?
- How many side actions, lifters and hand loads does the current design need, feature by feature?
- Can the USB-C port, buttons and microphone holes sit on one face so one slide forms them?
- Does the resin we picked allow bump-offs, or is it too rigid?
- At our volume, is a hand load cheaper overall than an automatic slide?
- Who owns the mold, and is it labeled with our company name?
To see how mechanical design, electronics and firmware decisions are made together, read how the mechanical work is organized.
Questions about undercuts and side actions
What is an undercut in injection molding?
It is a feature that prevents the part from being ejected from the mold (Protolabs, as of November 2026). Extreme Molding lists snap and latch features, threads, barb fittings and holes for buttons or wiring as examples (Extreme Molding, as of November 2026).
What is the difference between a side action and a lifter?
A side action, also called a slide, forms external undercuts. A lifter forms internal undercuts. Both move into place as the tool closes and away as it opens (Xometry, as of November 2026).
How could Duobay help with undercuts in my enclosure?
Enclosure design is part of our mechanical work. The EVT gate includes a first enclosure, and the DVT / PVT gate includes tooling at partner shops. Molds you pay for are labeled with your name. We do not publish prices. Tell us about your device to start.
If you want a housing reviewed for undercuts before you cut a mold, tell us about your device.
Sources
All pages accessed November 5, 2026. Each shop sets its own process limits.
- Protolabs, 6 Ways to Achieve Undercut Success in Molded Parts: https://www.protolabs.com/resources/design-tips/6-ways-to-achieve-undercut-success-in-molded-parts/ (accessed 2026-11-05)
- Protolabs, Conquering Undercut Design Issues with Side-actions, Inserts, and Shut-offs: https://www.protolabs.com/resources/design-tips/conquering-undercut-design-issues-with-side-actions-inserts-and-shut-offs/ (accessed 2026-11-05)
- Protolabs, Designing for Moldability: Complex Features in Injection Molding: https://www.protolabs.com/resources/guides-and-trend-reports/designing-for-moldability-complex-features/ (accessed 2026-11-05)
- Protolabs, Injection Molding Guide: Process, Design Tips, & Materials: https://www.protolabs.com/resources/guides-and-trend-reports/injection-molding-guide-process-design-tips-materials/ (accessed 2026-11-05)
- Protolabs, Injection Molding Design Guidelines: https://www.protolabs.com/services/injection-molding/plastic-injection-molding/design-guidelines/ (accessed 2026-11-05)
- Xometry, Types of Injection Molding Side Actions (updated September 11, 2025): https://www.xometry.com/resources/injection-molding/injection-molding-types-of-side-actions/ (accessed 2026-11-05)
- Hubs (Protolabs Network), How do you design undercuts for injection molding?: https://www.hubs.com/knowledge-base/undercuts-for-injection-molding/ (accessed 2026-11-05)
- Extreme Molding, Undercut Part Design for Injection Molding: https://www.extrememolding.com/blog/undercut-part-design-for-injection-molding (accessed 2026-11-05)
- Fictiv, How to Design Snap Fit Components (October 17, 2025): https://www.fictiv.com/articles/how-to-design-snap-fit-components (accessed 2026-11-05)
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