An undercut is any feature that blocks the part from being pulled straight off the core or out of the cavity — a side hole, a snap hook, an internal barb, an external thread. Molding it requires the tool to move in more than one direction.
Figure 1. Mold Slides and Lifters: Undercut Design Guide.
There are four practical ways to make an undercut, and choosing between them is one of the highest-leverage decisions in the whole tool design. Pick wrong and you get a tool that tears parts, wears out, or needs a person standing next to it.
1. The Four Options
| Method | How it works | Best for | Limitations |
|---|---|---|---|
| Bump-off / strip | The part is forced past a shallow undercut, flexing the plastic | Small undercuts ≤ 0.5 mm, flexible resins (PP, PE, TPE) | Only works with compliant geometry; risks scuffing and stress marks |
| Lifter (angled ejector) | An angled pin slides sideways as the ejector plate moves | Internal undercuts — snap hooks, internal barbs, undercuts on the core side | Angle limited to ~15°; travel limited by ejector stroke |
| Slide / side core | A cam-driven or hydraulic block retracts sideways before ejection | External undercuts, large side features, deep holes | Adds cost and maintenance; needs travel and pre-load design |
| Collapsing core | Segmented core collapses inward to release an internal thread or deep undercut | Internal threads, deep internal undercuts | Expensive; limited to suitable geometry |
Default guidance: if the undercut is on the inside of the part, reach for a lifter first. If it is on the outside, reach for a slide. If it is a thread, go to the unscrewing or collapsing-core route.
2. Slide Anatomy and Travel
A slide is a block that carries the forming steel for the undercut and retracts along a straight path before the part is ejected.
Core components:
- Slide block — carries the forming insert.
- Cam pin (angle pin / horn pin) — converts the opening stroke into slide travel.
- Wear plates — hardened plates that guide and carry the slide; consumable by design.
- Slide stop / positive stop — sets the forward position; the slide must be locked against injection pressure, never floating.
- Heel block / locking block — resists injection pressure in the closing direction.
- Spring or hydraulic cylinder — holds the slide retracted before the cam pin engages.
Travel calculation:
> Slide travel = Undercut depth + clearance (typically 3–5 mm) + safety margin
The cam pin geometry then sets the slide travel from the opening stroke:
> Slide travel = Opening stroke × tan(θ)
where θ is the cam pin angle. Rearranged:
> Required opening stroke = Slide travel / tan(θ)
Worked example: an undercut 12 mm deep needs 16 mm of slide travel (12 + 4 mm clearance). With a 20° cam pin, the required opening stroke before the slide clears is 16 / tan(20°) = 16 / 0.364 = 44 mm. If the machine only gives 35 mm of safe opening before ejection starts, the slide will not clear and the part will tear.
Angle limits:
- Cam pin angle: 15° – 25°. Under 15° the slide is slow; over 25° the cam pin experiences very high side loads and the mechanism becomes prone to jamming.
- Include a delay (a straight portion on the cam pin) at the start of opening so the slide begins to move only after the mold has separated enough to clear the part.
- Lock the slide positively with a heel block angled 1°–2° past the cam pin angle, so injection pressure pushes the slide into its seat rather than out of it.
3. Lifter Design
A lifter does two jobs at once: it pushes the part off the core and slides sideways to clear an internal undercut. It is elegant, cheap, and very easy to get wrong.
Lifter angle:
> Lateral travel = Lifter stroke × tan(α)
where α is the lifter angle from vertical and the lifter stroke is the ejector plate travel.
- Keep α between 3° and 15°. Below 3° the lateral movement is too small to be useful; above 15° the lifter bends, binds and wears rapidly.
- For a typical 40 mm ejector stroke at 10°, lateral travel = 40 × tan(10°) = 7.0 mm.
Design rules:
- The lifter must have a guided sliding seat in the ejector plate — usually a slot with a hardened wear insert. The lifter is a lever, and its base sees a large bending moment.
- Provide clearance between the lifter and the mold steel of 0.05–0.10 mm on the non-forming faces, and a proper shut-off on the forming face.
- The lifter tip should be drafted to match the part and radiused so it does not shear the undercut.
- For long lifters, add a guide bushing partway up to control buckling.
- The lifter head must be able to rotate slightly in its seat, or it will break at the base. A spherical or slotted head is standard practice.
When a lifter is wrong: if the undercut is deep (say over 1.5 mm on a stiff resin), or the part is large and heavy, the lifter force becomes a side load the mechanism cannot take. Go to a slide or a collapsing core.
4. Slides: Practical Details
Forming steel. The forming insert in a slide should be a separate piece, replaceable, and made of a wear-resistant steel — hardened to 48–52 HRC for abrasive resins. Slides are one of the highest-wear areas of any tool.
Venting. Slides create a trap for air. Provide venting at the shut-off surface, and remember that the vent must be cut into the sliding face without creating a leak path for melt.
Cooling. Slides are notoriously hard to cool because the forming steel is a moving island. Route cooling through the slide with flexible hoses or a rotary joint, or accept longer cycle time on that local feature. Undercooled slides are the most common cause of warpage near an undercut.
Ejection interference. The slide must be fully retracted before the ejector plate moves. Verify the sequencing in the opening curve: many tools use a mechanical interlock rather than relying on the operator.
Hydraulic slides. Use where travel is long, the angle would be excessive, or the slide must move at a programmable time. Add a position sensor — a hydraulic slide that fails to retract will destroy the cavity on the next shot.
5. Failure Modes to Design Out
| Failure | Root cause | Design fix |
|---|---|---|
| Slide jams / does not retract | Inadequate guide clearance, thermal growth, no lubrication path | Add wear plates with lubrication grooves; design for thermal growth |
| Cam pin bends or breaks | Angle too steep, insufficient pin diameter | Reduce angle below 25°; increase pin diameter; harden pin |
| Part torn at undercut | Slide or lifter does not clear before ejection | Increase travel/clearance; verify opening sequence; add delay |
| Lifter breaks at the base | No rotational freedom, over-tight seat | Use a slotted or spherical head; add hardened seat insert |
| Wear on the shut-off faces | Soft forming steel, abrasive resin | Harden forming insert to 48–52 HRC; specify replaceable inserts |
| Flash on the undercut feature | Slide not positively locked against injection pressure | Add an angled heel block; check lock-up in the closed position |
| Galling on the sliding surfaces | Same material on both faces | Use dissimilar steels or bronze wear plates |
Figure 2. Quick reference: the key checks and reference values from this guide.
6. Design Checklist
- [ ] Undercut depth and location documented from the 3D model
- [ ] Method chosen against the table in section 1, with a written reason
- [ ] Slide travel = undercut + 3–5 mm clearance
- [ ] Cam pin angle between 15° and 25°; opening stroke re-checked against the machine
- [ ] Delay on the cam pin so the slide starts after the mold has cleared
- [ ] Angled heel block for positive locking against injection pressure
- [ ] Lifter angle ≤ 15°, with a guided, rotatable head
- [ ] Lifter base seat hardened and wear-lined
- [ ] Forming inserts replaceable and hardened to 48–52 HRC
- [ ] Venting and cooling designed into the slide, not bolted on later
- [ ] Sequencing verified against the opening curve; interlocks where needed
- [ ] Hydraulic slides fitted with position sensors
- [ ] Lubrication points accessible from outside the mold
Conclusion
Undercuts are a mechanism problem, and mechanisms fail for mechanical reasons: backlash, side load, wear and sequencing. Choose the simplest method that meets the geometry — bump-off before lifter, lifter before slide, slide before collapsing core — and then design the travel, the lock-up and the lubrication properly.
Our tooling engineers review undercut geometry and simulate the opening sequence before the mechanism drawings are released. Send us a part with undercuts and we will tell you which of the four methods it actually needs.
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