Undercut Guide
for Silicone & Rubber Molding
Undercuts increase tooling cost 30-50%. Sometimes you can design them out. Sometimes silicone’s flexibility lets you snap them out. Here’s how to decide.
What Is an Undercut?
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An undercut is any feature that prevents the part from being ejected straight out of the mold in the direction of the draw. In rigid plastics, undercuts always require a side-action or loose core. In silicone and rubber, the material’s flexibility sometimes allows snap-out ejection — but this depends on hardness, undercut depth, and geometry. The image shows flexible silicone parts being flexed and snapped out of the mold, demonstrating how soft silicone can tolerate small undercuts that would lock a rigid plastic part. |
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Undercut Handling Methods
| Method | Cost Impact | When to Use |
|---|---|---|
| Design Out | None (best DFM) | Always try this first. Rethink geometry to eliminate the undercut. |
| Snap-Out Ejection | None (slightly higher scrap) | Shore A <40, undercut depth <0.5mm only |
| Loose Core (Hand Load) | Medium — slower cycle time | Low-volume, complex undercuts where cycle time is acceptable |
| Side-Action (Slide) | High (+30–50% tooling cost) | Deep undercuts, high-volume production |
| Post-Mold Machining | Medium — extra operation | When undercut cannot be molded and side-action is too expensive |
Silicone Snap-Out Undercut Rules
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Shore A 10–30
Up to 0.3–0.5mm undercut depth may be snap-able |
Shore A 30–60
Up to <0.2mm — discuss with engineers. Borderline. |
Shore A >60
Do not rely on snap-out. Use side-actions or redesign. |
Design Guidelines
- Avoid undercuts on sealing surfaces — side-actions leave witness marks that compromise sealing.
- Taper undercut walls — Adding 3–5° draft to undercut walls reduces ejection force and tear risk.
- Limit undercut depth — For snap-out, keep undercut depth <0.5mm and wall thickness uniform around the undercut.
- Communicate to engineers early — If a feature must be an undercut, mark it on the drawing and specify which method is acceptable.
- Consider parting line relocation — Sometimes moving the parting line eliminates the undercut entirely.
- Volume threshold for side-actions: <5,000 pcs/year — side-actions are rarely cost-justified.
Side-Action Mechanism
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When snap-out is not possible, side-actions (sliders) are used. The image shows a precision steel mold with a sliding core mechanism for undercut features. Side-actions add 30-50% to tooling cost and introduce additional maintenance points. The sliding surface must have its own draft angle (1-2 degrees) to prevent binding. For LSR injection molding, side-actions must seal tightly to prevent flash at the slide interface. |
Side-Action Design Notes
- Side-actions add 30–50% to tooling cost and introduce additional maintenance points
- The sliding surface must have its own draft angle (1–2°) to prevent binding
- For LSR injection molding, side-actions must seal tightly to prevent flash at the slide interface
- Discuss with our engineering team: Is the volume high enough to justify a side-action?
Other DFM Guides:
| Wall Thickness → | Draft Angle → | Parting Line → | Shrinkage & Tolerance → |
Have an Undercut You’re Not Sure About?
Upload your drawing — our engineers will evaluate snap-out feasibility and recommend the lowest-cost solution.