What Is an Undercut?
Any geometry that locks the part into the core or cavity after molding. Common examples:
Snap-fit hooks
Side holes or openings
Threads (internal or external)
Recessed ribs or bosses
Solutions for Undercuts
| Method | How It Works | Best For | Cost Impact |
|---|---|---|---|
| Sliders (Side Cores) | A moving core retracts before ejection | Large undercuts, deep side holes, external features | Moderate to high |
| Lifters | An angled component lifts the part as it ejects | Internal undercuts, small snap-fits | Moderate |
| Bumpoff (Stripping) | Flexible part geometry is deformed slightly during ejection | Flexible materials (PP, PE), small undercuts | Low |
| Hand Loads / Manual Inserts | Loose pieces inserted and removed by hand | Low volume, prototypes, complex undercuts | Low tooling but high labor |
| Core Pullers (Hydraulic/Pneumatic) | Actuated slides for large or multiple undercuts | Large molds, high production | High |
Best Practices
1. Design for Moldability First
Whenever possible, avoid undercuts by reorienting the parting line or modifying the part geometry. A small design change can eliminate a slider and save thousands of dollars.
2. Choose the Right Mechanism
Sliders for external undercuts on the cavity side.
Lifters for internal undercuts on the core side.
Bumpoffs only for flexible resins with sufficient elongation.
3. Consider Production Volume
Low volume (<10,000 parts): Bumpoffs or hand loads often make sense.
High volume (>100,000 parts): Invest in durable sliders or lifters with wear-resistant components.
4. Account for Slide and Lifter Clearance
Ensure enough space in the mold base for the mechanism to travel. Crowded tooling leads to premature wear and maintenance issues.
5. Use Appropriate Steels
Moving components like slides and lifters require hardened steel (e.g., H13, S136) to withstand repeated action. Softer steels will gall or wear quickly.