Key Considerations: Undercuts in Molded Parts
Summary
Undercuts – features that prevent a part from being ejected along a simple straight line – require specific tooling solutions that add cost and complexity. This guide covers the key considerations engineers should weigh when a design includes undercuts.
Key Takeaways
- Undercuts require specialized tooling mechanisms like side actions or lifters to enable part ejection.
- Side actions and lifters add meaningful tooling cost and complexity compared to simple straight-pull tooling.
- Some undercuts can be designed out through alternative geometry that achieves the same function.
- Collapsible cores offer an alternative solution for certain internal undercut features.
- Weighing redesign against tooling complexity is a key early decision for any part with undercuts.
What Makes a Feature an Undercut
An undercut is any part feature that would physically block the mold from opening and ejecting the part along a simple straight-line motion, such as a side hole, an internal clip feature, or a recessed groove. Recognizing undercuts early in the design process is essential, since they require specific tooling solutions that wouldn’t be necessary for a part with straightforward pull-apart geometry.
Common Undercut Solutions
| Solution | Best Fit |
|---|---|
| Side actions | External undercuts like side holes or protrusions |
| Lifters | Internal undercuts, often with limited space for mechanism |
| Collapsible cores | Internal threads or complex internal undercut geometry |
| Design redesign | Features where the undercut function can be achieved differently |
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Side Actions for External Undercuts
Side actions are mechanical tooling components that move perpendicular to the main mold opening direction, pulling away from external undercut features like side holes or protrusions before the mold opens and the part ejects. Side actions are a well-established, reliable solution for external undercuts, though they add tooling cost and some additional maintenance complexity compared to simple tooling without moving components.
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Lifters for Internal Undercuts
Lifters handle internal undercut features, moving at an angle as the mold opens to clear the undercut before full ejection occurs. Lifters are often used where space constraints make a side action impractical, though they require careful design to ensure reliable, consistent operation over the tool’s full production life without binding or premature wear.
Collapsible Cores for Complex Internal Features
Collapsible cores, which mechanically contract inward during ejection to clear complex internal undercuts like internal threads, offer a solution for geometry too complex for standard lifters to handle. This tooling approach is more expensive and mechanically complex than side actions or lifters, reserved for situations where the internal undercut genuinely can’t be addressed through simpler methods.
Evaluating Whether Redesign Makes Sense
Before committing to the added tooling cost and complexity an undercut requires, it’s worth evaluating whether the undercut’s underlying function could be achieved through alternative geometry that doesn’t require specialized tooling. Sometimes a minor design change eliminates the need for a side action or lifter entirely, meaningfully reducing tooling cost and complexity without compromising the part’s actual function.
A Practical Undercut Evaluation Checklist
- Identify all undercut features early in the design process.
- Evaluate whether the undercut’s function could be achieved through alternative geometry.
- Determine whether a side action, lifter, or collapsible core best fits each specific undercut.
- Factor added tooling cost and maintenance complexity into program budgeting.
- Discuss undercut solutions with your molder during the DFM review process.
Maintenance Considerations for Undercut Tooling
Tooling with side actions, lifters, or collapsible cores has more moving parts than simple straight-pull tooling, meaning it generally requires more attentive maintenance to prevent wear-related issues over an extended production life. Programs with significant undercut tooling complexity should factor this ongoing maintenance consideration into their total cost of ownership expectations, not just the initial tooling investment.
Balancing Undercut Complexity Against Production Volume
For very low volume programs, the added tooling cost of side actions or lifters may outweigh the benefit compared to evaluating whether a simpler, two-piece assembly approach could replace an undercut feature altogether. At higher volumes, the tooling investment for proper undercut handling is more easily justified by the cost savings of producing the feature directly in a single molded part.
Validating Undercut Tooling Through Simulation
Mechanism simulation tools can validate that side actions, lifters, or collapsible cores will move through their intended range of motion without interfering with other tooling components or the part itself, catching potential mechanical conflicts before the tool is actually built. This simulation step is particularly valuable for complex multi-undercut parts where several moving tooling components need to operate in close coordination with each other.
Quality Control Considerations for Undercut Features
Undercut features produced by side actions or lifters sometimes show more dimensional variation than features produced by the fixed cavity surfaces, since moving tooling components can introduce slight positional variation over repeated cycles. Establishing specific quality monitoring for undercut features, rather than assuming they’ll behave identically to fixed-geometry features, helps catch any emerging dimensional drift specific to these mechanisms.
How This Connects to Other Molding Topics
Understanding carbon footprint considerations is increasingly relevant when evaluating tooling complexity trade-offs; our key considerations for carbon footprint of plastic parts covers that topic. Broader cost reduction strategies often intersect with undercut tooling decisions; our how to reduce costs in plastic injection molding explores that connection.
Shrinkage behavior can affect how precisely undercut features need to be toleranced; our pros and cons explained for shrinkage in injection molding covers that topic. And piece-part cost estimation should explicitly account for undercut tooling complexity; our guide to choosing the right approach for piece-part cost estimation is a useful companion resource.
Choosing a Molding Partner for Complex Undercut Tooling
A molder experienced in plastic injection mold design should be able to recommend the right undercut solution and discuss its cost and maintenance implications clearly. A plastic injection molding company in Louisiana with genuine side action and lifter tooling experience helps ensure reliable, consistent undercut feature production.
Documenting Undercut Tooling Decisions
Documenting why a specific undercut solution was selected, including any alternative geometry considered and rejected, creates a useful reference for future design revisions or related part development. This documentation helps avoid re-evaluating the same trade-offs from scratch each time a similar undercut challenge arises in a new project.
Conclusion
Addressing undercuts in molded parts comes down to identifying them early, choosing the right tooling solution – side actions, lifters, or collapsible cores – and evaluating whether redesign could eliminate the need for specialized tooling altogether. Weighing these considerations carefully helps engineers make informed decisions that balance part function against tooling cost and complexity.
Frequently Asked Questions (FAQs)
What is an undercut in a molded part?
Any feature that would block the mold from opening and ejecting the part along a simple straight-line motion.
What is a side action?
A mechanical tooling component that moves perpendicular to the mold opening direction to clear external undercuts.
What is a lifter used for?
Handling internal undercut features, especially where space constraints make a side action impractical.
When are collapsible cores used?
For complex internal undercuts like internal threads that are too complex for standard lifters.
Can undercuts sometimes be designed out?
Yes, alternative geometry can sometimes achieve the same function without requiring specialized tooling.
Does undercut tooling require more maintenance?
Generally yes, since side actions, lifters, and collapsible cores have more moving parts than straight-pull tooling.
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