How Undercuts Are Molded Without Complex Side Actions
Summary
An undercut, any feature that would otherwise block a part from releasing straight out of the mold, usually calls for side-action tooling, but not always. This guide covers the design and tooling approaches that let certain undercuts mold without the added cost and complexity of a full side-action mechanism.
Key Takeaways
- Not every undercut requires a side action; some can release using part flexibility or strategic parting line placement.
- Collapsible cores allow internal undercuts to release by retracting inward before ejection.
- Flexible lifters work for shallow undercuts on parts made from resins with enough elasticity to flex over them.
- Repositioning the parting line can sometimes eliminate an undercut from the mold’s perspective entirely.
- The right approach depends on part geometry, resin flexibility, and how much tooling complexity the budget allows.
Checklist Item: Evaluate Whether the Part Can Simply Flex Free
Some undercuts, particularly shallow snap-fit features on parts molded from flexible resins like polypropylene, can release by flexing momentarily over the undercut as the part ejects, without any mechanical side action at all. Whether this works depends on the undercut’s depth and angle relative to the resin’s flexibility, so it’s evaluated case by case rather than assumed.
Using Collapsible Cores for Internal Undercuts
A collapsible core is a segmented core pin that retracts inward on itself before the part ejects, allowing it to mold an internal undercut, like an internal thread or snap groove, without a traditional side action. Collapsible cores add tooling complexity and cost compared to a straight core, but far less than a full hydraulic or mechanical side-action slide.
| Technique | How It Avoids a Full Side Action |
|---|---|
| Flex-to-release | Works for shallow snap-fits on flexible resins, no side action needed |
| Collapsible cores | Mold internal undercuts by retracting inward before ejection |
| Flexible lifters | Handle shallow external undercuts with diagonal mold-opening motion |
| Parting line repositioning | Can eliminate an undercut from the mold’s perspective entirely |
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Applying Flexible Lifters for Shallow External Undercuts
A lifter is an angled tooling component that moves diagonally as the mold opens, and for shallow external undercuts it can be designed with enough flexibility to let the part release as the lifter clears, rather than requiring a rigid, separately actuated slide. This approach works best on smaller, shallower undercuts rather than deep or tightly toleranced features.
Repositioning the Parting Line to Eliminate the Undercut
Sometimes what looks like an undercut in a part’s current orientation disappears entirely once the parting line, the split between the mold’s two halves, is repositioned relative to that feature. This requires design flexibility and sometimes compromises on where a visible seam line falls on the part, but it can eliminate the need for any side-action tooling at all.
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Common Mistakes
A common mistake is assuming every undercut automatically requires expensive side-action tooling, without evaluating whether flex-to-release or a repositioned parting line could work instead. Another is applying a flexible lifter to an undercut that’s too deep or tight for the approach, leading to part damage during ejection. A third is overlooking how parting line placement affects visible seams when repositioning it to eliminate an undercut.
How This Connects to Other Molding Topics
Strength considerations often intersect with undercut and rib design; our look at what happens when glass-filled plastics add strength to molded parts covers that related topic, as does our explanation of how weld lines weaken a molded part’s strength.
Turnaround expectations vary with tooling complexity like side actions; our piece on what happens when quoting turnaround times vary between molders is a useful companion resource. And testing a design with these features early matters; our guide to how a prototype mold validates part design before production covers that connection.
Choosing a Molding Partner for Undercut-Heavy Part Designs
A molder experienced in mold injection design should evaluate every undercut for the simplest viable release mechanism rather than defaulting to side actions. For programs near Texas, a plastic injection molding services in Bee Cave partner with tooling design depth can often find a lower-cost path for features that look like they need a slide.
Conclusion
Not every undercut requires the cost and complexity of a full side-action mechanism. Evaluating flex-to-release, collapsible cores, flexible lifters, and parting line placement against the specific part geometry often reveals a simpler, lower-cost path to the same finished feature.
Frequently Asked Questions (FAQs)
Do all undercuts need side-action tooling?
No; shallow or flexible-resin undercuts can sometimes release without a side action, depending on geometry.
What is a collapsible core used for?
It molds internal undercuts, like threads or grooves, by retracting inward on itself before ejection.
When does a flexible lifter work instead of a slide?
For shallow external undercuts where the lifter’s diagonal motion and some part flex allow clean release.
Can repositioning the parting line remove an undercut?
Sometimes; a feature that’s an undercut in one orientation may not be once the parting line shifts.
What determines which approach is right for a part?
Undercut depth, resin flexibility, part tolerance requirements, and tooling budget all factor into the choice.
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