Few design features have a greater impact on injection mold complexity than undercuts.
From a product designer’s perspective, an undercut may appear insignificant:
- A snap hook
- A retention groove
- A side opening
- A locking feature
In CAD, these features are often created in seconds.
In tooling, however, they can fundamentally change how a mold must be built.
A small undercut can introduce:
- Side actions
- Lifters
- Hand-loaded inserts
- Collapsible cores
- Additional shutoffs
- More maintenance requirements
The undercut itself is rarely expensive.
The tooling required to release it often is.
For this reason, experienced mold designers evaluate undercuts differently than product designers.
The first question is not:
“Can this be molded?”
The first question is:
“How will this feature be released?”
Most Undercuts Are Release Problems
An injection mold operates through separation.
The mold opens.
The part ejects.
The cycle repeats.
Undercuts interfere with that process because they create geometry that mechanically traps the part.
Once this happens, the mold requires a strategy to release the feature.
Every undercut solution ultimately serves the same purpose:
Allow the molded part to leave the tool without damage.
The challenge is selecting the simplest and most reliable approach.
Sometimes the Best Solution Is No Mechanism at All
Before considering slides or lifters, we often ask:
Can the geometry itself solve the problem?
Many small undercuts can be eliminated by:
- Adjusting draft
- Moving a feature to the parting line
- Splitting geometry across mold halves
- Modifying retention geometry
One of the most cost-effective tooling solutions is the feature that no longer requires tooling complexity.
Minor design changes frequently eliminate thousands of dollars of mold cost while maintaining the same functional performance.
This is often the first option we explore during DFM review.
Shutoffs Can Replace More Mechanisms Than Many Designers Expect
When an undercut intersects the parting line, shutoffs can often create the feature without requiring moving components.
A shutoff occurs when opposing mold surfaces seal against one another to form geometry that would otherwise require additional tooling mechanisms.
Examples include:
- Side windows
- Open slots
- Retention notches
- Ventilation openings
Compared with side actions, shutoffs are generally:
- Less expensive
- Easier to maintain
- More reliable
Whenever a feature can be achieved with a shutoff instead of a moving mechanism, it is often the preferred solution.
Hand-Loaded Inserts Are Useful in Low Volumes
Not every undercut requires automation.
For prototype tooling and lower-volume production, hand-loaded inserts can provide a practical alternative.
In this approach:
- An insert is manually positioned in the mold.
- Plastic is injected around it.
- The insert is removed before the next cycle.
Hand-loaded inserts avoid the cost of slides and lifters, but they introduce manual labor and longer cycle times.
As production volume increases, their economic advantage often disappears.
This is why they are commonly used for prototypes and bridge tooling rather than high-volume manufacturing.
Side Actions Solve Problems Efficiently—At a Price
Side actions are among the most common solutions for molded undercuts.
A side action moves perpendicular to the mold opening direction.
This motion allows the mold to create geometry that would otherwise lock the part in place.
They are widely used for:
- Side holes
- Snap features
- Retention grooves
- External undercuts
Side actions are reliable and proven.
However, they also introduce:
- Larger mold bases
- Additional machining
- Wear components
- Maintenance requirements
For many projects, the side action itself becomes one of the most expensive elements of the mold.
Lifters Solve Internal Undercuts
External undercuts are often addressed with side actions.
Internal undercuts create a different challenge.
Features such as:
- Internal hooks
- Internal grooves
- Retention tabs
may require lifters.
A lifter moves upward and outward during ejection, allowing the molded feature to clear the undercut.
Lifters are highly effective but generally require:
- Additional space
- Precise alignment
- Increased maintenance attention
Whenever lifters are introduced, mold complexity increases significantly.
Material Flexibility Can Sometimes Replace Tooling
Not every undercut requires a mechanical release mechanism.
As discussed in our previous article on bumpoffs, certain materials can deform temporarily during ejection.
Examples include:
- Polypropylene
- Polyethylene
- TPE
With appropriate geometry, these materials can flex over a small undercut and then recover their shape after ejection.
A successful bump-off design can eliminate:
- Slides
- Lifters
- Additional mold components
The challenge is ensuring that material properties, geometry, and production conditions all support reliable release.
Collapsible Cores Solve Some of the Most Difficult Undercuts
Certain internal geometries cannot be addressed through slides, lifters, or bump-offs.
Examples include:
- Internal threads
- Deep retention grooves
- Circular undercuts
In these situations, collapsible cores may provide a solution.
A collapsible core contracts during ejection, allowing the molded feature to release from the tool.
These systems are highly capable but also among the most complex and expensive tooling options available.
As a result, they are typically reserved for applications where alternative solutions are not practical.
The Cheapest Undercut Is the One You Never Create
One lesson repeatedly observed in tooling projects is that undercuts tend to multiply.
A small retention feature becomes a slide.
The slide requires additional mold space.
The larger mold increases machining time.
Maintenance requirements increase.
Lead time extends.
None of these consequences are visible when the feature is first created in CAD.
This is why undercuts should be evaluated as manufacturing decisions rather than purely geometric decisions.
Every undercut carries a cost.
The goal is ensuring that the value provided by the feature exceeds that cost.
Mold Engineering Perspective
When reviewing undercuts, our decision process is generally:
- Can the undercut be eliminated?
- Can a shutoff create the feature?
- Can material flexibility support a bump-off?
- Can a hand-loaded insert solve the problem?
- Is a side action or lifter required?
- Is a collapsible core justified?
The best solution is usually the simplest one that reliably achieves the design intent.
Complex tooling should be the result of necessity—not the default response.
DFM Checklist: Reviewing Undercuts
Geometry Review
□ Does the feature truly require an undercut?
□ Can the geometry be moved to the parting line?
□ Can draft eliminate the need for a release mechanism?
□ Has feature functionality been challenged?
Tooling Strategy
□ Can a shutoff create the feature?
□ Would a hand-loaded insert be acceptable?
□ Is a side action required?
□ Is a lifter required?
□ Would a bump-off solution be viable?
□ Is a collapsible core justified?
Material Considerations
□ Is the selected material flexible enough for bump-off ejection?
□ Are glass fibers present?
□ Will shrinkage affect release?
□ Is stress whitening a concern?
Production Requirements
□ Is production volume compatible with the proposed solution?
□ Has maintenance been considered?
□ Will cycle time be affected?
□ Is automation required?
Cost Evaluation
□ Does the feature justify the added tooling complexity?
□ Has an alternative design been explored?
□ Can the same function be achieved more simply?
□ Has total manufacturing cost been reviewed?
Final Thoughts
Undercuts are not inherently problematic.
Many successful products rely on them.
The challenge is understanding that every undercut introduces a tooling decision.
Sometimes that decision is a simple shutoff.
Sometimes it is a slide, lifter, bump-off, or collapsible core.
The most successful molded products are not necessarily those without undercuts.
They are the products where each undercut has been intentionally designed, fully justified, and matched with the simplest possible tooling solution.