Undercuts are often viewed as one of the most expensive features in injection molded parts.
And for good reason.
A relatively small undercut can trigger the need for:
- Side actions
- Lifters
- Collapsible cores
- Manual inserts
- Complex mold mechanisms
In many cases, the tooling required to release the undercut costs significantly more than the feature itself.
However, not every undercut requires a moving mold component.
Some small undercuts can be released by allowing the plastic part to deform slightly during ejection and then return to its original shape.
This technique is known as a bumpoff. It allows certain undercuts to be molded in a straight-pull tool without side actions or lifters.
The challenge is knowing when it will work—and when it will not.
Most Undercut Decisions Are Cost Decisions
When reviewing a molded part, engineers often ask:
“Can this undercut be molded?”
In reality, that is rarely the most useful question.
Almost any undercut can be molded if enough tooling complexity is added.
The better question is:
“What is the simplest way to release this feature?”
For example, a small retention groove may be achievable through:
- A side action
- A lifter
- A collapsible core
- A hand-loaded insert
- A bumpoff design
All five approaches may produce the same finished part.
The difference is the impact on tooling cost, maintenance requirements, lead time, and long-term production reliability.
This is why experienced mold designers evaluate undercuts from a tooling perspective before selecting a molding strategy.
A Bumpoff Relies on Material Flexibility
The concept behind a bumpoff is relatively simple.
During ejection, the molded plastic temporarily stretches or flexes enough to pass over the mold feature that created the undercut.
Once released, the material returns to its original shape. Protolabs compares this behavior to a snap feature or plastic closure that flexes during use and then recovers without permanent deformation.
The success of a bumpoff therefore depends heavily on the material.
Materials commonly suited for bumpoff designs include:
- TPE
- Polyethylene (PE)
- Polypropylene (PP)
These materials possess sufficient flexibility to deform during ejection and recover afterward.
More rigid materials typically create problems.
Examples include:
- Glass-filled nylon
- Glass-filled polypropylene
- Polycarbonate
- Highly filled engineering resins
These materials often lack the flexibility required for reliable bump-off ejection and may crack, whiten, or permanently deform.
Undercut Shape Matters More Than Undercut Depth
One of the biggest misconceptions about bumpoffs is that success depends primarily on undercut size.
In practice, geometry is usually more important.
A properly designed bumpoff behaves like a speed bump.
A poorly designed bumpoff behaves like a hook.
To eject successfully, the leading edge of the undercut should contain:
- A ramp
- A generous radius
- A smooth transition
Sharp corners and hook-like geometries tend to lock into the mold and resist release. These designs often prevent ejection entirely or damage the molded feature during demolding.
In many projects, a small geometric modification eliminates the need for a side action altogether.
The Surrounding Geometry Determines Success
A bumpoff is never evaluated in isolation.
The surrounding part geometry plays a major role.
Once the mold opens, the part must have enough freedom to flex.
Features that can reduce flexibility include:
- Thick walls
- Heavy ribs
- Large bosses
- Reinforced structures
- Glass-filled materials
A flexible retention groove on a thin cylindrical wall may eject successfully.
The same groove embedded within a rigid structural housing may not deform enough to release.
This is why bumpoff feasibility is usually assessed at the part level rather than by looking only at the undercut itself.
A Side Action Is Not Automatically the Better Solution
When engineers discover an undercut, the default response is often:
“Let’s add a slide.”
Sometimes that is absolutely the correct decision.
However, side actions introduce their own costs:
- Increased tooling cost
- Longer build times
- Additional maintenance
- Larger mold bases
- More potential wear points
Experienced toolmakers frequently evaluate whether a small design change can eliminate the slide before committing to a more complex mold design.
A successful bumpoff can often remove an entire mechanism from the tool.
For high-volume production, this can have a meaningful impact on mold cost and long-term reliability.
Process Window Matters More Than Many Designers Expect
Even when a bumpoff design works in principle, production stability remains important.
The feature may eject successfully under ideal conditions while becoming unreliable when processing conditions drift.
Factors that influence bumpoff performance include:
- Mold temperature
- Cooling time
- Packing pressure
- Material batch variation
- Part shrinkage
Production molders frequently observe that parts with bump-off features can become sensitive to shrinkage and cooling behavior, particularly when the design margin is small.
A design that requires the material to deform excessively may have a narrow processing window and become difficult to manufacture consistently.
Mold Engineering Perspective
When evaluating an undercut, our first question is rarely:
“Can we mold this?”
Instead, we ask:
- Can the undercut be eliminated?
- Can it be redesigned as a bumpoff?
- Can material flexibility support reliable ejection?
- Is a side action truly necessary?
If a bump-off solution can achieve the same function without introducing additional mold mechanisms, it is often the most efficient option.
The best undercut is not the one with the most sophisticated tooling.
It is the one that achieves the design intent with the least manufacturing complexity.
DFM Checklist: Reviewing Bumpoff Features
Undercut Geometry
□ Does the undercut include a ramp or radius rather than a hook?
□ Are sharp edges eliminated?
□ Is the transition smooth enough to support deformation?
□ Has undercut depth been evaluated relative to material flexibility?
Material Selection
□ Is the selected resin capable of elastic deformation?
□ Is the material excessively brittle?
□ Are glass fibers or mineral fillers present?
□ Can the material recover after ejection?
Part Design
□ Is there sufficient wall flexibility around the feature?
□ Do ribs or bosses restrict deformation?
□ Will the surrounding geometry support repeated production?
□ Can the undercut be simplified?
Tooling Strategy
□ Can the feature be produced without a side action?
□ Would a lifter or collapsible core provide a better solution?
□ Is the tooling cost justified by the feature?
□ Has long-term mold maintenance been considered?
Manufacturing Review
□ Is the process window robust?
□ Will shrinkage affect ejection performance?
□ Is cooling balanced around the feature?
□ Can the design tolerate normal process variation?
Final Thoughts
Many engineers assume that every undercut requires a slide, lifter, or other complex mold mechanism.
In reality, some undercuts can be released through controlled material deformation using a bumpoff design.
The success of this approach depends on material flexibility, feature geometry, surrounding part structure, and process stability.
When applied appropriately, bumpoffs can eliminate tooling complexity, reduce mold cost, and simplify production without compromising part function.
Like many injection molding challenges, the most effective solution is often not the most