Injection Molding Undercuts: Types, Fixes and Design Checks

An undercut in injection molding is any feature that stops a solidified part from sliding straight out of the mold as it opens. Hooks, snap-fits, side holes, internal grooves and threads are the usual culprits. You have two options: redesign the feature so the mold opens cleanly, or add a mechanism to the tool (a slide, lifter, unscrewing core or collapsible core) that moves out of the way before ejection. The first costs you a design change. The second costs you tooling money, cycle time and maintenance for the life of the mold.

This guide shows how to tell which option fits your part, how each mechanism works, and which design checks to run before you release a CAD file for tooling.

Key Takeaways

• An undercut is a feature that blocks straight ejection along the mold’s opening direction, so it needs either a design change or a tool mechanism.
• Redesigning the part, often by moving the parting line or opening a window through the wall, is usually cheaper than building a mechanism into the tool.
• Slides and lifters handle most external and internal features. Unscrewing mechanisms and collapsible cores are for threads and round internal undercuts.
• Small, shallow undercuts in flexible materials can sometimes be released by bump-off ejection with no side action, but only after checking geometry and material.
• Ask your molder for a design-for-manufacturability review before you cut steel. Every mechanism you remove is one less thing to wear out.

Table of Contents

• What an undercut is and why it blocks ejection
• Four ways to form an undercut
• What undercuts do to cost and cycle time
• Redesign first: ways to design the undercut out
• Design checks before you release the file
• Prototype before you commit to tooling
• Our perspective
• Get your undercuts reviewed
• FAQ
• Sources

What an undercut is and why it blocks ejection

A mold splits along a parting line, and the two halves open in a single direction, the pull direction. Anything that sticks out sideways or dips inward relative to that direction can lock the part into one half. Material suppliers describe undercuts as features that prevent straight ejection at the parting line. The general definition is the same: an indentation or protrusion that stops the part from being withdrawn from a one-piece mold.

Think of a pen cap with a small bump inside. The cap can only leave the core if that bump has somewhere to go. Either the bump deforms, the core moves, or the bump does not exist.

External and internal undercuts

The two categories are simple. External undercuts sit on the outside of the part and internal undercuts on the inside. The distinction matters because it decides which part of the tool has to move.

Four ways to form an undercut

When redesign is not possible, you choose a mechanism. Trade-press reviews of undercut molding methods cover unscrewing mechanisms and collapsible cores in detail, using closures and threaded parts as examples. Here is how the main options compare in practice.

Slides (side actions)

A slide is a block that moves perpendicular to the pull direction, driven by an angled pin or a hydraulic cylinder. It forms an external feature, such as a side hole or a recess, while the mold is closed, then retracts before the part is ejected. Slides are the workhorse solution for side features. They add steel, machining time, wear surfaces and a point that needs lubrication and inspection. They also need room around the tool, so the mold gets larger.

Slides can also be built inside the part as inner sliders, forming holes or windows from within. These need extra clearance and careful timing so they retract before the part moves.

Lifters

A lifter is an angled rod or blade that rides with the ejector system. As the part is pushed off the core, the lifter moves up and sideways at once, pulling its hook or tab out of the undercut. Lifters suit internal features like snap hooks on the inside wall of a housing. They are compact, but they need enough room inside the part, and the angle of travel must clear the feature completely. A tight, deep undercut with a thin lifter is a recipe for breakage.

Unscrewing mechanisms

Threads that run all the way around a part cannot be pulled straight off. An unscrewing core rotates out of the part, driven by a motor, rack or gear train, before ejection. This is the standard route for high-quality closures and threaded fittings. It is a complex mechanism, so it is typically reserved for production tools where the volume justifies the build. For a prototype or low-volume part, consider whether the thread can be molded in a simpler way or machined afterwards.

Collapsible cores

A collapsible core has segments that fold inward after molding, letting the core shrink and clear a round internal undercut. It is the answer when an inside groove runs around a bore and a lifter will not fit. The trade-off is cost and care: collapsible cores are intricate, and they can leave witness lines where the segments meet. The trade press notes challenges specific to this approach, so discuss it early with your molder.

Bump-off ejection

Sometimes no mechanism is needed. Small undercuts can be released by forcing the part off, letting it deform temporarily during ejection, depending on the geometry and the material. This is called bump-off or stripping. It works for shallow, rounded features in materials with enough flexibility, and it depends on the plastic, wall thickness and the shape of the lead-in. It does not work in rigid, glass-filled or brittle resins, where the feature will crack, whiten or tear.

What undercuts do to cost and cycle time

Every mechanism adds complexity. Material suppliers advise that undercuts increase construction and maintenance costs. Cams add cost and complexity and can affect cycle time. Removable inserts, which an operator loads by hand or a robot places, can lengthen the cycle further.

This translates into a few practical consequences for a buyer:

• Higher tooling quote. More machined components, more fitting time, a bigger mold base.
• Longer lead time. More parts to design, cut, polish and test during trials.
• Slower cycles in some cases. A slide must travel, a lifter must clear, an insert must be loaded. Per-part cost rises with every second added.
• More maintenance. Moving components wear, can flash at their shut-off surfaces and need periodic service.

We do not publish tooling prices because they depend on part geometry, material, cavity count and tolerance, so the only reliable number is a quote on your actual file. What we can say is that two versions of the same part, one with a mechanism and one without, will not price alike, and it is worth asking for both.

Redesign first: ways to design the undercut out

The strongest advice from material makers is blunt: designing the part to avoid undercuts is preferred. Two common moves are repositioning the parting line and extending an opening through the surface. Below are the ones we see work most often.

1. Move the parting line. If a ridge or lip sits just above or below the split, shifting the line to the feature’s widest point can release it without side action.
2. Open a window through the wall. A snap hook inside a housing often needs a lifter. If you add a rectangular opening in the wall directly above the hook, a core can form it from the other half of the mold, along the pull direction. This is the classic through-core or shut-off approach.
3. Turn a hole into a slot that opens to the parting line. Side holes become notches the mold halves can form themselves.
4. Split the part in two. Two simple parts that snap, screw or weld together can be cheaper than one complex part with three slides.
5. Use a different joining feature. A living hinge, a screw boss or an ultrasonic weld joint may replace a snap-fit that needed an undercut.
6. Accept a small bump-off feature. Where the material and geometry allow it, a shallow rounded lip can pass without any mechanism.

We go deeper on these moves in our post on four DFM moves that design out undercuts and cut China mold costs.

Design checks before you release the file

Use this list on every part with a feature that looks like it might trap the tool.

1. Find the pull direction. Decide which way the mold opens, then check every surface for anything that faces sideways or folds back. Many CAD packages have a draft or undercut analysis that colors trapped surfaces.
2. Classify each flagged feature. Is it external, internal or threaded? Is it essential to function, or cosmetic?
3. Ask whether it can be removed. Try the redesign options above on each feature before accepting a mechanism.
4. Check draft. Vertical walls that lack draft can lock a part even when no true undercut exists. Our guide to injection molding draft angles explains why a simple rule of thumb is only a start.
5. Check the material. A flexible resin may allow bump-off. A rigid or filled one will not. Compare options in our injection molding materials list.
6. Check space for the mechanism. A slide needs room outside the part. A lifter needs room inside it. If neither fits, the design must change.
7. Check wall thickness around the feature. Thin walls near a forced release can tear or whiten. Heavy sections near a slide can sink.
8. Check the cosmetic impact. Slides and lifters leave parting lines and shut-off marks. Make sure they do not land on a visible surface.
9. Confirm part count and volume. A mechanism that pays for itself at 100,000 parts may be wasteful at 500.
10. Request a DFM report. A qualified molder will mark trapped surfaces, propose alternatives and quote the options before steel is cut.

A worked example

Take a plastic enclosure with four snap hooks inside the wall, which latch into a lid. As drawn, each hook points inward and needs a lifter, so the tool carries four lifter assemblies. Cut a small rectangular window in the wall directly under each hook and the tool’s core can form the hook from the other side. The window is hidden under the lid rim, the lifters disappear, and the quote gets simpler. The trade-off is a small opening to manage for dust or moisture, and that is a design decision only you can make.

Prototype before you commit to tooling

Undercuts are where design assumptions fail most often. A printed or cast prototype lets you test a snap-fit’s engagement, the strength of a hook or the fit of a threaded part before you pay for steel. For a quick look at form and fit, SLA 3D printing and SLS 3D printing can produce features that a mold could not. Keep in mind that a printed part can include undercuts without penalty, so a design that works in print may still need rework for molding. Use the print to validate function, then run the molding DFM review.

If you are sourcing from China, our post on supplier questions for Chinese molders about undercuts lists what to ask about mechanisms, steel, maintenance and trial parts.

Our perspective

We see undercut problems in nearly every project that arrives with a first-time design. The pattern is consistent: a part designed in CAD for how it looks and assembles, with no thought for how a mold opens. The fix is rarely heroic. Most of the time, a conversation between our engineers and yours about moving a split line or adding a window removes a slide and the cost that goes with it.

Our view is simple. A mechanism should earn its place. If a feature is functionally essential, we build the slide, lifter or unscrewing core and we build it to last. If it is a habit rather than a need, we say so before you pay for it. We have no minimum order quantity, so you can check a design change on a small run before committing to a larger one, and a dedicated project manager keeps your engineers and ours talking while the tool is designed.

One piece of advice we repeat often: send us the file while the design can still change. A DFM comment on a draft model costs you an afternoon. The same comment after tool steel is cut costs weeks.

— Nas

Get your undercuts reviewed

If you have a part with hooks, side holes, threads or internal grooves, upload it to our injection molding service for an instant quote and an engineer’s review. We will flag trapped surfaces, suggest alternatives where they make sense, and quote the mechanisms that cannot be avoided. For process background, see our step-by-step injection molding guide.

FAQ

Can you injection mold an undercut without a slide?

Sometimes. Small, shallow undercuts in flexible materials may release by bump-off, where the part deforms briefly during ejection. You can also redesign the feature by moving the parting line or adding an opening through the wall. Rigid or filled resins usually need one of those redesigns or a mechanism.

What is the difference between a slide and a lifter?

A slide moves sideways, perpendicular to the mold opening, and normally forms external features. A lifter is an angled rod that moves with the ejector system and usually releases features inside the part. Both retract the feature before or during ejection so the part can leave the tool.

How do undercuts affect injection mold cost?

They add machined components, fitting time and maintenance, and some mechanisms add to cycle time. Cams and removable inserts are named in supplier guidance as sources of extra cost or longer cycles. Request quotes for the design with and without the undercut to see the difference on your own part.

How do you mold threads on a plastic part?

External or internal threads are often formed with an unscrewing mechanism that rotates the core out of the part before ejection. Where volume is low, you can sometimes redesign the thread to be split at the parting line or add the thread in a secondary operation. Your molder can advise based on thread size and quantity.

Sources

• Undercuts, Envalior
• Undercut (molding), Wikipedia
• Best Methods of Molding Undercuts, Plastics Technology

Recommended

• Supplier Questions for Chinese Molders on Injection Molding Undercuts
• Injection Molding Draft Angles: 1° per Inch Is Only a Start
• Slash China Mold Costs: 4 DFM Moves to Design Out Undercuts
• Mastering Injection Molding Workflow for Precision Prototyping