Undercuts, Slides, and Lifters in Injection Molding: What They Cost You

When two suppliers quote the same part and the prices are far apart, undercuts are often why. One shop saw a feature that needs a moving mechanism and priced it. The other missed it or assumed a design workaround.

Know how molds handle undercuts, what they cost over the tool life, and which undercuts are functional versus CAD accidents. Broader context sits in the design for manufacturing guide.

What an Undercut Is

An undercut is any feature that stops the part from pulling straight out of the mold in the primary draw direction. A standard mold opens on one axis, so the part must clear the steel as the halves separate. Anything that hooks behind, wraps around, or projects sideways relative to that opening is undercut by the steel.

Common examples: snap-fit hooks, side holes, side-action ports, threads, and internal clips. Many are needed for function. Each one forces the tool to move beyond a simple open-and-eject.

How Molds Handle Undercuts

When a feature cannot release in a straight pull, the mold needs a mechanism that moves steel out of the way before ejection. The two common ones:

• Slides (side actions): Move in from the side to form an external undercut (side hole or external clip) and retract laterally as the tool opens.
• Lifters: Pull on an angled path during ejection to clear an internal undercut (internal snap hook) before releasing the part.

These are mechanical assemblies: moving steel driven by the tool’s actuation that must be engineered, machined, fitted, and maintained. A complex part may need multiple slides and lifters. Each adds cost and a wear/failure point.

Mechanism	What it forms	Typical application	Cost & Risk Impact
Slide (side action)	External undercuts	Side holes, external clips, ports	Adds significant tooling cost, maintenance, and leaves a witness line on the exterior.
Lifter	Internal undercuts	Internal snaps, recesses	Adds tooling cost, requires lubrication, can leave witness marks on internal faces.
Design it out	N/A	Undercuts created by arbitrary design	Eliminates mechanism cost and risk.

The True Cost of an Undercut

Cost is not only the initial quote. It spans the tool life:

• Tooling Cost & Lead Time: Each slide or lifter needs more design and precise machining. More mechanisms mean a more expensive, slower-to-build mold.
• Maintenance & Wear: Moving components need lubrication and wear out. Heavy action means more upkeep and higher downtime risk.
• Cosmetic Impact: Slides and lifters create parting lines where they meet cavity steel. A slide on a cosmetic A-surface means managing a witness mark. See parting lines.

Necessary vs. Accidental Undercuts

Separate functional undercuts from accidental ones:

• Functional Undercuts: The product needs them (a port for a connector, a thread that must engage). Goal: mold them reliably, not erase them.
• Avoidable Undercuts: They exist because of how the CAD was modeled. A recess that could open to the draw, a hook that could be reoriented, or a different split. These add tooling cost with no functional benefit.

The Alternative: Stripping the Undercut (Bump-Off)

Between a straight pull and a mechanical slide sits another option: let the plastic flex over the steel during ejection. That is a “bump-off” or stripped undercut. No moving mechanisms, so lower cost and maintenance.

Whether a feature can strip depends on resin elasticity and geometry. Eastman’s mold design guidelines suggest stripped undercuts (snap rings or threads) can work if the undercut stays within 2 to 3 percent of the part diameter on relatively thin-walled parts, and the feature is heavily radiused and well-filleted so plastic rides over steel instead of catching a sharp edge.

That percentage limit means a small undercut on a large diameter is more likely to strip than the same undercut on a small diameter. Material stiffness is the gatekeeper: a soft TPU (BASF notes short-term overstretch up to about 5 percent without permanent deformation) will strip an undercut that would fracture a rigid, glass-filled polycarbonate.

When you review a minor undercut, ask: “Can this be stripped, and what wall thickness or resin grade is required for that to work?”

Communicating Undercuts in the RFQ

Surface undercuts in the quote package. Point out features you know need side action or lifters, mark which are functionally critical, and invite geometry changes that simplify the tool. “Here is what must stay; here is where I am flexible” produces more accurate quotes and usable design feedback. See the RFQ template.

Buyer FAQs

What is an undercut in injection molding?

Any feature (side hole, snap hook, internal thread) that prevents straight ejection. It usually needs a moving mold mechanism. In the RFQ, list known undercuts and whether each is functional or open to redesign so quotes are comparable.

How do slides and lifters work?

A slide forms an external undercut from the side and retracts as the mold opens. A lifter clears an internal undercut on an angled path during ejection. Both add cost and maintenance. Ask which mechanism each undercut needs before you compare quotes.

Why do undercuts increase mold cost?

Each undercut needs a dedicated slide or lifter engineered, machined, and fitted into the mold base. That raises build cost and ongoing maintenance. If a quote is silent on actions you can see in the CAD, treat the quote as incomplete.

Can undercuts be designed out?

Often yes. Reorient a feature, open a recess to the parting line, or change the split to make a straight pull. If the feature is small and the resin flexible enough, it may strip over the core without a mechanism. Require a written strip-vs-action recommendation before locking the tool concept.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, provide engineering services, or certify suppliers. Design and tooling decisions are part-specific; confirm them through your supplier’s engineering review.