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 frequently the cause. One supplier identified a feature requiring a moving mechanism in the mold and priced it accordingly; the other missed it or assumed a design workaround.

This guide explains how molds handle undercuts, why they add tooling cost and risk, and how to identify which undercuts are functionally necessary versus those that are simply artifacts of the CAD drawing. (See the design for manufacturing guide for broader design context).

What an Undercut Is

An undercut is any feature that prevents the part from being pulled straight out of the mold in the primary draw direction. Because a standard mold opens along a single axis, the part must clear the steel as the halves separate. Any feature that hooks behind, wraps around, or projects laterally relative to that opening direction will be “undercut” by the steel.

Common examples include snap-fit hooks, side holes, side-action ports, threads, and internal clips. While often necessary for product function, each undercut forces the tool to execute a movement beyond a simple open-and-eject.

How Molds Handle Undercuts

When a feature cannot release in a straight pull, the mold requires a mechanism that moves steel out of the way before the part ejects. The two most common are slides and lifters:

• Slides (side actions): These move in from the side of the mold to form an external undercut (like a side hole or external clip) and retract laterally as the tool opens.
• Lifters: These pull at an angled trajectory during ejection to clear an internal undercut (like an 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 require multiple slides and lifters, each adding cost and acting as a potential point of failure.

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

The financial impact of an undercut extends beyond the initial quote. It affects the total lifecycle of the tool:

• Tooling Cost & Lead Time: Each slide or lifter requires additional design and precise machining. More mechanisms equate to a more expensive, slower-to-build mold.
• Maintenance & Wear: Moving components require lubrication and wear out over time. A tool with heavy action requires more frequent upkeep and runs a higher risk of downtime.
• Cosmetic Impact: Slides and lifters create their own parting lines where they meet the main cavity steel. If a slide is required on a cosmetic A-surface, you must manage the resulting witness mark. (See parting lines).

Necessary vs. Accidental Undercuts

The most valuable distinction a buyer can make is between functional undercuts and accidental ones:

• Functional Undercuts: These exist because the product requires them—a port that must pass a connector or a thread that must engage. The objective is to mold them reliably, not eliminate them.
• Avoidable Undercuts: These exist merely because of how the CAD was modeled. A recess that could be opened to the draw direction, a hook that could be reoriented, or a feature that could be split differently. These add tooling cost for zero functional benefit.

The Alternative: Stripping the Undercut (Bump-Off)

There is a third option between a straight pull and a mechanical slide: allowing the plastic to flex over the steel during ejection. This is called a “bump-off” or stripped undercut. It requires no moving mechanisms, saving cost and maintenance.

Whether a feature can be stripped depends entirely on the resin’s elasticity and the geometry. Eastman’s mold design guidelines suggest that stripped undercuts (like snap rings or threads) can be achieved if the undercut is limited to 2 to 3 percent of the part diameter on relatively thin-walled parts, provided the feature is heavily radiused and well-filleted so the plastic can ride over the steel rather than catching on a sharp edge.

This percentage limit means a small undercut on a large diameter is more likely to strip successfully than the same-sized undercut on a small diameter. Material stiffness is the ultimate gatekeeper: a soft TPU (which BASF notes can endure short-term overstretching up to 5 percent without permanent deformation) will easily strip an undercut that would immediately fracture a rigid, glass-filled polycarbonate.

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

Communicating Undercuts in the RFQ

Surface undercuts explicitly during the quoting process. In your RFQ, point out the features you know require side action or lifters, note which are functionally critical, and explicitly invite the supplier to suggest geometry changes that would simplify the tool. This framing (“Here is what must stay; here is where I am flexible”) yields more accurate quotes and actionable design feedback. (See the RFQ template).

Buyer FAQs

What is an undercut in injection molding?

An undercut is any feature—like a side hole, snap hook, or internal thread—that prevents a molded part from ejecting straight out of the mold. It requires the mold to have moving mechanisms to release the part.

How do slides and lifters work?

A slide moves in from the side to form an external undercut and retracts as the mold opens. A lifter moves at an angle during the ejection stroke to clear an internal undercut. Both are moving steel components that add cost and maintenance.

Why do undercuts increase mold cost?

Each undercut requires a dedicated mechanism (a slide or lifter) to be engineered, machined, and fitted into the mold base. This adds upfront build cost and ongoing maintenance overhead.

Can undercuts be designed out?

Often, yes. Reorienting a feature, extending a recess to the parting line, or changing how the part splits can sometimes turn an undercut into a straight pull. If the feature is small and the resin is flexible enough, it may also be “stripped” over the core without a mechanism.

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.