Undercuts in Injection Molding: Side Actions, Lifters & Collapsible Cores (2026)

An undercut is any feature on a molded part that blocks straight-line ejection — a snap-fit hook, a side hole, an internal groove, a thread, a boss with a retention ring. The part cannot simply be pushed off the core, so the mold must open in more than one direction. Undercuts are the single most common reason a mold quote jumps by thousands of dollars, and they are also the feature most often added by accident. This guide explains how to handle undercuts properly: when to redesign them away for free, and — when they are functionally required — how to pick between side actions, lifters, collapsible cores and shut-offs without overpaying.

What Actually Counts as an Undercut?

Run a draft analysis in your CAD system along the mold-opening direction. Every surface with negative draft is either an undercut or a wall with no draft. Both cost money. The fast test is a single question: can this feature be pulled straight out of the mold? If the answer is no, the tool needs a mechanism.

Undercuts fall into three practical categories:

• External undercuts — side holes, outer snap-fits, external threads. These are reached from outside the part.
• Internal undercuts — inner snap-fits, inward-facing bosses, internal grooves and internal threads. These sit inside the part, away from the mold's opening axis.
• Circumferential undercuts — a continuous groove or ring around the perimeter of a cylindrical part.

Location decides the mechanism. External undercuts are usually handled with a side action; internal undercuts usually need a lifter or a collapsible core; circumferential grooves can sometimes be solved by moving the parting line instead.

First, Try to Eliminate the Undercut for Free

Before accepting any mechanism cost, run the four-step audit that a good DFM analysis performs on every new part:

• 1. Define the parting line and draw direction. Where the two mold halves meet sets what can and cannot be pulled straight out.
• 2. Run draft analysis. Flag every negative-draft surface automatically.
• 3. Challenge each flagged feature. Can you add draft and remove the undercut? Can a shut-off form it instead? Is the feature actually necessary?
• 4. Only then plan a mechanism for the features that survive — and budget for its count.

This is not a theoretical exercise. On a typical first-pass part from an engineer who has not molded before, three to five unintentional undercuts are normal — and the majority can be removed or converted to shut-offs at zero added tooling cost. Changing the parting line is often the cheapest fix of all: a groove that would need a slider can frequently be split across the two mold halves and released by straight ejection.

Undercut Solutions Compared

When an undercut must stay, four mechanisms are available. They differ sharply in cost, cycle time and reliability.

Mechanism	Best for	Typical cost adder	Trade-off
Shut-off / parting-line move	Features that can be split between core and cavity	≈ $0	Constrain surface finish and layout; may leave a witness line
Side action (slider)	External undercuts — side holes, outer clips, external threads	$500–$2,000 per slider; $4,000–$10,000 for complex side holes	More moving parts, larger mold, longer cycle, higher maintenance
Lifter	Internal undercuts — inner snap-fits, ribs, inward bosses	$1,500–$4,000	Geometry limits; wear if angle or cross-section is wrong
Collapsible core	Internal threads, deep internal grooves	$10,000–$25,000	Expensive, tight tolerances, restricts material and geometry
Rotating unscrewing	External threads on cylindrical parts	$8,000–$20,000	Slow cycle; special gear or hydraulic drive

Two rules of thumb save the most money. First, one side action typically adds 20–40% to the affected tooling cost, and a part with four undercuts can double the tool price before a single cavity is cut. Second, an internal undercut is almost always more expensive to tool than an external one — so if a design can flip a mechanism to the outside, it usually pays.

Side Actions: How They Work and How to Design Them

A side action (also called a slider or side core) is a block of steel that slides in perpendicular to the mold-opening direction. It forms the undercut while the mold is closed, then retracts before the part is ejected. Most production tools use an angled pin to drive the slider as the mold opens; complex or heavy undercuts use a hydraulic cylinder for controlled travel.

Key parameters that a competent mold builder will specify:

• Travel — undercut depth plus a 1–2 mm safety margin, every time.
• Angle-pin angle — typically 15–25°. A steeper angle moves the slider faster but raises friction and wear.
• Wear plates and locking blocks — hardened steel wear plates cut friction; locking blocks stop the slider deflecting under injection pressure and flashing.
• Draft — 1–3° on the slider contact surface so the part releases cleanly.
• Cooling — keep cooling lines away from sliding surfaces, or use baffles, so the slider does not warp or bind.

The most common failure modes are all design-driven: insufficient travel scuffs the part, a worn locking surface causes flash, and poor lubrication makes the slider stick. These are avoided at the drawing stage, not on the press.

Lifters: The Tool for Internal Undercuts

A lifter is essentially an angled ejector pin. As the ejector plate pushes forward, the lifter's angle makes it travel sideways as well as upward — a combined motion that releases an internal undercut the part would otherwise stay locked onto.

Lifter design has hard limits that buyers should understand:

• Angle: the working range is roughly 5–15° from vertical. Below 5° there is too little lateral travel; above 15° wear on the lifter rail and core pocket rises sharply, risking jams.
• Travel: allow at least 2 mm of movement beyond the undercut depth to prevent drag marks and scratches.
• Cross-section: size the lifter to carry at least 2× the expected ejection force — an undersized lifter will gall and seize.
• Material: specify hardened steel such as H13 (around 50–54 HRC) for production tools, and support the lifter base with a guide block to stop vibration and flash.

Because lifters move with the ejection system, they are self-timed and need no separate hydraulic circuit — which is exactly why they are the default choice for internal clips, bosses and inward-facing retention rings when space is tight.

Silicone and Elastomer Parts: A Special Case

For LSR and silicone parts, flexible material changes the calculation. A soft elastomer can often deform enough to strip off a shallow undercut during ejection with no mechanism at all — a "bump-off" feature. This only works for small, simple, shallow undercuts and low-to-medium volumes; deeper or tight-tolerance undercuts still need a steel mechanism. If you are molding silicone, ask your supplier to confirm whether a given undercut can bump off before paying for a slider.

How Undercuts Change the Mold Economics

Undercuts raise cost in four separate ways, not just one:

• Tooling — each mechanism is extra design, machining and assembly hours, plus hardened components.
• Cycle time — every extra mold motion (slider travel, unscrewing) adds seconds to each shot, and seconds multiply over millions of cycles.
• Maintenance — moving parts wear. Sliders need wear plates replaced; lifters need correct material and lubrication; a seized mechanism can halt a line.
• Risk — a mis-timed or under-sized mechanism can damage the part, leave flash, or stop the press, and the fix is rarely cheap.

For a part that will run for years, the mechanism's reliability matters more than its purchase price. A $1,500 lifter that gallops and seizes costs far more in downtime than a correctly sized $3,000 one. This is one of the clearest reasons to buy tooling from a supplier who has actually run the mechanism in production — see our guide on choosing a mold supplier in China.

Undercut Design Checklist

• ☐ Have you run a full draft analysis in the mold-opening direction?
• ☐ Can each flagged feature be redesigned with draft, or converted to a shut-off, at zero tooling cost?
• ☐ Can the parting line move to split a groove between core and cavity?
• ☐ Is the undercut external (→ slider) or internal (→ lifter / collapsible core)?
• ☐ Is 1–3° draft specified on all mechanism contact surfaces?
• ☐ Does slider travel equal undercut depth + 1–2 mm?
• ☐ Is the lifter angle within 5–15° with ≥2 mm excess travel?
• ☐ Is the lifter cross-section sized for at least 2× ejection force?
• ☐ For silicone parts: has bump-off been confirmed before paying for a slider?
• ☐ Does your quote itemise the number of mechanisms, so nothing is hidden?

Need Help With a Difficult Feature?

Precimold designs and builds injection molds with side actions, lifters and collapsible cores for plastic and silicone parts — from simple open-and-shut tools to multi-slider production molds. Send us your 3D model and annual volume, and our engineering team will flag every undercut, tell you which ones can be eliminated for free, and quote the mechanisms that genuinely need to stay. We reply within 12 hours.