What an undercut actually blocks

An undercut is any feature that stops a part from leaving the mold in a straight line. A side hole, a snap hook, an internal thread, and a ring groove all qualify. The mold cannot simply open and push the part out. Some steel must move out of the way first.

That extra motion is where the money goes. It adds steel, machining, fitting, and wear parts to the tool. It also adds a step to every cycle. A buyer who ignores the undercut at RFQ stage usually finds it later as a price increase.

One fact sets the frame. Every undercut needs a moving steel solution, a design change, or a second operation. Only the first two keep unit cost down at volume. Injection molding undercuts show up on housings, caps, and fittings alike.

Start by naming the undercut in plain terms. Is it on the outside or the inside of the part? Does it run all the way around, or only on one side? How deep is it, in millimeters? Those three answers narrow the mechanism before any quote is compared. For background on what a mold maker needs from you at this stage, see our guide to mold design information.

Slides for external undercuts

A slide is a side-action core that moves sideways before ejection. Mold opening drives it through an angle pin, a cam, or a hydraulic cylinder. It forms side holes, windows, hooks, and grooves that face outward.

The slide must travel far enough to clear the feature, plus a small safety margin. If it stops short, the part drags across the steel and scratches. If it locks weakly, injection pressure pushes it back and causes flash.

Slides cost the most of the two common side actions. One published comparison puts a single slide group at two to four times the cost of a lifter group. The gap comes from parts count: slide body, wear plate, locking wedge, angle pin, and a larger mold base.

Slides also demand the most care in service. Guides wear, lubrication dries out, and debris collects in the slide pocket. A slide that runs dry will gall and score the parting surface. Budget for inspection of wear plates and guide surfaces at set intervals, not only when parts start to fail.

One fact matters for slide buyers. A slide needs positive locking against injection pressure. An angle pin alone cannot hold it shut.

Slides still win when the feature is large, external, and hard to reach any other way. A long side window or a deep external hook leaves little choice. In that case, pay for the slide and protect it with a maintenance plan. Our mold structure guide explains where side actions sit inside the tool.

Lifters for internal undercuts

A lifter rides with the ejector system at an angle. As the ejector plate moves forward, the lifter head moves forward and sideways at once. That combined motion pulls the head out of an internal hook, snap, or pocket.

Lifters suit small internal undercuts. Published design guidance puts the practical limit near 6 mm of undercut depth for both lifters and slides. Beyond that depth, the lifter head travels too far and starts to drag on the part.

Lifter angles commonly run from 5 to 15 degrees. A shallow angle gives gentle sideways motion but needs longer ejection travel. A steep angle clears faster but puts more side load on the guide block. The mold maker must balance travel, strength, and clearance for each feature.

Lifters cost less than slides because they use fewer parts. A lifter group is mainly a head, a rod, and a guide block. It needs no larger mold base and no separate locking wedge. Tuning is also simpler: adjust ejection travel and clearance, then recheck the part.

One fact matters for lifter buyers. A lifter asked to clear a deep undercut will scratch parts. Its sideways motion grows with travel.

Watch one failure mode closely. If the lifter head is too thin for the resin, it bends or breaks under ejection load. Glass-filled resins raise this risk because they grip the steel harder. Ask how the head thickness was sized for your resin and your undercut depth.

Collapsible cores for round internal features

A collapsible core solves a different problem: a full-circle internal undercut. Think of an internal thread, a bottle-cap thread, or a ring groove inside a fitting. No single lifter can release a feature that wraps all the way around.

The core is built from segments around a tapered center pin. During molding, the segments lock together into a solid core. During ejection, the pin withdraws and the segments collapse inward. The outside diameter shrinks, and the part slides off cleanly.

Published guidance puts collapsible-core capacity near 20 mm of undercut depth. That is roughly three times the practical lifter limit. Standard cores come in diameters from 25 to 90 mm, with collapse of 1.20 to 3.75 mm per side. Smaller mini-cores cover 13 to 24 mm diameters, with collapse of 1.32 to 1.50 mm per side.

Collapsible cores also beat unscrewing molds on cycle time. Plastics Technology reports reductions of as much as 30 percent against unscrewing mechanisms. The core collapses inside the normal mold sequence, so no separate rotation step is needed. See the Plastics Technology guide to molding undercuts for the mechanism in detail.

One fact matters for collapsible-core buyers. A collapsible core trades a higher tool price for a full-circle release. No lifter can copy that release.

Two limits deserve attention. First, segment fit must stay tight or flash appears on the thread. Head space between the core flange and the pin flange must be held exactly, or the core can jam or break. Second, mini-cores suit interrupted threads better than a full 360-degree thread on very small diameters. Confirm the collapse per side against your thread depth before approving the design.

Injection molding undercuts cost compared: tooling, parts, and upkeep

Tooling cost is only the first line of the bill. A fair comparison covers tooling, cycle time, scrap, and maintenance across the whole production run. A cheap mechanism that slows every cycle can cost more than a dear one.

Use rough bands, not false precision, when comparing quotes. Our own design checker notes that each slider or lifter typically adds 5 to 15 percent to mold cost. A slide group can run two to four times a lifter group for the same size feature. Those bands move with part size, cavity count, and steel choice.

Volume changes the answer. One published comparison of forced release against a full slider solution found a break-even near 90,000 parts. Below that volume, the simpler method won on unit cost. Above it, the slider won. Your volumes, scrap rate, and labor cost will move that line.

Maintenance belongs in the quote comparison too. Slides need guide and wear-plate attention. Lifters need head and guide-block checks. Collapsible cores need segment fit and pin condition checks. Ask each supplier what spare parts they include and what a single repair visit costs.

One fact matters for quote comparisons. The lowest tool price wins only when cycle time, scrap, and repair cost stay equal. They rarely do.

How to choose without overpaying

Match the mechanism to the feature, not to the habit of the shop. A shop that builds slides all day will suggest a slide. A shop that favors lifters will suggest a lifter. Your part geometry should decide.

Try design changes before accepting any mechanism. Move a hole to the parting line if the design allows it. Open a relief window behind a snap so the core can pass through. Split a feature into an insert or a second part. Each change that removes a moving action cuts tooling cost and cycle risk at once.

For external features, ask whether a slide is needed at all or a parting-line shift will do. For internal hooks, check lifter travel against the undercut depth on paper first. For round internal features, compare a collapsible core against an unscrewing mold and against a design change. Do not let a supplier skip that comparison.

Resin choice also matters. Flexible resins such as PP and PE can sometimes strip off a shallow undercut without any mechanism. Stiff or glass-filled resins cannot. If your resin is flexible and the undercut is shallow, ask whether forced release was ruled out for a stated reason.

One fact matters for design reviews. Removing an undercut beats any mechanism. The cheapest side action is the one never built. Injection molding undercuts reward early design changes more than late tooling fixes.

For a second view on slide design and upkeep, see the slide side-action guide from Cavity Mold. It covers locking, venting, and wear surfaces in practical terms.

What to put in your RFQ

A vague RFQ produces vague quotes. Suppliers will price different mechanisms, and you will compare totals that hide different risks. Put the undercut facts in writing instead.

List each undercut with its location, direction, depth, and the resin you plan to run. State the annual volume and the expected mold life. State the cosmetic rules for the area near the undercut. Ask each supplier to name the mechanism they priced for each feature.

Ask for three numbers per mechanism: tooling adder, cycle-time effect, and included spares. Ask how the mechanism is timed with ejection and mold opening. Ask what happens if the mechanism fails mid-run: does the mold stop cleanly, or does it risk damage?

Also ask about inspection. How is slide travel or lifter clearance checked at T1? How is collapsible-core collapse measured? A supplier with a clear check at T1 will catch a travel error before it becomes a production defect.

One fact matters for RFQ writers. A quote that names no mechanism for an undercut is not a lower price. It is an unfinished quote.

For a sense of how tooling choices move the total project price, see our injection molding cost overview. Bring the undercut list to your quote review, and ask suppliers to price the same features the same way.