Undercuts and Side Actions in Injection Molding

Undercuts are where a lot of tooling budgets quietly blow up. A part looks simple on the screen, then someone traces the ejection direction and finds a snap fit, a side hole, or an internal thread that no straight pull can release. That single feature can add a slide, more moving steel, and another wear point to the mold. This guide walks through what undercuts are, how side actions release them, and when a small part change removes the mechanism entirely.

What is an undercut in injection molding?

An undercut is any part feature that blocks the part from ejecting in a straight line along the mold opening direction. It hooks onto the core or the cavity steel, so pushing the part off would either shear the feature or lock the mold. Common undercuts include snap fit hooks, side holes, external threads, internal threads, and molded in clips.

The mold opens along one axis, called the draw or the pull direction. Anything that sits perpendicular to that axis, or that wraps around steel it needs to slide past, is an undercut. Picture a bottle cap with a side window. The core forms the inside, the cavity forms the outside, and that window has an edge that catches on the steel when you try to push the cap off. That catch is the undercut.

Undercuts come in two families. External undercuts sit on the outside of the part, like a side snap or a barb on the wall. Internal undercuts sit on the inside, like an internal thread or a retention rib. External undercuts usually get solved with slides. Internal undercuts usually get solved with lifters or collapsible cores. The location drives the mechanism, and the mechanism drives the cost.

What are side actions and how do they work?

Side actions are moving mold components that pull away from the part before or during ejection so an undercut can release. The two workhorses are slides and lifters. A slide moves sideways across the parting line to clear an external undercut. A lifter moves at an angle as the ejector plate pushes forward, clearing an internal undercut while it lifts the part off the core.

A slide is driven by an angle pin, sometimes called a horn pin or a cam pin, that is fixed in the opposite mold half. As the mold opens, the angled pin cams the slide outward. Based on MoldMinds experience, angle pins commonly run 15 to 25 degrees, since a steeper angle moves the slide faster but loads the pin harder. A locking heel holds the slide closed against injection pressure so it does not creep open and flash the part.

A lifter is a blade or post mounted on the ejector system at an angle. When the ejector plate advances, the lifter travels forward and sideways at the same time. That sideways travel clears an internal undercut, like a snap detail on an inside wall, while the part comes off the core. Lifters are compact and fit inside the part, which is why they are the standard fix for internal features.

Both mechanisms add moving steel, tighter fits, and maintenance. Every side action is a surface that wears, a shutoff that can flash, and a place where debris can jam. That is real, but it is also routine. A well built slide or lifter runs for hundreds of thousands of cycles when the steel, the angles, and the lubrication are right.

Slides vs lifters vs other undercut methods: which do you use?

You match the mechanism to where the undercut sits and how deep it is. Slides handle external undercuts. Lifters handle internal undercuts. Bump offs handle shallow undercuts on flexible resins with no moving parts at all. Collapsible cores and unscrewing mechanisms handle internal threads. Here is how they compare.

Method	Best for	How it moves	Relative cost and risk
Slide (cam action)	External undercuts: side holes, side snaps, external threads	Angle pin cams the slide sideways as the mold opens	Moderate cost, proven, needs a locking heel and maintenance
Lifter	Internal undercuts: inside snaps, retention ribs	Angled blade travels forward and sideways on the ejector stroke	Moderate cost, compact, wear surface to watch
Bump off (stripper)	Shallow, rounded undercuts on flexible resins like PE and PP	Part flexes over the steel during a stripper ejection	Lowest cost, no moving steel, limited to forgiving geometry
Collapsible core	Internal threads and internal ring features	Segmented core collapses inward to release, then resets	Higher cost, precise, more parts to maintain
Unscrewing mechanism	Deep internal or external threads	Rack, gear, or motor rotates the core out of the thread	Highest cost, slower cycle, strong when threads must be perfect

Read that table as a decision path. Start by asking whether the undercut is external or internal. Then ask how deep it is and how flexible the resin is. A shallow snap on a polypropylene lid may bump off with no mechanism. A precise external thread on a rigid part needs a slide or an unscrewing unit. The wrong choice either fails to release or costs far more than the part needs.

When can you avoid a side action with a design change?

You can often delete a side action by moving the undercut to the parting line, adding a pass through window, or splitting the part. These are the three moves that remove a mechanism instead of paying for one. Each trades a small design compromise for a simpler, cheaper, more reliable mold.

The cleanest fix is to relocate the feature so the main parting line forms it. If a side hole can shift to face the mold opening direction, the cavity forms it directly and no slide is needed. The second fix is a shutoff, also called a pass through or a core pin kiss off. You let steel from one half touch steel from the other half through the wall, which molds a hole or a pocket without a moving slide. Based on MoldMinds experience, a shutoff surface wants at least a few degrees of angle so the steel seals cleanly and does not wear a witness line into the part.

The third fix is to split the part into two moldable pieces and join them later with a snap, a weld, or a fastener. That sounds like more work, and sometimes it is, but two simple molds can beat one complicated mold with three slides and a lifter. This is exactly the kind of call to make during design for manufacturing review, before steel gets cut. We cover the wider list in our DFM checklist for injection molding.

Draft angle matters here too. Undercuts and low draft often travel together, because a wall with no draft behaves like a shallow undercut and drags during ejection. Getting draft right can turn a feature that needed a mechanism into one that pulls straight. Our draft angle guide walks through the numbers, and our rib and boss design guide shows how internal features can be shaped to eject without a lifter.

How much do undercuts and side actions add to tooling cost and cycle?

Side actions add tooling cost, add mold build time, and can add cycle time. The exact number depends on the mechanism, the cavity count, and the tolerances, so treat any figure as a planning range, not a quote. Based on MoldMinds experience, a single slide or lifter adds meaningful cost to a mold, and that cost multiplies fast in a multicavity tool because every cavity needs its own mechanism.

Cavity count is the multiplier people miss. One slide on a single cavity prototype tool is manageable. That same slide on a 16 cavity production tool becomes 16 slides, 16 locking heels, and 16 wear points, plus the room to fit them all around the cavities. That is why an undercut that looks trivial on the print can drive the whole tooling strategy, including how many cavities you can even fit in the mold base.

Cycle time and maintenance are the running costs. Some side actions add a fraction of a second to each cycle, and over a long production run that adds up. Every moving mechanism is also something to inspect, lubricate, and eventually rebuild. When we review a part, we weigh the piece part savings of a molded in feature against the tooling and upkeep of the mechanism that makes it. Sometimes the feature is worth it. Sometimes a small redesign pays for itself in the first tool.

MoldMinds is vendor independent. We hold no referral arrangements with shops or suppliers, so the call to add or delete a side action is made on the engineering, not on who builds the mold. If you want that read before you commit steel, our plastic part design service reviews the part for undercuts, draft, and ejection early, and our Moldflow analysis confirms the part fills and packs the way the mechanism assumes.

The bottom line on undercuts and side actions

Undercuts decide how a mold gets built, how much it costs, and how reliably it runs. Trace the ejection direction on every part before you approve the print. If a feature cannot pull straight, decide early whether a slide, a lifter, a bump off, or a simple design change is the right answer. The cheapest side action is the one you engineer out before the steel is cut.