Undercuts in Injection Moulding: Side Actions, Lifters & Redesign Options

Updated: 2026-10-22

An undercut is a feature that traps a moulded part relative to the proposed opening or ejection direction. A side hole, internal lip or retaining hook is not a tooling problem in isolation: its orientation, function and allowable design changes determine the release options. Start by agreeing the part's functional boundaries before comparing mechanisms.

First ask whether the feature really needs moving steel

Mark the proposed mould opening direction on the model, then inspect the obstructed surfaces. Rotating the part, moving the parting line or exposing the back of a hook through a window can sometimes remove the obstruction. A shut-off lets mating tool surfaces form a feature without a separate moving core, but those surfaces still require suitable geometry, wear control and a permitted witness line. It is a design choice, not a free feature.

Check what the redesign changes for the product. A window behind a latch may compromise dust protection, appearance or cleaning. Moving a seam to a sealing face can solve release while creating a more important problem. Keep surfaces that may change separate from interfaces that must remain fixed.

Compare the release approaches

Approach	Useful application	Review before committing
Side action or slider	Side opening or external feature obstructing straight release	Direction, travel, space, shut-off and sequence before ejection
Lifter	Internal feature released with combined ejection and lateral movement	Available travel, part support, wear and collision clearance
Collapsible core	Suitable internal circular profiles	Available collapse, segment marks and compatibility with the exact feature
Unscrewing mechanism	A thread needing rotational release	Thread lead, drive, part restraint and cycle sequence
Controlled stripping or bump-off	A feature that can deform and recover during release	Interference, grade, wall flexibility, temperature and permanent damage
Hand-loaded insert or machining	A practical alternative for a defined batch	Handling, inspection, labour, repeatability and total delivered cost

The geometry sets the mechanism; a lifter is not always cheaper than a slider, and an unscrewing system is not automatically necessary for every thread. Ask the tool designer to show how the feature becomes fully free before removal. Protolabs' release examples illustrate why parting-line changes, side actions and controlled deformation solve different situations.

Why does the movement sequence matter?

A side core must clear the feature before the part is ejected. A lifter combines motions, so its path needs to remain compatible with both the part and surrounding steel throughout the stroke. Confirm locking during injection, return sequence, position detection where required and clearance to other moving components. A mechanism that appears clear in the final position can still collide halfway through travel.

Collapsible cores have product-specific limits, not unlimited freedom to release internal geometry. Their segment movement, cooling and permissible profile must match the design. DME's collapsible-core information is an example of manufacturer guidance for a particular system, rather than evidence that any internal groove is feasible.

Ask which moving surfaces will leave seams on the part. Their location may influence appearance, seal performance or dimensional inspection. Discuss access for maintenance and replacement of wear components; mechanism cost includes how the tool remains usable, not only its purchase price.

When can a feature be stripped over the core?

A rounded, shallow feature may release by temporary deformation, but the complete part must have room to flex. A flexible material around a heavily reinforced wall can still resist release. Conversely, changing grade, filler or colour formulation may change behaviour enough to require another trial. Check ejection support and inspect for cracking, whitening, permanent deformation and loss of retention after release.

Do not assume the deformation permitted during hot ejection is also acceptable during service, or that a successful manual removal proves repeatable automatic ejection. The snap-fit guide helps separate assembly flexing from the tooling question. For threaded features, compare the thread and insert routes as well.

Worked comparison: mould a side hole or drill it?

This is a fictional budgeting exercise, not a supplier quote or market-price claim. Assume both alternatives already meet the drawing. Route A adds EUR 2,400 in tooling and EUR 0.10 per accepted part; route B adds no dedicated tooling and EUR 0.90 per accepted part. These are invented incremental costs on top of the same base moulding cost.

For quantity Q, A = 2,400 + 0.10Q and B = 0.90Q. Equality occurs at Q = 2,400 ÷ (0.90 − 0.10) = 3,000 parts. At 1,000 parts the additional totals are EUR 2,500 and EUR 900; at 10,000 they are EUR 3,400 and EUR 9,000. The calculation excludes finance, maintenance, transport, inspection, scrap, fixtures and changes. Include those items in real proposals; an apparent cost advantage does not waive a tolerance or burr requirement.

What should the RFQ make explicit?

Supply CAD and a controlled drawing showing the feature's purpose, load, datums, fit and surfaces that cannot change. State material grade, forecast quantities, texture, sealing needs and acceptable seam or ejector marks. Ask for separate options where redesign or machining is permitted, with any added fixture, inspection and maintenance scope identified.

For approval, check hole position and size from the drawing datums, hook function, flash, burrs and damage after release. Identify cavity, material batch and process conditions. Agree the sample plan and functional test before comparing results. A part that can be removed from the mould has not necessarily passed its product requirements.

Use the RFQ brief builder, the sample approval pack and a project enquiry to document the release proposal and acceptance criteria together.