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How do you injection mould a part with an undercut?
An undercut is moulded either by adding a moving piece to the mould (a side action or a lifter), by letting a flexible part spring off a small bump, by loading a removable insert by hand, or by redesigning the feature so the mould can form it with a pass-through shutoff. Each option adds cost or cycle time, so it pays to remove undercuts you do not need before the mould is designed.
A simple mould opens in a straight line: one half moves away from the other and the part is pushed off. An undercut is any feature that would stop the part coming out in that direction, such as a hole in a side wall, a snap hook, a groove around a cap or an internal lip.
The main options
| Method | How it works | Best for | Trade-off |
|---|---|---|---|
| Side action (cam, slide) | A block slides in sideways as the mould closes and is pulled out by an angled pin as it opens | Holes and slots on outside walls | Adds tooling cost; must be reachable from the parting line |
| Lifter | A core piece moves at an angle with the ejectors, releasing an internal feature | Internal lips and clips in production tools | More complex; not offered by some prototype moulders |
| Bump-off (stripped undercut) | The part flexes over a small rounded bump during ejection | Caps, lens covers, snap rings in flexible resins | Only works for shallow, rounded features |
| Hand-loaded insert | A metal piece is placed in the mould each cycle and removed with the part | Prototypes and low volumes | Slower cycle and operator handling |
| Pass-through shutoff | Steel from one half reaches through the part to form the hook from the other side | Snap hooks and clips | Needs a hole behind the feature and enough draft |
Published limits
- Protolabs lists maximum automated side-action dimensions of about 8.419 in (213.84 mm) wide by 2.377 in (60.38 mm) high, with travel around 2.9 in (73.66 mm), and says it can use up to four side actions on a part. Production toolmakers have their own limits, which are often larger.
- Eastman's moulding guide allows stripped undercuts such as snap rings or threads of up to about 2 to 3 percent of the part diameter in relatively thin-walled parts, provided they are rounded and well filleted.
- Protolabs recommends at least 3 degrees of draft on telescoping or sliding shutoffs, so steel does not rub on steel, flash or wear early.
- For hand-loaded inserts, Protolabs suggests pieces of roughly 0.5 in square or larger, and no bigger than about a deck of cards, so a gloved operator can handle them.
Material matters
Side actions suit rigid resins such as nylon, polycarbonate and acetal, Protolabs notes, because very soft parts can cling to the side core as it retracts. Bump-offs are the opposite: they need a material that can flex past the bump without tearing, and Protolabs names low-density polyethylene, TPE and TPU as good candidates. Rigid resins such as polycarbonate are much less forgiving of undercut depth.
Designing undercuts away
- Ask whether the feature is really needed, or could be added by a secondary operation such as drilling, if volumes are low.
- Move the parting line. A hole that is an undercut in one orientation may form naturally if the part is split differently.
- Use a pass-through shutoff for snap hooks, accepting a small window behind the hook.
- Increase draft. Protolabs shows that a window part-way up a drafted wall can sometimes release from the core with enough draft, without any moving steel.
- Draft the faces a side action forms in the direction it withdraws, so it can pull out without dragging the part.
Limits here are published by specific suppliers and differ from toolmaker to toolmaker. Confirm the method with whoever builds the mould.
Questions buyers ask
How much does a side action add to a mould?
It depends on size and the toolmaker, so ask for the mould quoted with and without each action. Protolabs notes that cams raise mould cost but can remove secondary operations, which may lower the total project cost.
Why can my prototype supplier not use a lifter?
Some rapid-tooling services work only with straight-pull moulds plus simple side actions and inserts. Protolabs, for example, states that it does not offer lifters. A production toolmaker usually can.
Are threads undercuts?
Yes. External threads are often formed by splitting the mould along the thread axis, while internal threads may need an unscrewing core, a collapsible core or a stripped thread in a flexible resin.
Does an undercut slow the cycle?
Automated side actions and lifters usually add little time. Hand-loaded inserts add more because an operator has to place and remove them every shot.
Sources
- Protolabs, Using side-actions in molding design
- Protolabs, 6 ways to achieve undercut success in molded parts
- Protolabs, Managing undercut design advisories
- Eastman, Keys to successful molding (PDF)