An undercut is any feature that would prevent the part being pulled straight off the core or out of the cavity. Every undercut needs something in the tool to move out of the way first, and each of those mechanisms adds design, components, maintenance and a failure mode. Understanding what each costs is what lets a designer decide which undercuts are worth having.
The mechanisms and what each suits
A side action, or slide, moves perpendicular to the tool opening and is driven by an angle pin or a cylinder; it suits external undercuts such as side holes and snap features. A lifter moves at an angle during ejection and suits internal undercuts. A collapsing core folds inward to release internal threads or ribs. An unscrewing mechanism rotates a core out for moulded threads.
What they add to the tool
Design time, precision components, assembly, and wear surfaces that need maintenance. They also consume space in the mould base, which can force a larger base or fewer cavities, and that second effect is often larger than the mechanism itself. Every moving part is also a thing that can jam and stop production.
Designing undercuts out
Most snap features can be redesigned to open through the wall, so a core passes through and forms the underside from the other half of the tool. Side holes can sometimes be moved to the parting line. Internal threads can be replaced by a threaded insert. A designer who spends an hour on this before the tool is quoted regularly removes a substantial fraction of the tooling price.
When to keep them
When the feature does real work and the alternatives compromise the product. A side action on a high volume tool is a well understood, reliable mechanism and there is nothing wrong with using one deliberately. The mistake is arriving at a tool quote with three undercuts nobody had noticed, which is what a manufacturability review exists to prevent.
Questions people ask about undercut injection molding
What is an undercut in injection moulding?
Any feature that stops the part pulling straight off the core or out of the cavity, so something in the tool has to move before the part can be released.
What does a side action cost?
It varies with size and complexity, and the real cost includes the space it consumes in the mould base, which can reduce cavity count. Ask for it as a separate line in the tooling quote.
Can undercuts be avoided?
Often. Opening a window through the wall so a core forms the underside is the classic fix for snap features, and it is usually free in the design.
Are moulded threads worth it?
Sometimes. An unscrewing mechanism is expensive and slow. A threaded insert or a self tapping screw boss is frequently the cheaper answer for the whole product.