Designing for Complexity: An Engineer's Guide to Undercuts

By Ichiplas Design Engineering Team · Oct 2025 · Updated Nov 2026 · Reviewed by the Ichiplas engineering team

So far, we've covered rules for parts that can be easily pulled from a mold in one direction. But what about features like snap-fit clips, side holes, or recessed ports?

These are all examples of undercuts—features that block the part from being directly ejected.

Many designers assume undercuts are to be avoided at all costs. This isn't true. Undercuts are a normal part of complex part design. However, they require special mechanisms in the mold — learn more about how these are engineered in our mold design and fabrication services. Your job as a designer is to understand the different ways to achieve them, because one method is significantly more cost-effective than the other.

The Standard Solution: Side-Actions (Slides and Lifters)

The most direct way to form an undercut is with a side-action in the mold. This is a moving component, like a slide or a lifter, that extends into the cavity to form the undercut feature. Before the part is ejected, this component retracts out of the way, clearing the path.

This solution works perfectly, but it comes with costs:

Increased Tooling Cost: The mold is now a more complex machine, requiring precision mechanisms that add thousands of dollars to the upfront cost.
Maintenance: Moving parts wear out. Side-actions require regular maintenance to function reliably.
Slower Cycle Times: The time it takes for the action to move in and out adds a few seconds to every cycle, which can add up over a production run of millions of parts.

The Clever Solution: Shut-Offs

For many undercuts, there's a more elegant and affordable solution: designing a shut-off. A shut-off is a feature where you use a portion of the A-side of the mold to pass through an opening in your part and seal (“shut off”) against the B-side.

Imagine you need a snap-fit clip on the inside of a housing. Instead of using a slide to form the hook, you can design a hole in the housing wall directly behind the clip. This allows a piece of steel from the A-side to pass through the hole and form the underside of the clip. No moving parts are needed.

The trade-off is that you now have a pass-through opening in your part. If that is acceptable for your design, you have just saved significant money on tooling and complexity.

Ways to Mold an Undercut, Compared

There is almost always more than one way to form an undercut. The cheapest option is usually to redesign it as a shut-off; the most expensive is a collapsible core.

Comparison of methods for molding undercuts
Method	Best for	Added tooling cost	Cycle time impact	Trade-off
Shut-off / pass-through core	Snap hooks, clips and holes	Low	None	Leaves an opening in the part
Bump-off (stripping)	Shallow undercuts in flexible resins (PP, PE, TPE)	Low	Minimal	Only works for small undercuts the part can flex over
Slide (side-action)	External side holes, ports, recesses	Moderate – high	Adds a few seconds	Wear and maintenance on moving steel
Lifter	Internal undercuts and inward-facing hooks	Moderate – high	Small	Limited travel; adds ejection complexity
Hand-loaded insert	Low volumes, prototypes	Low	High (manual)	Labour cost and operator variation
Collapsible core	Continuous internal threads and rings	High	Small	Expensive, specialised mechanism

Relative costs. The actual premium depends on mold size and the number of actions — see our tooling cost guide for real pricing data.

Frequently Asked Questions

What is an undercut in injection molding?

An undercut is any feature that prevents a part from being pulled straight out of the mold in the opening direction — for example a side hole, a snap-fit hook, an internal groove or a thread. Undercuts need either a moving mechanism in the mold or a design change that lets fixed steel form the feature.

What is the difference between a slide and a lifter?

A slide is a side-action that moves sideways, usually driven by an angled pin as the mold opens, and typically forms external undercuts. A lifter is mounted on the ejector system and moves at an angle as the part is ejected, which makes it suitable for internal undercuts such as inward-facing hooks.

How much does a slide or lifter add to the cost of a mold?

Each side-action adds design, machining and fitting work, so the premium scales with mold size and the number of actions. A single simple slide commonly adds a couple of thousand US dollars, depending on the mold size and the mechanism, and it also adds ongoing maintenance and a little cycle time.

Can undercuts be avoided?

Often, yes. Common strategies are to convert a hook into a shut-off with a pass-through hole, move a side hole so it lines up with the opening direction, replace a feature with a separate part or fastener, or use a shallow bump-off undercut in a flexible material. A DFM review will usually identify which undercuts can be designed out.

What is a bump-off undercut?

A bump-off is a shallow undercut that the part is simply pushed over during ejection, relying on the plastic flexing while it is still warm. It works only for small undercuts in flexible materials such as PP, PE or TPE, and needs rounded lead-in angles so the part isn't damaged.

What's Next?

Understanding how to manage undercuts opens up a world of design possibilities. Now, let's zoom in on how to apply these principles to create functional features. Our next guide is How to Design Functional Features: Ribs and Bosses.

Getting your design ready for production can feel complex. Our team in Johor Bahru lives and breathes this process every day. If you want to ensure your design is optimized for manufacturing from the start, reach out to us. We're here to help.