Designing for Complexity: An Engineer's Guide to Undercuts

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.

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.