Design for Manufacturing / Undercuts
DfM deep dive
Undercuts: the biggest cost driver after product size
An undercut is any feature that blocks straightforward opening and ejection of the mold: a hook, a cross hole, an internal groove, a thread. Undercuts are the biggest cost driver in the mold after product size itself.
Step 1: design out
The cheapest undercut is the one that isn't there. In order of preference:
- Choose parting line smartly: by shifting or staggering the parting line (stepped parting line), many undercuts disappear automatically.
- Breakthrough instead of undercut: a window in the wall lets a mold part protrude. The classic: a snap-fit hook with a rectangular hole underneath, so that the two mold halves together form the hook without any movement.
- Tilt feature to the outside: replace an internal groove with an external one, let a cross rib run out in demolding direction.
- Forcing (snap ejection): small, round, shallow undercuts in flexible materials like PP and PE can be elastically forced off the core, with generous radii. Only for tough materials, never for filled or brittle ones.
Step 2: moving mold parts, eyes open
| Solution | For | Effect on mold |
|---|---|---|
| Slide | External undercuts, cross holes | Extra cost per slide, extra maintenance and parting line marks |
| Core pull | Deep or large cross-cores | More expensive than a mechanical slide |
| Lifter | Internal hooks and clips | Compact but prone to wear, limited stroke |
| Loose insert | Prototypes, very low quantities | Cheap in steel, expensive in labor per shot |
| Unscrewing unit | Internal threads | Heaviest option, only if thread really needs to be molded |
Every movement in the mold means higher purchase price, longer build time, extra witness marks on the product and more chance of malfunction during production run. With low quantities, mold price weighs heavily; with high quantities, susceptibility to malfunction does.
View the full DfM pillar or go to parting line and mold construction.
What does this mean for your design?
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