A negative angle or inward-leaning surface on a casting pattern or forging die. Because the bottom of the feature is wider than the top, the pattern locks into the sand or die cavity. Removing it tears the mold or ruins the part. Designers must either add positive draft, use loose pieces on the pattern, or design a separate core to form the undercut area.
Heard on the job
You designed this boss with severe underdraught, so the molder will rip the sand out every time they pull the pattern.
Insider tip
The classic rookie engineering mistake is designing a cast part with zero draft or underdraught, making it physically impossible to mold without expensive multi-part tooling.
Also heard as
- undercut
- back draft