underdraught
The situation where part of a formed or cast shape has a reverse taper or inward‑leaning surface that makes it difficult to remove from the die or mold.
Underdraught: a reverse taper that traps castings in the mold
An underdraught is a surface feature on a casting or forging that angles inward, creating a mechanical lock between the part and its mold or die. Where a normal draft angle slopes outward to permit easy withdrawal, an underdraught slopes inward. Even when the mold or die opens correctly, the part cannot be removed without either breaking it, damaging the tool, or inserting a separate core to fill the void and allow extraction.
Underdraughts occur most often in sand casting and die casting, particularly in parts with complex internal features or thin-walled sections. A simple example is a recessed pocket or undercut on a casting surface. If that pocket has walls that lean toward the parting line rather than away from it, the casting is locked. Similarly, a through-hole with flanges or bosses on both ends can create underdraughts if the hole direction does not align with the primary draw direction of the mold.
Detection and prevention
Designers and pattern makers must analyze draw direction early in the process. For sand castings, core prints and core shifts can introduce unintended underdraughts if positioning is careless. In die casting, movable inserts or sliding cores are routinely designed into the die to accommodate internal features that would otherwise lock the part. These mechanisms add cost and complexity but are sometimes unavoidable.
When an underdraught cannot be avoided, the mold or die must include a retractable element, a mechanically actuated slide, or a core that supports the trapped region during casting and withdrawal. Without such provision, the only alternative is manual breakout, which damages the tool and the part. Precision casting shops and tool designers spend considerable effort in the design phase to eliminate underdraughts before they reach production.
The term reflects the foundry perspective: the draught (or draft) is the angle that permits removal, and an underdraught is the failure of that angle, literally a shortage of the taper required for free extraction. It remains a leading cause of mold sticking and casting damage, especially in high-volume production where tool wear and metal buildup can gradually introduce unplanned underdraughts over thousands of cycles.