Cavity design and stress concentration: radii, draft, flash land, gates

Draft is the taper on vertical cavity walls that lets the forging eject without seizing. External draft, on outside walls where the forging shrinks away from the die during cooling, runs 5 to 7 degrees. Internal draft, where the forging shrinks onto an internal die feature like a boss or punch, runs 7 to 10 degrees because the part grips the feature on cooling. Deep cavities with depth greater than about 1.5 times the local width push internal draft to 10 to 12 degrees. Aerospace work sometimes specs 3 degrees external with post-forge machining stock to compensate, which is a customer-side trade.

Too little draft causes pickup and galling on the cavity wall during ejection, which scars the surface and pulls compound layer off in patches. Too much draft adds material to the forging that has to be machined off downstream on every piece for the life of the program, and a degree of unnecessary draft on a 2 kg forging at 500,000 parts adds up. Draft also interacts with radii. A generous draft on a deep pocket spreads stress over a longer transition, which lets the bottom radius run smaller without raising Kt. A vertical wall forces the bottom radius larger to compensate.