Step 3 of 5
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Cavity design and stress concentration: radii, draft, flash land, gates
Where forge dies actually crack and the geometric rules that move the failure point: minimum radii at stressed corners, draft angles by cavity depth, flash land and gutter dimensions, and gate placement that does not dump metal into a corner.
Step 3 of 5Flash land and gutter
The flash land is the thin annular gap around the cavity that creates the back pressure forcing material into the corners. The relief gutter sits outboard and lets excess material escape. Standard flash land geometry: the land thickness (the gap between upper and lower die) runs 2 to 5 percent of local part thickness, thinner for high-flow alloys like aluminum and brass and thicker for harder-flowing steels. Land width, the length of the thin region in the direction of metal escape, runs 2 to 5 times the land thickness. The gutter is 3 to 5 times the land thickness in depth, with a width 3 to 6 times the land thickness so material flows in without backing up.
The inside edge of the flash land is where most thermal-fatigue cracks initiate on a closed-die forge, not in the cavity proper. The 90-degree transition from cavity wall to thin land is a stress riser. Radius this transition R 1 to 2 mm minimum and treat the flash land as a critical zone for nitride and PVD specs. Underspec the land width and the flash freezes too fast, back pressure spikes, and the die seizes against the workpiece. Overspec it and material wastes out the flash without filling the cavity. Both failures show on the part as missing features, with opposite root causes. Leave 0.5 to 1 mm of stock on the flash-land face to support resharpening once or twice during die life.
Quick check
A closed-die press for a 1.2 kg steel forging is missing fill at the corners on the first production run. The flash land is 1.2 mm thick by 12 mm wide on a part with 25 mm local thickness. Is the land geometry the cause of the missing fill, and which dimension moves first on rev B?