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Lesson 03·Cavity design

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.

8 min readLesson 3 of 13

Tying it together

What this means on the shop floor

For a hot-forge connecting-rod cavity, open all internal radii at the rib base and the parting transition to R 3 mm minimum, with R 4-5 mm where geometry allows. Spec spherical radii at every 3D intersection.

For a hot-trim die, the cutting-edge geometry sets the stress field. Sharp inside corners on the trim profile are not optional, so the substrate has to carry the load. CPM 1V at 58-60 HRC with a thin γ' compound layer is the standard call when the radius cannot be opened.

For a press die with a deep central cavity, allocate internal draft at 10-12 degrees on the deepest features and verify ejection load before nitride. Pickup on the deep walls is a draft problem, not a polish problem.

For a hammer die where the impact load runs through a central rib, the parting line and the rib orientation decide whether the rib base sees compression or tension on the load stroke. Layout-stage decision, not a finishing-stage one.

For an Inconel hot-forge die, the workpiece temperature is hot enough that scale-trapping undercuts and tight radii create immediate hot spots on the cavity face. The geometric rules tighten because the local thermal load is higher.

Pushback questions for the cavity design

What is the smallest internal radius on the cavity in 3D, not 2D, and what is the calculated Kt at that radius for the expected peak contact stress?

Where is the parting line, and does it place the highest-stress internal radii on the die that takes compression at the feature rather than tension?

What is the flash land thickness and width, and what is the predicted peak press load at that flash restriction for the billet temperature in spec?

If the gate has to be relocated for fill reasons, does the new gate direction point at a sharp corner or a relieved one?

Common confusions

Part radius and die radius are not the same number. The part contracts on cooling and the die has to be larger by the contraction plus an allowance for the case if the case is on the dimensional surface. Copying the part radius onto the die produces an as-forged radius smaller than the part print.

Print radius and as-built radius are not the same number either. EDM, finish grind, and heat-treat distortion all shift the corner, and the as-built radius at a deep blind corner can run 30 to 50 percent smaller than print. Inspect the actual radius before nitride.

Mirror-polished cavities are not always better. A high-polish surface in a corner with high Kt cracks just as fast as a rougher one, and the polishing operation itself can tighten the radius. Polish to the working Ra the application requires (Lesson 9) and no further.

Up next: pre-hard versus through-hard.

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