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

The gate is where metal first enters the cavity. Gate placement sets the direction of metal flow and therefore the orientation of the stress field on the cavity. Two rules cover most of it.

Do not gate into a corner. Metal entering at a sharp internal corner hits the corner at full velocity, transfers thermal energy directly into the radius root, and concentrates impact stress at the place least able to absorb it. Gate into a flat face or a generous radius and let metal flow toward the corners rather than into them.

Orient the parting line so high-stress features sit on the die that supports them best. A boss or rib that sticks up from a flat parting plane should be on the upper die, where the impact load is compressive on the rib, not on the lower die where the load is tensile across the rib base. On an upset die where the part is a thick disc, the parting line runs across the disc face and gating is radial. On a hot-trim die the "gate" is the direction the flash sheds during the trim stroke, and the trim should be oriented so flash sheds away from the cavity. A trim die that pulls flash back across the cavity wall on the return stroke abrades the surface and chews up nitride case on every hit.