Polishing and surface finish for forge service

A press shop sends three hot-forge aluminum dies to a polisher with one instruction on the print: SPI A-1 mirror finish across the cavity. The dies come back at Ra under 0.1 µm with cavity walls polished to a reflective finish. The first production run galls at the deep pockets within 200 hits, aluminum pickup builds on the cavity walls, the lubricant beads off the polished surface instead of carrying, and the soft billet extrudes into direct contact with bare H13. The shop scraps two of the dies and re-cuts the third with a deliberate cross-hatch texture on the cavity walls. The textured die runs 40,000 hits.

Working surface finish on a forge die depends on what the die does, what flows across it, what lubricant it carries, and what failure mode dominates. The target that is right for an extrusion bearing is wrong for a forging cavity, and the target that is right for the gate is wrong for the flash land. The print needs to differentiate by feature.