Polishing and surface finish for forge service

Production Ra targets by feature.

Forging cavity surface: Ra 0.4-0.8 µm. This is the working surface that contacts the hot billet. The finish needs to be smooth enough to release the forging cleanly and resist adhesive pickup, but rough enough to hold a lubricant film. Below Ra 0.4 µm on aluminum and brass forging the galling risk climbs. Above Ra 1.6 µm the cavity acts like an abrasive against the workpiece and accelerates wear.

Flash land: Ra 0.8-1.6 µm. The flash land sees the highest contact pressure and the highest sliding velocity on the die. A slightly rougher finish here helps the lubricant carry and reduces the risk of cold welding at the land-to-cavity transition. The inside edge of the flash land still needs a radiused transition, but the working face does not need to be polished.

Gates and runners: Ra 0.2-0.4 µm. Where material flows fast through a constricted feature, a smoother surface reduces flow resistance and lowers the pressure required to fill the cavity. Gates and runners benefit from polish because they are flow channels rather than contact surfaces.

Extrusion bearing: Ra 0.1-0.2 µm. The bearing is the parallel land that defines the extrusion profile. The finish here transfers directly to the surface of the extruded section, so the bearing has to be polished to the part's surface finish target with a small margin. This is the one feature on a forge or extrusion die that needs mirror finish.

Contact surfaces want a moderate finish that holds lubricant. Flow surfaces want a smoother finish. Forming surfaces want whatever finish transfers to the part. The forging cavity is a contact surface.