Meaning
Resolidified metal layers form on the surfaces of injection mould cavities during thermal machining processes such as electrical discharge machining. This recast layer consists of altered, highly stressed material that has melted and rapidly quenched in the dielectric fluid. The resulting microstructure is brittle, hard, and prone to micro-cracking under the cyclic pressures of moulding runs.
Toolmakers remove this layer through polishing, chemical etching, or micro-peening to prevent premature failure.
Surface Quality
Microscopic cracks within the quenched steel can propagate into the bulk material of the mould under high clamping forces. The hard recast layer also increases the difficulty of achieving a mirror finish on optical tooling. Polish operations must penetrate below this zone to reach the unaffected substrate steel.
Machining Effect
Machining variables like pulse duration and peak current dictate the thickness and hardness of the thermal zone. Higher discharge energy increases the thickness of the melted material, requiring more manual benchwork to restore the required tool dimensions.
Tooling Endurance
Thermal fatigue from cyclic heating and cooling of the cavity leads to heat-checking cracks that originate in this brittle zone. These cracks transfer onto the surface of moulded plastic parts as raised, unsightly lines. Removing the damaged metal before putting the tool into service extends the production run life.
