Meaning
Brittle surface zones formed during electrical discharge machining consist of molten metal that has solidified onto the tool surface. The edm recast layer is a byproduct of the intense heat generated by the sparks which melt the steel while the dielectric fluid rapidly cools it. This layer often contains microcracks and has a different hardness than the base material.
It must be removed or modified to prevent premature failure of the mould.
Thermal Modification
Rapid quenching creates a white layer that is extremely hard but prone to chipping. Formation of the edm recast layer modifies the heat affected zone where the tempering of the steel has been altered. These changes in the metal structure can lead to stress concentrations during the moulding cycle.
Tool Durability
Cracks in the machined surface can propagate into the bulk of the tool steel when subjected to high injection pressures. Presence of the edm recast layer leads to catastrophic failure or the chipping of sharp corners in the cavity. Removing the layer through polishing or chemical milling extends the life of the tool.
Finishing Requirement
Polishing the mould surface removes the brittle material and restores the intended dimensions. Removal of the edm recast layer ensures that the grain is clean and free of metallurgical defects. Proper surface preparation is necessary to achieve the high gloss finishes required for consumer electronics.
