Electrode Material Microstructure Is One Important Determinant of Mould Cavity Surface Integrity and Finish
Industry Trends: Developments in Technology
 The degree of surface integrity of an EDMed mould cavity depends on how the EDM process alters the surface layers of the workpiece thermally. Poco Graphite notes that several factors need to be considered by the mould maker, with the type and quality of electrode material playing a big role in obtaining a desirable, cost-effective surface finish.
Thermally Altered Layers
The EDM process changes not only the surface of the metal, but also subsurface layers that collectively constitute the altered-metal zone. This zone comprises the recast, or white, layer and the heat-affected zone (image).
The white layer consists of metal that has been made just molten by the electrical discharge, but not hot enough to be ejected into the gap and flushed. In this layer, EDMing has altered the metal’s structure and characteristics; the molten metal has been cooled by dielectric and resolidified in the cavity, and includes some expelled particles that then solidified and were redeposited on the surface. Densely infiltrated by carbon resulting from the breakdown of hydrocarbons in the electrode and dielectric fluid, the white layer’s structure thus differs from that of the base material.
Beneath the white layer is the heat-affected zone, only minimally affected by carbon enrichment. It has been heated, but not to its melting point; its metallurgical structure is unchanged.
Microcracking
Mould makers are concerned about microcracking, resulting from thermal stresses created during the EDM cycle’s on-time phase and prominent in the white layer. If this layer is too thick, the microcracking present can cause premature tool failure. Further, microcracks lower the metal’s corrosion and fatigue resistance.
The depth of the microcracking (and the white layer), which varies directly with spark intensity, is partially controllable via the EDM programme. The parameters that affect mould-cavity surface integrity are voltage, amperage, on-time, and the duty cycle, all of which can be manipulated to optimize roughing, semifinishing and finishing efficiencies—and thus surface integrity.
Along with spark intensity, the metal’s thermal conductivity contributes to the mould’s surface integrity. Highly thermally conductive metals (e.g., copper alloy), owing to energy dissipation throughout their surface, usually have a thinner white layer and less microcracking than less-conductive materials (e.g., tool steel).
Burning carbide creates another concern, as this brittle material exhibits high levels of thermal cracking. The cobalt binder that holds the tungsten carbide or silicon carbide particles together is highly conductive; thus, spark energy disintegrates it, causing carbide particles to be released into the gap.
Electrode Material
Surface finish is as important a mould cavity characteristic as surface integrity. Just as they affect the integrity of the cavity’s sublayers, machining parameters govern surface finish. Another factor is the electrode material, which has little effect on the cavity’s subsurface integrity but is important for surface finish. Here, manufacturing costs and lead time might be reduced without preventing delivery of a quality mould.
A mould with a fine EDM finish can be put to use right out of the tank, saving time and expense. Such a finish is achieved by employing a high-frequency EDM programme with low amperages and short on-times, along with the optimal electrode material. Taking the programme out of the equation, the cavity surface varies directly with the material quality of the electrode used to finish the cavity.
Its particle and pore sizes play a key role in the electrode material’s ability to produce a smooth finish. If the material structure cannot produce the specified finish, the EDM machine will continue to run in a vain attempt to achieve that surface quality. If two electrodes made of different materials erode a cavity at the same parameters, the electrode with the finer material structure yields the smoother surface finish, which will require less, if any, subsequent polishing.
An electrode material with a poor microstructure (i.e., large or irregular particles) will wear unevenly. This is bad for cavity surface finish generally, but especially critical with multicavity moulds. When electrodes are graphite, all should exhibit a consistent quality. Because graphite grades from different manufacturers wear differently, the same EDM machine may not produce the programmed surface finish with all electrodes. This is seen when moulds are produced using a variety of electrode materials or EDM machines of different make.
Poco Graphite SARL
Limonest, France
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