EDM: machining the impossible shapes of a mould
A milling cutter is round: it cannot make a perfectly sharp inside corner. Hardened steel wears tools down. For such shapes, mould makers use a method that does not cut metal: it eats it away, spark after spark.
By Tecmom editorial team · 2 min read
Machining without touching
Electrical discharge machining, or EDM, removes metal with electrical discharges. An electrode and the workpiece are immersed in an insulating liquid, the dielectric, and brought close without touching. A very rapid succession of sparks jumps between them; each melts and vaporises a tiny amount of metal, which the liquid carries away.
Since nothing cuts, the hardness of the metal hardly matters: EDM works hardened steel as well as soft steel, which makes it a valuable tool for moulds.
Two techniques
- Sinker EDM uses an electrode shaped like the negative of what is to be hollowed out, often in graphite or copper. It descends slowly into the steel and imprints its shape. This is the technique for cavities, sharp inside corners, and deep, narrow ribs.
- Wire EDM uses a thin metal wire, tensioned and fed continuously, which cuts through steel like a toothless saw. It traces through-contours with great precision.
The electrode wears too as the sparks fly: its edges round off, and the mould maker allows for it, sometimes by planning several electrodes for one shape.
A texture too
EDM is not only for shapes. By adjusting the discharges, an even surface texture is obtained, from very fine to coarse. The German VDI 3400 guideline grades these textures, and the grain is reproduced identically on every moulded part. See one shape, dozens of variants.
What it means for a cap
A fine thread, a tamper-evident notch, a clean sealing surface: these details are shapes a cutting tool reaches poorly. Each machining method has its field, and a complex mould combines several. See the mould maker's trade and our expertise.
We imagine moulds being carved by powerful machines tearing metal away. Yet the finest details come from a more patient method: countless small sparks that engrave the steel without ever touching it.
Glossary
- EDM
- Machining by electrical discharges, with no contact between tool and workpiece.
- Electrode
- A tool, often graphite or copper, whose shape is reproduced as a hollow in the steel.
- Dielectric
- The insulating liquid in which the discharges occur.
- Hardened steel
- Steel hardened by heat treatment, hard to machine by cutting.