Mold Parts Electrical Discharge Explained

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  Key takeaways
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  How the mold parts electrical discharge process removes metal
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  Sinker EDM for blind cavities and sharp internal corners
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  Wire EDM for through profiles, split lines and ejector holes
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  Where EDM stops being the right choice
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  Settings, flushing and inspection that keep the burn on size
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  From CAD model to burned cavity in six steps
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  Choosing between sinker EDM, wire EDM and CNC milling

The mold parts electrical discharge process does not cut. It erodes. An electrode shaped to the negative of the cavity is brought close to a conductive workpiece, both submerged in dielectric fluid, usually deionized water for wire EDM and hydrocarbon oil for sinker EDM. A servo drives the gap down until the breakdown voltage of the fluid is exceeded and a spark jumps across.

Each spark lasts a few microseconds and reaches local temperatures above 8,000 °C. That is hot enough to melt and partly vaporize a small crater of steel. The fluid immediately quenches the crater and flushes the debris away as a cloud of fine spheres. Thousands of these discharges per second remove material at a rate set by the discharge energy.

The gap is small. On a finishing pass it can sit near 0.01–0.05 mm, which is why the electrode must be undersized by the spark gap plus the intended oversize. Nothing touches. There is no cutting force, no chatter, and no tool deflection pushing the wall off position.

That last point is the reason EDM holds a place in mold shops. A long, thin rib in a hardened cavity insert would spring away from an end mill. In the tank, it simply erodes to shape.

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  Dielectric does two jobsIt insulates until breakdown, then carries debris out of the gap.
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  Discharge energy sets the finishLow energy gives a finer surface but a slower burn.
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  Electrode wear is realAllow for it on roughing; use a separate finishing electrode for tight detail.

Sinker EDM, also called ram or die-sinking EDM, uses a formed electrode that plunges into the workpiece along one axis. Copper and graphite are the usual electrode materials; copper-tungsten handles the finest detail and the worst wear. The electrode is machined slightly small so the burned cavity lands on nominal size.

This is the process for geometry a rotating cutter cannot produce. A square internal corner, a deep rib, a text engraving, a shut-off edge with a 0.05 mm land. If the corner radius of your end mill is larger than the corner in the drawing, sinker EDM is the answer.

Typical mold applications are cavity inserts, cores, slide details, gate pads and parting-line shut-offs. Tool steel at 48–62 HRC burns without any softening of the surrounding material beyond the thin recast layer.

The trade-off is speed and electrode cost. A deep cavity may need several electrodes, and each one has to be milled, measured and indexed. On large pockets it is almost always cheaper to rough with a CNC mill and burn only the last 0.3–0.5 mm.

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  Blind pocketsNo through-hole needed, so the electrode can bottom out on a flat floor.
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  Sharp cornersInternal radii down to a few hundredths of a millimeter are practical.
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  Hardened insertsBurn after heat treatment and avoid the distortion of post-hardening milling.

Wire EDM feeds a thin brass or coated wire through the workpiece while the table moves on a programmed path. The wire never touches the part; the spark does the cutting. Because there is no mechanical force, a 0.25 mm wire can slice through 200 mm of hardened D2 with the same accuracy as through a soft plate.

In mold work this covers through-cavities, insert pockets, split lines, ejector-pin holes, water-line cross-drillings and the outside profile of cavity plates. If the feature goes all the way through and the wall is conductive, wire is usually faster and cheaper than sinking.

Taper cutting lets the wire tilt up to roughly ±30°, which produces draft on a mold wall or a relief angle on a punch in one setup. Four-axis and five-axis wire machines add the ability to cut different top and bottom profiles, useful for tapered water lines and angled shut-offs.

The limits are geometric. Wire cannot cut a blind pocket with a flat floor, and it cannot cut a feature that does not break through the part. Very small internal radii are also off the table: the minimum corner radius equals the wire radius plus the spark gap, so a 0.25 mm wire gives you roughly a 0.15–0.20 mm inside corner at best.

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  Through features onlyThe wire must enter and exit, so a start hole is drilled first.
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  Corner radius floorMinimum inside radius is set by wire diameter plus gap, not by the servo.
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  Taper in one passDraft angles come free on a four-axis wire machine.

EDM is slow at removing volume. A sinker burn might take out 5–20 mm³/min on a roughing setting, while a carbide end mill in a 40-taper spindle removes hundreds. If your cavity is mostly open pocket and only a few corners need burning, mill the pocket and burn the corners. Reversing that order wastes machine hours.

The recast layer matters on fatigue-critical or polished surfaces. Sparks leave a thin remelted skin, typically 1–20 μm, that is harder and more brittle than the base metal and can carry microcracks. On a mold that will be mirror-polished, plan a light finishing pass or a post-EDM polish to remove it. On a structural part under cyclic load, specify the same.

Materials must conduct. Aluminum, tool steel, stainless, copper alloys, titanium and Inconel all burn. Plastics, ceramics and most composites do not. That single fact decides whether EDM is available to you at all.

Finally, electrode and wire costs are per-part. For a one-off prototype, a five-axis CNC cut may be cheaper and fast enough. For a hardened production insert with sharp internal detail, EDM is usually the only route that holds tolerance.

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  Rough with a cutterLeave 0.3–0.5 mm for the burn and cut your EDM time by half or more.
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  Plan for the recast layerSpecify the finishing pass and any polish step on the drawing.
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  Check conductivity firstNon-conductive parts rule EDM out before anything else is discussed.

Discharge energy is the main dial. It is set by peak current and on-time; the off-time controls how fast debris clears before the next spark. High current and long on-time remove volume fast but leave a coarse, deep recast layer. Low current and short on-time give a fine finish at a fraction of the removal rate, so production burns run a roughing pass, one or two semi-finish passes, then a finishing pass.

Flushing decides whether the burn stays stable. If debris cannot leave the gap, the spark repeats in the same spot and you get arcing, which damages both electrode and workpiece. Sinker EDM uses pressure flushing through holes in the electrode, vacuum pull on the back side, or a controlled jump cycle that lifts the electrode to let fluid sweep the gap. Wire EDM uses upper and lower flushing nozzles close to the workpiece.

Thermal growth is small but not zero. The dielectric bath holds the part at a near-constant temperature, which helps, but a long burn still warms the workpiece. On a tolerance call of ±0.005 mm, let the part stabilize in the tank or in a controlled room before the final measurement.

Inspection on a mold insert usually means a CMM check of the cavity profile, the shut-off lands and the position of ejector holes against the drawing. Surface finish gets verified with a profilometer on the polished areas. Reports are issued on request, and every part is checked before it ships.

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  Rough, semi-finish, finishThree passes is the normal sequence for a tight cavity.
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  Flushing prevents arcingWeak flushing shows up as dark burn marks and corner washout.
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  Measure after stabilizationLet the part cool to room temperature before final CMM work.