EDM machining for injection mould tooling — sinker & wire
What is EDM machining, and why does it matter for injection mould tooling?
EDM — electrical discharge machining — removes metal from hardened steel using precisely controlled electrical sparks rather than cutting tools. Because there is no mechanical contact, EDM can machine cavity features in fully hardened tool steel that conventional milling and grinding cannot reach: sharp internal corners, deep narrow ribs, complex three-dimensional contours, and fine surface textures. Nordmould's partner toolroom uses EDM as a core step in building and maintaining injection mould tools, which is why short revision cycles and tight-tolerance cavities are achievable within the standard 4–11 week tooling window.
How does electrical discharge machining work?
The workpiece (the mould steel) and the electrode are separated by a small gap — typically 0.01–0.5 mm — filled with a dielectric fluid, usually deionised water or oil. Precisely timed voltage pulses across the gap produce sparks that vaporise and eject tiny particles of metal from the workpiece surface. The process repeats thousands of times per second. The net effect is controlled metal removal with no cutting force and minimal heat transfer to the surrounding steel.
Material removal rate, surface roughness, and accuracy are controlled by adjusting pulse duration, peak current, gap voltage, and flushing conditions. Roughing passes remove material quickly at the cost of surface finish; finishing passes run at low energy to achieve the final dimension and surface texture.
What are the two main EDM processes used in mould making?
Sinker EDM (die-sinking EDM)
In sinker EDM, a shaped electrode — machined from graphite or copper — is fed into the steel to erode a mirror-image cavity. The electrode is consumed during the process, so a series of electrodes may be needed for a single feature: rough graphite electrodes for the bulk of material removal, fine copper electrodes for finishing.
Sinker EDM is used for:
- Mould cavity forms that are three-dimensional or have draft angles
- Deep ribs and slots narrower than any available milling cutter
- Sharp 90° internal corners (a milling cutter always leaves a radius)
- Textured cavity surfaces, such as leather-grain or fine stipple, applied directly in the steel
- Blind pockets inaccessible to milling from above
Achievable surface finish with finishing passes: Ra 0.1–0.4 µm. Positional accuracy: typically ±0.005 mm for production-grade cavity work.
Wire EDM (wire-cut EDM / wire erosion)
Wire EDM uses a thin brass wire — typically 0.1–0.3 mm diameter — fed continuously from a spool and held under tension between upper and lower guides. High-frequency pulses arc between the wire and the workpiece, eroding the steel along a programmed two-dimensional or contoured path. The wire never touches the steel, so there is no deflection and no cutter wear.
Wire EDM is used for:
- Parting-line profiles cut precisely through core and cavity steel
- Mould slides, lifters, and core pins where the profile must match the cavity exactly
- Punches, ejector retainer plates, and die sections cut to final dimension in hardened steel
- Through-features — windows, vents, and registration slots — where the full section must be cut cleanly
- Splitting electrode blanks before sinker EDM finishing
Achievable tolerance: ±0.002–0.005 mm in a controlled environment. Surface finish after fine cutting: Ra 0.2–0.4 µm.
What precision does EDM deliver in practice?
| Process | Typical position tolerance | Surface finish Ra | Suitable hardness |
|---|---|---|---|
| Sinker EDM — rough | ±0.02 mm | 1.6–3.2 µm | Up to 60 HRC |
| Sinker EDM — finish | ±0.005 mm | 0.1–0.4 µm | Up to 60 HRC |
| Wire EDM — standard | ±0.005 mm | 0.4–0.8 µm | Up to 60 HRC |
| Wire EDM — fine | ±0.002 mm | 0.2–0.4 µm | Up to 60 HRC |
These figures apply to stable, temperature-controlled toolroom conditions with calibrated equipment. Nordmould's partner toolroom operates to these benchmarks and provides dimensional inspection records on request.
Why does in-house EDM capability matter for your tooling project?
When EDM is separated from the rest of the toolroom — whether outsourced to a third-party shop or handled across different sites — each revision cycle requires steel to be transported, scheduled externally, inspected on return, and re-qualified. For a mould that needs two or three cavity corrections during T1 trials, this can add a week or more per cycle.
Nordmould's partner toolroom combines milling, turning, surface grinding, EDM, and assembly under one roof. When a trial shot reveals a flow mark caused by a sharp rib corner, the toolmaker can sink a corrective electrode the same day and re-trial within 24–48 hours rather than waiting for an external EDM slot. This is one of the main reasons that Nordmould can commit to firm tooling lead times rather than open-ended estimates.
A further advantage is that electrode geometry and cavity corrections are designed and machined by the same people who built the tool. Electrode fit, spark gap allowance, and dielectric flushing strategy are planned as part of the original tool design — not solved after the fact by a supplier who has never seen the full assembly.
What mould features specifically need EDM?
Some cavity features cannot be produced by milling regardless of cutter size:
- Sharp internal corners — a milling cutter always leaves a radius matching the tool diameter. EDM sinks a true 90° corner (or radius below 0.1 mm) limited only by the electrode edge.
- Deep narrow ribs — ribs below 0.8 mm wide and deeper than 3–4× their width deflect or break milling cutters. EDM erodes them without any cutting force.
- Post-hardening cavity corrections — once tool steel is hardened to 50–60 HRC, milling is impractical. EDM is the standard route for all corrective work and final sizing after heat treatment.
- Complex parting surfaces on slides — the mating faces of side-action slides and lifters must match the cavity contour exactly. Wire EDM cuts both mating faces in the same setup to guarantee the fit.
- Fine surface texture — textures applied directly in cavity steel by sinker EDM match from tool to tool and cannot migrate the way applied chemical textures can.
How does EDM fit into the overall tooling process at Nordmould?
The typical toolmaking sequence places EDM after rough milling and heat treatment:
- Rough milling — bulk steel removal to near-net shape in pre-hardened steel (P20 at ~30 HRC) or in soft steel before hardening
- Heat treatment — cavity steel hardened to 50–60 HRC for Production-tier tools
- Sinker EDM — cavity forms, ribs, and textured surfaces machined to final dimension in hardened steel
- Wire EDM — parting-line profiles, slides, core pins, and ejector features cut to tolerance
- Surface grinding — flat reference surfaces and shutoff faces ground to flatness
- Polishing — optical or cosmetic surfaces hand-polished after EDM as required
- Fitting and assembly — tool components assembled, tested for movement, and aligned before T1 trial
Nordmould coordinates this sequence across its partner toolroom network and confirms the planned manufacturing route as part of every written tooling quote.
Frequently asked questions
What is EDM and why is it used in injection mould making? EDM (electrical discharge machining) removes material from hardened steel using precisely controlled electrical sparks rather than cutting tools. It produces features — deep ribs, sharp internal corners, fine cavity textures — that milling or grinding cannot reach. Nordmould's partner toolroom uses EDM as a standard step in building high-precision injection mould tools.
What is the difference between sinker EDM and wire EDM? Sinker EDM uses a shaped graphite or copper electrode to erode a matching cavity into the steel — ideal for blind pockets, textured surfaces, and three-dimensional cavity forms. Wire EDM uses a thin brass wire to cut through steel along a programmed path — ideal for punches, slides, parting-line profiles, and through-features with tight positional accuracy.
What tolerances can EDM hold in mould tooling? Wire EDM typically holds ±0.002–0.005 mm on feature positions and profiles. Sinker EDM with fine finishing passes achieves surface roughness of Ra 0.1–0.4 µm and cavity dimensions within ±0.005 mm. Nordmould confirms the tolerance requirements for your tool at DFM review stage.
Can EDM machine hardened steel without distortion? Yes. EDM is a non-contact, thermally controlled spark erosion process with virtually no cutting force. This means hardened tool steels (50–60 HRC) can be machined after heat treatment without the dimensional spring-back or distortion that conventional milling would cause. It is the standard route for finish-machining cavity steels.
How does in-house EDM capability affect mould lead time? When EDM is available in the same toolroom as milling, turning, and assembly, cavity corrections, electrode trials, and fine finishing happen without shipping steel between suppliers. This typically removes 3–7 working days per revision cycle compared to outsourcing EDM, which directly shortens the 4–11 week tooling window that Nordmould quotes.
What materials can be EDM machined? EDM works on any electrically conductive material. In mould making, the main materials are P20, H13, S136 (stainless tool steel), and hardened grades up to 60 HRC. Copper and graphite are used as electrode materials for sinker EDM. Aluminium can be EDM machined for Bridge-tier tools.
Does EDM leave a surface that needs further polishing? It depends on the application. Fine EDM finishing produces Ra 0.1–0.4 µm, which is acceptable for most mould cavity surfaces and can reduce hand-polishing time significantly. Optical or high-gloss surfaces require additional hand polishing after EDM to reach Ra < 0.05 µm. Nordmould specifies the finishing route in the tooling quote.
What mould features specifically require EDM rather than milling? Features that require EDM include: sharp internal corners (milling leaves a radius equal to the cutter radius), deep narrow ribs below 1 mm width, fine surface textures applied directly in the steel, complex cavity profiles in fully hardened steel, and precise contoured parting surfaces on slides and lifters.