EDM small-hole drilling for injection mould tooling features
What is EDM small-hole drilling for injection moulds?
A note on terminology: the capability described here is known in the toolmaking industry as EDM small-hole drilling, fast-hole EDM drilling, or start-hole erosion. The phrase "volumetric perforation erosion" is not standard industry terminology; it most accurately maps to this process — the creation of precise perforations in solid mould-steel volumes by controlled spark erosion.
EDM small-hole drilling produces clean, accurate holes in hardened injection-mould steel using electrical discharge erosion rather than mechanical cutting. A rotating tubular electrode — typically brass or copper, Ø0.3 mm to Ø6 mm — is fed toward the workpiece while a pulsed high-voltage current passes between electrode and steel, eroding the material at the tip by repeated spark events. The result is a precise, burr-free hole that can be drilled in fully hardened tool steel without cracking, distortion or cutting force.
Nordmould includes EDM small-hole drilling in its tooling partner network for cooling-channel work, ejector-pin hole preparation, venting and start-hole drilling during both initial mould build and subsequent modifications.
How does the process work?
The electrode rotates continuously while advancing into the workpiece. Dielectric fluid — typically deionised water — is pumped at high pressure (commonly 5–20 bar) through the hollow centre of the electrode directly into the machining zone. The fluid performs two functions: it flushes eroded debris (swarf) out of the hole to prevent secondary arcing, and it cools the electrode and workpiece.
Each electrical discharge lasts microseconds. The spark reaches a localised temperature of several thousand degrees Celsius, melting and vaporising a tiny volume of steel at the electrode tip. Millions of these controlled events in sequence build the hole progressively, advancing at speeds that can exceed 10 mm per minute in steel — far faster than slow-speed sinker EDM, which is why the process is also called fast-hole EDM.
Because there is no mechanical contact between electrode and workpiece, the process introduces no cutting forces, no work-hardening and no micro-cracks — all critical properties when working in hardened mould steel that is already at final hardness.
What mould features are produced by EDM small-hole drilling?
| Feature | Typical diameter range | Why EDM drilling is used |
|---|---|---|
| Cooling channels | Ø1.0 mm – Ø6 mm | Hardened steel; deep and angled channels; conformal layouts |
| Ejector-pin holes | Ø1.0 mm – Ø6 mm | Precise diameter and position; no distortion of surrounding steel |
| Vent holes | Ø0.3 mm – Ø1.5 mm | Sub-millimetre venting that conventional drills cannot reliably produce |
| Start holes for wire EDM | Ø0.3 mm – Ø2 mm | Wire EDM requires a pre-drilled entry point; EDM drilling creates it in hardened steel |
| Sensor and probe pockets | Ø1 mm – Ø6 mm | Temperature or pressure sensors embedded at cavity wall |
| Conformal cooling inlets/outlets | Ø2 mm – Ø6 mm | Entry and exit points for complex cooling-circuit layouts |
Why does this matter for injection-mould quality?
Three cavity-level problems in injection moulding are directly solved or prevented by precise small-hole features:
1. Cooling uniformity. The temperature distribution across the cavity surface determines dimensional stability, cycle time and warpage. Cooling channels drilled close to the cavity wall — and at depths and angles only achievable by EDM drilling in hardened steel — remove heat more evenly than channels cut before hardening, which must follow straight, compromise paths.
2. Trapped gas. Polymer melt advancing through a cavity compresses air ahead of it. If that air cannot escape, it creates a short shot (part does not fill), a burn mark (the compressed gas combusts) or a surface blemish. Vent holes at Ø0.5–1.0 mm evacuate gas while the diameter is too small to admit melt at injection pressure, so no flash is produced.
3. Ejector-pin precision. Ejector pins must slide freely within their holes to push the finished part off the core, yet the clearance must be tight enough to prevent flash between pin and bore. EDM drilling produces the bore to a consistent diameter, roundness and position even in hardened H13 or D2 steel where a conventional drill would deflect or break.
EDM small-hole drilling versus conventional drilling in hardened steel
| Factor | EDM small-hole drilling | Conventional twist drilling |
|---|---|---|
| Works in fully hardened steel | Yes | No — drill breaks or deflects |
| Minimum reliable diameter | ~Ø0.3 mm | ~Ø1.5–2 mm at depth |
| Depth-to-diameter ratio | Up to 300:1 achievable | Typically 3–10:1 before drill wanders |
| Cutting force on workpiece | None | High — risks micro-cracks in hard steel |
| Burrs at hole exit | None | Present — require deburring |
| Effect on hardness | None | Risk of localised work-hardening |
| Speed in hardened steel | Fast (~10+ mm/min) | Very slow or impossible |
Conventional drilling remains the right choice for soft pre-hardened steel in larger diameters. EDM drilling is specified when the steel is at final hardness, the diameter is small, the depth is significant, or the position tolerance is tighter than a drill can reliably hold.
How does EDM small-hole drilling fit into the mould-build process?
In a typical Nordmould Production-tier mould build, EDM small-hole drilling is applied at two stages:
During mould build: cooling channels, vent holes and ejector-pin bores are drilled in the hardened cavity inserts and core blocks after heat treatment. Drilling after hardening eliminates the dimensional shift that occurs during the hardening process, so channel positions and vent locations are accurate relative to the final cavity geometry.
During modification and repair: if a part is warping, a cooling circuit can be extended or redirected by adding new channels with EDM drilling — without disturbing the rest of the tool. If a vent is in the wrong position, it can be welded shut (via micro TIG) and a corrected vent drilled at the right location. The combination of micro TIG welding and EDM small-hole drilling is particularly powerful for tooling modification cycles.
What materials can be EDM drilled?
EDM small-hole drilling works in any electrically conductive material. For injection-mould tooling this includes:
- P20 (pre-hardened) and P20 HH — general mould steel
- H13 — hot-work tool steel used in cores and ejector systems
- S7 and D2 — high-hardness grades for high-wear tooling
- Stainless mould steels — for medical, food-contact and corrosive-material applications
- Beryllium-copper — used in inserts requiring accelerated heat extraction
- Carbide inserts — for extreme-wear applications
Aluminium tooling (Nordmould Bridge tier) is typically drilled conventionally; EDM drilling is used primarily for hardened-steel Production tooling where conventional drilling is limited.
Frequently asked questions
What is EDM small-hole drilling and why is it used in mould making? EDM small-hole drilling uses a rotating tubular electrode and controlled electrical discharges to erode precise holes in hardened mould steel. It is used in mould making because it works in fully hardened tool steel without mechanical force, producing clean, burr-free holes for vents, ejector pins, cooling channels and start holes for wire EDM.
What hole diameters and depths can EDM small-hole drilling achieve? The process covers holes from approximately Ø0.3 mm up to Ø6 mm, at depths up to 200 mm or more — aspect ratios that no conventional drill can reliably reach in hardened steel. Precise achievable depth-to-diameter ratios depend on electrode type and material, and are confirmed at project review.
Can EDM drilling be used on already-hardened mould inserts? Yes. This is one of its primary advantages over conventional drilling. EDM removes material by spark erosion — there is no cutting force — so fully hardened H13, P20 or carbide inserts are machined without cracking, work-hardening or distortion.
What is the difference between EDM small-hole drilling and sinker EDM? Sinker (die-sink) EDM uses a shaped graphite or copper electrode lowered into the workpiece to burn a cavity matching the electrode profile — it creates complex 3D forms. Small-hole EDM uses a rotating tubular electrode to drill precise cylindrical holes at high speed. Both use spark erosion; their geometry and purpose are different.
How does EDM small-hole drilling create cooling channels in a mould? Straight cooling channels are drilled from the back or side of the mould block into the steel close to the cavity wall. In conformal cooling layouts — where channels follow the part contour — EDM drilling reaches depths and angles that a conventional drill cannot navigate without breaking in hardened steel.
What is the role of EDM small-hole drilling in venting injection moulds? Trapped air and gas in a cavity cause short shots, burn marks and poor surface quality. EDM drilling creates vent holes as small as Ø0.3 mm in the cavity wall — small enough to vent gas without creating a witness mark on the part and without flash risk from the melt pressure.
Does Nordmould offer EDM small-hole drilling as part of the tooling service? Yes. Nordmould's EU partner network includes EDM small-hole drilling capability used during mould build, modification and repair. It is quoted as part of the overall tooling scope — customers do not source it separately.
What is the note about 'volumetric perforation erosion' on this page? The phrase "volumetric perforation erosion" is not a standard industry term. It most accurately describes the EDM small-hole drilling and spark-erosion process covered here — a process that creates precise perforations in solid tool-steel volumes through controlled electrical discharge erosion.