Basic Knowledge of CNC Mold Processing

A mold is a negative of the part you want to produce. CNC mold processing is the set of machining operations that cut that negative into steel or aluminum. The cavity forms the outside surface of the part, the core forms the inside. Everything else in the tool, ejector plates, slides, lifters, cooling channels, gate inserts, exists to support those two blocks.

The workflow starts from a 3D CAD model of the part. The mold designer adds shrink compensation, parting line, draft, and a runner and gate layout, then produces a mold assembly model. CAM software turns the cavity and core surfaces into toolpaths. A machinist picks the tools, sets the stock, and cuts. After machining, the tool goes to EDM for sharp internal corners, then to bench work for spotting and polishing.

The reason CNC matters here is repeatability. A mold may run 500,000 cycles or more. If the cavity is off by 0.05 mm, every part from that tool is off by 0.05 mm. There is no second chance to adjust the tool once it is hardened, so the machining tolerance on the cavity and core is usually tighter than the tolerance on the molded part.

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  CavityThe half that forms the visible or outer surface of the part.
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  CoreThe half that forms the inner surface, often with cooling and ejection.
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  Parting lineWhere the two halves meet; it sets draft and flash control.
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  Shrink compensationThe cavity is cut oversize to offset material shrinkage after cooling.

Most mold cavities are not simple. Deep ribs, tall cores, and curved parting surfaces force the tool into angles a 3-axis machine cannot reach without a long, thin tool that deflects. That is where 4-axis and 5-axis machining changes the plan. A 5-axis machine tilts the tool or the table so a short, stiff cutter can reach the same surface at the correct angle.

For a shallow part with a flat parting line, a 3-axis machine is usually enough and costs less per hour. For a deep box mold, a lift-rib mold, or a mold with undercuts that would otherwise need multiple EDM electrodes, 5-axis simultaneous machining is the faster route. On our floor, 16 simultaneous 5-axis machining centers handle that class of work, alongside 12 four-axis mills and 27 three-axis machines.

The trade-off is not just speed. Five-axis finishing produces a more uniform surface on curved walls because the tool contact point stays consistent. That reduces hand polishing, which is often the longest step in mold delivery. If your mold has large sculpted surfaces, specify 5-axis finishing early; if it is a plate with pockets and holes, 3-axis with a good fixture is the better use of budget.

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  3-axisFlat parting lines, plates, pockets, simple cores. Lowest cost per hour.
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  4-axisRotational features, angled holes, parts with a fourth-side datum.
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  5-axisDeep cavities, sculpted surfaces, undercuts, reduced EDM and hand work.

A common mistake is to tolerance every surface the same. That drives cost without improving the part. Split the drawing into three groups. First, the molded part features that must be held, typically ±0.05 mm on a small part. Second, the mold features that control those dimensions: cavity depth, core height, shutoff angles. Third, non-critical surfaces like the mold base, waterline bosses, and clamp slots, which can be held to ±0.1 mm or looser.

On our machines, the achievable machining tolerance is ±0.005 mm, and we hold ±0.0002 in when the drawing calls for it. But the mold does not need that everywhere. A shutoff surface that seals the cavity does need it. A cooling channel does not. Mark the critical surfaces on the drawing and let the shop spend time where it counts.

Surface finish follows the same logic. A cavity surface that forms a visible part face is usually polished to Ra 0.2–0.8 μm. A hidden rib or an internal core pin can stay at Ra 0.8–1.6 μm. As-machined surfaces at Ra 1.6–3.2 μm are fine for most structural cores. Specifying a mirror finish on every surface adds hours of bench work and rarely shows up in the part.

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  ±0.005 mmMachining tolerance for shutoffs, cavity depth, core height.
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  ±0.05 mmTypical molded-part dimension on a small technical part.
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  Ra 0.2–0.8 μmVisible part surfaces and optical cavities.
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  Ra 1.6–3.2 μmStructural cores, hidden faces, non-cosmetic areas.

Machining is only part of the timeline. After the cavity and core are cut, sharp internal corners go to EDM, usually with a copper or graphite electrode that was itself machined on a 3-axis mill. Then the mold is assembled and spotted. Spotting means applying bluing to one half and closing the tool to see where it contacts. High spots are stoned down by hand until the shutoff seals.

Next comes cooling and ejection. Waterlines are drilled or milled, then pressure tested. Ejector pins are fitted and checked for length. The tool goes into a press for a tryout shot. The first shots tell you whether the gate is too small, whether the part sticks, and whether the shrink compensation was correct. Most molds need one or two adjustments at this stage.

A realistic sequence for a small to mid-size mold runs through design review, DFM feedback, roughing, semi-finishing, finishing, EDM, bench work, spotting, and tryout. We quote and return a free DFM analysis within 12 hours, and production can start within 24 hours of an approved design. Parts from bridge tooling typically ship in 3–5 days.

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  CAMToolpath generation, tool selection, and stock setup.
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  EDMSharp corners, deep ribs, and hardened steel after heat treat.
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  SpottingBluing and hand fitting until the shutoff seals.
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  TryoutFirst shots in a press to check gate, warp, and ejection.

Mold components span more than cavity steel. Ejector pins, slides, and wear plates are often different alloys than the cavity. We machine 6061, 7075, 2024, and ADC12 aluminum for bridge tools and prototype molds. For production tooling we work with 718H, P20, H13, S136, and NAK80, plus 420 and 440C stainless for corrosion-resistant inserts.

On the plastics side, the mold design changes with the resin. ABS and PC need different shrink factors. POM and PA are more abrasive and wear gates faster. PEEK and glass-filled grades call for hardened steel and generous gate size. If you know the resin before the mold is designed, the tool will hold tolerance longer.

Titanium and Inconel appear in mold work less often, but they show up in fixtures, hot runners, and special inserts where wear or heat resistance matters. For those, we machine TC4 (Ti-6Al-4V) and Inconel on the same 5-axis centers used for cavity work.

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  Aluminum6061, 7075, 2024, ADC12 for bridge and prototype molds.
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  Mold steel718H, P20, H13, S136, NAK80 for production cavities.
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  Stainless420, 440C for inserts that resist corrosion and wear.
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  PlasticsABS, PC, POM, PA, PEEK, glass-filled grades.