Short answer for the drawing: NAK80 is a Ni-Cu-Al precipitation-hardening plastic mold steel, delivered prehardened at about 40 HRC. It fits injection molds, inserts and cores where you want a hard cavity without post-quench distortion, plus a mirror polish by hand. Do not confuse it with P20 (30–34 HRC, no age hardening), 1.2344 / H13 hot-work steel (quenched to 45–52 HRC, used hot), or stainless mold steels such as 420-type / S136. If the part needs more than ~40 HRC, or hardening after machining, NAK80 is wrong and no tolerance tightening fixes that.
Material identity first: what NAK80 actually is
NAK80 is a Japanese-developed special steel for plastic molds, of nominal composition 10Ni3MnCuAl. It belongs to the P20 family of prehardened mold steels, with one deliberate difference: a P20-style Cr-Mo base alloyed with roughly 3 % nickel, about 1 % copper and a controlled aluminium addition. That lets the mill age-harden the block to about 40 HRC in production, so the shop buys a block that is already hard and machines it to cavity finish.
NAK80 is not covered by one widely-used national standard the way Q235 is covered by GB/T 700. Composition and hardness live in mill datasheets and special-steel practice, and suppliers list slightly different windows. A drawing that says only “NAK80” is weaker than one naming the grade plus required hardness and inspection method.
Composition and what each element is doing
Typical as-supplied composition window for NAK80-type precipitation-hardening mold steel:
| Element | Typical content (mass %) | Function |
|---|---|---|
| C | ≤ 0.30 | Low; hardness from precipitation, not martensite carbon |
| Ni | 2.80–3.50 | Matrix toughness and hardenability; supports ageing |
| Mn | 1.20–2.00 | Matrix strength and hardenability |
| Cu | 0.80–1.20 | Copper-rich precipitates that raise hardness on ageing |
| Al | 0.70–1.30 | Nickel-aluminide precipitation; the other hardening mechanism |
| Mo | 0.20–0.50 | Hardenability and temper resistance |
| Cr | ≤ 0.30 | Residual; not a corrosion-resistant mold steel |
| Si | ≤ 0.50 | Deoxidation; high Si would impair polishability |
| P / S | ≤ 0.025 / ≤ 0.020 | Held low for polishability and cleanliness |
Typical grade composition as published by special-steel producers; verify against the mill certificate. Not a national-standard table.
With chromium at residual level there is almost no passive film — NAK80 will rust in a humid shop or on a mold water-line. And the carbon is low on purpose: the 40 HRC comes from Cu and Ni-Al precipitates, so the grade must not be re-austenitised or quenched like a carbon tool steel.
Condition of supply and hardness
NAK80 is normally sold prehardened and preground, hardness approximately 40 HRC (some datasheets give roughly 370–400 HV). Two rules:
- The hardness is set by the mill’s ageing treatment, before you cut. There is no legitimate in-house route from 40 HRC to 50 HRC. Re-quenching a low-carbon, Cr-poor, precipitation-hardened grade gives no useful martensitic hardening and risks cracking.
- Hardness is a specification, not an assumption. Ask for the supplied hardness and test method (usually HRC at the block surface, to a named standard such as ISO 6508 or ASTM E18). A block reading 36 HRC instead of 40 HRC changes tool life and polishing time.
Because the block arrives hard, you skip the heat-treat step that normally warps a P20 cavity after roughing.
Mechanical and physical properties at 40 HRC
Indicative room-temperature properties, prehardened:
| Property | Indicative value | Condition / note |
|---|---|---|
| Hardness | ~40 HRC (approx. 370–400 HV) | As supplied, prehardened |
| Tensile strength Rm | approx. 1250–1450 MPa | At ~40 HRC; datasheet figure |
| Yield strength Rp0.2 | approx. 1000–1200 MPa | Prehardened condition |
| Elongation A | approx. 8–12 % | Low compared with annealed P20 |
| Density | approx. 7.8 g/cm³ | Room temperature |
| Thermal conductivity | approx. 30–35 W/(m·K) | Room temperature |
Indicative values for prehardened ~40 HRC mold steel; use the supplier datasheet for the block in hand. Not guaranteed minimums; the test method changes the number.
Note the elongation: a 40 HRC mold steel is far less ductile than annealed P20, so a thin, tall core insert tolerates little unsupported cutting or clamp error.
Why it polishes — and why that is the point
NAK80 is chosen mainly for two surface properties a plastic mold must have:
- Mirror polishability. Low carbon, controlled sulphur and a homogeneous precipitation-hardened matrix let the surface take a very fine hand or diamond polish for optical, transparent and glossy cosmetic parts. Polish is a shop result, not a guarantee: it depends on grit progression, technique and the inclusion level of the block.
- Uniform hardness, no quench distortion. Because the block is hardened before delivery, there is no post-roughing quench and no distortion to chase with shims or re-datum work.
With a glass-filled resin in high volume or a hot-runner stack, wear and heat resistance become limiting and a different grade is called for.
Machining a 40 HRC prehardened block
At 40 HRC you are not cutting soft stock. Expect light depths of cut, real chatter risk on slender tools, and tool wear that climbs if the feed is too low. Two traps:
- Residual stress and distortion. Prehardened blocks carry stress from forming and ageing. Remove material heavily on one side and the block can bow; on a long, shallow cavity that shows as flatness error after the last cut. Balance stock removal with roughing, semi-finish and finish passes and re-clamping between stages.
- Work-hardened smearing at the polished surface. Cutting with a dull edge or too light a feed “rubs” the matrix and can bury fine, worked metal that is harder to polish out. Keep edges fresh and feeds consistent.
Conditional starting points (not guaranteed parameters)
Starting points for coated carbide on a rigid machining centre with good workholding. Results depend on machine rigidity, tool grade and coating, tool overhang, coolant, and whether the block was stress-relieved. No figure is a production commitment.
| Operation | Starting speed | Starting feed | Comment |
|---|---|---|---|
| Rough milling | 80–130 m/min (coated carbide) | 0.08–0.20 mm/tooth | Conservative DOC; minimise tool overhang |
| Semi-finish milling | 100–160 m/min | 0.05–0.12 mm/tooth | Constant-engagement toolpaths reduce load spikes |
| Finish milling | 120–180 m/min | 0.03–0.08 mm/tooth | Fresh edge; climb milling for surface quality |
| Drilling / piloting | 25–45 m/min (HSS-E or carbide) | 0.05–0.15 mm/rev | Peck deep holes; clear chips with coolant |
| Fine boring / reaming | 60–100 m/min | 0.05–0.15 mm/rev (ream) | Rigid set-up; watch hole-size drift from tool wear |
Tooling choice. For milling, use a coated carbide grade with a hard, wear-resistant coating (for example TiAlN or AlCrN) and a geometry that keeps cutting forces low. For drilling and reaming at 40 HRC, cobalt HSS-E or solid carbide with a stiff, short overhang beats an economical HSS drill. Flood or high-pressure coolant helps chip evacuation and keeps the edge from overheating in deep pockets. Where a finished cavity has a hard requirement, leave a grinding or EDM allowance rather than expecting a milled surface to hit a lapped-mirror finish. Related notes: design for manufacturability, tolerance selection, and surface finish requirements.
How NAK80 compares with the grades it is confused with
Compare the same condition and metric before substituting.
| Grade | Family / condition | Typical hardness | Practical difference |
|---|---|---|---|
| NAK80 | Ni-Cu-Al precipitation-hardening mold steel | ~40 HRC as supplied | Hard block, no post-quench distortion, excellent polish |
| P20 (1.2311) | Cr-Mo prehardened mold steel | ~30–34 HRC as supplied | Cheaper, easier to machine, softer and less wear-resistant |
| 718 (P20+Ni, 1.2738) | Ni-alloyed prehardened mold steel | ~30–36 HRC, thick sections | Better through-hardness on thick blocks; no age hardening |
| 1.2344 / H13 | Cr-Mo-V hot-work steel, quenched + tempered | ~45–52 HRC after heat treatment | Hardens only after a quench you perform; for hot work |
“P20+Ni” is a family description, not a synonym for NAK80: 718-type steel is hardened by its alloy content as supplied, whereas NAK80 depends on a controlled Cu / Ni-Al ageing treatment to reach 40 HRC. Near-equivalent mold steels from different mills are not automatically interchangeable — match composition window, supplied hardness and cleanliness before swapping.
Where NAK80 fits — and where it does not
Good fits: injection molds and inserts for transparent, optical and high-gloss cosmetic parts; cavities and cores needing a mirror polish without an in-house hardening step; medium-volume tools where a 40 HRC wall resists denting better than a 30 HRC P20 block.
Poor fits: high-volume molds running aggressive glass- or mineral-filled resins, where abrasive wear outruns a 40 HRC surface; hot-work and die-casting inserts needing red-hardness above 500 °C; parts to be quenched and tempered to 50 HRC or more after machining; and any service needing real corrosion resistance — NAK80 is not a stainless mold steel, so a wet or chemical environment calls for a different family (420-type / S136).
What to put on a drawing or RFQ for a NAK80 part
- Full designation: the grade “NAK80” plus the required hardness range and, if known, the intended supplier grade.
- Condition of supply: prehardened block, supplied hardness (e.g. ~40 HRC) and the test method.
- Mill certificate: composition and supplied hardness, heat number traceable to the block.
- Drawing: cavity/core geometry, critical dimensions, tolerances, datum scheme, and which faces are functional polish surfaces.
- Surface finish: required polish level on optical or cosmetic faces, and a separate Ra for non-cosmetic faces.
- Draft, shrinkage and quantity: resin and shrinkage allowance, cavities per tool, and spare inserts.
- Inspection: dimensions to be checked, and whether a report ships with the tool.
Getting a quote
If you have a NAK80 cavity, core or insert and want a manufacturability review and quote, send the drawing with the material specification and grade, the condition of supply and supplied hardness, the quantity, the tolerances and the surface finish requirements. Those details let a shop confirm the grade and plan the roughing, finishing, EDM and polishing sequence. Request a custom quote.
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