In the world of plastic mold steel, NAK80 has a well-known nickname—"mirror steel." Many mold shops making transparent parts or high-gloss appearance parts think of it first when mirror polishing comes up. But oddly enough, with the same NAK80, some get a mirror finish while others get haze, pinholes, or a surface that gets worse the more they polish. Where does the problem come from? Today we skip the composition table and talk about the three biggest traps in NAK80 machining.
1. Mirror Polishing: More Force Isn't Better
NAK80 is pre-hardened, typically supplied at HRC 38-42, with a very uniform microstructure—that's the foundation for its mirror polishability. But many polishers have a "brute force" habit: high speed, heavy pressure. The surface heats up, local hardness drops, and you get orange peel and haze instead of a mirror.
The right approach: step down gradually, light pressure, slow speed. From coarse sandpaper to fine, then to diamond paste—each step must completely remove the previous step's scratches before moving on. Skipping ahead too fast leaves deep scratches that can't be removed later, forcing rework. Also, always use coolant or dip in water frequently; don't let the surface get too hot to touch.
2. EDM: The White Layer Must Go
Mirror mold cavities often have complex details requiring EDM. But EDM leaves a "white layer" (recast layer) on the NAK80 surface—brittle, hard, and micro-cracked. If you polish directly, the white layer peels off in patches, creating pinholes and pits. If you texture without removing it, the etch pattern becomes uneven.
So after EDM on NAK80, you must grind away the white layer with oilstone or sandpaper before polishing. In‑shop visual inspection under magnification is the typical method to confirm complete removal. Nital etching is a metallurgical lab test method and should not be applied directly to finished mold surfaces. Skip this step, and you'll pay for it later.
| Comparison Items | NAK80 Mold Steel | S136 Mold Steel |
| Hardness (Delivery) | 38-42 HRC, pre-hardened | Annealed (S136H pre-hardened available), needs hardening |
| EDM White Layer | Must be removed before polishing | Must be removed before polishing |
| Polishing after EDM | Excellent, but white layer sensitive | Excellent, more forgiving |
| Corrosion Resistance | Moderate | Excellent |
| Weldability | Possible, strict process required | Fair |
| Main Use | High-gloss plastic molds, transparent parts | Corrosive plastics, precision mirror molds |
As shown above, both NAK80 and S136 require white layer removal after EDM, but NAK80 is more sensitive to incomplete removal because of its pre-hardened mirror-grade microstructure. S136, while needing heat treatment, is more forgiving in EDM and polishing, and offers far better corrosion resistance. If your plastic mold involves corrosive resins or long-term rust prevention, S136 is the safer bet; if you want to skip heat treatment and still achieve a mirror finish, NAK80 is the faster route—just don't skip the white layer step.
3. Welding: Proceed With Extreme Caution
NAK80 can be welded, but improper welding procedures create serious surface‑quality risks. Without post‑weld age‑hardening treatment, the heat‑affected zone will have lower hardness than the base material. During polishing, this hardness mismatch may produce ghost lines or uneven surface finish. During texturing, welded areas will show inconsistent etch depth and leave visible patch marks.
If welding is foreseeable at the design stage, consider switching to P20 or 718 for simpler repair work. If welding cannot be avoided: use matching NAK‑W filler rod, apply proper preheat, and perform the required post‑weld age‑hardening treatment. Most importantly, keep all welded regions away from mirror‑finish cavity surfaces.
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FAQ
Q1: What is the delivered hardness of NAK80 mold steel?
A: Typically HRC 38-42. It is a pre-hardened mirror plastic mold steel that can be machined directly without heat treatment.
Q2: Can NAK80 be polished directly after EDM?
A: No. The white layer left by EDM must be removed with oilstone or sandpaper first, otherwise polishing will produce pinholes and pits.
Q3: Can NAK80 mold steel be welded?
A: Welding is possible but requires strict process control. Without matching filler metal and post-weld age-hardening treatment, heat-affected zones will show hardness mismatch, causing ghost lines in polishing and uneven texturing. Avoid welding on mirror surfaces whenever possible. If welding must be done, use matching NAK-W filler rod, preheat, and complete post-weld age-hardening.
Q4: How to choose between NAK80 and S136 for mirror molds?
A: NAK80 is pre-hardened, saving heat treatment time, suitable for small-to-medium batch high-gloss molds. S136 has better corrosion resistance, suitable for corrosive plastics like PVC or molds requiring long-term rust prevention.