NAK80 Steel for Injection Molds

What NAK80 is

NAK80 is an age-hardened plastic mould steel supplied at roughly 37–41 HRC. It is a Japanese grade with no exact DIN equivalent, often classified nominally as a modified 1.2738 or P21. Its defining property is a very clean, uniform microstructure that takes a mirror polish, which is why it is the default for a high-gloss visible surface that has to be cut pre-hardened.

Age hardening is what makes the grade practical. The block is supplied already hard, so a large cavity can be machined, polished and run without the distortion of a hardening cycle. That is the same advantage P20 and 718H offer, taken to a higher hardness and a finer polish.

It is not a stainless steel and it is not a wear steel. Those two limits decide most of the applications it is wrong for, and they are worth stating before the block is ordered.

Hardness, and what it does not buy you

At 37–41 HRC, NAK80 is the hardest of the common pre-hardened cosmetic grades. That hardness holds a mirror surface through a long campaign and resists light indentation at the parting line and the ejector seats. On a high-gloss appliance face or a clear cosmetic cover, it is a genuine improvement over 718H.

It does not buy abrasion resistance. Glass fibre will dull the polish and wear the gate regardless of the extra hardness, and a mirror cavity in a glass-filled tool is a contradiction: the finish that justifies the steel is the first thing the fibre destroys. A hardened hot-work grade is the answer there.

It also does not buy corrosion resistance. The alloy is not stainless, and a humid tool room or a corrosive resin will attack it. A mirror surface makes corrosion more visible, not less: a rust bloom on a polished NAK80 face means a re-polish, and a pit means a weld and a re-polish.

Corrosion and polishability

Polishability is the reason to buy NAK80. The steel is clean enough to take a true mirror finish, and it holds that finish better than a standard pre-hardened block because the structure is uniform and the hardness is higher. On a part that has to read as a glossy black or a clear lens, the surface is the product, and this is the grade that delivers it without a hardening cycle.

Corrosion resistance is low to moderate. It is somewhat better than P20 in a humid environment, which makes it more forgiving in storage, but it is not a stainless grade. PVC, some flame retardants and any wet storage will still cause damage. The practical routine is to dry the cavity after every run, protect it with a film, and keep the cooling circuit treated.

The combination matters: a mirror cavity is the most expensive surface in the tool, and it is also the most visible when it fails. A corrosion pit on a textured 718H wall might be hidden; on a NAK80 mirror face it is a defect on the part. That is why a NAK80 tool is usually specified with a storage and cleaning procedure, not just a steel grade.

Where the part is both high-gloss and exposed to a corrosive resin, NAK80 is the wrong choice and a stainless mirror-grade steel is the right one. The polish requirement does not override the corrosion requirement.

By production volume

Below roughly 100,000 shots, NAK80 is usually more steel than the campaign needs unless the surface is genuinely optical or mirror-grade. A 718H cavity will hold a high-gloss finish for that volume at a lower block price.

Between 100,000 and a million shots, NAK80 is in its element for an unfilled cosmetic part: the hardness holds the polish, and the pre-hardened condition avoids a hardening cycle on a large cavity. Above that range, the maintenance of a mirror surface becomes the limiting factor, and a stainless grade with comparable polish and better corrosion resistance is often the better long-term choice.

Volume alone is not the test. A 50,000-shot clear lens may justify NAK80 because the surface requirement is absolute; a 500,000-shot matte housing may not, because the texture hides the polish difference.

By resin

NAK80 suits unfilled cosmetic resins: ABS for high-gloss housings, acrylic for light covers, polystyrene for display parts, and unfilled PC where a clear or high-gloss surface is required and the resin is not aggressive.

PC is a qualified match. The steel can run the higher mould temperature, and the polish suits a clear part, but a true optical requirement still points to a stainless grade. For a pigmented high-gloss PC housing, NAK80 is a strong choice.

Glass-filled resins such as PA+GF are a mismatch. The fibre dulls the mirror and wears the gate. PVC and aggressive flame-retardant grades are also a mismatch, because the corrosion resistance is not there.

The pattern is simple: NAK80 is for a surface-critical, unfilled, non-corrosive part. Move outside any one of those three and a different grade is the honest recommendation.

Heat treatment, welding and repair

NAK80 is age-hardened and supplied ready to use, so the normal tool never sees a hardening cycle. Re-hardening is not part of the routine and should not be attempted without the supplier's procedure, because the age-hardening response is specific to the grade.

Welding is possible but less forgiving than on P20 or 718H. The weld heat affects the hardness around the repair, and a mirror face will usually show the transition after polishing. Use a matching filler, preheat, control the heat input and cool slowly, and expect to re-polish a wider area than the weld itself.

For a wear location such as a gate, a replaceable insert is a better design than a repair. The insert can be made in a harder grade, replaced when it wears, and kept away from the mirror surface that the rest of the tool is protecting.

Nitriding is not normally applied to NAK80 for polish retention, because the treated surface polishes differently. Where extra wear resistance is needed at a specific spot, a separate hardened insert is the cleaner solution than a surface treatment on the mirror cavity.

Surface treatment and coatings

Surface treatments are used sparingly on NAK80 because the polished surface is the point of the grade. Nitriding hardens but changes the polish response, and chrome plating adds a layer that can chip at an edge and must be stripped before a repair.

A release coating is sometimes used on a difficult draw, but the same result is usually achieved by polishing and draft. On a mirror cavity, adding a coating is a last resort rather than a default.

Where corrosion protection is needed for storage, a protective film is preferable to a permanent coating: it can be removed before the run and does not change the cavity dimension.

Cost per shot vs cost of steel

NAK80 is the most expensive of the pre-hardened cosmetic grades, and the premium is justified by the surface, not by the volume. The block price difference is small compared with the polishing time on a mirror cavity, so the real question is whether the part needs that surface at all.

If it does, NAK80 usually removes a re-polish stop from the campaign and protects the appearance that the part is sold on. If it does not, 718H delivers a high-gloss finish at a lower cost and the NAK80 premium buys nothing.

Against a stainless mirror grade, NAK80 is usually cheaper and easier to supply in a large plate, but it gives up corrosion resistance. Where the resin is corrosive, the stainless premium is the cost of not repairing a mirror cavity mid-campaign.

Common selection mistakes

• Buying NAK80 for a textured or matte part. The mirror polish is hidden by the texture, and the premium buys nothing the part can show.
• Using it in a glass-filled tool. The fibre dulls the surface and wears the gate. A hardened grade with a replaceable insert is the correct answer.
• Assuming it is stainless. It is not, and a mirror surface makes corrosion damage more visible and more expensive.
• Welding a mirror face without planning the re-polish. The heat-affected zone polishes differently and the repair usually shows.

What to send us

Send the 3D part, the resin grade and filler content, the surface standard including the gloss level or the SPI/VDI texture, the annual volume and campaign length, and the cavity count. If the part is clear or mirror-finished, say so explicitly, because that requirement decides between NAK80, a stainless mirror grade and a standard cosmetic steel.