NAK80 Steel: Composition, Properties, and Engineering Applications in Precision Mold Making

NAK80 mold steel is a type of steel that has already been hardened and is usually supplied with a hardness of 37–43 HRC. NAK80 is different from regular tool steels in that it can be machined directly to its final dimensions without the need for rough machining and heat treatment (which can cause distortion). This feature does away with heat treatment after machining, which cuts down on lead times and lowers the risk of manufacturing. This means that designers can count on the surface quality of clear parts, and procurement teams can make a strategic trade-off by paying more for raw materials but getting them processed faster and with fewer mistakes.

What Is NAK80 Steel and How Is It Classified?

To really understand NAK80 steel, we need to look at more than just its brand name; we also need to look at how it is classified as a metal. NAK80 is a type of plastic mold steel that has been pre-hardened and is made for applications that require very high precision.

Defining Materials and the Benefits of “Pre-Hardened”

From a classification point of view, NAK80 is a type of precipitation-hardening steel. It comes from the mill already heat-treated to its working hardness (HRC 37–43). This sets it apart from other mold steels that are commonly used:

• When compared to S136 (stainless steel), S136 is often delivered in an annealed state and needs to be quenched and tempered after roughing to get the hardness, which can cause distortion.

• SKD61 (H13) is a hot-work tool steel that needs to be heated to a high temperature to reach working hardness, which makes it take longer to process.

Engineering Implication: NAK80 has great dimensional stability because it doesn’t need to be quenched after machining. A complicated mold core made from NAK80 will keep its shape without the warping or size growth that comes with heat-treated steels.

Brand Steel vs. Generic Versions

Daido Steel (Japan) first made NAK80, which is a proprietary grade. There are a lot of materials on the market that say they are “NAK80 equivalent,” but engineers should be careful.

• Original NAK80: Has tightly controlled chemistry and vacuum arc remelting (VAR) or strict refining to keep the number of inclusions low.

• Generic Substitutes: They often don’t have the cleanliness needed for real mirror polishing. Inclusions and segregation in generic grades can cause “pinholes” or “orange peel” during the last polishing stage, which could mean throwing away a mold insert that is worth a lot of money.

The main point is that NAK80 is chosen because it is predictable. In high-stakes optical or cosmetic tooling, the extra cost of real or high-quality certified NAK80 is like insurance against polishing failures that happen late in the process.

What Is the Chemical Composition of NAK80 Steel?

The reason NAK80 works so well in precision molds is because of a very specific Nickel-Aluminum-Copper alloying system. NAK80 doesn’t harden like carbon does; instead, it uses precipitation hardening (age hardening), which gives it special properties.

Table 1: Typical Chemical Composition (Engineering Ranges)

Note: Different manufacturers may have slightly different proprietary compositions.

Nickel and aluminum are important because they are the aging mechanism.

Nickel (Ni) and Aluminum (Al) work together to make NAK80 work well. During the manufacturing process, these elements come together to make intermetallic precipitates, such as 𝑁𝑖3𝐴𝑙 Ni3​ Al was spread out throughout the steel matrix.

• Uniform Hardness: Precipitation hardening happens evenly throughout the cross-section, unlike quenching, which hardens from the outside in (which can leave a softer core in thick blocks). The hardness of a 300mm thick block of NAK80 is almost the same in the middle as it is on the outside.

• Better Polishability: Because it has a low carbon content, the microstructure has very few hard, jagged carbides. The uniform matrix polishes evenly, which means that it can be used to make optical-grade finishes without “grain pull-out.”

What Are the Mechanical Properties of NAK80 Steel?

Engineers choose NAK80 not for its strength, but for its accuracy. Its properties are set up so that it won’t change shape when it’s injected molded, but it can still be worked with.

Table 2: Mechanical Properties (Common at Room Temperature)

Consistency of Hardness

The “sweet spot” for making molds is between 37 and 43 HRC.

• Machinability: You can cut it with regular carbide tools (coated) without the extreme wear that happens with steels with a hardness of 50 HRC or higher.

• Durability: It is strong enough to handle millions of cycles of non-abrasive plastic injection (like Polycarbonate or ABS) without losing the integrity of the parting line.

Polishability: The Most Important Trait

NAK80 and mirror finish are almost the same thing.

• Compatible with 14,000 grit: can reach surface roughness levels of Ra < 0.02 µm.

• No Pinholes: The vacuum degassing and clean alloy design keep non-metallic particles to a minimum, which stops the tiny pits that ruin optical molds.

This means that NAK80 is the best choice for automotive lenses, LCD bezels, and cosmetic containers.

Is Heat Treatment Required for NAK80?

“Do we need to heat treat this after machining?” is one of the most common questions that junior engineers and procurement professionals ask.
The short answer is NO.

Why it is not a good idea to re-harden

NAK80 comes in a state that has been hardened by precipitation. Trying to quench and temper it like a regular tool steel will probably ruin its properties.

• Risk of Dimension Change: When you reheat something, it expands and changes phases, which ruins the precise tolerances that were achieved during machining.

• Risk of Microstructure Damage: Heating that isn’t controlled can make the precipitates coarser, which makes them less tough and less easy to polish.

When do you need to relieve stress?

Stress Relief may help in some situations, but heat treatment for hardness is not needed.

• Heavy machining: If a lot of material is taken away (like deep pocketing), residual machining stresses can make things move a little.

• EDM (Electrical Discharge Machining): Heavy EDM can cause stress on the surface.

• Process: A low-temperature stress relief (usually around 500°C, check with your supplier) can be done to keep the dimensions stable without changing the hardness.

How Does NAK80 Perform in Machining and EDM?

NAK80 is different from annealed steels (like P20 at ~30 HRC) or fully hardened tool steels because it is pre-hardened to about 40 HRC.

Guidelines for CNC Machining

• Cutting Forces: NAK80 makes cutting forces that are stronger than those of P20. Machines need to be strong.

• Tools: Use carbide tools with TiAlN or AlCrN coatings. High-speed steel (HSS) will wear out quickly.

• Work Hardening: NAK80 doesn’t work-harden as quickly as stainless steel, but “rubbing” the tool will make it last less time. Keep the chip load steady.

• Use carbide drills when drilling. Peck drilling cycles help get rid of chips and keep heat from building up, which can damage the surface.

Features of EDM (Electrical Discharge Machining)

Because its microstructure is uniform, NAK80 is great for EDM because it erodes evenly.

• The “White Layer”: Like all steels, EDM leaves a layer of recast (hard, brittle) surface.

• Ease of Removal: It’s usually easier to grind or stone away the recast layer on NAK80 than on high-carbon steels.

• Finish: Can get very fine EDM finishes right away, which cuts down on the work needed to polish textured surfaces.

Tips for Designing for Manufacturing (DFM)

• Direct-to-Finish: If you can, plan machining strategies that let you go from roughing to finishing in one setup, taking advantage of the material’s stability.

• Corner Radii: NAK80 is tough, but don’t make the internal corners perfectly sharp. This will keep stress from building up during the injection cycle.

• Cooling Channels: Because NAK80 is clean, drilling long, straight cooling lines is easy and doesn’t cause the drill to drift.

What Are the Typical Engineering Applications?

NAK80 is a material that is only used for certain things. It is not the cheapest choice, so it is chosen when its unique features are useful.

1. Plastic Injection Molds with High Precision

• Clear parts: Lenses, light guides, visors, and medical vials. It’s very important that the mirror finish stays clear.

• Cosmetic Housings: Smartphones, audio equipment, and high-end appliance control panels where surface flaws are not allowed.

2. Molds for Making a Lot of Things

• Long Runs: NAK80 molds can run 500,000 to 1,000,000 shots with plastics that don’t scratch.

• Thin Walls: The stiffness of the material helps keep thin wall parts in place in electronic bobbin molds and connectors.

3. Making molds out of rubber and elastomer

• People often use NAK80 for O-ring and gasket molds where the parting line needs to stay clean to keep flash from happening.

What Are the Limitations and Failure Modes?

NAK80 is not a one-size-fits-all solution, even though it is great. To stop mold from failing, it’s important to know what its weaknesses are.

1. Limited Wear Resistance (Abrasive Plastics)
There are no hard carbides in NAK80, unlike D2 or CPM steels.

• Risk: If you mold glass-filled (GF) materials like PA66 + 30% GF, the glass fibers will quickly wash out the gate areas and cavities.

• Solution: Use NAK80 for the mold base and housing, and put Carbide or Hardened Tool Steel (like H13 or CPM 9V) in places that wear out quickly.

2. Not resistant to rust
NAK80 is NOT made of stainless steel.

• Risk: If it gets wet, gets condensation from cooling lines, or comes into contact with corrosive gases (like PVC off-gassing), it will rust.

• Solution: If the environment is corrosive, switch to S136 (Stainless Mold Steel) or use electroless nickel plating on the NAK80 (but plating can change the look of mirrors).

3. Problems with welding

• Risk: The weld zone will be different in hardness and microstructure than the base metal, even though it can be welded. After polishing, this can look like a “ghost line” or other flaw on mirror surfaces.

• Mitigation: To reduce the risk, use matching NAK80 welding rods and follow strict pre- and post-heat procedures. If you can, don’t weld on important optical surfaces.

NAK80 vs P20 vs S136: How Do You Choose?

Choosing the right mold steel means balancing cost, performance, and lead time.

Table 3: Matrix for Choosing Mold Steel

Selection Logic

• Scenario A: A mold for general use, parts that don’t need to look nice, and a tight budget. -> Pick P20.

• Scenario B: A cosmetic part with a high gloss, a tight tolerance, and a short timeline. -> Pick NAK80.

• Scenario C: Medical part made of PVC in a wet place. -> Pick S136.

Summary — Engineering Takeaways for NAK80 Steel

The mold maker uses NAK80 steel as a precise tool. It was made to fix certain issues, like time, stability, and surface quality.

• Use NAK80 when dimensional stability and surface finish are the most important factors for success.

• Ready to Run: Because it is pre-hardened, heat treatment is no longer a problem, which speeds up the delivery of molds.

• Mirror Finish: This is the standard for non-stainless optical polishing in the industry.

• Know the Limits: Don’t use it on glass-filled plastics that are very rough or PVC that is corrosive without protecting the surface.

Engineers can use NAK80 to make world-class molds that work the same way from the first shot to the last by choosing the right material for the plastic part and the number of parts that need to be made.

FAQ

Q: Is NAK80 a type of stainless steel?
A: No. NAK80 is a tool steel with a low amount of alloy. There isn’t enough chromium in it to make it stainless. It needs to be protected from rust (with sprays or coatings) while it is being stored and used. Use S136 or 420ESR if you need stainless steel.

Q: Is it possible to texturize (etch) NAK80?
A: Yes. NAK80 has a very even microstructure, which makes it great for photo-etching and texturing. The pattern will be clear and even all over the mold surface.

Q: Does NAK80 have a magnetic field?
A: Yes, NAK80 is ferromagnetic. This is helpful for holding things in place with magnets while grinding.

Q: What is the difference between NAK55 and NAK80?
A: They are both from the same family. NAK55 has sulfur in it to make it easier to work with (free-machining), but the sulfur inclusions make it not good for mirror polishing because they make pits. The clean and polishable version is NAK80. Use NAK55 for the bases of structural molds and NAK80 for the cavity/core.

Q: Is it possible to cover NAK80 with TiN or DLC?
A: Yes. PVD coatings like Titanium Nitride (TiN) or Diamond-Like Carbon (DLC) stick well to NAK80 and can make a big difference in how well it resists wear when molding abrasive plastics.