For previous reading, please refer to A Literary Perspective on Plastic Mold Steel - H13.

 

NAK80 is a pre-hardened plastic mold steel developed by Daido Steel Co., Ltd. of Japan. Its ready-to-use, heat-treatment-free design and high mirror finish have made it a star material in high-end plastic mold industry. Its composition is exquisitely designed: carbon content is kept within a low range of 0.06%-0.20%, combined with approximately 3.20% nickel, 1.0% aluminum and copper, and reinforced with 0.2%-0.5% molybdenum. This combination ensures matrix toughness while achieving a pre-hardening effect through precipitation of nickel-aluminum intermetallic compounds. Steel reaches a factory hardness of HRC 37-43, eliminating need for subsequent quenching.

Mirror Polishability: "Guarantee of Beauty" for Plastic Exterior Parts
Vacuum-degassed and refined NAK80 steel is pure, with minimal pinholes and a uniform, fine microstructure. It can easily achieve a mirror polish of 10,000-12,000 grit. From transparent cosmetic bottles to television filters, molds for products requiring stringent surface finishes are almost exclusively used for this purpose. Polishing should proceed from coarse to fine without skipping steps. A 1μm diamond paste finish is sufficient to achieve a high-gloss mirror finish.
Machinability: A Labor-Saving and Efficient "Lazy Man's Steel"
Although pre-hardened NAK80 has slightly higher cutting resistance, spheroidizing annealing improves its machinability compared to ordinary hardened steel. SKH9 high-speed steel or P40 super-hard alloy tools enable efficient machining. More importantly, its electrical discharge machinability produces smooth, fine-grained surfaces with no hardened layer. It can even directly replace pear-skin etching process, significantly shortening mold production cycles.
Mechanical Stability: A "Hard Strength" for Multi-Scenario Adaptation
At room temperature, it boasts a tensile strength of 128 kgf/mm² and a 38% reduction of area. Even at 300℃, it maintains a hardness of 360 HV, meeting temperature requirements of most plastic molding applications. After welding, aging at 500℃ restores its hardness to that of parent material, making it far easier to repair than comparable steels.

NAK80's applications are specifically focused on high-visibility plastic molds that don't require rust prevention:
Beauty Industry: Molds for high-gloss housings such as lipstick tubes and powder compacts;
Automotive Industry: Molds for reflective components such as instrument panel covers and dome light shades;
3C Products: Molds for precision components such as tablet housings and display frames. It forms a clear distinction with popular S136 mold steel: NAK80, with its pre-hardened state, is suitable for medium-to-high-mirror finishes (above 10,000 grit). Its import price is around 60-70 yuan/kg, while domestic Fushun refined steel is priced at around 30-45 yuan/kg. S136 requires quenching and hardening, while it can achieve a super-mirror finish of 18,000 grit and is corrosion-resistant, its cost fluctuates more. If mold does not come into contact with corrosive plastics (such as PVC), NAK80 is the most cost-effective choice.

Polishing Taboos: Skipping rough grinding and proceeding directly to fine polishing can easily leave pinholes. A three-step process is required: "grindstone → sandpaper → diamond paste," with each level of polishing perpendicular to previous one.
Heat Treatment Risks: While full hardening is not required, cryogenic treatment (-70℃) requires immediate tempering, otherwise complex molds may crack.
Storage Tips: Semi-finished products should be cleaned and stored in a dry environment to prevent surface oxidation that may affect polishing effect.
Origin of name NAK
Ni+Al+Cu(German: Kupfer)
NAK is a special steel made by adding nickel (Ni), aluminum (Al), and copper (Cu: Kupfer in German) to iron. Name "NAK" comes from initials of alloying elements.
6 charms of NAK80

 

High Hardness
Excellent Wear Resistance
Molds are subject to high loads both inside and outside. Consequently, repeated molding processes can easily lead to wear and deformation of mating surfaces and wear of molding surfaces. NAK80, with a pre-hardened hardness of 40 HRC, offers excellent wear resistance and is ideal for high-volume plastic molding molds.
Furthermore, as an age-hardened steel, its hardness is uniform from surface to center, making it relatively easy to machine despite its 40 HRC hardness. This achieves a perfect balance between ultimate hardness of pre-hardened steel and its machinability.

 

Easy Cutting
Machinability Enables Fine Machining
Melted resin material reflects shape of mold surface like a mirror. Therefore, molding plastic products that prioritize both performance and aesthetics requires high-precision, high-quality molds with precise processing.
NAK80's excellent machinability allows for fine die-cutting. Cut surface has sharp contours and minimal burrs, making it widely accepted as a steel for precision plastic molds.

 

No Heat Treatment Required
Pre-hardened steel is easy to use.
Pre-hardened steel is heat-treated at optimal conditions before shipment, allowing for immediate die-cutting.
Without concerns about dimensional changes or deformation caused by heat treatment, mold manufacturing time can be significantly reduced.

 

Easy to Grind and Polish
Stable Mirror Finish

 

Molds require mirror polishing to impart a vibrant gloss, aesthetically pleasing appearance, and a sense of transparency to plastic molded products. A smooth molded product surface provides a pleasant touch, resists dust and dirt, creating functional aesthetics.
During final stages of mold production, where polishing often relies on hands of craftsmen, mold steel must exhibit stable mirror finish properties. NAK80 steel, with its high purity, is easy to grind and rarely causes problems, making it a popular grade with repeat customers.
Excellent Hardfacing Weldability
Plastic molds often require hardfacing during trial production phase, design changes, and mold modifications. Furthermore, even beautifully finished molds can suffer wear and damage from repeated forming processes, sometimes requiring welding repairs.
NAK80 steel undergoes an aging treatment after welding to achieve uniform hardness, eliminating need for weld marks or uneven etch marks. Further polishing can also restore surface to a mirror finish.

 

Easily Etched
Enhancing Product Aesthetics

 

Mold surface decoration adds texture and expression to plastic molded products. It not only enhances aesthetics and luxury of exterior, but also improves tactility and extends product life, making it an indispensable element in plastic products.
NAK80, with its high purity and uniform microstructure, is suitable for etching and other texturing processes, contributing to production of products requiring high aesthetics.
Etching: A surface treatment method that uses chemicals to etch patterns into metal.
Texturing: A process that imparts patterns and relief (inverted patterns are processed on mold).
High-Performance Precision Plastic Mold Steel
Features: Age-hardening, specially fused 40HBC grade pre-hardened steel.
◆Solution-hardened and age-hardened, hardness 37-43 HRC; Can be used directly for die-cutting without heat treatment.
◆Ni-Al-Cu age-hardening steel: Excellent machinability; Easily grinds after cutting; Easily grinds after electrical discharge machining; Excellent weldability; Maintains high precision during use; Uniform hardness on the surface and center.
◆ Specially fused; Excellent mirror polishability, achieving a satisfactory gloss; Excellent texturing performance; Effective, beautiful surface finishes from electrical discharge machining; Can replace pear ground texturing.
Main Applications: High-performance, precision plastic molds; rubber molds; stamping molds (bending, etc.); molds for transparent products requiring mirror-polish finishes; molds requiring EDM surfaces; various parts for industrial machinery.
Chemical composition
 
Nitriding Properties

 

Surface treatment temperatures exceeding 520℃ may result in a decrease in hardness and dimensional changes.
Physical Properties
Coefficient of Thermal Expansion
Thermal Conductivity
Specific Heat
Young's Modulus
Overlay Welding Repair
1. Mold Pre-cleaning
· Thoroughly remove grease, dirt, and scale;
· Thoroughly remove cracks and surface treatment layers
· Bevel corners with a minimum of 3R
2. Overlay Welding Rod
· NAK-W
3. Preheating
· 300-400℃
· Slow heating in a furnace or with propane or natural gas
4. Welding
· DC positive polarity, step-back welding method
· Conditions

5. Post-Heat Treatment
450-500℃ x 1h
[Precautions]: When using propane or natural gas spray for preheating and post-heat treatment, it is advisable to perform full-heating to prevent cracking.
When overlaying or welding crack repairs during use, weld layer is often thicker. Depending on amount of weld buildup, stress relief (870℃) and aging treatment (500~520℃) may be performed immediately after weld buildup, even at expense of slight dimensional changes.
Mirror Polishing
Produces an excellent polished surface with a satisfying gloss.

 

[General Polishing Process]
Cutting and Milling → Oilstone (#220-#320-#400)
→ Sandpaper (#320-#400-#600-#800-#1000-#1200-#1500)
→ Diamond Lapping Paste (#1200-#1800-#3000-#5000-#8000)

 

Weldability
Welded area does not experience increased hardness. Aging treatment after welding prevents uneven etching.
Hardness distribution of deposited steel and base metal

 

Welding conditions
Welding method: TIG welding (shielded gas)
Preheating: 300-400℃
Current: 150-170A
Welding rod: Homogeneous metal (NAK-W)