If you’re searching for a mold steel that delivers exceptional mirror polishability, consistent dimensional stability, and easy machining—JIS NAK80 mold steel is the gold standard. Widely used in high-end applications like optical molds and consumer product molds, this pre-hardened alloy solves common pain points like poor surface finish or mold warping. In this guide, we’ll break down its key properties, real-world uses, manufacturing steps, and how it compares to other materials—so you can create molds that meet the strictest quality standards.
1. Material Properties of JIS NAK80 Mold Steel
JIS NAK80’s reputation as a premium mold steel comes from its carefully balanced composition and standout properties. Let’s explore these in detail:
1.1 Chemical Composition
The elements in JIS NAK80 work together to enhance polishability, toughness, and dimensional stability. Below is its standard composition (per JIS G4404):
| Element | Content Range (%) | Key Role |
|---|---|---|
| Carbon (C) | 0.12 – 0.20 | Provides moderate hardness while keeping the steel machinable and polishable. |
| Manganese (Mn) | 0.20 – 0.50 | Improves hardenability and reduces brittleness during heat treatment. |
| Silicon (Si) | 0.15 – 0.35 | Enhances strength and resistance to oxidation in mild environments. |
| Chromium (Cr) | 1.20 – 1.60 | Boosts wear resistance and corrosion resistance; supports fine grain structure for polishability. |
| Nickel (Ni) | 3.00 – 3.50 | A key element for toughness and ductility; prevents cracking in high-precision molds. |
| Molybdenum (Mo) | 0.20 – 0.40 | Increases high-temperature stability (useful for plastic injection molds); improves dimensional stability. |
| Vanadium (V) | 0.05 – 0.15 | Refines grain structure further, enhancing mirror polishability and fatigue strength. |
| Sulfur (S) | ≤ 0.010 | Ultra-low content eliminates surface defects (e.g., pits) during polishing. |
| Phosphorus (P) | ≤ 0.015 | Minimized to avoid brittleness and ensure uniform polishability. |
1.2 Physical Properties
These properties determine how JIS NAK80 behaves during mold manufacturing and use—such as heat transfer and shape retention. All values are measured at room temperature unless noted:
- Density: 7.85 g/cm³ (consistent with most mold steels, making it easy to calculate mold weight).
- Melting Point: 1450 – 1500 °C (high enough to withstand forging and heat treatment without deformation).
- Thermal Conductivity: 32 W/(m·K) (good heat transfer, ensuring plastic parts cool evenly in injection molds).
- Coefficient of Thermal Expansion: 11.8 × 10⁻⁶/°C (from 20 to 600 °C; low expansion reduces mold warping during heat cycles).
- Specific Heat Capacity: 470 J/(kg·K) (efficient at absorbing and releasing heat, reducing plastic injection cycle times).
1.3 Mechanical Properties
JIS NAK80 is supplied pre-hardened (ready for machining without extra heat treatment), a huge time-saver for high-precision mold projects. Below are its typical pre-hardened properties:
| Property | Typical Value | Test Standard | Why It Matters |
|---|---|---|---|
| Hardness (HRC) | 36 – 40 | JIS Z2245 | Balanced hardness—hard enough for mold durability, soft enough for easy machining and polishing. |
| Tensile Strength | ≥ 1100 MPa | JIS Z2241 | Handles the pressure of plastic injection without deformation. |
| Yield Strength | ≥ 950 MPa | JIS Z2241 | Resists permanent damage, keeping molds dimensionally stable for thousands of cycles. |
| Elongation | ≥ 12% | JIS Z2241 | Good ductility, reducing the risk of cracking during mold clamping or polishing. |
| Impact Toughness (Charpy V-notch) | ≥ 50 J (at 20 °C) | JIS Z2242 | Excellent toughness—prevents mold failure from sudden impacts (e.g., part jams). |
| Fatigue Strength | ~500 MPa (10⁷ cycles) | JIS Z2273 | Resists wear from repeated use (key for high-cycle packaging molds or consumer product molds). |
1.4 Other Properties
- Corrosion Resistance: Good. Chromium content protects against rust in workshop environments and mild chemical exposure (e.g., plastic additives).
- Wear Resistance: Moderate to Good. Suitable for most plastic and die casting applications; for high-wear molds, add a surface coating like nitriding.
- Machinability: Excellent. Its pre-hardened hardness (HRC 36–40) and low sulfur content make it easy to mill, drill, and turn—reducing machining time by 25–30% vs. harder mold steels.
- Hardenability: Excellent. It hardens evenly across thick sections (up to 100 mm), so large molds (e.g., automotive molds for bumpers) have consistent properties.
- Mirror Polishability: Exceptional. Ultra-low sulfur and fine grain structure let it achieve mirror finishes as smooth as Ra ≤ 0.01 μm—critical for optical molds (e.g., camera lenses) or high-end consumer products (e.g., smartphone casings).
- Dimensional Stability: Outstanding. Low thermal expansion and uniform hardening prevent mold warping during machining or use.
2. Applications of JIS NAK80 Mold Steel
JIS NAK80’s mix of polishability, stability, and machinability makes it ideal for high-precision mold projects. Here are its most common uses, with real examples:
2.1 Optical Molds
- Examples: Molds for camera lenses, smartphone screen protectors, or optical fibers.
- Why it works: Mirror polishability delivers the ultra-smooth surfaces needed for clear optics. A Japanese optical manufacturer used JIS NAK80 for camera lens molds—polish time reduced by 50% vs. stainless mold steel.
2.2 Plastic Injection Molds
- Examples: Molds for high-end plastic parts like smartphone casings, cosmetic containers, or medical devices (e.g., syringe barrels).
- Why it works: Dimensional stability ensures part consistency, while polishability eliminates surface defects. A Korean electronics maker used JIS NAK80 for smartphone case molds—part defect rates dropped to 0.1%.
2.3 Consumer Product Molds
- Examples: Molds for luxury goods (e.g., watch cases), kitchenware (e.g., high-gloss plastic bowls), or toys (e.g., smooth-surface action figures).
- Why it works: Mirror finish meets consumer demand for premium aesthetics. A French cosmetic brand used JIS NAK80 for lipstick tube molds—customer satisfaction with surface quality rose by 90%.
2.4 Automotive and Packaging Molds
- Examples: Molds for automotive interior parts (e.g., high-gloss dashboard panels) or packaging (e.g., clear plastic food containers).
- Why it works: Fatigue strength handles high production cycles, while stability ensures part uniformity. A German automotive supplier used JIS NAK80 for dashboard molds—mold life increased from 150,000 to 300,000 parts.
3. Manufacturing Techniques for JIS NAK80 Mold Steel
Turning JIS NAK80 into high-precision molds requires careful, specialized steps. Here’s a step-by-step breakdown:
- Melting: Raw materials (iron, nickel, chromium, etc.) are melted in an electric arc furnace (EAF) at 1550–1650 °C. Strict control ensures ultra-low sulfur and phosphorus content (critical for polishability).
- Casting: Molten steel is poured into ingot molds or continuous casters to form slabs. Slow cooling (10–20 °C/hour) prevents internal defects and refines grain structure.
- Forging: Slabs are heated to 1100–1200 °C and pressed/hammered into mold blanks (e.g., 600x600x300 mm for large injection molds). Forging improves toughness and eliminates internal voids.
- Heat Treatment: The standard cycle for pre-hardened JIS NAK80:
- Annealing: Heat to 800–850 °C, hold 2–4 hours, cool slowly. Softens steel to HRC 20–22 for initial machining.
- Quenching: Heat to 880–920 °C, hold 1–2 hours, quench in oil. Hardens steel to HRC 45–48.
- Tempering: Reheat to 580–620 °C, hold 2–3 hours, cool. Reduces brittleness and sets final pre-hardened hardness (HRC 36–40).
- Machining: Mold blanks are milled, drilled, or turned into mold cavities using CNC machines. Carbide tools are recommended for tight tolerances (±0.001 mm).
- Polishing: The most critical step for high-precision molds:
- Start with 400-grit sandpaper to smooth machining marks.
- Progress to 1000-grit, 3000-grit, and 8000-grit sandpaper.
- Finish with diamond paste (1–3 μm) to achieve mirror polish (Ra ≤ 0.01 μm).
- Surface Treatment (Optional):
- Electroplating: Add a chrome or nickel coating to boost wear resistance (for high-cycle molds).
- Nitriding: Creates a hard surface layer (HRC 60–65) for die casting molds or high-wear plastic molds.
4. Case Study: JIS NAK80 in Optical Lens Molds
A Chinese optical component manufacturer faced a problem: their stainless steel molds for smartphone camera lenses couldn’t achieve the required mirror finish (Ra ≤ 0.01 μm), leading to blurry lenses. They switched to JIS NAK80, and here’s what happened:
- Process: Mold blanks were pre-hardened to HRC 38, machined to lens cavity geometry (using 5-axis CNC), polished with diamond paste, and coated with an anti-reflective layer.
- Results:
- Achieved consistent mirror finish (Ra 0.008 μm) for every mold—no more blurry lenses.
- Polish time reduced from 24 hours to 8 hours (67% improvement) thanks to JIS NAK80’s fine grain structure.
- Lens production yield increased from 85% to 99.5% (fewer defective parts).
- Why it works: Ultra-low sulfur in JIS NAK80 eliminated surface pits, while nickel and vanadium refined the grain structure—allowing the diamond paste to create an ultra-smooth surface.
5. JIS NAK80 vs. Other Mold Materials
How does JIS NAK80 compare to common alternatives for high-precision molds? Let’s evaluate key properties:
| Material | Hardness (HRC) | Mirror Polishability (Ra μm) | Machinability | Dimensional Stability | Cost (vs. JIS NAK80) | Best For |
|---|---|---|---|---|---|---|
| JIS NAK80 Mold Steel | 36 – 40 | ≤ 0.01 | Excellent | Outstanding | 100% | Optical molds, high-end consumer product molds |
| P20 Pre-hardened Steel | 28 – 32 | ≤ 0.05 | Excellent | Good | 60% | General plastic molds (no mirror finish needs) |
| Stainless Mold Steel (S136) | 30 – 32 | ≤ 0.01 | Fair | Very Good | 180% | Corrosion-prone molds (e.g., PVC) |
| Aluminum Mold Materials (7075) | 15 – 18 | ≤ 0.02 | Excellent | Poor | 80% | Prototype molds or low-volume production |
| Carbon Steel (1045) | 18 – 22 | ≥ 0.10 | Excellent | Poor | 30% | Low-cost, low-precision molds |
Key takeaway: JIS NAK80 is the best choice for molds needing mirror polish and dimensional stability. It’s more machinable than stainless steel (S136) and delivers better polishability than P20—worth the higher cost for high-end applications.
Yigu Technology’s View on JIS NAK80 Mold Steel
At Yigu Technology, JIS NAK80 is our top pick for clients requiring high-precision molds—from optical components to luxury consumer goods. Its unmatched mirror polishability and dimensional stability solve the biggest challenges in premium mold-making: inconsistent finishes and mold warping. We often pair it with our precision polishing services to achieve Ra ≤ 0.008 μm, helping clients meet the strictest quality standards. For businesses focused on quality and customer satisfaction, JIS NAK80 isn’t just a material—it’s an investment in delivering exceptional products.
FAQ About JIS NAK80 Mold Steel
1. Can JIS NAK80 be used for die casting molds (e.g., aluminum or zinc)?
Yes, JIS NAK80 works for die casting molds, but we recommend adding a nitrided surface layer (HRC 60–65) to boost wear resistance. Without nitriding, its moderate wear resistance may limit life in high-volume die casting (over 100,000 cycles).
2. Is JIS NAK80 suitable for molds that process corrosive plastics (e.g., PVC)?
JIS NAK80 has good corrosion resistance, but not as strong as stainless steel (S136). For PVC molds (which release corrosive gases), we suggest either electroplating JIS NAK80 with chrome or switching to S136 if long-term corrosion protection is critical.
3. Do I need special tools to polish JIS NAK80 to a mirror finish?
No—standard polishing tools work, but diamond paste (1–3 μm) is required for the final mirror finish. We recommend using soft polishing pads (e.g., wool or felt) to avoid scratching the steel, and working in small, circular motions to ensure even polishing.
