718H Mold Steel: Properties, Uses & Manufacturing for High-Precision Molds

If you need a pre-hardened mold steel that delivers exceptional dimensional stability, mirror polishability, and durability for demanding molds—718H mold steel is the top choice. Designed for high-end applications like optical molds and medical device molds, this alloy solves common pain points like mold warping, poor surface finish, or slow production timelines. 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 and efficiency standards.

1. Material Properties of 718H Mold Steel

718H’s reputation as a premium pre-hardened mold steel comes from its optimized composition and standout properties. Let’s explore these in detail:

1.1 Chemical Composition

The elements in 718H work together to enhance precision, polishability, and toughness—tailored for high-precision mold-making. Below is its typical composition (per industry standards):

1.2 Physical Properties

These properties determine how 718H 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 pre-hardening heat treatment).
• Thermal Conductivity: 30 W/(m·K) (good heat transfer, ensuring plastic parts cool evenly in injection molds).
• Coefficient of Thermal Expansion: 11.7 × 10⁻⁶/°C (from 20 to 600 °C; ultra-low expansion reduces mold warping—critical for precision parts).
• Specific Heat Capacity: 470 J/(kg·K) (efficient at absorbing and releasing heat, reducing plastic injection cycle times).

1.3 Mechanical Properties

As a premium pre-hardened mold steel, 718H is supplied ready for machining—no post-machining heat treatment needed. Below are its typical mechanical properties (in pre-hardened state):

1.4 Other Properties

• Corrosion Resistance: Very Good. Chromium content protects against rust in workshop environments and mild chemicals (e.g., plastic additives or medical disinfectants).
• Wear Resistance: Good to Very Good. Suitable for most plastic and die casting applications; nitriding can boost wear resistance further for high-wear molds.
• Machinability: Excellent. Despite its higher hardness (HRC 36–40), its fine grain structure makes it easy to mill, drill, and turn—machining time is only 10–15% longer than lower-hardness pre-hardened steels.
• Hardenability: Outstanding. It maintains uniform hardness across thick sections (up to 120 mm), so large molds (e.g., automotive molds for bumpers) have consistent performance.
• Dimensional Stability: Exceptional. Ultra-low thermal expansion and pre-hardened state eliminate post-machining warping—critical for optical molds (e.g., camera lenses) or tight-tolerance medical parts.
• Mirror Polishability: Excellent. Low sulfur content and fine grain structure let it achieve mirror finishes (Ra ≤ 0.01 μm)—perfect for consumer product molds (e.g., cosmetic containers) or optical components.

2. Applications of 718H Mold Steel

718H’s mix of precision, durability, and efficiency makes it ideal for high-end mold applications. Here are its most common uses, with real examples:

2.1 Optical Molds

• Examples: Molds for plastic lenses (camera, smartphone), light guides (LED panels), or optical fibers.
• Why it works: Mirror polishability delivers ultra-smooth surfaces for clear optics, while dimensional stability ensures lens accuracy. A Japanese optical manufacturer used 718H for smartphone lens molds—polish time reduced by 40% vs. stainless mold steel.

2.2 Medical Device Molds

• Examples: Molds for plastic syringes, surgical instrument handles, or diagnostic test kits.
• Why it works: Corrosion resistance stands up to medical disinfectants, and precision ensures part consistency (critical for regulatory compliance). A U.S. medical supplier used 718H for syringe molds—part defect rates dropped to 0.05%.

2.3 Automotive High-Precision Molds

• Examples: Molds for automotive interior parts (e.g., high-gloss dashboard panels) or sensor housings (tight tolerance ±0.02 mm).
• Why it works: High yield strength handles high injection pressure, and dimensional stability prevents fitment issues. A German automotive supplier used 718H for sensor molds—mold life increased from 200,000 to 500,000 parts.

2.4 Consumer Product Molds

• Examples: Molds for luxury goods (e.g., watch casings), high-gloss plastic bowls, or cosmetic jars.
• Why it works: Mirror finish meets consumer demand for premium aesthetics, while durability handles high production volumes. A French cosmetic brand used 718H for lipstick tube molds—customer satisfaction with surface quality rose by 95%.

3. Manufacturing Techniques for 718H Mold Steel

Turning 718H into high-precision molds is streamlined thanks to its pre-hardened state, but requires attention to detail for precision. Here’s a step-by-step breakdown:

1. 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).
2. Casting: Molten steel is poured into ingot molds or continuous casters to form slabs. Slow cooling (20–30 °C/hour) prevents internal defects and refines grain structure.
3. 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 voids.
4. Pre-hardening Heat Treatment:
   • Annealing: Heat to 800–850 °C, hold 2–4 hours, cool slowly. Softens steel to HRC 22–25 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 560–600 °C, hold 2–3 hours, cool. Sets final pre-hardened hardness (HRC 36–40) and eliminates internal stress.
5. Precision Machining: Mold blanks are machined using 5-axis CNC machines to create complex cavities. Carbide tools with sharp edges are recommended for tight tolerances (±0.001 mm).
6. Polishing: The most critical step for high-gloss or optical molds:
   • Start with 400-grit sandpaper to remove 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).
7. Surface Treatment (Optional):
   • Nitriding: Creates a hard surface layer (HRC 60–65) to boost wear resistance for die casting molds.
   • Electroplating: Adds a chrome or nickel coating to improve corrosion resistance (ideal for medical or food-contact molds).

4. Case Study: 718H in Optical Lens Molds

A Chinese optical component manufacturer faced a problem: their P20 mold steel couldn’t achieve the mirror finish (Ra ≤ 0.01 μm) needed for smartphone camera lenses, leading to blurry optics and high scrap rates. They switched to 718H, and here’s what happened:

• Process: Mold blanks (pre-hardened to HRC 38) were 5-axis CNC-machined to lens cavity geometry, polished with diamond paste, and coated with an anti-reflective layer.
• Results:
  • Achieved consistent mirror finish (Ra 0.008 μm) for every mold—scrap rate dropped from 15% to 0.5%.
  • Polish time reduced from 24 hours to 10 hours (58% improvement) thanks to 718H’s fine grain structure.
  • Lens production yield increased by 20% (faster polishing + fewer defects).
• Why it works: Ultra-low sulfur in 718H eliminated surface pits, while nickel and molybdenum refined the grain structure—allowing the diamond paste to create an ultra-smooth surface for clear optics.

5. 718H vs. Other Mold Materials

How does 718H compare to common alternatives for high-precision molds? Let’s evaluate key properties:

Key takeaway: 718H is the best choice for high-precision molds needing mirror finishes and ultra-stable dimensions. It’s more cost-effective than stainless steel (S136) and delivers better precision than 718 Pre Hard or P20—worth the investment for premium applications.

Yigu Technology’s View on 718H Mold Steel

At Yigu Technology, 718H is our top recommendation for clients with high-precision mold needs—from optical lenses to medical devices. Its unmatched dimensional stability and mirror polishability solve the biggest challenges in premium mold-making: inconsistent finishes and warping. We often pair it with precision 5-axis machining and diamond polishing to achieve Ra ≤ 0.008 μm, helping clients meet strict industry standards. For businesses focused on quality and efficiency, 718H isn’t just a material—it’s a way to deliver products that stand out in competitive markets.

FAQ About 718H Mold Steel

1. Can 718H be used for die casting molds (e.g., aluminum or zinc)?

Yes, but we recommend nitriding to boost wear resistance. 718H’s toughness handles die casting pressure, and its dimensional stability ensures part consistency—we’ve helped clients use nitrided 718H for aluminum die casting molds with life up to 300,000 cycles.

2. Is 718H more difficult to machine than 718 Pre Hard Steel?

Slightly—718H’s higher hardness (HRC 36–40 vs. 32–36) adds 10–15% to machining time. But using sharp carbide tools and high-quality cutting fluid minimizes this difference, and the time saved from no post-heat treatment still makes 718H more efficient overall.

3. What’s the maximum mold size 718H can handle while maintaining uniform properties?

718H has excellent hardenability, so it maintains uniform hardness and stability for molds up to 120 mm thick. For thicker molds (over 120 mm), we recommend a slow tempering cycle (3+ hours) to ensure the core and surface have consistent properties.