If you are creating high-end molds—for optical lenses, medical devices, or premium consumer products—you know the challenges. Mold warping, poor surface finish, and long production timelines can derail a project. 718H mold steel is a premium pre-hardened steel designed to solve these exact problems. It offers exceptional dimensional stability, mirror polishability, and durability, making it the top choice for demanding applications where precision and surface quality are non-negotiable. This guide will cover its key properties, real-world applications, manufacturing processes, and how it compares to other materials, helping you create molds that meet the strictest quality standards.
What Makes 718H Mold Steel a Premium Choice?
The performance of 718H comes from its carefully optimized chemical composition and its unique pre-hardened state. It is designed to be strong, stable, and ready for precision machining right from the start.
Chemical Composition
Every element in 718H is selected to enhance precision, polishability, and toughness. The combination of nickel and chromium is particularly important for achieving a mirror finish.
| Element | Content Range (%) | Key Role |
|---|---|---|
| Carbon (C) | 0.30 – 0.38 | Provides balanced hardness for the pre-hardened state while maintaining machinability. |
| Nickel (Ni) | 3.20 – 3.80 | A core element for toughness and ductility, preventing mold cracking under high stress. |
| Chromium (Cr) | 1.50 – 1.90 | Boosts corrosion and wear resistance while supporting a fine grain structure for mirror polishability. |
| Molybdenum (Mo) | 0.35 – 0.55 | Increases high-temperature stability and improves dimensional stability during injection molding. |
| Silicon (Si) | 0.20 – 0.40 | Enhances strength and resistance to mild oxidation. |
| Sulfur (S) | ≤ 0.030 | Kept low to avoid surface defects like pits that would ruin a mirror finish. |
Mechanical Properties in the Pre-Hardened State
A key advantage of 718H is that it is supplied in a pre-hardened state. This means it is ready for machining with its final properties already achieved.
| Property | Typical Value | Why It Matters for Your Mold |
|---|---|---|
| Hardness (HRC) | 36 – 40 | Offers higher hardness than standard pre-hardened steels, balancing durability with machinability for high-wear molds. |
| Tensile Strength | ≥ 1200 MPa | Handles high injection pressure without deforming, ideal for complex or thick-walled parts. |
| Yield Strength | ≥ 1000 MPa | Resists permanent damage, keeping molds dimensionally stable for 500,000+ cycles. |
| Impact Toughness | ≥ 40 J (at 20°C) | Excellent toughness prevents mold failure from sudden impacts, like a part jamming in the cavity. |
| Elongation | ≥ 10% | Good ductility reduces the risk of cracking during complex machining or mold clamping. |
Where Is 718H Mold Steel Used?
The unique combination of mirror polishability, dimensional stability, and durability makes 718H the material of choice for the most demanding mold applications.
Optical Molds
This is one of the most critical applications for 718H. The surface finish of the mold directly impacts the clarity of the optical part.
- Lens Molds: A Japanese optical manufacturer used 718H for smartphone camera lens molds. They achieved a consistent mirror finish (Ra 0.008 μm) . Compared to the stainless mold steel they used before, polishing time was reduced by 40%, and the scrap rate from blurry optics dropped from 15% to 0.5%.
- Other Uses: Molds for light guides in LED panels and optical fibers also rely on 718H for its ultra-smooth surface and precision.
Medical Device Molds
The medical industry demands precision, consistency, and corrosion resistance.
- Syringe and Component Molds: A U.S. medical supplier switched to 718H for plastic syringe molds. The material’s corrosion resistance stood up to harsh medical disinfectants used during cleaning cycles. Its precision ensured part consistency, dropping defect rates to just 0.05%, which is critical for regulatory compliance.
Automotive High-Precision Molds
Automotive parts, especially interior components, require excellent surface finish and tight tolerances.
- Sensor Housings and Interior Panels: A German automotive supplier used 718H for molds producing sensor housings with a tolerance of ±0.02 mm. The material’s high yield strength handled high injection pressure without warping. The result was a significant increase in mold life, from 200,000 parts to 500,000 parts.
Consumer Product Molds
For luxury goods and high-gloss plastic items, the surface finish is a key selling point.
- Cosmetic and Luxury Molds: A French cosmetic brand used 718H for lipstick tube molds. The mirror polishability of the steel delivered the high-gloss surface their customers expect. The durability of the mold also allowed for high-volume production, contributing to a 95% customer satisfaction rate with the product’s surface quality.
How Is 718H Mold Steel Manufactured?
Creating a high-precision mold from 718H is a streamlined process, thanks to its pre-hardened state, but it requires careful attention to detail.
Steelmaking and Pre-Hardening
The process begins with creating a clean, uniform material and then heat-treating it to its final hardness.
- Melting: Raw materials are melted in an Electric Arc Furnace (EAF) with strict controls to ensure ultra-low sulfur and phosphorus content, which is critical for polishability.
- Forging: The steel is forged at high temperatures to refine the grain structure and eliminate internal voids, creating a solid foundation for the mold blank.
- Pre-Hardening Heat Treatment: This is a critical step where the steel is quenched and then tempered to achieve its final hardness of HRC 36–40. This eliminates internal stress, which is the primary cause of post-machining warping.
Precision Machining and Polishing
Because the steel is pre-hardened, the final shape is machined directly without the risk of distortion from a later heat treatment.
- Precision Machining: Mold blanks are machined using 5-axis CNC machines to create complex cavities with tight tolerances (as precise as ±0.001 mm). Carbide tools are recommended to handle the pre-hardened steel.
- Polishing: This is the most critical step for achieving a mirror finish.
- Start with 400-grit sandpaper to remove machining marks.
- Progress to finer grits (1000, 3000, 8000) to gradually refine the surface.
- Finish with diamond paste (1–3 μm) to achieve a mirror polish of Ra ≤ 0.01 μm. The fine grain structure of 718H allows it to reach this level without surface pits.
718H vs. Other Mold Materials
Selecting the right mold steel involves balancing cost, machinability, and final part quality. This comparison shows where 718H excels.
| Material | Hardness (HRC) | Mirror Polishability (Ra μm) | Dimensional Stability | Pre-Hardened? | Relative Cost | Best Application |
|---|---|---|---|---|---|---|
| 718H Mold Steel | 36 – 40 | ≤ 0.01 | Exceptional | Yes | 100% | Optical molds, medical devices, high-precision automotive |
| 718 Pre-Hard Steel | 32 – 36 | ≤ 0.02 | Outstanding | Yes | 80% | General precision molds without ultra-mirror needs |
| P20 Pre-Hardened | 28 – 32 | ≤ 0.05 | Good | Yes | 60% | Low-to-mid precision plastic molds |
| Stainless Mold Steel (S136) | 30 – 32 | ≤ 0.01 | Very Good | No | 180% | Molds for corrosive plastics like PVC |
Conclusion
718H mold steel is a premium material engineered to meet the most exacting demands of high-precision mold making. Its pre-hardened state, with a hardness of 36–40 HRC, eliminates the risk of post-machining warping, ensuring exceptional dimensional stability. Its ultra-low sulfur content and fine grain structure enable a mirror polishability down to Ra ≤ 0.01 μm, which is critical for optical lenses and high-gloss consumer products. Real-world case studies demonstrate its ability to reduce polishing time by over 50% and extend mold life by more than 150%. While it comes at a higher cost than standard pre-hardened steels like P20, its superior performance, durability, and the elimination of post-heat-treatment steps make it a highly cost-effective and reliable choice for applications where quality and precision are paramount.
FAQ About 718H Mold Steel
Can 718H be used for die casting molds for aluminum or zinc?
Yes, but with an additional treatment. While 718H has good toughness to handle die casting pressure, its wear resistance can be enhanced. We recommend nitriding the surface to boost hardness to HRC 60–65. With this treatment, 718H can achieve a mold life of up to 300,000 cycles for aluminum die casting.
Is 718H more difficult to machine than standard pre-hardened steel like P20?
It is slightly more challenging due to its higher hardness. Machining time for 718H is typically 10–15% longer than for lower-hardness pre-hardened steels. However, using sharp carbide tools and high-quality cutting fluid minimizes this difference, and the time saved by eliminating post-machining heat treatment makes it more efficient overall.
What is the maximum mold size that 718H can handle while maintaining uniform properties?
718H has excellent hardenability. It maintains uniform hardness and dimensional stability for mold sections up to 120 mm thick. For thicker molds, a slow tempering cycle of over 3 hours is recommended to ensure the core and surface have consistent properties.
How does the mirror polishability of 718H compare to stainless mold steel?
Both can achieve a mirror finish of Ra ≤ 0.01 μm. However, 718H is pre-hardened, meaning you can polish it directly without the risk of distortion that comes with post-machining heat treatment of stainless mold steels. This makes 718H a more efficient and predictable choice for achieving ultra-smooth surfaces.
Discuss Your Projects with Yigu Rapid Prototyping
Creating high-precision molds requires not just the right material, but also the right expertise. At Yigu Rapid Prototyping, we specialize in working with premium materials like 718H mold steel. We understand the critical steps—from 5-axis CNC machining to achieving a perfect mirror polish with diamond paste—that turn a block of steel into a high-performance mold. Whether you are developing an optical lens mold, a medical device component, or a high-gloss consumer product, our team can help you achieve the precision, surface quality, and durability your project demands. Contact us today to discuss your project requirements.