Choosing The Right Steel for Plastic Injection Molds: A Complete Guide

1. Pre-Hardened Steels (P20, 718)

Hardness: HRC 30-40 (pre-hardened, ready for machining)
Key Features:

• No heat treatment required → reduces lead time.

• Good machinability and cost-effective for low-to-medium production.

• Limited wear resistance compared to hardened steels.

Common Grades:

• P20 (3Cr2Mo, AISI P20) – Standard choice for general-purpose molds.

• 718 (3Cr2NiMnMo, ASSAB 718) – Improved toughness and polishability.

Best For:
✔ Prototypes & low-volume production (<500k cycles)
✔ Consumer goods, housings (PP, ABS, PS)

2. Tool Steels (H13, D2, SKD61)

Hardness: HRC 48-60 (requires heat treatment)
Key Features:

• High wear resistance and thermal stability.

• Suitable for high-volume production (>1M cycles).

• More expensive and harder to machine.

Common Grades:

• H13 (4Cr5MoSiV1, AISI H13) – Excellent for high-temperature plastics (PC, PA+GF).

• D2 (Cr12MoV, AISI D2) – High hardness, good for abrasive plastics (POM, PBT+GF).

Best For:
✔ Engineering plastics (nylon, glass-filled resins)
✔ High-wear applications

3. Stainless Steels (420, S136)

Hardness: HRC 45-55 (corrosion-resistant)
Key Features:

• High polishability (mirror finish possible).

• Resistant to corrosive plastics (PVC, flame-retardant resins).

• More expensive and difficult to machine.

Common Grades:

• 420 (4Cr13, AISI 420) – Standard corrosion-resistant steel.

• S136 (ASSAB S136, AISI 420 modified) – Superior polishability for optical parts.

Best For:
✔ Transparent parts (lenses, light guides)
✔ Medical devices & corrosive plastics

4. Aging Hardening Steels (NAK80, 18Ni Maraging Steel)

Hardness: HRC 40-50 (after aging treatment)
Key Features:

• Ultra-high precision with minimal distortion.

• Excellent polishability for fine details.

• Very expensive, used for high-end applications.

Common Grades:

• NAK80 (Japan JIS NAK80) – Popular for high-gloss surfaces.

• 18Ni (300 Grade Maraging Steel) – Extreme strength for aerospace/medical molds.

Best For:
✔ Micro-precision parts (connectors, gears)
✔ High-gloss automotive/electronics parts

5. Special Materials (Beryllium Copper, Aluminum)

Beryllium Copper (BeCu)

• High thermal conductivity → Faster cooling.

• Used for mold inserts to reduce cycle time.

Aluminum (7075, 6061)

• Lightweight & easy to machine → Great for prototypes.

• Lower durability (suitable for <10k cycles).

How to Choose the Right Mold Steel?

Conclusion

Selecting the right steel for your injection mold depends on:
✅ Plastic material (standard, abrasive, or corrosive)
✅ Production volume (prototype, medium, or high-volume)
✅ Surface finish requirements (textured, polished, or mirror-like)

By choosing the optimal steel, you can extend mold life, reduce costs, and improve part quality.