How to Choose the Right Steel Material for Plastic Injection Molds
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- Jul 9,2026
How to Choose the Right Steel Material for Plastic Injection Molds
Selecting the right steel depends primarily on production volume, plastic material (corrosiveness & abrasiveness), and surface finish requirements. At LONGXIANG, we guide clients through three main steel categories: pre-hardened (P20, 718H) for general-purpose molds under 500K cycles, through-hardened (H13, 420SS) for high-volume molds up to 2M cycles, and high-polish steels (S136, NAK80) for optical-grade surfaces.
Steel Comparison Guide
| Steel Grade | Hardness (HRC) | Max Cycles | Best For | Polishability | Corrosion Resistance |
|---|---|---|---|---|---|
| P20 (1.2311) | 28–32 | 500K | General purpose, low-medium volume | Good (SPI B1) | Low |
| 718H | 32–36 | 800K | Larger molds, automotive parts | Good (SPI B1) | Low |
| H13 (1.2344) | 48–52 | 1.5M | High-wear, high-temperature molds | Fair (SPI B2) | Medium |
| 420SS | 48–52 | 2M | Corrosive plastics (PVC, flame-retardant) | Good (SPI A2) | High |
| S136 (1.2083) | 48–52 | 2M | Optical, medical, food-contact parts | Excellent (SPI A1) | High |
| NAK80 | 37–42 | 1M | Texture, mirror finish, lens molds | Excellent (SPI A1) | Medium |
3-Step Selection Framework
Under 100K cycles → P20 or 718H. 100K–500K → 718H or NAK80. Over 500K → H13 or 420SS. xox recommends 718H as the cost-effective default for most projects.
PVC, POM, and flame-retardant grades release corrosive gases → use 420SS or S136. Glass-filled nylons are abrasive → use H13 with TiN coating. Standard materials (ABS, PP, PE) work well with P20 or 718H.
SPI A1 mirror finish → S136 or NAK80. SPI B1 fine texture → 718H or P20. SPI C satin → most steels. Chemical etching → NAK80 for best consistency.
Pro Tips from LONGXIANG Engineers
For molds running ABS or PP under 300K cycles, P20 is the most cost-effective choice with excellent machinability.
Always consider surface coating (TiN, TiAlN, DLC) — it can extend mold life by 2–3x without upgrading the base steel.
At LONGXIANG, a comprehensive DFM analysis includes steel recommendation with lifecycle cost projection — free of charge.