How to Choose Mold Steel for Injection Molding: P20 vs NAK80 vs S136
Selecting the right mold steel is one of the most critical decisions in injection mold manufacturing. The wrong choice can lead to premature mold failure, poor part quality, or unnecessary expense. This guide compares the most common mold steel grades to help you make an informed decision.
Common Mold Steel Grades
| Steel Grade | Hardness (HRC) | Mold Life | Best Use |
|---|---|---|---|
| P20 | 28-32 | 100K-300K shots | Prototype, low to medium volume |
| 718 (1.2738) | 30-34 | 200K-500K shots | General production molds |
| NAK80 | 38-42 | 500K-1M shots | Mirror finish, high volume |
| S136 (1.2083) | 48-52 | 1M+ shots | Corrosive materials, medical |
| H13 | 48-52 | 500K-1M shots | Abrasive materials (glass-filled) |
P20 Steel — The Standard Choice
P20 is a pre-hardened steel delivered at 28-32 HRC. It is the most commonly used mold steel, providing adequate hardness at the lowest cost. Easy to machine and weld-repairable. Limitations: not suitable for high-polish SPEI A1 finishes or extended production runs beyond 300K shots.
718 Steel (1.2738) — The Workhorse
718 offers slightly higher hardness (30-34 HRC) and contains nickel for improved toughness. Excellent for medium to high volume general production molds. Good machinability and polishability. The recommended upgrade from P20 for most production applications.
NAK80 — For High-Gloss Finishes
NAK80 is pre-hardened at 38-42 HRC with exceptional polishability. Can achieve SPI A1 mirror surface finishes. Ideal for automotive interior parts, consumer electronics, and cosmetic products. Approximately 50% more expensive than P20.
S136 (1.2083) — Stainless Steel
S136 is through-hardened stainless steel (48-52 HRC) with excellent corrosion resistance. Essential for molding PVC, POM, flame-retardant grades, and medical devices. Approximately 2x the cost of P20 but lasts over 1 million shots.
Selection Decision Tree
- Under 100K shots + standard materials → P20
- 100K-300K shots + standard finish → 718
- 300K+ shots or mirror finish required → NAK80
- Molding PVC, POM, or medical → S136
- Molding glass-filled nylon or PEEK → H13
Need help choosing the right steel? Contact RCH Plastic engineers for a free consultation.