Mold Steel Selection Guide: Which Steel for Your Injection Mold?
Why Mold Steel Selection Matters
The choice of mold steel directly impacts mold lifespan, part quality, cycle time, and tool cost. Selecting the wrong steel can lead to premature mold failure, poor surface finish, and excessive maintenance. Mold material costs represent 10-20% of total tooling cost.
Common Mold Steel Grades
| Steel Grade | Hardness (HRC) | Wear Resistance | Polishability | Corrosion Res. | Rel. Cost |
|---|---|---|---|---|---|
| P20 (1.2311) | 28-32 | Fair | Good | Poor | 1.0x |
| H13 (1.2344) | 46-52 | Very Good | Good | Fair | 1.5x |
| S7 (1.2358) | 50-56 | Good | Fair | Poor | 1.3x |
| 420SS (1.2083) | 48-52 | Good | Excellent | Excellent | 2.0x |
| NAK80 (1.2767) | 38-42 | Good | Excellent | Fair | 1.8x |
| Stavax | 50-52 | Very Good | Excellent | Excellent | 2.5x |
Selection by Application
High-Volume Production (>500,000 cycles)
Use premium H13 or S7 steel hardened to 50-54 HRC for excellent wear resistance and thermal fatigue. P20 not recommended beyond 200,000 cycles.
Corrosive Plastics (PVC, POM, PTFE)
Use 420SS or Stavax stainless mold steels for corrosion resistance when molding corrosive materials.
Optical and Clear Parts
Select NAK80 or 420SS for superior polishability (Ra < 0.05 um) in transparent applications.
Surface Finish Capabilities
| Surface Grade | Ra Value | Achievable With | Application |
|---|---|---|---|
| Diamond Polish | < 0.05um | 420SS, NAK80, Stavax | Optical lenses |
| SPI A1 | 0.05-0.1um | NAK80, 420SS | Automotive, cosmetic |
| SPI A2 | 0.1-0.2um | P20, H13 | Consumer electronics |
| SPI B1 | 0.2-0.4um | H13, S7 | General appearance |
Cost-Per-Cycle Analysis
| Steel | Mold Cost | Expected Life | Cost/Cycle |
|---|---|---|---|
| P20 | 5,000 | 150k cycles | bash.167 |
| H13 | 0,000 | 1M cycles | bash.040 |
| NAK80 | 5,000 | 800k cycles | bash.056 |
| 420SS | 0,000 | 1M+ cycles | bash.050 |
H13 provides the lowest cost-per-cycle for high-volume production. For medium-volume runs (<200k), P20 remains most cost-effective.