Choosing the wrong mold steel is expensive. Too soft and your mold wears out before the production run finishes. Too hard and you are paying for capability you do not need. This guide compares the six most common injection mold steels and tells you exactly when to use each one.
Quick Decision Matrix
| If you need... | Choose | Hardness |
|---|---|---|
| Lowest cost, low volume (<50K shots) | P20 | 28-32 HRC |
| Medium production (50K-300K), general purpose | 718H | 33-37 HRC |
| High polish, transparent parts, optical | NAK80 | 38-42 HRC |
| Medical, food-grade, corrosion resistance | S136 | 48-52 HRC |
| High-temp resins, die-casting | H13 | 46-50 HRC |
| Glass-filled, abrasive materials, >1M shots | M333 | 50-54 HRC |
P20 — The Workhorse
Hardness: 28-32 HRC | Cost: $ | Best for: Low-volume production, prototypes, simple parts
P20 is the most commonly used mold steel for a reason — it is affordable and easy to machine. It is pre-hardened, which means no heat treatment distortion after machining. Use P20 when your production volume is under 50,000 shots and your plastic is non-abrasive (PP, PE, ABS, HIPS).
Limitations: Cannot achieve high polish. Poor corrosion resistance. Wears faster with glass-filled materials. Do not use P20 for medical or food-contact parts.
718H — The All-Rounder
Hardness: 33-37 HRC | Cost: $$ | Best for: Medium production, commercial parts, good surface finish
718H is the most popular choice for commercial injection molds. It offers better polishability than P20, higher hardness for longer tool life (100K-300K shots), and good machinability. This is the default recommendation for most injection mold projects — it balances cost, durability, and finish quality.
Limitations: Not suitable for corrosive materials. Not hard enough for abrasive glass-filled compounds above 30% glass.
NAK80 — The Polisher
Hardness: 38-42 HRC | Cost: $$$ | Best for: High-gloss, transparent, optical parts
NAK80 is a precipitation-hardening steel with exceptional polishability. It can achieve a mirror finish (SPI A-1) that P20 and 718H cannot. It is the steel of choice for lenses, transparent covers, cosmetic surfaces, and any part where surface appearance is critical. It also holds fine detail well — important for textured surfaces.
Limitations: More expensive. Softer than S136 and H13, so not ideal for very high volume or abrasive materials.
S136 — The Medical Grade
Hardness: 48-52 HRC | Cost: $$$$ | Best for: Medical devices, food contact, corrosive materials, high volume
S136 is a stainless mold steel with high corrosion resistance. It is required for medical devices under ISO 13485, food-contact parts, and any application where the mold will run PVC or other corrosive materials. The high hardness (48-52 HRC) gives it excellent wear resistance for high-volume production (500K+ shots).
Limitations: Most expensive common steel. Harder to machine, which extends lead time. Overkill for simple commercial parts.
H13 — The Hot Worker
Hardness: 46-50 HRC | Cost: $$$$ | Best for: High-temperature engineering resins, die-casting molds
H13 is a hot-work tool steel designed for high-temperature applications. If your plastic requires mold temperatures above 120°C (PEEK, PPS, LCP, PEI/Ultem), you need H13. It is also the standard steel for aluminum and zinc die-casting molds. H13 resists thermal fatigue cracking that would destroy P20 or 718H under sustained heat cycling.
Limitations: More difficult to polish than NAK80. Higher cost. Generally overkill for commodity thermoplastics.
M333 — The Tank
Hardness: 50-54 HRC | Cost: $$$$$ | Best for: Glass-filled materials, abrasive compounds, million-shot production
M333 is a high-hardness stainless mold steel designed for extreme wear resistance. If your material contains 30% or more glass fiber, or if you need the mold to run over a million cycles without significant wear, M333 is the answer. The combination of high hardness and stainless properties makes it ideal for high-volume medical and automotive applications.
Limitations: Premium cost. Longest machining time. Only justified for high-value production programs.
Common Steel Selection Mistakes
- Choosing P20 for glass-filled nylon — The glass fibers will erode P20 within 10,000-20,000 shots. You need H13 or M333.
- Choosing S136 for a simple PP cap — Paying 3x the steel cost for capability you will never use.
- Not specifying the steel grade in the purchase order — Some low-cost suppliers substitute lower-grade steel. Always confirm via spectrometer verification at incoming inspection.
- Using European steel prices for China sourcing — Chinese mold makers import European steel when specified, but the machining cost is lower. You get the same steel at a better total price.
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