P20, H13, NAK80, S136 Mold Steel Selection Guide & Complete CNC Milling Machining Solutions

Introduction

Mold steel accounts for 30%–40% of total tooling cost. Picking the wrong grade at RFQ stage may lead to up to $25,000 in mid-production cavity repair and unplanned shutdown losses. This guide comprehensively compares P20, H13, NAK80 and S136, paired with real-world milling parameters for vertical CNC machining centers. We also provide a 3-dimensional selection framework based on output volume, resin type and surface requirements, plus cost-saving hybrid insert mold design strategies to cut overall material and machining expenses.

1 Critical Losses Caused by Improper Mold Steel Selection

Most mold buyers and DFM engineers leave steel selection entirely to mold makers, ignoring how material grades determine long-term production costs:

• Molds made of P20 for glass-filled resins show obvious gate wear after 200,000–300,000 shots. Replacing worn cavities costs $5,000–$15,000 each time;
• Using regular pre-hardened steel for corrosive resins like PVC and POM causes pitting and rust on cavity surfaces, scrapping glossy finished parts and halting production for 3–4 weeks for repairs;
• Specifying premium hard steels (S136/H13) for entire molds unnecessarily raises raw material costs by 2.0–2.8 times, while longer heat treatment, CNC milling and polishing hours further inflate upfront investment.

90% of standard injection molding projects only require four steel grades: P20, H13, NAK80 and S136. Below we break down material properties, mass production suitability and CNC milling difficulty for targeted selection.

2 Comprehensive Performance Comparison of Four Mold Steels

Unlike reference articles limited to hardness, shot life and cost, this table adds exclusive machining-focused metrics unique to machine tool manufacturers for differentiated value:

3 In-depth Introduction to Each Mold Steel Grade (With Machining Tips & Internal Product Link)

3.1 P20 (1.2311): Cost-Effective General Mold Steel for Small & Medium Batches

As the industry baseline pre-hardened steel, P20 arrives fully hardened and can be directly processed on CNC milling machines without heat treatment, cutting mold lead time by 5–10 working days.

• Advantages: Outstanding machinability; down-milling delivers smoother surface roughness; easy welding and modification for frequent trial-mold revisions.
• Drawbacks: Poor wear and corrosion resistance. Gate erosion occurs after 200,000 shots with glass-filled resins, unsuitable for corrosive plastic materials.
• Applications: Home appliance housings, general structural parts with under 500,000 shots; widely used as base frame material for hybrid molds.

3.2 NAK80: Pre-Hardened Steel Exclusive for Class-A Cosmetic Surfaces

NAK80 is a proprietary precipitation-hardened steel from Daido Steel. No secondary hardening is needed after machining, and it can be polished directly to meet cosmetic standards for consumer electronics and automotive interiors.

• Machining merit: Consistent texture after EDM etching; local cavity defects can be repaired by welding without re-heat treatment, ideal for mass finishing of cosmetic inserts on vertical machining centers.
• Limitations: Non-stainless with zero rust resistance; insufficient abrasion resistance for glass-filled resins; cannot replace S136 under corrosive molding conditions.

3.3 H13 (1.2344): Wear-Resistant Hot-Work Steel for Glass-Filled Molding

H13 must be rough-machined under annealed status with a 0.2–0.5mm finishing allowance, then vacuum quenched and double tempered to HRC 48–52. Post-hardening finishing relies on high-rigidity vertical machining centers. Vanadium carbide in its microstructure delivers superior abrasion resistance, maintaining dimensional tolerance over 1,000,000 cycles for PA66-GF30, PPS and PEI high-temperature resins. Its 5% chromium content provides basic high-temperature oxidation resistance for molding temperatures above 300°C.

While material cost is 1.5–2.0 times higher than P20, a hybrid design using H13 only for gates and wear inserts recovers the premium after an extra 300,000 production cycles by eliminating cavity replacement costs.

3.4 S136 (1.2083): Stainless Steel for Medical, Optical & Corrosive Resin Molding

As a 13% chromium stainless mold steel, S136 is the only mass-production grade capable of mirror polishing down to Ra ≤0.025μm. It resists acidic gas released by PVC and POM, complying with hygiene standards for medical devices and food packaging.

Machining challenges: Severe work hardening during milling. Operators need 8%–10% emulsion coolant fully covering cutting zones and coated carbide tools with cutting speed controlled at 80–120m/min. Polishing takes 20%–30% longer than NAK80, so high-precision vertical machining centers are recommended to reduce tool marks and shorten post-processing polishing hours.

Upgrading from P20 to S136 costs an extra $3,000–$8,000 per mold, yet avoids $5,000–$15,000 repair fees and weeks of production shutdown caused by cavity pitting corrosion.

4. 3 Core Selection Framework

4.1 Selection by Production Volume

• Below 500,000 shots, unfilled resin & standard appearance: P20; NAK80 for cosmetic-focused parts
• Over 1,000,000 shots, glass/mineral-filled materials: H13 (only for wear inserts, not full mold)
• 500,000–1,000,000 shots, medical/transparent/corrosive resins: S136 stainless steel

4.2 Selection by Resin Chemical Properties

• Non-corrosive unfilled PP/ABS/PE: P20
• PVC, POM, halogenated flame-retardant plastics: Mandatory S136
• GF30 Nylon, PBT, PPS high-abrasion engineering resins: H13 wear inserts
• PMMA transparent cosmetic parts: NAK80 (general clarity) / S136 (optical mirror finish)

4.3 Selection by Surface Finish Standard

• Textured or painted surfaces: P20 / H13 fully adequate
• Class-A high-gloss consumer electronics housings: NAK80
• Optical lenses, transparent medical containers requiring mirror effect: S136 only

5 Cost-Efficient Solution: Hybrid Multi-Grade Mold Structure

Manufacturers waste significant capital by manufacturing full molds from a single steel grade. The industry’s most cost-effective method allocates materials based on functional zones, and our high-precision machining centers support one-clamp multi-insert processing:

• Mold base & low-wear large structural plates: P20 for fast machining and low cost
• Cosmetic cavity inserts: NAK80 for uniform polishing and easy post-trial revisions
• Gates, thin shut-offs and glass-fiber erosion zones: Removable hardened H13 inserts
• Corrosive molding areas & optical transparent cavities: S136 stainless inserts

Case study: PA66-GF30 connector housing with 650,000 projected cycles. A hybrid mold structure cuts raw material cost by 22% and CNC milling time by 18% compared to a full-H13 mold, and worn inserts can be replaced separately without remaking the whole mold.

6 Practical CNC Milling Machining Tips for Different Mold Steels (Exclusive Machine Tool Manufacturer Content)

• Pre-hardened P20 & NAK80: Direct rough and finish machining, feed rate 0.15–0.25mm/r, standard coated carbide tools deliver optimal efficiency
• Annealed H13 roughing: Deep cutting depth for high throughput; reduce cutting speed drastically for post-quench finish milling to prevent tool breakage
• S136 stainless steel: Small depth of cut with high feed rate, sufficient coolant to suppress work hardening, spindle runout controlled within 0.002mm; heavy-duty vertical machining centers are prioritized

Our self-developed high-precision CNC milling machines feature reinforced spindle rigidity optimized for hard mold steel processing. Tool service life rises by 25% when machining H13 and S136, with workpiece deformation controlled within 0.03mm, ideal for mass production of all types of injection mold cavities.

7 Mold Steel Selection Pitfalls to Avoid

• Require full mill certificates and hardness test reports; reject vague descriptions such as "P20 equivalent" or "generic S136 substitute" without official documentation
• Reserve sufficient machining allowance for heat treatment of hard grades H13 & S136 to avoid dimensional out-of-tolerance after tempering
• For long-term humid storage or frequent production shutdowns, use S136 cavity inserts even for ordinary non-corrosive plastics to prevent rust
• Choose P20 or NAK80 for projects with frequent post-T1 engineering changes; hardened steels have complex, costly welding repair processes
• Hardness alone does not determine mold service life. Toughness, heat treatment quality, mold geometry, cooling design and injection pressure all impact long-term durability

Conclusion

There is no universal "best" mold steel among P20, H13, NAK80 and S136. Selection must balance annual output, resin composition, surface finish standards and potential downtime repair costs.

• P20: General molds for low-to-medium volume non-abrasive projects
• NAK80: Pre-hardened inserts for consistent high-gloss cosmetic parts
• H13: Wear-resistant inserts for glass-filled and high-temperature molding
• S136: Stainless inserts for corrosive, optical and medical-grade applications

Combining hybrid multi-grade mold construction with high-precision CNC milling equipment optimizes machining processes, balancing upfront material investment and long-term maintenance costs. This eliminates expensive production losses stemming from incorrect steel selection at the RFQ stage.

FAQ Section (For Google Featured Snippet & AI Overview Capture)

Q1: Which one to choose between NAK80 and P20?

A: Pick P20 for tight budgets and molds requiring frequent revisions. Select NAK80 for consumer electronics and automotive interior parts demanding stable Class-A cosmetic texture. Neither requires post-machining heat treatment for fast delivery.

Q2: Is full H13 mandatory for molds processing glass-filled nylon?

A: No. Use P20 for mold bases, and only install replaceable H13 inserts at gates, core pins and shut-off areas exposed to heavy glass fiber erosion to slash material and CNC processing costs.

Q3: Is S136 stainless steel? Do all medical molds need S136?

A: S136 is 13% chromium stainless steel with acid resistance and high-temperature sterilization tolerance. S136 cavity inserts are required for any mold contacting medical raw materials, food media, PVC or POM corrosive resins.

Q4: Which is harder to machine: H13 or S136?

A: S136 is more challenging. Stainless steel generates severe work hardening during cutting, placing higher requirements on machine rigidity, cooling systems and cutting tools. H13 only raises machining difficulty after quenching and tempering.

Q5: Can one single mold adopt multiple steel grades simultaneously?

A: Yes. Hybrid insert design is the industry’s most cost-effective solution, matching material performance to zones with different corrosion, wear and cosmetic requirements. Our vertical machining centers support integrated processing of multi-grade inserts in one setup.

Q6: Does higher hardness guarantee longer mold life?

A: Not necessarily. Toughness, heat treatment procedure, mold cooling layout, CNC fillet design and injection pressure all affect service life. Over-hardened steel with insufficient toughness tends to crack or chip easily.