Injection mold steel is core foundation of product molding, directly impacting product quality, production efficiency, and cost. Faced with numerous grades like P20, 718H, NAK80, and H13, how can beginners and engineers quickly choose right material? This article explores practical selection strategies based on three fundamental dimensions: core performance, application matching, and cost optimization, helping you avoid common pitfalls.

 

Key properties of mold steel are concentrated in three key areas, providing primary considerations for selection:
Hardness (HRC): Determines mold's wear resistance.
Low hardness (28-35 HRC): such as P20, suitable for low-wear applications;
Medium hardness (35-42 HRC): such as 718H and NAK80, offering a balance of wear resistance and processability;
High hardness (48-70 HRC): such as H13, S136 (48-52 HRC), and ASP60 (68-70 HRC), suitable for high-wear and high-temperature applications.
Corrosion resistance: Suitable for use with wet or corrosive materials (such as PVC).
High corrosion resistance: S136 and HPM38 (containing 13% or more chromium), suitable for medical and food molds;
Medium corrosion resistance: 718H;
General corrosion resistance: P20 and NAK80, requiring careful surface protection.
Polishability: Affects the product's surface smoothness.
High Polishing (#8000-15000 mesh): HPM38 (#15000 mesh), NAK80 (#10000 mesh), S136 (#8000 mesh), suitable for transparent parts (such as optical lenses);
Medium Polishing (#6000 mesh): 718H;
Basic Polishing: P20, suitable for general exterior parts.

Different product scenarios correspond to different grades; direct matching is more efficient:
Common Plastics (PP, ABS): Daily necessities, appliance housings
P20 is preferred choice: low cost (8,000-12,000 yuan/ton), with a lifespan of 100,000-300,000 cycles, suitable for small and medium-sized batches;
Upgrade to 718H: higher precision, 30% reduction in polishing hours, suitable for precision parts such as automotive interiors, with a lifespan of 300,000-500,000 cycles. High-temperature engineering plastics (PEEK, LCP): Engine components, battery casings
Choose H13 or 2344: High-temperature resistance (500-650℃) and thermal crack resistance; 2344 offers superior thermal conductivity, making it suitable for rapid mold cycles and boasting a lifespan of 500,000-1,000,000 cycles.
Transparent/high-gloss parts (optical lenses, cosmetic packaging):
Economically choose NAK80: Pre-hardened for direct machining, with a lifespan of up to 1,000,000 cycles after nitriding.
High-end choose HPM38: Ultra-high mirror finish, reducing post-polishing efforts, suitable for optical lenses.
Corrosive/special applications:
PVC, medical parts: Choose S136 (cost 20,000-70,000 yuan/ton) to avoid mold cavity corrosion.
Glass fiber reinforced plastic (PA+GF30): Choose ASP60, featuring ultra-high hardness (68-70 HRC) and a lifespan of over 1,000,000 cycles, suitable for high-wear IC packaging and high-end components.
Common Injection Mold Cavity and Core Mold Steel Properties and Selection Principles

Surface Treatment Improves Lifespan: NAK80 doubles its lifespan after nitriding (500,000 → 1,000,000 cycles); PVD coating of H13 triples wear resistance, achieving a payback period of 3 months.
Processing Matching:
Pre-hardened steel (P20, 718H, NAK80): Direct machining reduces cycle time by 40%;
Hot-work steel (H13, 2344): Vacuum quenching and tempering are required; H13 also undergoes stress annealing at 550℃ for 2 hours to reduce deformation.
Inventory Strategy:
Common grades (P20, H13): Maintain 1-2 months of inventory to cover peak seasons;
High-end grades (S136, ASP60): Just-in-time (JIT) procurement reduces capital investment.

Accelerating Domestication: 718 series has an import substitution rate exceeding 60% (e.g., Baosteel products), offering lower costs and performance close to imported products.
Technological Upgrades: Powder metallurgy (e.g., ASP60) and AI temperature control technologies make molds more durable and precise.
Green Development: Increased scrap recycling reduces energy consumption, making future costs more manageable.
Selection Summary: 3-Step Quick Decision-Making
Determine Requirements: Identify product material (e.g., PP/PEEK), surface requirements (normal/transparent), and life expectancy.
Performance: Narrow selection based on hardness, corrosion resistance, and polishability.
Cost Consideration: Choose P20 for small and medium-sized batches, H13/S136 for precision/high-temperature applications, and ASP60 for extreme applications.
Steel Grade Characteristics: 718H
Delivered in a pre-hardened state of HRC 33-38.
This quenched and tempered plastic mold steel features refined and vacuum degassed treatments, resulting in pure steel with uniform hardness, excellent polishing and machinability, is suitable for nitriding and surface treatment. 718(H), also known as "P20+Nī," boasts improved hardenability and superior performance, with excellent polishing, electro-etching, weldability, and machinability.
3. Common Applications:
It can be used to produce large-scale, high-quality plastic molds requiring uniform hardness, high toughness, and excellent surface finish, such as automotive bumpers, interior panels, home appliance products, TV housings, copier housings, bumpers, shock absorbers, and instrument panels. It is suitable for plastic molds requiring polishing or texturing, heavy-duty molds, and thermoplastic injection and blow molding molds.
4. Chemical Composition (as-melted): Corresponds to DIN 1.2738 and GB 3Cr2NiM0.

5. Physical Properties:

ArcelorMittal (known as "Arcelor" in mainland China) is world's leading steel manufacturer, with an annual production capacity of 130 million tons. In 1999, company successfully developed Superplast product line. Thanks to its exceptional reliability and effectiveness, Superplast 1.2738mod.HH (SP2738mod.HH) has become industry benchmark for high-quality mold steel.
Alloy Composition (Typical Values, Major Elements, Weight Percentage) 2738HH

Applications: Injection Molds, Plastic Extrusion Molds, Polishing and Texturing
SP2738mod.HH is a plastic mold steel prehardened to 300 HB. SP2738mod.HH is specifically designed to meet demands of current plastic mold industry. Its unique chemical composition ensures specific properties: extremely high processability, uniform and optimized hardening (up to 910mm thick), excellent weldability, and high thermal conductivity. SP2738mod.HH is an effective alternative to conventional steel grades such as P20 and SP2738mod.HH. It can be used in injection mold cores and cavities for large-scale injection molds such as automotive bumpers, instrument panels, fenders, appliance housings, and bottle cases. It is suitable for thermoplastics, thermosets, ABS, and transparent melts. It can be used in injection molding, die casting, and reaction injection molding (RIM).
SP2738mod.HH also offers significant advantages for moldmakers and users, reducing both mold manufacturing and operating costs.
Due to its outstanding processability, SP2738mod.HH can be used for small to large mold cavities and cores, such as bumper molds. It can also be used for texturing and polishing applications where a mirror finish is not required.
Metallurgical Properties
Purity - Pre-hardened to HRC 34-38, it is suitable for medium- and large-sized molds and offers excellent machinability. Low sulfur content in its patented composition ensures stable texturing and polishing properties. It also offers weldability and excellent thermal conductivity. Industeel utilizes a combination of electric arc furnace and VD refining processes, resulting in a very low inclusion content (especially oxygen). Therefore, SP2738mod.HH is ideally suited for high-polishing requirements.
Machinability - SP2738mod.HH offers excellent machinability (drilling and iron cutting). Its machinability is significantly superior to that of 1.2738/P20. Using efficient machining parameters (cutting speeds and feeds) significantly reduces machining costs and time.
Electrodischarge Machining - SP2738mod.HH is suitable for all EDM processes (electrode or wire cutting) as supplied. If cavity surface is EDMed, stress relief or surface polishing at 530℃ is required to completely eliminate white layer caused by EDM. If cavity surface needs to be textured, it must be polished to remove white layer.
2. Steel Characteristics: NAK80
Age-hardened steel, pre-hardened to HRC 38-42, eliminating need for heat treatment and ensuring permanent deformation. Uniform hardness from surface to center. Special metallurgy ensures excellent mirror polish and etching properties.
3. Common Applications: Flat-panel displays requiring a mirror finish; cameras; audio equipment; transparent covers and film housings.
4. Chemical Composition:

5. Physical Properties:

2. Steel Characteristics: P20
Pre-hardened to HRC 28-36, eliminating need for heat treatment, resisting deformation, suitable for EDM, and easy to polish.
3. Common Applications: Injection and blow molds for thermoplastics.
4. Chemical Composition:

5. Physical Properties:
Critical Temperature of P20 Steel

1. Steel Grade Characteristics: S136H
Delivered in a pre-hardened state with HRC 30-36 or annealed with HB less than 235. This is a martensitic stainless steel.
Smelted in an electric furnace and produced according to stainless steel standards, it is suitable for molds with general polishing requirements and can achieve a mirror finish of 5000 grit. It is primarily used for mold A and B plates, hot runner plates, in equipment manufacturing and molds requiring less stringent polishing requirements.
2. Common Applications:
Suitable for plastic molds requiring corrosion resistance and a mirror finish.
3. Chemical Composition:

4. Available Specifications:
[Raw Material] Thickness 15-380mm x Width 20-610mm x Length 20-800mm
[Blank Material] Thickness 10-380mm x Width 20-610mm x Length 20-800mm
[Fine Material] Thickness 10-380mm x Width 20-610mm x Length 20-800mm
5. Physical Properties:

Critical Temperature of S136H Steel

 

There's no "best" mold steel, only "the most suitable." Save this guide for your next mold selection—it's a quick guide even for beginners!