A comprehensive comparison and application scenario analysis of S136 vs P20 mold steels.

Mold steel is a core material in plastic mold manufacturing, and its performance directly determines the mold’s lifespan, the quality of the molded product, production costs, and delivery cycle. In the plastic mold industry, P20 and S136 are the two most widely used pre-hardened mold steels, corresponding to low-to-mid-range general-purpose applications and mid-to-high-end precision/corrosion-resistant applications, respectively. This article will comprehensively compare them from the perspectives of material properties, hardness, toughness, rust resistance, corrosion resistance, polishing performance, fatigue strength, mold life, price, and applicable scenarios, providing professional guidance for mold customization and selection.

1. Basic Properties of Materials

1. P20 Mold steel

This is a pre-hardened plastic mold steel conforming to the American ASTM standard, with the domestic equivalent grade 3Cr2Mo. It is a medium-carbon chromium-molybdenum alloy structural steel. This steel is produced using an electric furnace smelting and forging process, and is pre-hardened at the factory, requiring no subsequent overall quenching. It can be directly machined by cutting, milling, and grinding.

Its core components are carbon (C), chromium (Cr), and molybdenum (MMo). The carbon content is moderate (around 0.35%), the chromium content is approximately 1.7%, and it contains no strengthening elements such as nickel or vanadium. The composition design prioritizes ease of machining, low cost, and versatility, making it suitable for most conventional plastic product molds.

2.S136 mold steel

This is a martensitic stainless mold steel conforming to the Swedish ASSAB standard, with a domestic equivalent of 4Cr13, belonging to the high-chromium stainless steel category. Its carbon content is approximately 0.4%, and its chromium content is over 13%. Molybdenum is added to improve hardenability and corrosion resistance. Produced using the electroslag remelting (ESR) process, it boasts higher material purity and is free of significant porosity and impurities.

S136 requires quenching and tempering to reach its usable hardness. Its core advantages are rust and corrosion resistance, high polishability, and high stability, making it the preferred material for high-end precision plastic molds.

2. Hardness and mechanical properties

1.  Hardness

-P20: Factory pre-hardened hardness is 28~35 HRC, the hardness range is fixed and cannot be significantly improved through heat treatment. The hardness is relatively low, and the wear resistance is limited, making it suitable for producing molds in medium batches without high wear resistance requirements.

-S136: Annealed hardness ≤25 HRC, easy for rough machining; after quenching and tempering, the hardness can reach 48~52 HRC, much higher than P20. The high hardness brings excellent wear resistance and can cope with the long-term wear of highly filled plastic raw materials.

2. toughness

– P20: Low carbon content, fewer alloying elements, superior toughness compared to standard S136, strong impact resistance, less prone to chipping and cracking in molds, suitable for complex mold cavities with undercuts/thin ribs.

– S136: High hardness after quenching, slightly lower toughness than P20, but a proper tempering process can balance hardness and toughness, meeting the structural strength requirements of precision molds, and posing no risk of brittle fracture under normal use.

3. Fatigue strength

Fatigue resistance determines the mold’s resistance to cracking after long-term opening and closing.
-P20 has low hardness and relatively loose structure, resulting in average fatigue resistance and making it prone to microcracks during long-term high-frequency production.
-S136, after refining, has a uniform and dense metallographic structure and still possesses excellent fatigue resistance even at high hardness. It can withstand hundreds of thousands of mold opening and closing cycles and is not prone to fatigue cracks.

4. Rust prevention and corrosion resistance

This is the most crucial difference between the two types of steel, which directly determines the mold’s operating environment and maintenance costs.

– P20: Low chromium content, no rust-proof design, and virtually no rust prevention capability. It is highly susceptible to rust spots and corrosion in humid environments, when using circulating cooling water, or when in contact with acidic plastic raw materials, leading to damage to the mold cavity surface and affecting the product’s appearance. Regular application of rust-preventive oil is necessary during daily production, and moisture protection is required when the machine is shut down.

– S136: Its high 13% chromium content forms a dense passivation film, providing excellent rust prevention and corrosion resistance. It can be used directly in humid environments without rust prevention treatment; it can withstand long-term contact with acidic/corrosive plastic raw materials such as PVC, POM, and flame-retardant plastics without corrosion or rusting, making it the only preferred choice for corrosive plastic molds.

5.Polishing performance and surface gloss

The appearance quality (transparency, smoothness, and flawlessness) of plastic products depends entirely on the polishing effect of the mold cavity.

– P20: The material purity is average, with a small amount of internal impurities and porosity. Polishing performance is limited, achieving a maximum mirror finish of #800~#1000, but not a super-mirror finish. The surface is prone to orange peel texture and pitting, making it unsuitable for transparent/high-gloss products.

– S136: The electroslag remelting process ensures the material’s purity and lack of impurities, with a uniform metallographic structure. Polishing performance is excellent, easily achieving a super-mirror finish of #2000~#5000. The surface smoothness is close to glass, without any flaws, making it a dedicated mold steel for transparent acrylic, PC, optical lenses, and other products.

6.Mold lifespan

The lifespan of a mold is directly related to the material’s hardness, wear resistance, and corrosion resistance.

– P20: Limited by hardness and abrasion resistance, its service life is relatively short. When producing conventional plastics (ABS, PP), its lifespan is approximately 300,000 to 500,000 mold cycles; when producing abrasion-resistant plastics with glass fiber reinforcement or mineral fillers, the lifespan drops significantly to 100,000 to 200,000 mold cycles.

– S136: High hardness + high abrasion resistance + corrosion resistance, with a service life 3 to 5 times that of P20. In conventional plastic production, its lifespan can reach 1 million to 3 million mold cycles, and even in the production of corrosive/abrasion-resistant plastics, it can guarantee over 800,000 mold cycles, resulting in lower overall long-term costs.

7.Price and processing cost

Cost is a key consideration in mold selection, and the price difference between the two types of steel is significant:

– P20: A general-purpose, economical mold steel. Raw materials are inexpensive, with excellent machinability (easy to cut, no heat treatment required), requiring less machining time and resulting in low overall mold manufacturing costs. Suitable for small-batch, low-cost molds.

– S136: Stainless steel with refining process. Raw material prices are 2-3 times that of P20; it also has high hardness, making machining more difficult and requiring additional quenching and tempering processes, resulting in higher machining time and costs. Suitable for mid-to-high-end molds with long service life requirements.

8.Precisely categorized applicable scenarios

Scenarios where P20 mold steel is preferred

1. Medium-volume (≤500,000 mold cycles) conventional plastic products, such as appliance casings, ordinary toys, daily necessities, and non-surface structural parts;

2. Raw materials are general-purpose plastics such as ABS, PP, PE, and PS, which are non-corrosive and do not require high wear resistance;

3. No high-gloss/transparency requirements for the product appearance, and the mold cavity does not need ultra-mirror polishing;

4. Limited mold budget, pursuing low cost, short delivery time, and a dry operating environment with proper maintenance;

5. Complex structures with many thin ribs, requiring mold toughness to avoid quenching cracking.

Scenarios where S136 mold steel is preferred

1. Transparent/optical grade products: such as acrylic lampshades, PC transparent shells, optical lenses, and medical transparent consumables;

2. Production of corrosive plastics: raw materials that easily corrode molds, such as PVC, POM, flame-retardant plastics, and glass fiber reinforced plastics;

3. Humid production environments: molds are in long-term contact with cooling water and high-humidity workshops, making frequent rust prevention treatments impossible;

4. High-gloss, flawless products: cosmetic packaging, electronic product shells, automotive interior parts, etc., requiring a mirror-like finish;

5. Long lifespan requirements: large order volumes (≥1 million mold cycles), requiring reduced mold repair and replacement frequency to lower long-term production costs;

6. Medical and food grade molds: requiring rust prevention, easy cleaning, no rust contamination, and compliance with hygiene and safety standards.

Summary and Selection Recommendations

P20 and S136 are not inherently superior or inferior; they are simply complementary materials with different target markets. P20 is the “cost-effective choice,” with core advantages of low cost, ease of processing, and good toughness. It is suitable for general-purpose, medium-volume plastic molds that do not require high gloss or corrosion resistance, and is the most widely used basic mold steel in the industry.

S136 is the “high-performance choice,” with core advantages of rust and corrosion resistance, ultra-mirror polishing, high wear resistance, and long lifespan. It is suitable for mid-to-high-end molds that require transparent, precision, corrosive materials, long lifespan, and high aesthetic appeal.

In actual customization, if the product is a common structural component with a limited budget and small batch size, choosing P20 can control costs. However, if the product is a transparent/high-gloss component using corrosive materials, and requires long lifespan and low maintenance costs, S136 should be chosen. Although the initial cost is higher, it can significantly reduce the risk of later repairs and scrap, and improve product quality and production efficiency.