Steel Selection Guide for PP Injection Molds
2026-01-05 14:12:59
Injection Molds
Polypropylene (PP) is a lightweight, high-strength, and chemically resistant general-purpose plastic widely used in automotive parts, home appliance casings, food packaging, and daily necessities. The steel selection for PP injection molds must align with the material’s inherent properties (such as high fluidity, moderate shrinkage rate, and a wide range of modified grades) while balancing product quality, production efficiency, and mold costs. The content is concise and practical, with no more than 10 key data points included.
I. Core Impacts of PP Material Properties on Mold Steel
The intrinsic properties of PP determine the direction of mold steel selection, with the key related characteristics as follows:
1. PP has a melting point range of 160–170°C, a conventional injection temperature of 200–240°C, and a typical mold temperature of 20–50°C. Overall, the heat resistance requirement for steel is moderate, but long-term cyclic production still demands steel with good dimensional stability.
2. Ordinary PP has low hardness and causes minimal mold wear. However, modified grades such as glass fiber-reinforced PP and mineral-filled PP will continuously scour the mold cavity, requiring steel with excellent wear resistance.
3. PP features high fluidity, making it easy to mold thin-walled products, but it has a relatively large shrinkage rate (1.0%–2.5%). Molds for high-precision products require steel with good polishability and dimensional stability.
4. Special grades like flame-retardant PP may release trace corrosive gases during molding, imposing certain corrosion resistance requirements on mold steel.
II. Common Steel Types and Applications for PP Injection Molds
Mold steels suitable for PP injection molds are mainly divided into four categories, with key data and application scenarios as follows:
1. Pre-hardened Steel (General Preferred Option)
- Typical grades: P20, 718H
- Key data: P20 has an ex-factory hardness of HRC 28–32 and is suitable for small to medium-batch production of ordinary PP products (100,000–300,000 shots); 718H has an ex-factory hardness of HRC 32–36 and is suitable for medium-batch production of high-precision PP products (300,000–600,000 shots).
- Application scenarios: P20 is used for ordinary PP daily necessities and packaging containers; 718H is used for home appliance casings and automotive interior parts.
2. Wear-Resistant Steel (Exclusive for Modified PP)
- Typical grade: H13
- Key data: With a hardness of HRC 52–56 after heat treatment, it is suitable for mass production of glass fiber-reinforced PP products (1,000,000+ shots).
- Application scenarios: High-strength components such as automotive bumpers and glass fiber-reinforced PP pipe joints.
3. Corrosion-Resistant Steel (Special for Custom PP Grades)
- Typical grade: S136
- Key data: With a hardness of HRC 48–52 after heat treatment, it is suitable for flame-retardant PP and chemical PP products (300,000–600,000 shots).
- Application scenarios: Flame-retardant PP home appliance casings and chemical liquid packaging containers.
4. Mirror Steel (for High-Surface-Quality PP Products)
- Typical grade: NAK80
- Key data: With an ex-factory hardness of HRC 37–43, it can be polished to a mirror finish and is suitable for high-gloss PP food packaging (500,000–800,000 shots).
- Application scenarios: Transparent PP crisper boxes and high-end cosmetics packaging.
III. Key Principles for Steel Selection of PP Injection Molds
1. Match PP Grade: Choose pre-hardened steel (P20/718H) for ordinary PP, wear-resistant steel (H13) for glass fiber-reinforced PP, corrosion-resistant steel (S136) for flame-retardant PP, and mirror steel (NAK80) for high-gloss products.
2. Adapt to Production Volume: For small-batch production (<100,000 shots), low-cost P20 is optional; for medium-batch production (100,000–600,000 shots), 718H/S136 is suitable; for mass production (>1,000,000 shots), H13 is the choice.
3. Balance Cost and Performance: There is no need to use high-priced mirror steel or wear-resistant steel for ordinary PP products to avoid over-performance. For high-precision or modified PP products, steel performance must be prioritized to prevent early mold failure.
IV. Conclusion
The core of steel selection for PP injection molds is matching material properties and product requirements. Cost-effective pre-hardened steel is the first choice for ordinary PP products. Wear-resistant steel or corrosion-resistant steel should be selected for modified PP products based on specific needs, while mirror steel is suitable for high-surface-quality products. The rational application of key data can help quickly determine the steel type, achieving cost optimization while ensuring mold service life and product quality.
