Steel Hardness Selection for Thin‑Wall Injection Molds

Thin‑wall injection molding, typically defined as parts with a wall thickness of 0.5–1.5 mm, places significantly higher demands on mold steel compared to conventional molding. Higher injection pressures, faster cooling rates, and more frequent cycles require steels with superior wear resistance, toughness, polishability, and thermal conductivity. Hardness is a critical indicator that directly influences mold life, dimensional stability, surface quality, and processing costs. Selecting the optimal hardness requires balancing material properties, part requirements, and production conditions.

1. Key Requirements for Thin‑Wall Mold Steels

Thin‑wall molds must withstand pressures often exceeding 100 MPa and rapid thermal cycling. Consequently, mold steels must possess high wear resistance to prevent surface degradation, sufficient toughness to avoid cracking under high stress, good polishability for high‑quality surfaces, and adequate thermal conductivity to ensure fast cooling. Hardness is the primary factor governing these properties. Insufficient hardness leads to premature wear, deformation, or sticking, while excessive hardness can reduce toughness and increase machining difficulty.

2. Selecting Hardness Based on Plastic Material

The abrasiveness of the plastic is the most important factor in determining the required steel hardness.

1. Commodity Thermoplastics

Materials such as PP, PE, ABS, and PS have low abrasiveness and good flowability. Molds for these plastics can use pre‑hardened steels with a hardness of HRC 30–36, such as P20, 718H, or Nak80. These steels offer good machinability and moderate wear resistance, making them cost‑effective for medium‑volume production.

2. Reinforced Plastics

Glass‑fiber, carbon‑fiber, or mineral‑filled plastics (e.g., PA+GF, PBT+GF, PC+GF) cause severe abrasive wear on mold surfaces. For these applications, hardness should be increased to HRC 48–54. Common choices include H13, S136H, and 2344. For fill contents exceeding 30%, higher hardness steels such as ASP23 or Vancron 40 (HRC 55+) may be necessary to ensure acceptable mold life.

3. Corrosive Plastics

Plastics containing chlorine, fluorine, or flame retardants (e.g., PVC, POM, flame‑retardant ABS) can chemically attack the mold surface, leading to pitting and reduced polishability. Stainless steels with good corrosion resistance and sufficient hardness are preferred. Typical selections include S136 and STAVAX ESR, hardened to HRC 48–52.

3. Selecting Hardness Based on Surface Quality Requirements

Thin‑wall parts often require high‑quality surfaces, which depend heavily on the steel’s polishability and hardness.

1. High‑Gloss or Mirror Surfaces

Transparent parts, automotive interiors, and consumer electronics often require mirror‑finish surfaces. Steels with high polishability and moderate to high hardness are ideal. Common choices include Nak80 (HRC 40–44), S136 (HRC 48–52), and H13 (HRC 50–54). Higher hardness improves surface retention but increases processing difficulty.

2. General Surface Requirements

For structural parts where appearance is less critical, pre‑hardened steels such as P20 or 718H (HRC 30–36) provide a good balance of cost, machinability, and durability.

4. Selecting Hardness Based on Mold Structure and Processing

Thin‑wall molds are often complex, with small inserts, deep cavities, and narrow gaps, which affect steel selection.

1. Complex or High‑Precision Molds

Complex geometries require steels that are easy to machine, EDM, and polish. Pre‑hardened steels in the range HRC 30–40 (e.g., P20, 718H, Nak80) are commonly used to reduce processing time and cost.

2. Simple or High‑Volume Molds

For molds with simple geometries and high production volumes, higher hardness steels such as H13, 2344, or S136 (HRC 48–54) are preferred to enhance wear resistance and fatigue strength.

3. Hardness Matching Between Inserts and Cavities

Thin‑wall molds often use a insert‑type structure. Inserts, which experience higher local stress, are usually harder than the cavity. For example, the cavity may use P20 (HRC 30–36), while inserts use H13 or S136 (HRC 48–52).

5. Selecting Hardness Based on Expected Mold Life

Mold life is closely related to steel hardness, toughness, and resistance to wear and fatigue.

1. Low Volume (1–100,000 shots)

Pre‑hardened steels such as P20 or 718H (HRC 30–36) are suitable due to their low cost and good machinability.

2. Medium Volume (100,000–500,000 shots)

Steels with higher hardness and better wear resistance, such as Nak80 (HRC 40–44) or S136H (HRC 40–45), are recommended.

3. High Volume (500,000+ shots)

High‑hardness, high‑wear steels such as H13, 2344 (HRC 50–54), or ASP23 (HRC 55+) are required. Surface treatments such as nitriding or PVD coating can further extend life.

6. General Selection Guidelines

The selection of steel hardness for thin‑wall molds should follow these principles:

• Start with the abrasiveness of the plastic to determine the minimum required hardness.

• Adjust based on surface finish requirements, with higher hardness generally improving polishability.

• Consider mold complexity and choose hardness that allows efficient machining.

• Finally, match the hardness to the expected production volume to optimize cost and life.

Common industry combinations include:

• PP/PE/ABS thin‑wall parts: P20 (HRC 30–36)

• PC/PMMA transparent parts: S136 (HRC 48–52)

• PA+GF/PBT+GF structural parts: H13 (HRC 50–54)

• High‑volume packaging molds: 2344 (HRC 52–56)

• High‑precision complex molds: Nak80 (HRC 40–44)

7. Conclusion

Selecting the correct hardness for thin‑wall injection mold steel requires balancing wear resistance, toughness, machinability, surface quality, and cost. The optimal hardness depends on the plastic material, part design, surface requirements, mold structure, and production volume. By following industry guidelines and considering these factors, mold makers can ensure longer mold life, better part quality, and lower overall manufacturing costs.