How to Choose Mold Steel for Plastic Injection Molds

Selecting the right mold steel is an important part of mold design and manufacturing. The choice should be based on factors such as the forming method, the type of plastic material, mold size, expected production volume, and required surface finish.

1. Forming method
Mold steel should first be selected according to the molding or forming process. In general, tool steels can be divided into two main categories:

(1) Hot-work tool steel, which is suitable for applications involving high temperatures, such as die casting, forging, and extrusion.

(2) Cold-work tool steel, which is commonly used for blanking, shearing, cold forming, cold extrusion, cold forging, and powder compaction.

2. Plastic material
The type of plastic being processed also affects mold steel selection. Some plastics, such as PVC, can produce corrosive by-products during molding. In addition, condensation, corrosive gases, acidic substances, cooling or heating conditions, water exposure, and long shutdown periods during storage may also lead to mold corrosion. In such situations, stainless mold steel is generally recommended.

3. Mold size and expected mold life
Mold size and production quantity are also important considerations. Pre-hardened steel is often used for large molds, while fully hardened steel is more commonly used for smaller molds.

For molds intended for long production runs of more than 1,000,000 cycles, high-hardness steel is generally recommended, with a hardness range of approximately 48-65 HRC.

For medium production runs of about 100,000 to 1,000,000 cycles, pre-hardened steel is commonly used, with a hardness range of around 30-45 HRC.

For short production runs of less than 100,000 cycles, mild steel may be used, typically with a hardness of about 160-250 HB.

4. Surface finish requirements
Surface finish is another key factor in mold steel selection. Many plastic mold manufacturers require good surface polishability. Although sulfur is sometimes added to steel to improve machinability, higher sulfur content can reduce surface finish quality and make the steel more brittle. Therefore, when high surface quality is required, mold steels with lower sulfur content are generally preferred.

In summary, proper mold steel selection should balance mold performance, corrosion resistance, mold life, machinability, and surface finish requirements to achieve reliable production and consistent part quality.