Mold Steel Selection: Getting the Right Performance Without Wasting Money

Raw materials typically account for only 10–15% of the total mold cost. Choosing lower‑grade materials may seem like an easy way to save money, but in long‑term production, molds must have extended service life. This often requires high‑performance materials, which come at a higher cost.

Material Selection Guidelines for Key Components

For mold plates in the mold base, imported S50C or domestic SM45 is typically used, with a hardness of HB 160–200. These materials offer uniform hardness, low internal stress, and minimal deformation.

Guide pins are generally made of GCr15 or SUJ2, hardened to HRC 56–62 for high wear resistance. Guide bushings, ejector guide pins, ejector guide bushings, and return pins can also be made from GCr15 or SUJ2 at the same hardness level, or from T8A or T10A at HRC 52–56, depending on requirements.

For general structural parts — such as locating rings, spacers, ejector limit blocks, stop pins, and lock modules — domestic SM45 steel in its normalized condition (HB 160–200) is sufficient. These parts have no special hardness or wear resistance requirements and do not need heat treatment.

Comparative Summary of Common Plastic Mold Steels

1. P20

• Key Characteristic: All-purpose, pre-hardened, good machinability.

• Typical Hardness: 30–35 HRC

• Best Suited For: General-purpose molds, low to medium volume, non-corrosive plastics like ABS, PP, PE.

2. 718H

• Key Characteristic: Upgraded P20, better polishability and toughness.

• Typical Hardness: 35–40 HRC

• Best Suited For: High-precision, medium-to-large molds; glass-filled plastics (PA+GF, PBT).

3. NAK80

• Key Characteristic: Superior mirror polish, pre-hardened.

• Typical Hardness: ~40 HRC

• Best Suited For: High-gloss/optical parts (PC, PMMA), not for corrosive plastics.

4. H13 / 2344

• Key Characteristic: High heat resistance and toughness.

• Typical Hardness: 48–52 HRC

• Best Suited For: High-temperature engineering plastics (PEEK, LCP, PPS), long-run molds.

5. S136 / 2083

• Key Characteristic: Excellent corrosion resistance and polishability.

• Typical Hardness: 48–52 HRC

• Best Suited For: Corrosive plastics (PVC, flame-retardant grades), medical/food applications.

Choose P20 for general use, 718H for higher precision/GF materials, NAK80 for mirror/optical, H13 for high heat, S136 for corrosion. When in doubt, prioritize corrosion resistance (S136) over polishability (NAK80) if both are needed.

Practical Considerations: Standardization and Cost

In addition to performance and processability, two practical factors must be considered:

1. Standardization

Mold steels are generally used in small quantities. To simplify procurement and inventory management, it is advisable to choose commonly available, versatile steel grades whenever possible.

2. Cost‑effectiveness

A comprehensive economic analysis should be considered — including mold manufacturing cost, production volume, and the cost per part. The final material selection should balance both technical and economic factors.