How to Select Mold Steel for PS Material Products
PS (polystyrene) is a widely used general‑purpose plastic with good flowability, low melt viscosity, and non‑corrosive characteristics. For injection molds producing PS parts, mold steel selection mainly depends on appearance requirements, production volume, whether fiberglass is added, and overall cost. A reasonable choice can effectively improve product surface quality, extend mold life, reduce maintenance, and ensure stable mass production.
Key Material Characteristics That Affect Steel Selection
PS resin itself does not produce corrosive gas during molding and contains no abrasive components under normal conditions. Therefore, ordinary PS parts do not require highly corrosion‑resistant steel. However, different product requirements lead to different steel choices. Transparent or high‑gloss PS parts require excellent polishing performance and must avoid rust spots, pinholes, or orange peel patterns. Structural internal parts focus more on cost effectiveness and dimensional stability. For PS materials with added fiberglass, mold steel must have higher hardness and wear resistance to resist abrasion during long‑term production.
1. Mold Steel for Standard Non‑Transparent PS Parts
For common non‑transparent products such as household items, internal structural parts, and general casings with medium or small production volumes, pre‑hardened plastic mold steels are the most practical choice. P20 and 718H are widely used in this category. They are supplied in pre‑hardened condition, do not require quenching, have good machinability, and can be directly polished after processing. These materials provide sufficient service life for production volumes below 500,000 shots. If a slightly better surface finish is needed, NAK80 is a reliable upgrade. It offers more uniform hardness, better polishing performance, and can achieve a bright surface without mirror requirements.
2. Mold Steel for Transparent and High‑Gloss PS Products
Transparent PS, light guide parts, cosmetic containers, and high‑gloss appearance parts have extremely high requirements for surface finish and cleanliness. In these cases, standard P20 or 718H are not suitable because they may contain impurities or porosity, which cause defects after polishing. The most widely recommended material is S136 or S136H stainless steel. These grades have high purity, good corrosion resistance, and can be polished to a high mirror finish. They do not rust easily and prevent water marks or rust spots from affecting transparency. For high‑end optical‑grade products, super‑clean S136 or 1.2083 series steels are preferred to ensure zero defects on the molded surface.
3. Mold Steel for High Volume or Fiberglass‑Filled PS
When PS is blended with fiberglass, usually between 10% and 30%, the melt becomes highly abrasive and quickly wears mold cavities. For high‑volume production or reinforced materials, pre‑hardened steels are not durable enough. H13 or SKD61 hot work die steels are commonly used after proper quenching to reach HRC 48–52. These materials have excellent wear resistance, thermal fatigue resistance, and stability under long production cycles. For ultra‑high volume applications exceeding one million shots, DC53 or D2 can be used, which provide higher hardness and wear resistance but are more difficult to machine and more costly.
Important Processing and Application Notes
Hardness control is critical for both performance and service life. Pre‑hardened steels are typically used at HRC 30–38, while quenched steels for wear resistance range from HRC 48–52. Transparent parts should not be excessively hardened to avoid brittleness. Polishing must follow a step‑by‑step process from rough to fine to mirror to avoid flow marks or uneven surfaces. In humid environments or for long‑term storage, stainless steels such as S136 are strongly recommended to prevent rust. Low‑purity or recycled steel must be avoided for appearance parts, as they often cause visible defects that are difficult to repair.
Summary
Selecting mold steel for PS products requires balancing appearance, production volume, cost, and material properties. Standard non‑transparent parts can use P20 or 718H. High‑gloss parts benefit from NAK80. Transparent and mirror‑finish products rely on S136 or S136H. For fiberglass‑filled or high‑volume production, quenched H13 or SKD61 are the best choices. Proper hardness, polishing, and anti‑rust measures ensure stable molding, high product quality, and extended mold service life.
