Selection Method of Corrosion‑Resistant Steel for Injection Molds

Many plastic materials release corrosive gases during melting and injection, including acids, halogens, and formaldehyde. Common examples include PVC, POM, flame‑retardant nylon, and brominated or chlorinated polymers. These gases attack mold surfaces, leading to rust, pitting, corrosion spots, and premature failure. Using corrosion‑resistant mold steel is essential to protect cavities, gates, runners, and vents. This article introduces a complete and practical selection method based on corrosion level, appearance requirements, production volume, and working environment.

Determine Whether Corrosion‑Resistant Steel Is Necessary

Not all molds require stainless or corrosion‑resistant steel. Non‑corrosive materials such as PP, PE, standard PS, and ABS can use regular pre‑hardened steels like P20 or 718H in dry environments. However, corrosion‑resistant steel is mandatory when molding PVC, POM, flame‑retardant materials, or halogen‑containing compounds. It is also required in high‑humidity workshops, coastal areas, or for molds that are idle for long periods. Transparent parts that cannot tolerate rust spots or water marks also need corrosion‑resistant grades.

Selection Based on Corrosion Intensity

For mild corrosion or general anti‑rust requirements, pre‑hardened stainless steels such as S136H or 1.2316 Pre‑hard are suitable. They offer good corrosion resistance, fair polishability, and ease of machining. These grades do not require quenching and are cost‑effective for medium‑volume production.

For moderate corrosion applications, such as standard PVC, POM, and general flame‑retardant materials, quenched S136, 1.2083, or STAVAX are widely used. These are martensitic stainless steels with high purity, excellent corrosion resistance, and the ability to achieve high mirror finishes. After heat treatment to HRC 48–52, they provide good durability and surface quality.

For severe corrosion environments, including highly concentrated PVC, strong flame retardants, and high halogen content, high‑purity stainless steels such as quenched 1.2316 or super‑clean S136 are recommended. These materials have high chromium content, low inclusions, and strong resistance to pitting and acid corrosion. They maintain stability even under long‑term exposure to corrosive gases.

Combined Corrosion and Wear Requirements

When materials are both corrosive and abrasive, such as fiberglass‑reinforced flame‑retardant nylon, the mold must resist both chemical attack and mechanical wear. In such cases, high‑hardness corrosion‑resistant stainless steels are used. These grades are designed to balance chromium content for corrosion resistance and sufficient carbon and alloys for hardness and wear resistance. Typical hardness ranges from HRC 50–54. These materials are more expensive but necessary for dual harsh conditions.

Key Selection Indicators

The most important factors are corrosion resistance, polishing performance, hardness, wear resistance, dimensional stability, and cost. Corrosion resistance mainly depends on chromium content and material purity. Steels with high Cr and low sulfur and inclusions provide better protection. Polishing performance is critical for appearance parts and requires uniform, clean microstructure. Hardness must match production volume and material abrasiveness. Dimensional stability is especially important for precision molds, as stainless steels can be sensitive to heat treatment. Cost must be balanced with performance to avoid over‑specification.

Practical On‑Site Selection Procedure

First, identify the corrosiveness of the plastic material. Second, confirm appearance requirements, including transparency and surface finish. Third, evaluate production volume and whether fiberglass is added. Fourth, consider environmental humidity and storage conditions. Finally, select the appropriate grade: pre‑hardened stainless for mild corrosion, quenched stainless for moderate corrosion, super‑pure stainless for severe corrosion, and high‑hardness grades for combined corrosion and wear.

Common Mistakes to Avoid

Using standard P20 or 718H for corrosive materials will lead to rapid rust and pitting. Low‑purity stainless steels may fail to achieve good mirror finishes or show inconsistent corrosion resistance. Focusing only on hardness while ignoring purity will still result in corrosion. Improper heat treatment significantly reduces both corrosion resistance and service life. Regular maintenance, including cleaning and anti‑rust treatment, is also necessary to maximize mold life.

Summary

Corrosion‑resistant mold steel selection is based on material corrosiveness, appearance, production volume, and environment. S136H is suitable for mild corrosion, S136 for general corrosive and mirror applications, high‑purity 1.2316 for severe corrosion, and high‑hardness grades for combined wear and corrosion. Correct selection prevents mold damage, ensures stable product quality, and reduces long‑term production costs.