Guide to Mold Steel Selection for Low-Temperature Injection Molding

Low-temperature injection molding, with mold temperatures controlled at room temperature to 40℃ (even 0℃～20℃ for cooling-sensitive materials), is widely used in precision engineering plastics, transparent optical parts and low-stress structural parts. It offers advantages such as low internal product stress, high dimensional stability and suitability for heat-sensitive materials. However, low-temperature molds work in long-term low-temperature, alternating hot-cold and high-humidity environments, requiring special mold steel properties. Improper selection causes deformation, cracking, rust and shortened life. This article details selection logic, recommended grades and precautions.

Key Working Conditions of Low-Temperature Injection Molds

Low-temperature molds operate in a long-term low-temperature environment, with alternating hot and cold impacts from molten plastic. This requires steel with good thermal fatigue stability to avoid deformation and cracking from repeated expansion and contraction. Moisture in the air easily condenses on the mold surface, leading to rust—corrosion resistance is critical, especially in humid workshops and during long shutdowns.

Low-temperature injection is mostly used for precision and optical parts, requiring high surface finish and dimensional accuracy. Steel must have excellent polishability, machinability and dimensional stability to ensure no deformation or product scratching during long-term production. Low temperatures affect steel hardness and toughness: brittle steel easily cracks at stress concentration points; low-hardness steel wears, collapses and forms flash. A balance between hardness and toughness is essential, with a recommended HRC range of 30～52.

Core Principles of Mold Steel Selection

Prioritize pre-hardened or quenched stainless steel—ordinary P20 steel rusts easily in low-temperature condensing environments, affecting product appearance. Ensure excellent dimensional stability and small heat treatment deformation to avoid parting surface gaps, insert displacement and product size drift.

Balance hardness and toughness: avoid excessive hardness in low temperatures; ensure sufficient toughness to prevent cracking. Match steel performance to product type: high-polish, high-stability steel for precision/optical parts; high-wear, high-compression steel for large-batch, glass fiber reinforced parts; cost-effective pre-hardened steel for stable mass production.

Recommended Mold Steel Grades and Applications

For Small-to-Medium Batch, Ordinary Precision Parts (Economical)

718H/718S pre-hardened steel (HRC30–36): good machinability and weldability, suitable for dry, low-temperature environments and non-transparent shell/structural parts—low cost and easy processing.

NAK80 pre-hardened mirror steel (HRC38–42): no heat treatment needed, excellent polishability and dimensional stability, moderate toughness. Suitable for medium-precision, medium-batch parts with basic rust resistance—ideal for PC, PMMA and PET low-temperature molds, a cost-effective choice for precision parts.

For Large-Batch, High-Precision, High-Rust Resistance (Stainless Steel)

S136/420ESR martensitic stainless steel (HRC48–52): high purity, strong corrosion resistance and excellent mirror polishability. Suitable for long-term low-temperature, high-humidity environments, transparent optical parts, food-grade and medical products—good thermal stability, no rust or deformation, long life.

STAVAX/S136MOD improved stainless steel: better toughness and dimensional stability than S136, suitable for long-term alternating hot-cold low-temperature molds—excellent for PET preforms, high-transparent boxes and precision gears, with outstanding anti-collapse and anti-rust performance.

For Large-Batch, Glass Fiber Reinforced, High-Wear Parts

H13 (SKD61) hot work steel (HRC48–52): excellent high-temperature strength, thermal fatigue resistance, toughness and wear resistance. Poor rust resistance, but can be improved by chrome/titanium plating—suitable for large-batch, glass fiber reinforced parts, with strong pressure and deformation resistance.

DC53 cold work steel: high hardness, toughness and wear resistance, small quenching deformation. Suitable for high-wear, large-batch, stable low-temperature precision molds—note workshop humidity and 做好 rust prevention.

Conclusion

Low-temperature mold steel selection focuses on low-temperature deformation resistance, rust resistance, dimensional stability and hardness-toughness balance. Choose 718H for small-batch common parts, NAK80 for medium-batch precision parts, S136/420ESR for high-transparency and long life, and H13/DC53 for glass fiber reinforced, large-batch parts. Comprehensive selection based on working conditions, product material and batch ensures long mold life, stable products and low defect rates, achieving efficient low-temperature injection molding.