Mold Steel Selection for PET Injection Molding

PET (Polyethylene Terephthalate) is widely used in packaging, transparent parts and precision components due to its excellent transparency, mechanical strength and chemical stability. However, PET injection molding has high requirements: high molding temperature, fast crystallization rate, and sensitivity to mold performance. Choosing the right mold steel is crucial to ensuring product quality, mold life and production stability. This article details mold steel selection principles, recommended grades and practical suggestions for PET injection molding.

PET injection molding characteristics determine mold steel requirements: high temperature resistance (melt temperature 260~290℃), wear resistance (to resist friction from plastic flow), high polishability (for transparent products), corrosion resistance (to resist moisture and chemical reactions), and good dimensional stability (to avoid deformation under high temperature and pressure). Improper selection leads to mold deformation, wear, rust, short life and poor product appearance.

Mold Steel Selection Principles

First, prioritize wear resistance and high temperature resistance. PET’s high molding temperature and fast flow cause severe mold cavity wear—choose steels with high hardness and wear resistance. Second, ensure high polishability: transparent PET products require a mirror mold surface, so select steels with high purity, fine grain and no impurities. Third, consider corrosion resistance: PET contains trace moisture, which may cause mold rust—choose corrosion-resistant steels for long-term production.

Fourth, focus on dimensional stability: high temperature and pressure during injection molding easily cause mold deformation—select steels with small heat treatment deformation and good thermal stability. Finally, balance cost and performance: choose grades based on product batch, precision and appearance requirements.

Recommended Mold Steel Grades and Applications

For Small-Batch Common PET Parts (Economical Choice)

P20/718H pre-hardened mold steel: pre-hardened to HRC30–36, good machinability and weldability, low cost. Suitable for non-transparent, small-batch PET parts (such as ordinary shells and structural parts) with general wear and corrosion requirements, and dry production environments.

For Medium-Batch Precision Transparent PET Parts

NAK80 pre-hardened mirror steel: pre-hardened to HRC38–42, no heat treatment needed, excellent polishability (achieving high-gloss mirror surface), good dimensional stability and toughness. Suitable for medium-batch, medium-precision transparent PET parts (such as PET bottles and transparent boxes), balancing performance and cost—it is the most commonly used grade for precision PET molds.

For High-Transparency, High-Life PET Parts

S136/420ESR martensitic stainless steel: quenched and tempered to HRC48–52, high purity, strong corrosion resistance and excellent mirror polishability. Suitable for high-transparency PET parts (such as optical components and food-grade packaging), long-term production and humid environments. It resists rust and deformation, ensuring long mold life.

STAVAX/S136MOD improved stainless steel: better toughness and dimensional stability than S136, suitable for long-term high-temperature PET molding (such as PET preforms and precision transparent parts), with outstanding anti-collapse and anti-rust performance.

For Large-Batch Glass Fiber Reinforced PET Parts

H13 (SKD61) hot work mold steel: quenched and tempered to HRC48–52, excellent high-temperature strength, thermal fatigue resistance, toughness and wear resistance. Poor corrosion resistance, but can be improved by chrome plating or titanium plating for dry environments. Suitable for large-batch, glass fiber reinforced PET parts (such as PET structural components), with strong pressure and deformation resistance.

Conclusion

Mold steel selection for PET injection molding depends on product requirements, batch and production environment. Choose P20/718H for small-batch common parts, NAK80 for medium-batch precision transparent parts, S136/420ESR for high-transparency and long-life parts, and H13 for large-batch reinforced parts. Scientific selection ensures stable product quality, long mold life and efficient production, reducing costs and enhancing market competitiveness.