Scientific Guide to Mold Steel Selection for Polyethylene (PE) Injection Molding

Polyethylene (PE) is one of the most widely used resins in injection molding. Its low viscosity and non-corrosive nature result in minimal wear on mold steels, but its high coefficient of thermal expansion and tendency to stick to mold surfaces mean that mold steel selection must balance toughness, polishability, and cost-effectiveness. The choice of steel directly impacts mold life, part quality, and production efficiency, and should be based on part type, production volume, and precision requirements.

I. Core Principles for Mold Steel Selection for PE

The key to selecting mold steel for PE is “performance matching + cost optimization.” For standard PE parts, pre-hardened steels are preferred due to their ease of machining and lower cost. For high-volume or precision parts, steels with good thermal fatigue resistance and dimensional stability are needed to prevent cavity deformation during long-term production. For reinforced PE grades containing abrasive fillers, wear resistance becomes the primary concern. Food-contact applications require corrosion-resistant steels that meet relevant hygiene standards. All selections should be based on well-established material properties to ensure technical feasibility.

II. Scenario-Based Mold Steel Selection for PE

Industry practice has established clear guidelines for mold steel selection based on different PE applications:

Standard Consumer GoodsFor medium-to-low volume production of everyday items such as LDPE bottle caps and HDPE storage containers, the standard choice is pre-hardened mold steel P20. It offers excellent machinability and sufficient performance for PE’s low-wear characteristics at a competitive cost. For parts requiring better surface finish, such as thin-walled HDPE containers, the improved P20 variant 718 is commonly used for its superior polishability.

High-Volume and Precision PartsFor high-volume, precision components like HDPE crates and industrial pipe fittings, hot work die steel H13 (or its equivalent SKD61) is the preferred option. Its excellent heat resistance allows it to withstand the thermal cycling of high-output production. For parts requiring a high-gloss finish, such as food containers, mirror-polish stainless steel S136 is ideal. It provides exceptional corrosion resistance and dimensional stability, ensuring consistent part quality.

Reinforced PE ApplicationsGlass fiber reinforced PE and other filled grades are highly abrasive, necessitating the use of high-hardness wear-resistant steels. Cr12MoV and powder metallurgy steel ASP23 are industry standards for such applications. Their high hardness and superior wear resistance effectively counteract the erosive effects of fillers, significantly extending mold life.

Non-Cavity ComponentsFor structural elements like guide pillars, bushings, and mold bases that do not form the part geometry, high-cost specialty steels are unnecessary. Carbon steels such as 45# or T8 are commonly used. When properly heat-treated, they provide adequate structural strength at a fraction of the cost of mold-specific steels, helping to reduce overall tooling expenses.

III. Key ConsiderationsWhen selecting and using mold steels for PE, three critical factors must be addressed:

Surface TreatmentDue to PE’s toughness and tendency to stick, all cavity surfaces must be polished to a high standard. Reducing surface roughness minimizes ejection resistance and prevents surface defects on the finished part.

Material ComplianceFor food-contact products, cavity materials must be corrosion-resistant and meet strict safety standards, such as S136, to prevent contamination and ensure regulatory compliance.

Heat TreatmentWear-resistant steels like Cr12MoV should undergo vacuum heat treatment to minimize thermal stress and prevent cracking. Conversely, pre-hardened steels such as P20 and 718 should not be re-heat-treated, as this can degrade their hardness and toughness.

IV. Summary

Mold steel selection for PE requires aligning material properties with specific application needs: P20/718 for balancing cost and quality in standard parts, H13/S136 for durability and precision in high-volume production, Cr12MoV/ASP23 for wear resistance in filled grades, and 45# steel for cost-effective structural components. Adhering to these industry principles ensures optimal mold longevity, part quality, and production efficiency.