How to Select Mold Steels for LDPE

LDPE (Low-Density Polyethylene) is one of the most widely used general-purpose polymers, favored for its excellent flowability, toughness, chemical resistance, and low cost. It is commonly used in packaging, containers, caps, toys, films, and household products. LDPE has low viscosity, no corrosive behavior, and no abrasive fillers such as glass fiber. Therefore, mold steel selection for LDPE focuses on machinability, polishability, dimensional stability, cost efficiency, and appropriate service life, rather than extreme hardness or high corrosion resistance.

Key Requirements of LDPE for Mold Steels

LDPE’s material and processing characteristics define four core selection principles.

First, low corrosion requirement. LDPE itself is non-corrosive and does not release acidic or corrosive gases during molding. Therefore, expensive high-corrosion-resistant stainless steels are generally unnecessary unless used for food-grade or humid environments.

Second, low abrasion requirement. LDPE melts smoothly and contains no hard fillers. It causes minimal wear to mold cavities. High-hardness, high-wear-resistant steels like H13 or Cr12MoV are not required and will only increase cost and machining difficulty.

Third, good machinability and polishability. Most LDPE products are daily commodities with moderate to good surface requirements. Mold steels must be readily machined, textured, or polished to achieve a smooth or semi-high-gloss finish.

Fourth, dimensional stability. LDPE has relatively high shrinkage (1.5%–3.0%). Mold steels must maintain stable dimensions under long-term heating and cycling to ensure consistent part size and precision.

Steel Selection Based on Production Volume and Application Scenarios

Small-Batch Production, Prototype Molds

For trials, samples, or volumes under 50,000 shots, cost and speed are priorities.Recommended steels: 45# carbon steel, standard P20.45# steel is low-cost and easy to machine, suitable for very simple structures. P20 offers better stability and life. Both eliminate the need for heat treatment and support fast delivery.

Medium-Volume Production (Most Common for LDPE)

For 50,000–500,000 shots, such as daily containers, caps, and toys.Recommended: P20 / 718S, hardness HRC 28–32.These are pre-hardened mold steels with uniform structure, good polishability, and stable performance. They can achieve Ra 0.4–0.8 μm, ideal for standard LDPE parts. They offer the best balance of performance and cost and are regarded as the standard choice for LDPE molds.

High-Volume, High-Life, High-Gloss Parts

For 500,000–1,000,000+ shots or high-gloss products.Recommended: 718H, NAK80, hardness HRC 35–40.718H provides higher hardness and wear resistance for extended mold life. NAK80 offers superior polishability up to mirror finish (Ra ≤ 0.2 μm) without plating. Both ensure long-term stability and high-quality surfaces.

Food-Grade, Humid, or Corrosive Environments

For food-contact products or high-humidity workshops.Recommended: S136 stainless steel, HRC 40–45.S136 provides excellent corrosion resistance and will not rust or contaminate food-grade LDPE parts. It offers outstanding polishability and is widely used in medical and food packaging applications.

Mold Base and Accessory Steels

Mold base: S50C, HRC 25–30, providing sufficient strength and rigidity.Ejector pins, guide pins, bushings: SUJ2 bearing steel, HRC 60–65, for high wear resistance.Inserts: use P20 or 718H for cosmetic surfaces; standard P20 for structural areas.Cooling components: copper alloys for improved thermal conductivity.

Practical Guidelines and Common Mistakes

Avoid over-specification. Do not use expensive wear-resistant or hot-work steels for LDPE. They are unnecessary and increase cost.

Avoid inferior steels. Low-quality steels cause deformation, poor polishing, and short mold life. Always use certified grades.

Match surface requirements. Use P20/718S for standard surfaces; NAK80 or S136 for high-gloss or food-contact parts.

Match production life. Small batches use 45# or P20; high-volume production uses 718H or NAK80 for lower long-term cost.

Conclusion

For LDPE injection molds, the optimal steel selection follows the logic of material adaptation, production matching, and cost optimization. LDPE’s low-corrosion, low-abrasion nature eliminates the need for premium high-performance steels. For most medium-volume applications, P20 / 718S remains the most practical and cost-effective choice. For high-volume or high-gloss products, 718H and NAK80 are preferred. For food or humid environments, S136 ensures hygiene and rust prevention.

Scientific steel selection improves mold stability, surface quality, and service life while reducing overall manufacturing and maintenance costs, supporting efficient and reliable LDPE production.