In-depth Analysis of High-Toughness PE Injection Molding Die Materials

1. Core Requirements of PE Properties for Die Materials

As one of the most widely used thermoplastics, polyethylene (PE) imposes strict toughness demands on injection molding die materials. Performance differences between PE types directly dictate material selection logic: high-density polyethylene (PE-HD) has a density of 0.94-0.97 g/cm³ and tensile strength of 25-30 MPa, offering good rigidity but limited impact toughness; ultra-high molecular weight polyethylene (PE-UHMW) has a molecular weight exceeding 3.5×10⁶ g/mol, with elongation at break of 300%-500% and notch impact strength over 150 kJ/m², generating significant impact loads on die cavities during molding.

Common challenges in PE injection molding further increase reliance on high-toughness materials: its thermal expansion coefficient (10×10⁻⁵/°C-20×10⁻⁵/°C) causes thermal stress-induced microcracks in dies via thermal cycles; shear forces during melt flow and pressure fluctuations in the packing stage repeatedly impact cavity surfaces. Data from an injection molding enterprise shows that premature cracking due to insufficient toughness accounts for 62% of die failures in PE-UHMW molding, directly undermining production stability.

2. Performance and Application Scenarios of Mainstream High-Toughness Die Materials

(1) Compact Pre-hardened Die Steels

Represented by M2833D and M238, compact steels are primary choices for PE molding. After vacuum heat treatment, they achieve 30-35 HRC hardness while maintaining 45-55 J/cm² impact toughness, effectively resisting cyclic impacts from PE melts. Optimized alloy compositions control grain size above grade 10, reducing impurity-induced stress concentration—performing exceptionally in thin-wall PE-HD molding, with die life 3x longer than conventional 45# steel.

M238 offers corrosion resistance for general PE applications, showing no obvious rust after 3,000 consecutive cycles in 80°C humid environments. A home appliance manufacturer used M238 for washing machine PE inner tub dies, resolving parting line cracking from low-toughness materials and achieving over 80,000 cycles per die.

(2) Corrosion-Resistant High-Toughness Die Steels

Precipitation-hardening stainless steel S136H excels in PE molding, especially for modified PE. Quenched at 1020°C and tempered at 480°C, it reaches 40-45 HRC with 38 J/cm² impact toughness and superior chemical resistance, exhibiting a corrosion rate of only 0.002 mm/year in acid immersion tests simulating PE processing conditions.

In food-grade PE-UHMW production, S136H’s polishability achieves Ra0.02μm cavity roughness; combined with high toughness, it prevents cavity damage from product sticking during demolding. A medical device factory used S136H for PE-UHMW joint prosthesis dies, achieving 120,000 crack-free cycles and reducing product brittleness from 1.8% to 0.3%.

(3) Powder Metallurgy High-Speed Steels

For high-filled PE (e.g., glass fiber-reinforced PE), powder metallurgy high-speed steels like ASP-60 stand out. Their uniform composition via powder metallurgy delivers 60-65 HRC hardness and over 25 J/cm² impact toughness, with 5-8x better wear resistance than conventional steels. In 30% glass fiber-reinforced PE-HD molding, ASP-60 dies last 2.7x longer than S136H without cavity chipping.

3. Material Performance Optimization and Process Adaptation

(1) Heat Treatment Control

Vacuum heat treatment is critical for die steel toughness. M2833D undergoes quenching (860°C, 2h, oil-cooled) and triple tempering (550°C), enabling uniform carbide precipitation and avoiding toughness loss from conventional heat treatment. Tests show vacuum-treated M2833D has 40% higher impact toughness, with hardness variation within ±2 HRC.

For S136H, stepwise solution treatment (850°C preheating + 1050°C solution) reduces thermal stress; precise aging (4-6h) balances hardness and toughness. A die factory’s tests demonstrated 75% lower cracking in PE-UHMW molding with optimized heat-treated S136H.

(2) Surface Strengthening

For high-impact scenarios, gas nitriding forms a 5-10μm nitride layer (700-800 HV surface hardness) while preserving core toughness, creating a "hard shell-tough core" structure. In large PE-UHMW molding, nitrided M238 dies double cavity impact wear resistance.

For corrosive modified PE, PVD TiN coatings (2-3μm thick) achieve 1800-2000 HV surface hardness without compromising toughness. A chemical enterprise reported TiN-coated S136H dies lasting 3.2x longer than untreated ones in flame-retardant PE molding.

4. Material Selection and Quality Control

(1) Scientific Selection Methodology

Selection requires a 3D matching model: "PE type-product characteristics-molding conditions". M238 is optimal for general PE-HD (best cost-performance); S136H (≥35 J/cm² impact toughness) for high-impact PE-UHMW; powder metallurgy steels like ASP-60 for glass fiber-reinforced PE (meeting ASTM D4060: volume wear ≤0.5 mm³).

Key performance verification is mandatory: hardness 30-45 HRC (pre-hardened) or 55-65 HRC (quenched-tempered); Charpy V-notch impact toughness ≥20 J/cm²; non-metallic inclusions ≤ grade 2.0 (GB/T 10561).

(2) Quality Inspection and Maintenance

Ultrasonic flaw detection ensures no ≥φ0.5mm pores/cracks before use; regular inspections (every 2,000 cycles) check for microcracks via magnetic particle inspection (detecting ≥0.1mm early cracks).

Routine maintenance controls die temperature below PE’s heat deflection temperature (PE-HD ≤80°C, PE-UHMW ≤90°C) to avoid toughness degradation from thermal shock. An injection molding enterprise established a "material performance-process parameters-maintenance cycle" system, extending average high-toughness die life from 50,000 to 150,000 cycles.

5. Conclusion

Selecting and applying high-toughness PE injection molding die materials is essentially a system engineering balancing hardness, toughness, and corrosion resistance. Mainstream materials like M238 and S136H have clear adaptation standards; vacuum heat treatment and surface strengthening further unlock performance potential. Future R&D will focus on new die steels with higher toughness and temperature resistance as PE modification advances, while precise selection and strict quality control remain core to resolving PE product brittleness.