What Steel is Suitable for PE Injection Molds?

I. Core Requirements of PE for Mold Steel

• Ordinary PE causes little wear, but modified PE with high-hardness fillers scours mold cavities—so modified PE molds need wear-resistant steel.

• PE has a low melting point (110–135°C); injection temp is 150–220°C, mold temp 20–60°C (70–90°C for thick/wide products). Long-term cyclic temp changes require steel with good heat resistance and dimensional stability.

• PE’s high fluidity eases cavity filling but causes adhesion (especially for thin/complex products); high-surface-quality items (food packaging, medical supplies) need steel with good polishability and compatibility with surface treatments (e.g., nitriding) to improve demoldability.

• Ordinary PE is corrosion-free, but special products (flame-retardant PE, chemical containers) release corrosive gases—such molds need corrosion-resistant steel.

• Mold manufacturing involves cutting/grinding; steel must have good machinability to reduce production difficulty and costs.

  
  

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II. Common Steel Types & Applications for PE Molds

1. Pre-hardened steel (general-purpose, cost-effective)

   Delivered with HRC 28–35 (no post-heat treatment):

• P20: Uniform hardness, good machinability; for ordinary PE products (films, pipes); 100,000–500,000 shots.

• 718H: Improved P20 (HRC 32–38), better polishability/wear resistance; for high-precision/lightly modified PE; 300,000–800,000 shots.

• S50C: Low cost, HRC 25–30 after pre-hardening; for simple PE products (low precision); <100,000 shots.

2. Mirror steel (for high-surface-quality PE)

   High-purity, low-impurity steel (Ra ≤ 0.01 μm polishability, good demoldability):

• NAK80: HRC 37–43 ex-factory, polishable to 10,000+ mesh without heat treatment; for food packaging/medical supplies; 500,000–1,000,000 shots.

• S136H: Pre-hardened (HRC 30–36), combines mirror finish and corrosion resistance; for high-surface-quality corrosive PE; 300,000–800,000 shots.

3. Wear-resistant steel (for modified PE)

   Alloy steel (Cr/Mo/V) with HRC >50 after heat treatment:

• H13: HRC 52–58, good wear/heat resistance/toughness; for glass fiber-reinforced PE; >1,000,000 shots.

• SKD11: HRC 58–62, extreme wear resistance (poor toughness); for high-filler modified PE (note impact resistance design); large batches.

• STAVAX ESR: Electroslag remelted (HRC 50–55), combines wear/corrosion resistance and polishability; for high-surface-quality modified PE; 800,000–1,500,000 shots.

4. Corrosion-resistant steel (for corrosive PE)

   High-alloy steel (Cr/Ni) with good chemical resistance:

• S136: 13% Cr, HRC 48–52 after heat treatment; for flame-retardant PE; 300,000–800,000 shots.

• 2083: Pre-hardened (HRC 28–32), good machinability (no heat treatment); for low-corrosion PE; 100,000–500,000 shots.

• HPM75: Better corrosion resistance than S136 (HRC 50–55 after heat treatment); for highly corrosive PE; 500,000–1,000,000 shots.

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III. Core Principles for Steel Selection

• Match PE type: Pre-hardened steel for ordinary PE; wear-resistant steel for modified PE; corrosion-resistant steel for flame-retardant/chemical PE; mirror steel for high-surface-quality products.

• Align with product quality: Dimensional stability steel for high-precision items; mirror steel for high-finish products; tough/machinable steel for complex/thin-walled products.

• Adapt to production volume: Low-cost pre-hardened steel for <100,000 shots; balanced pre-hardened/mirror steel for 100,000–800,000 shots; wear/heat-resistant steel for >800,000 shots.

• Balance cost & performance: Avoid over-specification (e.g., use P20 for small-batch ordinary PE instead of expensive NAK80); consider machining costs (machinable steel reduces expenses).

• Consider processing & maintenance: Machinable steel shortens cycles; factor in heat treatment costs for heat-treated steel; polishable steel eases later repair.

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IV. Key Notes for Selection

• Prioritize steel purity/structural uniformity (electroslag remelted steel for high-requirement molds).

• Match mold structure: Use tough steel for complex/uneven-walled cavities; inlay wear-resistant steel for sliders/ejector pins.

• Use surface treatments: Nitriding pre-hardened steel improves wear resistance; chrome-plating ordinary steel enhances finish/corrosion resistance (reduces cost).

• Refer to supplier support: Choose well-known brands (same-grade steel varies by manufacturer).

• Reserve performance margin: Plan for future product upgrades (e.g., higher filler content) to avoid mold obsolescence.

V. Conclusion