Which Injection Mold Steel Should You Use for Each Plastic Material? A Complete Matching Standard

1. Non-Corrosive Plastics (ABS, PS, HIPS, PP, PE, TPE, TPU)

ABS, PP, PE, PS, and HIPS are neutral plastics — their melts do not release acidic compounds, so corrosion resistance is not the deciding factor. Selection is driven by production volume and surface appearance class.

• Under 100,000 shots — small-batch matte housings and toys: P20 pre-hardened steel. No heat treatment, fast machining, lowest cost.
• 100,000–500,000 shots with textured (grain) appearance: 718H. Denser material, resists whitening on textured surfaces, less cavity galling.
• High-gloss mirror finish: NAK80. Polishable directly to mirror finish in the pre-hardened state, low distortion.

• Trial-run / temporary molds: 45# carbon steel or S50C.
• Standard matte production: P20 as default.
• High-gloss modified PP: 718H (best cost-performance).

• All production volumes: 718H or NAK80 — dense steel resists sticking and deposit buildup; stainless steel not required.

2. Corrosive Plastics (PVC, POM, Flame-Retardant Grades)

PVC, POM, and flame-retardant modified plastics decompose at high melt temperatures and release acidic gases. With standard P20 or 718H, cavities rust and pit within a few thousand shots — stainless mold steel is mandatory.

• PVC (rigid & flexible), any volume: cavity inserts in S136 stainless steel, hardened to 48–52 HRC. Rust-proof, acid-resistant, maintains mirror finish over long production, prevents pitting and haze.
• POM: small trial runs may temporarily use 718H; long-term production must switch to S136 to prevent cavity corrosion that enlarges clearances and causes continuous flash (burr).
• Flame-retardant ABS / flame-retardant nylon: contain halogenated flame retardants (corrosive). Volume appearance molds must use S136 to prevent premature rust and mold scrapping.

3. Glass-Fiber Reinforced Plastics (PA, PC, PBT)

Glass-fiber filled PA, PC, and PBT contain hard fibers that scour cavity surfaces at high injection speeds. High-hardness, wear-resistant grades are required.

• GF PA6 / PA66 (10%–30% glass content): small-to-medium batches → STAVAX; high-volume precision connectors → SKD61 / H13 hot-work tool steels (high hardness, erosion-resistant, stable dimensions over long runs).
• GF PC / GF PBT electronic precision parts: cavity inserts in STAVAX with nitriding treatment — higher surface hardness boosts glass-fiber abrasion resistance for long-term precision production.

4. Transparent High-Gloss Parts (PC, PMMA)

• Recommended: cavity inserts in S136 or NAK80 — high purity, uniform structure, polishable to super mirror finish; parts transmit light evenly with no haze or blemish.
• Forbidden: P20 and 718H must not be used for transparent cavities — polishing defects appear easily and cause batch scrap.

5. Quick Selection Rule by Production Batch

• Small batch / trial production (under 100,000 shots): neutral plastics → P20; corrosive & transparent plastics → NAK80 / S136.
• Medium batch (100,000–500,000 shots): standard appearance parts → 718H / NAK80; glass-fiber materials → upgrade to STAVAX.
• Large batch / long-term production (over 500,000 shots): corrosive plastics → S136; glass-fiber wear-resistant plastics → SKD61 / H13.

6. Selection Pitfalls to Avoid

• Do not blindly choose premium stainless steel for ordinary plastic housings — it only raises tooling cost without adding value.
• Do not cut costs with ordinary pre-hardened steel for corrosive or glass-fiber materials — corrosion and wear accelerate; rework, repair, and reject losses cost more than the saving.
• Core logic: no corrosion → choose by volume · corrosive → stainless steel · glass-fiber → high-hardness wear-resistant steel · transparent → high-purity mirror-grade steel.