What Steels Are Used for PPE Injection Molds

PPE (polyphenylene ether, usually blended with PS) injection mold steel selection is mainly based on high-temperature stability of melt at 280–330℃, whether it contains glass fiber reinforcement, product appearance/accuracy requirements and production volume. The following are clear selections and process points by scenario, directly suitable for mold production and technical communication.

Basic General Grade (Unfilled PPE, Medium & Low Batch, Ordinary Appearance)

718H (1.2738, 3Cr2NiMo)

As an improved pre-hardened steel upgraded from P20, it has a factory hardness of HRC35–40 and can be machined without heat treatment, with excellent dimensional stability. It is suitable for medium and large molds and multi-cavity molds. Its polishability is better than P20, meeting the surface requirements of ordinary PPE structural parts, suitable for 500,000 to 1 million molding cycles, making it the cost-effective first choice for unfilled PPE.

P20 (Pre-hardened State)

With hardness HRC28–32, it has good machinability and the lowest cost, suitable for small batches (≤500,000 molding cycles) of PPE products with low appearance requirements. The disadvantage is average softening resistance at high temperatures, prone to dimensional drift during long-term production, so it is only recommended for prototype testing or short-term mass production.

Precision High-Gloss Grade (Unfilled PPE, High Appearance/Transparent Parts, Medium & High Batch)

S136 (4Cr13, Quenched Hardness HRC48–52)

It has excellent mirror polishability (up to #A1 mirror surface) and corrosion resistance, not easy to oxidize and accumulate carbon during high-temperature PPE molding, suitable for medical, optical PPE components and high-gloss shells. After vacuum quenching, it has high dimensional stability, service life ≥1 million molding cycles, and can be polished and repaired repeatedly, making it the benchmark choice for high-end unfilled PPE molds.

NAK80 (Age-hardening Steel, HRC37–43)

It can be directly etched with fine textures in pre-hardened state, with polishability close to S136 and excellent dimensional accuracy control (tolerance ≤0.01mm). Suitable for PPE products requiring high precision and textures such as precision electronic connectors and relay housings, suitable for 800,000 to 1.2 million molding cycles, with higher processing efficiency than quenched steel.

Special Grade for Glass Fiber Reinforced PPE (High Wear Resistance, High Load, Mass Production)

H13 (4Cr5MoSiV1, Quenched Hardness HRC48–52)

A classic hot work mold steel with outstanding high-temperature strength and wear resistance, able to resist long-term scouring of the cavity by glass fiber. Suitable for PPE reinforced materials with 10%–40% glass fiber content, maintaining stable hardness above 300℃ melt temperature, service life ≥1 million molding cycles, making it the core choice for PPE molds for automotive and industrial structural parts.

8407 (Improved H13)

Higher purity, better thermal fatigue resistance than H13, suitable for high-load scenarios of long-term high-temperature cycle, such as continuously produced glass fiber reinforced PPE pipes and engine peripheral components. After nitriding treatment, surface hardness can reach above HV1000, further extending mold life.

SKD61 (Similar Performance to H13)

More balanced heat resistance, suitable for glass fiber reinforced PPE molds with high toughness requirements at high temperatures, such as complex sliders, lifters and other stressed parts, avoiding cracking under long-term high temperature.

Economical Corrosion Resistance Grade (Flame Retardant PPE, Slight Corrosion, Medium Batch)

2083 (1.2083, Pre-hardened HRC30–35)

An economical stainless mold steel with basic corrosion resistance, good processing performance, able to cope with slight corrosion caused by PPE flame retardants. Suitable for PPE products with basic corrosion resistance requirements such as food contact and outdoor electrical housings, suitable for 500,000 to 800,000 molding cycles, with lower cost than S136.

Key Process and Surface Treatment Recommendations

Heat Treatment: Hot work steels (H13/8407/SKD61) must adopt vacuum quenching + tempering to avoid oxidation and decarburization and ensure high-temperature stability; cryogenic treatment is recommended for large molds to further stabilize dimensions.

Surface Treatment: Glass fiber reinforced PPE molds prefer gas nitriding (layer depth 0.3–0.5mm) to improve wear resistance and anti-sticking performance; flame retardant PPE molds can be treated with PVD coating (TiAlN) or hard chrome plating to enhance corrosion resistance and demolding performance.

Matching Clearance: PPE has good high-temperature fluidity, mold matching clearance must be strictly controlled at 0.01–0.02mm to avoid flash, especially suitable for processing characteristics of high-precision steels such as S136 and NAK80.