Choosing the Right Steel for PPS Injection Molds

1. Core Requirements for PPS Injection Mold Steel

1. High temperature resistance

   PPS molding temperatures typically range from 300 to 380℃. Molds operating continuously at high temperatures require steel with excellent thermal stability to prevent softening, deformation, and ensure consistent cavity dimensions.

2. Wear resistance

   PPS parts often contain 30%–50% glass fiber or mineral fillers. The high-speed flow of filled materials causes continuous cavity erosion, necessitating steel with high hardness and superior wear resistance to minimize surface damage.

3. Corrosion resistance

   Certain flame-retardant or reinforced PPS grades may release small amounts of corrosive gases during molding. Steel must possess basic corrosion resistance to prevent cavity rust and part defects.

4. Machinability and heat treatment performance

   PPS molds frequently feature complex cavities and cores. Steel should be easy to machine and polish, with minimal deformation after heat treatment to maintain mold precision.

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2. Common Steel Types and Applications for PPS Molds

1. Hot work tool steel

   Represented by H-series steels, this category offers good high-temperature strength and thermal stability, suitable for PPS high-temperature molding conditions. With relatively low processing costs, it is ideal for medium-volume production of PPS parts with filler content ≤20%.

2. Powder metallurgy steel

   Produced via powder metallurgy technology, this steel has a uniform microstructure and high purity, reaching a hardness of HRC 58–62. Its wear and heat resistance outperform conventional hot work steel, making it suitable for high-volume production of highly filled (≥30%) PPS parts and effectively reducing cavity wear rates.

3. Corrosion-resistant steel

   Martensitic stainless steel combines corrosion resistance with moderate high-temperature performance, suitable for molding corrosive PPS grades such as flame-retardant types. However, its high-temperature hardness is slightly lower than hot work steel, making it more appropriate for low-to-medium volume production.

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3. Key Principles for Steel Selection

1. Match production volume and filler content

   For small-batch, low-filler PPS parts, prioritize cost-effective hot work tool steel. For high-batch, highly filled parts, select wear-resistant powder metallurgy steel to reduce long-term maintenance costs.

2. Align with part precision requirements

   For high-precision PPS parts with tolerances ≤0.02mm, use powder metallurgy steel with a uniform microstructure to avoid precision loss caused by heat treatment deformation in conventional steels.

3. Adapt to technological trends

   Current PPS parts are developing toward thin-wall and complex structures, increasing demands for mold steel machining precision and thermal stability. Powder metallurgy steel, with its balanced performance, is seeing a growing application share in such molds.

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4. Critical Post-Selection Considerations

1. Heat treatment optimization

   Steel requires targeted quenching and tempering. The tempering temperature for hot work tool steel should be 20–30℃ higher than the PPS molding temperature to ensure stable hardness at operating temperatures.

2. Surface strengthening treatment

   Nitriding treatment can be applied to mold cavities to form a 0.15–0.3mm nitrided layer, further enhancing surface hardness and wear resistance and extending PPS mold service life.

3. Cooling system compatibility

   High-temperature PPS molding generates significant mold heat. Steel thermal conductivity must match the cooling system to prevent local overheating, which degrades steel performance, while improving part molding efficiency.