Polyphenylene sulfide (PPS) is a high-performance engineering plastic with exceptional thermal stability, chemical resistance, and mechanical strength, making it a material of choice for automotive, electronic, and aerospace applications. However, its demanding molding conditions—including high processing temperatures (300–330°C) and the common use of glass fiber reinforcement (up to 50%)—pose significant challenges to mold materials. The selection of mold steel for PPS must prioritize high temperature resistance, wear resistance, corrosion resistance, and dimensional stability.
1. Core Requirements for PPS Mold Steels
PPS molding at high temperatures releases acidic gases that can corrode standard mold steels. Additionally, glass fiber reinforcement drastically increases the abrasive wear on the cavity and core surfaces. Therefore, the ideal steel must possess:
High temperature strength: To resist softening and thermal fatigue under continuous high heat.
Superior wear resistance: To withstand the erosive action of glass fiber-filled melts.
Excellent corrosion resistance: To prevent surface degradation from acidic decomposition products.
Dimensional stability: To maintain tight tolerances despite thermal cycling.
2. Primary Steel Grades for PPS Molds
1. Preferred Choice: S136 (1.4313) Martensitic Stainless Steel
S136 is the premium choice for PPS molds, especially for high-precision and corrosion-resistant applications. This high-purity, martensitic stainless steel, when heat-treated to HRC 48–54, offers an exceptional balance of corrosion resistance and wear resistance. Its high chromium content effectively resists the acidic gases generated during PPS processing, preventing rust and surface degradation. Furthermore, its excellent polishability allows for mirror finishes (Ra ≤ 0.2μm), making it ideal for high-gloss electronic components and medical devices. It is the go-to material for medium to high-volume production (500,000+ cycles) of high-quality PPS parts.
2. Cost-Effective Workhorse: H13 (1.2344 / SKD61) Hot Work Tool Steel
H13 is the most widely used and cost-effective option for PPS molds. As a classic hot work steel, it exhibits excellent high-temperature strength, toughness, and thermal fatigue resistance. When heat-treated to HRC 48–52, it can withstand the rigors of continuous high-temperature molding. For standard glass fiber-reinforced PPS (20–30% GF), its wear resistance is generally sufficient. However, to enhance its corrosion resistance and extend service life, surface treatments like gas nitriding (HV 800–1000) or PVD coatings (e.g., CrN) are highly recommended. H13 is the industry standard for large-volume production of automotive structural components and connectors.
3. High-Wear Alternative: DC53 (Modified SKD11) Cold Work Tool Steel
For highly filled PPS materials (40–50% GF), the abrasive wear is extreme, and DC53 is the preferred choice. This high-performance cold work steel, when hardened to HRC 58–62, offers significantly superior wear resistance compared to H13, effectively resisting the constant scouring of glass fibers. Its high chromium content also provides a degree of corrosion resistance. However, due to its lower toughness, strict control of the heat treatment process (vacuum quenching, deep cryogenic treatment, and multiple tempering) is essential to ensure dimensional stability and prevent cracking. DC53 is ideal for high-load, high-precision molds requiring exceptional longevity.
4. Low-Volume/General Purpose: 718H / NAK80 Pre-Hardened Steels
For low-volume production (≤500,000 cycles) of unfilled, general-purpose PPS parts, pre-hardened steels like 718H or NAK80 can be a viable option. These steels are supplied in a pre-hardened condition (HRC 30–35), eliminating the need for post-machining heat treatment, thus reducing lead times and costs. However, their lower hardness and corrosion resistance make them unsuitable for glass fiber-reinforced PPS or long-term production. They are best suited for prototype molds and non-critical internal components.
Conclusion
The selection of steel for PPS molds is a critical decision that directly impacts part quality, mold life, and production efficiency. By matching the steel grade to the specific PPS formulation, production volume, and performance requirements, manufacturers can optimize their molding operations. For high-end applications, S136 is unparalleled; for general production, H13 with surface treatment offers the best balance; for extreme wear, DC53 is the solution; and for low-volume needs, pre-hardened steels provide a cost-effective entry point.
