Matching Standard of Wear-Resistant Mold Steel for POM Plastic
Polyoxymethylene (POM) is a high-crystallinity engineering plastic widely applied in gears, sliding components, precision fasteners and automotive functional parts. Formaldehyde and formic acid generated by thermal decomposition during molding continuously corrode mold surfaces. Meanwhile, POM components often operate under mutual friction, imposing strict requirements on surface finish, dimensional stability and wear resistance of mold cavities. Improper steel selection will lead to pitting corrosion, rapid surface abrasion, dimensional drift and demolding scratches. Steel matching should distinguish unfilled POM and glass-fiber-reinforced POM, considering production volume, appearance grade and ambient humidity. A standardized selection system covering cavity, core, inserts and moving components is required.
1. Basic Steel Performance Requirements for POM Molding Conditions
Unfilled POM and fiber-reinforced POM impose different demands on mold steel. Non-reinforced POM focuses on corrosion resistance, polishing capacity and surface wear resistance; glass-filled POM requires extra anti-abrasion performance against particle erosion. Hardness, purity and uniform microstructure determine long-term production stability. Copper and brass materials must be prevented from contacting molten POM, as copper ions accelerate thermal degradation and result in cracking and silver streaks on finished products. Basic specifications are defined clearly: materials should resist long-term erosion of weak acid; mirror-grade components need surface roughness Ra ≤ 0.2 μm. Base hardness is classified by shot quantity: pre-hardened steel HRC32~38 for low-volume production, quenched steel HRC48~54 for mass production. Angle lifters, sliders and other moving inserts require sufficient toughness to avoid corner breakage and fatigue cracks under reciprocating movement.
2. Graded Steel Matching Specification for Unfilled Pure POM
Material selection is divided into three grades according to production shots, appearance requirements and workshop environment. For trial production within 500,000 shots with ordinary texture non-appearance parts under dry conditions, 718H pre-hardened steel (HRC33~38) can be adopted. Passivation treatment should be carried out on molding surfaces, and regular polishing maintenance is necessary. This material is not suitable for continuous long-term production in humid workshops. For medium-to-long term mass production with precision or mirror appearance parts ranging from 500,000 to 1,200,000 shots, S136H pre-hardened stainless steel is preferred. It integrates corrosion resistance and polishing performance without integral post-heat treatment, effectively reducing pitting defects. For large-volume automatic production over 1,200,000 shots including high-precision gears and mirror parts, quenched and tempered S136 or STAVAX steel (HRC48~52) is recommended. Uniform microstructure improves anti-corrosion and wear capacity, supporting A1 mirror polishing and lowering frequent maintenance frequency. Under budget limits, NAK80 mirror pre-hardened steel can be used only for low-volume pure POM production and forbidden in humid environments.
3. Graded Steel Matching Specification for Glass-Fiber-Reinforced POM
Plastics filled with glass fiber or mineral additives bring severe abrasive erosion. Reliance only on stainless steel matrix cannot satisfy wear requirements. Material selection must combine corrosion resistance and anti-scouring performance. For components with glass fiber content below 15% and medium output, quenched S136 combined with PVD TiN or CrN coating can resist continuous fiber abrasion. For parts with 15%~30% glass fiber under long-term production, SKD61/H13 with hard coating is adopted, and high-wear zones can be designed as independent inserts. For materials with fiber content exceeding 30% under high-speed injection, powder metallurgy mold steel greatly extends service life. Ordinary pre-hardened alloy steel is prohibited for long-term production of fiber-filled POM, as cavities will quickly form erosion grooves leading to flash and dimensional out-of-tolerance. Coating temperature must be controlled to avoid mold deformation and dimensional error of precision products.
4. Differential Steel Matching Rules for Different Mold Components
A single steel grade cannot be adopted for the whole set of mold. Material selection varies according to contact status with melt. Cavities and cores directly contacting molten POM follow the selection rules for pure or reinforced POM. For angle lifters, sliders and side core pulling mechanisms, SUS440C stainless steel is preferred for friction pairs to prevent rust and jamming. Stainless steel ejector pins are used for pure POM, while high-hardness alloy ejector pins with coating are selected for fiber-reinforced materials. Mold base, clamping plates and back plates without melt contact can adopt normalized S50C or 45# steel to control manufacturing cost. Parting surfaces and vent grooves are high-risk corrosion areas. Stainless steel inserts can be embedded if base steel lacks sufficient corrosion resistance to avoid initial rust and difficult carbon deposit cleaning.
5. Matching Specification of Auxiliary Surface Treatment Processes
Surface treatment serves as supplementary anti-corrosion and anti-wear measures after base steel confirmation. S136 stainless steel generally requires no chrome plating, as thick electroplating easily causes coating peeling. Low-temperature gas nitriding with 0.03~0.08mm penetration depth can improve surface wear resistance for long-term pure POM molding. PVD coating with thin thickness and minimal deformation is prioritized for glass-filled materials to guarantee dimensional accuracy of precision components. When 718H pre-hardened steel is used for POM molding, passivation or chrome plating is compulsory, yet only applicable for short-run production due to limited coating durability. Thick nitriding is prohibited for mirror cavities to prevent cloudy surface defects. Polishing inspection is required after all surface treatments to eliminate micro-defects where corrosive medium accumulates and generates pitting.
6. Steel Selection Taboos and Production Adaptation Requirements
A negative checklist should be established to avoid common wrong selections. Ordinary P20 pre-hardened steel cannot be used for continuous POM molding; massive rust appears within tens of thousands of shots in humid environments. Mixing different material inserts should be avoided because galvanic corrosion accelerates metal erosion. Steel corrosion grade cannot be reduced for molding conditions with high temperature and poor venting. Production management extends mold service life: material residence time in the barrel should be controlled, vent layout reduces accumulation of acidic gas. Cavities need thorough cleaning and anti-rust spray after shutdown. Cooling channels should be sealed to avoid long-term condensate infiltration on parting surfaces and molding zones.
Conclusion
The matching standard of mold steel for wear-resistant POM components takes raw material type, production shots, workshop environment and precision grade as core judging conditions. Two application scenarios including pure POM and glass-fiber-reinforced POM are distinguished, while differential material selection is implemented for cavity cores, moving mechanisms and ejection parts. The overall selection principle follows priority of corrosion resistance, wear reinforcement, graded selection and matched auxiliary processes. Martensitic stainless steel is preferred in humid mass-production environments, and hard surface coatings are added for filled materials. Material selection should not merely pursue low initial cost. Full-life maintenance cost must be comprehensively evaluated to reduce cavity corrosion, erosion scratches and appearance defects, stabilize dimensional consistency of precision POM products and prolong mold service cycle.
