XPM is a high-performance pre-hardened plastic mold steel designed specifically for precision plastic molding dies. It is renowned for its excellent polishability, machinability, and dimensional stability. Supplied in a pre-hardened state, it requires no subsequent quenching or tempering, making it ready for direct mold processing. It is widely used in the production of plastic molds requiring high surface quality and complex structures.
Basic Information
Definition: XPM is a pre-hardened plastic mold steel optimized with alloying elements (primarily chromium, nickel, and molybdenum) and pre-hardening heat treatment to balance hardness and machinability. It is ideal for manufacturing large, precision plastic molds.
Key Characteristics: Supplied in pre-hardened condition (typically 30–35 HRC), good machinability, excellent polishability (capable of mirror finishes), high hardenability, dimensional stability, and moderate corrosion resistance.
Application Positioning: Replaces traditional pre-hardened steels (e.g., 718H, NAK80) for high-precision plastic molds in household appliance casings, automotive interior parts, medical devices, and optical lenses.
Chemical Composition
XPM’s alloy design focuses on polishability, hardenability, and toughness in the pre-hardened state. Typical composition ranges are as follows:
Element Content Range Role Description
Carbon (C) 0.28–0.35% Ensures strength and hardness, provides a basis for carbide formation, while avoiding excessive carbon that impairs machinability.
Chromium (Cr) 1.40–1.80% Improves hardenability and corrosion resistance, refines grain structure, and enhances polishability.
Nickel (Ni) 1.40–1.80% Enhances toughness and ductility, reduces notch sensitivity, and improves overall mechanical properties in the pre-hardened state.
Molybdenum (Mo) 0.20–0.40% Increases high-temperature strength and temper stability, refines microstructure, and reduces temper brittleness.
Manganese (Mn) 0.50–0.80% Assists in improving hardenability, ensuring uniform hardness in large-section molds.
Silicon (Si) 0.20–0.40% Acts as a deoxidizer, improves steel purity, and aids in strength enhancement.
Physical Properties
Density: ~7.85 g/cm³ (consistent with most mold steels).
Thermal Conductivity: ~30–35 W/(m·K) at room temperature, ensuring uniform heat dissipation in molds and reducing temperature fluctuations during molding.
Coefficient of Thermal Expansion: ~11.5×10⁻⁶/°C (20–100°C), a low expansion rate helps maintain dimensional stability during molding cycles.
Elastic Modulus: ~210 GPa at room temperature, ensuring structural rigidity of molds under high injection pressure.
Mechanical Properties
XPM is supplied in a pre-hardened state with balanced properties to meet core requirements of plastic molds:
Property Typical Values (Pre-hardened, 30–35 HRC)
Tensile Strength (Rm) 900–1100 MPa
Yield Strength (Rp0.2) 800–950 MPa
Elongation (A) 12–15%
Impact Toughness (Akv) ≥35 J (at room temperature)
Hardness 30–35 HRC
Polishability Mirror finish achievable (Ra ≤0.02 μm)
Machinability: Low cutting resistance in the pre-hardened state, compatible with high-speed steel or carbide tools for milling, drilling, and grinding, with easy control of surface roughness.
Weldability: Repairable via welding; preheating to 200–300°C and post-weld slow cooling with low-temperature tempering are recommended to avoid cracking.
Heat Treatment Characteristics
A key advantage of XPM is its pre-hardened supply status—final heat treatment is completed before delivery, eliminating the need for secondary quenching by users and simplifying mold manufacturing:
Pre-hardening Process: Achieved via quenching and high-temperature tempering to reach 30–35 HRC, ensuring uniform microstructure (tempered sorbite) and dimensional stability after machining.
Aging Stability: Minimal changes in hardness and dimensions during long-term use in injection molding (operating temperature ≤150°C), suitable for high-volume production.
Post-Processing: Local hardness enhancement (e.g., for cutting edges or parting surfaces) can be achieved via surface quenching (up to 45–50 HRC), but heating range must be controlled to prevent deformation.
Key Performance Advantages
Excellent Polishability:
Low non-metallic inclusion content and uniform microstructure enable ultra-high surface 光洁度 after grinding and polishing, meeting requirements for optical-grade plastic parts (e.g., lenses, light guides).
High Dimensional Stability:
Low residual stress after pre-hardening minimizes deformation during machining and use, suitable for high-precision complex molds (e.g., multi-cavity molds, thin-wall part molds).
Good Processing Efficiency:
Moderate pre-hardened hardness (30–35 HRC) allows direct milling, engraving, and EDM machining, reducing heat treatment steps and shortening mold manufacturing cycles.
Moderate Corrosion Resistance:
Chromium addition provides better resistance to mold release agents and resin additives than ordinary carbon mold steels, extending mold service life.
Application Fields
XPM plastic tool steel is suitable for mid-to-high-end plastic molding scenarios, particularly:
Household Appliance Molds: Molds for refrigerator, air conditioner, and washing machine panels requiring high-gloss surfaces.
Automotive Plastic Part Molds: Dashboard, interior trim, and lamp housing molds, balancing precision and wear resistance.
Medical Device Molds: Syringes, infusion set molds requiring clean, defect-free surfaces.
Optical and Electronic Molds: Mobile phone casings, lens modules, and light guide plate molds with strict polish and dimensional accuracy requirements.
Large Plastic Molds: Due to high hardenability, it is suitable for molds with thickness ≥100mm, such as automotive bumper molds.
Comparison with Similar Pre-hardened Steels
Property/Material XPM 718H (Traditional Pre-hardened) NAK80 (Mirror Steel)
Pre-hardened Hardness 30–35 HRC 28–32 HRC 37–43 HRC
Polishability Excellent (Mirror) Good Superior (Ultra-mirror)
Machinability Good Good Moderate (Higher hardness)
Hardenability High (Large Sections) High Moderate
Cost Moderate Moderate Higher
Ideal Applications General High-end Plastic Molds Large Molds Ultra-high Mirror Requirement Molds
Usage Notes
Processing Parameters: During pre-hardened machining, select appropriate cutting speeds (e.g., 80–120 m/min for milling) to avoid surface hardening due to heat generation.
Polishing Process: Follow a step-by-step 流程 of “rough grinding → fine grinding → progressive polishing paste refinement” to maximize mirror potential.
Maintenance: Clean residual resin and mold release agents after use to prevent long-term corrosion; store in moisture-proof conditions to avoid rust.
In summary, XPM plastic tool steel, with its pre-hardened supply, excellent polishability, and dimensional stability, is an ideal choice for mid-to-high-end plastic molds. It excels in scenarios balancing efficiency and precision, helping reduce mold manufacturing costs and improve plastic part quality.