How to Choose Ejector Pin Material for Injection Molds — SKD61 vs SKH51 vs High-Speed Steel

Ejector pins are the most heavily cycled mechanical components in plastic injection molds, undergoing continuous high-velocity reciprocating motion, frictional wear, and direct thermal shocks from molten resin. Selecting the incorrect grade of tool steel frequently leads to pin seizure, galling, surface flash on molded parts, or catastrophic fatigue failure.

1. SKD61 (JIS) / AISI H13 / DIN 1.2344 — The Universal Workhorse

SKD61 is a chromium-molybdenum hot-work tool steel and remains the global baseline choice for over 80% of standard injection tooling:

• Dual Heat-Treatment Architecture: Through-hardened to HRC 38–42 in the core to preserve flexible toughness, then vacuum ion-nitrided on the surface to achieve a micro-hardness of HV 950–1100 (equivalent to HRC 68–72).
• Thermal Shock Resistance: Excellent resistance to thermal cracking and softening at mold operating temperatures up to 550°C.
• Best Suited For: General-purpose engineering resins (PP, PE, ABS, PC, PA6) in standard cycle times.

2. SKH51 (JIS) / AISI M2 / DIN 1.3343 — High-Speed Steel for Severe Duty

SKH51 contains significant tungsten, molybdenum, and vanadium alloys, offering dramatically enhanced compressive strength and hot hardness:

• Uniform Through-Hardness: Heat-treated to a uniform HRC 60–64 across both surface and core without relying on a thin nitrided crust.
• Anti-Deflection Rigidity: Possesses a high Young's modulus, preventing elastic deflection when ejecting deep-rib or high-drag optical moldings.
• Best Suited For: Small diameters (≤ Ø2.5 mm), glass-fiber reinforced abrasive polymers (PBT+30%GF, PPS, LCP), and fast-cycling multi-cavity medical molds (e.g., syringe barrels).

3. Comprehensive Material Comparison Table

4. Solutions for Thin Pin Buckling & Fracture

When ejector pin diameters fall below Ø2.0 mm with stroke lengths exceeding 50 mm, straight pins often buckle under high release pressure.