Plastic Mold Ejector Pin Design Specifications

Ejector pins are critical components in the mold ejection system, responsible for smoothly and uniformly pushing the part out of the cavity or core. Proper design ensures part quality, prevents ejection marks or damage, and extends mold life. The following specifications are based on industry standards such as ISO 8020 and GB/T 4169, focusing on key principles and essential data for practical application.

1. Material Selection and Heat Treatment

Standard Material: SKD61 (or equivalent) is the standard choice for most applications. It offers a good balance of toughness and wear resistance. After heat treatment, the hardness should be HRC 52–58.

Surface Hardening: For extended wear resistance, SKD61 pins are often surface nitrided. The nitrided layer depth is typically 0.03–0.05 mm, resulting in a surface hardness of ≥ HV 800.

High-Precision Material: For slender pins or molds processing abrasive materials (like glass-filled plastics), SKH51 (or equivalent) is preferred. Its superior red hardness and wear resistance (HRC 58–62) significantly reduce breakage.

Micro-Pins: Pins smaller than 1.0 mm in diameter should use SKH51 with vacuum heat treatment to ensure straightness (≤ 0.01 mm/m).

Corrosive Resistance: For corrosive plastics, S136 stainless steel is recommended to prevent rust and degradation.

2. Dimensional Design Specifications

Minimum Diameter: The minimum practical diameter is 0.8 mm for standard use. For abrasive materials, this increases to 1.2 mm to prevent breakage.

Ejection Pressure: The contact area must be sufficient to keep the unit ejection pressure below 5 MPa to prevent part damage (such as blushing or cracking).

Length Proportions: The fitting length should be 3–5 times the pin diameter to ensure stability. The guide length should be at least half the fitting length.

Head Design: The pin head should have a small radius (R 0.1–0.3 mm) or a slight taper (1°–3°) to avoid scratching the part surface.

Appearance Surfaces: If a pin must be placed on a visible surface, the head should be recessed 0.02–0.05 mm below the part surface to hide ejection marks.

Surface Finish: The head should be polished to Ra ≤ 0.2 μm to minimize friction and prevent surface defects.

3. Layout Design Principles

Spacing: The distance between adjacent pins should be at least twice the pin diameter (and no less than 5 mm) to prevent interference and ensure even force distribution.

Deep Cavities: For deep parts (depth > 50 mm), the pin density should be increased to ≥ 1 pin per 10 cm² to prevent deformation during ejection.

Large Parts: For large parts, use a combination of edge and center ejection. Edge pins are spaced 80–120 mm apart, while center pins are spaced 50–80 mm apart.

Sensitive Areas: If pins must be near delicate features (threads/snaps), reduce the unit pressure to ≤ 3 MPa and use highly polished heads to avoid damage.

4. Fit and Relief Specifications

Fit Tolerance: The standard fit between the pin and the guide hole is H7/f7, ensuring smooth movement without binding.

Clearance: Clearance depends on diameter: 0.01–0.02 mm for pins ≤ 5 mm, and 0.02–0.03 mm for larger pins.

Relief: The non-fitting section should have a relief of 0.1–0.2 mm to reduce friction and heat generation.

Chamfering: A chamfer (C 0.5–1.0 mm) at the transition between fitting and relief sections aids assembly and protects the guide surface.

5. Special Pins and Maintenance

Special Types: Flat pins are used for thin walls (thickness = 1/3–1/2 width). Sleeve ejectors use a clearance of 0.01–0.02 mm with the core.

Maintenance: Inspect pins every 50,000–100,000 cycles. Replace pins if head wear exceeds 0.05 mm or if bending is detected.