Last updated: 2026-09-01 · 5 min read

How Do You Size and Place Ejector Pins — and What Happens When the Layout Is Wrong?

Ejector pins push the molded part off the core (B-side) of the mold after the cavity opens. The pin layout must distribute the ejection force evenly across the part — too few pins or poorly placed pins cause the part to stick, distort, or puncture at the pin contact point. The ejection force depends on the part geometry, the resin shrinkage onto the core, the surface texture depth, and the draft angle.
Getting the pin layout right at the DFM stage prevents weeks of ejection troubleshooting at T1.

Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. See also related guide.

Key Facts

  • Ejection force is proportional to the contact area between the part and the core, the resin shrinkage rate, and the coefficient of friction. A deep box part in PBT GF30 (shrinkage 0.3–0.5 %) generates less ejection force than the same part in unfilled PP (shrinkage 1.5–2.0 %) because PP shrinks more onto the core.
  • The standard ejector-pin diameter range is 2.0–8.0 mm. Smaller pins (below 3 mm) can bend or break under high ejection force and should be used only where space is constrained. Larger pins (6–8 mm) distribute force over a wider area and resist bending.
  • The pin should contact the part at a flat, non-cosmetic surface — ideally on a rib, boss base, or a dedicated ejection pad. Contacting a cosmetic surface produces a visible witness mark; contacting a thin wall (below 1.5 mm) risks puncture.

    • Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. The related guide covers adjacent topics.

    Calculating the Ejection Force

    The ejection force is the friction force between the shrinking part and the core surface. It is calculated as the product of the contact area, the contact pressure (from shrinkage), and the coefficient of friction. For a simple box part, the contact area is the total outer wall area that wraps around the core.

    Factor Increases Force Reduces Force
    Resin shrinkage PP, PE (1.5–3.0 %) PC, PBT GF30 (0.3–0.7 %)
    Core draft angle 0° (no draft) 1.5–2° per side
    Surface texture depth deep textures (MT 11030+) smooth or SPI polish
    Core height (draw depth) tall cores, deep boxes shallow parts
    Mold temperature low (part shrinks faster) high (part releases easier)

    Pin Placement Rules

    1

    Place pins at the deepest points first

    The areas farthest from the parting line have the most shrinkage grip on the core and the longest ejection stroke. Place the first pins at these locations — typically at the bottom corners of a box or at the base of deep bosses.

    2

    Distribute pins symmetrically

    Asymmetric pin placement creates a tipping moment — one side of the part ejects before the other, causing the part to cock on the core and stick or drag. For a rectangular box, place pins in a symmetric grid; for a cylindrical part, arrange pins on a circle centered on the axis.

    3

    Contact at structural features

    Place pins on ribs, boss bases, or thickened ejection pads — not on thin cosmetic walls. A 6 mm pin pushing on a 1.5 mm wall will puncture the part or leave a visible bulge. A pin pushing on a 3 mm rib base pushes against a rigid surface and leaves a clean witness mark on a non-cosmetic face.

    4

    Verify the pin count against the ejection force

    Divide the total ejection force by the number of pins to get the force per pin. Each pin should carry no more than 300–500 N for standard pins (3–6 mm diameter). If the force per pin exceeds 500 N, add more pins or increase the pin diameter. If the force per pin exceeds 1,000 N, the pin will bend or the part will puncture.

    When Standard Pins Are Not Enough

  • Sleeve ejectors are used around cylindrical cores (bosses, posts). The sleeve surrounds the core pin and pushes the part off the core uniformly around the entire circumference. Sleeve ejection eliminates the localized witness mark that a standard pin leaves on one side of the boss.
  • Stripper-plate ejection is used for parts that cannot tolerate any pin witness marks on the outer surface.
    The stripper plate contacts the entire parting-line perimeter and pushes the part off the core uniformly. This is the standard approach for cosmetic lens parts, medical device housings, and thin-wall containers.
  • Blade ejectors are used for thin ribs and narrow features where a round pin cannot fit. A blade is a flat, rectangular ejector that fits into a narrow slot in the core.
    The blade width matches the rib width (typically 1.5–3.0 mm), and the blade contacts the full rib base for even force distribution.
  • Air-pop (air-ejection) valves supplement mechanical ejection on parts with deep cores and high vacuum.
    When the mold opens, the vacuum behind the part resists ejection — an air-pop valve breaks the vacuum by admitting air behind the part at the moment of ejection, reducing the required pin force by 30–50 %.

    • Frequently Asked Questions

    How many ejector pins does a typical part need?

    A simple flat part with shallow walls may need only 2–4 pins.
    A deep box part (50 mm+ draw) typically needs 8–16 pins. A large automotive panel may need 20–40 pins. The count is determined by the ejection-force calculation, not by a rule of thumb — every pin must carry a calculable share of the total force.

    What causes ejector-pin witness marks to be visible on the cosmetic side?

    Witness marks appear when the pin pushes against a thin wall that flexes under the ejection force, or when the pin is not flush with the core surface (protruding or recessed by more than 0.02 mm). A pin that protrudes creates a raised bump on the part surface; a pin that is recessed creates a depression. Both are visible on textured or polished cosmetic surfaces.

    Does ejection speed matter?

    Yes. Fast ejection can crack brittle parts (especially glass-filled resins and thin walls) because the impact load on the pin contact point exceeds the local yield stress. Slow ejection reduces the peak force but increases cycle time. The ejection speed should be set to produce a smooth, controlled release — typically 50–100 mm/s for standard parts, slower for fragile geometries.

    Key Takeaways

    1. Ejector-pin layout is a force-distribution problem — the total ejection force must be divided among enough pins, placed symmetrically, so that no single pin carries excessive load and no area of the part is left without ejection support.
    2. Pin placement belongs in the DFM review, not in the T1 adjustment. The pin count, diameter, and contact location should be calculated from the part geometry, resin shrinkage, and surface texture before the core plate is drilled.
    3. When standard round pins cannot solve the ejection problem (cosmetic surfaces, narrow ribs, deep bosses), the alternatives are sleeve ejectors, stripper plates, blade ejectors, and air-pop valves — each solving a specific geometry constraint.

    A mold with a well-designed ejection system produces parts that drop cleanly from the core every cycle, with consistent witness marks and zero sticking. A mold with a poorly designed ejection system spends hours at T1 adjusting pin lengths, adding pins, and polishing drag marks — and the production cycle time includes a 2-second ejection delay that compounds across every shot for the life of the tool.

    Sources
    DME — mold base and ejector components
    HASCO — standard mold components
    Plastics Technology — ejection design

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet