Ejection Stroke and Ejection Force: Calculation Guide

Most ejection problems are designed in long before the first shot. A part sticks because there is not enough ejection area, the pins are in the wrong place, or the plate cannot travel far enough to clear a deep core. Ejection works reliably when two numbers are right: the force needed to push the part off the core, and the stroke needed to clear it. This guide shows how to estimate both and how to turn them into pin counts, pin sizes and plate travel.

Figure 1. Ejection force formula with resin factors and pin sizing plus stroke clearance rules.

Why parts stick

Ejection force must overcome several mechanisms at once:

• Shrinkage grip — the part shrinks onto the core, creating radial pressure that resists release.
• Vacuum — a sealed pocket or a cup shape forms a vacuum between part and core.
• Friction — coefficient of friction between resin and steel, amplified by the grip pressure.
• Mechanical interlock — undercuts, texture or a rough core surface.
• Adhesion — resin clinging to a polished or hot surface.

Good design attacks the causes: draft angle, adequate cooling, venting at the core, and a core surface finish that releases.

Estimating ejection force

A practical first estimate uses the part's projected area and a resin-dependent factor:

F ≈ A × p_e

Where:

• F = total ejection force (N or lbf)
• A = projected contact area of the part on the core (mm² or in²)
• p_e = ejection pressure factor, typically 5–20 MPa (700–3000 psi) for common resins

The factor rises with stiffness and shrinkage: soft, low-shrink resins such as PP sit near the low end; stiff, high-shrink or glass-filled resins sit near the high end. Deep draws, thin walls and textured cores push the factor up further.

A more refined estimate adds a friction term:

F = A × (shrinkage pressure) × (coefficient of friction)

This is why draft angle matters so much — a small increase in draft can cut the required force sharply.

> Treat the estimate as a starting point. Always add a safety factor of at least 1.5–2× so the ejection system has reserve capacity for a cold shot or a slightly overpacked part.

From force to pin count and size

Once the total force is known, the pin layout follows:

• Ejection area — aim for the smallest pin area that keeps the contact stress below the resin's yield. Too few or too small pins dent or punch through the part.
• Pin count — distribute the force so no single pin carries a disproportionate load. Adding pins is cheaper than adding damage.
• Pin location — place pins where the part is stiff: at ribs, bosses, corners and side walls, never on a free span or a cosmetic surface.
• Pin diameter — size for buckling as well as stress; long, slender pins buckle before they push.
• Symmetry — balance the pin layout so the part lifts evenly and does not cock on the core.

For soft or thin parts, use a stripper plate or an air-assist poppet instead of pins; they spread the ejection force over a large area.

Ejection stroke

Stroke must clear the core completely:

Stroke = core depth + clearance

• Core depth — the deepest point the part must be lifted off. For a part with a 40 mm core, the stroke must exceed 40 mm plus a few millimeters of clearance.
• Clearance margin — add 5–10 mm so the part drops free rather than dragging.
• Part shape — a tall, thin part may need the stroke plus a tilt or pick-out device, because the part can grip the core along its whole length.
• Ejector plate travel — the machine's ejector stroke and the tool's ejector plate travel must both exceed the required stroke; the smaller of the two governs.

Undercuts, internal threads, rings and grooves add stroke or need a separate unscrewing or collapsing action. Never size the stroke to the bare minimum; leave room for the part to fall and for a robot or conveyor to take it.

Design rules that reduce the numbers

• Add draft — 1° minimum on core surfaces, 1.5–2° for textured walls; more on deep draws.
• Polish the core in the direction of ejection; a rough finish multiplies friction.
• Cool uniformly so the part shrinks evenly and does not grip one area harder.
• Vent the core to break the vacuum on cup-shaped parts.
• Round the pin ends slightly to spread load and avoid marking.
• Add ejector bosses (a small local thickening) where a pin must press, so the pin does not dent a thin wall.

Common mistakes

• Sizing ejection from one shot. A cold or overpacked shot needs far more force than a hot, well-set one.
• Too few pins. A large flat part on two pins will bow or crack.
• Pins on a free span. The part flexes instead of releasing.
• No vacuum relief. A cup-shaped part needs a poppet valve or air blow-off.
• Stroke too short. The part hangs on the core and is scraped by the next cycle.
• Ignoring buckling. A slender pin fails in compression before it moves the part.
• No return-spring or early-return check. Pins that do not retract before closing crash the tool.

Worked example

A 150 mm × 100 mm polypropylene cover, 40 mm deep, with a stiff core:

• Projected area ≈ 15,000 mm².
• Ejection factor for PP ≈ 8 MPa → F ≈ 120 kN.
• Apply a 1.8 safety factor → ≈ 216 kN.
• At a permissible 20 MPa contact stress, minimum pin area ≈ 10,800 mm²; spread over 16 pins of 8 mm diameter (≈ 800 mm² total) the stress is far lower, but the layout must sit under ribs and corners.
• Stroke = 40 mm core + 10 mm clearance = 50 mm minimum; set the ejector plate travel to 55 mm.

The numbers are estimates, but they turn a vague "add some pins" into a layout a toolmaker can build and a machine can be checked against.

Figure 2. Quick reference: the key checks and reference values from this guide.

Checklist

• Projected contact area and ejection factor estimated
• Safety factor of at least 1.5–2× applied
• Ejection area checked against permissible contact stress
• Pin count and diameter sized for load and buckling
• Pins placed under ribs, bosses and corners only
• Stripper plate or air used for soft or thin parts
• Stroke = deepest core + 5–10 mm clearance
• Machine ejector stroke verified against tool travel
• Vacuum relief designed for cup-shaped parts
• Draft, polish and core cooling optimized to cut sticking
• Early return / return spring checked before closing

Ejection is easy to get wrong and expensive to fix in a hardened tool. Estimate the force, size the stroke, and lay the pins out before the steel is cut — the numbers are simple, and the savings are real.