Last updated: 2026-09-04 · 6 min read

Where Do Ejector Pins Go? — Ejection Layout, Witness Marks and the Sticking-Part Fix

Ejection is a force-balance problem disguised as a hardware choice. The plastic grips every core as it shrinks, and the mold must apply enough force, spread across enough points, in the right places, to release the part without bending or marking it. Good layout concentrates push force against the features that hold hardest — deep cores, tall walls, bosses — and leaves witness marks only where the customer will never look.

Key Facts

A round pin leaves a small circular witness that must sit off show faces; an ejector sleeve lifts a boss by its full circumference; a blade handles a deep rib or thin wall a point load would crack.

Push on structure — under a rib, around a boss, on a gusset — never on a broad thin face that will dimple or stretch under a point load.

Sticking on one core rather than everywhere is almost always a local imbalance — too little ejector force, a polished core that vacuums, or an unbalanced pattern where the stuck core fires after its neighbors.

Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. The related guide covers adjacent topics.

Which Ejector Fits Which Feature?

Five ejector forms, each with a cosmetic cost:

Ejector Type	Best On	Witness and Limits
Round pin	General walls and ribs on hidden faces	Small round mark; must sit off show faces
Ejector sleeve	Bosses and small coring that need ring lift	Ring seam; can gall if core shrinks hard
Blade ejector	Thin walls, deep ribs, frame openings	Linear mark; more complex to machine
Step (shoulder) pin	Deep or fragile features needing broad push	Larger contact footprint on the part
Air / stripper assist	Soft, deep-draw or vacuum-held parts	Added system cost; not always needed

The reason a plain round pin is not always enough is shrinkage grip. A thermoplastic cools and contracts onto whatever core it formed around, and the deeper and smoother that core, the tighter the part clamps it. If you only put pins on a broad flat area, the plastic there releases easily while the deep core still holds, so the part stretches over the core instead of sliding off — and when it finally lets go it can snap, whiten, or stay distorted.

Correct layout puts ejector force as close to the holding features as geometry allows: pins under ribs, sleeves around the bases of bosses, blades along a deep thin wall, so the push and the grip fight each other head-on.
Each ejector form buys capability with a visible price — a round pin is inexpensive but leaves a witness; a sleeve lifts a boss cleanly but leaves a faint ring seam; blades and step pins spread force over a line or shoulder at the cost of harder machining.

How Do You Review Ejection on the DFM?

Four steps, in order:

Map the part’s grip points before choosing pins. Identify where the plastic will hold hardest — the deepest cores, the tallest walls, the largest flat areas that can vacuum-stick — and treat those as the features ejection must be designed around first. Estimate the holding force qualitatively from core depth, surface finish and material shrinkage: a polished deep core grips far more than a short textured one. Once you know the stubborn zones, place ejector force against them rather than scattering pins evenly and hoping.
Push on structure, not on thin unsupported skin. Land ejector force where the part is stiff enough to take it — under a rib, around a boss, on a gusset — and never on a broad thin face that will dimple or stretch under a point load. The rule that keeps ejection invisible is to put the witness on a feature already there to take load and already hidden: bosses are ideal because they exist to be fastened and sit on the non-show side. If the only available locations fall on a cosmetic face, the part needs a structural rib or boss added at DFM.
Balance the pattern so the part lifts evenly. Ejection is a rigid-body event: if pins fire unevenly or cluster on one side, the part tilts, binds on the remaining cores, and can crack or scar. Ask for a layout that spreads pins so the resultant force passes near the part’s center and every core releases together, with more and larger ejectors under the high-grip zones and symmetric placement for symmetric parts.
Confirm pin marks on a real T1 part. Examine the molded trial parts and confirm each ejector witness is where you intended, is small enough, and sits only on faces the customer will never see. Look for failure signatures — a dimpled skin from an over-long pin, a shiny ring from a galling sleeve, a stress mark beside an under-sized ejector. Ejection problems are inexpensive to solve on the DFM and expensive once the cavity is cut.

Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. The related guide covers adjacent topics.

Ejection is not a hardware choice — it is a force-balance negotiation between what the part grips and what the mold can push, resolved on the DFM before steel is cut.

Key parameter identified — critical variable confirmed against spec.

Tolerance within achievable range — commercial ±0.15mm or tighter as needed.

Material matched to application — resin grade selected for end-use environment.

Process window validated at T1 — parameters locked after first trial.

Inspection method defined — CMM, visual, or functional test specified.

Frequently Asked Questions

How many ejector pins does a part need?

Enough to spread the release force so no single pin is overloaded and the part lifts evenly — the answer is set by the part’s grip, not by a fixed count. A small shallow clip may release on two or three pins; a large tray with deep walls may need dozens placed against every stubborn core.

The sizing logic is: estimate the holding force at each sticking feature, choose an ejector type that can apply more than that force without marking the part, and repeat across all grip points so the pattern is balanced. On the DFM the right question is not “how many” but “which cores hold, and what is pushing them.”

Can ejector marks be hidden completely?

On a truly closed cosmetic face, sometimes not without a design change, which is why this belongs in the DFM. A pin has to touch the part somewhere, and that touch can leave a witness, so the craft is placing the touch where it is invisible by design — under a rib, on the back of a boss, inside a pocket.

If every functional ejector location would mark a Class-A surface, the clean answer is usually to add a small hidden boss or rib whose only job is to accept an ejector, or to route ejection to the core side where the part shrinks and is naturally held. Planning this early means the “hidden” ejector point reads as intended part geometry.

Why does my part stick on one core but not the others?

Usually because that core holds harder or is ejected later than the rest. Common causes are a deeper or more polished core that grips through shrinkage, a core that runs hotter and releases softer plastic, an undercut creating mechanical lock, or an unbalanced pattern where the stuck core has too little ejector force or fires after its neighbors.

The fix follows the diagnosis: add ejector force at the stubborn core, cool it more evenly, polish or texture the core to break the vacuum, add draft angle to reduce grip, or re-time the ejector pattern so all cores release together. Sticking on one location is almost always a local imbalance.

Key Takeaways

Ejection is a force-balance problem — match each feature’s shrinkage grip to the mildest ejector that clears it, and concentrate push force against deep cores, tall walls and bosses rather than scattering pins evenly.
Push on structure — under ribs, around bosses, on gussets — never on thin unsupported skin; if the only ejector locations fall on a cosmetic face, add a hidden boss at DFM whose only job is to accept the pin.
Sticking on one core is a local imbalance — too little force, a polished vacuum, or an unbalanced pattern — and the fix follows the diagnosis: add force, break the vacuum, or re-time the ejector sequence.

Ejector layout is a negotiation with appearance, structure and the parting line, resolved on the DFM before steel is cut.
Map the grip points, push on structure, balance the pattern, and confirm the witness marks on T1 parts. A part that ejects cleanly every cycle is not luck — it is the result of matching every feature’s grip to the right ejector in the right place.