DefinitionAn ejector pin is a steel pin in an injection mold that pushes the finished part off the mold core after the mold opens.
Explained
What Do Ejector Pins Do in Injection Molding?
As a part cools it shrinks onto the core, the mold half that forms its inside surfaces. When the mold opens, the part stays on the moving half, gripping the steel, and the ejector pins push it off. The cylindrical pins run through close-fitting holes in the core and are mounted on an ejector plate inside the mold base.
While the cavity fills and cools, the pin faces sit flush with the mold surface and form part of the cavity wall. Once the mold opens, the machine's ejector moves the plate forward, the pins protrude, and the part is pushed clear. The pins then retract before the mold closes for the next shot.
Pins travel in the direction the mold opens, perpendicular to the parting line, and they are almost always placed on the core side. Putting them in the cavity half is impractical for mold construction and would mark the part's show surfaces.
Ejector Pins vs Other Ejection Methods
Pins are the default ejection method, but the choice depends on where the part grips the mold and how much surface is available to push on.
| Method | How it works | Typical use |
|---|---|---|
| Ejector pin | A round pin pushes on a flat area of the part | General-purpose; the default for most parts |
| Ejector sleeve | A hollow tube slides along a core pin and pushes around it | Bosses and round features that grip a core pin |
| Blade ejector | A flat, rectangular pin | Thin ribs and narrow walls with no room for a round pin |
| Stripper plate | A plate pushes on the part's whole edge | Thin-walled cups, lids and round parts that pins would distort |
Where Should Ejector Pins Go?
Pin layout is a trade-off between pushing the part out evenly and keeping marks off the surfaces that matter.
- Push where the part grips hardest: next to ribs, bosses and deep walls, which shrink tightly onto the core.
- Spread the pins so the part lifts evenly. A warm part can bend if pushed from one side, and moving pins or adding draft is a standard fix for warpage on ejection.
- Put the marks on non-cosmetic faces, such as inside walls, the underside or an area hidden by a label.
- Keep pins and their holes in good condition; clearance around a worn pin is a common place where flash forms.
- Mark on the drawing any surface where a pin mark is not acceptable, so the mold designer can plan around it.
A DFM review before tooling is where pin positions and cosmetic surfaces get agreed. At NICE Rapid it is the first step of every plastic injection molding project.
Key takeaways
- Ejector pins push the cooled molded part off the mold core after the mold opens.
- The pins sit flush with the core surface during injection and advance together on an ejector plate to release the part.
- They are almost always placed on the core side because the part shrinks onto the core and stays there.
- Pin placement balances even ejection against hiding witness marks on non-cosmetic surfaces.
- Worn pins and their clearances are common locations where flash can form.
Related terms
References
- US Patent 5,731,014 — Injection mold with retractable ejector pins — Google Patents
- US Patent 6,348,173 — Mold ejector system — Google Patents
- MIT OpenCourseWare 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II (Spring 2025) — MIT OpenCourseWare