An ejector pin is a steel pin in an injection mold that pushes the finished part off the mold core after the mold opens. The pins sit flush with the mold surface while plastic is injected, then an ejector plate drives them forward to strip the part free. Each pin leaves a small witness mark on the part.
In short: ejector pins are how a molded part gets out of the mold, and where they push decides where the marks end up.
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. So when the mold opens, the part stays on the moving half, gripping the steel, and something has to push it off. In most molds that job falls to ejector pins: cylindrical pins that 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, but they are not the only way to eject a part. 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 part that is still warm can bend if it is pushed from one side, and moving the pins or adding draft is a standard fix for parts that warp 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. The clearance around a worn pin is one of the places flash forms.
- Mark on the drawing any surface where a pin mark is not acceptable, so the mold designer can plan the layout 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.
Frequently Asked Questions
What is an ejector pin?
It is a pin built into an injection mold that pushes the molded part out once the mold opens. The pins sit flush with the core while the part is molded, then move forward together on an ejector plate to release it.
What are ejector pin marks?
They are the small witness marks left where each pin face touched the part, usually a faint circle the size of the pin. Every pin leaves one, so pins are placed on hidden or non-cosmetic faces wherever possible.
Why are ejector pins on the core side of the mold?
Because the part shrinks onto the core as it cools and stays on that half when the mold opens. Pins in the core can push it straight off, while pins in the cavity half would be hard to build and would mark the visible outside of the part.
Related Services at NICE Rapid
- Plastic Injection Molding — every project starts with a DFM review of the production design files, before any tooling is cut.
- Rapid Tooling — aluminum molds from 1–2 weeks for bridge production, so ejection and pin-mark positions can be seen on real parts before production steel.
- Request a Quote — send a STEP file and note any faces where pin marks are not acceptable.
Sources
- US Patent 5,731,014, Description — "The part 12 will usually shrink onto and remain with the core"; "retractable ejector pins" that "are usually cylindrical members" mounted on an ejector plate, moving from a retracted position "flush with the first mold face" to an extended position to "push the part" away — patents.google.com
- US Patent 6,348,173, Background — "the finished part is held tightly against the core half of the mold"; "core mounted ejector pins movable perpendicular to the plane of the mold parting line"; "cavity ejector pins are impractical from the standpoints of machine and mold construction and appearance of the finished part" — patents.google.com
- MIT OpenCourseWare, 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II (Spring 2025) — flash develops "at the mold parting line or ejector pin installation point"; warpage fix: "Adjust the ejector pin position or enlarge the draft angle" — ocw.mit.edu
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