Front page / Where do ejector pins go, and why do they leave marks?
Where do ejector pins go, and why do they leave marks?
Ejector pins sit in the half of the mould the part stays on when it opens, usually the inside or back of the part, and push it off the core at the end of each cycle. Because each pin is a flat steel face pressing on the plastic, it leaves a small round mark; the usual fix is to place pins on hidden faces and agree those positions with the toolmaker before steel is cut.
When the mould opens, the part tends to stay on the core side because plastic shrinks onto it. Protolabs describes this half as the B-side: its pins extend into the cavity, push the part free, then retract before the mould closes again.
Why the marks appear
A pin needs a flat pad to push against, square to the direction it moves. Protolabs explains the cosmetic results that follow:
- On a textured face, the smooth end of the pin shows as a shiny circle.
- On a curved or angled face, the flat pin end cannot match the surface, so it leaves a pad that is slightly raised on one side and slightly sunk on the other.
- A pin set proud of the surface leaves a standing pad that thickens the wall and can cause sink on the other side; a pin set deep thins the wall and can punch through.
In a traditional steel production tool the pin end can sometimes be machined to follow the part contour, which reduces the mark on curved faces.
Published sizes and placement
- Fictiv's ejection design guide gives common pin diameters of about 3 to 8 mm (1/8 to 5/16 in), chosen by part mass and the force needed. Small pins can bend or dent thin sections; larger pins spread the load.
- Pins should be spread so the push is even, and concentrated near features that grip the mould: deep cores, tall ribs, textured areas and faces with little draft.
- Soft or sticky resins may need more or wider pins so they do not pierce or mar the part, according to Protolabs.
- Fictiv notes that even 1 degree of draft greatly reduces the force needed to release a part, which in turn reduces how hard the pins must push.
Other ways to eject a part
| Method | How it works | Used for |
|---|---|---|
| Ejector sleeve | A hollow tube pushes around a core pin | Bosses and round posts |
| Blade ejector | A thin flat blade pushes on a narrow rib | Ribs too thin for a round pin |
| Stripper plate | A plate pushes the whole edge of the part | Thin-walled cups and containers that would distort on pins |
| Air assist | A blast of air breaks the vacuum between part and core | Deep parts, usually alongside pins |
The list follows Fictiv's guide. A grille or grate with no flat areas may need small bosses added purely as places for pins to push, Protolabs notes.
What to put on the drawing
- Mark which faces are cosmetic and where no pin marks are acceptable.
- State whether pin marks may be raised, flush or slightly recessed, and by how much.
- Ask to approve a gate and ejector layout before the mould is cut. Many suppliers provide one as standard.
- If the cosmetic face must take pins, for example because a clip makes that side grip the mould, discuss texture matching or pin positions near edges.
Pin sizes and layouts depend on the part, resin and mould; the figures here are general published guidance.
Questions buyers ask
Can a part be made with no ejector marks at all?
Not usually on the side that stays on the core. The goal is to put the marks where nobody sees them, or to use a stripper plate or sleeves where pins would show.
Why are there pins on my cosmetic face?
Some features, such as clips formed through the part, make that face grip the mould harder, so it becomes the ejection side. Protolabs gives this as a common reason for pads on the cosmetic side.
Do pins do anything besides eject?
Yes. Protolabs notes they are sometimes used to vent deep features so air is not trapped at the end of fill.
Are ejector problems a design or a process issue?
Both. Sticking and drag marks often trace to too little draft, uneven cooling or trapped air, and Fictiv lists more draft, venting and better cooling among the fixes alongside pin changes.