When the tool opens, the part has shrunk onto the core and has to be pushed off it. Ejector pins are the usual way, and every pin leaves a witness mark where it pushed. Deciding where those marks fall, and making sure the force is spread enough not to distort the part, is a design conversation as much as a tooling one.
The mechanisms
Round ejector pins are the standard and the cheapest. Ejector sleeves push on the annulus around a boss, which is exactly where the part grips hardest. Blade ejectors push on thin ribs. Stripper plates push on the whole rim of a part at once and are used for deep thin walled parts such as containers. Air ejection assists on large or flexible parts by breaking the vacuum behind them.
Why marks appear and where to put them
The pin pushes the still warm plastic, so it leaves a slight depression or a raised ring. The mark is unavoidable; its position is not. Put pins on non cosmetic surfaces, inside the part, on ribs and bosses rather than on flat visible faces, and tell the toolmaker which faces are cosmetic before the design is finalised rather than after the first shot.
What goes wrong
Too few pins concentrate the force and leave deep marks or push through a thin section. Pins placed away from where the part grips let it distort before it releases. Insufficient draft makes the part grip harder than the ejection system was designed for. And ejecting too early, before the part is rigid, distorts it in ways that look like a moulding defect.
Design so ejection is easy
Draft everything in the direction of draw, including internal ribs and bosses. Keep wall thickness uniform so the part is rigid enough to be pushed. Provide flat areas where pins can bear. And where the part must be cosmetically perfect on both sides, raise it early, because that is a stripper plate or an air assisted design rather than a set of pins.
Questions people ask about injection molding ejector pins
Why do moulded parts have small circular marks?
They are ejector pin witness marks, left where the pins pushed the part off the core. They are unavoidable, but their position is a design decision.
Can ejector marks be eliminated?
Not entirely. They can be moved to non visible surfaces, or the ejection method can be changed to a stripper plate that pushes on a rim instead.
What causes parts to distort during ejection?
Too few pins, pins placed away from where the part grips, insufficient draft, or opening the tool before the part is rigid enough.
What is an ejector sleeve for?
Pushing on the annulus around a boss, which is where a moulded part grips the core hardest and where a plain pin would mark or push through.