Ejector pins in injection molding

Ejector pins are hardened steel pins in an injection mold that push the molded part off the core when the mold opens. They belong to the mold, not to the injection molding machine. The machine only supplies the push: its ejector cylinder moves the ejector rods, the rods push the mold's ejector plate, and the pins fixed in that plate push the part. Worn or broken pins are therefore replaced by the toolroom, the mold maker or a mold component supplier, but many ejection problems start on the machine side.

How the machine drives the ejector pins

Ejection is a chain, and every link in it matters:

1. The ejector cylinder behind the moving platen pushes the machine's connecting plate forward: see ejector cylinder.
2. The connecting plate drives the ejector rods through holes in the moving platen and in the mold's clamping plate: see ejector rods.
3. The rods push the mold's ejector plate, which is bolted to the retainer plate that holds the heads of the pins: see ejector plate.
4. The pins slide forward through close-fitting holes in the core and push the part off.
5. On the way back, coupled rods pull the plate back, or the mold's springs and return pins push it back as the mold closes.

While the plastic is injected, the face of each pin forms a small part of the cavity surface. Its length therefore has to match the core surface exactly when the ejector plate is fully back.

Types of ejector pins and other ejectors

• Straight ejector pins with a cylindrical head, the most common type.
• Stepped pins, thicker along most of their length for stiffness, with a thin end for small features.
• Blade (flat) ejectors for thin ribs.
• Ejector sleeves, tubes around a core pin, for bosses and other round features.
• Stripper plates and rings, which push the whole edge of a part, and air ejection, used where pins would mark or deform the part.

Pins are made of through-hardened or nitrided tool steel in standard metric and inch sizes. A pin whose face follows a shaped core surface has its face ground to that shape and a head that is keyed so it cannot turn.

Ejector pin marks

A small round mark where each pin touched is normal on the non-visible side of a part. Marks become a problem when they are deep, raised, white or flashed:

• Raised pad on the part: the pin face sits below the core surface, so plastic fills the pocket above it. The pin is too short, or the ejector plate does not return fully.
• Depression in the part: the pin stands proud of the core surface. The pin is too long, or the plate is held forward, for example by machine ejector rods that are too long.
• White stress marks, bulges or cracks around the pins: the ejection force is too high for the part at that moment. Causes include too little draft, undercuts, too much hold pressure (overpacking), too short a cooling time, ejection that is too fast, and too few or too small pins.
• Flash around the pins: worn pins or pin holes leave too much clearance, or the injection and hold pressures are too high.
• Pins pushing through the part: the part is too hot or too thin at the pins when it is ejected, or it is stuck on the core.

Process and mold causes are for the molder and the mold maker to fix, but machine settings such as ejector speed, force and timing are often part of the solution.

Machine causes of pin damage and marks

Check the machine side before blaming the mold:

• Ejector rods of different lengths, or a bent rod, tilt the mold's plate, so pins bind and break on one side.
• Rods that are too long hold the plate forward when the ejector is back, leaving the pins proud.
• A stroke set too long drives the plate hard into its stops; too short leaves the part on the core.
• Ejector speed or force set too high marks and cracks parts.
• An ejector that is not fully back when the mold closes crushes the pins against the fixed half. The ejector-back interlock exists to prevent this.
• A tilting connecting plate, from worn guide pins or broken return springs.

Pin wear and breakage in the mold

Pins wear and gall when they run dry or dirty, when the mold heats unevenly so the pin holes no longer line up, and when the ejector plate is not guided. Mold maintenance usually includes cleaning the pins, lubricating them lightly with a product that suits the molded material, and checking them for scoring and free movement. Replacement pins are cut and ground to length to match the core surface. This is mold work, done by the toolroom or the mold maker.

What to send when the machine side needs parts

If the cause is on the machine side, we can quote the parts that drive the pins: ejector rods, the connecting plate and pin kit, return springs and the auxiliary cylinder used to drive the ejector. Send the machine brand, model and serial number, the part and its dimensions, and photos, and request a quote. The ejector guide explains the whole system.