An injection molding machine ejector rod, also called a knock-out rod or KO rod, is the hardened steel rod that carries the ejector force from the machine to the mold. One end screws into the ejector's connecting plate behind the moving platen; the rod passes through a hole in the moving platen, and its other end pushes the mold's ejector plate. When the ejector cylinder moves the connecting plate forward, the rods move the mold's plate, and its ejector pins push the part out.
Where the ejector rods sit
The moving platen has a center bore and a pattern of extra ejector holes around it. A small mold may use only the central rod; a larger mold, or one with a long ejector plate, uses several rods so that the plate is pushed at several points and moves square. The mold design decides which holes are used: the mold's clamping plate has matching holes through which the rods reach the mold's ejector plate. The platen's hole pattern is shown on the machine's platen drawing; see the platens guide.
Coupled and uncoupled ejector rods
- Uncoupled (push only). The rods only push. After ejection they pull back, and the mold's ejector plate is returned by the mold's own return springs and, at the latest, by its return pins when the mold closes.
- Coupled. The rod ends are screwed into the mold's ejector plate, directly or through adapters, or locked to it with a quick coupling. The machine then pulls the plate back as well as pushing it forward. Coupling is needed when the pins must be fully back before the mold closes, for example where they sit under slides, and it is often used when the ejector makes several strokes per cycle.
A coupled mold must be uncoupled before it is taken out of the machine, and the ejector settings must suit the coupled mold.
Setting the rod length
The rod length decides where the mold's ejector plate sits at both ends of the stroke:
- With the machine's ejector fully back, the rod ends must not push the mold's ejector plate forward. On an uncoupled mold they should sit at or just behind the platen face, so the mold sits flat and its plate can return fully.
- With the ejector fully forward, the rods must move the plate through the travel the mold needs, but never beyond its stops. Set the ejector forward position on the controller to suit the mold.
- All the rods used on one mold must be the same length, so the plate is pushed square. One long rod tilts the plate, and the pins bind or break.
- Screw each rod fully into the connecting plate, and lock it if the design has a lock nut, so it cannot unscrew.
Signs of wear or failure
- A rod that drags or scrapes in its platen hole, or score marks along its surface: the rod is bent, or the hole has a burr.
- Damaged or stretched threads, or a rod that keeps unscrewing.
- A burred or mushroomed end where the rod meets the mold's plate.
- Rods of different lengths in the same set, often after a mold change with mixed rods.
- A broken rod, usually at the thread.
Common causes are ejecting with the mold closed or blocked, a stroke set longer than the mold allows, rods that were not tightened, a tilted plate that bends the rods sideways, and too much ejector force.
Inspecting and replacing ejector rods
- Retract the ejector, then switch off and lock out the machine.
- Unscrew the rods from the connecting plate and clean them.
- Roll each rod on a flat surface, or turn it between centers under a dial indicator, to see whether it is bent.
- Inspect the threads and the ends, and measure the rods of each set to check that they are the same length.
- Clean the platen holes and remove any burrs.
- Fit straight rods of the right length, tighten them fully, and run the ejector slowly with the mold to check the stroke before production.
The ejector rods we supply are ground and hardened, in 45# or 42CrMo steel, with the diameter made to suit the machine.
What to send for a replacement quote
Send the machine brand, model and serial number; the rod length and diameter; the thread size and pitch at each end and whether it is male or female; the shape of the push end; how many rods you need; and a photo. Request a quote, and see the ejector guide and the ejector plate page for the rest of the system.
Updated by the IMSPARES technical team.