The Injection molding process

There are many sizes and types of injection molding machines, but the machines all follow the sequence shown in this animation.


A mold is bolted into the Clamping section of the machine. The machine closes the mold, then applies a large force to “Lock” the mold closed. Inside the closed mold is a cavity that is the exact shape of the plastic part.

The Injection section of the machine has a hopper to hold plastic pellets; a barrel with heater bands to liquefy the plastic pellets; a feed screw to move the pellets forward in the barrel; a check valve to force the liquid plastic into the mold, and a nozzle to seal the injection section to the mold. The liquefied plastic is forced into the cavity of the mold with high pressure.

Once the liquid plastic has been injected into the mold, the machine goes into the Cooling phase. The liquid plastic must cool enough to turn solid so it takes on the shape of the cavity and stays that way. While the cooling take place, the screw will rotate, bringing in more pellets for the next part.

When the part is ready to be removed from the mold, the clamp will open, and the part will be removed from one half of the mold. Then the part will be Ejected from the other half of the mold, and the machine will start a new cycle.