Basics of Plastic Injection Process

Injection molding is the most common way we use to realize mass production of complex, high precision plastic parts.

It is a method of forming plastic products from pallets by feeding the raw material into the hopper and push the material into the mould by the use of the screw.

Generally, the injection process can be divided into five stages shown as below.

Injection molten flow into the mould; keep the holding pressure if required. Then the plastic part cooled down and solidified, open the mould and eject the part out. Close the mould and the molten plastic is ready for next shot. This is one injection cycle finished. And the process will repeat itself.

Following figure can give a brief idea what are the main parts that consist an injection machine.

The plastic pallets go through the hopper and gradually melt into the molten flow that will be injected into the mould to form the plastic parts. The screw rotates powered by the drive unit and it has two main functions: transfer material forward and shear the plastic with friction. The mould clamping unit ensures enough clamp force to form the desired shape.

Next we will see a very important component of the injection machine, the screw.

In the feed zone, the screw will rotate to push the plastic pallets fallen from the hopper to move forward. And in the transition zone, the plastic pallets gradually melt with the shear friction from screw against barrel and heat from the heater bands. Finally, in the meter zone, the molten flow is well mixed and ready to inject into the mould.

Also, there is one component of the screw we cannot ignore, which is the check valve.

As we can see, when the screw rotates to drive the molten flow forward, the check valve is open to let the material flow from metering zone to the front and be ready for injection. When the injection happens, the check valve close to prevent the molten flow back. And this prevention of back flow is vital during pressure holding stage.