One of the most common methods of shaping plastic resins is a process called injection moulding. The popularity of this method is its ability to be used in mass-production. Injection moulding is accomplished by large machines called injection molding machines.

Injection moulding machine

0 – Hydraulic motor and gears, 1 – Mould, 2 – Hopper, 3 – Injection cylinder with screw, 4 – Heaters, 5 – Nozzle, 6 – Fixed platen, 7 – Moveable platen, 8 – Clamping unit, 9 – Ejector

The key process in injection moulding is when plastic resin in the form of granules is melted until soft enough to be injected under pressure to fill a mould where it cools into the shape of product. Once the product has cooled sufficiently to harden the mould opens releasing it. The result is a product that is an exact copy of the mould shape. The whole injection moulding process then repeats.

The process can also be thought of in its most simplistic terms. The three phases:

1. Heating – melting resin, usually granules which soften
2. Shaping/forming – where force contains it
3. Cooling – that enables it to retain its shape


Closed injection mould

Resin is fed to the machine through the hopper. The resins enter the injection barrel (injection cylinder) by gravity though the feed throat. The barrel has heater bands around the outside. Upon resin entrance into the barrel, it is heated to the appropriate melting temperature. The moving and fixed platens of the injection moulding machine holds the mould together under pressure.

Injection of molten resin

The rotating screw forces the resin along the barrel. This in return forces the screw back as the molten resin collects at the end of the screw. When the right amount of resin for the shot has accumulated the screw stops rotating. The screw then acts like a plunger and with his axial movement inject the molten resin under pressure into the mould.

Open injection mould and eject product

The mold is cooled constantly to a temperature that allows the resin to solidify and be cool to the touch. Once the shape has cooled sufficiently to harden, the moving platen moves away from the fixed platen opening the mould tool. Rods, a plate or air blast then aids ejection of the completed plastic shape from the injection mould tool.