Injection Moulding

An injection moulding* machine can be split into two main parts:  the clamp and the injection unit. The clamp holds the mould tightly together while the injection unit squirts hot, molten plastic into it.

* Animation from British Plastics Federation www.bpf.co.uk

The main parts of an injection moulding machine are the hopper, heated barrel, nozzle, and the mould.

The polymer is fed to the machine through the hopper.  In the hopper they can be mixed with additives, such as colour pigments, anti-static agents, and slip agents.  The polymer mix enters the injection barrel by gravity though the feed throat. 

Upon entrance into the barrel, the rotating screw (screw apparatus is shown above) moves the polymer forward towards the nozzle.  At the same time the polymer is heated both by the shearing action of the screw and the electrical energy from the heater bands which surround the barrel.  When enough polymer has been plasticized (melted) in this way the whole screw itself moves forward like a piston and injects the melt into the mould through the nozzle. To allow the screw to change function from a screw to a piston, there is a non-return valve at the tip of the screw which allows plasticized polymer to pass through it during screw-back, and shuts off as soon as it starts to inject so that material cannot flow back along the screw.

The mould is the part of the machine that receives the plastic and shapes it into the component.  It has cavities cut into it which are the shape of the component you want to produce. The mould is cooled constantly to a temperature that allows the polymer to solidify and be cool to the touch.  The mould plates are held together by hydraulic or mechanical clamping force.  Once the plastic inside the cavities cool and solidify, the clamping unit pulls the moving half of the mould back, the mould opens and the components are ejected out.