Injection Plastic Moulding – The Process and its Issues

Even though you possibly don’t realise it a lot of your world is shaped by plastic injection moulding. This form of manufacturing is used to create a whole variety of items and goods that we use on a daily basis. So, how does it work?

Injection moulding machines make items from plastic and the manufacturing process involves a large number of plastic pellets being placed inside an injection moulding machine. These pellets are melted down in a chamber of the device and then are injected into a variety of moulds where the plastic cools and hardens into the shape of the item that you wish to create. This plastic cools and then is removed from the mould, where it is then cleaned, polished and fixed up. The excess parts of plastic are removed and recycled for later use.

The Process

The initial step of the process involves feeding the plastic pellets into the machine and then these are fed into a barrel. The barrel is heated and also has a second part known as a reciprocating screw. This device will crush the pellets and makes it easier for the plastic to be liquefied.

At the fore of the barrel the screw is used to push the now liquid material forward and in essence injecting it through a nozzle into a space shaped into the end item or a mould. This mould is cool and so encourages the plastic to harden. Moulds are held together by moveable plates which are attached to a hydraulic piston – this will help to keep a certain amount of pressure on the area or mould. This need to be tight to prevent any of the plastic escaping from the mould area – if this happens then the mould does not end up in the desired shape.

There are a whole host of issues that can come about during the process and cause issues for the end result. These can include anything from brittle parts, to burning of plastic, to deformities. These issues can come about when the moulds in the machine are too cool or the heat of the plastic in the barrel is too high.


Other problems occur if the reciprocating screw is not rotating at a fast enough paces or the plastic is in the barrel for too long and ends up burned. Other issues occur if the mould is not clamped properly, if the temperature is too high at melting or the mould is not even on both sides. Also, if too much resin ends up in the chamber beforehand the mould will be too brittle and this causes issues for the end result. The only way to ensure this does not happen is to regularly test the mould to ensure that it’s all working well and running smoothly without issues.

However, injection moulding has shown that though issues can be caused by it, it is a fantastically accurate manner with which to create and manufacture items. It’s fast, great for mass production or unique prototypes and also low in cost and one of the most popular techniques out there.

Cormac Reynolds is lover of manufacturing techniques. He has worked in the manufacturing area for a number of years.