Injection moulding research:

Injection moulding is an industrial process where a plastic is fed into the machine, carried along the screw that rotates around in order to travel through, where it gets to the heater. In the heater, the plastic is melted and carried along further to the end (being the mould cavity) where the plastic which is now liquid is injected into the mould, and then cooled in order for the plastic to become solid and the desired shape (depending on the mould used).
FYI, the diagram below is American. I spell it as "mould" because that is how we spell it in England before anyone screams at me.

Now you may be wondering,what is a mould used for specifically? A mould consists of 2 plates which are put together. One is the injection plate, the other the ejection plate. The plastic is fed into the injection plate while the ejection plate is attached. The plastic will solidify, then the ejection plate is detached in order to take out the made product. If both moulds do not fit together perfectly though, there will be a split line between the produced product showing where the moulds didn't fit together precisely.