Materials such as polystyrene, nylon, polypropylene and polythene can be used in a process called injection moulding. These are thermoplastics - this means when they are heated and then pressured in a mould they can be formed into different shapes.

The DVD Storage unit seen opposite has been made in one piece using this process

An animation of an injection moulding machine is shown below. The product being produced is the DVD / CD storage unit seen opposite.

1. Granules of plastic powder (note the plastics listed above) are poured or fed into a hopper which stores it until it is needed.

2. A heater heats up the tube and when it reaches a high temperature a screw thread starts turning.
 

3. A motor turns a thread which pushes the granules along the heater section which melts then into a liquid.  The liquid is forced into a mould where it cools into the shape (in this case a DVD storage unit).
 

4. The mould then opens and the unit is removed.