EPS is produced in a three stage process:
Upon contact with steam the pre-foaming agent found within the polystyrene beads (usually a hydrocarbon such as pentane) starts to boil and the beads are expanded to between 40 to 50 times their original volume.
After expansion the beads undergo a maturing period in order to reach an equilibrium temperature and pressure.
The beads are placed within a mould and again reheated with steam. The pre-foamed beads expand further, completely fill the mould cavity and fuse together. The beads are moulded to form boards, blocks or customised products.
There are two moulding processes for EPS.
Block moulding produces large blocks of EPS which can then be cut into shapes or sheets for use in both packaging and construction applications.
Shape Moulding produces parts which have custom designed specifications. Electronic product packaging in particular is where shape moulded EPS is used extensively. When moulded, nearly all the volume of the EPS (~98%) is air.