The conversion process is carried out in three stages:

Pre-expansion  The tiny spherical polystyrene beads are expanded to about 40 times their original size, using a small quantity of pentane (typically 5% by weight) as a blowing agent. This process involves the heating of beads, using a flow of steam, which causes the blowing agent to boil and thus a honeycomb of closed cells is formed.  

Final Forming  In this final stage the stabilized pre-expanded beads are transported to moulds where they are again subjected to steam so that the beads bind together. This produces large blocks (that are later sectioned to the required shape like boards, panels, cylinders etc) or products that have a shape and profile. In this form EPS is made up of 98% air.

Maturing When the steam is removed the residual pentane returns to liquid form and, as it cools, the recently expanded particles form a semi-vacuum in their interior and this must be compensated for by air diffusion. The pre-foamed material is therefore conveyed to large storage hoppers where it is left to stand for approximately 12 hours.