The EPS raw material is transported as granulate to the pre-expander and heated there by wet steam. Hot water vapour - as medium - softens the polystyrene beads and activates the blowing agent released in them. This blowing agent evaporates and causes the beads to expand. Based on the raw material weight and the volume of the expanded material the density is adjusted. The pre-expanded EPS-beads are cooled down and stabilized in the fluid bed before a cellular wheel sluice transports them into the silos by a blower.
Fusing of finished products
During the subsequent intermediate storage the blowing agent diffuses from the pre-expanded beads and air gets in. This "packed air" is further processed in shape moulding machines or block moulds. For lower densities, EPS can be expanded again in a second cycle. For expandable polypropylene, EPP/HP pre-expanders for low-density foaming are available.