SPAN-PACK® particles contain a volatile blowing agent . When heated with steam above 78ºC / 172ºF the resin begins to soften (glass transition temperature) and the volatile  blowing agent causes expansion creating a multitude of small cells. Normal processing temperature is between 92ºC / 198ºF to 100ºC / 212ºF  Use of saturated steam is  highly recomended. Please check your steam quality.

Exiting the expander the foam cools and the foaming agent condenses creating a vacuum inside the cells. At this stage the foam is very sensitive  and easily compresible. Cells are permeable to atmospheric air that will  diffuse slowly into the cells balancing the preassure. This stage is called  the maturing period and can last 24 to 48 hours depending on density and wall thickness of the cells.

After maturing cells contains air plus the condensed blowing agent that makes further expansion possible.  SPAN-PACK® can be expanded up to three times. The final product will show excellent features.

As the beads are warmed by the steam to above the glass transition temperature of the material, they soften and the blowing agent boils at a large number of nucleation points, forming cells which grow so that the whole bead is foamed throughout. Typically there will be hundreds of cells formed within each bead, sometimes even more. The key variables in prefoaming are the steam pressure (temperature), the amount of dilution air and time – some preexpanders have a level sensor which stops the expansion when a desired volume has been reached.

Following expansion, the beads (now a ‘prepuff’) are dried and discharged and allowed to mature for a period of several hours (often overnight or even longer). This allows them to cool and the cell walls to become rigid, able to support the negative pressure once the residual blowing agent has condensed. As maturing progresses, air diffuses into the beads, and they become stable enough to be processed further. It is well known that for a given bead type a lower final density can be reached in two or more expansion stages than can be achieved in a single steaming. In the case of multi-stage expansion, maturing is required between expansions – usually these expansions are individually less severe than a single-stage process (lower steam 8 Expandable Polystyrene (EPS) – A Mature Technology? pressure, or shorter steaming time), but ultimately result in a lower foam density.

Continuous pre-expander:
Continuous pre-expanders are cylindrical vessels that operate at atmospheric pressure, thus the temperature of the chamber is limited to 100°C. The raw material is introduced continuously at the bottom of the vessel by a screw feed and the pre-expanded beads leave the machine through an overflow chute that is adjustable in height. Steam is introduced continuously at the bottom of the vessel. The screw feed rate, steam pressure and the height of the overflow chute allow control of the density of the prefoamed beads.

As the temperature inside the vessel increases with the introduction of the steam, to between 90 and 100°C, the polystyrene softens and the vapor pressure of the pentane increases. These two processes cause the beads to expand by up to 50 times their original volume and the formation of the foam structure of closed cells within the beads. This cell structure contributes to the excellent insulation and mechanical properties of this foam material.

Usually such shrinkage or partial collapse of the foamed particle results in the formation of a plurality of folds occurring in the cell walls and struts of the expanded styrene polymer. Such folds are referred to as "accordion folds". At least on a short term basis, such folds resemble someneewhat the folds occurring in the bellows of an accordion.