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