EPS beads are processed in three essential stages;
- PRE-EXPANDER
During pre-expansion by direct steam, the polystyrene bead softens, and the liquid pentane vaporizes creating microscopic closed cells within the polystyrene.
Steam Generator.
The density is controlled by adjusting the steam pressure or the EPS beads residence time in the steam expansion chamber.
As soon as the prefoamed beads cool, the blowing agent and the water vapor begin to condense. At this stage, a partial vacuum within each cell is developed. - MATURING
Before the pre-expansion beads can be used in a molding process, this partial vacuum must be replaced by air which diffuse in the cells of the beads from the atmosphere.This process is called maturing or ageing.
Whilst the beads are absorbing air, they are losing blowing agent and the rate of this diffusion is temperature dependent.
Maturing process generally takes several hours (generally 6 to 24 hours) depending on the bead size, expanded density and exposed temperature. - MOLDING MACHINE
During molding, the expanded polystyrene beads are injected into a mold generally made of aluminium alloy. The EPS is heated by injecting steam under pressure in the mold chambers, which penetrates into the mold cavity through vents (core-vents).
The residual pentane will again vaporize and the EPS beads will expand against each other and fuse together. Before the mold is opened, the heat must be removed. The rate of this cooling is mostly dependant on steam diffusion through the EPS molded part. Several factors affect this diffusion, namely molding machine vacuum suction assistance, number of core-vents, molded part thickness, voids between the EPS beads and bead size.
