From Raw Material to Foam

The conversion of EPS beads into foam consists of three major steps [124]  

The continuous pre-foamer normally consists of an open stirred tank with baffles. The raw material is fed by an adjustable feed screw near the container bottom. Steam is introduced at the bottom of the pre-foamer. The EPS beads move slowly upwards during pre-foaming and discharge into the fluidized bed via the overflow. The foam density attained depends on the residence time in the pre-foamer, which is adjusted by the feed screw or the weir height. High foam densities ( 30g/l) require temperature control by air addition. For low bulk densities (special, 15 g/1), the material pre-foamed once is pre-foamed again, after temporary storage. 

Discontinuous pre-foamers are more costly, but because of the higher steam pressure, lower foam densities can be achieved. In Europe, more and more discontinuous pre-foamers are being used, the reason being the availability of low pentane grades for reduced pentane emission. 

As a rule, the molds are filled pneumatically. This can be accomplished with a blower centered over the filling injectors. The air blown in is removed through perforated walls, or through special devices. It is also possible to suck in the material by applying a vacuum in the mold. It is practical to add to the prefoamed material the foam accumulated during cutting, as reclaim rates up to 20% are possible without quality deterioration. 

Cooling is accomplished by applying a vacuum. Heat is removed intensively from the mold due to vaporization of water under a vacuum. 

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