Pre-expansion:
The Pre-expansion phase of manufacturing is simply the swelling of the small bead to almost 50 times its original size through heating and rapid release of the gas from the bead during its glass transition phase.
Pre-expanders
Dr. Fritz Stastny did not only invent EPS in 1949, he is also known as the father of pre-expanders. His original constructions of continuous and discontinuous pre-expanders, which he described in a device patent, were already designed in the form of standing and insulated agitator tanks. The design of the agitator in cooperation with the breaker rods guarantee an evenly loose bead bulk without hardly any agglomerations. In order to achieve that the raw material is evenly supplied with heat energy, steam flows through the tank from bottom to top.


Continuous Pre- expanders
Continuous pre- expanders are operated without pressure and are thus equipped with an open- top agitator tank in which raw material is continuously fed from the bottom by an adjustable screw conveyor.. Steam is "also fed continuously into the pre- expanding room through openings that are positioned shortly above the tank bottom. An agitator and fixed breaker rods make it more difficult for bead clusters to develop; they keep the beads in motion and make sure that al ready pre-expanded beads of lower density move to the top of the bead bulk and fall into a shaft through a height- adjustable discharge opening. Pre-expanders for the so- called "second pass expansion" of already expanded materials, are equipped with an especially large feed screw in order to transport the already expanded material to be steamed again. All pre- expanders have in common that the completely expanded beads trickle into a so- called fluid bed dryer where they are dried and stabilised before they are transported into the storage silos.
Continuous pre- expanding
First of all, a screw conveyor or a bucket conveyor transports the material as carefully aspossible into the store tank. A conveying screw continuously feeds the material into the pre- expander where steam flows through it. The continuous pre- expansion process, can be achieved after a second or third pass. The capacity of a continuous pre- expander ranges from 200 kg to 3000 kg per hour.Here, the throughput capacity depends mainly on the type and quality of the raw material and the volume of the pre- expander.
Continuous pre- expanders show some disadvantages compared to discontinuous pre- ex-panders: first, in regard to the minimum densities that can be achieved, second, when it comes to the processing of fine- beaded materials and third, in the processing of lowpentane material. The major advantage of continuous pre- expanders, however, is their low investment. Moreover, they are simple to install and to operate. In the field of second pass expansion, however, discontinuous pre- expanders are superior.
Discontinuous Pre- expanders
These kinds of pre- expanders differ in some details to the continuously operating machines.
Pressurized pre- expanders are often equipped with tanks that are approved for a maximum operating pressure of 0.49 bar. Some series allow pressures up to 1 bar while special pre- expanders allow pressures up to 5 bar.
Different from continuous pre- expanders, the raw material is dosed in batches. It is important to measure the raw material exactly in order to achieve consistent densities. The best results are generally achieved by using gravimetric dosing systems. The beads are fed into the expansion tank via an opening in the lid of the pressure tank.

As usual, the steam flows through the expansion tank from bottom to top. A homogeneous distribution over the total floor space is important. This is why high- quality machines are equiped with wedge wires over the total floor space. The control of the steam pressure is especially important because it adjusts the temperature in the expansion tank. If EPS of a high density is to be pre- expanded, air is added to the steam flow. In this way, the temperature of the steam- air mixture is decreased and the heat transmission is drastically pressure) Numerous mechanical and physical characteristics of EPS expanded plastics are, among others, influenced by the expansion pressure. This makes the expansion pressure an important process parameter and thus it is used as a measured variable for the process control (e. g. generally in the case of block moulds). When standard material brands are moulded, steam pressures of pe = 0.5 to 1.3 bar are used. These pressures are applied to the mould cavity and the moulding material for a few seconds. When material brands remain stable under heat, are moulded, higher steam pressures are necessary depending on the type and density of the mould, which also requires corresponding machines, moulds and process conditions.