Generally speaking, there are two types of EPS pre-expander machine, which were continuous pre-expander machine and batch pre-expander machine. The main difference is that continuous EPS pre-expander machines expand polystyrene through continuous cycle operations and that batch EPS pre-expanders are performed in batches. Another difference is that continuous EPS pre-expander machines are allowed to achieve bead densities between 40g/litre and 15g/litre in the first expansion, but this kind of machines can also be integrated with the second expansion unit to achieve lower densities up to 10g/litre. The batch pre- expanders are specially designed to sacrifice continuity to obtain maximum uniformity and a wide range of material density,therefore in the first expansion, densities level are allowed to be between 100g/litre and 12g/litre. Also, the batch EPS pre- expanders can be integrated with a second expansion unit to achieve lower densities up to 8g/l.