A resin compound of polystyrene and styrene copolymers, blowing agent, and other additives is known generically as expandable polystyrene or EPS. Pentane is frequently used as the blowing agent. EPS can be used to produce low density foam products. The largest applications for EPS are thermal insulation, disposable drinking cups, and cushioned packaging materials.

EPS is typically supplied as beads having a diameter from 0.01 – 0.1 inches. These beads are mostly made by suspension polymerization, but are sometimes larger particles are produced using pelletization. The choice of particle size is based on the desired characteristics of the foam product. The wall thickness of the resulting foam will be equal to the diameter of the starting particle. The starting particle size can influence the final product in other ways as well. For instance, lower densities foam is easier to obtain with larger particles. Small particles, on the other hand, make it easier to uniform fill a part.

Many properties of foam are strongly influenced by the density of the product. Some of the properties affected by foam density include energy absorption, resistance to heat flow, stiffness to weight ratio, buoyancy, and cost per volume. Because these properties functions of product density, manufactures have fine control of the final product simply by adjusting the particle size of the input resin and some processing changes. Often, there is no need for any retooling or other equipment changes.