The primary raw material used for manufacturing EPS geofoam is called a polystyrene polymer.

The word Polymer comes from the Greek "poly" meaning many, and "meros", parts or units. A polymer is a group of many units. You combine many "monomers" to create a polymer. Polymer is often used as a synonym for "plastic", however many biological and inorganic molecules are also polymeric. Plastic more commonly refers to the way a material behaves under applied forces, or behaves when it melts and flows.

Polystyrene is a "thermoplastic" that can be heated and formed, then re-heated and re-formed repeatedly. The shape of the polymer molecules is generally linear, or slightly branched, allowing them to flow under pressure when heated above the effective melting point.

Polystyrene is an "amorphous" polymer with bulky molecular chains or large branches or functional groups that tend to be stiff and will not fold up tight enough to form crystals. Other common amorphous polymers include polycarbonate, acrylic, and ABS. Amorphous polymers have low shrinkage, good transparency, gradual softening when heated, average to poor chemical resistance, high surface friction, and average to low wear resistance.

Commodity polymers (EPS geofoam is a good example) have relatively low physical properties compared to engineering or performance polymers. They are used for inexpensive or disposable consumer or industrial products or packaging. They have limited stress and low temperature resistance, but are well suited to high volume production.

Polystyrene is formed by chemical reaction in large vessels (shown at left) under heat and pressure. Other ingredients are added to control how the polymer is formed and to produce the proper molecular length and desired properties. This chemical process is called "polymerization".

An expanding agent (typically pentene) and a fire retardant are added to the polymerization process during manufacture to produce "modified" resin beads used for manufacturing EPS geofoam. The polymerization reaction begins with the addition of a catalyst that breaks a double bond between molecules and creates two bonding sites. These sites quickly bond to other monomer or polymer molecules. The polymerization reaction continues until the catalyst is used up. The length of molecules is determined by the number of monomer molecules which can attach to a chain molecule before the catalyst is consumed and all molecules with available bonding sites have reacted.

Resin beads manufacturers are able to supply EPS geofoam manufacturers with a range of resin bead diameters and additives. Resin beads that contain pentane are typically shipped in corrugated boxes with a specially designed liner as required by the Federal Department of Transportation.

EPS geofoam is manufactured from modified resin beads that are typically 0.01 in. to 0.1 in diameter. A finite amount of resin beads is added to an expansion vessel. The resin beads are expanded into "pre-puffs" by injecting steam that softens the polymer and partially or completely vaporizes the expanding agent. The photograph below shows the relative size difference between resin beads and expanded resin beads, or pre-puffs.

The amount and duration of heat application determines the density of the pre-puff and the amount of residual expanding agent left in the pre-puff. The figure shown below shows measurment of the pre-puff density.

Automatic controls determine when the desired pre-puff size or volume has been reached. The expanded beads are then released into a bed dryer to remove moisture that is detrimental to the molding process. The pre-puffs are then stored in fabric storage bags (shown below) to allow them to stabilize to atmospheric temperature and pressure.

Most thermoplastic polymers, including EPS geofoam, can be recycled. Thermoset polymers, however, can only be recycled for use as inert filler placed within another material. Manuacturers of EPS geofoam typically recycle by grinding and mixing a pre-determined percentage of industrial scrap, called regrind, with virgin pre-puff material prior to block molding. The percentage of recycled material is typically limited to 5 percent or less for EPS geofoam, resulting in a final product with predictable engineering properties.

The block molding process involves forming the pre-puffs into a solid block mass. An example of a modern block mold is shown below.

First, a pre-determined amount of pre-puffs are injected into the block mold. Steam is then injected into the mold under controlled pressure to soften the polymer structure of the pre-puff. The heat from the steam and the applied pressure causes the pre-puffs to further expand. Since the block mold is a confined environment, the expanding pre-puffs reduce the void space between individual particles and fuse (bond) together to form an expanded polystyrene (EPS) block. The block is then ejected from the block mold onto a conveyor system (photograph below) where it is commonly measured for density and/or trimmed using a hot wire cutting system.