EPS Expanded PolyStyrene is one of the most widespread foams used in our society. It is the white picnic cooler foam that you see eggs and stereo gear packed in. It is the peanuts in your mail order package. It is the white food carton or drink cup you get at a carry-out. It is cheap to manufacture, light, and has almost ideal crush characteristics with no bounce-back to make the impact more severe. It can be reliably manufactured with reasonable quality control procedures.

EPS is formed by placing polystyrene beads (granules) about the size of table salt in a pressure mold shaped like the helmet liner and expanding the bead from 2 to 50 times with a blowing agent like pentane under pressure and heat. The beads expand to form the cells and fill the mold. The cells are tightly bonded–under ideal conditions. Under poor conditions the steam/pentane temperature is not just right or the pressure is off a little and the foaming may not be uniform, or there may be hidden recesses where the granules did not expand correctly. (A helmet liner with such a recess “rattles” with unexpanded beads inside when shaken.) The foaming is often done by a “foam shop” outside the manufacturer’s plant, and the challenge for helmet quality control programs is to design testing that will catch any problem liners. Foam density is measured by weighing the liner, then placing it in water and weighing the amount of water displaced, and comparing the two weights.

The version of EPS you see in a helmet is several quality grades above what normally is used for picnic coolers. It can be tuned to produce optimal crush for a given impact level by varying the density of the foam cells. Additives can provide increased cell adhesion, cutting down the splitting of helmets in very hard impacts. (GE’s GeCet foam is an example of a product that adds a resin to make the EPS more resistant to cracking.) Additives can also be used to color the foam, although they may change the impact characteristics. Manufacturers can add internal reinforcing using nylon, carbon fiber or various types of plastics to reduce cracking as well, enabling designers to open up wider vents and still pass the lab impact tests.

Molding techniques for EPS have evolved over the half century that it has been used for helmets, enabling manufacturers to push the envelope by producing a helmet liner with harder and softer foam in layers (variable density foam). That lets the softer inner layer of foam crush in a lesser impact, where harder foam would just resist and pass the energy on to the head. The harder outer layer is still there when the soft foam bottoms out to keep managing the energy in a hard impact. Over the years there have been several helmets that used this technique.