Foam vs TPU - Why spend more on superior materials?
As a naturally curious engineer, I've always questioned convention. In the case of helmets, the obvious question is, "Why foam?" Expanded Polypropylene (EPP) and Vinyl Nitryl (VN) foams are the standard material of choice in hockey helmets as they are inexpensive and relatively light. However, they lack durability in a sport that demands it. Foam compresses and molds to your head over time which may feel good, but it also changes the protective characteristics of the helmet. The sweat and hardening of foam over time also have adverse implications. Foam is an isotropic material and therefore does little to reduce angular accelerations that correlate to traumatic brain injuries. Finally, foam is an insulator retaining heat and leading to that odd and somewhat ironic sensation of sweating profusely while being in a freezing ice rink.
At KAV, we zoomed in early one class of flexible polymers called thermoplastic polyurethane (TPU). As a class of materials, their high modulus of elasticity translates into the ability to create energy-absorbing structures. They're resistant to a variety of classes of chemicals, and because they're non-porous much easier to clean. The ability to use them in additive manufacturing creates the foundation for not only a more protective helmet but also one with better heat dissipation.
TPUs do have two downsides:
1. Our blend of TPU is more expensive to manufacture and fabricate
2. As a class of materials, TPU is much denser than foam
Raw TPU materials is more expensive than foam and we test each spool of polymer before fabricating a custom helmet, which improves the consistency of the end product. The foams we sampled varied in density by 10% or more. In fact, while foam costs between $2-3 per kg we spend upwards of $80 per kg for our proprietary blend of TPU. So while a $300 hockey helmet has a $1 worth of foam in it, the KAV helmet has roughly $35 of energy-absorbing polymers to protect your head. It's not all that different than buying the top of the line carbon fiber stick and comparing it to a wood stick, except our helmets don't break!
As for creating a lighter helmet and overcoming the density disadvantage of TPU - you'll have to wait for the next post.