The material is EP1150, a 50 durometer urethane from Eager Polymers. The molds are made on a FDM 3-D printer. The mold is made in 3 sections to facilitate removal of the finished wheel. I also printed a small funnel to facilitate pouring the urethane into the small crevice (it seems that a shorter sidewall holds up better). The performance wheel is a COTS item, and I wrap the wheel with a couple of wraps of fiberglass drywall tape before overmolding to help adhesion and strengthen the tread.
