Polymer sealing elements can be manufactured by open-cast moulding. This process consists of mixing the constituent components of the polymer together, and pouring the mixture into the mould at ambient pressure. Open-cast moulding is one of the simplest and most common moulding methods for thermoset polyurethanes, and is good for parts in relatively small quantities. In fact, the mould itself may be made from polyurethane.
Source: API RP 17L2, Recommended Practice for Flexible Pipe Ancillary Equipment, First Edition, March 2013. Global Standards