I came across this and I know it’s a few years old, but it’s still really cool…
JESTER of J6Studios documented a toy project he did using Smooth-On Smooth-Cast 325 which is a castable liquid polyurethane. After designing his toy in sketches, he built up the form in clay and created silicone molds for molding the polyurethane.
This is a bit different from the kind of rotomolding used to make the kids toys you’re used to seeing in your back yard, but the design approach is pretty much the same. The big difference is that this uses a chemical process to convert a liquid material (the Smooth-Cast) to a rigid part using a catalyst. Traditional rotomolding uses heat to melt (actually sinter if you want to be really accurate) a powdered plastic using heated molds. But in both cases you do need to rotate the molds (thus the name) while the material distributes itself around the mold.
There’s lots of images showing his technique. Enjoy.