I discovered this stuff about 8 years ago when I was into making one off customs and was just teaching myself how to make two part molds.
I didn't want to invest in polyurethane and silicone and the rest to find out I couldn't use it, so I made two part plaster molds, and bought some of this stuff.
The first batch I bought was white. I got this blue stuff from a hobby shop in Japan. You just need to heat it to make it malleable.
It actually worked. I wet my finger to avoid getting it burned and pushed the molten resin all around the inside of the mold. Making a really tough hollow cast. You can throw it against a wall and it'll bounce back.
Since I discovered resin and got a pot - I didn't think i'd ever need thermoplastic. I've never once read about a toymaker using it.
Using a heated up pair of scissors as a sculpting tool, it's been possible to shape to plastic just enough to be a good filler. It's drillable and i've given the joints a good hard tug (I've got a 250 kilo grip, by the way) and they're holding up fine.
Heat it up enough and it's re-usable too. Which makes it great for small one part molds.