I recently bought a bag of InstaMorph Moldable Plastic, and spent some time with it.
You dunk the stuff in hot water and pull it out and mold it with your hands. It hardens after 2-3 minutes at room temperature. If you still need to work on it some more, dunk it in hot water again.
I decided that wanted to make some Spartan spears for some skeletons, in their size. The shafts are made from wood, with real leather wrapped around it. For the spear heads, I used InstaMorph.
It doesn't hold detail as well as Sculpey, but it has a HUGE advantage over wood, styrene, ABS plastic, putty, Sculpey, etc... it's almost unbreakable. The fragile part of a spear head where the spear head meets the socket is actually very strong and isn't going to snap off. The sockets can be made hollow to slip over the wooden spear shafts (like real life). Bows can be made completely functional, and can shoot arrows a few feet. There is a slight flex to InstaMorph, which can be tapped as an advantage. The texture of the weaponry is a little rough. Don't expect the elegance of Valyrian steel swords for the Lannisters using InstaMorph. The results are more akin to the rough ironwork made by a field blacksmith.
Examples of weaponry made with InstaMorph:
Functional bow and arrows made from InstaMorph:
To dye the stuff, I used chalk pastels. Basically, I shave off some powder from a chalk pastel stick and knead it into the plastic while it's warm. The color possibilities are endless, since a huge set of chalk pastels (36 colors) can be bought at Michael's for $4.99. Since the colors become part of the plastic, there is no paint rubbing off. The colors are permanent.
The moldability of InstaMorph varies based on the water temperature you use. If the water is at a rolling boil, then InstaMorph becomes VERY moldable... my preferred temperature to knead in colored pastel powders. At the "barely boiling" water temperature, you can put a bend on an already-molded piece, or flatten a tip. I'd say that the greatest risk for burns is from the boiling water, not the InstaMorph itself. I use tongs to pull it out of the hot water, then pat-dry it and it's reasonably safe to mold by hand. I wouldn't recommend it for children under 12.
It gets better... InstaMorph BINDS TO PVC! So, whatever
soft PVC heads you have... you can add hair, moustaches, beards, horns,
etc. and the stuff will bind itself to the heads, or hands or boots,
etc. It could possibly be used to repair body and joint cracks.It
doesn't stick to all types of plastic, so you'd need to experiment and
find out what it does/does not bind to.
In the hands of skilled sculptor, an endless array of plastic action figure accessories can be made. It's not quite like having your own home plastic injection molding machine or vacuform kit, but it succeeds in placing some awesome plastic molding capabilities in the hands of your average kitchen hobbyist.