I once built a very simple one in my backyard using a shock absorber from a car as the barrel, which just worked like a syringe with no screw. It was driven by

I once built a very simple one in my backyard using a shock absorber from a car as the barrel, which just worked like a syringe with no screw. It was driven by a threaded rod powered by an electric drill. The heater was a spiral stove element or a hot-air paint stripper or both - I forget which. The mold was a stack of aluminium plates that I'd cut with hand tools and bolted together. It was very fiddly to operate but worth the experience! Driving a car without shock absorbers is also a real experience! With alternating braking and accelerating, you can build up an oscillation that's big enough to bounce the wheels off the ground with almost no forward speed.