Very small hollow metal spheres
Tiny metal spheres are needed for tiny ball valves and tiny ball bearings, which are needed for all kinds of miniaturized machines. Hollow spheres are lighter, and thus have less inertia, and thus can be made to move faster in these very small applications, where response time is often critical. But how do you make a hollow metal sphere 2mm across? Turns out you can do it with one of the lost foam processes I’m always going on about. Tiny styrofoam beads are first coated with fine metal powder and a binder, then heat-treated to evaporate both binder and bead, leaving only a fragile hollow metal powder shell, which is then sintered into a continuous shell at higher temperature. The sintered shells can then be polished in a tumbler to the same exterior finish as regular ball bearings.
