Common packaging styrofoam is actually puffed polystyrene, the same polymer that’s used to make things like CD cases and plastic model parts. When acetone is used as a solvent, the expanded polystyrene will easily dissolve and you’ll be left with a liquid acetone/polystyrene solution. As the acetone evaporates, the polystyrene becomes increasingly viscous: first pourable, then moldable, and finally solid polystyrene plastic.
Solid polystyrene will dissolve in acetone as well, but with the vastly larger surface area (due to all the little gas bubbles) styrofoam dissolves much faster. That, and it’s readily available if you have a basement full of old packaging materials.
There must be an easy method for pouring or spin-casting custom plastic parts or action figures using dissolved polystyrene. Have any of you ever tried this or have any ideas on the subject? Please let us know in the comments!