If you can use a 3D printer, you’re not limited to making things out of hot plastic. If you have the imagination to design the inverse of the shape you need (or if you can locate the right setting in your design software), you can use your printer to create moulds for papier mâché, food (as long as you’re using food-safe materials)… anything really, including concrete.
MattyGyver92 has used this fact to design and build this lava lamp.
It’s a blend of space-age Soviet visuals with brutalist raw concrete, and we love it. There’s no microcontroller involved; everything is old-fashioned analogue electronics but, despite that, it’s still a complex build. Did you know, for example, that the wax used in lava lamps is paraffin wax, which floats in water, and so, in order to use it, the maker is obliged to add tetrachloroethylene in order to make it more dense?