Prepare nanofoam by: (a) providing a mold (10) with a mold cavity (12) defined by mold walls defining a sealable port (32); (b) providing a foamable polymer mixture containing a polymer and a blowing agent at a pressure at least 690 kilopascals above the saturation pressure for the polymer and blowing agent; (c) introducing the foamable polymer mixture into the mold cavity (12) while maintaining a temperature and pressure at least 690 kilopascals above the saturation pressure and controlling the pressure in the mold cavity (12) by expanding a wall of the mold; and (d) releasing pressure around the foamable mixture by moving a mold wall (20) at a rate of at least 45 centimeters per second, causing the foamable polymer mixture to expand into nanofoam having a porosity of at least 60 percent, a volume of at least 100 cubic centimeters and at least two orthogonal dimensions of four centimeter or more.