An apparatus for mixing and molding a chemically foamable reaction mixture is comprised of a mold with a mixing chamber which has its outlet aperture in a wall of the mold and a displacement piston mounted in the opposite wall. An ejector piston is displaceable within the mix chamber and may be so channelled as to serve as a control piston. This ejector piston is displaceable from a position wherein the injection apertures lie between it and the mold cavity to a position wherein its end surface is flush with the mold wall. The displacement piston is so mounted that it may extend across the mold cavity and intrude into the outlet aperture of the mixing chamber. This displacement piston has a diameter smaller than the diameter of those portions of the mixing chamber into which it protrudes so that when this piston is within the mixing chamber an annular cavity is formed between it and the side walls of the mixing chamber. It is withdrawable to a position flush with the mold wall it protrudes through. This piston may comprise a hollow sleeve and a core which together form a cavity when the sleeve is disposed in the mixing chamber. This piston may be mounted in a mold wall insert which can be moved toward the opposite mold wall to locally decrease the mold cavity width. The mix chamber and displacement piston may also be mounted in opposite walls of a sprue system which communicates with a mold cavity or mold cavities.