A toroidal core is produced on a rim 1 comprising a central portion connecting two annular lateral anchoring zones. The rim has an air passage orifice which can be selectively closed or opened and connected to a source of pressure, to the atmosphere or to vacuum. A conduit 3 passes through the rim and has a valve 30 which can be selectively closed or opened, the conduit extending to an orifice to which a pipeline can be connected by a suitable flange 13. A stretchable membrane 2 is mounted on the radially outer side of the rim and is fastened to it in air-tight manner on anchoring zones over the entire periphery of said rim so as to define a closed space on the radially outer side of said rim between the rim and the membrane. A fluid 6 fills said closed space to overflowing, said membrane being pressed against a mold 5. After filling, the membrane is pressed against the mass of filling material under the effect of the vacuum maintained in said closed space, thus producing a substantially rigid core supported by the rim 1.