Abstract: Apparatus for constraining the flow of molten metal from a tundish to a mould along the inner periphery of a tube mounted between them. A vertically movable stopper rod is seated at its lower end in a nozzle at the entrance to the tube the end being so designed as to cause an annular discharge of metal into the tube upon the stopper rod lifting from its seating, gas being injected through the stopper whereby to generate a gaseous void within the interior of the tube to promote the maintenance of the metal flow path of along the length of the inner periphery of the tube.