A cavity in a body of particulate moulding material has at least one rib-like formation for gripping an object received in the cavity. In the case of a metal casting mould a riser sleeve (1) is inserted into a cavity (9) in the top part of the mould and the cavity (9) is produced by means of a tapered former (5) the exterior of which has a plurality of radially spaced apart recesses (6) extending between the top and towards the bottom of the former and which has a negative taper from bottom to top. The depth of the recesses (6) is such that circles geometrically inscribed inside the base of the recesses (6) have diameters smaller than the corresponding mean outside diameters less three times the standard deviation of the riser sleeves (1) to be used. The recesses (6) form ribs (11) from the moulding sand and the ribs (11) grip the outer surface of the sleeve (1).