Abstract: A mould for forming a cap having a cylindrical skirt, the skirt being provided with at least one screw thread, a top and a tamper-proof ring linked to the free end of the skirt by breakable linking elements includes a movable part or mould top, a stripping part or stripper ring and a core. The stripper ring has, on the side facing the top of the mould, on the lower edge of the skirt and in at least two separate places, at least one notch or at least one projection so as to create a mortise-and-tenon linkage between the skirt and the stripper ring during moulding and demoulding of the cap.