A method and an apparatus form aluminium screw capsules with a safety ring made of synthetic plastics for tamperproof closing of containers. An aluminium capsule body is inserted into a moulding cavity of a mould conformed and arranged for the overmoulding of a safety ring on an edge of an annular wall of the capsule body. An annular sealing zone is defined for the synthetic plastic material between a first surface of the capsule body and a mould first portion inserted into the central cavity of the capsule body to form a safety ring anchored to the capsule body.