A mould and method for the injection-moulding of an assembly for creating a closable fluid connection, and an assembly. The assembly comprises a female element (20) and a stopper (30) which is used to close off a bore (27) in the female element (20). The mould comprises a core with a core (2) with a core body (3) and a head (4). Furthermore, the mould comprises a mould ring which is composed of a plurality of mould parts (10, 11) are designed in such a manner that the mould cavity for the stopper (30), is located at a distance from the mould cavity for the female element (20). The mould parts (10, 11) of the mould ring which lie around the core (2) together delimit an annular sealing surface (15), by means of which the mould ring, in a zone between the mould cavity for the stopper (30) and the mould cavity for the female element (20), bears in a sealing manner against the core (2) during the injection moulding, so that the mould cavities are completely separated from one another.