A mould assembly (60) and a method of use in injection moulding of a part having a functional cavity therein. The mould assembly has a recess (72) to receive a hollow tubular insert (48) intended to be placed in the mould assembly before the injection of a mouldable material into the cavity and to remain in the part after moulding. A gas supply arrangement supplies pressurised gas to the hollow tubular insert at a selected stage during the injection process whereby to form the functional cavity in the part.