the neck opening of a filled plastic bottle is sealed by a sealing unit ultrasonically welded to the top of the neck . the sealing unit can be a one - piece member having an upwardly projecting hollow portion arranged to be broken off to provide an opening for dispensing the contents of the bottle . alternatively , the sealing unit can be composed of an inner cap , an outer cap and a sealing disc interposed between the caps , this unit being arranged to be pierced by a cannula for withdrawing the contents of the bottle .