a plastic cap assembly with a base cap and a cap that is formed in one piece and may be installed on the neck of the container that is sealed with a foil or a membrane . a projection in the flat wall of the base cap has on its underside a penetrator with which the foil can be penetrated . the contents of the container can then flow through the hole in the foil from the penetrator to the pour opening . the cap assembly is simple to operate and to produce ; it supports the foil and in addition can be safety sealed . it is particularly well - suited for containers used for readily oxidizable foodstuffs such as ketchup , for example .