a plastic cap assembly has a base cap that can be installed on the neck of a container . within the base cap there is a pour spout that serves as a sealing pin , can be moved axially upwards and downwards , and is closed off at the top . on the base cap there is a screw cap that can be turned . the screw cap has a central opening in which the pour spout that serves as the sealing pin engages when in the closed position . the pour spout has an outside thread that engages with an inside thread beneath the central opening in the screw cap . the advantges of this cap assembly lie in its good sealing characteristics and in the fact that , provided with a suitable penetrator , it can be used on containers sealed with a membrane or a foil .