The invention relates to a plastic closing device comprising a bottom part, a closing cap, and a cutting ring which is mounted inside the neck of the bottom part so as to be helically movable. A driving cam (16) which is axially disposed in the closing cap acts upon the non-continuously circular cutting ring when the closing cap is unscrewed such that the cutting ring perforates and cuts through the receptacle in a helical cutting movement. The cutting ring forms a ventilating concavity (6) which cooperates with the driving cam (16). The non-circular cutting ring, the bottom side of which is provided with a sharp cutting edge extending at an obtuse angle relative to the bottom edge of said ring, performs a helical cutting movement similar to an advancing knife when the closing cap is unscrewed such that the receptacle wall is easily cut by applying a minimum amount of force because a new sharp point constantly attacks the packaging material, thus preventing shredding. The recess (6) that is provided on the non-circular cutting ring acts as a ventilation duct (26) when the content is poured, obtaining an extremely steady, non-gushing pouring action.