said foaming die device comprising:

a stationary die having an inner die surface;

a movable die having an inner die surface, said movable die movable into engagement with said stationary die to define a foaming space between said inner die surface of said stationary die and said inner die surface of said movable die;

said stationary die having an inner region adjacent to the inner die surface, wherein said inner region faces said one side of said three-dimensional trim cover assembly when the three-dimensional trim cover assembly is placed in said foaming space;

an injection element for injecting said foaming agent into an inside of said three-dimensional trim cover assembly, said injection element engageable with said foaming die device and said three-dimensional trim cover assembly when the three-dimensional trim cover assembly is placed in said foaming space subsequent to said movable die being engaged with said stationary die, thereby allowing said liquid foaming agent to be injected from said injection element into said inside of the three-dimensional trim cover assembly;

a closure element for insertion into said opening of said three-dimensional trim cover assembly to close said opening and prevent leakage of said liquid foaming agent therethrough, said closure element being rotatably provided in said inner region of said stationary die substantially at a point for alignment with said opening when the three-dimensional trim cover assembly is placed in said inner die surface of said stationary die, with such an arrangement that the closure element is rotatable relative to said point at said inner region of the stationary die, thereby allowing the closure element not only to be directly inserted into said opening prior to said three-dimensional trim cover assembly being placed in said inner die surface of said stationary die, but also readily removable from the opening when the three-dimensional trim cover assembly is removed from the foaming die device subsequent to said foaming process.