There is provided an improved valve mechanism for use with a casting or injection mold which is quick acting and simple to construct. The valve includes an inlet coupled to the mold cavity and an outlet for venting gas from the mold cavity. The valve further includes a closure oriented perpendicularly to the inlet and movable between open and closed positions for closing off the outlet. The closure has a central axis and the valve is configured to move the closure into the closed position by directing the melt passing through the inlet to impinge substantially coaxially upon the closure so as to transfer a majority of the momentum of the melt to the closure.