Abstract: A plastic closure for use on a container having an externally threaded neck is disclosed wherein the skirt portion of the closure cap includes an internal structural thread formation. The structural thread formation comprises a helical thread ramp portion which is configured for load-bearing engagement with the thread of the associated container, and further includes means for reinforcing the thread ramp portion, in the form of a plurality of spaced-apart reinforcing gussets. The provision of the structural thread formation desirably facilitates high-speed molding of the closure, and desirably acts to reduce the quantity of plastic material required for molding the closure.