A closure cap comprises a base having at an inner side thereof an annular gland for receiving an annular seal. The annular gland has a bottom surface and an annular side surface depending from the bottom surface. A skirt depends from base and has an internal fastening thread for attaching the cap to a correspondingly but externally threaded body. Additionally, the annular side surface of the annular gland has a venting thread depending from the bottom surface of the gland to enable venting of air that may otherwise be trapped behind the annular seal when fitted in the gland. Also disclosed is a method of molding the cap.