a screw plug self - taps itself into an open filler stem of a thin plastic cylindrical centrifuge tube to press an underside of the plug against the annular top surface of the stem after the solutions to be centrifuged have been placed in the tube to seal the tube . a support crown may support the stem during the screw plug insertion into the stem and supports an upper portion of the tube adjacent the stem when the tube is inserted in a centrifuge rotor . auxiliary seals may also be provided to ensure sealing of the tube stem . an annular fractionating plug is screwable into the tapped aperture in the stem after centrifuging , to remove the centrifuged tube contents .