in the extrusion of plastic pipe wherein the pipe is in a self supporting but semi - molten state on exiting a die orifice , the inner surface of the extrudate is cooled by a device which applies a suction for establishing flow of a cooling fluid which contacts the inner surface of the extrudate . in a further embodiment the flow of cooling fluid established for cooling the extrudate additionally cools a cavity within a mandrel of the die orifice .