A steel extrusion tool which forms extruded closed profiles containing one or more voids such as rectangular tubing. The tool generally consists of a die cap which generates the outer surface of the profile and the mandrel or core which generates the inside contour. Hollow or semi-hollow profiles are produced usually with either bridge, porthole or spider (taper seal) type dies or variants thereof. Extruded sections produced on such dies have seams or longitudinal weld lines, due to the metal flow around the web supports (bridges) that hold the mandrel. The latter determines the inside contour of the profile being extruded. After flowing around the supports, the metal is fused in a weld chamber before passing through the die (die cap) proper.