Abstract: Rigid thermoplastic articles having sections of different thickness are extrusion welded together, the part of the thicker section to be contacted by the extruded bead being preheated before being thus contacted, to a temperature higher than that of the thinner section. The thicker section may be heated above its softening point, e.g. by the tip of the extruder, while the thinner section remains below its softening point until contacted by the extruded bead. The method is useful for welding thin sectioned profile extruded board to thicker sectioned header pipes during manufacture of heat exchangers such as solar panels.