An improvement in the process of producing a double-walled synthetic plastic tube having a concentric outer tube and an inner tube wherein an outer tube of molten synthetic plastic is extruded from an annular die and passing to a mold cavity formed by a divided mold having two cooperating endless recirculating trains of mold halves which close to complete the mold cavity and an inner tube of molten synthetic plastic is extruded from a die orifice located in the interior of said mold which improvement comprises introducing a first gas into the space between said inner tube and said outer tube before said synthetic plastic has fully hardened, said gas being at a pressure above atmospheric and at a temperature at or above the softening point of said synthetic plastic and permitting said outer inner tubes to cool; an apparatus for carrying out the process comprising an extruder having a first die disposed over a first mandrel to define a first annular space through which molten synthetic plastic can flow en route to a mold cavity of the type described, a second annular die concentric to a first mandrel and a second mandrel concentric to said first die, said second mandrel and said second die disposed to define a second annular space through which additional molten synthetic plastic can flow wherein a first gas channel entering the region between the first annular space and the second annular space is provided to supply fluid gas between the outer wall of the inner tube and the inner wall of the outer tube as the tubes emerge from the respective dies.
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