Extrusion of laminate pipes

An extrusion assembly for extrusion of a triple walled pipe, in which respective inner and outer layers of plastics material are bonded to, by face to face direct cohesive contact with, a metallic layer therebetween, which extrusion assembly comprises

• a first extrusion apparatus for extruding an inner layer of the pipe, which inner layer is of plastics material, which first extrusion apparatus comprises an extruder having an extrusion sleeve and an extrusion die disposed substantially coaxially within the extrusion sleeve, which extrusion sleeve and extrusion die together define, at an axial end of the extrusion sleeve, a first, annular, exit orifice for the plastics material which is to provide the inner layer of the pipe,
• a shaping sleeve for shaping a metallic layer, which shaping sleeve surrounds and is substantially coaxial with the extruder of the first extrusion apparatus and is disposed axially between opposed axial ends of the extruder, which shaping sleeve has a first axial end adapted to receive a U-section form metallic layer, a second axial end adapted to allow exit therefrom of the shaped metallic pipe and an elongate aperture disposed axially between the said axial ends and passing through a wall of the shaping sleeve to allow access through the wall of welding means the shaping sleeve being capable of shaping the U-section form metallic layer passing through it into a tubular configuration whereby opposed longitudinal edges of the metallic layer overlap with one another to provide a leaf seam in register with the said elongate aperture,
• welding means for welding the metallic layer of tubular configuration, which welding means is capable of passing through the elongate aperture in the wall of the shaping sleeve to weld the metallic layer at the said overlap and thereby provide a metal tube defining a metallic layer of the pipe,
• means for supplying a pressurized gas downstream of the first, annular exit orifice whereby extruded plastics material emerging from the said first exit orifice is forced against, so as to promote cohesive bonding thereof with, an internal surface of the metal tube to provide the inner layer of the pipe, and
• a second extrusion apparatus for extruding an outer layer of the pipe, which outer layer is of plastics material and is disposed radially outwardly of the metallic layer of the pipe, which second extrusion apparatus comprises a body part surrounding the said first, annular exit orifice for the plastics material which is to provide the inner layer of the pipe and a supply channel within the body part for supplying plastics material and terminating in a second exit orifice, downstream of the shaping sleeve and surrounding the metal tube, for the plastics material which is to provide the outer layer of the pipe,
• which extrusion assembly has positioning means selected from at least one of,
• (a) an essentially annular section recess in the body part of the second extrusion apparatus which recess is capable of receiving a second axial end of the shaping sleeve remote from the said first axial end, which allows accurate positioning of the shaping sleeve relative to the first and second extrusion apparatuses, and
• (b) a universal joint to which the extruder of the first extrusion apparatus is operatively connected, which connection is adjustable to adjust the position of the said first exit orifice of the first extrusion apparatus relative to the said second exit orifice of the second extrusion apparatus, thereby allowing accurate setting of the said position.