Extrusion die for co-extruding tubular preforms, e.g. fuel tank

An extrusion die for production of tubular preforms for subsequent inflation into hollow products comprises either: (a) two channels offset at 180 deg to each other at their inlets with a circumferential width which progressively increases until it covers 360 deg at the outlet or (b) four channels (10-13) offset at a 90 deg spacing to each other with a circumferential width which increases to 180 deg at the outlet. A nozzle (17) in each channel (10-13) allows extrusion of a different material (24) from that which flows along the channels (10-13) and forms the basis of the tube. Also claimed are: (1) an extruder with the die; (2) a thermoplastic tubular product in which the cross-section displays two half-moon shaped parts of one thermoplastic welded together at 180 deg to each other, each with a central circumferential layer(24) of a different thermoplastic which bonds the inner and outer layers together; (3) a similar product which has four half-moon shaped parts welded together at 90 deg to each other, each part comprising inner and outer layers as before; and (4) a fuel tank produced utilising the claimed tubular product or die.