Abstract: A technique for extrusion of plastic including a first step of extruding which includes extruding at least one layer of a thermally cross-linkable material in an extrusion apparatus (20) having at least one die wall (22); a second step of extruding, performed simultaneously with the first step of extruding, and which includes extruding at least one layer of thermoplastic material in the extrusion apparatus between the thermally cross-linkable material and the at least one die wall (22) and heating the thermally-cross-linkable material, thereby to provide at least a predetermined minimum rate of cross-linking thereof during the steps of extrusion of the plastic.