A continuous process for producing oriented plastic tube is disclosed having the steps of extrusion (12) of a tube to an initial extruded diameter, temperature conditioning (18), diametrical expansion (20) and cooling (30), characterised by control of the circumferential draw ratio by the step of adjusting the diameter of the extruded tube to an adjusted diameter by means of a variable diameter calibrator (13) located between said extrusion and temperature conditioning steps. A process line for carrying out the invention is also disclosed. Further embodiments of the invention include adjustment of the extruded wall thickness and compensatory adjustment of the adjusted diameter so as to effect a change in final wall thickness with unchanged draw ratio, a process for changing final product diameter and a start-up procedure for the process.