The invention provides a die assembly comprising a nozzle (11) and at least one mandrel (24) which defines with the nozzle an annular die orifice. The nozzle has a longitudinally extending stem (18) terminating in the sleeve (25), the mandrel (24) being located coaxially within the sleeve (25) and coupled to the stem (18) of the nozzle (11) by an axially extending screw threaded coupling. The mandrel includes an internal flow passage (36) having an inlet positioned to receive thermoplastic material from a supply passage (36) extending along the nozzle (11) and an outlet (37) positioned to deliver the material to an annular distribution chamber (27) defined by the opposing surfaces of the mandrel (24) and the sleeve (25). The distribution chamber (27) provides an uninterrupted annular flow path extending from the outlet (37) to the die orifice, thereby avoiding the disadvantage of imperfect material distribution. For the production of multiple-walled tubing, a plurality of die stages coupled together in coaxial relationships are provided, the die components being so coupled by axially extending screw threaded couplings so positioned as not to interfere with the flow of plastic materials.