FORMAL TRAINING AND PROCEDURE AND PREPARATION FOR SUCH RECOMMENDATION

摘要

Tubular preform comprising several layers of coaxial thermoplastic materials. Said preform includes at least one three-layered inner structure comprising an inner (1), median (2) and outer (3) layer and is characterized in that the median layer (2) consists of first and second alternating transverse rings (2ua/u, 2ub/u) made of first and second thermoplastic materials (A, B), and separate longitudinal rods (2uc/u) formed from a third plastic material (C), distributed about the preform axis. Said rods intersect the alternating transverse rings (2ua/u, 2ub/u) of the median layer (2) as they delimit cells with the first transverse rings (2ua/u), said cells (2ub/u) being made of a second thermoplastic material (B) and having a longitudinal size exceeding that of the first transverse rings (2ua/u). The invention also concerns a method and apparatus for the manufacture of said preform by coextrusion.