Multi-wall cylindrical structure, method of making such structure, and apparatus for carrying out such method

A method of forming an elongated cylindrical article, comprising at least one inner cylindrical member, at least one outer tubular member at least partially radially spaced from said inner cylindrical member, and a plurality of ribs disposed between said inner and outer members in supporting relation to said members, wherein all elements of said tubular article have a composite of a longitudinal and circumferential orientation, wherein said method comprises:

• extruding, through at least partially arcuate first and second extrusion dies, that are at least partially radially spaced apart, at least a molten and deformable smaller diameter cylinder and a molten and deformable larger diameter tube at least partially radially spaced from said smaller diameter cylinder, wherein said molten smaller diameter cylinder is within said molten larger diameter tube;
• simultaneously extruding, through a plurality of third extrusion dies, that are disposed between and in operative contact with said first and second extrusion dies so that extrudate emitted from said first and second extrusion dies is continuous with extrudate emitted from said third extrusion dies, a plurality of molten ribs so as to dispose them between, in adherent contact with and in supporting relationship to, said smaller cylinder and said larger tube whereby forming a molten extrudate comprising said inner cylinder, said outer tube and said plurality of supporting ribs;
• cooling said molten extrudate an amount sufficient to freeze said extrudate into a substantially rigid, plural walled structure having solidified ribs between and in supporting relationship to said inner cylinder and said outer tube;
• and
• moving said molten extrudate and said solidified extrudate away from said extrusion dies;
• while moving said molten and solidified extrudate away from said extrusion dies, rotating the whole of said substantially rigid structure relative to said extrusion dies whereby applying a circumferential twist to said molten extrudate while it is still in a formable condition thereby causing the whole of said molten extrudate to axially rotate while simultaneously proceeding away_from said extruder dies, whereby causing the whole of said structure to take on a composite of a circumferential and a longitudinal orientation along at least a portion of its axial length.