An apparatus produces a tubular structure having a longitudinal axis and at least one form extending helically inward from the structure relative to the longitudinal axis. The apparatus includes an extruder including at least two spaced apart extrusion die surfaces adapted to extrude plastic state material therethrough. At least one extrusion die has at least one nonplanar channel having facing nonplanar surfaces configured and disposed such that during extrusion of the material through the at least one extrusion die, material flowing into the channel creates the at least one form. The channel surfaces direct material flow such that the at least one form and the structure exiting the extruder and last contacted by the at least one die of the extruder collectively helically rotate about the longitudinal axis.