Method and apparatus for fluting a web in the machine direction

A method of forming a longitudinally-corrugated web, comprising:

• a) feeding a web of medium material having an initial width in a machine direction through a longitudinal fluting labyrinth defined between opposing sets of at least partially interlaced flute-forming bars, wherein pluralities of the flute-forming bars in each set are curved such that the bars in said respective pluralities converge in a cross-machine direction as a result of their curvature as they proceed toward an exit end, said opposing sets of flute-forming bars having a degree of interlacement that increases in the machine direction beginning at a choke point where said sets just begin to interlace and continuing until said exit end, wherein longitudinal corrugations are thereby gradually formed in said web of medium material between said choke point and said exit end, and b) adjusting a spacing between said opposing sets of flute-forming bars at an entry end where said web approaches said labyrinth to thereby adjust a machine-direction location of said choke point.

A corrugating die having an entry end and an exit end spaced apart along a machine direction, the die comprising a continuous smooth first forming surface having a first sinus contour viewed in lateral cross-section adjacent the entry end, the first forming surface smoothly evolving in the machine direction to a second sinus contour viewed in lateral cross-section adjacent the exit end, the first sinus contour having a larger amplitude and lower frequency than the second sinus contour.