Method and apparatus for fluting a web in the machine direction

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

• uniformly introducing into a web of medium material a full-width array of longitudinal flutes of a first geometry as the web travels along a web-travel pathway in a machine direction, thereby reducing the width of said web to substantially a final width that corresponds to a take-up ratio for preselected longitudinal corrugations or other three-dimensional structure to be formed in the web, wherein at any selected location along said web-travel pathway said longitudinal flutes define a sine wave that propagates in a cross-machine direction and has a constant zero-crossing point, and after introduction of said first-geometry flutes into said web, introducing therein longitudinal corrugations having a lower amplitude and higher frequency than said first-geometry flutes.

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;
• and
• b) reducing the width of said web to a substantially final width by forming longitudinal flutes of a first geometry in said web as it passes through said labyrinth.

A forming device having an entry end and an exit end spaced apart along a machine direction, the forming device comprising first and second sets of flute-forming bars opposing one another and defining therebetween a longitudinal fluting labyrinth, each said set of flute-forming bars extending from adjacent said entry end toward said exit end, at least a subset of said flute-forming bars having a variable-tangent configuration such that imaginary tangents to each of said subset of bars, at spaced locations along a length thereof, become successively nearer to parallel with the machine direction such that said subset of bars converge in a cross-machine direction as a result of said variable-tangent configuration as they proceed toward said exit end, wherein a lateral distance between adjacent flute-forming bars decreases in a direction toward said exit end.