Method of making high strength paperboard panel

A method of making a paperboard panel comprising the following steps:

• (a) positioning a first flat paperboard layer (51) having grain lines (G) such that said grain lines are arranged in a first predetermined direction,
• (b) positioning a second flat paperboard layer (52) having grain lines (G) such that said grain lines are arranged in a first predetermined direction and such that said second flat paperboard layer lies beneath and spaced from said first flat paperboard layer,
• (c) providing an undulating midstratum layer (53) having upper and lower edges and plural channels (55, C) formed therein,
• (d) applying adhesive means (58, 59) to the upper and lower edges of said undulating midstratum layer (53),
• (e) positioning said undulating midstratum layer (53) between said first flat paperboard layer (51) and said second flat paperboard layer (52) such that said plural channels (55, C) are arranged in a second predetermined direction lying substantially perpendicular to said first predetermined direction,
• wherein said first flat paperboard layer (51) is comprised of chipboard material having short fiber lengths contained therein,
• wherein said second flat paperboard layer (52) is comprised of chipboard material having short fiber lengths contained therein,
• wherein said undulating midstratum layer (53) is comprised of paperboard material having long fiber lengths contained therein,
• wherein said first and second flat paperboard layers (51, 52) have average short fiber lengths contained therein of 2 mm or less,
• wherein said undulating midstratum layer (53) has average long fiber lengths contained therein of 5 mm or more,
• wherein the upper undulation portions of said midstratum layer (53) are secured directly to said overlayer (51) by adhesive means (58), and
• wherein the lower undulation portions of said midstratum layer (53) are secured directly to said underlayer (52) by adhesive means (59).