Crease Formation
Mechanical deformation zones localize shear stresses along designated score lines in paperboard during creasing and folding operations. Structural evaluation of plane strain shear concentration explains how paperboard plies delaminate intentionally along crease lines to allow clean, uncracked ninety-degree bends in folding boxboard cartons. Male creasing rules press paperboard into female matrix channels, inducing high shear stress confined within a narrow transverse plane.
Ply Delamination
Controlled internal delamination reduces the bending stiffness of the crease line without tearing the outer printable paperboard liner. High plane strain shear concentration forces middle ply fibers to slide past one another, absorbing strain energy and forming an internal bulge on the inside of the fold. If shear stress distribution spreads too widely beyond the crease channel, the top liner ruptures, exposing raw fibers and ruining printed graphics.
Crease depth, channel width, board caliper, and moisture content of the board govern the magnitude and distribution of localized shear strains. Board with insufficient ply bond strength suffers chaotic cracking, whereas overly stiff board resists creasing and causes box squareness defects on high-speed cartoning machines.
Carton Squareness
Proper localized deformation ensures precise box geometry on automatic packaging lines. Precise creasing minimizes opening force and prevents side-wall bulging in filled cartons. Uniform plane strain shear concentration maintains clean fold lines and high structural integrity across finished board packaging.
