Angle Deviation
Angular deviation of maximum tensile stiffness from the machine direction indicates how far the primary fiber orientation tilts away from the centerline of the paper machine. This tensile stiffness angle is a critical quality metric for ensuring that the paper will run straight through printing and converting equipment. Ideally, this angle should be zero, meaning the strongest direction of the sheet aligns perfectly with the direction of travel.
Machine Alignment
Discrepancies in the flow of pulp slurry from the headbox onto the forming wire can cause fibers to settle at an angle. When the tensile stiffness angle deviates from zero, the sheet exhibits unbalanced mechanical forces when subjected to tension on a printing press. This imbalance can cause the web to wander from side to side, resulting in poor color registration and increased risk of edge tears.
Twist Prevention
Controlling this fiber deviation is essential to prevent warping and twisting in printed board and paper products. If the tensile stiffness angle is high, sheets of paperboard will curl diagonally when exposed to changes in ambient humidity. This dimensional instability is particularly troublesome for multi-ply board or laminated packaging, where different layers might curl in opposite directions and tear the glued joints apart.
Paper mill technicians use ultrasonic transducers to measure this property across the web profile and make rapid adjustments to headbox slice screws to maintain a flat sheet.
