Measurement Standard
Mechanical resistance to deflection across paper and paperboard specimens determines carton rigidity and handling stability during automated packing. The l&w bending resistance test evaluates the force in millinewtons required to deflect a rectangular test specimen through a specified fifteen-degree bending angle. A pneumatic or mechanical clamping jaw secures one end of the strip while a motorized load cell applies deflection at a standard test span of five, ten, or fifty millimeters.
The resulting measurement reflects the elastic flexural behavior of the board rather than its ultimate rupture point. Testing laboratories conduct these evaluations in controlled atmospheres under standard temperature and relative humidity conditions to prevent ambient moisture from softening cellulosic bonds.
Instrument Mechanics
The testing instrument rotates the clamping head smoothly through the pre-set angle while the stationary sensor records the maximum bending force. Short fifty-millimeter spans serve standard paperboard gauges, while ten-millimeter spans accommodate lighter paper grades and corrugated components. The l&w bending resistance apparatus eliminates gravitational drag artifacts by orienting specimens vertically or through electronic tare balance compensation.
Grain direction significantly alters the test output, with machine direction specimens routinely recording three to four times the bending resistance of cross-machine direction strips. Multi-ply boards show high resistance figures because their outer plies absorb large tensile and compressive loads during initial displacement.
Specification Boundary
Carton designers specify this mechanical property to verify that carton side panels will resist bulging under internal product pressure. Insufficient bending resistance leads to carton collapse inside packaging magazines, misfeeds in automated vacuum pickers, and excessive bowing on retail shelves. The method ceases to provide meaningful data once structural ply delamination or localized creasing fractures occur during deflection.
L&w bending resistance provides a reliable quality control parameter for verifying mill furnish consistency and predicting finished packaging performance.
