Downgauging Folding Boxboard without Losing Compression Strength
Downgauging folding boxboard demands increasing core bulk and outer ply elastic modulus to preserve caliper and bending stiffness under McKee formula laws.
Caliper Cubed Law
Stiffness Relationship
Bending stiffness in multi-ply paperboard scales exponentially with the thickness of the material. This fundamental mechanical rule, known as the caliper cubed law, states that doubling the thickness of a sheet yields an eightfold increase in its bending resistance. The relationship governs how converters choose board grades for cartons that must resist bulging under the weight of the contents.
Layered Structure
The distribution of raw materials through the thickness of the board determines how the sheet responds to folding stresses. Standard manufacturing processes place dense, high-tensile chemical pulp in the outer plies and high-bulk mechanical pulp in the center. Because the outer layers experience the highest tension and compression, this distribution maximizes the effect of the caliper cubed law.
A lighter core layer holds the outer plies apart, which yields the highest possible stiffness for a given sheet weight.
Converting Response
Package performance on the high-speed cartoning line depends directly on this thickness exponent. In practice, a small reduction in caliper during calendering or embossing causes a rapid drop in board rigidity. This loss of stiffness leads to problems during gluing, where the folding flaps must resist pressure.
If the sheet is crushed, the box will fail to square up properly.