Internal Density
Maximizing the thickness of the center layer in multi-ply paperboard without increasing its weight is essential for creating stiff, lightweight packaging. The middle ply bulk is the volume per unit mass of this central layer, which determines the overall caliper of the board. This property is measured in cubic centimeters per gram.
Stiffness Influence
Increasing this parameter allows manufacturers to produce a board that resists bending while using fewer fibres. The middle ply bulk determines how far apart the two strong, dense outer plies are held, which increases the moment of inertia. A board with a high bulk in the center will be much stiffer than a single-ply board of the same weight.
This structural efficiency reduces the amount of raw material needed for each carton, which lowers transportation costs and environmental impact.
Furnish Selection
Achieving this high volume requires the use of specific wood pulps and processing techniques. The middle ply bulk is usually created by using mechanical pulps, such as chemithermomechanical pulp, which have stiff, springy fibres that do not collapse easily. It can also be enhanced by using chemical additives that increase the spacing between the fibres during sheet forming.
This selection must be balanced with the need for internal bond strength to prevent the board from delaminating during printing.
