Mechanical Pulp Core

Stiffness and Density

Wood logs or chips are ground mechanically, retaining almost all the natural lignin and yielding stiff, rigid fibers. A mechanical pulp core maintains a high volume because these stiff fibers do not pack together as tightly as chemically refined fibers. This bulky structure gives the paperboard its required stiffness, allowing it to withstand the compression forces encountered in stacking.

However, the presence of lignin makes this layer sensitive to yellowing and loss of strength when exposed to light or heat over time.

Converting Process Constraint

Multi-ply boards must have sufficient internal bond strength to prevent the core from delaminating during high-speed printing and converting operations. Chemical liners are bonded to each side of this mechanically produced core to provide a smooth, bright surface for printing. Measuring the core thickness and bulk allows the mill to guarantee that the board will meet the rigidity requirements of the packaging line.

Adjusting the ratio of mechanical to chemical pulp helps the manufacturer balance the trade-off between board stiffness, surface finish, and cost.