Stock Layering
Papermaking fiber suspensions stratified into separate fluid layers across the forming section produce engineered board structures with distinct surface and core properties. A multi-ply furnish combines different fiber grades across outer and core plies on specialized cylinder or multi-fourdrinier forming machines. Bleached chemical softwood or hardwood pulps form smooth outer printing surfaces, while recycled mechanical pulps build bulk and bending stiffness in internal plies.
This layered approach optimizes strength and printability while reducing total fiber cost.
Forming Mechanics
Separate headboxes deliver tailored fiber slurries onto individual forming wires before wet-pressing joins the wet webs. Interply bonding relies on hydrogen bonds formed between adjacent fiber layers during pressing and drying stages. Starch additions at interply spray stations enhance ply bond strength.
Structural Limitation
Incompatible fiber lengths between adjacent layers cause internal stresses during web drying. Chemical pulp layers shrink at different rates than mechanical pulp cores, inducing curl and twist in finished paperboard sheets. Poor interply adhesion leads to delamination under converting loads such as creasing or die-cutting.
Excess fine particles in core plies impede water drainage through wet webs, forcing lower machine operating speeds and raising thermal drying energy requirements. Creasing operations fail when core plies lack sufficient compressibility, causing top liner tearing along fold lines. Control of interply bonding prevents ply separation during high-speed printing press runs.
