Dimensional Distortion
Thickness expansion of a paper or board sheet when exposed to moisture represents a primary source of structural and dimensional instability in packaging. This material response, known as z-direction swell, occurs when the cellulose fibres absorb water and expand outward perpendicular to the sheet surface. This swell alters the caliper of the paperboard, which can lead to converting errors.
Mechanism Analysis
Paper thickness changes because the internal fibre bonds break and the dried fibre network releases its built-in drying strains. In measuring z-direction swell, researchers note that the percentage of thickness expansion is far greater than the expansion in the length or width directions of the sheet. This difference is due to the layered orientation of fibres created on the paper machine forming wire.
Printing Consequence
Uneven thickness expansion across the web causes serious ink-transfer and registration issues during multi-color offset printing. When paperboard exhibits high z-direction swell, the surface becomes uneven, which leads to mottled ink coverage and poor resolution. This swelling of paperboard also alters the pressure in the printing nip, which requires operators to adjust press settings to prevent excessive sheet feeding problems and waste, and ensures a clean finished package with sharp graphics.
