Thickness Increase
Dimensional changes describe the growth in the thickness of a paper or board sheet following exposure to environmental factors or mechanical stress. While caliper expansion is often a temporary response to humidity, it can become permanent if the internal fiber bonds are disrupted by moisture or heat. This change alters the bulk of the material.
Moisture Influence
Cellulose fibers absorb water from the surrounding air, causing them to swell and push against neighboring fibers. This internal pressure results in caliper expansion that can compromise the flatness of a stack or the registration of a print run. High-yield pulps containing more lignin often show a greater degree of swelling than fully bleached chemical pulps.
Converting Impact
Precise control of thickness is required for the proper functioning of high-speed folder-gluers and automated filling equipment. If caliper expansion exceeds the tolerances of the machinery, the board may jam or the score lines may crack during the folding process. This effect is particularly problematic in chilled or humid storage environments where the packaging must maintain its structural dimensions.
Manufacturers use sizing agents to limit the rate of water absorption. They also monitor the storage conditions to keep the relative humidity within a stable range. Consistent conditioning of the board before use helps to mitigate these issues.
