Moisture Expansion
Dimensional instability in paperboard occurs when exposure to humidity alters the physical dimensions of the sheet. The rate of cross-directional swell measures the percentage increase in width as the fibrous structure absorbs moisture from the surrounding environment. Paper fibres expand up to four times more in their diameter than in their length.
Because the papermaking process aligns most fibres along the machine run, the cumulative widthwise expansion is far greater than the lengthwise change.
Fibre Alignment
Mechanical processes during sheet formation determine the orientation of cellulose fibres on the wire. Wet fibres align primarily along the direction of the run, leaving the cross direction vulnerable to lateral expansion as moisture enters the amorphous regions of the cellulose. This differential expansion can distort printed packaging during high-speed run cycles.
Press operators observe that cross-directional swell alters registration on multi-colour offset lines, making precise alignment difficult when humidity varies. The paperboard suffers from waviness along the edges which creates feeding failures on folder-gluers. Controlling moisture during storage helps prevent these register shifts before converting.
Conversion Tolerance
Production standards on folding carton lines require close control over dimensional fluctuations to prevent glue line misalignment. Carton blanks affected by cross-directional swell struggle to fit the feed gates of packaging lines. Process limits are usually set below one percent variation.
