Dimensional Shift
The expansion or contraction of a paper sheet in the cross machine direction occurs in response to changes in the relative humidity of the air. This transverse hygroexpansion is typically much greater than the dimensional change in the machine direction because of the way fibers are oriented. It is a primary cause of registration errors and curling in the printing process.
Fiber Physics
Individual wood fibers swell significantly more in diameter than in length when they absorb water. Because more fibers are aligned with the machine direction, the cumulative effect of this swelling is seen as transverse hygroexpansion.
Process Stability
Controlling the moisture content of the stock and the humidity of the pressroom is necessary to minimize this movement. Transverse hygroexpansion can cause the board to ripple or develop a wavy edge, which interferes with the feeding of the sheets. In multi color printing, even a small shift in dimensions between passes through the press can ruin the alignment of the colors.
Understanding this property allows converters to compensate for dimensional changes during the design and layout of the packaging. Proper conditioning of the board before printing is the most effective way to manage this behavior.
