Paper, like wood, has a grain. Grain direction refers to the way the paper fibers are laid down in the paper making process – typically, the fibers settle in the way the paper making machine is moving. With the grain, there is greater strength. Against the grain, there is greater expansion and contraction which makes it less stable.
Have folding in your next piece? Make sure you take the paper grain direction into consideration when selecting your paper type! Not thinking about the grain, could end with disastrous results.
A fold is cleaner and more resilient when the grain runs parallel to the fold. Folding against the grain can cause cracking (rough appearance) most noticeable in areas with high ink coverage, and it may not lay as flat as well. Against the grain is also less resilient, and the pressure exerted by the rollers on the folder must be reduced to avoid excessive weakening of the paper along the fold line. A die-score can be utilized if you must fold against the grain to alleviate some of the aforementioned issues.
Here are some simple tests to determine paper grain direction:
- FOLDING TEST: When folding a small sheet of paper you will notice that it folds more easily and smoothly when the fold is parallel to the grain. If folded against the grain, the small fiber particles break and make an unsatisfactory fold. (This is better seen on your thicker stocks)
- MOISTURE TEST: If moisture is applied to one side of a sheet of paper, it immediately starts to curl in one direction. The expansion is on the cross-grain edge, the curl indicating the paper grain direction.
- BENDING TEST: Thicker papers are best tested by bending them in both directions. One direction offers considerably more resistance than the other. Parallel to the grain direction the resistance is far less than against the grain.
- TEARING TEST: A sheet is torn in longitudinal and transverse directions. The tear pattern will be straight when parallel to the grain and jagged across the grain.