Paper Grain is the direction of fibres in a sheet or web of paper, generated during paper making process.
When paper is cut into sheets, it will be either long-grain (if the fibres are aligned parallel to the sheet's longer dimension) or short-grain (if the fibres are aligned parallel to the sheet's shorter dimension). The direction of the majority fibres is called "Parallel to Grain" or "With Grain". The other direction is called "Perpendicular to Grain" or "Across Grain".
Think of paper fibres as tooth-picks that are lined up. The direction the toothpicks point is the grain direction. You can understand now that the stiffness will be higher from top to bottom in this image, and the folding will be easier when done parallel to this direction.
Paper will tear and fold more easily with the grain and with greater difficulty across the grain. It has more strength/stiffness across the grain direction. Especially in thicker papers, it can crack when folding against the grain. Finally, when paper gains or loses moisture, it expands or contracts much more across the grain (think of it as each toothpick getting thinner or fatter!). This expansion / contraction can also cause it to curl.
With Grain: Easier
Against Grain: Tougher, Cracking
With Grain: Less
Against Grain: More
With Grain: More
Against Grain: Less
With Grain: More
Against Grain: Less
How to tell the grain direction of a paper?
There are a few tricks to find out the grain of a sheet of paper. It is easier to tell with thicker papers and less so with thinner papers.
1. Bend Test – Take a sheet of paper, bend the paper (don’t fold or crease it) horizontally and vertically. There will be less resistance in with grain than against the grain.
2. Tear Test – Take a sheet of paper and tear it horizontally and then vertically. The tear that was straighter is parallel to the grain, the more jagged tear is across the grain.
3. Wet Test – cut a small square (around 2 x 2cm) and dip the paper into a glass of water. In a few seconds, the paper should curl. The edge that curls is across the grain and the edge that is straight is parallel to the grain.
Know which grain direction to use!
Cartons: Usually the grain direction is kept horizontal (assuming the opening flaps are on the top and bottom). This is because cartons are usually held by the sides, and stiffness in that direction is required functionally.
Books: Usually the grain direction is kept parallel to the binding edge. The two main reasons are: When turning pages, there is less stiffness perpendicular to the binding edge, due to which the pages tend to fall to the left or right, rather than stick up. Secondly, paper expands or contracts if it picks up or loses moisture – having the grain parallel to the spine ensures this change of dimension is not at the spine, where it is fighting with the adhesive which is trying to prevent it from moving.
Folders: Usually the grain is kept parallel to the folding direction – this minimises cracking at the folds.
Labels: With self-adhesive labels, the direction of the label on the reel is determined more by the applicator machine used for dispensing the labels. Do find out the requirement of your applicator. If pasting the labels manually (as is usually done for smaller lots), it shouldn’t matter much.
With wet labels, it should be parallel to the direction of the turn of bottle/product during application – this is so that there are less problems during application. However, there are different types of applicators, so it would be best if you find out from the people running the applicator and inform us of the requirement.