What is  "grain direction" of paper?

LGSG
Long grain and short grain sheets cut from reel

During  the manufacture of paper, the paper fibre orient in the direction of flow of pulp and this is known as the grain direction or machine direction. As you know, paper is manufactured in the form of large reels.  The sheets are then cut out of these reels. Depending on whether the longer or shorter side of the sheet is in the grain direction, the sheet is called LG (Long  Grain) or SG (Short  Grain)

What is the significance of grain direction in printing & binding?

Paper expands across / against the grains. While printing, this can cause problems in colour registration. It is possible to compensate for this, in the circumferential direction; but not laterally. This would mean that the grain direction should be parallel to the printing cylinders. In other words, for multi colour printing, long  grain  is  preferred.

In binding,  the grain direction should be parallel to the book spine. That means the final fold should be in line with the grain direction.  In such a case,  the book will open more easily and lay flat. Books, adhesive bound with wrong grain direction (perpendicular to the spine) have a better chance  of  falling  apart.

Let us elaborate this a bit more:  Paper is anisotropic in nature and has different stiffness along length and width.  It is stiffer along the grain direction;  this explains why a book with grain perpendicular to spine behaves like ‘mouse trap’, shutting with a clap-sound when its opened and left. Even in thread-sewn books,  the signatures of the open book tend to stand up and refuse to lay flat. The same holds true for covers as they crease and fold better along the grain direction.

In order that the grain direction requirements of both printing and binding are met, the imposition of the pages will need to be planned properly.

How does one know the grain direction of the paper he or she is planning to print on?

Paper supplies: In most cases the supplied sheets mention the type of grain – whether LG or SG. In some cases, the direction is marked by underlining the side (28'' x 40'' indicates long grain and 28'' x 40'' is short grain )

Droop test:  A piece of paper is placed on the edge of a table so that it overhangs by six inches. The angle of droop is noted and the same paper is turned 90 degrees and extended by the same overhang again. The orientation that shows the greatest droop has the grain aligned with the table  edge.

droop test

Droop test

It is important for the spine to be stiff, while the pages are flexible. The only way is to have the grain direction along the spine. 

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.  

moisture test
Moisture test

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 fibre particles break and make an unsatisfactory fold.

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 latitudinal directions. The tear pattern will be straight when parallel to the grain and jagged across the grain.

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