The direction in
which the web moves through the paper machine is said to be
with the grain, whereas the direction across the web is said
to be against the grain. With the grain, there is greater strength,
and against the grain there is greater expansion and contraction
which makes it less stable. This strength with the grain occurs
because during the manufacturing process, the fibers are stretched
so that they lose virtually all further capacity for expansion,
which creates relatively good dimensional stability.
Depending on the
quality of the paper, folded sheets retain a certain degree
of resilience after folding so that they have a tendency to
reopen. The angle of opening is least with parallel folds, and
most noticeable with right angle folds.
A fold is cleaner
and more resilient when the grain is parallel to the fold. A
fold against the grain may not lay as flat, and can cause cracking
most noticeable in areas of heavy ink coverage. A fold against
the grain is less resilient, and the pressure exerted by the
rollers must be somewhat less to avoid an excessive weakening
of the paper along the fold line. If folding against the grain
is a must, a die-score can alleviate some of these problems,
as well as careful paper selection.
Testing the Paper
Grain
The following tests can be used to determine grain direction:
Fingernail Test
With the fingernails of the thumb and middle finger, pinch
and slide down the vertical and horizontal edges of the sheet
of paper. With the grain, there is virtually no change but,
against the grain, a wave will be clearly visible.
Bending Test
When a square test-sheet is bent in both directions, there
will be less resistance parallel to the grain than against
it.
Tearing Test
Tear a sheet on the vertical dimension, then tear it horizontally.
With the grain, the tear will be relatively straight. Across
the grain, there will be greater resistance to tearing, resulting
in a jagged edge.
