Lesson #1: How Paper Affects Folding
Paper is, by far, the most important factor in the folding process. Paper choice is not only for aesthetics, but also for practicality. The wrong paper choice on a folded piece can produce a disappointing result. For example, a large-sized brochure printed on thin paper is floppy, and a big, folded poster printed on thick paper is wrinkled and won't lay flat. Much consideration and often the advice of your printer or paper representative is required to make the right choice. It is essential to understand the characteristics of paper before we can understand how it performs in the folding process.

Paper manufacturing processes
Paper fibers have certain characteristics that depend upon the nature of the composition of the fiber and the way in which it is milled. As climate changes, the fibers can expand and contract considerably in their thickness, but, in terms of length, they remain virtually unaltered.

The way that paper fibers are laid down in sheets during manufacturing determines the certain characteristics of expansion and contraction or grain direction. These characteristics vary with each of the processes normally used in the production of paper.

Machine-manufactured paper
Paper produced by machinery—vat machines or long web machines—has a pronounced grain direction because the fibers align themselves parallel to the direction of movement on the machine.

Hand-made paper
The highly diluted pulp of a hand-made paper is evenly spread over a web through manual shaking which causes the fibers to follow a random orientation. For this reason, there is no discernible grain direction in hand-made paper, and the effects of expansion and contraction are virtually identical in both directions.

Grain direction
Paper, like wood, has a grain direction. Paper is made out of cellulose fiber material, and the direction in which these fibers line up during the manufacturing process determines the grain of the paper. 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.

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 (a rough appearance) most noticeable in areas of heavy ink coverage. A fold against the grain is less resilient, and the pressure exerted by the rollers on the folding machine must be reduced 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.

Hands-on exercise: Testing the paper grain
Do these exercises with your students to illustrate paper grain. All you need is a stack of 4 x 6 sheets (or other size) of paper. Distribute three per student.

"The 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.

"The Bending Test" — When a square test-sheet is bent in both directions, there will be less resistance parallel to the grain than against it.

"The 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