Paper

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Paper

Technical Bulletin TRM/95

Paper is the most widely used commodity in the world and one of the press operator's most important printing tools. However, it can wrinkle, curl, shrink, stretch, wave and practically dare you to run it through the press. Its unruly behavior is simply part of its natural disposition. By understanding its properties, you can learn how to better control it.

Properties of Paper

There are four basic properties of paper about which the press operator should know and understand.

Paper Grain

The alignment of fibers in the pulp is known as the grain of the paper. Paper tears more easily with the grain than against it. Lightweight stocks are likely to feed with less trouble if the sheets are cut with the grain running in the direction in which the sheet will feed through the press. Checks process better if they are run through the sorter equipment with the grain following the length of the check.

Static Electricity

When paper gets too dry, it can become charged with static electricity. Static causes the sheets to cling to one another or to the press cylinder. Static may also cause the freshly printed sheets to stick together so tightly that the wet ink from one sheet will offset onto the back of the other.

Because of this and for optimum print performance, we suggest you keep the relative humidity (RH) above 45%.

Instability

Paper is unstable. When it absorbs moisture it expands more across the grain than in the long direction and shrinks in the same manner as it dries. Unbalanced moisture causes the paper to curl. When the edges absorb moisture and the centers remain dry, waves occur. This is why we suggest you keep the RH at 65% or lower.

Wire Side & Felt Side

Modern technology enables suppliers to produce sheets with equally smooth wire and felt sides. This is what is known as a "twin wire" sheet. Wire and felt sides are equally important since stock is printed on both sides.

Seasonal changes often contribute to instability and static electricity. These properties can be controlled by proper storage in a humidity controlled atmosphere.