Tass Print - TIPS and ARTCLES 2: The importance of grain direction.
Paper Grain: The direction of fibres in a sheet or web of paper, generated during paper formation.
As paper moves forward along the forming wire on a papermaking machine, the fibres align themselves in a direction parallel to the direction of wire travel through the machine. 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. Paper will tear and fold more easily with the grain and with greater difficulty against the grain.
Grain direction is an important consideration in printing processes such as sheet fed offset lithography (in particular multiple-colour work), especially in connection with dimensional stability (how well a particular sheet of paper will retain its original length and width upon exposure to moisture and pressure). When paper fibres absorb water, they will expand in width, but not in length. Therefore, the direction of greater dimensional change will be in the cross-grain direction. Long-grain paper is sometimes preferred to short-grain paper for colour offset lithographers as the total dimensional change due to moisture will be less if the expansion is in the shorter dimension. This is an important consideration when separate colours in multiple-colour jobs need to align properly. However in black-and-white and single-color jobs, short-grain paper is preferred as register is not such a concern.
In addition, since paper is stronger against the grain, paper that is fed into the press against its grain will be less likely to suffer from structural deformities such as curling, stretching, or waffling as a result of tensile forces. When pages are to be bound, as in books and catalogues, the grain should be parallel to the binding edge. Pages bound with the grain perpendicular to the binding edge do not lie flat or turn easily.
Paper will also fold or crease more easily with the grain and tear straighter with the grain.
There are a variety of ways of determining the grain direction in a particular sheet of paper.
One test is to moisten one side of a paper square. It will curl toward its dry side, and the two opposite
edges that curl up will be parallel to the grain direction.
Long grain provides the best fit/least stretch but feed and curl issues will create register problems of their own that short grain will reduce!
Questions for the paper procurer:
- Is the printing press gripping the sheet on the long or short edge?
- What is the imposition ( how many up is the image being printed) and what is the final
orientation of the finished trimmed sheet?
- What is the best sheet grain for image fit (least paper stretch)?
- What is the best sheet grain for feed and curl?
- Are both sides of the sheet being printed?
If both sides of the sheet are being printed at the same time (perfecting), then the issue of sheet feeding
and paper curling is reduced but if the work is being printed separately (sheetwork), then the grain direction
and operation needs to limit sheet curl that will make second side printing difficult, resulting in feed related
register problems, scratching and crinkling problems and more.
The main reason a printed sheet curls is that it is stacked . Moisture and air affect the edges of the sheet,
encouraging it to grow while the rest of the sheet is mostly sealed off to those elements, creating a
distortion or wave in the sheet.
What finishing processes ( fold, crease, perfect bind, saddle stitch) are to be applied after print?
Digital printing presses and laser printers require long grain paper for best feed.
Observing the best finishing practices mentioned above, an A3 sheet, folded to A4 would require
creasing before folding to avoid fibre and image cracking whereas an A4 sheet printed 2 up ad folded to A5