What is grain direction, and how can it help us identify the direction of grain in paper or cardboard?

Today we return to the topic of grain direction and the question: how can the swelling of the fibre help us to determine the grain direction of the paper or board?

Paper and board are, first and foremost, hygroscopic, i.e. the paper fibre absorbs moisture and also releases it again. It is interesting to note that, as a result of moisture absorption, the paper fibre expands (grows) three times faster in the cross-direction than in the machine direction.

As the paper fibres on the paper or board machine align primarily in the machine direction, i.e. along the direction of movement of the wire of the paper/board machine, the paper web effectively bends upwards at the outer edges of the paper/board machine during our moisture absorption test (paper sample placed on hot coffee or tea).

We are therefore dealing here with a sheet of DIN A6 paper in a long grain.