The grain direction of paper is in the direction the web of paper travels through the paper machine (machine direction). Cross grain is referred to as the direction across the web. The strength of the paper is stronger in the machine direction then in the cross grain direction. The machine direction has good dimensional stability due to the manufacturing process. Cross grain also has greater expansion and contraction characteristics resulting in it being less stable.
Paper quality will have an effect on folding however folded paper will retain most of its flat characteristic and will have a tendency to reopen. This reopening characteristic is less apparent in the machine direction but can be very noticeable in the cross grain direction.
A fold in the machine direction is crisper and more resilient. A fold in the cross direction may not lay flat and can cause cracking especially if there is heavy ink coverage. We recommend scoring if cross grain folding is necessary especially if a heavy stock is used.
There are several ways to determine grain direction.
- The first is to simply fold the paper in both grain directions. The fold in the machine direction will be cleaner and crisper. Cross grain will be bulker and may show cracking.
- The second is to cut a narrow strip from the paper in both grain directions. Place over the edge of a table. The one that has more stiffness is the machine direction. The cross grain will be limp.
- The third way is to wet a strip of paper with water. The machine direction will be in the opposite direction of curl.
- The grain direction can also be identified using a burst test, TAPPI T 403, with the main rupture occurring in a perpendicular direction to the grain direction.
- The TAPPI T 404 tensile test can be used to identify grain direction.