Paperboard does not have equal resistance when folding in every direction. With most of the fibers more heavily aligned in one direction across the sheet during production there is what is commonly referred to as grain direction. Folds made with the grain are often easier to form, while those done against the grain are often more difficult to form and may have whitening roughness or cracking on the surface. It is easiest to see this problem with thick or coated substrates that have heavy ink coverage, varnish or lamination.
You can experience this with your own small paperboard sample. Get two similar pieces of the same paperboard from the same sheet. Position one at ninety degrees to the other. Gently bend each without scoring. You will find that one will be more flexible, and the other will resist or bend less uniformly. Next add a scoring line with a blunt edge tool to each, then bend each. Scoring will increase the control, but will not overcome the effects of grain direction.
It can be problematic to a dieline structure when you have a very long carton panel or a narrow glue or tuck flap, or many close-together folds. A carton that is folded against the grain, especially if there are multiple long folds on the same line, may require extra force to assemble and may deform or spring open slightly after being closed. A tuck flap may be difficult to insert into the slot, and a narrow carton panel or printed area may crack over the fold. A flat artwork may look fine on a computer screen because grain direction only has an impact on the actual physical material, not the digital art layout.
Often the physical damage is only blamed on insufficient crease depth or rule width. Crease depth, rule width, channel size, board thickness, moisture level and coating are factors that influence fold quality; grain direction is one more factor to consider. A crease cannot always force a fold to go against the natural stiffness of the grain. Check printed artwork and/or laminates on a fold in a sample, especially dark printed areas or heavily coated regions, for fine white lines or flaking. This effect is often more noticeable in the printed or laminated areas than on the board itself.
Not every packaging structure can have every fold parallel to the grain. There could be a number of folds in different directions within a single carton; the laydown must be optimized for the paperboard and must also consider the paperboard use, printing and die-cutting. The goal should not be to find the absolute right way, but to know which folds are most critical to the packaging structure and ensure that the board selection, creasing settings and grain direction result in a good package in testing.
Whenever testing samples, keep track of the grain direction. Note if the carton folds easily, if the carton holds its shape, and if the carton has cracking or spring-back. Look at the finished product under the light and observe the printed side as well as the interior of the folds. A less resistance, a cleaner fold and better carton panel alignment are clues to an acceptable carton structure and grain.