What is a crease in packaging?

When board folds, the outside of the fold has to stretch and the inside has to compress. Left to itself, coated board resists both and the coating splits along the fold — the familiar white line through a dark printed area. A crease prepares the board by pushing a channel of fibre out of the way in advance, so the fold has somewhere to go.

The crease is made by a rounded rule in the die pressing the board into a matching groove in the counter beneath it. The width of that groove is set by the caliper of the board: thicker board needs a wider groove and a deeper crease. Get it wrong in either direction and the result is visible — too narrow and the fold cracks, too wide and the fold wanders and the box does not square up.

Which way the crease faces matters too. A crease folded away from the creased side is doing the job it was designed for; folded the other way it fights the displaced fibre and is much more likely to crack. That is decided at the dieline stage, by which face of the board is printed and which way each panel folds.

Cracking on a fold is one of the most common in-process quality checks on a carton run, precisely because it is the failure that shows up under handling rather than on the press sheet.