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What is a crease line? Folding lines on boxes and covers

Fold heavy paper or board without preparation and it cracks along the fold, the ink breaks and the edge ripples. To prevent that, a crease line is pressed in first: the sheet is not cut, only pushed into a channel. On boxes, folders, covers and invitations, that line is what makes a clean fold.

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Crease versus cut

A cutting rule cuts through the sheet. A creasing rule is a round-edged steel strip, slightly lower than the cutting rule, that only presses: it pushes the board into a channel on the other side, and the fold then follows that groove.

There is also perforation, a broken cut meant for tearing by hand. A crease is for folding, not tearing, and the three should never be mixed up in the file.

When do you need it?

Thin paper such as leaflets usually folds on a folding machine without creasing. As the stock gets heavier, and especially on board, folding without a crease leaves cracks.

The main cases where you should ask for a crease:

  • Boxes and packaging: every fold and tuck flap is creased.
  • Heavy covers: the spine and hinge lines on a perfect-bound book.
  • Full-bleed or dark backgrounds: ink and lamination crack on the fold and show white.
  • Folders, invitations and menus on heavy card, with a single or double crease.

Grain direction and cracking

Paper fibres run in one direction. Folding parallel to the grain meets less resistance and cracks less; folding across the grain cracks more easily.

So when the job is laid out on the sheet, the fold direction and the grain direction are considered together. Where that is not possible, the crease width and depth are adjusted to the board thickness. The finishing shop usually sets that, but putting the crease in the right place in the file is your job.

How a crease is shown in the file

In a dieline file, cut and crease come as separate lines in separate spot colours. The usual convention is a solid line for the cut and a dashed line in a different colour for the crease. These lines never go to the printing plate; they are for making the die and for checking.

Common mistakes:

  • Cut and crease in the same colour: the shop cannot tell which one to cut.
  • Die lines left as a printing colour: the lines print on the job.
  • Text or a logo sitting right on a crease: the fold splits it in two.
  • No allowance for board thickness: panels rub against each other when the box closes.

See the fold before the press

The easiest way to know the creases are in the right place is to fold the box on screen. Do the panels close in the right order, does the tuck flap seat, does any text land on a fold? All of that shows on the folded box, not on the flat file.

For a new box design, a sample cut before the run also shows cracking and fit problems before they reach the whole job.

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