Cleft is a process that is carried out after printing and consists of making a groove or mark by pressing the paper or cardboard at the point where it is to be folded in order to facilitate subsequent folding.
The aim of the designer and printer is to reproduce a job as close to the original as possible, in any quantity. Many jobs must be folded after printing, and clients will not tolerate a finished, folded piece that is marred by any “cracks” or splits in the sheet. Some printed papers fold without problems (usually text papers and lightweight covers that fold with the fibre).
First, let’s look at 3 factors that affect the paper. Secondly, we’ll look at how cleft works and why it’s an important solution.
- The structure of the paper varies according to the thickness of the coating, the fibre content, the length of the fibres, the bond between the base and the coating and the pulp treatment. Heavy coatings and low fibre content mean less stability and a greater tendency to crack. No sheet is immune to cracking, but because the coated sheet is more complex than the uncoated, it is more prone to fibre cracking. There are simply more things that can go wrong. But coated papers tend to reproduce originals better than uncoated sheets; that’s why you see more full-colour work printed on coated sheets.
- The printing process makes the paper less flexible with the addition of countless combinations of inks, varnishes and coatings. The heat applied to quickly dry the printed sheet as it comes off the press affects the moisture content of the sheet, further reducing flexibility.
- The environment, especially with regard to the moisture content of the paper, influences the likelihood of paper cracking. A printing or bindery room with a relative humidity of 50% will have fewer cracking problems than an environment with a relative humidity of 25%.
Why use cleft in printing?
Many folding jobs are susceptible to cracking and scoring is often not sufficient to ensure crisp, accurate folds to eliminate this problem. Digitally printed products are at even greater risk of cracking at the fold because the process involves heating the sheets to fuse the toner to the paper, which tends to dry out the paper. Dried paper is more prone to cracking.
The cleft creates double folding points that reduce the tension in the fibre at each point. The paper is folded a total of 180 degrees, but each stress point is folded to a lesser degree. This seemingly small difference will reduce or completely eliminate cracking in most digitally printed jobs.
When the sheet is folded (cleft) at the cleft, all the outside force that would normally split or crack the sheet is directed inward toward the weak, delaminated line. As the fold of the paper delamination is extended, (on the left) the sheet delaminates further inwards and a rounded bead forms on the inside of the fold, which absorbs all the force, while the outside of the folded piece remains smooth and intact. Cleft can be done by the platen method (both the sheet and the die are flat) or by the rotary method (the paper is passed between cylinders or wrapped around a cylinder). Both methods use a male die to force the sheet into a female channel, the dimensions of which vary according to the density of the paper.
Sometimes, the simple cleft eliminates fibre splitting and cracking, but as sheets become heavier, this is less likely and the stronger compressive forces of the cleft are needed to prevent cracking. In high quality commercial printing, the cleft is therefore the second critical strategy used to combat the problem of fibre cracking. Not only does it aid folding, but, when done successfully, it achieves the commercial printer’s and designer’s goal of faithfully reproducing the print.
On which products is the cleft used?
The cleft process is often used for products that need some kind of folding, be it brochures, boxes, books. Thanks to the cleft process, more perfect folds are achieved, which guarantees a much more professional result for our products.
If you want to know more about other types of finishing to make your printing projects perfect, take a look at our finishing section where we detail different processes to make your products stand out from the rest.