Why does graphic board warp?

Market research indicates that warping of book covers is one of the more frequent problems encountered by bookbinders.

It´s a phenomenon that people in the industry know all too well. Even though you are using the same materials, the same equipment and the same procedures as usual there are certain production runs that inexplicably warp and lift.

Most often the blame is placed on the board rather than the cover; but why do covers warp? Why does the weather or season affect the cover? Why does the warp disappear?

Let´s start with one basic fact: provided the paperboard and other materials used are of good quality, warping is the binder´s problem. There are parts of a book that are prone to warp. One board may be somewhat less sensitive to warping than another, one case-making glue more resistant than another and some cover materials more prone to warp than others.

Most binderies implement controls to eliminate warping and curl. Although effective, these measures don’t always counteract the swings in weather or the change of seasons. The problem? You guessed it: moisture.

Paper and paperboard are made of long hair like fibers which at one time were composed of living cells. While no longer literally living they can still act very much alive thanks to the fluctuations in humidity and moisture. This fluctuation can cause the cover to destabilize causing warp and curl.

The absorption of moisture expands the fibers in their diameter across the grain of the paper or board. Conversely, loss of moisture causes the fibers to shrink mostly in diameter and mainly across the grain. The movement caused by moisture (“hydro-expansivity”) is greater as the moisture exchange is greater or faster.

The key controlling warping is to prevent large swings in the humidity of the environment. If this cannot be avoided, then the key is to make sure the changes in humidity take place as slowly as possible.

Be sure to use a paperboard that is made with as many types of wastepaper as possible. Each type of paper has a different balance between absolute and relative moisture; composite recycled board will be in balance with a very broad range of rates of relative humidity.

Carte graphique CFM is produced on a twin-wire Fourdrinier board machine with in-line lamination. This technique makes our board very stable and this results in a very good stiffness, flatness and smoothness.

Using cover materials with a low shrinkage factor will also help. Case-making glues can and will contribute moisture to the cover. Often the lower the quantity of glue, the less water is introduced into the cover. Note that the shrinkage factor of the glues of different bookbinders may be completely different.

Finally, one of the most important factors is the ambient relative humidity. With low air humidity the bookbinder must prevent the board, cover material, book blocks, books etc. from losing their moisture. Conditioning is the ideal solution, but very expensive and difficult.

Ensure that during dry periods a moisture exchange with the environment is prevented by screening the board and the books. This is easily done with a sheet of polyethylene or even paper. In the winter months only process the materials after allowing enough time for them to reach the temperature of the converting room.



How do you handle Graphic Board?

What can you do to make sure the graphic board / paperboard you buy will perform as it should when processed?

Here is some advice on proper storage and handling:

  • Keep the shrink wrapping on the paperboard until you´re ready to begin processing.
  • Let the board acclimatize for at least one day in your processing area.
  • Do not store it near hot or cold air flows, such as heaters or air conditioners.
  • After processing, re-wrap with plastic.
  • The ideal processing climate is 18 to 22° Celsius (64 – 72° Fahrenheit), at a relative humidity of 55% to 65%.