The Primary Causes of Warped Cover Boards, Part I

01 Oct 2012

Board warpage

Board warpage

Board warping problems, which most of the time occur in the winter, arrive at our book-testing laboratory from all over the world. The purpose of this article is to furnish you with some basic information on why such problems do occur.

Mirror, mirror on the wall, who has the biggest warp of them all?

A Primer on paper and paper boards

When paper or paperboards are made, approx. 70 to 80 percent of the fibres float in the machine direction (MD). Paper fibres are hygroscopic. They pick up water when the moisture is high, they lose water when the percentage of moisture is low.

Fibre expansion
Fibre expansion

Paper fibres expand 4 to 5 times more in their width than they do in their length. Therefore, one must make sure that all cover-materials, boards, endpapers have the grain parallel to the binding edge.

How paper and paperboards do react to moisture in an ever changing environment, is shown in the adjacent figure:

Are there guarantees against paperboard warping?

What are the basics of a paperboard? All paperboard manufacturers rely on paper-waste which, being a re-cycled product, is hard to control for consistency. Some use newsprint, others magazine papers etc. For example, a high amount of corrugated box content attracts more moisture, whereas groundwood or thermo-mechanical furnish, i.e. soft fibres, respond to less moisture. In addition, papers and boards with short fibres tend to saturate more rapidly and therefore expand and contract more quickly. Long fibred stocks are more stable in this respect and in certain instances, it is possible to exercise considerable control, a factor every mill tries to achieve in a best possible manner. Unfortunately, with today's trends toward recycling, we must cope with shorter fibres. This is also true for the printed papers used as covering materials.

There are many different techniques being used to manufacture paper boards. Most important for them is to manufacture a paperboard product that is dimensionally stable. Some structures promise to offer more stability than others. That maybe true, but despite those claims made by some paperboard manufacturers, this bookbinding expert has experienced and analyzed warping problems on all brands and qualities of paper boards made. I have never come across a single warping incident where, at least in my opinion, where the paper boards were at fault.

How moisture or an adhesive application affects papers and boards

The most fundamental relationship in a print-finishing process does lie in the reaction of papers, boards etc. on each other in the presence of water-based adhesives, that is animal or protein glues, cold-emulsion PVA's, starches etc. To understand these effects fully, it is essential to have a good understanding of the construction of the fibrous materials and how they react to moisture during the drying and manufacturing processes. These factors greatly influence the performances of the finished products in the ever changing climatic environments. Example, books made in Asia during the summer in high humidity, then transported to a bookstore in Minnesota, where, during the winter, the air is very dry.

Paper and paperboards are all based on cellulose fibres, which have a tubular shape. Given the opportunity, these will absorb water by capillary action and as a result, the individual fibres will considerably increase in their diameter, but gain only a fraction in the fibres lying in a more or less parallel direction. As each of the fibres expands in its diameter, expansion will occur. The expansion is across the machine direction.

Now, if a binder applies a water-based adhesive onto one side of a paper-board, this will cause the fibres on the surface to expand rapidly. All this does happen, because there is a time lapse before the moisture has penetrated evenly through the board. Therefore, and that is important, expansion will initially be uneven and a curl will be induced into the paperboard. If the same amount of moisture is applied to the other side, the board will flatten out.

The following schematics are self-explanatory:

The interplay between moisture and paperboards
The interplay between moisture and paperboards

Now, try to solve a problem sent to our laboratory. A manufacturer produced puzzle boards. They mounted a color printed sheet only onto one side of the board and then they did wonder why the boards warped! Now, as a consultant and the necessary knowledge of the forces at work, you can give them the following advice: “In order to make a board to lie flat, one must mount identical papers onto both sides, same grain direction, same amount of adhesive!”. Timing is critical as well. As you can see for yourself, it is a relative simple solution! Just try to understand Mother nature at work.



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