Tips & Tricks: Expansion of the Book Block

Outline and impact of problem
One speaks of expansion of the book block when the entire book block, beginning from the spine, shows strain that increases correspondingly up to the outer edge. This effect depends on the grain of the paper, the printing process, and the potential mixture of different papers in the book block and can become more pronounced when copies are stored in climatically unfavorable conditions. In this regard, the changes in paper dimensions in drier ambient air are very significant. This happens most frequently during the heating period of the colder season of the year.

Challenge and solution
Papers are hygroscopic and consequently react to moisture. Their dimensions alter by a factor of 3 to 5 at a right angle to the grain compared to the length of the grain. In case of moisture absorption, this manifests itself as expansion; in case of moisture release, this manifests itself as shrinkage. If different papers were printed with various running directions and/or using different methods, strong warping may result, leading to a loss of f latness. In order to avoid warping in a product, the same paper, with the same running direction, and the same printing method should be used for the core. Protecting the product for as long as possible from climatic inf luences by using appropriate airtight packaging is another safeguard.

Case Study
A product was printed in several different colors using the sheetfed offset printing method. In order to achieve a particular aesthetic effect, partially printed transpa- rent paper was combined with likewise partially printed coated cardboard. The product had an oblong format of 28.5 × 15.0 centimeter. A visual evaluation of the rejected exemplar indicated pronounced warping that increased up to the outer edge. The product fanned out, and was baggy and swollen (see illustration). It emerged that the grain of the transparent paper within the book block was uneven. The printer had attempted to cut as many sheets as possible from a delivery of the sheet format to use in the printing. The resulting paper had different running directions. In the lab, measurements made on the dimensional stability of the paper indicated that the hygroexpansivity of the transparent paper compared to the cardboard was three times higher. In a further test, the rejected book block was stored in a warming cupboard set at 40 degree Celsius (104 degree Fahrenheit). This made it clear what had caused the expansion in question:

• The book block was made from a mixture of different print stocks.
• The grain of the transparent paper was uneven.
• Compared to the cardboard, the transparent paper had a much higher hygroexpansivity.
• When the product dried during the winter months, the book block shrank in varying degrees.