Web growth is a phenomenon that happens during the manufacturing process for books. We get many inquiries about it from customers who receive books thinking a mistake has been made. While we do our absolute best to prevent this from occurring, it can still happen.
A common issue with heat set printing methods is the discrepancy in width between the pages and covers of books due to fluctuations in moisture — a phenomenon known as web growth. Web growth can occur on any press that uses heat to set ink including some digital printing presses, not just traditional offset presses. When paper loses moisture and gains it back later, it can appear like the cover of a book was trimmed too short. Web growth can be visible hours after a book is trimmed to its final size however the full extent of expansion is not realized until days or even weeks have passed. Web growth is more apparent in soft cover books and some paper stocks are more susceptible to it.
So why does this happen? All paper is hygroscopic, which refers to paper’s tendency to gain or lose moisture based on the surrounding environment. After ink has been pressed onto paper with a heat-set press, it passes through a dryer at one end of the machine. Paper can reach between 200 and 300 degrees Fahrenheit in a matter of seconds causing it to lose moisture. Paper maintains heat of about 250 degrees upon exiting the press.
After paper has been printed on a heat set press, it is cut, folded and bundled into logs. Signatures remain compressed until the moment they are fed into a binder leaving them little time to regain the moisture they have lost. In contrast, covers or inserts that are printed on a sheet fed press have not been exposed to heat and have not lost moisture. When covers and pages are combined in the binding process, both are trimmed to the same size. Pages printed on a heat set press can now potentially gain moisture growing wider than the covers in soft cover books. Some paper stocks are more susceptible to web growth than others. For example ground-wood stocks tend to fluctuate in size more than hard white paper stocks.
While web growth can be minimized to a certain extent, a cost-effective solution to eliminate its occurrence has not yet been developed. Edwards Brothers Malloy utilizes silicon application systems on many of its heat set presses that help restore some moisture to paper after it has been printed. Dryer temperatures on heat set presses are optimized to avoid an excess loss of moisture. Although these measures help prevent web growth it, is a phenomenon that is difficult to control. If you have more questions about web growth and why it occurs, please contact your customer service or sales representative.