Educational Programs

Print Buyers Guide

Web Growth or Creep

Web creep results in a difference in the size of the pages and covers of a trimmed book. This problem is caused by low moisture content of some signatures, usually associated with heat set web printed signatures expanding beyond sheetfed (covers with normal moisture content) after binding and trimming. This phenomenon is often referred to as, "web creep", or "web growth". The GATF book "What the Printer Should Know about Paper"' contains a good explanation of the hygroscopic nature of the cellulose fiber. Changes in the dimension of the cellulose fiber are greater in the diameter of the fiber than in the length of the fiber, and explains the rather drastic changes in paper size when the paper is subjected to significant changes in the paper's equilibrium moisture content. The change in moisture content is due to the low moisture content of the web signatures having run through a heatset dryer. The paper actually shrinks in the dryer, and as the printed signatures sit and recondition by absorbing moisture from the atmosphere, the signatures expand. If the dry heatset signatures are bound with other signatures or a cover and trimmed before they reach an equilibrium with the moisture in the atmosphere around them, they will continue to expand beyond the signatures or covers that are at a RH equilibrium. This problem is very apparent when a bindery binds and trims a sheetfed printed cover to heatset signatures with very low moisture content. To prevent this, the signatures must be the same RH as the sheetfed covers which will have basically the same RH as the atmosphere around them.

Suggestions to reduce this phenomena would include:

  1. Run the press dryer temperature as low as possible.
  2. Use an in-line re-moisturizing unit after the dryer to replace some of the moisture lost in the dryer will reduce the conditioning time.
  3. Allow the signatures time to condition after printing.
  4. Some will place the signatures in a small enclosed room with humidifiers to accelerate the conditioning process.---other than this forced conditioning, there are no shortcuts.

I know of no official standards as to what is an acceptable amount of "growth" or how much a customer should except. The printer and/or binder should be on the look out for this problem especially when the cover has been printed with a dark solid or is expected to be of high quality. The understanding of the customer is important to allow the necessary time for the heatset signatures to condition so as to minimize the web creep phenomena.

Courtesy of Dillon Mooney
Research Technical Information Desk
Graphic Arts Technical Foundation

Return to Top