Have you ever seen a catalog with the inside pages protruding further than the cover? In appearance the cover looks slightly too small. You might assume the printer trimmed the cover incorrectly or perhaps there was an error during perfect binding. What you see was not a mistake or intentional. It is called Web Creep or Web Growth and is a common problem when catalog covers are printed sheetfed and inside pages are run on a heatset web press. The effect of Relative Humidity (RH) on paper fiber, the use of dryers in web offset printing, and insufficient time for conditioning are the main causes. Controlling environmental conditions of paper storage and pressroom are important for preventing web creep and other undesirable effects caused by unstable temperature and humidity.
Paper is subjected to different temperatures and levels of RH at the paper mill, during transport, in storage at the paper supplier, in storage at the printer, in the pressroom and bindery. Relative Humidity (RH) is the amount of water vapor content in the air in relation to how much moisture the air can hold at that temperature. When RH is high the paper fibers in the sheet or roll will absorb moisture and expand. When low, the fibers shrink. For optimum print quality it is critical that the paper achieves RH equilibrium, a balance between the temperature and moisture content of the paper with that of the surrounding air. Allowing time for paper to achieve RH equilibrium is called conditioning.