Have you ever noticed that on some magazines, the text pages appear just slightly wider than the cover pages? The explanation lies not in the finishing process, but in the printing process.
A large percentage of short-run magazines have their cover pages printed on a sheet-fed press and their text pages printed on a web press. Paper is formed from wood fibers which are hygroscopic, which means that they naturally absorb or release water molecules in an effort to equalize with the moisture in the air that surrounds them. When fibers absorb moisture, the paper expands, primarily in width. When fibers lose moisture, the paper decreases in size, again, primarily in width.
Paper printed on a sheet-fed press is manufactured to a moisture content of approximately 4.5% to 5%, a level intended to insure stability in pressrooms with controlled humidity (in order to maintain register, it is important that sheet-fed paper does not expand or contract during the printing process). Paper that is printed on a heatset web press runs through a dryer to speed drying time. In order to prevent blistering (moisture seeking to escape through the surface of the paper while passing through the dryer), web paper is manufactured to a lower moisture content; generally 3% to 3.5%. That moisture level is further reduced as the paper passes through the dryer.
Once the cover and text are printed, they are bound together and trimmed. At that point in time, the cover and text are perfectly even. However, as the fibers in the text pages seek to equalize with the surrounding humidity, the text pages tend to “grow” to match the higher moisture content in the air, while the cover pages remain more stable. The amount the text paper expands depends in part on the humidity in the surrounding air and the type and weight of the paper.
Self-cover magazines, where the text and cover are both printed on the same stock, all on a web press, will not show text pages expanding beyond the covers. Likewise, magazines where the run length is long enough to justify printing both the text and cover on a web press, or short enough that both text and cover are printed on a sheet-fed press, also will not show expanding text pages. The phenomenon – also known as web creep – is commonly accepted in the industry, and is generally unique to those magazines who print their text and cover on the two different types of presses.