Why it happens:
Paper is hygroscopic — meaning it absorbs or loses moisture depending upon its environment. When paper is exposed to changing moisture conditions, the fibers will contract or expand more in diameter than in length. This means when paper loses moisture, it contracts and decreases in size; when it absorbs moisture, it expands and grows larger.
Since the fibers expand more in diameter than in length, the potential for size change is primarily in the cross-grain direction.
What it means in the printing process:
When heat-set web signatures of text pages are subjected to high temperatures in the dryer, the paper loses moisture and shrinks. Web signatures start with less moisture than sheetfed covers to begin with. After the final trim process, the web text may grow slightly beyond the finished cover size as the paper achieves relative humidity equilibrium.
Since the industry-accepted best practice is to run paper grain direction parallel to the spine, the web growth beyond the sheetfed cover will primarily be visible on the face edge of the book. Self-cover magazines and catalogs will not show this. Neither will covers printed on a heat-set web press.
At this time, due to current paper availability, staffing constraints, and scheduling challenges, shorter-run projects will be produced using sheetfed covers with web signatures. Please be aware of the potential for web growth and design around it to help minimize its appearance.