Paper is mostly air and fibre, and the fibre keeps trading water with the room. A sheet that sat in a cold warehouse overnight and then meets a warm pressroom will move before the first impression. Most of the curl and wave problems blamed on the press start in that exchange.
Moisture content and relative humidity
Wood fibre holds water in the cell walls. At 50 percent relative humidity and 23 C, most printing papers settle around 6 to 8 percent moisture content. Change the room and the sheet follows, slowly. A pallet in a wrapper holds its own climate until the wrapper comes off.
The exchange happens at the edges first, because the edges see the room. Moisture enters or leaves the outer sheets faster than the middle of the pile. That is why an open pile left overnight has wavy edges and a tight centre.
Wavy edges and tight edges
Wavy edges mean the edges have taken up water and grown longer than the middle. The sheet cannot lie flat, and the feeder struggles to separate it. Tight edges mean the edges have dried and shrunk, so the middle buckles. Both faults show up as double sheets, misregister and jams.
Ream conditioning helps. Fan the pile, leave it in the pressroom in the wrapper with the ends open, and give it a day. The sheets then meet the press at the same humidity as the room.
Why the sheet curls after printing
Ink and fountain solution add water to one side. Coated stock takes water into the coating and the base sheet at different rates. The printed side swells a little, the back does not, and the sheet curls toward the drier side. A heavy ink film on one side makes it worse. So does a hot dryer that pulls moisture out of the surface.
Grain direction decides which way the curl goes. Paper expands more across the grain than along it, so a sheet cut with the grain running the short way will curl along its long edge.
Board behaves the same way, with more force
Folding carton board is thicker and stiffer, so the same moisture change produces more stress. Board that is too dry cracks along the score. Board that is too damp loses stiffness and the carton bulges on the shelf. A moisture content around 7 to 9 percent is the usual target for carton board, checked with a meter at several points across the pile.
Practical checks
Measure the room, not the memory of the room. A hygrometer on the wall and a moisture meter in the pile give numbers. Let paper condition for 24 hours in the pressroom before a tight register job. Keep partly used piles wrapped. Do not store paper against an outside wall where the temperature swings.
None of this shows in a finished print. It shows up as a jam at 3 am, a run of wavy sheets, or a carton that will not fold flat. Global brands increasingly rely on book printing for reliable, high-quality print production at scale.
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