Vapor Balance
Atmospheric vapor balance describes the ambient state where a material neither gains nor loses moisture to its surrounding environment. When paperboard reaches equilibrium relative humidity with the surrounding air, its internal moisture content remains stable because the rate of absorption equals the rate of evaporation. This state depends heavily on the temperature of the storage area since warm air holds more water vapor than cold air.
If the paperboard is stored in a sealed protective wrap, it preserves its internal moisture balance regardless of external seasonal fluctuations.
Moisture Exchange
Stacking unconditioned sheets in a room with a different ambient humidity initiates rapid water vapor transfer between the paperboard and the surrounding air. If the pile is colder than the room, moisture from the air condenses on the exposed edges of the sheets, which expands the fibers. Conversely, when dry room air draws water out of the pile, the edges of the sheets shrink while the center remains unchanged.
This unequal volumetric change causes severe physical distortions like tight edges or wavy board. Protecting the paperboard pile with plastic stretch wrap until it reaches the ambient temperature of the pressroom prevents these localization effects.
Dimensional Stability
Maintaining consistent ambient conditions during the printing and converting processes minimizes register shift and feed jams. Handlers utilize specialized hygrometers inserted into the paperboard pile to measure the internal humidity before removing the protective wrapping. This measurement ensures the stock is compatible with the printing press environment.
