Cold Foil Register Drift as the Stack Warms across a Shift
Stack thermal buildup causes substrate shrinkage and mechanical expansion, driving cold foil register drift across continuous production shifts.
Pile Moisture Migration
Moisture Distribution
The transfer of water vapor within stacked sheets of paperboard occurs when there is a difference in temperature or humidity between the stack and the room. Pile moisture migration is the movement of moisture from the warmer center of the stack to the colder edges, or from the humid outer atmosphere into the dry sheet margins. This shifting of moisture levels changes the dimensions of the paperboard fibres, which can cause the sheets to warp or wave.
The boundary of this migration is determined by the temperature equilibrium of the stack.
Atmospheric Effect
Temperature differentials between the paperboard stack and the ambient air in the pressroom are the primary driver of this physical change. Pile moisture migration is accelerated when cold pallets of board are brought from cold warehouses directly into a warm printing shop without being allowed to acclimatize. Rapid exposure to a different climate causes the edges of the sheets to absorb or release moisture quickly, while the center remains unchanged.
This uneven moisture profile leads to feeding problems on the press.
Flatness Management
Moisture barriers such as plastic wrapping are used to protect the pallets during storage and transport. The wrapping must remain sealed until the board has reached the same temperature as the pressroom. Proper climate control in the pressroom keeps the sheets flat.