Substrate Adhesion
An ink transfer phenomenon occurs when wet or under-cured films migrate from a print face to the rear of an adjacent sheet within a stack during post-press finishing. Set off blocking represents an unwanted bonding of surfaces that ruins substrate finish and destroys image integrity. It governs the relationship between ink drying rates, stacking pressure and pile height.
The condition ceases to apply once the ink film reaches a state of total oxidative or ultraviolet polymerization that precludes transfer.
Thermal Mechanics
High ambient temperatures at the delivery end of a press lower the viscosity of binders and synthetic resins. Set off blocking happens when the residual heat in a fresh pile exceeds the glass transition point of the ink vehicle. Molecular mobility allows the liquid phase to bridge the gap between the printed surface and the back of the next sheet.
Mechanical pressure from the weight of the stack then forces this migration across the interface. Cooling sections or the addition of anti-set off powders modify the separation distance to arrest this movement. Mills formulate substrates with specific surface energy characteristics to mitigate the risk when high-coverage jobs require intense drying cycles.
Conversion Liability
Production lines demand precise control over stack configurations to prevent physical damage to the paper surface during transport. Set off blocking indicates a catastrophic failure of the drying process that leaves permanent scarring upon the fibre structure. Operators adjust the fount solution and fountain rollers to prevent excessive moisture, because trapped water hinders the reaction of chemical additives.
Drying tunnels that lack sufficient airflow fail to remove solvent vapours and invite adhesion between sheets. Proper management of the moisture content and ink film thickness prevents the formation of these solid bridges between individual layers of the stock. A well-executed print run maintains a stable surface chemistry that resists unwanted bonds under normal warehouse conditions.
