Blocking is what happens when two coated surfaces stick together in the reel or the stack — the film tears or the coating transfers when they are separated. In flexible packaging it is a routine and expensive failure.
It is fundamentally a Tg problem. A coating soft enough to have excellent adhesion and flexibility is usually soft enough to flow under pressure and stick to whatever it is wound against. Heat and pressure both make it worse, which is why blocking often shows up in summer storage or in tightly wound reels.
It is addressed by raising Tg, by adding waxes or slip additives that create a low-friction surface layer, by ensuring complete drying or cure before winding, and by controlling winding tension.
The tension between block resistance and adhesion is one of the fundamental trade-offs in packaging ink formulation.