(Quoted from Nazdar Ink Technologies. Full article here.)

What is ink blocking?

SGIA defines ink blocking as "an undesired adhesion between layers of materials placed in contact under moderate pressure and/or temperature in storage or use, usually occurs in a stack of printed material that is stacked prior to thorough drying." This can occur with single- or double-sided prints.
What causes ink blocking? Blocking can be caused by a number of factors, and finding a solution to this production issue often requires adjusting more than one variable.

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