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Viscosity, often defined as "resistance to flow," is one of the most important properties of a printing ink. The higher the viscosity, the greater is the resistance to flow. Rheologists have models that can describe viscosity in terms of the force required to move molecular layers at specific shear rates.
Since most inks do not follow ideal Newtonian flow, the description and measurement of ink viscosity is a study in itself. High-viscosity materials, such as lithographic inks, are typically measured by measuring the time it takes a falling rod to pass through the ink. Low-viscosity materials, such as flexographic and rotogravure inks, are measured by comparing the time it takes a given amount of ink to empty from an efflux cup.
Many printing processes have narrow viscosity requirements. Typical viscosities of printing systems are shown in the table below:
The viscosity of a printing ink is primarily a result of the resins used in the...