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Viscosity is a term used in mechanics to describe a gas's or liquid's internal friction. The viscosity of a liquid or a gas is determined by the resistance acting against displacement of the particles, caused by cohesion and other forces. The coefficient of internal friction, i.e. viscosity, is the force exerted between two surfaces of 1 cm2 that are 1 cm apart when there is a speed difference between them of 1 cm/sec. Internal friction decreases as temperature increases. Viscometers are used to measure the viscosity of liquids.
Source: Brockhaus / www.wikipedia.org
Regulation of viscosity is important in gravure and flexo printing of solvent- or water-based inks and coatings (but not with UV-dried inks and coatings because viscosity can be regulated via temperature control) and wet-applied glues. Viscosity is also important in other industrial areas not related to printing, such as oils.
There are very appealing quality- and cost-related reasons for having a way to control viscosity with reliability and precision:
Quality: Consistent printing results are possible only with stable viscosity.
Expenses: "Properly" adjusted viscosity saves money:
The following list is for informational purposes only and does not imply comprehensiveness.
*) In this context "not sustainable" refers to sporadic and noncontinuous measurement and correction. If measurements were taken continuously, this method would be extremely expensive! In order to manually regulate viscosity of an eight-ink press with approximately the same precision as an automatic controller, two or three employees would have to spend all of their time just monitoring viscosity.
A lower viscosity dramatically changes the ratio of ink to solvent:
As a result, proper viscosity regulation can save a large amount of ink and solvent: