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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.
- Inks and coatings are often delivered to the machinery in a viscosity unsuitable for printing.
- Evaporation of the solvent during the printing process will alter viscosity. Water-based inks and coatings are subject to much less evaporation.
Expenses: "Properly" adjusted viscosity saves money:
- If viscosity is too low, quality will suffer (too thin) while saving ink, but at the cost of increasing waste expenses as well as higher expenses for solvent.
- If viscosity is too high, quality will suffer (too thick) with excessive and costly levels of ink consumption that can never be offset by the solvent savings.
The following list is for informational purposes only and does not imply comprehensiveness.
- Very low cost, but imprecise and subject to personal inconsistencies, not sustainable *.
- Low-cost, relatively precise (approx. Â± 1 second, practiced printers often achieve much greater precision), but time-consuming and not sustainable*).
- Economical, theoretically accurate, but lower long-term stability, not suitable when a small amount of ink is left in the ink tank or if tank contents are not completely homogenous due to sedimentation or thixotropy.
- Economical, high measurement accuracy thanks to permanent self-cleaning, equally accurate with residual amounts of ink, lack of homogeneity in the ink tank, or with thixotropic ink. Easy to handle, no need to carry around measurement instruments. Automatic washing.
- Good measurement accuracy, suitable for process inks whose composition must be consistent at all times, because inconsistency would require recalibration; straightforward cleaning; but expensive.
*) 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.