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Due to the diverse nature of the printing equipment used (inking units, anilox screen rulings, doctor blades, etc.) and print jobs carried out, Flexo and Gravure inks are delivered in concentrated form.
The printer therefore has to prepare the ink before each print run in order to optimise the quality of the print, by adjusting:

• The viscosity
• The intensity
• The drying speed

The viscosity

The viscosity of a Flexo / Gravure ink is measured by the time it takes, in seconds, for a precise volume of ink to run through an orifice of a given diameter. The higher the viscosity, the slower the run off.
Several parameters are involved in the adjustment of the viscosity:

• The inking unit
Certain former generation “rising” inking units require high viscosity inks (above 60 seconds) to ensure the ink is properly transferred from the roller to the plate.

• The type of work
Fine screen printing (four colour process) requires quite a low viscosity (20 to 25 seconds) to optimise dot sharpness and limit dot gain.
In the case of solid prints, a higher viscosity (above 40 seconds) will result in smoother prints.

• The nature of the substrate
A relatively high viscosity is recommended on very absorbent substrates (vellum, kraft paper, etc.) to avoid the phenomenon of ink “burying” itself by penetrating too quickly into the substrate.

These various parameters should be kept in mind when determining the most suitable viscosity to ensure optimal print quality.

Viscosity adjustment should always be carried out on inks at room temperature just before they are used on the press.

Solvent based inks
Viscosity adjustment must be carried out with the recommended thinning solvents. Normally, the amount of thinner added varies between 20 and 30 %. The thinner(s) should be added slowly, while agitating the ink. It is preferable to adjust the ink to a higher viscosity than the planned viscosity in order to be able to fine tune this viscosity at the start of the print run.
Once the inks have been correctly adjusted, the viscosity should be checked during the print run. The viscosity has a tendency to increase in proportion in ambient temperature, and when there is low consumption of ink. It then needs to be re-adjusted as explained previously.

Water based inks
In normal conditions of use, viscosity adjustment is carried out using tap water. Since the viscosity drops very rapidly, the ink should be diluted in small dosages of 2 to 5 %.
Alternatively, viscosity reducer (5 to 10 %) should be used instead of water.
One of the principal characteristics of water based inks is that they have good stability on the press. However, when the temperature is high or the ink consumption is low, it is nevertheless necessary to incorporate a small amount of regenerator (1 to 2 %) every hour in order to maintain the viscosity and the pH at their initial levels.
The intensity

The intensity of the print is influenced by the speed of the press. Only carry out intensity adjustments at the true printing speed.
Optimum intensity is obtained with inks that are correctly adjusted in terms of viscosity and evaporation speed.
To reduce the intensity:
• Reduce the amount of ink deposited by making the appropriate adjustments to the press or by using a lower capacity cylinder screen.
• Add the recommended decolorant to the ink, adjusted beforehand to the same viscosity as the ink. Decolorant may be added in any proportion and does not alter the physical and chemical characteristics of the ink, with the exception of the light fastness .
See “Fastness of inks”

The drying speed
• Solvent based inks
Three types of thinning solvents are used:

- Normal solvent
This does not modify the initial drying speed of the ink. It should be used in most cases for viscosity reduction.

- Retarding solvent
This improves the stability of the ink on the press. It should be used for printing texts and, more generally, for print runs with low ink consumption. Normally, the amount that should be added is between 10 and 25 %.

- Accelerating solvent
This is used for cases where drying is difficult (solid prints) or for increasing printing speed. It can be used in place of the normal solvent.

• Water based inks

The drying speed of water based inks is only slightly influenced by the incorporation of solvents. The addition of a retarding solvent can inprove stability.
> See “Auxiliary products”
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