From the beginning, the printer of flexible material, by means of flexographic or rotogravure techniques, has had the need to control the color obtained in this process called conversion. Today we will talk about the ink viscometer. The ink is, in essence, a mixture of pigment that provides the color, resin that fixes the pigment on the substrate and the solvent that gives the ink a viscosity or ability to flow adequately to be able to move from the chrome cylinder to the gravure material, as well as from the anilox cylinder to the plate and from the plate to the flexo printing support. Of these three main components, the proportion between resin and pigment is fixed, since they do not evaporate and are consumed at the same time. The same does not occur with the solvent, which does evaporate and, therefore, changes the proportion between it and the solids responsible for the color, modifying, therefore, the tone or intensity obtained. From all the above we understand that maintaining the solids – solvent ratio allows us to obtain a constant color and this ratio is directly proportional to the viscosity of the ink. Therefore, by keeping the viscosity constant, we will have a constant concentration of solids and, therefore, the same color in the printed substrate. We have then an easy way to control and maintain the color by using viscometers.

The evolution of the ink viscometer

Viscometers, over time, have been adapted to the needs of the industry. At first they were manual systems such as the Ford or Zahn cups, based on the time required for an ink to be discharged from a container, the cup. The cup has different names and numbering, depending on the shape and size of the discharge orifice. Currently, more sophisticated electronic systems are used in different technologies to measure viscosity: – Systems based on the working frequency of a pneumatic pump when sending the ink to the printer. This is the simplest but most inaccurate system. – Line pressure reading systems, a sensor in the duct that transports the ink and measures the pressure exerted by it. The higher the viscosity, the higher the pressure.
– Vibration viscometers (also called ultrasonic), a paddle vibrates inside the ink reaching a different frequency for each viscosity.
– Viscometers for drop-body inks measure the time taken by a body immersed in the ink and confined in a tube, usually glass, where longer times correspond to higher viscosities. Once an automatic viscometer is installed, maintaining color is very easy, as long as the ink is kept at a constant temperature. This is not normally the case, as the printing process itself raises the temperature of the ink.

Principle of viscosity in inks

The problem is that the viscosity of the inks can vary not only because of the difference in the amount of solvent in the mixture, but also because of temperature changes. Let’s see what happens with an example:
An ink at 25 ° Celsius, diluted by 20% (for example), has a viscosity of 28“ in Ford 4 cup, that same ink with the same degree of dilution at 30 degrees will have a viscosity of 24”, lower.
This happens in countless materials, such as oils, plastics, wax, greases of all kinds: at higher temperatures they flow better (they have a lower viscosity).

What is the result of working with normal viscometers at constant viscosity when the ink temperature changes?

Let’s see it again with another example: A job with an ink at 25° Celsius and 25“ in Ford 4 cup (let’s assume with a dilution of 20%), when adding cooler ink at the same dilution, for example at 20°, we will obtain a viscosity of 30”, higher not because of the change of dilution which remains at 20%, but because the ink at lower temperature flows worse (its viscosity increases). At this point, the viscometer detects the increase in viscosity and adds solvent to reach the 25” setpoint, which increases the proportion of solvent from 20% to 25%. We then have a less intense tone! The solution to the error, introduced by viscometers, is to measure and compensate for the difference in viscosity caused by temperature changes, so that they only correct for variations in solvent proportion, but not for those caused by temperature variations.
In this case, we have temperature-compensated viscometers, also called solids concentration controls (pigment and resin), which guarantee a uniform colour throughout the run. An additional advantage that more sophisticated concentration controls can offer is to assign a numerical value to the set concentration (not the viscosity). By using this value in successive repetitions of the same job, we will obtain the same colour in the first printed metre, reducing colour adjustment times to zero as long as our anilox cylinder has the same volume (is clean) in flexography or, in the case of gravure, our engraving cylinder is in good condition (not too worn).