Viscosity control systems: models, functioning and advantages

An introduction to viscosity control systems, an instrument of innovation and a precious ally in printing in multiple sectors, from flexography to converting.

Viscosity control systems are divided into three main categories:

Off-line systems (typically rotational viscosity control systems inserted in a tank)
Bypass systems (typically falling ball viscosity control systems)
In-line systems (typically vibrating systems).

Off-line viscosity control systems

Offline systems are simple immersion instruments that sample the measurement continuously inside the tank containing the fluid to be measured. These systems require frequent and often costly maintenance, and they are bulky, in the sense that they take up a lot of space that is necessary to the operator to add ink. They often need to be extracted from the tank for cleaning and maintenance, and, as a secondary consequence, they are prone to frequent breaks which coupled with the physical modifications to the measuring element cause the loss of the correct calibration.

Bypass viscosity control systems

Bypass systems are the evolution of offline systems. They have the advantage of freeing up space in the tank and, since they are installed in a fixed position which is not modifiable, allow a more accurate measurement compared to an off-line system. Unfortunately, bypass viscosity control systems require valves to interrupt the flow during the measurement, causing increases in pression in the doctor blade chamber in flexographic printing, with the risk of ruining the printed material. The ink valves are subjected to stress (they perform at least a cycle per minute) and are therefore prone to wear. Furthermore, the bypass system increases the ink circulating and consequently the number of pipes to be cleaned during machine downtime, thus increasing the consumption of solvent used for washing. This type of viscosity control systems generally interferes with the printing pressure and therefore is prone to malfunctioning due to insufficient or excessive pressure. Given also that bypass systems are susceptible to the presence of clots and impurities, and that their measurement becomes unreliable over time, it is only natural that they have been outclassed by in-line systems.

In line viscosity control systems

The new in line viscosity control systems greatly simplify the installation on the machine because they avoid inserting invasive and difficult to install elements in the tank. In line systems reduce the ink circulating and therefore are the ideal solution. Installing an in-line system does not result in an increase in pipe length and so allows a higher level of cleanliness. They do not need reading sampling valves and thus guarantee a higher wear resistance.

The principle of resonance

Virtually all the line instruments that are now being commercialized are based on the principle of resonance. Resonant viscosity control systems make a body vibrate, and said body changes its frequency of resonation when the sampled fluid’s viscosity varies. They have the advantage of sampling a high number of measurements and take up a little space. Their limit is, however, their sensitivity to the vibrations produced by the machine. These systems need to be fitted with anti-vibrating solutions for the correct installation of the instruments. The measuring probe has a junction point that is very delicate for the measurement and as such cannot use Teflon based surface treatments to reduce ink deposit on the measuring body. The probe becomes increasingly heavier and the measurement drifts over time. This is the reason why high frequency vibrating probes cannot correlate with the measuring cup in the long run. We return then to the need to recalibrate the high frequency vibrating instruments regularly. This does not impede the maintenance of control over the viscosity of the machine but still forces the printer to rely heavily on the operators’ prowess, requiring them to recalibrate the instruments on the machine continuously.

Low frequency vibrating viscosity control systems

Low frequency vibrating viscosity control systems use a blade that vibrates without searching for resonances, and, like a fish’s caudal fin, measure the oscillation frequency of an ample and prolonged movement. Since the movement is ample, the micro-vibrations become inconsequential and thus there is no need to employ complicated anti-vibrating solutions. Moreover, considering also that the blade can be coated in PTFE (Teflon), it is understandable how measurement reliability is extremely high and the recalibration of the instrument is rarely performed and not usually recommended. Obviously, the objective of improvement is the same for all in line instruments: completely avoid the recalibration process, thus keeping a perfect correlation with the measuring cup. In this way the printer can use the measuring cup to verify the correct functioning of the installed viscosity control systems.

Advantages in consumption reduction

The viscosity control system is an extremely useful instrument, if correctly used, and allows to sensibly reduce ink consumption. A study conducted on site by our technical department observed a reduction in ink consumption of over 20% for solvent-based inks and over 10% for water-based inks. Furthermore the viscosity control system allows to keep an optimal repeatability of the printing quality and simplifies the whole work process.

Viscosity control systems are, therefore, the ideal instrument to control the printing process and allow production performance measurement.

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