Why temperature is important in viscosity determination?
Temperature is the single most important parameter for obtaining accurate and precise viscosity measurement. This is especially true for inks and coatings, as their rate of viscosity change per unit temperature is high.
Why does the ink temperature rise?
While ambient temperature does have an impact, its significance primarily emerges during the initial phase of the print run, often referred to as the ‘cold start.’ As ink transitions from the tank to the tray or chamber, then onto the printing plate, and finally onto the substrate, it encounters friction. Simultaneously, the oscillating movement of the doctor blade on the print cylinder or anilox roller also creates friction. The entire process of transferring ink from the ink fountain to the printing plate and the subsequent interaction with the substrate entails the application of shear forces. These frictional and shear forces contribute to a rise in temperature, generated because of the dissipation of energy in the form of heat.
What happens when the ink temperature rises?
As temperature rises, one will expect ink viscosity to decrease. However, the opposite occurs on a rotogravure or flexographic press. Solvent-based inks, in such instances, undergo a paradoxical behaviour: their solvents evaporate more rapidly, necessitating additional solvent to maintain the desired viscosity.
This phenomenon leads to an increase in total solvent consumption, encompassing both the solvent added during the makeready phase and that topped-up during the print run, as temperature rises. Consequently, the overall mass, which includes ink and solvent, also rises with increasing temperature.
Ink consumption too, excluding the solvent added during makeready, may also have slightly increased as temperature climbs. This uptick in ink utilization at higher temperatures may be attributed to the warmer ink’s higher pigment-to-solvent ratio, resulting in the application of a thicker layer.
How is ink temperature maintained?
To effectively regulate ink temperature on gravure and flexo presses, printers employ various cooling systems designed to keep the ink within the optimal temperature range for superior printing performance.
A temperature sensor is strategically positioned near the point of ink utilisation, ensuring continuous monitoring of ink temperature. It relays real-time feedback to the control system, enabling immediate adjustments to the cooling components.
These cooling systems, usually comprise a liquid-to-liquid heat exchangers, are seamlessly integrated into the ink recirculation system, positioned just before the ink enters the ink tray or chamber. Chilled water is used to precisely control the ink’s temperature. By modulating the flow of cold water, the ink temperature is maintained at the desired setpoint.
Can ink cooling replace viscosity controls?
No, an ink temperature controller should not be considered a substitute for a viscosity controller when it comes to rotogravure and flexographic presses. These two devices serve distinct purposes.
When ink is too thick, it can lead to issues like poor ink transfer and uneven coverage, while overly thin ink can result in problems like dot gain. In either case, print quality suffers. A viscosity controller adjusts the ink’s consistency to ensure it flows smoothly through the press and onto the printing substrate.
Conversely, an ink temperature stabilizer is specifically designed to maintain a consistent ink temperature throughout the printing process. This is crucial because temperature fluctuations can directly impact the ink’s viscosity, which, in turn, affects its performance on the press.
For optimal control and consistency throughout the printing process, it’s important to use both a viscosity controller and an ink temperature stabilizer. If budget constraints or other factors necessitate a phased approach, particularly in tropical climates, it is advisable to prioritise the implementation of an ink temperature stabiliser first, followed by the addition of a viscosity controller.
In the next and concluding section, we will delve further into the advantages of ink cooling, provide an example illustrating these benefits, and address operational challenges, among other topics.