Viscosity and surface tension are two physical characteristics of liquid.
Viscosity is simply the measurement of how resistant to flow a liquid is, while surface tension is defined as the resistance a surface to liquid penetration. Viscosity and surface tension are affected by temperature changes. Understanding and controlling ink viscosity remains one of the most significant variables and flexo printing process. If this process is not controlled continuously in closely, uniform ink coverage and accurate color matching will not be achieved. Given the cost of solvent and water-based inks, printing operations of any size can save significant cost by controlling and viscosity. But understand that viscosity is not merely adding additional vehicle to the ink to account for evaporation, it's also controlling temperature.
Viscosity is determined by the time that it takes a given amount of fluid to flow through an instrument. When a fluid is heated, its molecules begin to move. The energy of this movement is enough to overcome the force that bind these molecules together allowing the ink to become more fluid and decreasing it's viscosity. For example, when syrup is cold it has a high viscosity and can be difficult to pour. However, when heated in the microwave the viscosity decreases and the syrup flows more freely.
So why does heat make ink more fluid or runny?
Molecules on the surface of the ink bind to the molecules beside and below them, but have nothing above them to bond to. Because of this in-balance, the molecules on the surface of the liquid will be drawn more strongly to those around it. As the temperature of an ink increases, it's surface tension decreases. When the ink heats up, the movement of its molecules disrupts the imbalanced forces on the surface of the ink and can form a sheet like barrier of tightly bound molecules thereby lowering the surface tension. This is why hot water is more effective when cleaning; it's low surface energy allows it to easily penetrate the fibers of the fabric and wash away stains.
So now that we understand how temperature affects the surface tension of our flexo inks, how can we further control delivery of the ink to the substrate? Understanding that ambient conditions can fluctuate greatly in some flexographic printing environments, some press manufacturers, including Bobst, utilize a system that runs hot water to the anilox rolls to bring the core temperature above the fluctuating ambient temperature of the pressroom. What this means is the anilox is always warmer than its surroundings and therefore fluctuation in ink delivery is greatly minimized.
But the state of the art press manufacturers don't stop at just heating the core temperature of the anilox. Some presses now utilize infrared heat to control the heat applied to the surface of the anilox not only to regulate ink temperature, but to maximize the ink transfer from the anilox to the printing plate. What this means is that as the temperature applied to the anilox roll increases, the operator can then see an increase in solid ink density (SID) by adjusting this temperature. While this technology is very noticeable when UV ink start being used, the effects can be seen in both solvent and water-based inks as well. Who would've thought that when we look to slightly increase or decrease SID, in some cases we would first look to adjust anilox surface temperature?
Tim Reece - All Printing Resources, TSG Team Member – Has over 34 years experience in narrow to wide web flexo. Tim joined APR eighteen years ago with his primary focus being on technical services and training. Tim has certified and re-certified 3 times by the NCSS as an Expert Press Operator and is Level III FIRST Certified Implementation Specialist. He has spoken for the TLMI on training and process improvement and has presented at FTA Forums and InfoFlex as well as serving as a judge for the FTA printing awards on four occasions, and now serves on the committee. Specialty areas include photopolymer platemaking processes and development, primary pressroom converting, and sustainability.