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FLEXO Magazine : June 2013
Technologies & Techniques Viscosity Sensing in Printing Applications Measurement of Aqueous & solvent-Based inks Can Benefit From the latest Advancements in Technology By Justan steichen Maintaining the right viscosity for printing ink is es- sential to ensuring a high-quality printed product. Mixing the printing ink too thin may result in color inconsistencies, mottling and weak color print. On the other hand, mixing the printing ink too thick may result in adhesion issues, feathering, smearing or fill-in. Furthermore, printing product with excessively thick ink results in unnecessary costs and wasted ink. Techniques for determining ink viscosity vary considerably ranging from manual measurement instruments to microsen- sors with electrical output. The traditional manual viscosity measurement method uses a viscosity cup and a stopwatch to determine whether the ink is too thick or too thin. Within the past couple decades, various viscosity instru- ments with an electrical output have been developed to re- place the manual method and have allowed the measurement to become an integral part of the automated printing process. More recent advancements in viscosity sensing have result- ed in the development of in-line, compact microsensors with the added capability of algorithm adjustments to optimize the sensing technique. Additionally, printing press original equip- ment manufacturers (OEMs) can now design their own ink viscosity control systems for specific customer requirements and realize considerable cost savings. This paper discusses the existing viscosity measurement techniques and reviews the latest advancements in acoustic wave viscosity sensor technology. VISCOSITY MEASUREMENTS In the printing industry ink viscosity measurement using a viscosity cup and a stopwatch is still considered to be the historical standard against which all other viscosity measure- ment techniques are referenced. There are numerous viscos- ity cups available, however, two viscosity cups, EZ Zahn #2 and Din 4, are the most commonly used. Even today, some printers still make adjustments to viscosity with manual addition of solvent or base ink based on a viscos- ity cup reading. This approach has two major shortcomings: Improved vIscosIty measurement • In-line viscosity measurements provide real-time feedback for improved viscosity control • Quality and consistency of ink measurement is improved through intrinsically safe solid state sensor design • No moving parts helps increase the ink delivery system’s reliability, repeatability and robustness • Due to the sensor’s small size, ink delivery systems can be configured in ways that were not previously possible • Viscosity measurement is achieved by correlating the sensor’s BAW electrical parameters to the acoustic viscosity (AV) of the fluid • Closed loop control of viscosity is reduced through the use of modern output interface options • High-throughput flexographic printing is an ideal application example of the solid-state viscosity sensor www.flexography.org June 2013 FLeXO 91