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FLEXO Magazine : June 2013
it increases setup time and reduces throughput. In addition, the accuracy of manual viscosity cup measurements is very dependent on how conscientiously the operators start and stop the stopwatch and how much margin for error they allow. Since numerous types of cups exist, the resulting cup-second measurement is only meaningful for the specific cup used. While electromechanical viscometers and automated vis- cosity controls have been available for over a decade, many OEM printing press suppliers have not incorporated them into their systems. In contrast, more sophisticated printing press companies use an automated approach in their press. As in other industries and other aspects of printing, automation improves throughput and quality. MEASUREMENT INSTRUMENTS Three electromechanical techniques are most commonly used for determining ink viscosity: the falling piston, the falling ball and the vibrating rod viscometers. The falling piston viscometer is composed of a cylinder and piston assembly. The piston is raised drawing the ink to be measured into the cylinder through an inlet path. The piston is allowed to fall by gravity, expelling the ink sample out through the same inlet path. The time of fall in seconds is a measure of viscosity that can be correlated to other units of viscosity, such as cup seconds or centipoise. Falling ball viscometers operate in a bypass line from the ink pumped to the printing press. Stopping the ink flow allows the ball to fall providing a timed measurement proportional to the terminal velocity and inversely proportional to the viscosity. The measurement is taken periodically and is not a continuous measurement. The separate fluid bypass is quasi- independent of the main loop to the printing press. It requires additional space to accommodate the separate flow path and associated instrumentation. The third approach consists of a straight metal rod main- tained in resonant vibration by a continuously applied power source. Installed in-line to the fluid flow, the sensor is between the ink pump and printing deck. The operating frequency is in the audible range (typically 300-500 Hz). High-pitched sounds and vibration in the press that is close to the resonant or harmonic frequency of the rod can affect the reading. Based on the me- chanical operation of printing presses, it may not be uncommon to encounter these frequencies, but they can be suppressed with mechanical isolation to allow acceptable operation. While falling piston, falling ball and vibrating rod viscometers have been successfully utilized in the industry, a new solid-state solution further simplifies the integration of the viscometer due to its small size, ease of use, and output interface options. Equally important, the sensor allows users to configure the viscoity control solution that is optimal for their application. SOLID-STATE SENSOR A solid-state viscosity sensor, based on bulk acoustic wave (BAW) technology, uses a piezoelectric sensing element excited by a high-frequency oscillator and operates in the thickness shear mode (TSM) of vibration. In this mode, shear displacement occurs on the crystal faces in the plane of the crystal plate. As shown in Figure 1, the displacement profile occurs throughout the thickness of the plate and is a maximum at the surfaces. Because the displacement motion is parallel to the plate, the TSM continues to operate in fluids making it ideal for fluid sensing. Figure 1: Increasing viscosity of a fluid placed on the top electrode results in an increase in the damping of the viscosity sensor and a decrease in the BAW’s oscillator’s frequency. 92 FLEXO June 2013 www.flexography.org