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FLEXO Magazine : March 2010
20 FLEXO MARCH 2010 www.flexography.org TECHNOLOGIES & TECHNIQUES Special Notes on Zahn Cup Measurement • Hole sizes will result in different stream break char- acteristics. • Always include the numbered hole size of the cup when communicating viscosity. • If there is surface foam, swirl it out of the way before taking a measurement. • Microfoam in the ink (tiny bubbles in the body of the ink) can cause inconsistent viscosity readings. • Ink sitting still can have a different viscosity reading than the same ink under shear and flow on the press. • If this is your first time using a Zahn cup, take three or four readings of the same ink to check yourself for consistency in measurement. Viscosity is an important ink attribute. While modern technology allows ink viscosity to be automatically measured and regulated on its way from the pump to the metering system, many flexographic printers/converters still check their inks manually. This article will explain viscosity and the correct use of the Zahn cup to consistently measure ink viscosity. VISCOSITY DEFINED Viscosity is simply the resistance of a liquid to flow against itself. The more resistance a liquid exhibits, the slower it flows. In printing we tend to think of slower flow as thicker ink. Why is this flow property of interest? The reason is, viscosity changes can cause changes in color density, affecting how well an ink flows through the metering system, how well ink transfers from the anilox roller to printing plate and then from the printing plate to the substrate. It also affects ink build-up on the printing plate, as well as final clean up of ink. With all that viscosity can influence, the press crew deter- mines what the operational viscosity of an ink should be on press. All presses can behave differently and have a sweet spot for viscosity depending on what anilox rollers are in the press, how fast the press is running, what the graphics are and what substrate is being printed on. For these reasons, the press crew knows best. The ink supplier can only suggest a recommended viscosity range. MEASUREMENT TOOLS A stopwatch and an efflux cup are the only two tools needed to measure ink viscosity press side. Efflux is a term describing a family of viscosity cups. Three common styles of efflux cups in printing are the Zahn, Shell, and DIN cups. All cups have different sized holes in the bottom to allow the ink to flow out. Common hole sizes in printing are No. 2, No. 3 and No. 4. The higher the number, the faster the ink will flow out of the cup. For this reason, it is important to include the hole size along with the style of cup when communicating viscosity numbers. The Zahn No. 2 and No. 3 cups are the most widely used efflux cups in printing. PROCEDURE The best practice for using a Zahn cup is as follows: • Rinse the Zahn cup with water to remove dust and debris. • Inspect the Zahn cup for damage or dried ink. • Submerge the Zahn cup in ink then pour the ink out. This ensures a consistent baseline • Submerge the cup in the ink again. • Raise the Zahn cup up to where the rim is just noticeable below the ink surface. • Pick up the Zahn cup while starting the stopwatch pre- cisely as the cup rim breaks the ink surface. • Allow the cup to dangle straight up and down while the ink flows out. Do not tilt the cup. This will cause inaccu- rate results. • Stop timing when ink stream breaks approximately 1in. below the bottom of the cup. • Record the time. ABOUT THE AUTHOR: David Stokes is a junior in Graphic Communications at Clemson University. Zahn Cup 101 Proper Use Ensures Accurate Measurements By David Stokes Stop the watch when ink stream breaks approximately 1in. below the cup. BEGINNER FLEXOGRAPHER
Sustainable Winter 2010