Viscosity in flexo inks

Viscosity is the measure of a substance's resistance to flow at a specific temperature. In the final printed results, the viscosity of the ink is a critical variable that greatly impacts quality.

Among its main characteristics is that it can be affected by temperature and must be continuously reviewed during printing to make the corresponding adjustments.

For flexo printers, controlling viscosity during printing is of utmost importance because it allows them to maintain three important aspects in the composition of the ink.

The first one is the color, which gives the nuances and intensity; the second corresponds to the Print quality, where it is reflected in the coverage, point gain and the quality of the trapping. And the last aspect is their performance properties, which affects the thickness of the ink layer, the drying speed and how much it retains solvents.

Do you know how to measure viscosity?

Ink viscosity can be measured in two ways, manually or automatically. Manual measurement is the most common in the flexographic industry, using gravimetric analysis, submergence, or cups, called flow cups. Automatic measurements are made with viscometers that are attached to the machines.

Manual method

The manual method used in the flexo industry is through the flow cup, or it is generally called the Zahn Cup.

This cup is a metal container with a hole in the bottom, which is dipped in the ink and later removed for measurement. The time it takes for the ink to drain through the hole in the cup is measured in seconds using a stopwatch, and this would be the viscosity value of the ink. Generally the measurement of is taken in seconds (secs).

There are Zahn cups numbered 1 through 4, but No. 2 and No. 3 are used in flexo.

Zahn Cup No.3 has a larger size in the recess at the bottom, and is recommended for water-based inks. While the smallest gap between these two is that of the Zahn Cup No. 2 which is preferably used for solvent inks.

The most recognized brands are Zahn, Shell, Din and Ford. But if you ask in the industry, they will surely refer you to the Zahn brand.

If you want to buy any of these versions, we share two options so you can get them at a good price.

Zahn Cup # 2

Suggested for measuring inks solvent base.

Container Volume: 1.5 fl oz, Step Time (sec): 20-80s.

Measurement range: 20cm to 250cm.

Certified by the ASTMD 4212 standard for the use of dip-type cups to measure viscosity.

The stainless steel cups are precision molded and the holes are precision drilled using high speed CNC drilling.

Zahn Cup # 3

Suggested for measuring inks water based.

Container Volume: 1.5 fl oz, Step Time (sec): 20-80s.

Measuring range: 10 cm to 800 cm.

Certified by the ASTMD 4212 standard for the use of dip-type cups to measure viscosity.

The stainless steel cups are precision molded and the holes are precision drilled using high speed CNC drilling.

How to do the test with Zahn cup No. 2?

It is very simple, you must have the ink that you will use to print, the Zahn cup and a stopwatch.

  1. Fill the Zahn Cup with ink without letting it begin to drain through the bottom hole.
  2. As you let the ink drain, start timing.
  3. Keep the cup in an upright position.
  4. Stop the timer when the Zahn Cup is unoccupied and the last drop has drained.
  5. Record that time, and take another sample again to verify the measurement.

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