Let’s talk shop. Checking ink viscosity isn’t flashy—it doesn’t make anyone a printing rock star. But if you’ve ever dealt with streaky print, ghosting, or ink that starts drying in your ink pan liner before it hits the web, you already know: this stuff matters.

How Do Most Flexo Shops Measure Viscosity?

Simple: Zahn cups. These stainless steel cups with a small hole at the bottom have been pressroom staples for decades. Why? They’re affordable, easy to use, and—when used consistently—surprisingly accurate. You dip it into your ink, lift it, and time how long the stream takes to break. That’s your drain time—your viscosity baseline. Most flexo shops using water-based inks aim for 20–30 seconds using a Zahn #2 or #3, depending on substrate, anilox volume, and press speed.

Pro tip: Whatever works for the job, write it down. A job log or spec sheet saves time, avoids headaches, and makes repeat work easier.

How to Use a Zahn Cup (The Right Way)

  • Stir the ink thoroughly—settled pigments = false readings.
  • Dip the cup vertically, submerging it completely.
  • Lift it smoothly and start your stopwatch the second the stream starts flowing.
  • Stop the timer when the stream breaks into droplets.
  • Clean it immediately—dried ink inside the cup? Your readings are toast.

Total time? Less than 30 seconds.

Other Tools Some Shops Use

While Zahn cups are the go-to, some flexo shops use:

  • Brookfield viscometers for lab precision
  • DIN or Ford cups (especially outside the U.S.)
  • In-line viscosity control systems for high-speed, long-run setups

But unless you’re printing massive runs with ultra-tight tolerances, a Zahn cup and a stopwatch still get the job done.

Don’t Forget the Environment

Ink viscosity isn’t static. It shifts with:

  • Temperature changes in the pressroom
  • Humidity levels (especially in summer)
  • pH drift for water-based inks

What ran perfectly at 9 AM might turn into molasses by 2 PM. If you’re not checking mid-run, you’re flying blind.

Bottom Line

No one’s bragging about checking viscosity—but it’s the kind of small, behind-the-scenes detail that keeps your press run smooth, your print sharp, and your team out of trouble. Ignore it, and you’ll be cleaning up more than just ink.

Coming Next in the Series: “Troubleshooting Ink Viscosity Issues for Better Print Quality”. We’ll cover how to spot bad viscosity while you’re running, and what to do about it before things spiral.