Viscosity is a liquid’s resistance to flow that is measured by time and temperature, and it is paramount to how coating performs on press.
If a coating’s viscosity is too thick, the pumping system won’t be able to pump the liquid from the barrel causing the system to shut down. This will result in no coating being transferred to the sheets.
If the coating is too thin, it will leak from the coating chamber. Since presses run at incredibly high speeds, the thinner liquid will splash all over the press.
Some of the performance issues that can be a result of not measuring your coating’s viscosity include the coating layer being invisible, uneven lay down, and too little coating due to it being watery.
A brief overview of coating viscosity:
Two different cups are used to measure coating viscosity based on your location. The Zahn 3 cup is primarily used in the United States, and the Din 4 cup is used in the rest of the world.
Time and Temperature
If you are using a Din 4 cup, the viscosity should fall between 35-45 seconds. If you are using a Zahn 3 cup, the viscosity typically falls in between 18-20 seconds.
NOTE: In some cases, stating that a Din 4 cup should always have a velocity between 34-45 seconds can be misleading. An example of this is in Asia where our Saphira coatings are measured to meet the 60 second viscosity with a Din 4 cup.
We have two different viscosity numbers because we have a Two Roller System and a Chamber Doctor Blade System. For the Two Roller System, we select the 35 second viscosity. For the Chamber Doctor Blade System, we select the 45 second viscosity.
When using the Din 4 cup, standard temperature for measuring is 77°F. When using the Zahn 3 cup, the standard temperature of measuring is 20°C.
NOTE: Both of these temperatures do not translate to the same thing. 77°F is equal to 25°C, and 20°C is equal to 68°F. When the coating is measured at the same temperature with both cups, the Zahn cup will be approximately half of the Din cup.
Temperature can play a huge factor in viscosity levels. When the environment is warmer, the viscosity will be lower. When the environment is cooler, the viscosity will be higher.
Other Factors to Consider
Another huge factor in the results of viscosity is if the drum has been opened and circulated. If the coating was circulated through a pump system, the viscosity reading may not be accurate because of the formation of microbubbles. Coating with microbubbles may cause an extremely high viscosity reading that may be considered a false reading. Therefore, all viscosity readings should be done prior to using the coating.
Tools used to measure viscosity:
Din 4 cup (Europe) or Zahn 3 cup (USA)
Small stirrers can be used for small drums
Paddles can be used for large drums
Mechanical agitation device can be used for very large drums
Remember: All readings should be made prior to the use of the coatings and should be conducted at the correct pressroom temperature prior to using the cup.
Step 1: Gently agitate the coating for 10 minutes.
Step 2: Fully submerge the cup into the coating.
Step 3: Have the stopwatch ready to start. Once the cup is lifted out of the coating, start the stopwatch.
Step 4: Stop the stopwatch as soon as the stream breaks for the first time. The time it ends at is the viscosity number.
It’s important to check viscosity levels because unusual levels may lead to application issues. As long as your viscosity is in between 18 and 50 seconds, the application process should be trouble-free.
Check out Peter Halbert’s How To video on Measure Coating Viscosity below.
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