Wink of Knowledge: Measuring the density and viscosity of ink in industrial processes

Density and viscosity measurements of ink for inkjet printers using the VLO-M2 viscosity sensor show a high degree of consistency with control measurements in laboratory analyzers over a temperature range of 10 to 65 °C.

Why this test?

The measurement parameters density and viscosity are excellent indicators of ink quality and can also be used to optimize process parameters in the actual printing process.

The density value can be used to derive the concentration of color particles. If the ink is too diluted, this can lead to color deviations in the print, as individual dyes are too low in concentration. It is also important that the ink has a consistent density throughout the entire printing process, as otherwise this can lead to uneven printing results.

Viscosity, on the other hand, has a direct influence on several critical aspects: the flow behavior in the nozzles and nozzle openings, the formation of drops after exiting the nozzle, the drying time of the finished print, the adhesion to the printing medium, and the quality of fine details and resolutions.

Overall, the density and viscosity of inks are crucial for the throughput and quality of industrial printing processes. To achieve consistent, high-quality printing results, these parameters must be carefully monitored and controlled.

Results

The density measurement results are shown in Figure 1. The blue circles and the linear trend curve of these measured values show the measurement results of the laboratory analysis. These were determined using the DSA 5000 M laboratory measuring device from Anton Paar. The squares shown in orange and their trend line show the measured values recorded using the VLO-M2 viscosity sensor. A linear change in density can be seen across the entire temperature range.

Figure 1: Density from 5 to 65 °C

Congruent with the density measurement, Figure 2 shows the viscosity measurements and their trend line, which is represented by a polynomial instead of a linear dependence.

Figure 2: Viscosity from 5 to 65 °C

The viscosity measurement shows that the sensor’s measuring points do not completely match those of the laboratory measuring device. With a measurement deviation of -0.185 mPas, just under ±2%, in viscosity, these results are still well within the sensor’s specification, which is ±[0.2 mPa s + 5% of the measured value]. As mentioned at the beginning, viscosity is a fundamental parameter for printing processes, and viscosity is often adjusted by heating the print head. The VLO-M2 allows both the temperature and viscosity of the ink to be measured in real time, enabling perfect control of the printing process.

These deviations can also be explained by a temperature gradient in the sensor that occurs during heating and cooling in the oven. The parts in direct contact with the warmer (or cooler) ambient air assume a different temperature than the parts and components inside the sensor.

Under real process conditions, where the temperature of the medium and the ambient temperature are stable, a homogeneous temperature can be established throughout the sensor. This means that deviations in viscosity measurement of <2% are plausible.

Procedure

The density and viscosity of an ink for use in industrial printing systems were measured using the VLO-M2 and the DSA 5000 M and SVM 3001 laboratory devices (Anton Paar).

To do this, the VLO-M2 viscosity sensor was completely filled with ink and subjected to a temperature ramp using an oven. Measurement values were recorded continuously. Temperature levels in the range of 5 to 65 °C were set at intervals of 5 °C using the laboratory measuring devices.

These measurements were carried out immediately one after the other to minimize changes in physical properties, for example due to aging effects or moisture ingress.

To ensure comparability of the data, the measurement points recorded in the VLO-M2 viscosity sensor were averaged within a temperature range of ±0.2 °C around the respective level of the laboratory measuring device. This allows the changes in density and viscosity to be graphically represented and estimated.

Conclusion

This test shows that both the density and viscosity of the ink provided for test measurements can be determined very accurately. Compared with the results of the laboratory analysis, a maximum measurement error of ±2% in viscosity and ±0.125% in density can be identified across the entire temperature range measured.

It should be noted that the measurement deviation from the reference can be further reduced by optimizing the measurement setup, making measurement errors in the range of ±1% of viscosity realistic.

With the help of these two parameters, any printing processes where high value is placed on quality and reproducibility can be better monitored and continuously improved.

Which sensors were used?

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Viscosity

Viscosity sensor VLO-M2

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