Rotational viscometer
The rotational viscometer method, based on the Couette flow model in Newtonian fluid mechanics, determines viscosity by measuring the torque subjected to rotation of a rotor immersed in a sample at a constant speed. Its basic relationship can be expressed as follows:
η = K × (T / ω)
where η is the dynamic viscosity, K is the instrument constant, T is the torque, and ω is the angular velocity. This method is suitable for non-Newtonian fluid characterization, and the rheological behavior of inks can be studied by changing the shear rate. During operation, the sample with constant temperature control needs to be placed in the measuring container, select the appropriate rotor and speed, and record the data after the reading is stable. Calibrate the instrument before measurement and ensure that the sample is uniform and free of bubbles.
Viscosity cup method
The viscosity cup method is an empirical method of determining viscosity by measuring the time it takes for a fixed volume of fluid to flow through a standard pore size, and is often used for rapid detection in the field. Its basic formula is:
η = k × (t - c)
where η is the kinematic viscosity, k is the cup constant, t is the outflow time, and c is the correction coefficient. Commonly used cup types such as ISO cups and Ford cups have corresponding standard apertures and capacities. During operation, the full sample should flow freely at a constant temperature to record the time when the flow is first interrupted. This method assumes that the fluid is Newtonian fluid, so its applicability may be limited for inks with obvious shear thinning properties.
Method comparison
There are essential differences between the two methods in principle, applicable scenarios and data properties. Rotational viscometers provide accurate absolute viscosity values and rheological curves for R&D and quality control. The viscosity cup rule provides relative outflow time and is more suitable for quick comparison on the production site. The following is a comparison from multiple dimensions:
| Measurement principle | Rotational torque measurement |
| Data output | Kinetic viscosity (mPa·s) |
| Fluid type suitability | Newtonian and non-Newtonian fluids |
| Shear rate control | Precise control and scanning |
| degree of standardization | GB/T 17473, ASTM D2196 |
| Typical application scenarios | Laboratory precision analysis |
| Measurement principle | Gravity outflow time |
| Data output | Outflow time(s) |
| Fluid type suitability | Close to Newtonian fluid |
| Shear rate control | Single approximate shear rate |
| degree of standardization | GB/T 1723, ISO 2431 |
| Typical application scenarios | Fast inspection at the production site |
Key influencing factors include temperature control (recommended ± within 0.5°C), sample preparation (homogenization, de-bubbling), instrument calibration (using standard oils), and operational specifications (e.g., viscosity cup cleaning and horizontal placement). For thixotropic inks, the resting time and the time record of the measurement process should be indicated.
Application Recommendations
The rotational viscometer data can draw rheological curves for analyzing the rheological characteristics of inks, such as thixotropy and yield values. The viscosity cup data is mostly used for batch-to-batch consistency comparison. When the results of the two methods need to be correlated, empirical conversion relationships should be established for similar Newtonian fluid samples at the same temperature and their limitations should be noted. In ink research and development, it is recommended to use a rotary viscometer as the main focus, and cooperate with the viscosity cup method for production monitoring. The report should contain measurement conditions, instrument model, standard method, temperature, and result units.
References
GB/T 17473-2022 Test method for precious metal slurry for microelectronic technology
ASTM D2196-2022 Standard Test Method for Rheological Properties of Non-Newtonian Materials by Rotational Viscometer
GB/T 1723-2022 Coating viscosity determination method
ISO 2431:2022 Coloured paints and varnishes Determine the outflow time by means of the outflow cup
"Printing Ink Performance Detection Technology", China Light Industry Press