Principle
Rotational viscometer is a rheological measurement instrument widely used in industry and scientific research, and its core principle is to evaluate the viscosity of fluids based on the shear force generated by rotational motion. The instrument typically contains a rotor (or measuring cylinder) and a measuring cup in a coaxial configuration. When the rotor rotates at a set speed in the fluid to be tested, the fluid creates viscous resistance to the rotor surface, forming torque. The instrument calculates the dynamic viscosity of the fluid by accurately measuring this torque value and based on the known geometry and rotational speed. For non-Newtonian fluids such as inks, their viscosity tends to change with the shear rate, so the rotational viscometer can simulate the flow behavior of the ink during the actual printing or coating process by changing the rotational speed (i.e., changing the shear rate) to fully interpret its thickness and thinness characteristics.
ink density and thinness
In the printing process, the "consistency and thinness" of ink is a comprehensive concept, which not only refers to the apparent viscosity under static states, but also involves the viscosity change behavior under shear action, that is, rheology. Excessive ink thickness may lead to transfer difficulties and enlarged outlets; Too thin may cause transparent printing and insufficient color intensity. Rotational viscometers can quantify key parameters such as plastic viscosity, yield values, thixotropy, and other key parameters of inks by providing controlled shear conditions. For example, by measuring the viscosity value at different shear rates, the flow curve can be drawn to analyze whether the ink is pseudoplastic, expandable, or has a thixotropic ring, which directly determines the transfer efficiency of the ink on the ink roller of the printing machine, the spreading behavior on the substrate, and the final print quality.
Measurement method
When measuring inks with a rotational viscometer, standardized procedures need to be followed to ensure comparable and accurate results. Before measurement, the ink sample needs to be sufficiently constant temperature at the specified temperature, as temperature has a significant effect on viscosity. A suitable rotor and measuring cup combination is usually selected and a low to high speed scanning range is determined. During the measurement, the steady-state torque value is recorded and calculated according to the instrument constant or calibration with standard viscosity oil. Relevant test methods can refer to domestic and foreign standards, such as general test standards related to viscosity of coatings and inks. These standards detail sample preparation, instrument calibration, testing procedures, and report content, and are the cornerstone of reliable data.
Data analysis
Rotary viscometer testing provides a range of key parameters that define the rheological properties of inks. The following table lists a few of the core ones:
| Plastic viscosity | It reflects the flow resistance of ink after the internal network structure is destroyed at high shear rate, which is related to the transfer performance during printing. |
| yield value | The minimum shear stress required for the ink to start flowing. Too high a value may lead to difficulty in starting a pen, and too low a value can easily cause blurred dots. |
| Thixotropic index | Characterize the strength of the ink's thixotropy, i.e., the ability of viscosity to recover over time-shear history, affecting the leveling and clarity of the blot. |
| Flow curve index | Describes the sensitivity of viscosity to changes in shear rate, and is used to determine whether a fluid belongs to a shear thinning or shear thickening type. |
The comprehensive analysis of these parameters helps formulation engineers adjust the ratio of resins, solvents and pigments, and also provides a scientific basis for printers to set the appropriate machine speed and pressure.
Notes:
In actual production quality control and R&D, the application of rotational viscometers is carried out throughout. In the development phase of new inks, it is used to screen the base formulation; In batch inspection, it is an essential tool for monitoring product consistency. To ensure the validity of the measurement, the following points should be noted: First, the measurement temperature must be strictly controlled, and viscometer accessories with temperature control systems are recommended. Secondly, choose the appropriate rotor model and speed range according to the approximate viscosity range of the ink to avoid the increase of error caused by the torque exceeding the range or being too low. Finally, for inks with obvious thixotropy, the time for sample pre-shear and resting recovery should be strictly specified, and the test conditions should be noted in the report. By standardizing the operation, the data provided by the rotary viscometer becomes an effective technical language for communicating R&D, production and printing applications.
References
ASTM D2196, Standard Test Method for Rheological Properties of Liquid Coatings, Inks, and Related Products.
GB/T 9269, Determination of viscosity of architectural coatings - Stormer viscometer method (the principle part of which has reference value).
Industry Technical Manual: Printing Ink Performance and Testing.