Rationale
The ink viscosity value refers to the viscosity properties exhibited by the ink under dynamic shear conditions, which reflects the ink's ability to resist separation during the printing process. This parameter is crucial for controlling print quality and preventing pilling, ink flying, and other phenomena. The measurement is usually based on how a viscometer works: two rollers with a specific speed difference touch the ink film layer, measuring the torque or force required to maintain that motion, and then calculating the viscosity value. The viscous value is a relative quantity, no unit, and its numerical magnitude is directly related to the instrument model and test conditions.
Standard basis
At present, rotary viscometers are commonly used for measurement in the world, and common standards include ASTM D4361 and ISO 12634. These standards specify key conditions such as instrument structure, test temperature, ink film thickness, drum speed, and cleaning procedures. During the test, an appropriate amount of ink is applied to the instrument drum, and the drum is run at a fixed speed ratio at a set temperature, and the instrument sensor will record the viscous reading. The test is carried out in a constant temperature and humidity environment, usually controlled at 25°C ±0.5°C, because the viscosity value may decrease by about 5-10% for every 1°C increase in temperature.
Factors affecting viscosity values
The viscosity value of ink is affected by various factors, mainly including ink composition, environmental conditions, and instrument parameters. Formulation factors such as resin type, solvent volatilization rate, pigment content and other formulation factors will change the cohesion of the ink. Ambient temperature and humidity will affect solvent volatility and fluidity. The hardness, speed, and cleaning status of the instrument drum directly affect the repeatability of the measurement results. In order to obtain comparable data, the test conditions must be strictly uniform.
| Categories of influencing factors | Typical direction of influence |
| Resin type and content | High resin content usually has an increased viscosity |
| Solvent volatilization rate | If the volatilization is fast, the viscosity will increase quickly with time |
| Ambient temperature | As the temperature rises, the viscosity decreases |
| Drum speed difference | The viscosity value increased when the speed difference increased |
| Ink film thickness | Too thick or too thin can affect the stability of the reading |
Operational points
Standard measurement procedures include instrument preheating, ink specimen preparation, coating, data reading, and cleaning. First, the viscometer needs to be kept at a constant temperature to the standard temperature, and then the ink is evenly applied to the lower drum with a quantitative applicator, and the instrument is started to make the ink evenly distributed and a stable reading is recorded. Pay attention to the consistency of ink dosage during operation, and thoroughly clean the drum with the specified solvent after each test to avoid residue affecting subsequent tests. It is recommended to measure the same specimen three times to take the average.
Data interpretation
The viscosity value itself is a relative value, which needs to be interpreted in combination with the printing speed and the type of substrate. Generally, when printing at high speed or using paper with low surface strength, lower viscosity ink should be selected to prevent lining; For printing with high dot restoration requirements, high viscosity is required to ensure transfer accuracy. The rate of change of viscosity over time can also reflect the drying characteristics of the ink, and its changes can be roughly described by the following relationship:
ΔT ≈ k · ln(t), where ΔT is the amount of viscosity change, t is the time, and k is the coefficient related to the ink formulation.
In actual print shops, viscosity is often fine-tuned by adding ink mixing oils or solvents to accommodate changes in production conditions.
Maintenance and calibration
To ensure measurement accuracy, viscometers need to be maintained and calibrated regularly. Routine maintenance includes drum surface inspection, bearing lubrication and temperature sensor verification. Calibration can be verified using a standard viscosity reference material or by a torque system with standard weights. A full calibration is recommended every six months, with a quick functional check before each important test. The storage environment of the instrument should avoid dust, corrosive gases and violent vibration.
Summary
Ink viscosity measurement is an important part of printing process control, and the viscosity value obtained through standardized operation can provide a reliable basis for ink selection, formula adjustment and printing parameter setting. Understanding the measurement principle, strictly controlling the test conditions, correctly interpreting the data, and analyzing it in combination with the actual printing conditions are the key to effectively using viscometers to improve print quality.
References
ASTM D4361-21, Standard Test Method for Determining Viscosity of Printing Ink and Color Carriers Using a Rotary Viscometer.
ISO 12634:2023, Printing technology - Determination of printing ink viscosity using a rotary viscometer.
Printing Ink Performance Test Manual, China Light Industry Press.