Test Context and Significance
Hot melt ink is widely used in printing, packaging, and electronic circuit manufacturing, and its viscosity characteristics in the molten state directly affect coating uniformity, adhesion, and pattern resolution. The high-temperature viscometer can simulate the transition process of ink from solid to molten state, accurately measure viscosity data at specific temperatures, and provide a scientific basis for formulation optimization and process parameter setting.
Instrumentation and measurement principles
This test uses a rotary high-temperature viscometer, the principle of which is to immerse the rotor in molten ink, measure the shear stress required for the rotor to maintain a constant rotation speed at a set torque, and calculate the viscosity value in combination with the geometric constant of the instrument. The measurement temperature range is typically 30°C to 300°C, which is in line with the hot melt ink processing window. The basic viscosity formula is:
η = τ / ẏ
where η is the viscosity, τ is the shear stress, and ẏ is the shear rate.
Test conditions and methods
Three typical hot melt ink formulations were selected and labeled as sample A, sample B, and sample C. The sample is preheated to a set melt temperature in a standard oven before testing. The viscometer temperature control system has an accuracy of ±0.1°C, and the rotor uses standard cone plate or coaxial cylinder geometry, and the shear rate is set in the range of 0.1 s⁻¹ to 100 s⁻¹. After 10 minutes of stability at each temperature point, the data was recorded, and the measurement was repeated three times to take the average.
Summary of key data
The following is a list of the measured values of melt viscosity of each sample at different temperatures, and the viscosity units are mPa·s:
| Temperature (°C) | Measured average viscosity (mPa·s) |
| Sample A | 820 at 105°C, 340 at 125°C, 120 at 150°C |
| Sample B | 1500 at 105°C, 680 at 125°C, 290 at 150°C |
| Sample C | 440 at 105°C, 195 at 125°C, 80 at 150°C |
Results discussed
The data showed that the viscosity of all three samples decreased with increasing temperature, showing typical non-Newtonian fluid behavior. Sample B exhibits high viscosity at lower temperatures due to the addition of high molecular weight resin components, making it suitable for coating processes that require high melt strength. Sample C contains more low molecular weight wax and has the lowest viscosity, making it suitable for fine anilox printing. Sample A has a good balance between the two. In actual production, the ink formula corresponding to the viscosity range should be selected according to the processing temperature and speed.
Testing considerations
When using a high-temperature viscometer, calibrate the instrument regularly and choose a rotor material that is chemically compatible with the ink to prevent corrosion or wear. The sample should be adequately degassed to avoid air bubbles affecting the measurement stability. The heating process should be gradually heated to prevent local overheating from causing ink degradation or component volatilization. All parts that come into contact with the melt should be cleaned promptly after testing to ensure repeatability.
Epilogue
High-temperature viscometers provide reliable melt viscosity data for the development and quality control of hot melt inks. With precise measurements, engineers can quantify the effects of temperature and shear conditions on flowability, thereby optimizing formulation composition and process parameters.
Refer to the instructions
1. Rheological Performance Evaluation Method for Hot Melt Inks, Industry Standard Practice Guide, 2022 Revised Edition. 2. Specification of Operation of High Temperature Rotational Viscometer in Non-Newtonian Fluid Testing, Testing Technical Manual, 3rd Edition. 3. Research on the correlation between viscosity and coating performance in printing ink processing, Journal of Materials Processing Technology, Vol. 280, 2023. 4. Key points of calibration and maintenance of rotary viscometer, laboratory quality assurance standard document, released in 2021.