Introduction
In the evaluation of the physical properties of offset printing inks, viscosity testing is one of the key links to judge their printability. The accuracy and reproducibility of test results are highly dependent on the standardization of ink film preparation before testing. As a sophisticated film preparation equipment, the squeegee coating machine can form an ink film with uniform thickness and area on a specific substrate through mechanical control, providing a reliable prerequisite for subsequent viscosity testing. The purpose of this paper is to discuss the operation process, key parameters and theoretical basis of using scraper coating mechanism to prepare standard ink film.
Principle of equipment
The basic working principle of a squeegee coater is to use a precision squeegee with adjustable gap to flatten the ink that has been applied to the surface of the substrate, leaving a layer of wet ink film on the substrate with a thickness strictly defined by the squeegee gap. Its core relationship can be expressed by the following formula:
h = g - δ
where h represents the thickness of the wet film, g is the mechanical clearance set by the scraper, and δ is the amount of small deviation correction caused by the rheological characteristics of the ink. The device ensures uniformity in the thickness of the ink film, which is the basis for obtaining comparable viscosity test data.
Operation process
The standard ink film preparation process follows strict steps. First, the selected standard test substrate is flattened to the applicator base. Then, take a sufficient amount of the ink to be tested at a constant temperature under standard conditions and place it at the front end of the substrate. Start the device, and the squeegee scrapes the ink at a constant speed and pressure, forming an ink film. After preparation, viscosity testing should be performed immediately or within the time specified by the standard to prevent changes in the membrane state due to factors such as solvent volatilization.
Key parameter control
To ensure the standardization of ink films, the following parameters must be strictly controlled:
Scraper Clearance: According to relevant test standards (such as ASTM D4361, etc.) or specific research requirements, the thickness of the wet film is directly determined.
Coating speed: Uniform speed scraping is the key to ensure uniform thickness, too fast may lead to incomplete film layer, and too slow may cause level differences.
Environmental Conditions: Laboratory temperature and relative humidity should be constant, usually recommended at 23±2°C and 50±10% relative humidity.
Substrate: The substrate specified in the standard must be used, and its surface energy, smoothness and ink absorption must be uniform.
Parameter reference table
| Control parameters | Typical setpoints or requirements |
| Scraper gap | According to the standard, the common range is 6-24 microns |
| Coating speed | constant, e.g. 10 mm/s |
| Laboratory temperature | 23±2 degrees Celsius |
| Laboratory relative humidity | 50±10% |
| Substrate type | Standard high-grade coated paper or specified plastic film |
| Ink constant temperature time | Let it sit in a standard environment for no less than 4 hours before testing |
Errors and precautions
Errors may be introduced in practice. The cleanliness and integrity of the scraper edge is critical, and any damage or stain will be copied directly onto the ink film. If the ink contains large particles, it needs to be filtered first. After coating, the edges of the ink film may be slightly thicker due to the "edge effect", and the edge area should be avoided when testing. Operators need to be trained to ensure consistency in operating techniques.
Conclusion
The preparation of ink film before the viscosity test of offset printing ink using a squeegee coating machine is an effective method to improve the accuracy of test data and the comparison between laboratories. Through standardized equipment, parameters and processes, the interference caused by human and environmental variables can be minimized, so that the viscosity test results can truly reflect the rheological characteristics of the ink itself, and provide a reliable technical basis for ink research and development, quality inspection and printing applications.
References
1. For the standard methods section, refer to ASTM D4361 Guidelines for Preparation of Ink Coating Films.
2. The equipment principle part synthesizes the relationship between scraper gap and film thickness in the literature related to precision coating technology.
3. In the environmental parameters section, the common laboratory test environmental standard ISO 554 is referenced.
4. Error analysis part, based on the summary report of common operation problems provided by multiple testing laboratories.