Introduction
In the field of coating preparation and analysis, desktop automatic coating machines have become a key equipment for standardized coating film preparation in laboratories due to their convenient operation and controllable programs. The repeatability of coating thickness is one of the core parameters to evaluate the comparability between coating quality and subsequent test results, and the stroke accuracy of the coating machine is a potentially important factor affecting this repeatability. The purpose of this paper is to explore the correlation between the stroke accuracy of the coating machine squeegee or applicator and the repeatability of the obtained coating thickness through design experiments, so as to provide a reference for laboratory instrument operation and result quality control.
Experimental principle
The coating thickness is mainly affected by factors such as the thickness of the wet film, the coating speed, the properties of the substrate and the rheological characteristics of the coating solution set by the coating machine. From the perspective of mechanical motion, the stroke accuracy of the coating machine is directly related to the relative position stability between the scraper and the substrate plane. If there is unexpected vibration or deviation in the stroke, it will lead to inaccurate control of local wet film thickness, which will affect the uniformity and repeatability of coating thickness after drying or curing.
In this experiment, a desktop automatic coating machine that meets the relevant mechanical accuracy standards is selected. The actual position of the coating squeegee in the set stroke is monitored by a high-precision displacement sensor, and its position deviation Δx can be defined as the difference between the set position and the actual measured position. At the same time, a series of parallel samples were prepared, the thickness of the cured coating was measured using a calibrated coating thickness gauge, and its standard deviation was calculated as a repeatability evaluation index. The experiment was conducted in a constant temperature and humidity environment, and the same batch of model coatings with stable rheological properties was used to exclude interference from other variables.
Data analysis
The typical position deviation data in the coater stroke at different set coating speeds was recorded. The analysis shows that the deviations in the stroke are not uniformly distributed and are usually more pronounced at the start, end or steering phases of the stroke. The root mean square value (RMS) of the position deviation was correlated with the standard deviation of the coating thickness measurement of the corresponding batch.
Assuming that the coating thickness T has a functional relationship with the set wet film thickness H, the coating speed v and the position deviation Δx, it can be simplified as follows:
T = k * H + f(v, Δx)
where k is the coefficient related to the shrinkage rate of the coating liquid, and f is the function that characterizes the influence of the introduction of velocity and mechanical deviation. Experimental data show that the repeatability (in standard deviation) of coating thickness is significantly improved when the RMS value of stroke position deviation is controlled within a certain threshold.
| Stroke position deviation RMS (μm) | Coating thickness standard deviation (μm) |
| ≤ 2.0 | ≤ 0.15 |
| 2.1 - 5.0 | 0.16 - 0.30 |
| 5.1 - 10.0 | 0.31 - 0.60 |
| > 10.0 | > 0.60 |
The table above shows a typical association trend. It should be noted that the specific threshold range varies depending on the equipment structure, coating characteristics and substrate, but the trend is the same.
Discussion
The experimental results verify that the stroke accuracy of the coating machine is a necessary condition to ensure the repeatability of coating thickness. Stroke deviations can arise from factors such as the straightness of the guide rail, the stability of the drive system, the wear of mechanical components, or the assembly clearance. For high-precision coating applications, it is necessary to regularly check and maintain the stroke accuracy of the coating machine. In addition, optimizing the coating speed parameters can sometimes avoid mechanical resonance points, thereby reducing the travel deviation amplified by vibration at specific speeds.
In practical operation, in addition to focusing on the mechanical properties of the equipment itself, the skill of the operator is also crucial, such as ensuring that the substrate is flat and accurately aligned with the initial alignment, which reduces the additional impact on the repeatability of the final coating thickness.
Conclusion
Through the experimental verification of the system, it can be confirmed that there is a clear correlation between the stroke accuracy of the desktop automatic coating machine and the thickness repeatability of the prepared coating. Keeping travel position deviations low can help achieve more repeatable coatings, which can improve the reliability of data comparison and process studies between labs. It is recommended that laboratories include the stroke accuracy of the coating machine in the daily quality control and instrument verification items, and establish corresponding acceptable standards in combination with the specific coating material system.