Coating speed impact analysis
Adjustable film coating machines have been widely used in the preparation of functional films, optical films and flexible electronic materials. As one of the key process parameters, the coating speed has a significant impact on the uniformity of wet film thickness and the consistency of the final dry film. In this paper, based on the coating solution with different viscosities, the effect of coating speed on film thickness consistency was systematically investigated under fixed scraper gap, substrate tension and environmental conditions.
The experiment uses a laboratory-grade adjustable film coating machine, with a coating width of 100mm and a squeegee gap of 50μm, 100μm and 150μm, respectively. The coating solution was a polyester model system, and the viscosity ranged from 200 mPa·s to 1500 mPa·s. The coating speed gradient is set to 0.5 m/min, 1.0 m/min, 2.0 m/min, and 4.0 m/min. No less than 10 samples were prepared under each condition, and the film thickness was tested by non-contact white light interferometer, and the consistency was characterized by the coefficient of variation (CV%) of thickness.
Based on fluid mechanics, there is a relationship between the thickness of the wet film h, the coating speed U, the scraper gap G, and the fluid viscosity μ as follows:
Wet film thickness expression
The thickness of the wet film is approximately in line with the following formula:
h = (μU / τ)^(1/2) + k· G
where τ is the shear stress and k is the correction coefficient related to the rheological properties of the coating solution. When the coating speed U increases, the previous ratio term increases, and the tendency of wet film thickness to deviate from the scraper set gap increases, especially in low-viscosity systems.
The typical distribution of measured data is shown in the table below (under the condition of scraper gap 100μm and viscosity 650mPa·s):
Data comparison at a glance
Coating Speed (m/min) | Average film thickness (μm) | Coefficient of Variation (CV%)
0.5 | 96.7 | 2.1
1.0 | 99.2 | 2.5
2.0 | 102.8 | 3.8
4.0 | 108.6 | 5.9
It can be seen from the table that with the increase of coating speed from 0.5 m/min to 4.0 m/min, the average film thickness increases by about 12 μm, the coefficient of variation increases from 2.1% to 5.9%, and the film thickness dispersion increases significantly. The film thickness changes smoothly and consistently in the lower velocity range (0.5 to 1.0 m/min), while after 2.0 m/min, the thickness offset increases and the variability worsens.
Further analysis of the effects of different viscosity conditions shows that the high-viscosity system (>1000 mPa·s) is prone to coating fringes and irregular thickness at high speed, while the low-viscosity system (<400 mPa·s) has thickness fluctuations and increases due to the uneven stretching and thinning of the liquid film at high speed. Based on the experimental data, it is recommended to adjust the coating speed within the following ranges to obtain the best thickness consistency:
Recommended speed range
Viscosity range (mPa·s) | Suitable coating speed (m/min)
200 – 400 | 1.0 – 2.0
400 – 1000 | 0.5 – 1.5
1000 – 1500 | 0.3 – 0.8
In the practical application of coating film, it is recommended to try coating at a lower speed first, and then gradually increase the speed according to the film thickness measurement results, and then sample and analyze after stabilizing for more than 30 seconds after each adjustment. In addition, the scraper clearance, substrate tension and coating solution temperature should also be controlled and recorded at the same time.
In conclusion, the coating speed significantly affects the consistency of film thickness, especially at medium and above speeds, the thickness deviation and variability show a nonlinear increasing trend. Optimizing the coating speed requires a combination of coating solution viscosity, target film thickness, and equipment accuracy.
Briefly cited sources
1. Based on the basic theoretical analysis of fluid coating mechanics, please refer to the Proceedings of Functional Coating Technology for related research.
2. For the experimental research content of coating speed and film thickness uniformity, please refer to the relevant discussions in surface coating engineering journals in recent years.
3. Some of the data related to the influence of viscosity comes from the special report on the coating process of flexible electronic substrates.
4. The actual recommended interval is summarized with reference to the experimental data in a number of coating process optimization literature.