Slot Coating Research Articles

Analysis of the Influence of Inlet Position on the Transverse Homogeneity of Slot Coating

Slot coating is a widely used method for high performance material production. Coating quality mainly depends on whether the coating liquid can be evenly coated on the material. To be evenly coated, the coating liquid is required to have good transverse velocity uniformity at the exit of the die. The position of the inlet of the die has an important effect on the horizontal uniformity of the die. In order to quantitatively analyze the influence of the inlet position of the die head on the transverse uniformity of the die, we respectively simulated the internal flow field of two different shapes of uniform cross section die head, and finally got the preliminary law. It is found that no matter the shape and structure of the cavity, the transverse uniformity of the coating can be greatly improved using the intermediate feeding method.

Modelling and Simulation of Transverse Homogeneity of Slotted Coating Process

Slot coating is a kind of precise coating process, which is widely used in the production of membrane materials. The thickness deviation of coating along the transverse direction is the key index to measure the coating quality. The homogeneity of the transverse distribution of the coating velocity at the exit of the die is the most direct factor leading to the transverse thickness deviation of the coating. To predict the transverse homogeneity of slot coating, a mathematical model for the internal flow field in the die was established, where the incompressible Navier-Stokes equations were used to describe the fluid flow and generalized Newtonian fluid model was used to describe the rheological behaviour of non-Newtonian coating. An efficient explicit splitting algorithm based on finite element method is proposed to solve the model. To verify the effectiveness of the model and the algorithm, the internal flow field of a trapezoidal double cavity die with equal section was simulated. The transverse homogeneity obtained was close to the experimental measured value, which verified the validity of the model. Furthermore, the influence of measuring slot width on the transverse homogeneity of outlet velocity is studied, and the simulation results are consistent with the theoretical expectations.

Simplified model for operability window of slot coating without vacuum

Slot coatings are widely used to fabricate films with highly uniform thicknesses. Stable operating conditions are critical for achieving desired thickness uniformity. When the slot coating is operated with a vacuum chamber to produce thin films of a Newtonian fluid, the visco-capillary model of Higgins and Scriven (1980) has been successfully used to predict the operability window, i.e., the range of stable operating conditions. The model assumes a channel flow under the slot-die and describes the pressure balance inside the channel to determine the operability window, particularly for the vacuum pressure range. Currently, battery electrodes and membrane coatings require relatively thick films that can be processed without a vacuum. Such operating conditions are compatible with high aspect ratio channels. We show that the model describes the downstream pressure jump contribution as having a negligible magnitude compared to the total pressure drop inside the channel. Consequently, a simplified model can be obtained by omitting the downstream contribution from the visco-capillary model. The simplified model is consistent with the experimental results for a thick film production of a high viscosity Newtonian liquid. Furthermore, because the simplified model is expressed as an explicit function of the operating conditions, it enables straightforward estimation of the operability window and the solution of inverse problems, such as predicting the desired viscosity range from the target operating condition.

Simultaneous Analysis of Die Internal and External Flows in Slot Coating Process

Simultaneous Analysis of Die Internal and External Flows in Slot Coating Process

CFD Technology for Slot Coating

化学工学の分野で塗布は重要な要素技術であり，日本の産業界ではVOF法を用いた塗布ビードの自由表面解析が盛んに行われてきた。この解析手法にて，空気同伴，リビング，リビュレット，段ムラ，塗布エッジ膜厚不均一等の各種塗布故障が解析結果として再現された。また，これら塗布故障の発生状況と塗布条件の相関を定量的に整理したコーティングウィンドウにおいても，実際の塗布試験結果と良好な一致が見られた。このように，塗布解析を活用した塗布故障発生原理の解明，未知の操業条件における塗布故障発生予測が実用化され，近年では主に電子材料分野の塗布最適設計に役立っている。現状この分野では，スロット塗布方式による単層塗布が一般的だが，多層同時塗布技術の応用も期待されている。

Flow and Film-Forming Characteristics Analysis of Non-Newtonian Fluid Slot Coating

The slot coating process in the production of polymer films has a wide range of applications. However, the process cannot be systemized. This study used the computational fluid dynamics software Polyflow to analyze the slot coating process and investigate the influence that the process parameters have on the characteristics of thin-film coating to reduce the time and cost consumed in the experimental methods. The rheological characteristics of the non-Newtonian fluid used in this study were first identified by conducting experiments, and then configured in the simulation software for fitting with mathematical models. In addition, the models of the slot coating process were constructed, and the Arbitrary Lagrangian-Eulerian (ALE) calculation methods were then used in the Polyflow software. The simulation results were then compared to the experimental results and the findings reported in relevant literature, to determine the influence that the process parameters have on the characteristics of thin-film coating. The simulation results were represented graphically in a coating window plot. The comparison results indicate that the viscosity-shear rate characteristic of the material in the shear rate range of optical film coating is an excellent fit for the Cross Law. When the coating speed is too high or the amount of fluid supply is insufficient, phenomena such as break lines, uneven thickness, and air entrainment can occur.

Improved Coating Window for Slot Coating

Slot die is a premetered coating operation in which the coating solution is extruded from a feed slot onto the surface of a substrate passing through. The defect-free region of operation is known as the “coating window”, and the wider it is, the more versatile the operation will be. The slot-coating experiments reported here were carried out with two types of coating solutions, a hard coating solution with low viscosity and a polysilica solution with high viscosity. The coating windows were measured as flow rates q versus minimum and maximum coating speeds, Vmin and Vmax, respectively. Therefore, at a given flow rate of the coating solution, there exists a minimum speed and a maximum speed that border the corresponding coating window, outside which coating defects occur. Also, in our experiments, the coating windows shifted to higher coating speed and obtained thinner coating film thicknesses through different settings of the die angle to the substrate and different geometries of die lips. The data showed that the coating windows shifted to higher speed and thinner film thicknesses when the die angle and the geometry of the die lips were varied appropriately. These results have important practical implications. The fundamental principles involved here are also applicable to other premetered coating processes.

Coating Flows of Power-Law Non-Newtonian Fluids in Slot Coating

One-dimensional flow equations were derived for non-Newtonian fluids in slot coating and a simple numerical method was proposed to solve the equations. We investigated non-Newtonian effects on coating performance and coating quality in slot coating. In some cases, computational results and experimental data were compared and both agreed well. A series of numerical computations showed that the one-dimensional approach can describe non-Newtonian flow under lips in a few minutes and that a pressure field equation proposed by Higgins can determine the location of an upstream meniscus. Moreover, it was found that the non-Newtonian characteristics of coating liquids affect both coating performance and coating quality. Because each non-Newtonian effect is quite different, we should optimize the conditions of slot coating and die geometries according to the target problem.

Dynamic Response Analysis and Control of Slot Coating

A slot coating application is an attractive coating method for creating a uniform coating thickness over a substrate material. However, a variety of disturbances in operational conditions (e.g., pumping fluctuations, motor system cogging) may lead to unacceptable variations in the final coating thickness. The dynamic response characteristics of the final coating thickness resulting from such disturbances can be computed using a numerical analysis technique that employs a visco-capillary model. In addition, there are two different methods for controlling the thickness of the final coating. One passive control method changes die lip geometry, and the other active control method uses a flow feedback system coupled with a final coating thickness sensor. The passive technique can suppress final thickness variations by 20-30% in a limited number of cases, whereas the active technique substantially reduces thickness variations in every case where the disturbance frequency is less than 10 Hz.

Comparison of vertical and horizontal slot die coatings

AbstractComparison of vertical and horizontal slot coatings was carried out experimentally using aqueous glycerin solutions with varying viscosity on a laboratory‐scale slot coater. The maximum coating speeds achieved in vertical slot coating were found to be higher than those of horizontal slot coating for low‐viscosity solutions. The coating defects observed below the minimum wet thickness changed from ribbing to air entrainment at a critical solution viscosity of around μ = 75 mPa s. Above this critical viscosity, a lower minimum wet thickness was obtained for the horizontal coating but not for vertical coating. The difference between vertical and horizontal coating was mainly the length of the coating bead. The dimensionless minimum wet thickness was correlated as a function of Reynolds number. There existed three regions of minimum wet thickness depending on Reynolds number. Initially, the dimensionless wet thickness increased in the low Reynolds number region (Region I), then reached a plateau in the intermediate Re region (Region II), finally dropped off sharply in the high Re region (Region III). The boundaries of these three regions were identified by two critical Reynolds numbers, i.e., Re = 1 and 20. Only Regions I and II of the wet thickness could be found for horizontal coating, while mainly Regions II and III were obtained for vertical coating, depending on the viscosity of the coating solution. POLYM. ENG. SCI., 47:1927–1936, 2007. © 2007 Society of Plastics Engineers

Effectiveness of Stress Grading Coatings on Form Wound Stator Coil Groundwall Insulation Under Fast Rise Time Pulse Voltages

In this paper, the effectiveness of stress grading coatings (SGCs) on form wound stator coil groundwall insulation under fast rise time pulses is analyzed. The combined performance of the stress grading coating and the semiconductive slot coating is studied under fast rise time transient voltages. During the rapid rise of voltage, the high stress moves from the stress grading coating to the semiconductive coating at the end of the slot. According to the results, it is evident that the design of both the stress grading and semiconductive slot coatings must be done together for effective stress relief. The use of materials with varistor behavior in the stress grading coating, combined with a high conductivity semiconductive slot coating, is analyzed as a solution to the high stress that develops at the slot end of form wound stator coil groundwall insulation of inverter fed drive (IFD) motors.