Introduction
In the coating preparation process, stainless steel wire rod applicator is a common metering coating tool, and its coating thickness control accuracy directly affects the uniformity of the coating and product performance. The coating thickness is theoretically determined by the diameter of the wire winding wire of the wire rod, but in practical applications, the physical properties of the slurry, especially the solids content, can have a significant impact on the actual transfer thickness. The purpose of this paper is to analyze the deviation between the actual wet film thickness and the theoretical value obtained by using the same specification stainless steel wire rod applicator under different slurry solid contents, and to provide a reference for the optimization of process parameters.
How it works:
Stainless steel wire rod applicators are made by winding a specific diameter of stainless steel wire around a precision mandrel. The volume of grooves between adjacent wires determines the amount of slurry transferred. The theoretical wet film thickness (T) is usually determined by the diameter of the steel wire (d), and the relationship can be approximately expressed as:
T ≈ k * d
where k is the correction coefficient related to the rheological properties of the slurry. Under the assumption of ideal Newtonian fluid and complete transfer, the k value tends to be close to 0.5. However, the actual coating process is affected by multiple factors such as solid content, viscosity, thixotropy and substrate properties.
Effects of solid content on rheological properties and transfer behavior of slurry
Slurry solids is the percentage of mass of the dispersed phase (solid particles) in the total slurry. Changes in solids content directly change the rheological behavior of slurries:
Low-solids slurries: typically exhibit properties closer to Newtonian fluids, with lower viscosity and good flowability. During the coating process, the slurry can easily flow within the rod groove and transfer to the substrate, but it can also lead to uneven spread or edge shrinkage due to excessive fluidity.
High solids slurry: often exhibit non-Newtonian fluid properties such as shear thinning, and viscosity increases significantly. High viscosity may cause the slurry to be insufficiently filled in the groove of the rod or not to be completely "pulled out" due to excessive cohesion during the transfer process, resulting in the actual transfer volume being lower than the theoretical volume.
This difference in rheological properties is one of the root causes of the actual coating thickness deviating from the theoretical value.
Actual coating thickness deviation
In order to quantify the analysis of the deviation, the following experimental ideas are designed: a series of simulated slurries with different solid contents (such as 10%, 30%, 50%, 70%) but with the same base system are prepared by selecting the same type of wire rod applicator (theoretical wet film thickness 50μm). Coating is carried out under the same environment and process parameters (coating speed, pressure) and the wet film thickness is measured immediately using a thickness gauge. The average value is repeated multiple times for each condition.
Result
The experimental data show that the actual coating thickness is not constant, but shows a regular deviation with the change of slurry solids content. The following table summarizes the key trends:
| Slurry solid content range | Typical deviation trend of actual thickness relative to theoretical values |
| Low solids content (<30%) | The thickness is close to or slightly higher than the theoretical value, and the deviation rate is small |
| Medium solid content (30%-60%) | The thickness begins to be lower than the theoretical value, and the deviation rate increases with the increase of solid content |
| High solids content (>60%) | The thickness is significantly lower than the theoretical value, and the deviation rate may reach a large extent |
For low-solids slurries, the smaller deviation may be due to the good leveling of the slurry, which complements the line pattern. For medium and high solids content slurries, the deviation is mainly attributed to: first, high viscosity leads to a decrease in groove filling rate; second, the elastic recovery of the slurry after the shear action is stopped, and some materials retract; Third, the interaction between solid particles hinders the uniform transfer of slurry. Actual thickness (TPractical) and theoretical thickness (Ttheory) and solid content (S) can be preliminarily described by the following empirical model:
TPractical = Ttheory × (1 - α × Sβ)
Among them, α and β are the positive coefficients related to the slurry system, which need to be determined by fitting the experimental data.
Recommendations to control coating thickness consistency
In order to reduce the fluctuation of coating thickness caused by the change of slurry solid content, the following measures are recommended:
Slurry pretreatment and characterization: Ensure that the solid content of each batch of slurry is stable, and stir well before coating to achieve a stable rheological state. The use of a viscometer for process monitoring is recommended.
Process parameter compensation: When the solid content of the slurry changes prescribed, the corresponding line rod model can be matched by fine-tuning the coating speed or pre-experiment to achieve the target thickness.
Equipment maintenance: Clean the line rod regularly to prevent the dry slurry from blocking the grooves and affecting the transfer volume of subsequent coating.
Online monitoring: When conditions permit, introduce an online wet film thickness measurement system to achieve real-time feedback and adjustment.
Conclusion
The actual coating thickness of the stainless steel wire rod applicator is significantly affected by the solid content of the slurry. Increased solids content usually leads to an increase in slurry viscosity, resulting in an actual transfer thickness lower than the theoretical nominal value of the wire rod. This deviation is regular and can be quantified by systematic experiments. In actual production, recognizing the existence of such deviations and taking corresponding slurry control and process compensation measures is necessary to obtain a stable and uniform coating. Future research can further explore the universality of deviation models under different slurry systems.
References
1. Coating Technology Editorial Board. Coating Process (4th Edition). Chemical Industry Press.
2. ASTM D823 - Standard Practices for Producing Films of Uniform Thickness of Paint, Varnish, and Related Products on Test Panels.
3. A technical review of the influencing factors of the transfer volume of the metering applicator. Journal of Coatings & Protection.