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For larger coating volumes and rates, these smaller systems are typically not well suited. Instead, rotary pumps connected to a large solution reservoir are used.
In these systems, displacement is provided by a rotating element placed in a way that can restrict the flow of solution.
The volume pumped into a system is then directly related to i the speed at which the element rotates, and ii the volume of solution passed through in each rotation.
In comparison to peristaltic pumps, the rotary pump method reduces the impact of pulses in the flow rate. However, as the displacement mechanism is in line with the solution protecting the mechanism against damage and also protecting the solution, contamination can become a problem.
One of the most complex aspects of a slot-die coating system is the design of the slot-die head. The head controls the distribution of solution across the width of coating, the actual coating width of the film, and also helps determine the stability of the coating process.
Figure 3 shows the inside of a slot-die head design. The head consists of several critical components - these include the inlet, manifold, land, slot, shim, and lip.
Some heads can have multiple manifolds and lands called pre-lands to improve the distribution of the solution.
However, increasing the size and number of these will also increase the dead volume within the system. For the inlet, the position is typically in the middle of the system positioned near the top of the manifold.
For slot-die heads with a large width, multiple inlets can be used to reduce the size of the manifold needed, as well as the length of lands, pre-lands, and slots.
Once the solution enters the system, it begins to fill the manifold. There are multiple designs that can be used for this section of the slot-die head, and it is this section which most strongly influences the distribution.
Figure 4 shows the most common designs for distribution manifolds and the different cross sections available. The simplest design is the T-shaped manifold, where the bottom and top edges of the manifold run parallel to the exit of the slot-die.
This leads to lower solution flow rates closer to the ends of the manifold, uneven solution distribution over the width of the head, and different travel times for solutions.
To achieve equal flow rates across the width of the slot-die head, the manifold can be shaped into a coat-hanger design.
Although the flow rate of the solution becomes more uniform across the width when utilising a coat-hanger design, the shape of the coat-hanger must be re-optimised for different flow rates and solution viscosities.
A modified form of coat-hanger design called a 'constant shear manifold' can be used, where the reduction in distance between the bottom edge of the manifold and the exit of the slot-die is not linear.
This design allows for a constant flow rate across the width of the slot-die head independent of the solution viscosity and flow rates. The constant shear manifold, however, requires longer landing lengths than the other designs - and at the same time, the volume of the manifold is significantly larger.
This increases both the size of the slot-die head needed and the dead volume in the system, which leads to an increase in initial setup costs and operational costs over simpler designs.
The pre-lands, lands, and slots within a die head are all regions where the flow of the solution is restricted via a narrow channel.
The width of this channel and the length of the channel are crucial in controlling the drops in pressure within the slot-die head, which helps control the stability of the wet film coating.
The flow of solution through these narrow channels is determined by the Pouiselle Flow equation given by Equation 1 where the drop in pressure within a slot-die head delp is determined by the flow rate of the solution V , the viscosity of the solution mu , the channel length L , and the channel width b.
From this, it can be seen that the parameter with the largest amount of control over the pressure drop is the channel width.
However, the cost of changing the length and width of channels is very high as would require the milling of a new head each time they are changed.
It is possible to get around this by using shims of different thicknesses, which can expand the width of the channel. By using shims of different sizes for different flow rates or solution viscosities, the pressure drop can be maintained at a fixed value.
In addition to controlling the pre-land, land, and slot thickness, shims can also be used to set the width of the coating.
They can allow for the deposition of stripe patterns, and can be used as a meniscus guide to improve the definition of edges. By having the final slot thickness determined by the shim, it is possible to stop solution from flowing out of certain areas of a slot-die head.
Figure 5 shows how a shim design can deposit 4 stripes of material onto a substrate using a single head.
When depositing a stripe where the coating width is less than the width of the slot-die coater head, the coating meniscus can spread out along the width due to capillary forces.
This reduces the accuracy of the coating pattern, and can cause multiple stripes to bleed together. To improve the edge quality, meniscus guides can be added.
These guides are shims where a thin protrusion below the slot-die head lip is used. The protrusions are placed where the stripe pattern is needed, and act to pin the meniscus to the shims rather than the lip.
This stops the meniscus from spreading along the width of the head. The final aspect of slot-die head design is the lip.
This is the area upstream and downstream of the exit slot of the head. In these regions, the meniscus is pinned to the slot-die head, and the stability of these menisci can be strongly influenced by the design and positioning of these lips.
The simplest change in the design of the lips is a change in the length of the lips, by increasing the length.
Figure 6 shows the three different geometries available for the lips. These are:. The advantage of changing the configuration is that the stability of the upstream meniscus can be improved without changing the stability of the downstream meniscus.
The positioning of the slot-die head relative to the substrate is important in helping to stabilise the coating bead.
In both sheet-to-sheet and roll-to-roll systems, the machining and tolerances of the carriage or roller that holds the substrate can have a great impact on the gap height.
Either localised defects due to surface roughness, issues with the flatness of a bed, or the concentricity of the roller can cause changes to the gap height and coating bead stability over the length of the substrate.
The exact tolerances for the system will be dependent upon the solution you are coating and the processing parameters used.
When operating the system, the movement of the slot-die head into position over the substrate can be controlled either manually or automatically.
The advantage of automated control is that inline monitoring of the film quality can be used as feedback for the adjustment of the slot-die head height.
Typically, the height of the gap is determined via the use of digital height gauges or micrometers. These are fixed to the head carriage, and measure the position relative to the top of the roller or carriage.
Knowledge of the underlying theory behind slot-die coating is crucial in understanding how the operating parameters and slot-die geometry interact to create a stable coating.
High-quality coatings can be achieved only within a specific coating window, and moving outside of this stable coating window will result in the formation of defects - until eventually, the film will no longer coat.
By knowing the origin of defects within coated films, it is possible to know which processing parameters and slot-die geometries need changing to return to the stable coating region.
In this section, we will talk about i the theoretical basis behind the design of manifolds for improved distribution, ii how the solution flows through restricted channels producing large pressure gradients, and iii how the shape and position of coating menisci are influenced.
Distribution of solution through a slot-die manifold is determined by several competing processes that drive the movement of fluid, and others that retard the movement of fluid.
These can be categorised as: i the hydrodynamic pressure of the solution entering the manifold, ii gravitational forces helping to drive the solution down the slot-die head, iii viscous losses, and iv inertial acceleration of the fluid.
The rate of flow of solution is determined by the pressure drop across a given section. Therefore, in order to understand the flow of solution across the manifold, the pressure drop across individual points needs to be calculated and the flow of solution determined.
This can be done via computer simulation using finite elemental analysis, where the pressure drop is calculated at each individual element.
These pressure drops can be calculated using Equation 2, where the pressure drop dP across a finite length of manifold dx is determined by four different terms:.
The first of these terms is the viscous losses in a fluid. These can occur due to molecule-molecule interactions inside the liquid itself, and are dependent upon the speed at which the solution is moving and the length over which the solution is being transported.
This term dominates the pressure drop for high viscosities and very high flow rates. The term in brackets is a correction factor that accounts for the direction of the solution flow with relation to the orientation of the cross section.
This is important for sloped or curved manifolds, such as the coat-hanger or constant shear manifold. The third term relates to shear forces at surfaces perpendicular to the flow of solution.
This term is strongest for elements near the walls of the die-head. The stress tensor tau relates to the average velocity of the finite elements.
Viscous losses occur when two adjacent regions have varying flow rates. Therefore, at cavity walls, the stress tensor is high due to the flow of particles being zero at this interface.
The final term of the equation is the pressure drop due to gravity. This term is only useful for cavities where the manifold is at an angle, such as coat-hanger and constant shear manifolds.
Figure 7 shows how finite element analysis across a coat-hanger manifold is used to break the system up into individual elements. Inside each element, the drop in pressure is calculated across the element.
Inside the lands and slots, the ratio of slot thickness to length is low enough that lubrication approximation can be used to predict the flow of solution.
This results in the flow being given by the Pouiselle equation Equation 3. At the boundary between the manifold and the slot, the pressure and flow rates must be continuous.
In reality, there can be a small loss of pressure during this transition between the manifold and the slot due to changes in the flow direction , causing viscous losses and inertial forces.
This can happen at abrupt interfaces that can cause vortices within the flow of the solution. These can be reduced by smoothing out the change in flow direction when transitioning from the manifold to the slot by using manifold cross sections such as the teardrop-shaped design.
The change in pressure can be given by the pressure difference between the manifold and the slot-die exit.
Due to the conservation of mass, if the flow rate is fixed by a metering system, the flow rate through the slot must remain the same. Therefore, the pressure drop is regulated by the viscosity of the solution, the length of the land, and the thickness of the channel.
The channel length in a slot-die coater is difficult to adjust, and requires the re-milling of an entirely new head to achieve. This makes using the slot length as a method of controlling the pressure drop between the manifold and the slot exit unrealistic, as the cost and time required to change this parameter is too high.
Alternatively, the viscosity of the solution can be modified, but this option is often not possible as many coating formulations require specific properties such as material composition, carrier solvents, surface tensions, and even viscosities to produce the optimal film properties.
The final method of controlling the pressure drop is to vary the channel thickness, as this term is cubic even small changes to this value will have a dramatic effect on the pressure difference.
In slot-die coating systems, varying this channel thickness is the best way to achieve the desired flow of solution through the slot-die head.
This is done by using thin metallic shims that work to space the two heads a small distance apart. By using shims of different thicknesses, or stacking multiple shims of a given thickness, the spacing can be increased to the desired channel thickness.
In slot-die coating, the head is placed close to the moving substrate so that once the solution exits the slot-die head, it enters the gap between the head and the substrate.
As the solution exits the slot-die head, it enters one of two constrained channels in the upstream and downstream directions. The flow of solution in these channels is given in part by the Pouiselle equation Equation 1 , where the channel thickness is given by the gap height between the slot-die lip and the substrate and the channel length is given by the length of the slot-die lips.
Due to the presence of a moving surface relative to the slot-die head, the flow of solution is not just determined by the Pouiselle equation.
Due to boundary conditions for fluids in contact with a solid surface the flow rate of the solution must be zero relative to the solid surface at the interface.
As the slot-die head is effectively stationary and the substrate moves at a set speed, this results in a varying flow rate between the bottom of the channel and the top.
The flow rate varies linearly up the profile of the flow channel, this type of flow is known as Couette flow and can be determined by the Navier-Stokes equation.
Due to the Couette flow, the overall profile of solution flow between the upstream and downstream lips will vary as it will be a summation of the flow due to the pressure gradient Pouiselle flow and the shear force Couette flow.
Figure 8 shows the superposition of the two flows for the upstream and downstream lips. It is the balance between these two flow dynamics that ultimately determine the positions of the upstream and downstream mensici, and the coating quality in slot-die coating.
The two menisci that form within the upstream and downstream lip channels are responsible for the quality of the coating of the wet film in slot-die coating.
Both menisci should be pinned within the channel between the lip and the substrate. If the menisci drift either towards the slot-die exit or swell outside of the channel, defects in the coating will occur see troubleshooting.
When the menisci are pinned within the channel, the coating is said to be within a stable coating window. The position of the menisci are ultimately determined by the balance between the pressure gradient and the flow of solution given by the Pouiselle equation and also the Couette flow from the shear force.
Figure 9 shows how the processing parameters of a slot-die coating system can alter the position of the upstream and downstream menisci relative to each other, and how a stable coating window can be achieved for a wide range of processing parameters.
The gap-to-thickness ratio of the system is a parameter that relates the flow rate of solution, the speed at which the underlying substrate moves, and the lip-to-substrate height.
The flow rate of solution and the gap height vary the pressure difference given by the Pouiselle equation. At the same time, the substrate speed will vary the shear forces and increase the Couette flow.
By increasing the gap-to-thickness ratio, the upstream meniscus is pulled back towards the slot-die exit as the Couette flow dominates the position of the upstream dynamic contact point.
In Figure 9 , it can be seen that a pressure difference exists between the upstream and downstream lips.
Under standard conditions, this value will be zero as the variations in atmospheric pressures over such small distances are insignificant.
However, in high-end slot-die coating systems, a vacuum chamber can be incorporated at the upstream lip - resulting in a lower pressure at the upstream lip compared to the downstream lip.
This causes an increase in the pressure difference in the Pouiselle equation for the upstream lip in comparison to the downstream lip.
This results in more material flowing upstream in comparison to downstream. This allows for faster substrate speeds for a fixed flow rate and gap height while staying within the stable coating window.
It can be seen that the stability of coating is a simple balance between the flow rate from the slot-die exit and the shear force due to the substrate moving.
Both the upstream and downstream lips must form stable menisci in order for the coatings to be defect free. Outside of the stable coating window, there are many defects that can be formed see troubleshooting.
The deposition of thin-films with high uniformity is of great significance for a variety of different technologies. Slot-die coating, with its ability to coat across a wide range of viscosities and at high web speeds, means that the technique can be used for advanced thin-film manufacturing and also for low-cost, high-volume products.
A wide variety of products use slot-die coating as a thin-film deposition method, including:. Although slot-die coating has many advantages, there are several technical challenges that make it more difficult than standard coating techniques such as spin coating.
This is due to the need to balance pressures at varying interfaces so that a stable meniscus can be formed during the coating process.
Defect-free coating can only be achieved by coating within a stable window, and the variation of one of many parameters can cause the process to exit this stable coating region.
Obtaining defect-free coatings requires an understanding of the various different defects that can appear during the coating process.
By knowing and being able to readily identify the defect, it is possible to pinpoint the origin of these defects. Once these have been noted, it is possible to identify which type of defect has occurred.
By changing the processing parameters, checking the equipment, and modifying the solution properties, these defects can be overcome - and users can begin to coat defect-free films using their slot-die coating systems.
The following section gives an understanding of the most commonly-found defects, and provides a broad overview of their characteristics, origins, and methods that can be used to eliminate their presence.
Slot-die coating relies upon the formation of two stable menisci that can be seen upstream and downstream of the slot-die exit.
The position and angle of the meniscus are important for obtaining defect-free coating. Figure 1 shows the position of the upstream and downstream menisci during coating of a defect-free film.
The upstream and downstream meniscus becomes pinned at the ends of the lips, and these are then classed as static contact points on the slot-die head.
The upstream meniscus also has a second contact point with the substrate. This contact point is free to move, and is called the dynamic contact point.
The shearing of the liquid due to the moving substrate causes a force directed downstream, which moves the dynamic contact point downstream towards the slot-die exit.
While downstream, the shear force causes a thinning of the liquid film. The second contact point for the downstream meniscus is assumed to be an infinite distance away, and is only a consideration during the start and end of coating.
The equations and parameters that determine the magnitude of these competing forces can be found in our slot-die theory guide.
The stable coating window is a region where the sets of parameters used for coating lead to the formation of upstream and downstream menisci similar to the ideal ones shown above.
Just outside of this window, specific defects are formed related to the shape of the meniscus. Going even further away from the coating window will lead to complete failure of the coating bead.
Figure 2 shows the stable coating window for a slot-die coating system. The upstream pressure is the difference in pressure at the upstream meniscus in comparison to the downstream meniscus.
In a standard slot-die coater, this value will be zero - as at the boundary between the atmosphere and the fluid, the pressure must be equal. Therefore, both menisci have a pressure equal to atmosphere.
However, with the addition of a vacuum box at the upstream lip, a pressure difference can be present between the upstream and downstream meniscus.
The gap-to-thickness ratio is the ratio of the height the downstream lip is above the substrate, and the thickness of the wet film.
This value is a maximum of two when no vacuum is present on the upstream lip - meaning that the thinnest the film can be is half the gap height.
Below the Coating Window - When the process drops below the stable processing window, the upstream meniscus begins to move towards the slot-die exit.
This starts with a gradual movement of the dynamic contact point and eventually leads to the static upstream contact point moving down the lip.
When an air gap becomes present underneath the slot-die exit, the presence of bubbles can occur through air entrapment.
When the static contact point recedes to the slot-die exit ribbing can occur as the downstream flow becomes disturbed by the formation of vortices.
Above the Coating Window - When the coating process goes above the stable coating window, in the presence of a vacuum box, the upstream static contact point begins to go past the confined channel of the lip.
This results in a swelling of the meniscus and a formation of swelling defects where excess material becomes present on the upstream lip - causing severe variations in the thickness of the coated film and a poorly defined coating width.
To the Right of the Coating Window - When the coating process goes to the right of the window, the wet-film thickness is significantly lower than the gap height.
Due to the high shear forces relative the to the pressure of flow downstream of the slot-die head, the wet film becomes significantly thinner than gap height.
The meniscus begins to recede towards the slot-die exit, and the formation of bubbles occurs as air begins to become entrapped within the film.
Further reducing the wet-film thickness relative to the gap height results in the static upstream contact point receding.
This leads to the coating bead becoming destabilised locally - thus the film no longer coats, resulting in the formation of ribbing defects.
It can be seen that the formation of two stable menisci situated within the lips of the slot-die coater results in stable coating of films.
When there is an imbalance between i the shear forces induced by the moving substrate, and ii the pressure drop of a solution flowing through the constricted channel, these menisci change.
Slot-die coating is a complex process, and obtaining a stable coating of a film requires a deep understanding of the physics behind the deposition techniques.
There are two types of defects that can occur:. The following section looks at the two categories of defects and shows commonly-occurring problems, the characteristic features of these defects, where they arise from, and the methods that can be used to remove these defects.
Chatter is a defect which is present across the whole width of coating. This defect appears either at the same point in the coating, or at regular intervals.
The characteristics of chatter are:. Fluid Delivery Systems - If chatter defects are due to the fluid delivery system, the reason is typically due to the pulsed flow of solutions.
Displacement pumps rely upon the movement of discrete volumes of solution. This leads to a chatter defect frequency dependent upon the rate at which these discreet units are displaced.
Here are just a few specific applications:. Micro-electronics: Flat panel displays, thin circuits. Batteries and capacitors: Lithium-ion battery electrodes, multilayer ceramic capacitors.
Barrier films: In food and medical packages, liquid film coatings are being applied to food coating. Solar photovoltaic: Solar cells require thin coatings.
Medical diagnostics: Chemistries and other solutions need to be accurately applied. Variations in thickness can result in variations in results.
Transdermal and oral pharmaceuticals: The more accurate you coat the transdermal, the more accurate the dosage.
Stripe coating is the preferred method when you are producing a narrow strip with exposed foil along one or both edges, in battery and capacitor applications for example.
Proper cavity design is essential. You need proper die shim thickness for optimized die pressure; that produces the right uniformity for cross web production.
The shim sets width — it looks like a comb. Accurate shim design and fabrication is important to maintain the proper alignment. A die vacuum is used to fine-tune the width.
The die vacuum also ensures there is no saw tooth or waviness. The coating edges must be continuous, consistent and sharp.
There can be no heavy build-up at the start of the patch. Scott describes this method in the audio interview. The trailing edge should be straight and clean, no feathers or drip lines.
As we noted earlier, industries such as electronics, medical devices and pharmaceutical keeping raising the bar. Choose the right methodology for your next coating project.
Our monthly e-newsletter provides insights from Delta ModTech and industry experts. Douglas Darrow says:.
February 27, at pm.