Slot-Die Coating: Guide to Troubleshooting Defects
Slot-die coating is an extremely versatile deposition technique that is capable of being integrated into both roll-to-roll and sheet-to-sheet deposition systems. A major advantage of slot-die coating is its simple relationship between wet-film coating thickness, the flow rate of solution, and the speed of the coated substrate relative to the head. In addition, it is capable of achieving extremely uniform films across large areas, with variation in modern state-of-the-art equipment being less than 1% across several meters.
Although this process has many advantages, there are several technical challenges that make slot-die coating 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.
In the following guide, we hope to introduce the idea behind the stable coating window that slot-die coating operates within, and relate this to observable defects that can occur within the film. By knowing how the processing window relates to coating parameters, it is possible to vary these to return to the stable processing region and remove defects from your coatings.
Contents of Slot-Die Coating Troubleshooting Guide
- Stable Coating Window
- Slot-Die Defects
Stable Coating Window
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 shape of the menisci and the position of the contact points are determined by two factors:
- The interaction between the shear force
- The pressure drop of a liquid flowing through a thin channel.
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:
Defects due to instabilities in the coating bead meniscus where the coating process exits the stable coating window. Altering of coating parameters will result in the return to the stable coating region.
Or what could be classed as external factors either relating to the delivery of fluid, movement of the substrate, or the viscoelastic properties of the solution. These defects often require alterations to the coating system or fluid in order to overcome these issues.
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:
A line where the thickness varies in comparison to the rest of the coating.
A line where defects become more prominent in comparison to the rest of the coating.
A defect occurring at evenly spaced intervals or frequencies.
The origin of this defect is due to either:
The fluid delivery system having variations in pressure or flow rates.
The substrate roller/linear stage having pulsed movements or a defect on the roller.
Variations in the pressure within an upstream vacuum box.
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.
For rotary pumps, this is a function of the RPM of the system. Conversely for other displacement pumps based on linear motors (like syringe pumps), this is dependent upon the stepping rate of the motor. This can be mitigated by switching to delivering the discrete volumes of solution at faster rates, either by higher RPM or higher rate of micro-stepping.
Pulse-dampening elements can also be added to the solution feed to smooth out the output of displacement pumps. Metallic piping can be replaced with plastic piping, which undergoes expansion and relaxation during the feeding of new liquids - effectively smoothing out the pulses.
Substrate Stage - Stage chatter defects depend on the type of system being used. In a roll-to-roll process where a roller is used, the shape of the roller or the motor driving the roller could be the origin of the defect. By checking the distance between defects to the circumference of a roller, it is possible to determine if the origin of the defect is due to the roller.
For linear stages (typically used in sheet-to-sheet deposition), the chatter could be dependent upon a defect in the stage surface or an issue in the stepping rate of the motor. If the defect appears in the exact same position of the substrate, the issue will likely be due to a localised defect on the stage surface. For defects occurring at regular intervals across the length, the chatter will likely be due to the motor.
Vacuum Boxes - In some systems, a vacuum box is incorporated into the upstream lip of the slot-die head to overcome the minimum thickness limitations. Changes in the background pressure of the vacuum box will cause variations in the stability and positioning of the coating bead. Changes in the background pressure can be due to issues such as: i) chamber leaks, or ii) the problems with the vacuum pump being used. It can be difficult to relate the frequency of the defects in the coating to variations in vacuum pressure, as these may not always be regularly spaced if it is due to leaking.
Ribbing is similar to chatter. However, the defects appear along the length of the coating in regular intervals across the width of the coating. The characteristics of ribbing are:
Lines along the length of coating, where the thickness of the film reduces in comparison to the rest of the film.
Typically consists of multiple lines across the entire width of coating.
Some ribbing may occur as single-line defects. The origin of these ribs are different compared to multiple ribs.
Ribbing occurs when the upstream meniscus recedes towards the slot-die exit. This can be due to either:
High shear forces, due to fast substrate speeds moving the dynamic contact point downstream.
Low pressure at the slot-die exit, due to low viscosities or large shim thickness.
Reduced upstream pressure due to a wide gap between the substrate and slot-die head.
Localised defect on the head or slot-die feed forming vortices within the flow.
Shear Force vs Flow Pressure - The position of the meniscus is ultimately due to a balance between the shear forces at the substrate liquid interface and the pressure associated with flow through narrow channels. By balancing these two, the meniscus can be stabilised. The coating bead can be returned to the stable coating window by either reducing the shear force, or increasing the flow pressure. This can be done by:
Reducing the speed of the substrate to reduce the shear force.
Increasing the flow rate of solution to increase the flow pressure.
Reducing the distance between the upstream lip and the substrate to increase the flow pressure.
If it is not possible to change the above parameters, other methods can be used to reduce the presence of ribbing. These methods are:
Reducing the shim thickness to increase pressure at the exit of the slot.
Increasing the viscosity of the solution to increase flow pressure.
Adding a vacuum box to increase the upstream pressure gradient.
As these methods require a shut down of the coating process, they should be considered as the last options for removing ribbing defects.
Localised Defects - Sometimes the presence of ribs are due to localised defects on the slot-die head - these can either be on the lip, or within the feed slot - and can result in the formation of localised vortices which will cause a localised drop in the flow rate. These defects can be either due to damage of the slot-die head from mishandling, aggregation of material in the slot-die feed, or poor design of the slot-die head. Damage to the slot-die head can be repaired by polishing the surface to remove scratches. To reduce aggregation, the system will need to be cleaned and the solution reformulated.
Neck-in defects occur across the length of coating, at the very edges. The characteristics of neck-in defects are:
A gradual decrease in the coating width of the film across the length of travel of a substrate.
A thickening of the edges of the film, with the thickness increase at the edge becoming more predominant as the width of the coating decreases.
The origin of the neck-in defect is due to:
A transition from constrained flow of solution between the lips and substrate towards a plug flow results in a change in the flow dynamics.
Mismatch between the flow rate and the substrate speed causes acceleration of the fluid and shear forces.
Shear forces result in the contraction of the coating bead towards the centre leading to higher flow rates at the edges.
Changes in Flow Dynamics - The change from the constrained-channel Poiseulle flow towards a plug flow occurs as the gap height between the lip and the substrate increases. This occurs when the solution can no longer form a meniscus with the slot-die lip (when slot coating transitions to curtain coating). The greater the gap, the higher the degree of neck-in that occurs - therefore lowering the gap height can reduce the appearance of neck-in defects.
Solution Acceleration - Differences between the solution flow rate and the speed of the coated substrate can cause the solution to accelerate along the travel direction of the substrate. This results in the formation of shear forces that cause the edge of the curtain or slot bead to recede towards the centre of the coating resulting in neck-in. Lowering the speed of the coating web can reduce the appearance of neck-in if this is the cause, in addition a smaller shim thickness will increase the speed of solution exiting the gap allowing better velocity matching between the substrate and solution.
Solution Contraction - This can be affected by the properties of the solution. For example, solutions that have high viscosities and high surface tensions typically have a higher degree of neck-in occuring. These solution types have strong interactions between the solvent molecules. For solutions with high surface tensions, the addition of surfactants can help reduce these interactions.
Edge defects can occur on all edges of a coating. However, when under constant operation in a roll-to-roll system, the front and back edges of the coating are not seen as an issue. The characteristics of edge defects are:
Variations in the thickness at the edges of the coated films, typically appearing as a thickening of the edge areas.
Variations in the position of the coating edges. For leading and trailing edges, these are typically curved rather than straight; for side edges, these can go in and out along the length of the coating.
The origin of edge defects is due to either:
Transitioning between the coating being on or off is the main cause for edge defects in the leading and trailing edges.
Surface tension and viscoelastic properties of the solution can cause the movement of the wet film during the deposition and drying stages.
Differences within the surface energy of various regions on the substrate can alter how the solution wets to the surface.
Trailing and Leading-Edge Transition - The main reason for trailing and leading-edge defects is the rate at which the coating bead can be stabilised and destabilised. By changing the processing steps to purposefully destabilise the bead at the end, this edge can be well defined. This can be done by stopping the flow of solution to the head - or even retracting solution from the bead by drawing solution back. The speed of the substrate can also be increased to rapidly shear the coating bead. For more advanced systems, the slot-die head can be retracted from the surface to rapidly increase the distance between the slot-die and the substrate. Combining all these methods can result in a very well-defined coating edge - especially when working on intermittent coatings.
Solution Properties - At lower viscosities, the surface tension of the solution dominates this edge thickening. By reducing the surface tension and improving the wetting of the solution onto the substrate, the edge thickening can be kept within the region of a few millimetres. For high-viscosity solutions, the surface tension's contribution to edge defects is minimal, and varying the surface tension of the solution is not likely to improve edge quality. For viscoelastic materials where compression and expansion of the solution occurs, swelling at the ends of the slot-die exit can happen as the pressure drops. This can result in variations in the coating width and thicknesses.
Unfortunately, elimination of all edge defects is extremely difficult and requires complex engineering. Some examples of methods are: the use of air jets to remove the bead, doctor blades to shear the thick areas, or introducing solvent at the edge of the slot-die exit to dilute the solution at the edge of the bead. Generally, an acceptable size for a coating edge defect is around a few millimetres, and many substrates take account of this by having sacrificial coating areas which can be removed.
Surface Treatment - The wet film can be pinned to specific locations to improve the definition of coating edges. This can be done by treating the surface of the substrate in specific locations where the film needs to be wet. One method is to use UV ozone treatment to reduce the surface energy of the substrate in a specific location. This will result in preferential wetting in the treated areas leading to well defined coating locations.
Generally, edge defects are only a major problem when dealing with large-scale production. In small-scale prototyping, the use of sacrificial areas (to allow for the presence of edge defects) is commonly used. However, in roll-to-roll or sheet-to-sheet processes (where throughput is high), thicker areas will have slower drying times, which could result in contamination of the equipment if these areas are still wet. In roll-to-roll systems, thicker areas can result in the web not being able to be wound properly - and when using contact printing methods further down the production line, the centre of the coating area may not contact as well as the outer edges.
Streaks are a common feature in wet-film coating technique, and can arise from a wide variety of sources. The characteristics of streak defects are:
Individual line defects that originate from a point and continue upstream - appearing similar to a comet. A build-up of material appears on the downstream face of the defect, while upstream a line of reduced thickness appears.
An elongated line with reduced thickness that extends along the length of the coating.
The origin of streak defects are either:
Substrate defects such as dust or dirt particles that obstruct the flow of solution forming streak defects originating where the dirt particle is present.
Obstructions in the slot-die head or at the lip can result in extended lines of reduced thickness, or even uncoated areas. These obstructions are either aggregates within the slot-die feed, or dust particles big enough to become trapped between the lip and the substrate.
Damage to the slot-die head can result in the formation of permanent streak defects within the film, due to destabilisation of the coating bead at that particular location.
Substrate Defects - The presence of dust and dirt on the surface of substrates is an inevitable problem when it comes to wet film-coating techniques. Thorough cleaning of the substrates will remove the presence of dirt and dust. However, dust particles within the air will re-contaminate the substrate over time. The chance of this happening can be reduced by working within a clean room environment, where the presence of dust particles is minimised. Inline processes often integrate high-pressure air jets or rubber wipers just before the substrate reaches the slot-die coater, to blow or push away dust particles from the surface.
Obstructions - In areas where large dust particles are present and the gap between the slot-die lip and the substrate is roughly the same size of these particles, obstruction of the upstream or downstream lip can occur. This results in a destabilisation of the coating bead and the formation of a streak. The steps listed above to reduce and remove these dust particles from the substrate can be used however these are never 100% successful. Other methods (such as a rapidly oscillating slot-die head) can be used to dislodge these trapped particles.
The other source of obstructions that form streak defects are those within the feed slot of the slot-die coater - where the channel width is low. Here, any possible aggregates that form within the solution can become trapped and disrupt the flow of solution downstream. If aggregates become a significant problem with the solutions that you are working with, you may need to reformulate the ink to decrease the presence of aggregates. The simplest ways to reduce the chance of these obstructions is to increase flow rate through the head (to reduce dwell time), or increase the slot-die channel width. More advanced methods can involve the addition of a heating element to the slot-die head, or an internal wiper that can be quickly passed across the whole width of the feed channel to sweep away trapped aggregates.
One final source of obstruction is the presence of entrained air within the solution before it reaches the slot-die head. This entrained air results in the formation of bubbles that can get trapped in both the feed slot, and between the lip and substrate. To help prevent this, the solution should be thoroughly filtered and degassed before it enters the system. In addition, connectors and adaptors that are used to transport the solution should be checked for any potential leaks.
Slot-Die Head Damage - Although the slot-die heads are made from strong materials (such as stainless steel), damage to the head can occur in critical regions (such as the slot-die lip). Slot-die heads should be handled and stored with care to reduce the chance of any damage occurring - especially to the lip and feed channel. If any damage does occur, small scratches can be removed through the use of a low-grit lapping film. For large dents in the head, milling of the area can be done to smooth out the damaged region - or a complete replacement of the head may be needed.
Bubbles can appear in the film at any point in the coating. The characteristics of bubble defects are:
A film defect that is round or elliptical in shape. This size of these defects can vary.
The thickness of the film where the bubble defect appears will be significantly thinner, or may not contain any material.
Position of defects along the width of the coating is randomised.
The origin of bubbles are either:
Air entrainment in the original solution fed into the pumping system.
Entrapment of air occurring from leaks within the fluid delivery system.
Entrapment of air from destabilisation of the upstream or downstream meniscus.
Air Entrainment - Air entrainment is the trapping of air in the solution before it enters the solution-metering system. This can occur for several reasons during the solution preparation, such as loading of the solution into the pumping reservoir, or through incorrect setup and purging of the lines with solution. The presence of bubbles is often exaggerated for high-viscosity solutions, as they have a longer lifetime. In addition, low surface tensions can result in the formation of more bubbles - especially if surfactants are used to produce these low surface tensions.
The solutions can be degassed before being put into the fluid delivery system. If there are problems with bubble defects, additional care should be taken when loading the solution into the fluid pump reservoir. The solution-metering system should be kept at a lower height than the slot-die assembly, and bends in piping should be avoided in order to reduce trapping of air bubbles when initially flushing the lines.
Leaks in the Lines - When leaks within the line can occur, they can introduce bubbles into the solution before it enters the slot-die head. These leaks are most likely to occur where connectors and adaptors are used. Check that all of these are airtight - the use of PTFE tape (for sealing of threads) can help maintain a tight seal.
Meniscus Entrapment - When the coating process exits the stable coating region, the position of the meniscus will move inwards for both the upstream and downstream menisci. As the position of these menisci is fairly dynamic, the menisci can (within a localised region) oscillate between being stable and unstable. This can result in the trapping of air within the wet film, resulting in the formation of a bubble defect. By moving the coating process deeper into the stable coating window, these randomised fluctuations in the position of the menisci - and therefore the presence of the bubble defects - can be significantly reduced. Typically, to return the processing to the stable coating window, the gap height-to-thickness ratio should be reduced. This can be done by moving the slot-die coating lip closer to the substrate, reducing the web speed, or increasing the flow rate of solution.
Slot-die coating is a powerful processing technique for depositing highly uniform films via a scalable deposition process. By tuning the geometric parameters of the slot-die head, the solution properties, and the processing parameters uniform thin films can be deposited over very large areas. 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 pin point the origin of these defects.
In this guide, we have given users an understanding of the most commonly-found defects, and provided a broad overview of their characteristics, origins, and methods that can be used to eliminate their presence.
The most important things to observe when troubleshooting defects are:
- Where the defects occur
- Frequency of the defects
- Defect size & shape
- When in the coating process they occur
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.
To the best of our knowledge, the technical information provided here is accurate. However, Ossila assume no liability for the accuracy of this information. The values provided here are typical at the time of manufacture and may vary over time and from batch to batch.