In standard proximity coating with slot die equipment requires that a gap is maintained between the slot die lip face and the substrate. In proximity coating, the substrate is supported by a precision backing roll and the narrow gap is set by a feeler gauge. To maintain the proper balance of flow, develop a meniscus that keeps air from infiltrating the dynamic wetting line and encourage the fluid to wet the substrate, the slot die to substrate gap needs to be 1-2 times the wet coating thickness. For practical purposes, setting the gap between the slot die and substrate is limited by the feeler gauge thickness and the precision control of the slot die positioning equipment. Feeler gauges are typically made no thinner than 1 mil (24.5 microns), which limit the wet coating thickness to 0.5-1 mil (12-24 microns). With great precision in the resolution of the slot die positioning equipment, the slot die to roll gap can be run narrower than this, but it is not recommended.
The concept behind TWOSD is that when a slot die is engaged with a precision backing roll you have a metal roll and a metal slot die that are within microns of each other. The thinner you want to coat, the closer you have to get. This is a dangerous proposition for two expensive pieces of equipment. One option is to eliminate the roll and have the web run over the slot die instead. But how does this work? The convex nature of the web is reversed, and the web wraps around the slot die lip geometry instead, creating a concave structure for the fluid flow. The web is supported by 2 idler rolls and the slot die penetrates the space in between. Now the roll and slot die no longer specify the gap of the flow geometry with that pesky web caught in the middle. Instead the hydrodynamic force of the fluid set the gap between the web and the slot die. Ingenious! This web and slot die interaction without the roll geometry has a special name – elasto-hydrodynamic action.
Elasto-hydrodynamic interaction means the fluid is lubricating the substrate and slot die lip interface, keeping the equipment from scratching the web. The amount of fluid required and the space between the slot die lips and substrate are controlled by the pump rate, line speed and process conditions. The back pressure developed within the slot die in combination with the external flow control (and pressure balance) between the upstream and the downstream lips control the streamlined path of the substrate over the slot die lip assembly and precise coating. With elasto-hydrodynamic interaction, the wet coating thickness can be reduced from 0.5 mil (12 microns) to single digits micron applications of 0.04 mil (1 micron) and up.
In the high speed, thin coating applications that benefit from TWOSD, the hydrodynamic lift force balances the tension of the web to keep the slot die from touching the web. This process can be simulated and the important parameters are: web tension, line speed/flow rate, wet coating thickness, wrap angles, lip geometry and resultant web penetration. The resultant pressures experienced by the fluid outside the slot die are different (higher) than typical slot die roll coating applications. These higher pressures can help smooth the coating, or in some cases, penetrate into the substrate. This penetration coating can be a desired effect for some saturated products that want to have fluid coating throughout a porous substrate.
You can’t avoid all coating defects however. If the tension, geometry, or pressures are not within the coating window similar defects can occur to slot die roll coating. The mathematical computation of the defects is simply different than determining the ribbing over overspill defects associated with the standard geometry.
So how do you control the wrap angle, tension, and overall coating capability of TWOSD? Physically the slot die is positioned between two idler rolls and the web serpentines between the lower roll, the slot die, and then the upper roll. The spacing of these rolls, both vertically and horizontally, are important to the resultant tension, pressure and elasto-hydrodynamic action. If you are stuck with a specific idler placement, then turn to the external lip geometry of the slot die to control the resultant pressures and coating window. The more radius that is created, the more flexibility is provided. Be careful! While your coating window opens up with increased lip curvature, to coat thinner, a smaller curvature is required.
For TWOSD, the input data-
- Span = distance between idler rolls where TWOSD is occurring (centerline to centerline)
- Indent = what range can be push the slot die in?
- Tension = over the span of TWOSD coating
- Wet thickness = coat weight for various products
- Speed = line speed for various products
- Angle = is the slot die coming in perpendicular to the web, or at an angle
- Surface tension of the substrate (or substrate type)
- Fluid rheology
- Density of the fluids
- Surface energy of the fluids (or type)
For TWOSD (Tensioned Web Over Slot Die) coating, there are 6 major variables in the process:
- Web tension
- Line speed/flow rate
- Wet coating thickness
- Wrap angles
- Lip geometry
- Web penetration
TopCoat simulation software was utilized to analyze an optically clear coating application.
The following inputs were used for the TopCoat simulations-
- Rheology data = LOW, MED, & HIGH (Expert Mode)
- Span = 4.4375 inches
- Indent = 0-0.09375 inches
- Tension = 1.2 PLI
- Mag = 10
- Wet thickness = 0.54 mil [13.8 microns]
- Speed = 150 fpm
- Land width = 0.015 inch
- Angle = 0o
- Tension = 0.02-0.03 N/m [20-30 dyne/cm]
- Viscosity = (provided in rheology chart) Pa.s
- LIP GEOMETRY
- Lip offset = 0 mil
- X-Scale = 1
- Y-Scale = 10
- Pivot % = 0
- Slot-Pos = 0
- Inlet Length = 1516 mil [38.5 mm]
- Inlet Gap = 2 & 3 mil
- Up-Stream Settings:
- Chamfer = 0 inch
- Chamfer = 0o
- UpLand = 0.015 inch
- Angle = 0o
- Radius = 0 inch
- Down-Stream Settings:
- Chamfer = 0 inch
- Chamfer = 0o
- Step = 0 inch
- Radius = 0 inch
- Density = 0.85 (high) – 0.95 (low)
- Surface Energy = 0.04 N/m [40 dyne/cm]
- Iterations = the number of calculations the modeling software is making; the higher the number, the more inaccurate the results are (>300 yields a “no solution” error)
Taking these set points into account, the simulation software was run at standard operating conditions. The standard set points shows stable flow-
This image is consistent regardless of the solids content of the fluid. There is a recognizable amount of excess fluid developing on the downstream face of the slot die. If the slot die has physical damage (a burr or imperfection), then the slot die/fluid interaction on the downstream lip/body may be causing the thin streak.
The 3 most common variables adjusted for coating window development for TWOSD are-
When the penetration of the slot die is reduced, a coating defect is recognized (“Unstable inflow”)-
This is true for all fluids. This defect remains until a penetration of 0.06 inch is achieved, then the coating window is stable through 0.09 inch.
Increasing the span (the distance between the idler rolls) allows for a larger coating window-
This is an example where the span has been increased to 10 inches. This allows for a more gradual transition of the substrate over the slot die and more shallow indentations into the web have a stronger effect. This is only true for the low solids content fluid.
The higher content solid fluid prefers a shorter span between the rollers-
In this instance, the span of the rollers is reduced to 2 inches and the indentation can reduce to 0.03 inch. This position works well for the low solids fluid also.
The effect of tension was tested by increasing and decreasing with the other parameters remaining the same. When the tension was cut in half (0.6 PLI)-
The drop in tension did not cause a coating defect until 0.3 PLI was reached and “Unstable inflow” occurred. When increasing tension, the coating window was clear until 2.4 PLI was reached. A “No solution” defect occurred, which mean that the mathematical model failed to converge.
Tension ideally is in a range that controls tracking of the web without the tension being high enough to damage the substrate. Tensions are more important in TWOSD coating technique than proximity coating, but both coating techniques require analysis and understanding of the tension zones. It would be preferable to have the coating section isolated for tension control entering and leaving the space where the coating head is depositing the fluid onto the substrate. The worst case scenario is to have the entire web line be one tension zone controlled by the unwind and rewind exclusively. Even when the coating station tension zone is isolated, the effects of temperature, span and web handling control are important to the resultant coating.
Tension control can be isolated by an incoming nip prior to the slot die and a wrap of 180 degrees or more on an outgoing roll (downstream). The loads can then be monitored by a load cell at each location. This isolation of tension is critical and TWOSD wouldn’t work without it!
PROCESS & DESIGN
In addition to the standard variables to be adjusted, physical changes to the slot die, or process variation may lead to an improved product. Decreasing the span seemed to help performance, so alternatively you could decrease web speed or reduce the downstream land length.
Running at a reduced line speed allowed for reduced indent-
Reducing the downweb lip face (to 0.010 inch from 0.015 inch) allowed for increased line speed-
Lip face geometry has been studied in academia and shown that not only the reduced lip face geometry, but more curvature in the lip face can contribute to improved coating capability. The wide variety of lip geometries were not studied here, but in general, more curvature can provide a larger coating window. This lip geometry variation is very fluid and process dependent, so it would be recommended to experiment with lip geometries as part of an overall experimental design prior to production.
In addition to line speed and lip face variations, web handling needs to be paramount when running TWOSD coating technique. Skew between idler rolls is more important and if the web tends to wrinkle, curl or otherwise travel, the isolation of the coating station takes on more importance.
Unfortunately, the simulation software can only place the slot die at the middle of the span and cannot show the effect of shifting the slot die towards one roller or another.
Standard warnings in TopCoat:
- Ribbing = Pressure(Land Length) > Pressure(Land Length-1) > Pressure(Land Length-2)
- Unstable Flow = Inlet Gap – Upstream Meniscus < 0.2
- Upstream Overspill = Inlet Gap + Land Length – Upstream Meniscus < 0.1
Whether flow instabilities represent a real world defect witnessed at a current coating arrangement is outside the capability of TopCoat. A coating defect is not recognized in the standard arrangement. If a coating defect were recognized, this warning may point to a change being required either in the process parameters or the geometry of the slot die system. Please keep in mind that TopCoat software utilizes math and physics modules and real world data may differ, especially for fluids that show more elastic characteristics and non-uniform substrates that can ride the fluid differently than expected.
TopCoat shows the cross sectional plane of a pump-fed, tensioned-web slot coater. This is a variation of traditional slot die coating in proximity of a precision backing roll. As with traditional slot coating, the coating thickness is set by the pump feed rate and the web speed. Everything else develops the coating window. Some key factors to consider-
1.The upstream meniscus is comfortably within the upstream lip. If it is too close to the inlet, then transverse barring will occur. If the upstream meniscus overflows the upstream end, then there will be flow beyond the physical pinning of the slot die.
2. Pressure should be in a reasonable range of a couple bar (approximately 30 PSI). This pressure is smaller than with conventional slot die coating over roll. This is because of the web increases the local gap and decreases the pressure, and because the effective pressure from the web is very small for small wrap angles. If the pressure dips into a negative zone on the downstream end of the slot die, this would predict ribbing defect (but does not show as such on the TWOSD module). During this phenomena, the slot die and the web flow are fighting the counter-effects of Couette and Poiseuille flow. Couette flow is driven by the moving web and the Poiseuille flow is driven from the need to move the liquid through the narrow gap. From these, a pressure distribution is first calculated, assuming that the film is flat and at a distance from the slot equal to the wet coat thickness. The pressure produced in the slot then deforms the substrate. Because the gap between the substrate and the slot will have increased, the pressure will have decreased. So when the substrate deformation is re-calculated it will not be as large as before. After a number of iterations, the deformation will stabilize and the final simulation values are shown.
3. For a specific coating thickness, web speed and rheology, there are 3 process variables that control the coating window:
- Span – the distance between the supporting rollers Indent – the amount the slot pushes into the substrate
- Tension – the amount of tension on the substrate
- Span and indent are interrelated.
A large span is equivalent to a small indent and vice versa. The important parameter in the slot die is the amount of tension normal to the substrate. The normal tension = tension * sin (angle). Angle = arctan (indent/span/2).
4. Elastic behavior of the substrate is ignored in TopCoat. The modulus and thickness of the substrate are not used in the calculations. The extra force exerted by the substrate on the liquid arises from local curvature of the substrate.
5. The number of iterations of the simulation is a guide to the boundary conditions of the process point. If the number of iterations is high (>200), then the slot die is near a borderline regime of the fluid flow. If the iterations exceed 300, then the simulation assumes that no solution is possible.
6. Viscosity and speed are much more significant in TWOSD than conventional slot die coating. Increasing viscosity and speed increases the pressure in the downstream portion, increases the curvature of the web and the gap increases. To bring the system under control, the options are to increase tension, increase indent/decrease span, or decrease web speed. If these process parameters are not acceptable, then the slot die downstream land length can be reduced. For low viscosity fluids at low speeds the tension should be decreased, indent decreased/increased span, or increase slot die downstream land length.
7. Pressure distribution within the slot die is important also. A modest back-pressure is an advantage for leveling out pressure fluctuations in the delivery of the liquid. If the slot lip lengths are small, then pressure control is more critical. Under these circumstances, surface tension forces are more important than viscous forces.
8. Tension control is helpful because utilizing this process parameter for coating control eliminates the need for accurate gaps. However, using tension control requires accurate control of the tension with little variation during operation of the substrate. This includes controlling the upstream and downstream rollers. Alignment of these rollers becomes important for good control of the tension in this arrangement.
9.Roller setting versus angle. In TopCoat, the roller settings are not adjustable. The effective angle adjustment is utilized as a replacement for this function.
10. Lip offset moves the upstream lip relative to the downstream lip, where a positive value increases the gap between the upstream lip and the roller.
11. Inlet gap is the lip-to-lip gap through which the liquid travels to exit the slot die. Decreasing the inlet gap increases back pressure and evens out the flow within the manifold of the slot die. The effect of the inlet gap has a cubic effect on fluid flow. However, with a smaller inlet gap, the precision of the lip surfaces become more important and coating lines more apparent.
12. Inlet length is the path length through which the liquid travels before exiting the slot die. The longer the path, the greater the back pressure. Inlet gap has a stronger effect on pressure and fluid flow than inlet length.
13. Pivot% sets the pivot point in TopCoat for the downstream land. The default is to have the pivot at the downstream end of the land. This is the best position for considering the intrinsic science of slot die coating. However, the setup for an individual process may vary greatly from this arrangement – most slot die manufacturers tend to define the pivot as 0% (right at the top of the downstream lip). This is a critical component to the outcome of the simulation, with a small variation having a large effect.
14. Length is a term in TopCoat defined as the length of the upstream land independent of the downstream land.
15. Inlet pressure is the calculated pressure needed to pump the required fluid through the slot die for deposition. The goal is to have a small differential between the inlet pressure and the maximum pressure calculated between the lips and the web. Balancing these pressures will ensure a proper fluid balance and larger coating window.
While there is a lot to take into account when reviewing slot die coating with TWOSD coating technique, the resulting system can allow for thinner coating thicknesses and improved operability. Keep in mind that all slot die coating techniques are pre-metered, so the thickness of the coating is controlled exclusively by the pump rate and line speed. The slot die is simply the vehicle to precision coating. Also remember that a vacuum system cannot be employed to assist in coating because there is not both a roll and a slot die to pin against. These differences are not draw backs, but simply variations in precision coating. Embrace TWOSD and let the physics of elasto-hydrodynamic interactions be your friend and help you coat thinner, faster and more effectively!
Find this article in AIMCAL’s Converting Quarterly publication when you click here.