Patch Coating for Precision Fluid Placement

Authors:

The world has gone patch coating crazy for products including batteries, pharmaceuticals, and adhesives. The interest in reducing waste and providing functional coating that can be used right off the coating station is very high. With this increased interest in intermittent coating, we also must consider the technical challenges.

Every coating process has a start and stop. In patch coating, this occurs more often than at the beginning and end of a roll. It is a continuous, discontinuous process.  Of course, the steady-state continuous flow in between starting and stopping has been discussed at length and has its own issues to deal with. But what special issues are associated with the start and stop of a coating head? For this discussion, we will concentrate on slot die patch coating. Any shape can be coated–as long as it is a rectangle!

In the start-up and stop flow analysis, the challenge is to reduce waste and defects associated with the transition from fluid flow to not. The considerations of start-up include wetting of the fluid on the substrate, pump control of fluid dynamics, and physical position of the coating equipment to the substrate.

Wetting: The ability of the fluid to displace air at the fluid/solid interface in coating is critical to reducing defects. If the surface tension of the fluid and the surface energy of the substrate are not compatible, the start of the coating bead may be delayed or jagged. This would lead to an improper “head” of the coated patch. Surface energy modification may be required to produce a solid patch.

Pump Control: There are many techniques for intermittent coating, with the most widespread technique using valve control of the fluid and physical movement of the coating head. The valve acts as the immediate start/stop of fluid flow by redirecting the flow from the slot die back to the fluid delivery tank, while the mechanical movement of the coating head breaks the wetted bead. The timing of the valve control with the mechanical movement can produce a good head or a poor start that includes poor bridging of the fluid, streaks, or a heavy “head.”  Many systems also use some type of “suction” device to further influence the profile of the start/stop.  Many current systems incorporate this suction into the valve itself.

Physical Position: Where the coating head sits in relation to the substrate can determine the output flow of the fluid “head.” As a liquid surface approaches a substrate, the liquid boundary layer has vapor molecules that begin to adsorb to the substrate surface. This adsorption forms a bridge when the concentration is high enough. If the bridge occurs with a concentration that is even across the coating width, the coating will be uniform for the “head” or beginning of the coated patch. If the concentrated vapor is too far away, the substrate surface too rough, or the concentration of the fluid fluctuates, the coating will create a curved coating “head” to the patch.

The considerations of stopping are similar to start-up, but need to consider fluid reaction to mechanical and rheologic behavior. The resultant “tail” of the coated patch is a function of the timing between the valve for fluid control, mechanical movement of the coating head, and wettability of the substrate by the fluid. There is not just one way to coat patches of fluid onto a substrate, but as long as you are aware of the coating fundamentals, you will be successful in your intermittent coating operation.

INTRODUCTION

Intermittent coating (or patch coating) is coating with a start and stop shorter than the length of the full roll and defined by an uncoated border. In the case of slot die intermittent coating, the shape is always a rectangle. Intermittent coating can be completed in full web or lane coating, creating rectangles downweb from the coating source. The limit is based off patch control and reaction time.

Why would someone be interested in coating a discrete patch of coating instead of full web continuous coating? Money and performance. The most common example is in the world of battery coating. When coating an anode or cathode, fuel cells and lithium-ion batteries require an uncoated border to act as a current collection point either through the addition  of a welded “tab.” in the final construction. The uncoated current collection point serves a critical purpose to provide a very low electrical resistance point for the electrons to flow.  What this experiment was designed to evaluate are the limits of the coating width and spacing allowable with slot die coating technology. The application of lithium-ion battery technology, along with the product specifications, provided the outline for the experimental procedure and conclusions.

PATENTS

One of the earliest known patents regarding slot die technology and intermittent coating is credited to Edward Choinski (US Patent 4,938,994) from 1990. This patent describes the basis of intermittent coating required for printed electronics based off flow control and mechanical movement. This patent also describes multilayer fluid coating to allow for more than one fluid to be coated simultaneously on the same side of the substrate.

Further information (and the patent closest to the current arrangement) is described by a 3M patent (US Patent 5,360,629), in which a pattern of discrete patches are spaced on a moving substrate. These coatings are created with a slot die and metering pump where a valve directs the fluid to either the slot die or a holding tank. The three-way valve is important to most intermittent coating applications today. Description of the PLC control provides insight into the process utilized in this experiment.

Another patent application from Watanabe et al. (US Patent 5,824,156) describes a physical reduction in flow (shut-off bar positioned within the slot die) to eliminate flow through a position within the slot opening. This shut-off bar is located internal to the slot die, while some non-patented applications have considered a shut-off bar external to the slot opening.

A Durr patent included describes some additional details of the intermittent process similar to the 3M patent uses independent valves and a method that can include die or web movement to improve the starts and stops of the patches.  This patent uses a position-based control scheme rather than time-based.

SPECIFICATIONS

For lithium-ion battery applications, the following data set is common and was the basis for the experiment:

HEAD & TAIL DEVELOPMENT

The initial start and final stop flow control is what is considered the “head” and “tail” of an intermittent coating patch. Following industry protocol, the goal was to reduce this increased or decreased thickness to a maximum of 3 mm in length when coating a discrete patch at 10 mpm. The head and tail occur for reasons of typical start and stop phenomenon (crossweb caliper control, velocity gradient, pressure gradient, and volumetric flow).

The following graph shows the relationship between mass free length and coating speed.  As the coating speed increases for a given mass free length, the amount of time available to perform the start/stop of coating is reduced.  Current state of the art for patch coating requires at least 30-50 ms of time to complete the action of stopping and starting the coating.

EDGE EFFECTS

When the anode or cathode is coated onto a foil (aluminum and copper, respectively), besides the start and stop phenomenon of the head and tail, we also have to deal with edge effects. When assembling the final battery cell, any significant variation in coat weight will cause issues in the battery performance. Edge effects occur for three main reasons: surface tension, film stretching, and die swell.

The combined head and tail effects with edge effects may seem small in the coated and calendared product, but the stack of multiple layers can increase the variation by millimeters and cause unevenness, wrinkling, folding, and power differentials. None of which is allowable in energy storage devices. This current distribution variation is recognized, in the worst case, as a spike in energy with runaway heat. Heavy edges increase battery cell volume and decrease energy density.

PATCH CONTROL

There are two fundamental methods for creating discrete coatings on continuous web:

The response time of each is critical to the resulting patch quality. This is the realm of computer control, electrical response, and programing know-how. The intermittent programming logic needs to control as much as the die positioner movement, valve start and stop, web control, and possibly vacuum for fluid pinning. The response time of the valve and the time required for physical movement are the controlling factors of the patch length.

Flow Control: In a typical arrangement, a pair of two-way valves, a three-way valve, or in many cases a custom valve(s) controls the flow of fluid toward and away from the slot die. Flow control includes valves that work within the closed slot die system to allow for flow that is diverted when not needed for coating and shut-off bars within and external to the slot die to keep internal fluid flow within the slot die from escaping.  While the valves control the direction of flow, sometimes additional “suck-back” mechanisms are required to precisely form the beginning and end of the coated patch.  These suck-back devices can be in the form of a vacuum tank and valve, a diaphragm-type, or incorporated into the design of the valve itself.

Mechanical Movement: If the fluid flow control is not enough to break the bead, then mechanical movement of the equipment is required to create clean starts and stops for the rectilinear patch. The mechanical movement can include moving the slot die away from the substrate or moving the substrate away from the slot die. This movement can be controlled by pistons that react with air or mechanical positioning. Another option is to utilize a cam for controlled motion with valving to match the cam motion.  The cam can either be mechanical, or electronic with servo control.  Servo driven movement is more precise, repeatable, and quieter than pneumatic or other mechanical movement.  However, servo-driven systems add cost and complexity for hazard-rated areas of operation.

The mass free length is governed by the physics behind the timing for completing a cycle from stopping to starting the coating.  Most intermittent processes are limited to completing this cycle in 30 ms or greater.  Cycling valves or moving the web or slot die faster than this minimum time result in extreme G forces and stress on the mechanical components and also risk cavitation of the coating within the fluid delivery system.

Fluid Properties: The patch coating process works best with fluids that are shear-thinning in nature, non-elastic, and incompressible.  Shear thinning fluids enable the start/stop to occur efficiently.  Elastic coatings confound the start/stop of the patches because their elastic nature resists breaking of the coating bead for the stop of the patch.  Any kind of air in the coating whether entrained, dispersed, or dissolved creates a compressible coating that acts similar to elastic coatings.  Compressible coatings also create machine direction problems in unstable mass loading as the fluid is compressed and decompressed.

EXPERIMENTAL

Water-based anode and NMP-based cathode slurries were utilized with aluminum and copper substrates for coating understanding. These raw materials are typical for lithium-ion battery applications. Rheological data shows shear thinning behavior for both fluids across the shear rate range experienced in slot die coating (1-5,000 sec-1).

After anode and cathode were coated, cut-outs and cross sections were taken and analyzed to determine the relative heights to the average crossweb profile. These are final dried profiles, and not wet.

The slot die was of uniform dimensions for all applications:

RESULTS AND DISCUSSION

It is damaging to the final battery cell assembly if the head, tail, or edges of coating differ in height in comparison to the bulk of the coated anode or cathode slurry. The additional coating in edge beads or leading/trailing edges result in increased mass and density for those portions of the electrode.  The increased mass creates a localized imbalance of anode to cathode in the cell, but also create localized high-density areas that have lower porosity and higher impedance in the electrode.  In addition to height variation, the width of the unwanted bead and the parallel nature of the coated line is important.

What was found in experimental results was that the parallel line is a function of line speed. As line speed increased, the parallel nature was less difficult to maintain. This is a combination of the wetting effects of the shear thinning fluid and the reduced pulsation at the higher line speeds.  Pressure drop through the slot die has also been found to play an important role in controlling parallelism of the starts and stops in intermittent coating.  Different fluids require different pressure drops to provide the exact volume and velocity of coating as it exits the slot die to form the coating bead.  The same is true for stopping the coating at the end of the patch.  However, parallelism of the slot die to the backing roll can also create non-parallel starts and stops to the coated electrodes.

Another important phenomena is the effect of obtaining steady-state flow conditions for the fluid prior to engaging the slot die. The critical concern is making sure the flow rate is smooth at the exit and air is not trapped in the system. In the “head” of the coating, if there is a pressure overshoot, then the fluid comes out at an increased velocity. Even if the pressure is stabilized, air in the system will allow the fluid to flow excessively. Measuring and balancing pressure within the slot die valve system improved head and tail development.

In edge effects, whether considering surface tension, film stretching, or die swell, stresses are imparted in the fluid from the slot die. These stresses develop a draw ratio D, which is defined as a ratio of the substrate speed (vsubstrate) to the fluid speed (vfluid). The edge height is taller by a factor of the draw ratio (D1/2). Proper mechanical design of the slot die for the rheology and similar fluid to substrate speed improved edge effects.

CONCLUSIONS

Intermittent coating continues to be an important and growing field in the world of slot die coating technology in general, and the flexible electronics and energy storage industries in particular.

With lithium-ion battery coating applications, the ability to meet the current production requirements for a simultaneous dual-sided, intermittent coated anode and cathode are possible in the 10-30 m/min range. The question is where the upper limit is and what advances need to be developed for higher production speeds and improved energy density.

Fluid control and physical movement were investigated for this experiment. Utilization of both seemed to create the best effects for reduced head/tail development, reduced edge bead, and improved parallel edge effects. Surface modification and vacuum were not considered for this work.

Developing a deeper understanding of raw material rheology, process limitations will provide the framework for new developments and breakthroughs in the intermittent coating industries.

REFERENCES

An August 2025, article in US News and World Report, “US Companies Aren’t Ready for Baby Boomers to Retire,” outlines the significant amount of industry knowledge that has been and will be exiting the market and what a negative impact that loss of knowledge, or “Brain Drain” will have.

This is common in all industries, but how will this affect the roll-to-roll (R2R) industry? If you’ve been in R2R long enough, you’ve probably felt it: projects that used to move smoothly now stall on basics. The line can run, but it can’t run well. Troubleshooting takes longer. Quality drift gets normalized. And the experienced staff who used to handle the issues are no longer available.

That’s brain drain. And in R2R industries, expertise is leaving faster than it can be replaced, and often the training is insufficient. In addition, processes are becoming more complex with more raw materials, more SKUs, tighter tolerances, and more systems to interact with. The result isn’t just inconvenience, it includes real losses from lower yields, less uptime, slower scale-up, longer commissioning timelines, poor delivery, and lost customers.

R2R manufacturing is especially vulnerable because the industry relies on several complex disciplines. R2R manufacturing encompasses a chain of tightly coupled unit operations where a mistake in an early stage becomes a very expensive defect after further processing, or at the customer site. Many companies rely on a few “process whisperers,” experienced individuals who know how to tweak the line, because they’ve seen decades worth of successfully recovering from failures. When those engineers, operators, and mechanics leave, the ability to resolve these problems is often lost because much of the process knowledge was not documented.

R2R Manufacturing can be divided into eight sections which we refer to here as the eight Cs. The loss of experience will show in these eight areas and the handoffs between the stages:

Most companies still have pockets of excellence which might include strong chemistry or strong converting, but they struggle to connect the dots across all eight areas. Thinking of the entire system, referred to as ‘systems thinking,’ is often lost with the loss of experienced staff.

The costly loss to many companies when they lose skilled individuals is the loss of speed. Brain drain shows up as:

In systems as just described, the existing team often sacrifices time to learn the fundamentals of their processes because they are always in firefighting mode.

Brain drain in industry is more than just a headline. It is a slow leak that quietly turns good and stable manufacturing into reactive manufacturing. What can your R2R Manufacturing company do? If your experienced staff is still available to you, it is wise to assign your less experienced staff to document process details from the experienced staff. If that is not an option, your next best option is to work with experienced process engineers experienced in R2R Manufacturing Optimization AND Systems Development to help create guides, teaching materials, and a resource to support existing staff when needed.

Coating Manufacturing Technologies – An Experienced Team of R2R Experts

1) Rapid diagnosis that respects the full process chain. We map the defect and trace it across unit ops including material → delivery → coating → drying/curing → handling → converting → controls/data.

2) A structured way to rebuild lost tribal knowledge. We avoid guessing. We apply science and experience to:

3) On-demand experts for the gaps you can’t staff for. It is unlikely you would need a full-time specialist in each discipline year-round, but when your issues show up you need one now!

4) Project-based support that leaves your organization stronger afterward. The win isn’t that we are just fixing today’s problem. The real value is that your team will be more capable of resolving tomorrow’s problems, since we come along side your staff and resolve the issues together, teaching along the way.

In the pursuit of higher energy density lithium-ion and next-generation batteries, the field is shifting rapidly toward thick, high-mass-loading electrodes, a challenge that conventional wet coating struggles to meet. The recent work “Sustainable and cost-effective electrode manufacturing for advanced lithium batteries: the roll-to-roll dry coating process” (Park et al., 2025) makes a compelling case for replacing wet slurry coating with roll-to-roll dry coating. Find that technical resource, referred to in our article as “The Technical Review” here: RSC Publishing

Dry coating demands even more precise control over the deposition process and is inherently challenging. This complex process offers core advantages in battery manufacturing and can be best accomplished by integrating several precision technologies including slot die coating and extrusion technologies. Combining the precision of slot die with the efficiency of roll-to-roll manufacturing is the most precise and efficient way to develop the next-generation battery technology.

› Eliminating solvent-related drawbacks in thick electrodes

The review points out that the current most widely used technology, wet slurry coating, becomes problematic when electrode thickness grows. During the drying step, solvent evaporation causes binder migration leading to inhomogeneous microstructure. This means there can be uneven distribution of binder, active particles, and conductive additives. This condition tends to worsen for thick electrodes (e.g., ~200 µm), degrading ionic/electrical pathways and reducing performance.

› Sustainability & cost benefits

Dry coating removes the need for toxic organic solvents, reduces energy consumption (no solvent drying ovens), lowers environmental footprint, and can cut production costs materially compared with wet processes.

› Mechanical integrity via binder fibrillization 

In roll-to-roll dry coating, the polymer binder (commonly Polytetrafluoroethylene, PTFE) is fibrillized. This means that shear forces during the rolling/extrusion process stretch and reorganize PTFE particles into a porous, fibrous network. This network provides mechanical cohesion, structural integrity, and uniform distribution of active materials, conductive additives, and binder. The structural integrity of the network is crucial for performance in thick electrodes.

› Scalability and industrial viability

The review argues that roll-to-roll dry coating is among the most scalable and cost-effective approaches for next-generation lithium, lithium-metal, lithium-sulfur, or solid-state battery systems.

These strengths make roll-to-roll dry coating a powerful alternative to wet-slurry processes. The complexity of the dry coating process can make the up-front development hurdle a challenge for companies. Research and development needs to include a team with the scientific, technical, and process knowledge to develop a successful and powerful finished product. You may be surprised to that the best place to find those specific skills is to look to those experienced in slot die coating technology.

Even though dry coating replaces the liquid-slurry + drying sequence, the process remains a coating operation. That means many of the same underlying demands and risks apply including uniform film formation, consistent layer thickness, careful control of material flow and web dynamics, and scaling up from lab to roll-to-roll production.

Here’s how slot die coating and high-precision slot dies remain essential in battery manufacturing:

• Pre-metered, highly controllable deposition

In slot die coating, the wet (or in this case dry-mixed/fibrillized) “ink” is delivered through a precisely engineered narrow slit (the slot die) onto a moving substrate. The coating thickness (or more broadly, mass-per-area) is determined by the ratio of volumetric flow rate to substrate speed and die width in a pre-metered system that lends itself to predictable scaling and reproducibility.

For dry coating, this means that even if material is brought in a dry, shear-mixed form, the slot die geometry, flow control, and motion synchronization are critical to ensure uniform layer formation and avoid defects.

• Uniformity across wide webs and long runs (roll-to-roll ready)

Slot die coating is widely adopted in roll-to-roll manufacturing because it reliably and precisely delivers very uniform layers over large areas with high throughput.

In the context of energy-storage dry coating uniformity is not cosmetic, it determines the homogeneity of active material, binder, and conductive additive distribution, which in turn drives ionic/electronic conductivity, mechanical integrity, cycle life, and overall battery performance. In short, uniformity is crucial to performance and safety of the finished battery product.

• Avoiding defects and unstable coating behavior

Slot die processes operate within a coating window defined by parameters such as flow rate, coating speed, die gap (or coating gap), and material rheology. Deviating from that window can lead to defects such as uneven edges, beading, meniscus breakup, and non-uniformity. These defects compromise quality and safety of the finished battery product.

For dry coatings where material behavior may differ (e.g., PTFE fibrillized particles, different rheology) careful die design and slot die control is critical to successful coating and accomplished by utilizing experienced engineers specializing in both the scientific aspects and the process itself.

• Scalability and repeatability from lab to fab

Because slot die coating is a pre-metered, non-contact, roll-to-roll compatible process, one can develop a recipe on a small scale (e.g., lab coater) and reliably scale it to pilot or production lines by preserving die geometry, pump settings, web speeds. This relative ease of scalability is a key advantage for battery research and development to manufacturing workflows.

The Technical Review marks an important milestone: roll-to-roll dry coating is emerging as a viable, scalable, and sustainable route for next-generation battery electrode manufacturing, offering significant advantages over conventional wet-slurry methods.

However, “dry” does not equate to “simple.” The success of dry-coated, high-mass-loading electrodes hinges on precise control over deposition, uniformity, rheology, fibrillization, and web dynamics, all of which are governed by the design and performance of the slot die head and the broader roll-to-roll system.

About the Authors:

This article was written by Mark Miller, Co-Founder of Coating Tech Slot Dies; Niloofar Moradian, Technical Consultant and PhD, Coating Tech Slot Dies; and Erik Maki, Founder of Maki, LLC

Dr. Niloofar Moradian is a mechanical engineer specializing in simulation-driven engineering, with over thirteen years of combined academic and industry experience. Her work focuses on slot-die coating systems, where she applies physics-based modeling and computational analysis to understand material behavior, improve coating uniformity, and prevent defect formation in non-Newtonian fluid applications. She has broad experience across research and development, including concept development, engineering design, simulation-driven analysis, prototyping, testing, and product scale-up. In addition to her industry work, she serves as an adjunct professor, contributing to the integration of academic research with real-world engineering challenges.

Eric Maki is a consultant based in De Pere, WI, where he focuses on on-site and remote support for solving process problems in battery electrode manufacture.  He has 30+ years of experience in mixing and coating process engineering design and development from R&D through production scale-up. He has a bachelor’s degree in chemical engineering and master’s in business administration.

For Coating Tech Slot Dies (CTSD), a company that designs slot dies and process-critical hardware, this scalability and the ability to deliver comparable coating behavior at different scales is a core value proposition. At CTSD, we don’t just build slot dies. We build trust, performance, and partnerships. The emergence of roll-to-roll dry coating for advanced battery electrodes does not reduce the importance of slot die manufacturing. It amplifies it. Here’s how we see our role:

At CTSD, we believe that this is exactly where slot die expertise and process partnership deliver value. By combining world-class slot die design with deep process insight listening carefully, thinking critically, and collaborating intimately we equip battery innovators to “coat better, faster, and more confidently.”

In a world moving rapidly toward high-energy-density storage, sustainable processes, and industrial scale-up, slot die precision is not an optional extra. It is fundamental.

The 8C’s of Roll-to-Roll Coating: A Balanced Approach to Precision Coating

Precision roll-to-roll coating is a complex process that requires careful consideration of multiple interrelated factors. To achieve consistent, high-quality products, it is helpful to think in terms of the 8C’s: Chemistry, Compounding, Coating, Curing, Conveyance, Converting, Controls, and Computation. Each area contributes to the ultimate performance of the coated product and must be balanced to ensure success.

At the heart of every coating process is the chemistry of the fluid. Chemistry defines the molecular structure, rheology, and interactions of the coating material with the substrate. It provides feedback between the process engineer and product developer, ensuring that the final product achieves the desired optical, mechanical, or functional properties. The correct chemistry also dictates how the fluid responds to compounding, coating, and curing steps.

Compounding is the preparation and homogenization of the fluid prior to application. This includes mixing, blending, and degassing to prevent trapped air or inconsistencies that could create defects. Proper compounding ensures that the fluid maintains uniform properties as it is pumped to the coating head. A well-compounded fluid forms the foundation for all downstream processes.

The coating step applies the fluid to the substrate using precision equipment, such as slot dies, gravure rollers, or curtain coaters. The coating head determines the initial wet layer thickness and uniformity. Critical variables include line speed, fluid flow rate, surface tension, and substrate type. Effective coating replaces the air at the substrate interface with the fluid, forming the initial structure that will solidify into the final product.

Once applied, the fluid must be cured to transform it into a solid structure. Curing can be achieved using heat, light, electron beams, or other energy sources. The method, rate, and intensity of energy input directly influence the final microstructure and morphology of the coating. Proper curing ensures dimensional stability, mechanical strength, and functional performance.

Conveyance refers to the continuous movement of the substrate through the coating process. Whether handling discrete sheets or a continuous roll, maintaining steady-state motion is essential for uniform coating application. Conveyance integrates line speed, substrate tension, and alignment, ensuring that the fluid is delivered evenly along both the cross-web and down-web directions.

After curing, the product often requires converting to meet final specifications. This includes slitting, masking, laminating, or ablating the coated substrate into its usable form. Converting operations must be compatible with the cured coating to prevent damage or deformation, completing the transformation from coated material to final product.

Controls encompass the mechanical and process regulation necessary to repeat production consistently. Tension control, pump rates, line speeds, and environmental conditions must be monitored and adjusted to maintain uniformity. Robust control systems allow operators to respond quickly to variations, minimizing defects and ensuring reproducibility.

Modern roll-to-roll coating increasingly relies on computation for process optimization. Computational tools enable simulation of fluid flow, curing kinetics, heat transfer, and substrate interactions. Data-driven analysis helps engineers refine parameters across chemistry, coating, and curing steps, providing predictive insight into product performance and process stability.

Integrating the 8C’s

Successful roll-to-roll coating requires understanding the interplay of these eight domains. For example, the chemistry of the fluid impacts compounding, which affects coating uniformity and ultimately the microstructure after curing. Conveyance and control systems ensure consistent deposition, while computation allows for precise modeling and adjustment. Converting completes the process, delivering a product that meets functional and aesthetic requirements.

By considering Chemistry, Compounding, Coating, Curing, Conveyance, Converting, Controls, and Computation, engineers can develop a balanced approach to precision coating, optimizing both process and product performance for a wide range of industries, from optical films and adhesives to pharmaceuticals and battery technologies.  The key is to learn as much as you can about each individual unit operation as a stand alone technology, but also (and maybe more importantly) learn how they interact and influence one another to develop a precision coated product.  Keep digging into these areas, and the connections will make sense!

Edge Bead Defects

Have you heard of the common defect in slot die coating called edge bead? The defect can occur in either ambient temperature coating or hot melt adhesive coating.

Let’s define what edge bead is. When you are coating, the hope is that you can create the same coating thickness from edge to edge within that coated product. In proximity coating with backing roll, the fluid exiting the die goes on to the substrate and typically the material is coated with what we call a dry edge.  That area may be as narrow as a couple of millimeters or as wide as a centimeter on either side, depending on your product.

The area without liquid coating on the edges may be trimmed off or it may be necessary. In many cases, the fluid is even all the way across but builds up right at the edges. This buildup might be a millimeter or two. Once cured, it may still be a buildup that causes a solidified edge on your product. This can cause softening of your rolled product with hard ends and soft in the middle. This can also make it difficult or impossible to convert into smaller rolls, sheets, or whatever you are trying to develop. Therefore, reducing edge beads is helpful.

Even if you will be slicing off the final edge, the extra buildup of fluid at very least is a waste of fluid. Most likely it is causing more trim and handling. You can understand that reducing that fluid buildup is an economical solution.

To reduce the edge bead and make it less of an issue, understanding of why it is occurring is the key.

In ambient temperature coating, the internal design of die may result in forcing more material to the edges than allowing in the middle. All this excess fluid gets to the edge, and it hits the shim barrier. When this excess fluid exits, it has nowhere else to go and it just beads up on the end.

One possible cause for this may be that the internal manifold design is not appropriate, and you need to change the shape to encourage more flow to the middle than you have to the end. Does this mean that you need a new die? Not necessarily. One way to correct the problem is to use an insert inside the existing manifold to change the shape of it and encourage flow in a different way. Long term, you may want a manifold that is designed for your fluid, but if you are coating a variety of fluids, inserts are one way to overcome the issue.

Another cause for edge bead may be that the shim thickness is not accurate. When the shim is thinner and you bring the upper die body closer to the lower die body, you encourage the fluid to be pushed out to the end. If you’re getting edge bead and you feel confident that the manifold design is the best it can be, then the shim thickness might be off. You can change the shim thickness to have more flow in the middle. Assuming the shim is the control valve of the system, having thicker shim helps the fluid to naturally flow throw the middle of the die. If you have a center entrance fed die, flow coming into the die from the back middle is going to want to go out the front middle unless you squeeze it down and force it to the ends. For ambient temperature coating, typically you would have manifold shape and/or shim thickness control over the fluid flow from center to end.

The other solution for edge bead control is chambering. To understand the chambering solution, let’s assume that you are using 5 mil shim and you need to stage up to a size between 5 mil and 6 mil to not get the edge bead. If you can’t get the shim thickness accurate for what your fluid flow need is, then you probably need to look at chamfering the shim. But chamfering the shim or cutting kind of an edge release or constriction at exit for the die is something that does take computer modeling in order to get accurately right off the bat and just cutting off an edge of the shim is not going to necessarily solve your problem. It is kind of a micro or nano level control beyond the shim thickness.

Hot melt adhesive coating and ambient temperature coating are connected but have differences. In hot melt adhesive coating, you care about the viscosity of the fluid, the pressure inside the die, the flow material just like you do in the ambient temperature coating. But in addition, you also care about the elasticity of the polymer. So even in ambient temperature coating, you may have some elastic component, but the more fluidized, the more liquid that material is, the less the elastic character plays into that. In hot melt coating, the elastic component might actually be the bigger part of your flow control.

If you look at hot melt adhesive, you are basically taking 100% solids and you are controlling the flow of that 100% solid polymer by heating it. You are using temperature in order to make something that is rubbery to flow by heating it up. But depending on what your temperature is and how much sheer energy you are putting into it by forcing it through the slot die, you are going to either have the hot melt adhesive polymer act like a solid, act like a liquid or act like something that is rubbery in between that solid and liquid. The reason is not just the viscosity of the material, but also the G prime and G double prime, the loss of storage modulus, are effective parameters. One of those tells you the pure viscous nature of your polymer and the other tells you the elastic nature of your polymer.

If you are in the elastic nature of your polymer and you look at the final lip-land right before exit of the die, the point where the polymer is rheologically as it is exiting the die, if it is more elastic than it is purely viscous, then you may have a rubbery response or retraction of that polymer after it exits the die. This may create curl in your substrate and other types of defects like wrinkling. If you are avoiding edge curl at the point that the fluid exits the die, you have to be even more concerned about what state of the polymer is it in the viscous area, is it the rubbery plateau or is it moving through where it is acting like a solid, and how is it going to respond when it releases all that pressure, sees the outside area and then forms its final polymer?

Similar to ambient temperature dies, in hot melt adhesive dies, you can also control the shear rate by closing the lip faces down just like changing the shim thickness and you can encourage flow to the ends, and if you have too much flow in the middle, you can close down. If we are getting edge beads you may open up in the middle in order to encourage flow to the middle. Almost all hot melt dies have an adjustable lip instead of just one solid shim with fixed thickness, and you could make it variable from center to end in order to encourage flow differently as it exits the die which can be a benefit.

An operator may want to close down on the end of the die in order to stop flow completely so that the adhesive doesn’t leak out from the side, but so much compression force may push that polymer into a regime where it is actually acting more rubbery or if you haven’t pushed it enough, you may not have fluid. You should be doing calculations or running simulations on your flow to figure out what is the optimum gap in the die so that you are not forcing your hot melt adhesive to act like a rubbery material that might retract or cause issues.

In both ambient and hot melt coating, you are trying to look at the material and make sure that it is flowing to a point where it doesn’t have too much flow to the ends. In hot melt, specifically, you want to make sure that you are in the right flow regime for your polymer.  In ambient temperature coating, depending on how complex your fluid is, in addition to just getting the right manifold shape, shim thickness, and potentially chamber, you may also want to consider looking at if your material is acting more like a rubbery material, taking the same type of approach as hot melt.

A simplified explanation would be if you are trying to push rubber bands out and they are under a high amount of stress, they are going to retract when they exit as they are released from the stress. And then, when they retract, they are going to cause the issues stated earlier. The polymeric material behaves in a way similar to rubber bands in this instance. You want to have the fluid coming out of the die as relaxed as possible while still controlling flow.

In summary, in ambient temperature or hot melt coating to avoid an edge bead, first make sure your manifold shape is accurate. Secondly, consider the size of the opening and utilize shims to fine tune. Ultimately, understand the rheology of your material, so that you can determine the shear that is being placed on it by your equipment. This will help you make your material flow through the exit the way you want it to.

INTRODUCTION

Panel coating is a unique application of fluid coating technology that provides capability and functionality to a discrete substrate. But what is important to understand and what tradeoffs exist?

Unlike continuous coating application against a moving substrate, we are trying to develop steady state control over an intermittent process. The fluid needs to exit the slot die only when we want it to and stop flowing before a mess is made. In intermittent coating there are a number of variables to consider.

Like continuous coating, the math and physics involved to control flow and predict behavior for two dimensional laminar flow is the same. The trick is to develop quality coating immediately and stop the flow perfectly. So this head and tail development is more critical in rigid panel coating than it is in continuous coating.

FLUID RHEOLOGY

Many fluids can be coated onto rigid panels, so what are the key factors to be aware of in the fluid chemistry, surface energy and rheology to predict quality performance? One key factor is viscoelasticity of the fluid to be coated. In addition to the viscosity, elasticity can cause die swell, edge bead or ribbing. The external effects of fluid flow and viscoelasticity typically require some trial and error because predicting flow outside the slot die has fewer boundary conditions for simulation. Owning or having access to research and development level panel coaters are very helpful with new fluids or substrates.

There are two main techniques for coating rigid panels – proximity and curtain coating. Both utilize slot dies, but each has a different set of criteria to develop successful performance.

In proximity coating, the gap maintained between the slot die lip face and the rigid panel is 1-2 times the wet coating thickness. So, the lower the wet coat weight, the closer the slot die gets to the substrate. For rigid panels that have variation in surface caliper, this small gap can lead to variation in coating or damage to the substrate.

For very light coat weights (less than 1 mil or 25.4 microns), curtain coating should be considered. For curtain coating, the critical calculation of success is the Weber number. No heavy math is required, just a simple understanding of the fluid and the equipment setup.

We = ρQVσ

We = Weber number

ρ = density

Q = volumetric flow rate

V = impingement velocity

σ = surface tension

This simple equation helps us determine the success of coating based off a couple of easy to find numbers from the fluid and process. Success is typically predicted when the Weber number is greater than 2. One thing that you will notice with this equation is that the volumetric flow rate needs to be high and therefore the line speed needs to be high for low coat weights. This may limit the equipment capability if the coat weight is low because of the acceleration required to enter the coating curtain.

One of the big differences between proximity and curtain coating is the area coated. In proximity coating there is typically a dry border left on the substrate. With curtain coating, there will be overcoat that needs to be dealt with.

EQUIPMENT DESIGN

How about the equipment utilized? First of all, the slot die is going to be positioned vertically. Why does this matter? Air. Evil air. When the fluid initially fills the manifold of the slot die, the air in the system wants to rise to the highest point. For a vertically oriented slot die, this means that the air will travel to the back of the manifold or the joint of attachment for the fluid delivery pipe or tube. If the air has no release mechanism, then the air will slowly release over time, causing coating defects. Ideally, a purge valve will be placed either in the back line of the manifold or the fluid delivery tube, whatever the highest point in the system is. Once the slot die is filled and purged, the air will release and then operate correctly. Without a purge mechanism, intermittent coating will fail forever, due to pressure variations.

Intermittent coating is also subject to start and stop phenomenon. To relieve the pressure jolts that would occur with a valve with back pressure releasing a fluid into the slot die manifold and creating a heavy starting edge bead, a recirculation loop should be provided back to the day tank. Maintaining pressure within the entire circuit is critical to performance. The head and tail coated on the panel are a function of the stability of the pressure in the fluid delivery system in combination with the valve control and mechanical movement of the equipment.

OPERATION

After the equipment is set-up correctly, the coating of the rigid panel can still have issues. These issues can include the variation in the substrate, control of the travel and potential environmental factors. Let’s break these down a little.

Especially in glass substrates, the surface flatness can vary tremendously. If the final product requires micron level control, and the equipment is precisely manufactured to deliver micron level performance, the product may still fail if the substrate variation exceeds the coating specification. Make sure that the substrate flatness meets or exceeds the flatness requirement for coating caliper control both cross web and down web.

Another key factor in the coating of rigid panels is how to properly handle the substrate. Should the slot die move over the panel or should the panel move under the slot die? Can the panel be held stable with a vacuum table, or will the suction cause damage to the substrate? These factors need to be considered to properly design the equipment. Consider the stability and weight of the equipment traveling during the process. Also consider the ability to maintain precision as a heavy and stable piece of equipment traverses the coating area.

Environmental factors come into play for both proximity and curtain coating, but more so in curtain coating. Because of the larger gap created in curtain coating we have to be concerned with air in the room disrupting the curtain. With proximity coating, room air won’t cause a fluid disturbance to the same extent as curtain coating, but mechanical vibration and environmental debris can cause coating defects. Make sure the fluid and substrate are clean before coating.

CONCLUSION

Rigid panel coating is becoming more and more common place. Understanding the variables to consider will help your company attack this multi-variable process with the right tools for success. If you work through the questions presented here, you will have a deeper understanding of how to successfully coat panels and limit coating defects.

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-

For TWOSD (Tensioned Web Over Slot Die) coating, there are 6 major variables in the process:

TopCoat simulation software was utilized to analyze an optically clear coating application.

The following inputs were used for the TopCoat simulations-

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!

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:

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:

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.

Whether coating batteries or adhesive tapes, there are reasons why coating full width is not always optimal.  When an operation lends itself to coating wet and dry lanes down web across a substrate, what is required to understand what is involved?

Lane coating is the process of taking a fully formed precision flow of fluid and breaking it into areas of coated and uncoated areas as the substrate moves down web.  This appears as lanes of coated fluid with dry lanes in between.  This patterned coating can be achieved with any coating technique by placing barriers to the flow path.  In slot die coating, this means that the flow area needs to be controlled both inside and outside the flow cavity of the coating head.

Inside the slot die, the flow control can simply be the placement of a seal at the exit point of the slot die.  In a fixed lip style slot die, where the slot gap is held open by a body shim, the body shim would simply need to be manufactured with “fingers” that run from the manifold back line to the exit point of the slot die where the fluid would be kept from exiting.  The open spaces between the fingers would allow for fluid flow and the full manifold would still be employed to distribute the flow correctly.  In a flexible lip style slot die, where the slot gap is controlled by adjustable screws, a metal or plastic deckle would be placed in the front face of the slot die between the upper and lower lip and the adjusting bolt would be clamped down on the deckle.  Care needs to be taken with an adjustable lip slot die to make sure that the spacing of the adjusting lip and the deckle size correlate.  Adjustable lip bolts should work in groups of 3 across the solid lip assembly and spacing of the adjustable screws are typically 1 inch (25.4 mm) or more.  If a narrower spacing is required, an external shim may need to be considered.

The lane dimensions are limited to the internal flow, the external flow and the physical geometry of the shim being developed.  Once the shim geometry becomes narrow, a serrated lip may need to be considered.  A serrated lip would have the coated and uncoated areas cut and ground into the lip land itself.  This permanent feature should only be considered after the experimentation is complete to understand how the fluid reacts with the substrate at operating conditions of production.

When the fluid to be coated is reduced from full width to something less, the manifold is running sub-optimally.  With this sub-optimal flow, internal flow analysis will make sure that turbulence and residence time doesn’t become an issue, but experimentation will still be required to understand what is going on outside the coating head.  A rule of thumb is to have the manifold run optimally requires that full width coating takes place down to 20% less than full width coating.  In lane coating, these uncoated sections can add up and produce flow areas that reduce the flow control of the slot die manifold cavity design.

Computer modeling reduces guess work and provides precise understanding of the internal flow control, but the boundary conditions and mathematical analysis of external flow is limited.  The liquid can be understood according to the Navier-Stokes equation, but surface energy and surface tension interaction along with wetting and spreading phenomenon are not described by the math involved.  Because of this lack of capability to simulate flow outside the slot die, experiments should be run within the production process conditions to understand how the fluid will react to the substrate after exiting the slot die.  This may lead to shim dimension variations and iterative design and manufacturing of flow control features.

The ultimate goal is to reduce converting steps and not have wasted fluid coated onto the substrate of choice.  With simulation support and experimental guidance, lane coating and efficient coating can be developed.

INTRODUCTION

The slot die coating positioner.  That’s not even a real word!  If you type in the word positioner, the word processing program complains that you didn’t spell positioned correctly.  Well, the software developers didn’t work in the coating and converting industry.  For those of us who do, the positioner, or mounting equipment for the slot die is critical to fluid coating application.

In standard proximity coating, a slot die is held the wet coating distance away from the substrate, which is supported by a precision backing roll.  But what is holding the slot die in place?  The mounting system that holds the slot die within microns of the proper coating position is referred to as a positioner in coating manufacturing circles.  As products are coated thinner and more precisely, the potential variation in the slot die to substrate gap can be the difference in precision coated products and scrap.

The positioner has some key features that make it work properly.  Mechanically, the slot die mounting system should be designed to not twist or torque under the movement and pressures associated with the slot die interaction.  Finite element analysis of the mechanical structure is recommended during the design phase.  Movement along linear bearings allow for precision control and smooth transition from the off-coat to the on-coat position.  Pneumatic actuation is common, but servo motors can allow for improved repeatability and less settling time, although the strength of the motor versus the back pressure in the slot die to roll operation should be considered.

Setting the gap initially is important to flow control at the exit of the slot die, but repeatability is also important.  If the substrate requires a splice to pass, the positioner is the equipment that jumps the splice with a micro movement.  Having the positioner return to the coating position quickly and repeatably allows the coating to continue with very little interruption.  If the coating line needs to stop for an extended period of time, the positioner allows the slot die to be retracted a distance that allows the operator to clean the front face with no potential of interfering with the substrate or precision roll.  This coarse adjustment is typically 6 inches (152 mm) or more.  The splice jump movement is enough to allow the splice to pass without keeping the slot die disengaged too long.  Repeatability is typically 0.0001 inch (2.54 microns).

A positioner is not only critical in proximity coating.  In tensioned web over slot die (TWOSD) coating technique, a uniform placement of the slot die in relation to the web tensioned over the slot die is paramount.  Skew in the slot die to substrate position could lead to cross web coating non-uniformity, wrinkling or coating defects.

In hot melt adhesive coating, the positioner has the added function of controlling the attack angle of the slot die to the backing roll.  Because of the stronger elastic characteristic of hot melt adhesives, the variation of the attack angle plays an important role for successful coating.  The rotation of the slot die should be around the lower corner of the upper lip (coating in a horizontal position).  Rotating in another location of the slot die will lead to improper geometric changes and having to shift the die to coat correctly.

There are 2 critical gaps in slot die coating technology, the slot die opening itself and the gap between the slot die and the substrate.  While the slot die opening is more critical and controlling a cubic volumetric flow value, the positioner controls the linear relationship of flow control at the exit point of the slot die.  If the coating bead at the dynamic wetting line is not stable, neither will the flow control.

The positioner controls the coating bead development which keeps out air and develops the meniscus across the length of the slot die lip face.  The variations in pressure from the upstream to the downstream portion of the fluid flow can be successfully managed through this gap control and proper lip offset.  In the common coating defects seen in fluid applications to substrates, some of the most common are related to the slot die to substrate gap which the positioner controls.

Position your slot die properly and enjoy coating success!  Regardless of whether a positioner is a real word or not.

INTRODUCTION

Economies of scale occur when a coated product is manufactured wide, fast and in one pass. Substrates can be manufactured wide and multilayer coating can provided added functionality, but how do you make a coated product faster. The first hurdle is curing (typically the bottleneck of the process). Once you properly size the oven for the application, you need to consider the fluid flow dynamics within the coating system. As the fluid is coated faster and faster, the effects of film split, turbulence and air reek havoc on the coated product and leave a trail of coating defects in the path.

Some coating techniques, however, actually perform better under high speed scenarios. One coating technique that should be considered for high speed applications is curtain coating. Imagine taking a slot die, coating in close proximity, and raising it up in the air to allow the fluid to drop down onto the substrate. This rain shower of liquid can have some advantages, but the advantages can only be realized with higher speeds. Why is that?

The fundamental equation behind the success of a falling curtain of fluid is the dimensionless Weber number-

We = ρQV/σ 

We = Weber number

ρ = density 

Q = volumetric flow rate 

V = impingement velocity 

σ = surface tension 

The Weber number provides us with the height of the curtain that allows for success. A good starting point is to look for the coating conditions that allow for We > 2. This is considered a stability region for the fluid. As We < 2, the constant flow of fluid breaks up and disintegrates into droplets instead of a sheet. While it is possible to maintain a curtain under many conditions, this gets us started. Let’s break it down. The Weber number is a ratio of the density, volumetric flow rate and fluid flow rate to surface tension forces. The fluid flow rate is also termed the impingement speed of the fluid – the higher the fluid falls the faster the speed. Line speed and pump work together to develop the coat weight of the product – for a set pump speed (volumetric flow rate), a higher line speed creates a lighter coat weight product. Now, if the fluid is dropped at a high throughput, but the line speed is slow, the coat weight will be high. So if your product requires a light coat weight, you will need to adjust height or flow rate to meet our stability criteria (We > 2). Since density and surface tension are relatively set, volumetric flow rate or height of the curtain are the only variables that can be shifted to land the product in a reasonable coating window.

Now, there are many variables that affect the flow and stability of curtain coating a fluid, but the Weber number stability criteria is a good starting point. The coating window itself is a function of the Reynolds number and the web speed to impingement velocity-

Re = ρQ/μ 

Re = Reynolds number 

ρ = density 

Q = volumetric flow rate 

μ = viscosity 

U/V 

U = web speed 

V = impingement velocity 

If the U/V ratio is too low, disintegration of the curtain occurs (similar to a low Weber number). At a balance point between Re and U/V, the coating is stable. As U/V increases, we begin to see air be an issue because the line speed overcomes the fluid velocity.

With this handful of variables and simple equations, you can evaluate curtain coating to improve your speed and move into a new realm of coating capability.

ADVANTAGES

At its basic level, curtain coating differs from proximity coating only the distance from the substrate.  Besides this distance, many of the basic physical characteristics apply.  Both systems develop a meniscus, dynamic wetting line and free surface.  Both require precision machined edges and faces of the slot die to perform well.  Both techniques can coat as multi-layer systems to produce coated systems with multiple functional layers.  Also, both proximity coating and curtain coating technique need proper development of the fluid substrate interface to reduce air entrainment.

For proximity coating, a slot die is positioned the wet coating thickness away from the substrate.  For curtain coating, the gap is much larger and determined not only by the wet coating thickness, but the web speed, impingement velocity, density and viscosity of the fluid being coated.  This extra distance allows for coating irregular surfaces that would not normally be proximity coated.  Examples include nonwoven substrates and glass panels.

Hydrodynamic assist is a function of distance from the slot die exit to the substrate, gravitational orientation compared to direction of fluid flow, and momentum of the fluid at the impact with the substrate.

Whether coating a continuous sheet or a discrete part, curtain coating provides a speed advantage that may be a limitation for proximity coating.  In the early years of curtain coating, the liquid was released in a fluid film over parts and pieces and any excess liquid was collected and recirculated for continuous use.  This is still an option, but if solvents are released during operation, a recovery system should measure the density and viscosity to replace the proper amount of solvent during the recovery process.

So how fast do you need to run the line, how high does the slot die need to be from the substrate and how much fluid needs to be pumped through the slot die to form a stable curtain?  Every fluid and substrate interaction is different because of rheology and interfacial dynamics, but a good rule of thumb is that the flow rate of 0.5 cm3/s at a height of 250 mm (9.84 inches) works well for most applications.

Similar to proximity coating with slot die technology, finite element analysis can be completed to verify that the internal flow is appropriate for a stable cross-web caliper liquid curtain.  This information can verify that the liquid curtain will be robust enough to develop flow that has the same velocity, same pressure drop and same volumetric flow at exit.

DISADVANTAGES

The lengthy fluid bridge that is created in a curtain coating technique is fundamentally different than the short fluid  bridge created in proximity coating with slot die technology.  Instead of needing to cover the wet coating thickness through the formation of a meniscus at the substrate and slot die interface, curtains that are formed need to be self-supported until they reach the point of substrate interaction where the hydrodynamic assist develops the meniscus as required in all coating systems.

This lengthy curtain of fluid can be disrupted by air flow or vibrations.  To have the best possible flow control a couple items are required:  sharp lips at the slot die exit to control the static contact line of the fluid; edge guides to control curvature of the fluid during flow from the slot die exit to the substrate; and air shields to control the environmental impact of air currents on fluid flow.

To maintain a stable film formed to the complete width of the slot die coating exit, edge guides are required.  These can take the form of simple plates that the fluid rides from the slot die exit to the substrate or as complex as liquid fed tubes that help carry the curtain down.  The key is curtain stability at the edges.  Without mechanical edge guides, the liquid would neck in and have curvature in three dimensions.  This is most troublesome at the edges of the liquid formation, causing the fluid to curl in on itself and form edge beads that would be thicker than the rest of the coated substrate.

Air shields protect the formed curtain from disruption during operation.  A simple air current developed from an air duct or door movement can break up a stable curtain.  These mechanical air shields need to be concerned with the fluid formation zones at the slot die exit, the curtain stability zone during the travel from the slot die exit to the substrate and the coated film formation zone at the substrate.

In addition to the air and vibration issues associated with an unsupported fluid curtain, the hydrodynamic assist function creates an operating limitation and a reduced coating window.  The way the fluid develops a heel at the point of impact is important.  This heel development is a function of the contact angle which is subsequently dependent on the line speed and the wettability of the fluid onto the substrate.  This wettability can be encouraged through surface energy and surface tension match making between the liquid and the substrate, but is more powerfully controlled by the substrate speed and the volumetric liquid flow.

This ratio of volumetric flow (V) and substrate speed (U) is presented at the Reynolds number calculation. The ratio of U/V has a dramatic effect on the heel development of the impinged fluid flow.  While heel development needs to meet a critical level of stability for the removal of air, too large of a heel (too low of a substrate speed for the fluid flow) can lead to turbulent flow in the impinged heel and air defects downstream.  The exact Reynolds number and U/V ratio is empirically determined.

The Weber number calculation provides a great starting point to understand where the fluid will form a stable curtain, but this needs to be combined with the understanding of the hydrodynamic assist concept to produce a usable coated product.  Past experience has shown that while the literature states that a Weber number greater than 2 should produce a stable curtain, a Weber number greater than 7 typically is required for a robust stable curtain.  This theoretical versus reality based idea should be considered when designing equipment and coating liens with little empirical data.

Understanding the distribution of the liquid flow at the slot die exit is important for both proximity and curtain slot die coating operation.  This can be completed through finite element analysis and verified through experimentation.  However, developing a computer model of the flow external to the slot die in either arrangement is less simple.  The boundary conditions are less controlled which leads to a reduced understanding and replication with empirical data.  Navier-Stokes calculations provide a good starting point for a physics based concept of flow.  To further develop mathematical models, understanding the liquid to substrate interface should be empirically measured and compared to the FEA models being developed.

In the observation of the heel formation in curtain coating projects, the hydrodynamic assist can lead to a stable meniscus and coated film or not.  The worst case scenario looks like a pulled film without any heel formation while an intermediary flow looks like a heel is formed, but disrupted and unstable.  In either case the issue is air entrainment.  Air entrainment will lead to uneven flow and potential streaks developing in the coated surface.  The volumetric flow needs to be increased or the substrate speed needs to be reduced.  Keeping in mind that the ratio of the two develops the proper coat weight for the product.

CONCLUSION

Curtain coating slot die technology is an ideal solution for coating irregular surfaces at high speed.  It should be considered an option whenever proximity coating is not an option or thinner coatings and higher line speeds are required to meet the economics of the coated product.  With every equipment setup, however, considerations need to be reviewed.  Balancing the advantages of increased coating gap, higher line speeds and thinner coatings, with the disadvantages of air disturbances, vibration disruptions and the minimum line speed and flow rate requirements, leads to a process equipment decision based on economics and capability.  Curtain coating provides a strong case for precision coating from a distance.

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