Slot Die Coating and its basics

Slot Die Coating is one of the basic methods of applying a liquid ma terial to a “substrate”. This discussion will be confined to flexible substrates, or “webs”.

Most simply, a coating liquid is forced out from a reservoir through a slot by pressure, and transferred to a moving web. In practice, the slot is generally much smaller in section than the reservoir, and is oriented perpendicular to the direction of web movement.

Slot Die coating has many variations, including design of the die itself, orientation of the die to the web, distance from the die to the web (“slot die coating” versus “extrusion coating” and “curtain coating”), “on roll” versus “off roll”, “patch coating” versus “continuous coating”, “stripe coating”, and the method of generating the pressure which forces liquid out of the die.
“Slot Die” coating is generally recognized to be coating with a die “against” a web (actually separated from the web by a cushion of liquid being coated).
“Extrusion Coating” is generally recognized as using a die with a gap between the die and web, and the liquid is “self supported” as it travels from the die to the web.
“Curtain Coating” is generally recognized as using a die vertically above the web at some distance, usually with wires or fixed supports on the edges of the liquid “curtain” to keep the coating liquid in a “flat bubble” shape from collapsing as it travels down to the substrate.

Practical considerations for use of slot dies as a coating method are geared to the quality needs of the coated product. These needs include acceptable “performance”, uniformity of coating thickness, freedom from point or line defects, and a uniform surface finish with the desired characteristics. As is the case with many manufacturing processes, high efficiency and productivity are usually key driving forces.

Slot Die coating helps maintain a high level of cleanliness, as the entire liquid flow path can be sealed against the environment, until the moment the liquid meets the web. Wear in a Slot Die system is very low compared to most other coating methods, further reducing “contamination” of a coated product.

“Uniformity” and “Quality” of coated products demand:

1) A coating liquid of appropriate composition and flow characteristics for the Web, giving a ‘good’ product, using the Slot Die method. 2

2) Consistent coating liquid… constant in composition, viscosity, and temperature.

3) Consistent substrate web (thickness, flatness, surface properties, pretreatments, etc)

4) Uniform liquid handling and pressure feeding means.

5) Mechanically accurate and reproducible die support.

6) Uniform web speed and tension at the point of coating.

7) “Appropriate” drying (a subject unto itself).

8) Environmental cleanliness appropriate for the product being coated.

9) Methods, procedures, and measurements, to allow control of important variables.

10) Design and Precision of equipment.

In our company, we have developed, and continue to develop our Slot Die coating equipment to satisfy our Customers’ needs for quality, uniformity, performance, efficiency and productivity.

Profile Uniformity
Profile of a coated layer relates to the deviation from the average thickness both in the “cross web” direction, and in the “down web” directions. “Cross Web” profile is generally controlled by the slot die flow design. “Down Web” or “machine direction” profile is generally controlled by uniformity of web speed and web tension, uniformity of feed pressure to the slot die, and mechanical stability and freedom from vibration.

--Cross Web Profile

One branch of strategy to yield a uniform cross web profile is to use a control system of so called “continuous” measurement of profile (normally a scanning sensor which describes a “saw tooth” pattern of measurement of the coated material along it’s length), then some “smart” software to feed back a control action to a slot die with a number of thickness control points across the width of the die. The control action then “fixes” the profile for the next length of coating. For instance, a measured thick lane of coating 2 inches in from one side of the web causes a control action to reduce the thick lane by redirecting the extra coating liquid to “somewhere else” across the web.
The role of the “smart” software is to attempt to ignore changing “machine direction” variations, then redirect the coating liquid to a thinner area. The ultimate goal of this type of high performance measurement/control system, is to achieve a steady state, non-changing setting of the profile adjustment “bolts”, once a good profile is attained.
Another method is to use the same “sawtooth” scanning, then operator experience and technique is employed to judge which bolts to adjust, and how much to adjust each one.
--On the downside, this type of profile control system is very expensive, and generally requires a high level of technical support. In addition, a deviation is corrected after the fact, so the length of product made prior to a correction is different than after the correction. That length of product may or may not be within specifications.

Another strategy, used by our company and others, is to make slot dies without profile control “bolts”, but to match the internal flow design to the coating liquid/product coated. We make our dies to an extremely high standard of accuracy, and test to prove that consistent profiles are achieved. Profiles in Production are then measured off-line, for QA requirements.
--On the downside, each slot die is limited to some range of coating liquid properties, for instance, to a 5 to 1 range of viscosity…..However, our company has developed several techniques of “building up” a die offline, to greatly extend the range of application for a single die. The Customer builds their slot die in that same manner the next time that same product is produced.
For some bulk produced products, for instance ceramic tapecasting, the same product is produced continuously. Profile for many of our company’s tapecasting Customers is consistently less than +/- 2 pct, even for tapes with 2 to 20 micron dry thicknesses.

--Down Web Profile

Good “down web” profile, or “machine direction” profile, is in part, a result of good web speed and tension control. These two aspects of web handling are mutually interdependent. In effect, ideally, a slot die provides a constant supply of coating liquid per unit time, ie: 10 ml/second. If a web is moving slowly, the coating is thick. If the web is moving quickly, the coating is thin. So, as a web speeds up and slows down, the coating is thinner, then thicker, and so on. Average speed of the web determines the average thickness, and instantaneous speed determines the instantaneous thickness. So, a “chatter” in web speed creates a chatter problem in the coating.

Our company works diligently to attain the most uniform web speed and web tension to give the best coating for critical coated products, through using the best design and best components.

Likewise, supply of the coating liquid to the die must also be uniform for a uniform down web profile. Low flow gives thin coating , high flow gives thick coating. Average flow gives average thickness. If the flow varies, coating profile varies….For instance, a gear pump with a bad coupling will cause a surge in flow, and therefore a surge in coating thickness. A gear pump with ‘bad’ teeth will cause thickness variations every time the bad teeth mesh together (or 2 times for each tooth mesh).

Our company prefers a “pressurized tank” feed for slot die coating. Using a very accurate blanket gas pressure regulator, this provides and absolutely “pulse-free” supply of coating liquid to the slot die. For production applications, a flow meter is important, to allow compensation for changes in coating liquid viscosity from slight changes in composition, temperature, holding time, etc. from batch to batch.

“On-Roll” versus “Off-Roll”

“On-Roll” designates a slot die positioned to coat a web while the web is supported on a roll.
“Off-Roll”, or “Tensioned Web” describes slot die coating, without a physical support behind the web at the point of coating.
Generally speaking, On-Roll slot die coating is used for thicker, or less critical coatings, and for “patch coating”. With adequate tension control, Off-Roll coating is used for thin, high speed coatings.

With On-Roll coating, the “in-out” die position is kept fixed relative to the center of rotation of the “backing roll”. Therefore, coating thickness, although controlled by liquid flow and web speed, will vary, based on imperfections in the radius of the backing roll, and variations in thickness of the substrate. Substrate thickness variations of 2 or 3 percent are common…For a 75 micron base web thickness, this is a few microns. “Run-out” of the backing roll may also be a micron or more.
So, with On-Roll coating, for a wet coating thickness of 100 microns, the coating will vary 3 percent or more just from the backing roll and web variations. For a wet coating of only 6 or 8 microns for thin ceramic sheets, for instance, this situation creates a totally un-acceptable variation in coating thickness.

Off-Roll, or tensioned web use of a slot die can avoid the above problems. But, tension control and constant tension become more important than for On-Roll coating. Our company emphasizes uniform tensions by equipment design and individual component selection.
Our Customers achieve profiles generally less than 2 percent, even for ceramic sheets down to 2 microns (dry thickness), and at speeds in excess of 50 m/min with Off-Roll Slot Die Coating.

Die Orientation

There are 3 basic slot die orientations. These are: Die facing upwards, horizontal, and die facing downward. Our company normally offers a horizontal slot die orientation, often with an option for angling the die downward to some degree. Our Customers find that a die facing upwards (web over die), creates a bad mess, particularly during adjustments necessary for coating a new product. This position also is not good for thick coatings with low viscosity liquids, as the liquids tend to run back down the web towards the die. With a horizontal slot die position, or a slot die angling downward, the vertical rise of a web can be reduced, so there is less tendency for liquid to run backwards on the web.
Horizontal position gives the best combination of air purging at start-up, clean operation, and the finest degree of operator adjustment. Horizontal orientation also lends itself most easily to a shift from an On-Roll to an Off-Roll coating position.

Two Sided Coating

Our company receives a large number of requests for equipment for two sided coating. The idea here, and it is not a bad concept, is to simultaneously coat the front and back side of a web, then pass this double wet web through a single dryer, and have a highly productive coater……
Of course our company (as well as other coating companies) have tried this concept, because it does seem so attractive, particularly for lithium batteries, some types of fuel cells, and so on. The logic for two horizontally opposed slot dies, with a web moving vertically upwards between them, through a vertical dryer, seems good.
BUT, there are un-solved problems associated with this concept, a few of which are:
---The roller under the “back-side” die will invariably get messy.
---Vertical dryers are very expensive, and require a tall building.
---Drying materials that tend to “skin over” and crack, etc. require back-side heating with low airflow on the front side. There is no back-side when both surfaces of the web are wet. IR heaters can help, but this is difficult for solvent base coatings.
---Set-up time and start-up times are longer.
---Adjusting one slot die position affects the web position and therefore adjustment of the other side coating.
---First contact of the dried web will be with a roller at the top of the tall dryer. A web which has wet coating on both sides, inside a dryer possibly 8 meters or 12 meters tall can be very heavy. This weight must be supported by the web at the top of the dryer. Thin foils or films may stretch or break with this weight, requiring a thicker substrate.
---Vertically oriented webs with thick coatings of a low viscosity solution tend to develop streaks from the solution draining backwards down the web. So, simultaneous 2-sided coating requires coating liquids of high viscosity and reduced coating thickness.
---Metal foils should be dried in either an arch style or a “sine-wave” flotation dryer. Arch style dryers are for webs with only one side wet. Flotation dryers can require high air velocities, which can cause skinning, then cracking of problem coating liquids, if high velocities are used early in the drying process. So, metal foils may present a problem with simultaneous two-side coating.

For two side coating, our company recommends either:

1) Use a single side coater, and coat the product in 2 separate coating passes, or
2) Use a coating machine with 2 separate coating heads, and 2 separate dryers, or
3) Use a coating machine with 2 separate coating heads, and a “2-in-one” dryer.

Patch Coating

“Patch Coating” or “pattern coating” is a style of interrupted continuous coating, to leave a cross web line of substrate without any coating. This style of coating is often used for manufacture of electrodes for “prismatic” design lithium chemistry rechargeable (also known as “secondary”) batteries, super-capacitors, fuel cells, and other predominantly energy-related products.

Our company produces a slot die coating system with “patch coating”. This patented system uses a PLC control system for coating liquid flow control, die position, and a “suck-back” mechanism. Profile of the leading and trailing edges of each patch are adjustable to suit a variety of coating liquid characteristics. Patch length and un-coated length are likewise adjustable.

Coating Liquids

Many types of liquids are coated with slot dies. These liquids fall into descriptive categories, and are broadly classed as “homogeneous”, and “heterogeneous”.
In the homogeneous class of liquids are polymers and other solids dissolved in solvents, and monomers that are cured by cross-linking.
The heterogeneous class of liquids are dispersed hard particles, soft particles, and insoluble liquid phases in a carrier liquid. The carrier liquid may consist of a solvent or monomers with dissolved components (such as binders, plasticizers, processing aids, etc).

Examples of homogeneous coating liquids are:

Polymers dissolved in solvent, or “solutions”, such as ink adhesion primers, monomers for “hardcoats”, other clear protective coatings.

Examples of heterogeneous coating liquids are:

Lattices - a “latex” is a suspension of soft particles in a non-solvent carrier liquid, such as some paints, adhesion primers, some membrane casting liquids, and PSA coatings.

Emulsions – an emulsion is one liquid dispersed in another liquid, neither of which will dissolve the other, such as some water-base coating monomers. Milk is a common emulsion.

Slurries – (“colloids” are a stable, small particle type of slurry) - a slurry is a mixture of small particles suspended in a liquid. Examples of slurries are battery electrode coatings, fuel cell coatings, magnetic coatings, bonded abrasive coatings, inkjet receiver coatings, and so on. Slurries are the most popular liquids to coat with slot dies.

Coating liquids are critically important to high quality, efficient, high productivity coating. Broadly speaking, composition and preparation are the most important factors for high quality coating liquids. Without going into a text on this topic, it is an urgent matter to maintain a consistent supply of raw materials and an un-changing method for the preparation process of the coating liquid.

Coating Liquid Flow Properties

Slot Die coating can work well with coating liquids having a range of “flow properties”, or “rheological characteristics”. Slot die coating is generally understood to generate a low level of shear on a coating liquid.

“Newtonian” liquids maintain the SAME viscosity, regardless of whether the liquid is stationary inside a tank, or being subjected to a high shear. Slot Dies typically perform well with coating liquids with Newtonian behavior.

“Thixotropic”, or “shear thinning” liquids tend to have a higher viscosity at rest and at low “shear”, than when subjected to a high level of shear. Slot Dies work well with shear thinning liquids, in general….liquids with a high degree of shear thinning can be troublesome.

“Dilatent” or “shear thickening” coating liquids can be very difficult to coat. A common material with dilatent behavior is “Silly Putty®”.

A note on changes:

Every coated product producer should have a program for product improvements. This often includes changes in process “inputs” (materials, methods, equipment, etc). Changes should be made only in a deliberate, formal program, and not be made a part of “routine production”, until the “Customer” approves of the change.

Disclaimer: This information is offered in good faith, and does not necessarily pertain to a particular product or situation. No liability is assumed for either direct or indirect losses, or both, as a result of use of this information. Typographical or out-of-date information is subject to correction.

Suggestions and Comments are always welcome. Please send to coating@mirwecfilm.com