Slot Die Basics (Related Information: SD Applications & Patch Coating)
Slot Die Coating is one of the basic methods of applying a liquid material 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.
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.
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 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.
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.
--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”
designates a slot die positioned to coat a web while the web is supported on a
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.
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
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.
Two Sided Coating
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……
For two side coating, our company recommends either:
Use a single side coater, and coat the
product in 2 separate coating passes, or
Patch Coating (More information)
“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.
Many types of
liquids are coated with slot dies. These liquids fall into descriptive
categories, and are broadly classed as “homogeneous”, and “heterogeneous”.
Examples of homogeneous coating liquids are:
Examples of heterogeneous coating liquids are:
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.
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