SLOT DIE COATING TECHNOLOGY
Mark David Miller
Extrusion Dies Industries, LLC
911 Kurth Road, Chippewa Falls, WI 54729-1443
Prented at the 14th International Coating Science and Technology Symposium,
September 7-10, 2008, Marina del Rey, California1
ABSTRACT
Slot die coating technology is a function of the coating process, auxiliary system and fundamental technique. The decision to utilize a coating technology needs to be analyzed against the functions to determine best fit. In the era of clean, thin and preci converting operations, a customized coating system is necessary to have a technical edge. Building the system from the material up to the process ts the stage for a high precision tool designed around the process needs.
The dramatic increa in the expectations of coated products, in areas as diver as optical films to battery technology, has put substantial pressure on the systems ud to produce the next generation of coated products. It is time for the equipment suppliers to step up as new tools and techniques are required. With incread speeds, thinner coatings and incread functional performance, awareness of coating techniques and technologies is required. The world is changing and new tools are needed to compete in the changing world markets.
An awareness of coating techniques, technologies and systems allow for novel adaptations and application to new market opportunities. Slot die technology provides innovative tools that will help coating companies improve precision, tackle new markets, and develop new process.
INTRODUCTION
Slot die coating was developed as the need for a better and more efficient coating method was required. This process has been successful in replacing other coating methods in applying many types of solutions. A slot coating die is a device that is capable of holding a fluid’s temperature, distributing a fluid uniformly and defining a coating width. The die is comprid of steel body ctions that hou the fluid flow chamber.
Slot die coating has four main advantages:
1. Incread production speeds. In slot die coating a positive displacement pump is utilized to deliver a constant supply of coating fluid to the slot die. Modification of the coating fluid formula, such as an increa in the concentrate of solids, is allowable in a slot die. This 1
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decreas the workload of the drying units, leading to incread production rates.
2. Positive coatweight control. The fluid delivery system delivers a constant supply of coating fluid and variations in coatweight are controlled by regulating the pumping rate to line speed ratio.
3. Cross-web distribution control. The slot coating die’s manifold geometry is designed to uniformly distribute the coating fluid, bad on the fluid’s rheology. In the ca of an adjustable lip die, the lip adjustment system is ud to fine tune the cross-web distribution as the fluid exits the slot die. In a fixed lip slot die, the body shim works in combination with the manifold for distribution effects. The slot die support system is just as important as the slot die. The support system accurately and preci
ly positions the slot die relative to the back-up roll or substrate. For elevated temperature applications, dies are designed with multiple heating zones for optimum heat control. Incread heat control promotes uniform fluid viscosity contributing to uniform cross-web distribution
4. Improved waste management. All of the coating fluid is applied to the substrate via the positive displacement pump and the slot in the die exit (pre-metered coating). A slot die system is a clod system, which reduces coating fluid contamination. This is especially uful in a clean room environment.
The superior control over coating application, provided by the slot die method, is attributable to its constant application rate. An extruder or positive displacement pump feeds the coating fluid into the die at a pul-free, uniform rate, and all of the fluid that goes into the die is applied to the web. In roll coating, only a portion of the coating on the applicator roll is actually deposited on the web. The amount varies with such factors as the viscosity of the fluid, the speed of the web and the speed of the rolls.
The need to re-circulate the remaining coating fluid is a disadvantage of roll coating, attributable to partial u of the coating on the applicator roll. The contamination that can result damages product quality and leads to raw material waste.
Slot die coating systems can transcend limitations of conventional roll coating. Slot die coating emerges as an efficient, controllable alternative to roll coating and other conventional methods of applying fluids to web substrates. For a wide range of adhesives and low-viscosity coatings, today’s market and regulatory forces place a high premium on the advantages of the slot die coating technique. Coating applications where slot die methods provide new capabilities for meeting the challenges include pressure nsitive adhesives, transdermal patches, optical coatings, battery slurries and solar panel coatings.
PROCESS
The conditions most fitting for slot die coating technology when the coating is 0.00004 inch (1 micron wet) up to 0.05 inch (50 mil) in caliper; 1-100,000 cP in viscosity; up to 2000 fpm in line speed; with temperature control up 450 o F. The conditions are not mutually exclusive, as the rheology, surface energy and flow characteristics of each fluid are unique.
A slot coating die can be designed to run an individual fluid, or multiple fluids simultaneously. Other uful capabilities of a slot die include intermittent coating capability and lane coating.
SYSTEM
A slot die coating system is comprid of 5 main components:
1. Die. The slot die pre-meters the fluid onto the substrate. The fluid rheology is the fingerprint of the fluid. It is the comparison of viscosity vers shear rate at a specific temperature. Each fluid has its own rheology, which is ud to design the internal flow geometry. The die manifold is the contoured flow geometry machined into the body ctions to exacting tolerances. The body ctions may be disasmbled and split apart for cleaning. The function of a slot coating die is to maintain the fluid at the proper temperature for application, distribute it uniformly to the desired coating width, and apply it to the web. In a typical construction, the body of one type of slot die coating head (EDI UltraCoat®) is about 9 inches (230 mm) long in the machine direction and 5 inches (127 mm) high; widths up to 118 inches (3 m) are available, as oppod to a typical maximum of 85 inches (
2.1 m) for roll coating. The die is split into top and bottom ctions that are bolted together. Enclod within the ctions is a flow channel machined into one of the body ctions. The key portion of this channel is the coat hanger shaped manifold, who widest gment, the exit slot, corresponds to the coating width. The manifold distributes the coating fluid that enters the die to its full target width and is designed to generate a uniform, streamlined flow of material through the exit slot of the die.
2. Die Positioner. An adjustable carriage, that precily positions the slot coating die at the optimum angle and proximity to the roll and isolates the die from vibrations that can affect coating application, is an important support system that should be incorporated. The die positioner is important for stabilizing the interaction between die and moving web utilizing the angle of attack between die and substrate, the distance between the two and the degree of offt between the lips. The parameters can be adjusted to optimize the process.
3. Roll. A back up roll provides the precision surface for most coating techniques. The Total Indicated Run-out of the roll will provide the basis for the interaction between the substrate, the fluid and the slot coating die.
4. Fluid Delivery System. The fluid delivery system has to provide a non-pulsing, constant feed of fluid to the die. This critical piece of equipment works in combination with an accurate line speed control to determine coat weight of the fluid.
5. Substrate. This component to the coating uniformity equation is typically outside the control of the machine builder’s influence, but the influence of coating quality and caliper control is just as big.
Each of the components takes on a portion of the tolerance of the coating caliper control. The con
cern of the system designer is to minimize the influence of each component. TECHNIQUE
Slot dies are pre-metered heads that uniformly apply coatings to substrates. To provide coatings without defects, dies must be positioned to form an ideal geometry between the substrate and the die lips. This can be accomplished with numerous coating techniques:
1. Wipe/Contact (Direct & Indirect). Direct coating, in which material is applied directly to a substrate supported by a back-up roll, achieves coating thickness as low as 0.00075 inch (18 microns) and can replace roll, gravure, Meyer rod and some knife-over-roll/substrate applications. Indirect coating, in which material is applied to a precision metering roll and transferred to a substrate on a back-up roll, can achieve thickness down to 0.0005 inch (12 microns) in replacement of the same conventional methods as direct coating.
2. Draw. Draw coating, in which material is applied directly to a substrate supported by a back-
up roll from a distance of up to 0.012 inch (12 mil), achieves coating thickness as low as
0.00004 inch (1 micron) in replacement of the same conventional methods as direct coating. This coating technique is typically run at slower line speeds and is a function of the fluid’s elongational viscosity.
3. Curtain. Curtain coating allows for incread production speed.
4. Free span. Free span coating applies material directly to a substrate without the assistance of
a back-up roll. Replacing many knife-over-roll/substrate applications, this method is ud with solutions that adhere to the substrate without pressure or, in the ca of substrates that absor
b material quickly, in cas where penetration of the substrate is undesirable.
CONCLUSION
Implicit in this discussion is the importance of precision in the manufacture of slot dies. The smoothness and flatness of surfaces at or near the exit slot of the die are critical for product quality. The surfaces include the lip land just inside the exit slot and the face of the wet lip (the lip pasd by the moving web after it has pasd the exit slot). Improvements in dimensional tolerances made possible by precision machining of the surfaces may em minuscule, but in applications involving high volumes or costly coating materials, the economic gains can be great. The effect of flatness variations becomes progressively greater as coating weights become smaller.
The slot die approach provides many benefits, such as higher line speed capability and greater coat
weight uniformity. To realize the benefits, the coating system as a whole must be designed to exacting tolerances bad of the rheological characteristics of the fluid. The slot die can then achieve defect-free coatings with incread production speeds, positive coatweight control, controlled cross-web distribution and improved waste management.
