Discover slot-die coating

Slot-die coating is a highly scalable technique for rapidly depositing thin and uniform films with minimal material waste and low operational cost. slot-die coating technology is used to deposit a variety of liquid chemistries onto substrates of various materials such as glass, metal, and polymers. This is achieved by precisely metering the process fluid; dispensing it at a controlled rate while the coating die is moved relative to the substrate.

Industrial origins

Slot-die coating has historically been used in industrial-scale manufacturing due to its reliability, high throughput, and extremely low waste production. The patent for slot-die Coating first principles was filed by Eastman Kodak Co. in 1951 for the production of photographic films. Today, some of the world’s most important commercial products are made using slot-die coating techniques. This includes LCD displays, chemical release patches, touchscreens, food packaging, Li-ion batteries, and multi-layer ceramic capacitors among many other applications.

Precision hardware

Slot-die systems achieve their coatings by delivering the desired coating fluid (often referred to as “ink”) onto the substrate through a highly precise delivery component known as the “slot-die head.” The slot-die head lies at the core of any slot-die coating system. Its design and position relative to the substrate significantly influence the quality and stability of the resulting thin film.

A fully assembled slot-die head consists of several key components. Namely, the front plate, back plate, shim, and meniscus guide. The front and back plates are the exterior metal blocks that receive the ink from the delivery system and secure all of the slot-die head components in place. The shim and meniscus guide are two thin sheets of metal between the front and back plate that serve to guide the ink to the coating edge of the slot-die head.

FOM medium slot die head

Outstanding thickness control

The interior design of a slot-die head can vary. Typically, the ink is pumped into the system via tubing connected to an inlet on either the front or top of the slot-die head. Upon entering the slot-die head, the ink first fills into a hollow cavity that has been machined into one of the Slot Die plates. This cavity is often referred to as the ink reservoir, manifold, or distribution chamber. The ink is then guided downwards from this reservoir to the coating edge of the slot-die head by the shim, whose cutout shape determines the coating width or stripe pattern of the ink on the substrate. A meniscus guide may sometimes be used to stabilize the fluid meniscus between the slot-die head and the moving substrate. The meniscus guide typically appears as a metal tooth protruding from the bottom lips of the slot-die head, facilitating the flow of the ink out of the slot-die head and onto the substrate.

Inner workings of a slot die coating

A key benefit of slot-die Coating is its “pre-metered” nature. This means that the coated wet film thickness (t) can be easily controlled and calculated from the pump rate (Q; the rate at which ink enters the system), the substrate motion speed (U; the rate at which the substrate accepts ink from the system), and the coating width (W; the width across which the ink is dispersed upon the substrate. Not shown in the schematic).

t = Q / (W * U)

Unlike in “self-metered” coating techniques, the height of the Slot Die head does not physically control the thickness of the coated layer. Instead, the wet layer thickness in Slot Die Coating is entirely determined by the volume of ink applied to the substrate, divided by the area over which the ink is coated. This affords a highly flexible, “contact-free” coating process, with additional benefits such as closed fluid delivery and wide viscosity compatibility (up to tens of thousands of cP). These properties combine to provide significant process control and scaling advantages over common R&D coating techniques such as blade coating, bar coating, and spin coating.

High quality thin films

Coating thickness is often the primary parameter of interest for users of slot-die coating equipment. However, control over the quality of this coating must also be carefully considered. The quality of the coating is generally discussed in the context of the “stable coating window” of a process. This stable coating window is defined by all of the possible combinations of key process parameters (e.g. pump rate, coating speed, gap-to-thickness ratio, etc.) that support a stable coating meniscus, thereby avoiding defects in the final coated layer (e.g. ribbing, dripping, streaks, etc.).

Slot die coating technology, model of slot-die coating process

In practical terms, achieving a defect-free film of the desired thickness is often a matter of simply balancing the ink pump rate, substrate motion speed, and slot-die height against each other. However, the boundaries of the stable coating window can also be influenced by the position of the slot-die relative to the substrate, as well as the nature of the inks and substrates being used. Viscosity, surface tension, and surface energy are just a few of the material properties that can be controlled and optimized together with high-quality slot-die hardware and motion systems to achieve ultra-thin films with outstanding uniformity. By controlling these parameters, Slot Die Coating provides precise, repeatable, and scalable thin films with thicknesses ranging from tens of nanometers to hundreds of microns.

The forefront of research

Slot-die coating has seen a great increase in lab-scale R&D applications in recent years, as researchers strive for more consistent experimental results and rapid scaling of new additive manufacturing processes to roll-to-roll and sheet-to-sheet production. As a result, FOM Technologies coating equipment is now supporting cutting-edge research into 3rd generation solar cells, OLED and electrochromic devices, fuel cells, batteries, sensors, membranes, printed electronics, medical diagnostics, conductive thin films, and more.

Conductive foils
Printed electronics
Fuel cells
Thin film solar cells
Tandem solar cells
Water purification

Enabling functional materials R&D

At FOM Technologies we aim to bring industry-grade coating solutions to the R&D environment. We pride ourselves on providing high-quality hardware, user-friendly process control, and a researcher-focused design mentality across all of our products. Whether you are conducting fundamental research, establishing a pilot process, or moving into full-scale production, it is our mission to support you with a premium, precise and scalable Slot Die Coating platform to elevate your thin film research and production projects.

Slot die coating technology, model of slot-die coating process

Download the slot-die coating Technology brochure

Get to know slot-die coating. In this brochure you will get an introduction to slot-die coating, learn about film thickness control, film quality control, and find some of the R&D benefits of slot-die coating.

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FOM Technologies - Technology brochure

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