Printing, coating, metering & the slot-die process

Printing, coating, metering & the slot-die process

The landscape of modern printing and coating technologies

A wide variety of printing and coating technologies exists today, with applications ranging from domestic inkjet printers to industrial roll-to-roll converting lines, to semi-continuous vapor deposition processes and beyond. In all these instances, the intended outcome is controlled application of one or more specific materials onto a desired support or substrate. But beyond this shared definition, what sets these technologies apart from each other, and how should one go about choosing which technology to use for a given development or production project?

All these technologies offer their own unique benefits and drawbacks. Consequently, determining which printing or coating technology to use depends significantly on the cost, functionality, scale, and performance requirements of the specific process in question.

In the interest of providing new operators and researchers with an introduction to the landscape of modern printing and coating technologies, this article provides a qualitative description and categorization of these technologies for lab- and pilot-scale applications. Industrial assessment based on large-scale process modeling and ROI estimation may well be the topic of a future post.

“Printing” versus “coating”

The difference between “printing” and “coating” processes is deceptively simple, but nevertheless provides a meaningful perspective in the equipment selection process.
Slot-die coating

An example of a printing process (left) and a coating process (right), where printing produces complex shapes and coating produces large-area coverage of the substrate.

A coating process typically aims to apply a uniform layer of material over a significant portion of the substrate, where total coverage of the substrate surface is often the desired outcome. Coating equipment is therefore designed to achieve this large-area coverage with optimal speed, cost, and control over the layer thickness and uniformity.

A printing process typically aims to apply material only to selected portions of the substrate, where fluid deposition in dynamic shapes, patterns, and fine features is often the desired outcome. Through consecutive buildup of several such printing steps, complex images and patterns consisting of many different shapes and materials can be produced on the same substrate. The resolution of these features can reach down to the microscale, and printing equipment is therefore primarily designed to achieve this pattern printing functionality as efficiently as possible, rather than emphasizing uniform large-area coverage.

While printing equipment can technically be configured to achieve “coating” processes (i.e., by setting the printer to “print” a large-area layer of material as its target “pattern”), some aspect of efficiency is often lost in terms of speed, cost, or flexibility when using printing hardware to achieve a coating process rather than dedicated coating equipment.

Some examples of challenges encountered when using printing equipment for lab- and pilot-scale coating processes have been described below:

Printing technology
Inkjet printing
Gravure / flexo
Screen printing
Coating challenge
• Typically slow line speed and/or high cost to achieve large area coverage
• Limited low range for fluid viscosity and thickness per pass
• Limited adjustability (new roller often required for process adjustment)
• Best performance at high line speeds (not always feasible)
• Limited adjustability (new screen often required for process adjustment)
• Predicting and controlling process adjustments can be a challenge
• Typically limited to dry thickness of ca. 5 microns or more, coming from viscous slurry or paste-like inks (not ideal for low viscosity / very thin films)
A summary of common challenges experienced when using printing equipment for coating process development.
While printing and coating are complementary technologies that are often employed together as separate steps of an overall process, at FOM Technologies we specialize primarily in coating processes. The rest of this article will therefore focus on providing a deeper understanding of these technologies in the context of coating applications.

“Pre-metered” and “self-metered” coating

Controlling the quantity of fluid that is applied to the substrate during coating (often referred to as the “coating weight”) is a critical step in establishing a successful coating process with suitable thickness and quality. The process of controlling the rate of fluid application during coating is called metering, and coating technologies are frequently categorized as either “self-metered” or “pre-metered,” depending on how fluid metering is achieved in the process.
Slot-die coating

Examples of self-metered (left) and pre-metered (right) coating processes. The self-metered process controls coating weight at the point of application, while the pre-metered process controls coating weight via an upstream metering device.

In a self-metered process, metering of the final coating weight of the fluid on the substrate is determined by fluid flows between the substrate and coating applicator (knife, roll, comma bar, dip bath, etc.), induced by the applicator at or following the point of fluid application.

In a pre-metered process, metering of the final coating weight is determined by a precisely controlled rate of volumetric fluid application per unit area of substrate per unit time. This rate of fluid application is controlled by a dedicated metering device upstream in the process (i.e., a continuously running pump or finely calibrated anilox roller), typically with high material transfer efficiency (i.e., most or all the pumped fluid is applied directly to the substrate). The metering device is therefore primarily responsible for controlling the coating weight, rather than the configuration of the coating applicator itself (slot-die, extrusion die, slide, spray nozzle, etc.), and coating weight is pre-determined before the fluid reaches the substrate.

Note that here the term “coating weight” is analogous to “average wet film thickness.” Actual thickness may display localized uniformity deviations, though the average coating weight/wet thickness does not take these deviations into account.

Several common coating and printing technologies are categorized below according to their metering mechanism. It should be noted that this list is intended as an introductory overview, and is by no means exhaustive of all technologies in the modern coating and printing industry:

Coating
Printing
Pre-metered
Slot-die
Extrusion die
Curtain coating
Slide
Spray
Rotary rod
Inkjet
Flexo / gravure
Self-metered
Direct / reverse roll
Knife coating / tape casting
High speed doctor blade
Comma bar
Wound Mayer rod
Spin
Screen
Common printing and coating methods categorized via their coating weight metering mechanism.

So, as a basic summary: self-metered coating methods physically “press,” “scrape,” or “submerge” the coating fluid onto the substrate, with final coating weight depending on the complex flows of fluid on the substrate induced at the point of application. Process speed, fluid rheology, and applicator configuration (i.e., shape and distance from the substrate) can all influence the resulting coating weight. As a result, these processes are often relatively simple to begin using, but less flexible with respect to control, adjustment, and predictability compared to pre-metered methods.

Conversely, pre-metered coating methods precisely pre-measure the volume of fluid required to achieve a desired coating weight, upstream from the point of application, and then transfer the fluid to the substrate with high material efficiency. This precise pre-metering mechanism gives rise to a simple relationship between layer thickness (t), fluid volume (V), and coating area (A), where:

t = V / A

In a steady-state slot-die process, this relationship can be further expressed as:

t = Q / (U x W)

Where Q is the volumetric pump rate, U is the coating speed, and W is the coating width. Through this simple relationship, slot-die processes provide direct layer thickness control and predictability via simple pump adjustments. While in some instances, nonhomogeneous or viscoelastic fluids can make accurate metering difficult to achieve, this fundamental algebraic relationship holds true.

Pre-metered processes, such as slot-die coating, therefore provide a significant benefit in terms of precise coating weight control, adjustability, and predictability, by reducing the influence of process speed, applicator configuration, and material rheology on coating weight (though they still play a role in determining uniformity!).

The benefits of slot-die coating for scalable process development

Slot-die coating is a prime example of a pre-metered coating technique with a high degree of predictability, flexibility, and control. While it falls firmly into the category of “coating” technologies (i.e., without the potential for printing fine shapes or patterns), it is arguably one of the most versatile dedicated coating technologies available today.
PROCESS PARAMETER
COMPATIBLE VISCOSITY (Pa*s)
TYPICAL WET THICKNESS (μm)
ACHIEVABLE ACCURACY (%)
MAX. LINE SPEED (m/min)
SLOT-DIE CAPABILITY
0.001-20
10-250
2
400
Commonly cited operating limits for optimized industrial slot-die coating processes. Note that these are guiding values to communicate ideal industrial processing potential. They should not be considered as product specs for FOM Technologies lab-scale development tools.
With wide compatibility towards varied material viscosities and wet thicknesses, slot-die technology is suitable for use in a diverse range of research and production applications. When considered in the context of other pre-metered coating methods, the on-the-fly, pump-based coating weight adjustments make the slot-die process more flexible than coating with flexo or gravure, while the steady-state film deposition mechanism can provide superior uniformity compared to a spray coating process with a wider selection of materials.

These factors, combined with the rapid potential for scaling from small-area tests to large-area pilot production and beyond, make slot-die coating a powerful process for development and scaling of novel functional material coatings and devices, as well as a platform for rapid iteration between R&D and manufacturing for existing production processes.

We look forward to discussing how we can bring the benefits of predictable, flexible, and controllable pre-metered slot-die coating technology to your lab- and pilot-scale production activities soon!

For more information on these and related topics, we recommend the excellent works of Cohen and Gutoff.

How to choose your slot-die coater

Our customers often ask which slot die coat machine and options to choose for their applications. Although it is a trivial question, there are some essential considerations to have before answering. This post will provide some valuable hints to select the right equipment and options for your needs.

Let’s talk about scaling your research

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