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.

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 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:

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:

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!).

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.