- Slot-die coating
Slot-die coating is a reliable and versatile method of applying layers sized from 10 nm to 2 microns onto substrates, over 8x thinner than the diameter of a human hair. Relative to other techniques, slot-die coating offers greater consistency and precision at these tiny thicknesses and therefore is common to industries such as battery manufacturing, OLED manufacturing, barrier films, transdermal and oral pharmaceuticals, and solar panel construction.
Whereas traditional coating techniques such as reverse roll, knife over roll, and comma coating use a blade or wiping system to remove unused coating materials, slot-die coating doesn’t waste any coating liquids due to its complete application onto substrates. Coating liquids move through slot-die systems at precisely controlled flow rates; the final coating liquid distribution is determined by specifications of slot-die heads using a series of formulas, including the Poiseuille equation, providing consistent end-process coating thicknesses. Slot-die manufacturing achieves higher levels of accuracy and consistency than do other traditional coating methods for layers sized from 10 nm to 2 microns. However, as a trade-off for its higher levels of precision, slot-die coating also requires a higher level of process control.
Originally developed in the 1950s in order to apply photographs to paper at industrial scale, slot-die coating technology has now evolved to include an expansive range of coating fluids and flat substrates such as glass, metal, paper, fabric, and plastic foils. Typically, the main components of a slot-die machine include the following:
- Fluid reservoir for storing the main supply of coating fluid
- Fluid pump for moving materials through the system
- Flow meter or controller for ensuring accuracy of coating fluid flow rates
- Slot-die system to distribute the coating fluid to a desired coating width before applying onto the substrate
- Substrate mounting system to support the substrate in a controlled manner as it moves through the system
- Coating motion system to drive the relative speed of the slot-die and substrate in a controlled manner during coating
Differences in slot-die head design impact the final distribution of solution across the width of coating, the actual coating width of the film, and the overall stability of the coating process. Components of the slot-die head include the inlet, manifold, land, slot, shim, and lip.
Each of these has its own unique designs modified for different uses. For example, constant shear manifolds are specifically used when a constant flow rate across the width of the slot-die head (independent of the solution viscosity and flow rates) is desired whereas the T-shaped manifold is the simplest design and is used when an uneven solution distribution over the width of the head is acceptable.
Many variations in slot-die systems have been developed for specific applications. Differences in solution metering methodologies arise due to the individual characteristics of coating fluids, including:
- Differences in viscosities of liquids
- Volumes of solutions required
- Desired levels of accuracy of flow rates
- Cost constraints
- Other fluid attributes of specific solutions
Generally, at lower volumes and flows, peristaltic pumps are more commonly used for slot-die systems whereas at higher volumes and flows, rotary pumps are used more often.
Flow meters and controllers measure the consistency of coating fluid movements from fluid reservoirs to slot-die heads for distribution onto substrates and are critical components for ensuring that systems are working properly. If actual measurements relating to flow and pressure drop do not match expected measurements, then such attributes such as the thickness of layers can be inaccurate. Slot-die coating systems use a closed flow in order to help mitigate deviations in flow rates. Slot-die coating systems must also be checked and kept clean of any contaminates, such as dust or hair, to ensure greater long-term repeatability and accuracy.
Types of defects
While generally slot-die coating results in more precise layering than other techniques, defects can occur either because of instabilities in the coating bead meniscus that happen where the coating process exits the stable coating window or because of external factors related to the delivery of fluid, movement of substrate, and viscoelastic properties of solutions.
- Chatter defects occur across the entire width of coating and can be seen either at the same point in the coating or at regular intervals that happens when the fluid delivery system has variations in either pressure or flow rates, roller issues, or variations in the pressure of the upstream vacuum box.
- Ribbing defects occur when errors arise along the length of the coating in regular intervals and is caused when the upstream meniscus recedes towards the slot-die exits due to high shear forces, low pressures at slot-die exits, reduced upstream pressures, or localized defects to heads or slot-die feed vortices.
- Neck-in defects occur at the edges of lengths of coating, creating a thickening of edges and decrease in coating width along margins of the length of a substrate and occurs due to transitions from constrained flow between the lips and substrate towards a plug flow that results in changing fluid dynamics, shear forces that contradict coating beads towards the center that lead to higher flow rates at edges, or a mismatch between flow rates and substrate speeds causing acceleration of the fluid and shear forces.
- Edge defects can occur to all edges but are not seen as an issue when under constant operation in a roll-to-roll system.
- Streaking and bubbles are final types of defects that occurs either due to dust or particle obstructions, blocking of slot-die heads or lips, air entrapment, and damage to slot-die heads.
In summary, slot-die coating offers greater accuracy than other methods when adding coating layers between 10 nm to 2 microns onto substrates but is also more sensitive to any miscalculations that result in inaccurate layer thicknesses or system issues that result in different types of defects. Slot-die coating offers advantages for industries where precision is most important yet also requires routine system maintenance and that all variables in the process are calculated correctly and properly understood.