Doctor Blade Coating: Method, Coating Thickness and Design

Doctor blade coating is a common technique used to apply a thin, uniform layer of solution onto a substrate. The process involves using a blade to spread a coating of solution across a surface.

The method is widely used across various industries including to coat paper and packaging, textiles, electronics and medical devices. It is a simple, versatile and easy-to-use technique that can produce thin, consistent coatings. This is essential for high-quality finishes for a range of products.

Factors such as blade height, angle, substrate, speed, and material viscosity are carefully controlled to achieve the desired coating properties. The technique is valued for its precision, scalability, and adaptability to different materials and applications, this makes it a critical process in both research and industrial production.

Doctor Blade Coating Process

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The doctor blade coating process involves several key steps:

Solution Preparation: The coating material, liquid or slurry, must have the appropriate viscosity and consistency to ensure smooth application.

Equipment Set-Up: The substrate and blade must be properly set up. Doctor-blade coating is compatible with many different substrates including: one or multiple smaller substrates, one larger substrate, a a continuous roll (web) or individual sheets. Once the blade and substrate are properly aligned, the coating material is dispensed onto the substrate in front of the blade.

Spreading and Smoothing: As the substrate moves, the doctor blade spreads the coating material. The thickness of this layer is controlled by the gap between the blade and the substrate as well as the angle of the blade, the deposition speed and the properties of the coating material.

Drying/Curing: After the wet film has been coated, the coated substrate typically undergoes a drying or curing processes to solidify the coating. This curing can involve heat, UV light, air-blading or other curing method. The drying/curing method and the rate at which the solutions dries are both important factors that need to be optimized.

Doctor Blade Coating Design

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There are several different components needed for doctor blade coating:

• Doctor Blade

  The blade is one of the most important parts of the doctor blade coater. It is usually made from stainless steel, ceramic or plastic material. The choice of material depends on the required durability, resistance to corrosion and interaction with the coating material.

  The blade edge can be sharp, rounded, or bevelled, depending on the desired coating thickness and the properties of the material being applied. The blade height (the gap between the blade and substrate) is adjustable, along with the blade angle, and applied pressure.

• Solution Delivery System

  Some blade coaters have a reservoir to hold the coating material, helping to create a consistent supply. These are often more complex in large, automated systems, with sophisticated pumps and controls. In some cases, the material is directly applied to the substrate just before the doctor blade spreads it. In others, it may be applied via a secondary component, such as a roller. The choice of blade and other materials must be compatible with the coating material to prevent chemical reactions or wear.

• Substrate Base, Holder or Tensioning System

  The positioning and tension of the substrate must be precisely controlled to ensure uniform coating. The substrate must remain flat and aligned, preventing movement, wrinkles or deviations that could affect coating uniformity. The coater must be compatible with the type of substrate you wish to use, considering factors like the substrate's flexibility, thickness and sensitivity to pressure or heat.

• Blade Adjustment Mechanism

  Manual systems use screws to adjust the blade.Then the blade must be manually dragged across the substrate. Whereas automated systems use motors controlled by software to drive the blade across the substrate. This ensures consistent deposition speeds and leads to a more consistent thin film.

  Some systems also feature blade height control mechanisms which will control of the gap between the blade and substrate.

• Control system

  Basic systems may use manual controls for all adjustments, whereas more advanced systems feature digital control panels or computer interfaces, allowing for precise control of all parameters and automation of the process. Automated systems often use internal software to control these parameters.

Automated vs. Manual Coaters

In manual doctor blading, the user manually controls the doctor blade, and therefore the coating process. Here, the user manually adjusts the blade height, angle, and applies the coating material. Automated doctor blading is a process that is controlled by automated machinery, for example computer-controlled systems which can control the blade and substrate movement, allowing users to automate their experiments.

Manual doctor blading is often used in early research and development, and other small-scale production. It is useful where flexibility and low cost may be the priority factors.

Automated doctor blade coaters are preferred in industrial and high-volume manufacturing settings where consistency, efficiency, and precision are critical to experiments. This system is common in industries like electronics, medical devices and large-scale paper and packaging productions. Automated systems are also preferred where reproducibility and consistency are the most important factors.

Advantages of Doctor Blade Coating

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Large Coating Area

Doctor blade coating is designed to quickly and reproducibly coat large areas with thin films. Most experimental research uses small quantities of materials coated over a small area, usually deposited using techniques like spin coating. After proof of concept experiments, the next most important stage of research to explore how scalable your processes are. This is especially true if your goals are related to real-life applications, such as producing new solar cell or battery technology or producing medical equipment or devices. Doctor blade coating can be an incredible tool to help you test the scalability of your materials or processes.

High Versatility

This technique can handle a wide range of coating materials with different viscosities and chemical compositions. Doctor blade coating is used to coat thin layers of slurries, liquids and pastes.

Quick, Reliable and Reproducible

With doctor blade coating, you can coat large areas very quickly. This is advantageous for high-volume production.

The simplicity of doctor blade coating make it a very reliable process. It is a well-understood technology. This helps to ensure reliable and repeatable results

Relatively Low Wastage

Doctor blading allows you to conduct your experiments with a reduced amount of coating material compared to other coating techniques. For example, dip coating which requires a large reservoir of excess material. Also, with spin coating 90% of the material applied is cast off as waste. With doctor blade coating, any excess solution after coating can be collected and filtered for reuse.

Easy to Use and Optimize

Compared to other large area coating techniques, such as slot-die coating, there is relatively few factors that need optimizing. You can vary your thickness simply by changing the blade you are using. Also, once you find a process that works it's incredibly easy to do this again and again.

Doctor Blade Coating Disadvantages

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• Achieving uniform coatings can require precise set-up and calibration of the doctor blade. This, combined with the large coating areas, makes this more complex to optimize than say spin coating. Doctor blading technique is not as precise.
• Doctor blade coating is not suitable for coating extremely thin films. It is not possible to create films with thicknesses below 10s of microns for example.
• Coating materials with large particles or high solid content can cause issues like blade clogging or surface defects.
• Materials which have very low or very high viscosities can be challenging to apply uniformity, limiting the range of usable materials

Doctor Blade Coating Film Thickness

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Achieving accurate and consistent wet film thickness is crucial for achieving the desired properties in the final product, such as colour, conductivity, barrier properties and adhesion.

The formula describes a fundamental aspect of the doctor blade coating process: calculating wet film thickness. Wet film thickness is a critical parameter in ensuring that the coating meets the required specifications that you set for your experiments.

• Wet film thickness (h) – This is the thickness of the coating immediately after it has been applied to the substrate, before any drying or curing has occurred
• Flow rate (Q) – The volume of coating material applied per unit time, often measured in litres per minute (L/min). It represents the total amount of material being deposited in front of the doctor blade. This determines how much material is available for coating.
• Coating speed (v) – The speed at which the substrate moves relative to the doctor blade or vice versa. This speed is typically measured in metres per second (m/s).
• Substrate width (w) – The width of the substrate being coated, which is measured perpendicular to the direction of the coating movement. This width is often in metres or centimetres.

The formula essentially tells us the wet film thickness is directly proportional to the flow rate and inversely proportional to both coating speed and substrate width. In practical terms, this relationship should help in setting up the correct process parameters to achieve a specific coating thickness. Namely:

• Increasing flow rate (Q) will increase the wet film thickness, assuming other variables remain constant.
• Increasing coating speed (v) or substrate width (w) will decrease the wet film thickness.

You may need to adjust the flow rate, speed, or blade gap in a doctor blade coater to achieve a desired wet film thickness. Viscosity and surface tensions are also important properties to consider in doctor blade coating, because these properties of the coating material influence how it spreads and levels out once applied. Higher viscosity can lead to thicker films, however this might require more force to spread, while surface tension affects the smoothness of the film.

Blade configuration is also important; the angle of the blade, the force applied, and the blade’s flexibility can all influence the coating outcome. Adjusting these parameters can help control the thickness, uniformity, and smoothness of the coating.

Doctor Blade Coating Applications

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Doctor blade coating is a versatile technique widely used across both industrial and research settings. It allows for control over coating thickness, uniformity, and material deposition over large areas making it applicable to a range of materials and products.

Industrial Applications

Some of the common applications of doctor blade coating include in the printing and packaging industries. It is commonly used in the printing industry to apply uniform layers of ink, varnishes and protective coatings on paper, plastics and other substrates. This ensures consistent colour quality and durability in printed materials.

It can also be used to apply adhesives and coatings in laminating processes, which are critical in packaging to provide protective barriers and enhance appearance.

The doctor blading technique is also used to deposit conductive and insulating layers in the manufacturing of LCDs, OLEDs and touchscreens. Thin films of conductive materials are important for the functionality of these devices.

Industrial applications require strong quality control to ensure that the coating achieved meets the standards set, therefore monitoring film thickness, uniformity and surface quality is needed – often by using advanced inspection systems.

Research – Scaling of laboratory techniques

In laboratories, doctor blade coating is widely used to study thin film materials, these include polymers, composites and nanomaterials. Researchers use such techniques to explore new material properties, structures and applications.

A specific analytical technique where doctor blading is useful is in microscopy, where preparing uniform thin films is crucial.

It is important to consider that in laboratory methods, achieving the same level of uniformity and consistency may require advanced machinery and process control systems. Scaling up also means that users will have larger quantities of coating materials to manage, this could require specialised feeding systems and environmental controls. The purpose of this is to prevent contamination whilst ensuring a consistent quality.

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