Do You Know How Did Coating Blade Working?

In the coating process, the doctor blade coating method involves transporting the coating liquid to the front of the narrow gap between the doctor blade and the substrate. By utilizing the directional, uniform, or accelerated movement of the coating blade, the coating liquid is propelled and dragged to spread evenly on the substrate. After drying and curing, a desired thin film is ultimately formed on the surface of the substrate. As the core component of the coating equipment, the performance of the coating blade directly affects the uniformity, quality, and production efficiency of the coating, making it crucial for achieving high-quality coating output.

Coating Blade Performance and Control Factors

Selection of Coating Blade Materials

The coating blade material directly contacts and removes excess coating liquid, thus it needs to possess good corrosion resistance, heat resistance, and chemical resistance. Coating blades are divided into hard blades and soft blades based on their materials. Hard blades have a certain pre-grinding angle (such as 35 degrees), and when used, the pre-grinding angle of the coating blade is usually the same as or 3 to 5 degrees greater than the angle between the blade and the backing roll. Soft blades usually do not have a pre-grinding angle or have a very small angle (such as 4 degrees), and when used, hard supports and small angles are typically employed. Hard blades are easier to control the coating amount and more convenient to operate than soft blades; although soft blades are not easy to control, they can achieve a larger coating amount and better coating effect.

Common coating blade materials include metals (such as stainless steel and steel alloys), rubber, polyurethane, and ceramics.

Coating Blade Edge Shape

Common coating blade shapes for knife edges include beveled, slightly pointed, internally beveled, and flat blade edges. Beveled blade edges are the most widely used for coating; slightly pointed blade edges can precisely control the flow and distribution of coating liquid, making them more suitable for high-precision coating; internally beveled blade edges are designed to help reduce bubbles and scratches during the coating process; and flat blade edges are suitable for applications requiring large-area, high-efficiency coating.

The Impact of the Coating Blade Angle

The angle of the doctor blade has a significant impact on the flow direction and distribution state of the coating liquid, and needs to be adjusted comprehensively based on factors such as the viscosity, fluidity, solid content, substrate properties, coating speed, and coating thickness of the coating liquid.

The range of values for the doctor blade angle can be between 15 and 90 degrees. In the use of a doctor blade coater, the angle between the tangent of the contact point between the doctor blade and the anilox roller is usually recommended to be between 15 and 30 degrees, which can reduce the vibration or runout issues caused by an excessively large angle.

Due to the force exerted by the motion unit, the blade will undergo deformation, altering the actual blade angle during the scraping process. Even if the initial blade angle is the same, the actual blade angle may vary due to the displacement of the motion unit. Therefore, it is necessary to measure the angle using photogrammetry or other methods.

Selection of Coating Blade Pressure

Appropriate coating blade pressure ensures close contact between the blade and the coating surface, effectively removing excess coating liquid and achieving uniformity in the coating. Insufficient or excessive pressure can lead to coating quality issues. In practical operations, the doctor blade pressure is generally set at around 200~400kPa, which is usually applicable to various coating scenarios. Actual application may require adjustments based on specific circumstances.

Coating Blade Thickness

The thickness of the coating blade body affects its performance and service life. For high-precision coating applications, ultra-thin doctor blades may be required, typically ranging from 0.1 to 1 millimeter in thickness. In most coating applications, the thickness typically ranges from 1 to 6 millimeters. When dealing with high-viscosity coating liquids or requiring coating blades with higher strength and stability, doctor blades with a thickness exceeding 6 millimeters may be chosen.

Coating Blade Sharpness

The sharpness of coating blade depends on multiple factors such as material, edge angle, edge shape, manufacturing process, as well as usage and maintenance methods. A sharp blade can more effectively remove excess coating liquid, ensuring the uniformity and quality of the coating. However, there is currently no unified standard for measurement, and it is usually judged subjectively: if the coating blade can easily and smoothly remove excess coating liquid, and the coating surface is smooth and uniform, it indicates a high level of sharpness. Conversely, if the scraper experiences stalling, scratches, or uneven coating during the scraping process, it indicates insufficient sharpness of the coating blade.

Coating Blade Flatness

The flatness of coating blade refers to the degree to which its cutting edge or working surface remains smooth, without undulations or indentations, when it is in operation. An uneven blade can lead to inconsistent coating thickness.

Coating Blade Running Speed

The speed of the coating blade affects the residence time of the coating liquid on the substrate and the thickness of the coating. A faster speed can improve production efficiency, but excessive speed may lead to uneven coating or the formation of bubbles.