Key Things to Know About Coating Blade
The coating blade involves using a doctor blade to remove excess coating solution from the paper surface in order to control the coating amount and meet specified requirements.
In contrast to non contact coating using a pneumatic blade, this is a direct contact coating method that produces a coating with high flatness. So the coating surface is not affected by the original surface roughness of the substrate.
The operating principle of the coating blade is as follows: After the substrate is fed via feed rolls or a fountain feed system, the doctor blade meters and levels the coating solution, removing excess coating from the substrate surface.
Because the coating blade is non conforming, the coating solution can fill in depressions on the substrate's surface, resulting in a relatively smooth and uniform coating.
Therefore, this coating method is particularly suitable for high-viscosity, high-solids coatings and high-speed coating applications.
Coating blade is a traditional method. To overcome the drawback of limited coating doctor blade lifespan, an increasing number of new materials have been applied to coating blades, and in particular, many new technologies have been adopted for coating the tips of the blades.
Materials such as ceramic alloys and zirconium are now widely used for coating blades.
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How to Make Better Use of Coating Doctor Blade
Generally, a thicker doctor blade is used for the base coat.
A base coating blade primarily controls base application volume and finishes the coating surface. The base coating doctor blade helps ensure an even coating and provides a good foundation for the subsequent topcoat.
In particular, since liquid packaging paper has a high long fiber content, its surface is rough and uneven, resulting in significant friction with the coating blade during operation. Therefore, operators typically select a coating blade with a thickness of 0.457 mm.
Thicker coating blades are stronger, deform less under applied pressure, and make it easier to control the coating amount.
Therefore, generally speaking, the higher the paper machine speed, the thicker the coating blade.
In a coating system, the base coating doctor blade is one of the key components. It has a significant impact on coating performance and paper surface quality, and therefore, high standards are set for its selection and use. Common blade angles for the base coating doctor blades are 25°, 30°, and 35°.
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The Use Of Top Coating Blade
The blade angle of the top doctor blade is generally around 4°, although 0° or 2° may also be used in some applications.
The top coating blade helps control the amount of coating applied, improve coating uniformity, and enhance the surface quality of the paper.
Because the top coating directly affects the surface properties of the paper, the angle, pressure, and stiffness of the coating doctor blade, as well as the condition of its cutting edge, all have a significant impact on the final coating result.
Common Problems with Coating Blades and Their Causes
Why Does a Grainy Texture Appear
During the application of the top and base coating blade, a grainy texture may sometimes occur. In comparison, top coating doctor blades are typically finer in texture, so this issue arises less frequently, whereas primers are more prone to developing a grainy texture due to the nature of their raw materials.
If the particles flow along with the coating, the position of the particle streaks may also change. Therefore, when the position of the streaks is found to be constantly changing, the coating dispersion and filtration systems should be checked first, rather than simply attributing the issue to a problem with the coating blade.
Why Do Both Ends of the Coating Blade Wear Out Easily
The relatively short service life of the coating blade may be related to the presence of paper edges or protrusions at both ends of the paper during operation.
These protrusions increase friction at both ends of the coating blade, making the blade more prone to wear at both ends.
