Doctor Blades for the Paper Industry

Motivation

In the paper industry, doctor blades (English: Doctor Blades) are indispensable components for cleaning rolls and cylinders. These blades must be extremely resistant to abrasion and friction to work effectively under the demanding environmental conditions of paper production. Traditionally, metal doctor blades have been used, but the use of doctor blades made from composite materials has proven to be advantageous. The reasons for this include a combination of high flexural strength, low weight, and optimal adaptability to the surfaces of paper machines offered by laminated press materials. Explanations for this and many more details are given in the following sections of this article.

Manufacturing

Composite doctor blades are made from laminated press materials, a specific class of composite material. Their production begins with the selection of a suitable matrix material, typically a thermoset resin, which is reinforced by fiber fabrics, usually glass or carbon fibers. The manufacturing process involves making a prepreg, layering these prepregs, and pressing them on a semi-continuous press. This process can produce strips up to 150 meters in length. The desired strip width and length can then be cut to size. There are also hybrid constructions made from glass and carbon fiber fabrics. These hybrid constructions combine cost-effective glass fabric with the outstanding properties of carbon fiber, achieving good performance at an acceptable price. To optimally adapt the doctor blades to the operating conditions, an edge along the blade is often beveled. This can improve the cleaning pattern, as well as the friction and thus the abrasion behavior.

Properties

Doctor blades made from composite materials are characterized by high flexural strength and an excellent modulus of elasticity, allowing them to remain elastic under high load and not deform permanently. The fiber content and the choice of fiber material directly influence the mechanical properties of the doctor blade. The higher the fiber content, the higher properties such as flexural strength and the modulus of elasticity. Glass fiber fabrics are cheaper than carbon fiber fabrics, but the latter offer outstanding mechanical properties. Typically, epoxy resin is used as the resin matrix because it also exhibits very good mechanical properties. The density of the material is lower than that of metal doctor blades, which facilitates handling. Furthermore, these blades have low water absorption, making them less susceptible to swelling or deformation at high humidity levels. They can also be made resistant to various environmental conditions, such as high temperature and humidity values that necessarily occur in paper production, by choosing the right resin material. The doctor blades must withstand high relative speeds and intense friction, where the composite material has the advantage of causing less wear on the rolls and cylinders than metal.

Application

In practice, doctor blades made from laminated press materials are used in paper machines to remove dirt and deposits from the surfaces of the rolls and cylinders. This is a very important task, as a failure of the cleaning can result in the paper web tearing, requiring the machine to be stopped and restarted, which leads to production downtime of many minutes and corresponding economic losses.

Doctor blades are used at high speeds and must work precisely without damaging the paper or machine components. Thanks to their elastic properties, the blades flexibly adapt to the surface structure of the parts to be cleaned, enabling efficient and gentle cleaning. Furthermore, they have a lower coefficient of friction than comparable metal blades, resulting in less energy loss due to friction and making the manufacturing process overall more efficient.

Alternatives

Metal doctor blades continue to be used as alternatives to laminated press material blades. They offer higher hardness and are still indispensable in applications where extreme abrasion resistance is required. However, metal doctor blades also have disadvantages, such as a higher density, which makes handling more complex and expensive. In addition, metal doctor blades, due to their high stiffness and higher coefficient of friction, cause increased wear on machine components. Therefore, wherever possible, metal doctor blades should be replaced with blades made from composite materials to operate the paper machine as efficiently and cost-effectively as possible.

Conclusion

Doctor blades made from laminated press materials offer an innovative and efficient solution to the challenges of the paper industry. With advantages such as high flexural strength, low density, low water absorption, and optimal adaptability to various environmental conditions, they represent a valuable alternative to traditional metal doctor blades. By strategically using composite materials, paper manufacturers can extend the lifespan of their machine components while ensuring high cleaning performance. However, it remains important to consider the specific requirements of each application to make the best material selection for the composite product. The properties of the doctor blades can be varied widely by using different resin materials and fiber reinforcements and adapted depending on the application.

Our Selection of Doctor Blades