Why Does Forming Fabric Tension Decrease After Approach Flow?

Basic Principles and Objectives of Forming Fabric Tension

The forming fabric tension setting is not a fixed value, but rather a comprehensive balance based on three main principles: mechanical stability, process optimization, and cost control. Its core objectives are:

• Ensuring the forming fabric is flat and stable: Sufficient tension ensures the fabric remains absolutely flat in both the working and return sections, preventing wrinkles, deviations, or localized sagging. This is the fundamental basis of its function.
• Providing a stable foundation for dewatering elements: Stable tension at dewatering elements such as the breast roll, scraper, and vacuum boxes ensures close and uniform contact between the forming fabric and the elements, resulting in high and consistent dewatering efficiency.
• Controlling forming fabric vibration: At high paper machine speeds, the forming fabric is prone to lateral and longitudinal movement, affecting the uniformity of the paper sheet. High tension increases the natural frequency of the fabric, suppressing undesirable vibrations.
• Influencing fiber orientation and retention: Tension indirectly affects the orientation of fibers on the fabric surface and the retention rate of fine fibers and fillers by influencing the fabric’s minute deflections and vibrations.

Forming Fabric Basis and Considerations

• Paper machine speed: This is the most critical factor. The higher the speed, the more centrifugal force, inertia, and dewatering impact there will be, and the required tension will increase in a square relationship. For example, a low-speed paper machine might require 5 kN/m, while a high-speed paper machine often requires 10 kN/m or even higher.
• Forming fabric type and structure: Based on material, it can be divided into polyester monofilament mesh, multi-layer composite mesh, etc., with different tensile strength, elastic modulus, and fatigue performance. Forming fabrics with thicker yarns require higher tension to achieve the same flatness. When producing thin paper or specialty paper, higher demands are placed on forming uniformity, requiring more stable tension. When producing high-basis weight cardboard, although the requirements are slightly lower, the dewatering load is large, and sufficient tension is still needed to maintain contact between the forming fabric and the dewatering elements. If the wire section is equipped with a large number of vacuum boxes, vacuum tables, and other high-negative pressure dewatering elements, higher tension is required to resist the deformation caused by the negative pressure.
• Forming fabric tension trade-offs: High tension makes the fabric flatter, the operation more stable, and dewatering more uniform, suitable for high-speed, high-quality paper grades. However, it accelerates forming fabric wear (wear at contact points with dewatering elements and rollers, shortening the forming fabric life) and places high load requirements on guide rollers and tension roller bearings, increasing the risk of fabric breakage. Low tension can extend the service life of the forming fabric and reduce energy consumption, but it sacrifices operational stability and paper tension.
  While ensuring paper quality (uniformity, basis weight distribution) and stable operation (no wrinkles, no deviation), the lowest possible tension should be used to balance quality, lifespan, and cost.

Issues to Pay Attention to Regarding the Forming Fabric During Operation:

• Inspection and maintenance of the forming fabric: Regularly check for wear, focusing on the wear of the vacuum box panel, scraper contact area, and drive roll contact points. Abnormal wear may be caused by excessive tension, uneven dewatering elements, or doctor blade problems. Keep the forming fabric clean to prevent fillers, adhesives, and resins from clogging the mesh and altering local dewatering characteristics, which may also lead to distorted tension sensing due to uneven friction. Pay attention to the strength and flatness of the forming fabric joint; the joint is weaker at high speeds.
• Tension control and monitoring: Use an automatic tension control system to ensure minimal tension fluctuations during transient processes such as startup, acceleration, deceleration, and paper breaks. Manual control cannot meet the requirements. Check whether the tension is uniform across the entire width; this can be judged by observing the parallelism of the guide rolls, whether there is unilateral wear, or whether the forming fabric is running off-center. Uneven tension is a major cause of uneven basis weight and moisture content across the paper web.
• Coordination of related systems: The alignment and tensioning systems must ensure that the alignment and tensioning devices operate sensitively and reliably. Regularly check the cylinders, valves, and sensors. Check the condition of the dewatering elements; all table rolls, scraper, and vacuum box panels must be installed flat, without sharp edges or damage, to prevent severe local wear or scratches on the forming fabric under high tension. Check the wear of the surface coating of the guide rolls and drive rolls, ensuring good roundness and dynamic and static balance to prevent the rolls from causing periodic fluctuations.

The tension of the wire mesh is a dynamic and optimized process, not a fixed one. During paper machine operation, the principles of stability, uniformity, detectability, and system compatibility must be adhered to. Through meticulous operation and maintenance, while ensuring high-quality production, the service life of the forming fabric is maximized, achieving maximum efficiency.