In the Kraft paper bag automated production line, tension control is a key component in minimizing bag deformation and improving product quality. Kraft paper fibers are highly absorbent and are susceptible to humidity, temperature, and mechanical stress during processing, leading to paper expansion and wrinkling. Improper tension control can also lead to problems such as paper breakage, printing offset, and uneven sealing, directly impacting yield and production efficiency. Therefore, precise tension control strategies are essential to ensure constant tension throughout the unwinding, printing, laminating, and heat-sealing processes.
Tension control in the Kraft paper bag production line relies on a closed-loop feedback system. This system, comprised of a tension sensor, controller, and actuator, monitors the actual paper tension in real time, compares it with a preset target value, and dynamically adjusts the actuator output. For example, during the unwinding process, if the sensor detects excessive tension, the controller reduces the magnetic powder brake's excitation current, reducing braking torque; otherwise, it increases the current to enhance braking effectiveness. This closed-loop control system rapidly responds to tension fluctuations, avoids sudden tension changes caused by inertia or speed variations, and ensures stable paper movement at high speeds.
Coordinated control of multi-stage tension is key to minimizing deformation. Kraft paper bag production involves multiple processes, including unwinding, printing, laminating, and heat-sealing. Tension in each stage must be independently controlled and coordinated. For example, the tension in the printing stage needs to be slightly higher than that in the unwinding stage to offset paper shrinkage caused by ink absorption. The tension in the laminating stage must be adjusted based on the elongation of the laminated film to prevent internal stress caused by uneven stretching of the materials. By setting up tension controllers in separate stages and adopting a master-slave control mode, with the printing stage as the primary reference and other stages adjusting accordingly, tension balance can be achieved throughout the entire process, avoiding deformation caused by localized over-tension or under-tension.
Tension taper control technology effectively addresses deformation at the rewind end. During the rewinding process, as the roll diameter increases, maintaining constant tension will cause the outer layers of paper to stretch and deform due to accumulated stress. Tension taper control dynamically adjusts the set tension based on the roll diameter, reducing tension as the roll diameter increases, ensuring uniform tension across all layers of paper. For example, the tension can be set at 100% at the initial roll diameter. When the roll diameter increases to 50%, the tension automatically decreases to 80%, forming a linearly decreasing taper curve. This control method prevents paper from shrinking or bulging due to stress release after winding, thereby improving the flatness of the roll.
Material property compensation is a complementary method to tension control. Kraft paper's thickness, moisture content, and fiber orientation can affect tension transmission efficiency. For example, damp paper has a lower elastic modulus, requiring an appropriate increase in initial tension. For paper with stronger longitudinal fibers, the transverse tension should be 10%-15% lower than the longitudinal tension to prevent tearing. By presetting a material parameter library in the controller or using an online moisture meter to adjust tension settings in real time, customized control can be achieved for different batches of kraft paper, minimizing deformation caused by material variations.
Mechanical structure optimization can improve tension control stability. For example, the use of a floating roller structure can buffer transient tension shocks. When the paper speed changes suddenly, the floating roller absorbs tension fluctuations through displacement, preventing them from being directly transmitted to subsequent processes. The dancer, through the up and down motion of the oscillating roller, converts tension changes into position signals and feeds them back to the controller, creating a more sensitive closed-loop control. Furthermore, the use of low-friction guide rollers and anti-static coatings reduces friction between the paper and the equipment, minimizing tension fluctuations caused by uneven resistance.
Managing the production environment is an external guarantee for tension control. Kraft paper is sensitive to humidity. For every 10% increase in ambient humidity, the moisture content of the paper may increase by 1%-2%, causing deformation. Therefore, the production line must be equipped with a constant temperature and humidity system to maintain a humidity range of 45%-65% RH and a stable temperature of 20-25°C. Furthermore, regular inspections of equipment cleanliness to prevent dust from adhering to guide rollers or sensors, which could affect tension detection accuracy, are also important measures to minimize deformation.
