In-depth analysis of bag making machine tension control principles, methods and common problems

In the modern packaging industry, bag-making machines are core equipment, and their performance directly impacts the quality of flexible packaging products such as plastic bags and composite bags. During the bag-making process, tension control is a critical technical step in ensuring bag flatness, precise dimensions, and a secure seal. Improper tension control can lead to problems such as material deformation, misaligned patterns, uneven sealing edges, and even film breakage, severely impacting production efficiency and product yield. Therefore, a thorough understanding of the principles of bag-making machine tension control is crucial for equipment operation, maintenance, and process optimization.

1. Basic concepts and importance of tension control

In bag-making machines, tension refers to the tensile force applied to film or coils during transport. Ideally, the material should maintain a constant and appropriate tension during unwinding, pulling, and rewinding. Too little tension results in loose material, which can lead to wrinkles and misalignment. Too much tension can cause deformation or breakage. The core of bag-making machine tension control lies in maintaining stable tension at each stage of the process through real-time detection and adjustment, thereby ensuring consistent bag dimensions, pattern alignment, and mechanical strength.

This control process involves multiple units, including the unwinder, traction rollers, intermediate drive system, and rewinding mechanism. The dynamic balance of tension within each unit determines the quality of the final product. High-speed bag-making machines are particularly sensitive to tension fluctuations, necessitating a high-precision control system to rapidly respond to external disturbances (such as changes in material properties and speed changes).

2. Composition and working principle of tension control system

A complete tension control system usually consists of three parts: detection unit, control unit and execution unit.

1. Detection Unit:
The detection unit is responsible for measuring the actual web tension in real time. Commonly used devices include tension sensors and dancer roller potentiometers. The tension sensor is mounted directly on the guide roller bearing and uses strain gauges to detect the force applied by the material and convert it into an electrical signal. Dancer roller detection uses the position of a dancer roller to indirectly measure tension. When tension changes, the dancer roller moves, and a potentiometer or encoder outputs a signal. Both methods have their advantages and disadvantages. Direct sensors offer high accuracy but are more expensive. Dancer roller systems are simpler and suitable for most medium- and low-speed applications.

2. Control Unit:
The control unit is the "brain" of the tension system and is typically implemented as a PLC (Programmable Logic Controller) or dedicated tension controller. The controller receives the signal from the detection unit, compares it with the preset tension value, and calculates the adjustment command based on the deviation using a PID (Proportional-Integral-Derivative) algorithm. PID parameters must be optimized based on material type, width, and machine speed to achieve fast and stable response. Modern advanced controllers also include adaptive features that can learn material characteristics and automatically compensate for environmental changes.

3. Actuator:
The actuator adjusts the actual tension based on control signals. The primary mechanisms include a magnetic powder brake/clutch, a servo motor, and a pneumatic brake. During the unwinding phase, the brake's torque output is adjusted to control the resistance to material release. During the rewinding phase, the motor's torque or speed is controlled to maintain a constant winding tension. Servo motors, due to their high precision and fast responsiveness, are becoming the preferred choice for high-speed bag-making machines.

The entire system's workflow forms a closed loop: detection → comparison → calculation → execution → feedback, enabling continuous tension adjustment. For example, if high tension is detected, the controller reduces the braking torque or lowers the winding motor torque to restore tension to the set range.

3. Tension control strategies at different process stages

The tension control of the bag making machine needs to adopt differentiated strategies according to the characteristics of different production stages:

• Unwinding tension control : As the roll diameter decreases during unwinding, maintaining a fixed braking torque will result in increasing tension. Therefore, the system requires roll diameter detection and employs taper tension control, which reduces the torque setting proportionally as the roll diameter decreases, thereby maintaining constant tension.
• Draft and intermediate tension control : The tension between the draft rollers directly affects bag length accuracy and pattern alignment. Speed ​​matching is often used here, controlling tension by adjusting the slight difference in speed between the drive rollers. Synchronicity is extremely demanding and is typically achieved by a servo system.
• Winding Tension Control : As the winding roll diameter increases, taper control is also required. At the same time, the winding tension is usually slightly lower than the unwinding tension to prevent the inner material from being compressed and deformed. Modern systems also use a pressure control mode, which adjusts the cylinder pressure to achieve uniform coil tightness.

In addition, during acceleration, deceleration or roll changing, the system needs to preset tension ramp changes to prevent sudden changes that may cause material breakage or relaxation.

4. Factors and common problems affecting tension control accuracy

Despite continuous advancements in control systems, actual production still presents numerous challenges. First, material properties such as thickness, elastic modulus, and coefficient of friction directly impact tension stability. For example, PE film is highly ductile and requires lower tension, while BOPP is more rigid and can accommodate higher tension. Appropriate parameters should be set based on material testing before production.

Mechanical factors shouldn't be overlooked either: Parallelism of guide rollers, bearing wear, and surface wear can cause tension fluctuations. Drive belt slippage or excessive gear play can also introduce disturbances. Regular maintenance and equipment calibration are therefore crucial.

Electrically, sensor drift, inappropriate PID parameters, or signal interference can all reduce control accuracy. Common issues manifest as tension oscillation (fluctuating) or delayed response. In these cases, check grounding and shielding, re-tune PID parameters, or upgrade the control algorithm.

Operators must be familiar with the meaning of alarm codes, such as tension exceeding the limit and detection loss, and be able to adjust settings promptly through the HMI interface. Recording historical data and analyzing trends can help predict the source of problems.

5. Development of Advanced Control Technology

With the advancement of Industry 4.0, bag-making machine tension control is moving towards intelligence. IoT-based systems can monitor tension curves in real time and integrate with MES systems to achieve predictive maintenance. Adaptive control algorithms can automatically switch parameter plans based on different materials. Furthermore, sensorless tension control technology is beginning to be used. It estimates tension based on motor current feedback, reducing hardware costs and maintenance requirements.

These advancements not only improve the control accuracy of tension, but also significantly reduce energy consumption and scrap rates, providing a solid guarantee for the efficient and high-quality production of flexible packaging.

Bag-making machine tension control is a comprehensive technology that integrates mechanics, electrical engineering, and materials science. From basic closed-loop control to intelligent adaptive systems, its core goal remains to maintain stable tension during dynamic production. A thorough understanding of its principles, the appropriate selection of control solutions, and focused attention to routine maintenance and parameter optimization are essential to fully utilize the equipment and produce high-quality, reliable packaging products. In the future, with the further application of sensor technology and AI algorithms, tension control will undoubtedly become even more precise and efficient, propelling the entire bag-making industry to new heights.