Detailed explanation of paper bag machine tension control system: the key to ensuring smooth operation of paper

In modern paper bag automation production, paper undergoes a complex and precise journey from unwinding, printing, creasing, gluing to final forming and winding. Among them, tension control plays the role of an "invisible guardian", running through the entire process to ensure that the paper remains stable, flat, and wrinkle free during high-speed operation. Once the tension is out of control, problems such as paper breakage, inaccurate overprinting, uneven glue application, and skewed bag shape will arise one after another, which can result in downtime and waste, or even the entire batch of products being scrapped. Therefore, a deep understanding and mastery of the essence of the paper bag machine tension control system is an essential core technology to ensure efficient and high-quality production.

1、 Tension control: the lifeline of paper bag production

Tension, in short, is the pulling force that paper experiences when it is pulled and braked inside a machine. The ideal tension state on a paper bag machine should be uniform, constant, and of moderate size:

Excessive tension: Paper is excessively stretched, which can easily lead to paper breakage; For paper with printed patterns, the color registration position may be misaligned (inaccurate printing); The indentation line may be too deep or even broken; When winding, the paper roll will become hard and the inner layer of paper will deform under pressure.
Too low tension: The paper is loose and sagging, causing floating and wrinkling; Uneven glue application; There may be ghosting during printing; Unclear indentation; The winding is not neat, resulting in "chrysanthemum core" or collapsing phenomenon.
Tension fluctuations: Even if the average tension value is appropriate, frequent or severe tension fluctuations can still be fatal. It can cause instantaneous shaking or changes in the tightness of the paper, and is the culprit of quality problems such as printing blur, glue interruption, and uneven bag size.
It can be seen that precise and stable tension control is the cornerstone of ensuring smooth operation of paper, precise coordination of various processes, and ultimately producing high-quality paper bags.

2、 The core components of tension control system

A complete paper bag machine tension control system typically consists of the following key components working together to form a precise closed-loop or open-loop adjustment network:

Tension detection unit (the system's "sensory nerve"):
Floating roller sensor: This is the most classic and widely used method. A freely rotatable guide roller (floating roller) is subjected to a predetermined force by mechanisms such as springs or cylinders. When the tension of the paper changes, it will push the floating roller away from the equilibrium position. By accurately measuring this displacement (commonly using high-precision potentiometers or magnetostrictive displacement sensors), the actual tension value of the current paper can be calculated in real time. Its structure is relatively simple and reliable, with moderate cost and good adaptability to most paper bag materials.
Ultrasonic/infrared non-contact sensor: using sound waves or infrared to measure the sag (Sag) of paper during operation, indirectly calculating tension. Especially suitable for extremely thin, fragile, or surface sensitive materials (such as those that have been coated), avoiding damage that may be caused by physical contact. But its accuracy is easily affected by the environment (temperature, airflow) and material surface characteristics, and the cost is relatively high.
Tension measuring roller: This is a direct measurement method. The special guide roller is equipped with high-precision tension sensors (such as strain gauges) at both ends. When the paper is wrapped around the roller, the radial force (i.e. tension) borne by the roller body is directly measured. The highest accuracy and fast response, but the structure is complex and the cost is high. It is usually used in critical workstations with extremely high tension control requirements or as a calibration benchmark.
Control center (system "brain"): Paper bag machines are typically equipped with dedicated PLCs (programmable logic controllers) or motion controllers that integrate advanced tension control functions. It receives real-time signals from tension sensors and compares them with the target tension value set by the operator. The core control algorithm (most commonly PID algorithm - proportional, integral, derivative control) performs high-speed calculations to analyze the magnitude, duration, and trend of deviations:
Proportional (P): Generate an output proportional to the current tension deviation and respond quickly.
Integral (I): Accumulate historical deviations and eliminate steady-state errors of the system (such as long-term small deviations).
Differential (D): predict the future change trend of deviation, provide advance regulation and restrain system oscillation.
The controller accurately calculates the control amount (current, voltage, or speed command) that needs to be applied to the actuator based on the PID operation results, in order to achieve fast and smooth adjustment of tension.
Execution mechanism (the 'muscles' of the system):
Unwinding department: The main task is to provide braking torque to prevent the coil from loosening freely due to inertia.
Magnetic powder brake/clutch: By adjusting the excitation current, it accurately and linearly controls its output torque (braking force). Fast response, simple control, and good overload protection are the mainstream choices.
Pneumatic brake: uses air pressure to control the braking torque. The cost is lower and maintenance is relatively simple, but the response speed and linearity are usually not as good as magnetic powder.
Servo motor drive: In high-end or equipment that requires precise speed synchronization, the unwinding reel is directly driven by a servo motor. By controlling the torque mode of the motor (providing reverse torque) or precise speed mode (synchronizing and fine-tuning with the host speed), tension control can be achieved. The dynamic performance is optimal, but the cost is the highest.
The main task of the winding department is to provide traction torque and roll the paper neatly.
Magnetic powder clutch: commonly used to drive the winding shaft, controlling the torque (traction) transmitted to the winding shaft by adjusting the excitation current.
Servo motor drive: optimal solution. The servo motor operates in torque mode to provide constant winding tension, or in speed mode with taper tension control (automatically reducing the tension setting value as the coil diameter increases to prevent deformation caused by inner layer compression). It also has the best dynamic performance.
Intermediate traction parts (such as between printing units and adhesive units): usually driven by AC variable frequency motors or servo motors. Fine tuning the motor speed (acceleration or deceleration) can quickly compensate for tension changes caused by material elasticity, speed variations, or fluctuations in front/back tension. The precision and response speed requirements for speed control are very high here.
3、 The Implementation Principle of Tension Control

Closed loop control (feedback control - mainstream method):
Process: The tension sensor detects the actual tension of the paper in real time ->transmits the signal to the controller ->the controller compares the measured value with the set value, calculates the deviation ->the controller (through PID algorithm, etc.) issues control instructions ->the actuator (brake/motor) acts to change the force/speed applied to the paper ->the actual tension tends towards the set value ->the sensor detects again … form a continuously operating closed loop.
Advantages: High control accuracy, strong anti-interference ability (able to automatically compensate for tension disturbances caused by changes in material properties, speed fluctuations, mechanical resistance changes, etc.).
Application: This is the standard mode for tension control of modern high-speed and high-precision paper bag machines, especially suitable for unwinding, rewinding, and tension control between various process units.
Open loop control (indirect control):
Process: Does not rely on real-time tension detection. Common methods include "speed matching" or "torque control". For example, in a simple system, a fixed tension is generated by precisely setting the speed difference of each driving motor (the speed of the latter unit is slightly faster than the previous unit); Alternatively, at the unwinding end, the brake torque can be adjusted according to the preset curve based on the change in roll diameter (through sensors or calculations).
Limitations: Relatively low accuracy, unable to perceive and compensate for actual tension fluctuations in real time, poor anti-interference ability. High consistency is required for material properties and mechanical states.
Application: May be used for low-speed, simple equipment with low tension requirements or as an auxiliary/coarse adjustment means for closed-loop control.
4、 The key application and value of tension control system in paper bag machine

Stable unwinding tension: Ensure that the paper roll unfolds smoothly, avoiding paper shaking, breakage, or roll collapse caused by sudden tension changes. Especially when the roll diameter changes from large to small, the control system can automatically adjust the braking torque to maintain a constant tension.
Accurate synchronization of tension between printing units: In the multi-color printing process, even a tiny difference in tension between units can lead to inaccurate overprinting (color misalignment). High precision tension control is a prerequisite for ensuring the quality of exquisite printing.
Constant tension during the adhesive/composite process: Fluctuations in tension directly affect the uniformity of the adhesive amount (uneven thickness of the adhesive layer) and the bonding effect of the composite material (bubbles, wrinkles). Stable tension is the key to ensuring firm adhesion and a smooth appearance.
Indentation and die-cutting station tension control: Moderate tension ensures that the paper passes smoothly through the mold, making the indentation clear, consistent in depth, and precise in die-cutting position, avoiding displacement or wrinkling caused by paper looseness.
Precise winding tension and taper control: During winding, constant tension or taper tension control (tension decreases with increasing roll diameter) is applied to ensure that the paper roll is tightly and neatly wound, with consistent tension between the inner and outer layers, avoiding "chrysanthemum core", collapse, or outer paper being squeezed out by the inner layer, providing qualified semi-finished rolls for subsequent bag making processes.
5、 The core benefits brought by excellent tension control

Significantly improve product quality: reduce paper breakage, eliminate wrinkles, ensure accurate overprinting, evenly apply glue, clear indentation, consistent bag type standards, significantly reduce scrap rate.
Maximizing production efficiency: reducing downtime (handling paper breaks, adjustments) and speed reduction caused by tension issues, and improving overall equipment efficiency (OEE).
Reducing raw material loss: Reducing paper breakage and waste means directly saving costs such as paper, ink, glue, etc.
Extend equipment lifespan: Smooth tension reduces impact and abnormal wear on guide rollers, bearings, and transmission components.
Adapting to high-speed production: Strong tension control is the core guarantee for paper bag machines to achieve high-speed and ultra high speed stable operation.
Expanding material adaptability: Precise tension control enables the equipment to handle paper of different weights, materials (such as kraft paper, coated paper, and paper containing textile fibers), with large differences in thickness, and even stretchable thin film composite materials more easily.
6、 Key points for maintenance and optimization of tension control system

To fully utilize the efficiency of the tension control system, careful maintenance and operation are essential

Regularly calibrate sensors: Tension sensors are the "eyes" of control systems, and their accuracy is crucial. Regularly use standard weights or calibrators according to the manufacturer's requirements for calibration to ensure accurate measurements.
Maintenance of executing agencies:
Magnetic powder brake/clutch: Regularly check the magnetic powder status (replace according to the service life), ensure good cooling (clean heat sink), and check the coil resistance and insulation. Avoid working at maximum torque for extended periods of time.
Pneumatic brake: Keep the air source clean and dry, regularly check the cylinder seal and air pipe for leaks to ensure stable air pressure.
Motor (frequency conversion/servo): Regularly check the operating temperature, vibration, and abnormal noise of the motor, maintain good ventilation and heat dissipation of the drive, and perform maintenance according to the manual.
Mechanical transmission component inspection: Ensure that the floating roller rotates flexibly without jamming, and that the bearings are well lubricated; Check whether all guide rollers are straight, clean and free of stains (stains may affect tension or cause material contamination); The tension of the drive belt/chain is moderate.
Electrical connection reliability: Regularly check whether the sensor signal line, control line, and power line connections are firm and reliable, to avoid poor contact or interference that may cause signal jumps or control failures.
Parameter optimization and adaptive adjustment: For paper of different materials, weights, widths, and production speeds, it may be necessary to fine tune PID parameters (proportional gain, integration time, differentiation time) or tension settings in the controller to achieve optimal control effects. Accumulating experience data is very important.
Operator training: Operators should understand the basic principles and importance of tension control, master the daily inspection points, be able to identify common tension abnormalities (such as paper shaking, violent jumping of floating rollers), and know how to conduct preliminary investigations or notify maintenance personnel.
The tension control system of the paper bag machine, although not directly involved in printing patterns or adhesive molding, is the "behind the scenes hero" of the smooth, efficient, and high-quality operation of the entire production line. It is like a highly skilled 'kite pilot', able to accurately perceive and adjust the force in its hand no matter how the wind speed changes, allowing the 'paper' kite to fly smoothly along the preset trajectory. From the core sensors and controllers to the actuators, from the closed-loop feedback principle to the ingenious application of PID algorithm, every link of this system embodies the wisdom of industrial automation. A deep understanding of its working principle, and careful maintenance and optimization, is a solid technical guarantee for paper bag manufacturing enterprises to enhance their core competitiveness, achieve cost reduction and efficiency improvement, and win market trust. In the future paper bag production that pursues higher speed, more complex processes, and more diverse materials, tension control technology will continue to evolve and play an increasingly critical role.

Core points of the article:

Tension is the lifeline of paper bag production: excessive, insufficient, or fluctuating tension directly leads to a series of serious quality problems such as paper breakage, wrinkles, inaccurate overprinting, uneven glue coating, and poor winding.
Precise closed-loop operation of the system: relying on tension sensors (floating rollers, ultrasonic waves, etc.) for real-time monitoring, controllers (PLC/motion controllers, core PID algorithms) for fast calculation and decision-making, and actuators (magnetic powder brakes/clutches, servo/variable frequency motors) for precise adjustment of braking force or speed, forming closed-loop control to achieve high-precision, anti-interference and stable tension.
Throughout the entire production process: from preventing loosening during unwinding, synchronizing and preventing inaccurate overprinting between printing units, applying glue/composite to ensure even adhesion, ensuring smooth indentation and die-cutting, to preventing collapse during winding/chrysanthemum core, tension control is everywhere, ensuring precise connection between each process.
Bringing core benefits: significantly improving product quality (reducing waste), maximizing production efficiency (reducing downtime), reducing raw material loss, extending equipment life, supporting high-speed production, and expanding material adaptability.
Maintenance optimization is key to efficiency: regularly calibrating sensors, carefully maintaining actuators (magnetic powder parts, motors), inspecting mechanical conductive components, ensuring electrical reliability, optimizing parameters based on material processes, and strengthening operator training are all indispensable.