During the high-speed operation of the gravure printing machine and the coating machine, stable tension plays a crucial role in the registration accuracy. Therefore, identifying the causes of tension fluctuations in gravure printing machines and detecting and controlling the tension are the prerequisites for ensuring the quality of printed products.

1. Factors causing tension fluctuations

(1) The substrate material rolls are not wound evenly or are eccentric, and the material rolls are poorly managed, resulting in an irregular outer circle shape of the material rolls. The actual axis of the material rolls does not coincide with the theoretical axis, causing jumping when unwinding.

(2) If the parallelism between the guide rollers is poor, the guide rollers themselves are not round or the dynamic balance exceeds the standard, it will cause tension fluctuations when the gravure printing machine operates at high speed.

(3) The tension detection mechanism or tension control component malfunctions. If the tension is detected by the float roller potentiometer detection component, the air pressure of the cylinder must be stable. Generally, low-friction cylinders are used, and ultra-precision pressure regulating valves or electrical proportional valves are installed before the cylinder takes in air. If a magnetic powder clutch brake is used as the tension control component, the magnetic powder is prone to tension instability after aging. If a pneumatic brake is used at the unwinding point, uneven or excessive wear of the brake shoes can easily lead to errors in the control of the unwinding tension. Therefore, using a float roller potentiometer to detect the tension component is the best method.

(4) Paper slippage causes tension fluctuations. The slipping mentioned here mainly occurs at the point where the rubber roller and the steel roller are pressed together. If the cylinder pressure is insufficient or the precision of the rubber roller and the steel roller themselves is not good enough, it is easy to cause the paper to slip.

(5) During the acceleration and deceleration of the gravure printing machine, the tension may also fluctuate. In such cases, the tension can generally be stabilized by adjusting the operating parameters of the motor inside the equipment.

(6) Uneven thickness of the substrate or the presence of ruffles can also cause changes in tension.

2. Tension detection method

Sensor detection method. The tension sensor is installed at both ends of the detection roller. A load is applied to the tension sensor through the detection roller. The sensor transmits the obtained tension signal to the tension control component to adjust the tension magnitude, thereby achieving tension closed-loop control. The disadvantage of this tension detection method is that it is prone to tension oscillation during the tension adjustment process, causing local fluctuations in tension.

(2) Detection method of float roller potentiometer. Set an appropriate cylinder pressure based on the required tension of the substrate and control the cylinder diameter of the cylinder, and balance it with the tensile force of the substrate to achieve the purpose of stabilizing the tension. After the equipment is turned on and running, if there is a tension fluctuation, the gear rotating coaxially with the end of the swing arm shaft of the float roller will drive the potentiometer gear to rotate by a certain Angle. The potentiometer will then transmit the obtained electrical signal to the tension control execution part to adjust the tension in the opposite direction, thereby achieving closed-loop control of tension. It overcomes the shortcomings of the sensor detection method and can effectively absorb the oscillation fluctuations of tension.

3. Tension control method

There are three methods for tension control in the winding and unwinding sections: magnetic powder brake clutch method, pneumatic brake clutch method and motor drive method. Whether the magnetic powder braking is manual or automatic, it is achieved by directly controlling the rotational torque and indirectly controlling the tension. The control of torque is realized by setting the current of the tension controller.

The motor drive mode generally controls the motor speed by regulating the magnetic field strength of the asynchronous motor, which is known as vector frequency conversion control. There are also those that adopt DC motor control mode or servo motor control mode. The principle is to take one of the motors as the main motor in the system. By setting the speed of the main motor, other motors will follow it. During the following process, if the tension changes, the feedback signal from the potentiometer or sensor will be transmitted to the tension control center. The control center then issues instructions to make appropriate adjustments to the motor speed, thereby achieving the purpose of tension adjustment. The advantage of adopting the motor control method is that the diameters of each driving roller do not have to be exactly the same, the matching is flexible, the control accuracy is high, and it represents the development direction of tension control.