Tension Control in Gravure Printing (I)

In recent years, with the rapid development of the intelligent control and automation of gravure printing machines in gravure printing machines, some domestic high-end computerized color gravure printing machines have adopted microcomputer control systems. In addition, a new high-precision, repeatable tension control technology has also made a major breakthrough. This advanced tension control system uses a PLC programmable controller for complete machine control, touch-type computer screen man-machine interface operation, high degree of automation, easy operation, full parameter settings, with operability, high stability and Durability advantages. Therefore, due to the simultaneous development of the two, modern gravure printers have greatly improved the quality of prints. 
First, several factors that affect the tension control of gravure printing machine
The tension control system of a gravure press is essentially a special follow-up system in which the input amount changes according to an adjustable attenuation law. Tension control can be said to be the core of complete machine control. As long as the tension control is stable and the tension change is small, the color registration accuracy and reject rate of the gravure printer are easily controlled. 
Therefore, in order to ensure gravure quality and efficiency, a fully functional tension control system must be provided. However, in the printing process, the factors that cause fluctuations and changes in the tension of the gravure press tend to be more complex. The main influencing factors are roughly as follows:
1. In the process of collecting and unwinding the gravure roll, the diameter of the winding and unwinding is constantly changing, and the change of the diameter must cause the tension of the strip to change. When the unwinding torque is constant, the diameter decreases and the tension increases. On the contrary, if the winding torque is constant, the tension will decrease as the winding diameter increases. This is determined by the intrinsic characteristics of the gravure printing machine and is also one of the main factors that cause the tape tension to change. 
2. The manufacturing accuracy and assembly accuracy of the main components of the gravure printing machine such as the base, wall plates, and guide rollers are different. For example, the flatness and straightness of the base assembly, the verticality of the assembly of the wall plate and the base, the level of the assembly of the rollers and guide rollers, and the parallelism between them, their respective runout deviations, mass dynamic and static balance deviations, etc. And so on, all are very strict. Otherwise, when the tape runs on the plate roller and many guide rollers, the tension on the tape will change slightly. It will eventually be reflected on the entire machine, resulting in an irregular change in tension. In addition, the gears and gear boxes in the main drive system of the gravure printing machine should be precise and without gaps to ensure that the printing rollers of each printing unit run synchronously. If the printing process causes synchronization error, it is bound to cause the tension of each printing unit. Make a change. 
3. Inhomogeneity of the material inside the tape. For example, the fluctuation of the material elastic modulus, the change of the material thickness along the width and the length, the eccentricity of the material coil volume, and the temperature and humidity changes in the production environment will all have a subtle influence on the tension fluctuation of the entire machine.
4. In the gravure printing machine in the process of automatic reloading without stopping the machine, the feeding and cutting of the material will cause the original stable tension to suddenly change. The higher the speed of operation of the equipment, the greater the interference. At this time, the tension control system should be able to automatically and randomly adjust the tension according to the tension of the tape tension so that the tension can restore the original stable state in time.
Second, the type of gravure press tension control, detection methods and applications
1. The main types and characteristics of tension control
Tension control refers to the ability to permanently control the tension of the tape as it travels on the device. This control must be effective for any machine speed, including machine acceleration, deceleration, and uniform speed. Even in case of emergency stop, it has the ability to ensure that the tape does not produce any damage. Gravure press tension control is basically divided into manual tension control, open-loop semi-automatic tension control and closed-loop automatic tension control. Manual tension control means that when the winding diameter changes to a certain stage during the winding or unwinding process, the operator adjusts the manual power supply device to achieve the purpose of tension control. However, the manual tension control system of modern gravure press has basically been eliminated, but only exists as an operation mode in the closed-loop automatic tension control system. Open-loop semi-automatic tension control, also known as roll diameter detection type tension control, uses a proximity switch mounted on a reel to detect the rotation speed of the reel, and calculates the cumulative value by setting the initial value of the reel diameter and the material thickness. The current diameter of the rewinding or take-up reel and the change of the corresponding reel diameter output a control signal to control the rewinding torque or the rewinding braking torque, thereby adjusting the tension of the material tape.
Because each revolution of the reel, the coil diameter will occur twice as much as the thickness of the strip. This kind of tension control is not affected by external stimuli and can implement stable tension control. However, due to factors such as torque variation, linear change, and mechanical loss of the transmission, the absolute accuracy of such tension control is poor. Closed-loop automatic tension control is a tension sensor that directly measures the actual tension value of the tape, and then converts the tension data into a tension signal and feeds it back to the tension controller. By comparing this signal with the controller's preset tension value, the control is calculated. The signal, automatic control execution unit makes the actual tension value equal to the preset tension value, so as to achieve the purpose of tension stabilization. It is currently a more advanced tension control method. In addition, in the tension control systems for middle and high-grade printing presses manufactured and sold in China, higher tension and higher production speeds are required by the higher printing speeds and production processes, making magnetic powder clutches no longer competent for the execution units of such systems. Therefore, in the modern gravure printing machine, high-speed Slitting Machine, and high-speed coating compounding machine, it has been replaced by AC and DC servo motor execution units, achieving more advanced tension servo control.
If your machine requires very accurate tension control, you must use closed-loop automatic tension control. One of the important links is how to reasonably select the tension detection method. At present, the tension detection method is basically divided into three types: (to be continued)