On tension control in gravure printing process

Talking about the tension control in the process of gravure printing

in recent years, with the rapid development of intelligent and automatic control technology of gravure printing machine, some high-end computer chromatic gravure printing machines in China have adopted microcomputer control systems. In addition, a new high-precision and repeatable tension control technology has also made a major breakthrough. This advanced tension control system adopts PLC programmable controller to control the whole machine, touch-screen computer human-machine interface operation, high degree of automation, simple operation, all-round parameter setting, and has the advantages of operability, high stability and durability. Therefore, due to the synchronous development of the two, modern gravure printing machines have greatly improved the quality of printed matter

I. several factors affecting the tension control of gravure press

the tension control system of gravure press is essentially a special servo system whose input changes according to an adjustable attenuation law. Tension control can be said to be the core of the whole machine control. As long as the tension control is stable and the tension change is small, the chromatic accuracy and scrap rate of the gravure press can be easily controlled

therefore, in order to ensure the quality and efficiency of gravure printing, a fully functional tension control system must be equipped. However, in the printing process, the factors that make the tension of gravure press fluctuate and change are often complex, and the main influencing factors are roughly as follows:

1 In the process of winding and unwinding of gravure machine, the diameter of winding and unwinding is constantly changing, and the change of diameter must cause the change of belt tension. When the braking torque of unwinding is unchanged, the diameter decreases and the tension increases. On the contrary, if the winding torque remains unchanged, the tension will decrease as the winding diameter increases. This is determined by the inherent characteristics of gravure press, and it is also one of the main factors causing the change of belt tension

2. There is a deviation between the manufacturing accuracy and assembly accuracy of the main components of the gravure press, such as the base, wallboard, guide roller, etc. For example, the flatness and straightness of the base assembly, the perpendicularity of the wallboard and the base assembly, the levelness of the assembly of each plate roll and guide roll and their parallelism, their respective runout deviation, mass dynamic and static balance deviation, etc. are all strictly required. Otherwise, when the material belt runs on the plate roller and many guide rollers, the tension on the material belt will change slightly. It will eventually be reflected on the whole machine, resulting in irregular changes in tension. In addition, the gears and reduction boxes in the main drive system of gravure printing machine should be accurately driven without clearance to ensure that the plate rollers of each printing unit are transported synchronously. The scope of the industry is expanding day by day. If the transmission synchronization error is caused in the printing process, it is bound to change the tension of each printing unit

3. The non-uniformity of the internal material of the material belt. For example, the fluctuation of the elastic modulus of the material, the change of the material thickness along the width and length, the eccentricity of the mass of the coil, as well as the change of the production environment temperature and humidity, will have a subtle impact on the tension fluctuation of the whole machine, which is a well-known experimental machine manufacturer in the industry

4. In the process of automatic material receiving and changing without stopping of gravure press, material receiving and material breaking will suddenly cause interference changes to the original stable tension of the whole machine. The higher the running speed of the equipment, the greater the interference. At this time, the tension control system should be able to quickly adjust automatically and randomly according to the interference of the belt tension, so that the tension can be restored to the original stable state in time

II. Type, detection method and application of gravure machine tension control

1 The main types and characteristics of tension control

tension control refers to the ability to permanently control the tension of the material belt when it is transported on the equipment. This control must be effective for any running speed of the machine, including acceleration, deceleration and uniform speed of the machine. Even in the case of emergency shutdown, it has the ability to ensure that the belt will not be damaged at all. Gravure machine tension control is basically divided into three categories: dynamic tension control, open-loop semi-automatic tension control and closed-loop full-automatic tension control. Manual tension control means that in the process of winding or unwinding, when the coil diameter changes to a certain stage, the operator adjusts the manual electric FH FL scanning broadband source device, so as to achieve the purpose of tension control. However, the manual tension control system of modern gravure press has been basically eliminated, and only exists as an operation mode in the closed-loop full-automatic tension control system. Open loop semi-automatic tension control, also known as coil diameter detection tension control, uses the proximity switch installed at the reel to detect the rotation speed of the reel, and through the set initial value of the reel diameter and material thickness, calculates the current diameter of the winding or unwinding drum accumulatively, and outputs control signals corresponding to the change of the coil diameter, so as to control the winding torque or unwinding braking torque, so as to adjust the tension of the material belt. Because every turn of the reel, the coil diameter will change twice as much as the thickness of the strip. This kind of tension control is not affected by external stimulation, and can implement stable tension control. However, due to the torque change, linear change and mechanical loss of the transmission device, the absolute accuracy of this tension control is poor. The closed-loop full-automatic tension control is to directly measure the actual tension value of the material belt by the tension sensor, and then convert the tension data into a tension signal and feed it back to the tension controller. By comparing this signal with the preset tension value of the controller, the control signal is calculated, and the automatic control execution unit makes the actual tension value equal to the preset tension value, so as to achieve the purpose of tension stability. It is an advanced tension control method at present. In addition, in the tension control system of medium and high-end printing machines manufactured and sold in China, the higher printing speed and production process put forward higher requirements for tension control, so that the magnetic particle clutch is no longer competent for the executive unit of this kind of system. Therefore, in modern gravure press, high-speed slitting machine and high-speed coating compound machine, it has been replaced by AC and DC servo motor execution units to realize more advanced tension servo control

if your machine needs very accurate tension control, you must adopt closed-loop full-automatic tension control. One of the most important links is how to reasonably select the tension detection method. At present, there are basically three kinds of tension detection methods:

① tension sensor (load cell) detection method: it directly detects tension, which is closely combined with machinery, and there is no detection method of moving parts. The investment of new equipment will be promoted. Usually, the two sensors are used in pairs, and they are installed on the end shafts on both sides of the detection guide roller (as shown in Figure 1 (see the magazine for details)). The material belt applies load through the detection guide roller, causing the sensitive element of the tension sensor to displace or deform, so as to detect the actual tension value, and