The paper, cellophane, plastic film and aluminum foil used in the packaging of food, medicine, clothing, clothing, cigarette, etc., are mostly printed by gravure rotary printing machine. In recent years, printing diversification and high-speed printing, especially in the next process of equipment size accuracy requirements, and in order to save materials, improve the yield. Tension control method is widely used. At present, the use of tension control abroad has become mature and widely used.
Principle and characteristics of tension control
1、 Gravure printing characteristics
At present, there are three main types of printing: relief, lithography and gravure. Gravure is characterized by the concave part of the picture and text, immersing the whole page in the ink tray, and then scraping off the surplus ink with a scraper to transfer the remaining concave part of the ink to the printed matter, which is gravure printing.
Gravure printing features:
The concave ink can transfer enough ink to layered printed matter. The thickness of the printed matter is indicated by the depth of the plate. The printed matter has a three-dimensional sense, and the small lines can be printed clearly. It can freely and widely select and use various plate materials.
2、 Gravure tension control
Before describing this problem, it is necessary to understand what tension control is.
At present, the use of gravure press need to apply the tension control principle, there are mainly two parts:
The first part is from feeding to feeding traction roller. Overprint error, that is, the overprint accuracy difference between the colors of multi-color printing. The position relation of printing machine mechanism is correct, and the error is caused by material elongation or shortening. Tension control is the most effective way to control a certain elongation within the allowable range.
The second part is from the drawing roller to each part after receiving. The tension control should be paid attention to such factors as the tightness, evenness and reliability of the receiving coil.
Tension control method between feeding shaft and feeding traction roller

The action of the traction roller pulls the printing material out from the feeding shaft at a certain speed and tension, and the tension of this part is greater than that of the printing section. Usually, the brake on the paper feeding shaft is used to control the tension. As is known to all, the coil diameter gradually decreases at the same time of operation. In recent years, high-speed, diversified materials used, coil diameter changes greatly. In the past, the manual mechanical brake could not reach high quality printing. It is necessary to select automatic control mode, especially to use data management of tension measurement meter. The detection methods include spring rocking roller type and micro displacement type detection. Mechanical brake has been used for a long time, and its price is cheap, but the torque value is not accurate. Gradually other forms appeared. Magnetic powder brake uses magnetic powder as friction medium to generate braking force, and its torque current characteristics have good linearity. It is widely used. Air brake is widely used in printing machinery, but its torque characteristic is nonlinear and wear is big. It is also gradually replaced by large capacity magnetic powder brake.

The feeding tension is determined by the following factors:
Uneven thickness leads to tensile change;
Material slippage and even being caught slow down;
The coil is not installed properly, and the material and shaft are eccentric;
The cycle speed changes during the rotation of the overturning device;
The reaction force of press roll and cutter when connecting paper;
The roundness of each roller is not enough.
Most of the above changes occur in a short period of time and are transmitted to the printing department as tension changes. From the point of view of mechanical structure, it must be noted that the inertia of guide roller and manufacturing error between the feeding shaft and the traction roller are the causes of the tension increase, especially the low tension less than several kilograms can not be ignored.
Tension change and control between traction roller and plate roller
In gravure printing machine, the traction roller is adjacent to the first edition roller, and there is usually a circumferential speed synchronization mechanism. This is different from the feeding situation, the diameter does not change, and the tension is controlled by other principles.
Printing materials are elastic and plastic like general objects, and printing processing should be carried out within the elastic limit. The law of elasticity is needed to control the tension. This is to understand the properties of different materials.
The tension can be determined according to the ratio of material width and thickness
There are two ways to get the right tension:
① The diameter of the pressing roller is slightly changed;
② Correct the rotation number of hold down roll.
The time required for printing materials to reach the set tension from no tension state is to take the distance between rollers and the speed reduction value as the time constant of one delay response, which is usually called tension control.
The basic form of gravure press is the tension between the paper feeding traction and the plate roller, between the plate roller and the receiving traction.
In addition, there is a swing roller tension control method. The principle is that the tension changes with the change of weight and counterweight position. Under what circumstances, the swing roller has the effect of absorbing the change of tension. Generally, the swing roller adopts the roller with small manufacturing error, small inertia, light weight and large diameter (such as hollow aluminum tube). This method is commonly used to produce short-term synchronous tension change between the feeding roller and the traction roller to make the small tension The change has stabilized. This method can also reduce the paper loss.
In addition, in the receiving traction part, the drying between the plate roller and the receiving traction part usually increases the distance, the drying and heating, and the extension car changes, causing the material expansion and contraction changes. These are the reasons for inaccurate overprint. As with the feeding traction unit, it needs better accurate tension control. There are also very thin paper low tension materials are blown by the dryer wind, easy to electrostatic adhesion, and produce small changes, these must also be considered.
The tension control of the coiling part (from the take-up traction roller to the take-up shaft) is also an important topic
This part is to be printed material as the final product, to composite, cutting, bag making and other next process. At the same time, attention should be paid to the uniform coiling of products to avoid scratching and wrinkling. In plastic film and aluminum foil, pay attention to prevent thickness change. This is only a question of how much tension is used for coiling. It is important to adjust the tension at the same time as the winding diameter is increased, which is to adjust with constant tension and to adjust with dimensional tension.
There are generally two driving modes of coiler
（1） Surface drive;
（2） Center drive.
Although the center drive type is widely used, the surface coiling type also has good properties, and the roller speed is constant. Some industries use it more. Compared with the feeding part, the receiving part should pay more attention to taper tension and inertia compensation error.
The central drive is the direct coiling material on the paper tube. The friction control method was used in the past, but now the high-speed, automatic and high-performance electrical control method is adopted
① Magnetic powder clutch is used;
② Use torque motor;
③ Use DC motor.
The magnetic powder clutch can obtain excellent performance at the appropriate price; the torque motor is not easy to use when the coil diameter changes greatly; DC motor is often used for large capacity and automatic paper connection.
3、 Taper tension
In general, constant tension is used to take up the material, and the material moment inside the coil increases, which leads to slippage, that is, coiling shrinkage. Then the material shrinks and the air releases the pressure to the center, and the material is squeezed out or squeezed out laterally, resulting in the so-called bamboo shoot phenomenon. Crepe is produced near the roll core to make the surface uneven. To solve these problems, the tension should be gradually reduced when the coil diameter gradually increases, that is, the taper tension should be used. The initial tension during coiling determines the tension at the end of coiling. The degree of reduction is called taper. Generally use 10% - 50% taper. For example, 30% taper is used; that is, starting with 10kg tension, then ending with 7kg tension. At this time, the change ratio of material and mechanism performance are different. The manufacturer should make various types of test curves through test and adjust them in production.
4、 Inertia compensation
The so-called inertia compensation is to compensate for the tension change caused by coil inertia when the speed changes from static to the fixed speed when the machine starts. In practice, the moment of inertia has a nonlinear relationship with the diameter, and is proportional to the material density and material width. The average value is calculated by formula calculation by the printing manufacturer, and the compensation curve is accurately calculated to guide the production.
At present, the advanced tension control device is not widely used in China's printing machine industry. With the development of reform and opening up, all kinds of printing machinery and packaging machinery will adopt the method of tension control to improve product quality. Therefore, in gravure printing, the principle of tension control must be understood, and it will be more handy in application.
Dongguan Beite packaging materials Co., Ltd. - located in Dongguan City, the world's manufacturing city, is a subsidiary of Beite packaging (China) Co., Ltd., with a plant area of 6000 square meters. It is a flexible packaging enterprise integrating R & D, production and sales. It has a complete set of domestic advanced production equipment, a scientific and humanized management system and a group of experienced technical personnel For backing, to provide customers with quality products and first-class service.
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