Talking about the Understanding and Application of Tension Control Theory in Gravure Printing

Paper, cellophane, plastic film, and aluminum foil used in the packaging of foods, medicines, clothing, clothes, cigarettes, etc., are often printed on a gravure web printer due to their own characteristics. In recent years, due to the diversification of printing and the speeding up of printing, especially the requirements for the dimensional accuracy of the next process equipment, as well as the saving of materials and the improvement of the yield. Tension control methods are widely used. At present, the use of tension control in foreign countries has become mature and its use is very common.
Now try to analyze the tension control principle and characteristics.

First, the characteristics of gravure

At present, the main applications of printing layout are relief, lithography and gravure. The gravure is characterized in that the graphic is partly concave, the layout is completely immersed in the ink tray, and then excess ink is scraped off with a scraper to keep the ink remaining in the concave portion. The method of transferring to the printed matter is gravure printing.

The main features of gravure printing are:

The ink accumulated in the concave portion of the layout can transfer sufficient ink to obtain a layered print. The shade of the print is expressed by the depth of the print, so the print has a three-dimensional effect, and the fine lines can be printed clearly and freely and widely. Select and use various media.

Second, on the gravure tension control

Before describing this issue, understand what tension control is.

At present, the gravure printing machine used needs two main parts that apply the tension control principle:

The first part is from the feed to the feed roller. Overprinting error, that is, overprinting accuracy between colors of multi-color printing. If the position relationship of the printing press mechanism is correct and the error is caused by the elongation or shortening of the material, a certain amount of elongation is controlled within the allowable range, and the tension control is the most effective.

The second part is from the receiving traction roller to the receipt of all parts, should pay attention to the receiving material tight, neat, reliable and other factors tension control.

The tension control method from the feed shaft to the feed pull roller will be described in detail below.

Due to the action of the pulling roller, the printing material is pulled out of the feeding shaft at a certain speed and tension and sent to the printing section. The tension of this part is greater than the tension of the printing section. The tension is usually controlled by a brake attached to the feeding shaft. . As we all know, the winding diameter of the work gradually becomes smaller. In recent years, due to the increase in speed and the use of diversified materials, the coil diameter has changed greatly. Therefore, manual mechanical brakes used in the past have not been able to obtain high quality prints. Therefore, the use of automatic control methods, especially the use of tension measurement data management becomes a necessary practice. Its detection methods are spring-rolled roller type and micro-displacement type detection. Mechanical brakes have been used since very early, and they are inexpensive, but they are inaccurate. So gradually there are other forms. Magnetic powder brakes use magnetic powder as a friction medium to generate braking force through current, because of its good linearity in torque-current characteristics. So it is widely used. Pneumatic brakes, although some printing machines are used in large quantities, are due to the non-linear torque characteristics and the large wear factor. Also gradually replaced by large-capacity magnetic powder brakes.

In addition to the tension control of the feed, the tension of the feed is determined by the following factors:
Tensile change due to uneven thickness;
The slippage and even slippage of the material slow down;
Roll is not installed, material and shaft eccentric;
The change in the peripheral speed of the turning device during rotation;
The reaction force of the pressure roller and cutter when the paper is connected;
The roundness of each roller is not enough.

Most of these changes took place within a short period of time and were transmitted to the printing department as tension changes. From the mechanical structure point of view, it must be noted that the inertia of the guide roller and the manufacturing error between the feed shaft and the traction roller are the causes of the increase in tension, and in particular, the low tension of several kilograms or less must not be ignored.

Let's talk about the change and control of the tension between the traction roller and the plate roller (traction part).

In the gravure printing machine, the traction roller and the first plate roller are usually provided with a peripheral speed synchronization mechanism. This is different from the feed situation, the diameter does not change, so use other principles to control the tension.

The printed material has the same elasticity and plasticity as a general object, and the printing process is performed within the elastic limit. Therefore, the tension must be controlled according to the law of elasticity. This is to understand the characteristics of different materials.
In order to obtain the same elongation in use, the tension can be determined by various ratios of the width and thickness of the material.

There are two ways to get the proper tension:

1 Make the diameter of the pressure roller slightly change;

2 Correct the number of rotations of the pressure roller.

The time required for the printed material to reach the set tension from the tension-free state is a one-time delay response of the distance between the press rolls and the speed reduction value as a time constant. This type is usually called a stretch control.
For the basic form of the gravure printing machine between the feeding traction and the printing roller, the tension between the printing roller, between the printing roller and the receiving traction is based on this principle.

There is also a method of pendulum roller tension control. The principle is that the tension changes with the change of weight and the change of the weight position. Under any circumstances, the pendulum roller has the effect of absorbing the tension change. The pendulum roller generally adopts manufacturing error and small inertia. Small lightweight and large-diameter rollers (such as hollow aluminum tubes) are usually used to remove the short-term tension fluctuations that occur between the feed material and the take-up roll, so that small changes in tension are stabilized. This method can also reduce the loss of the paper.

Then describe the receiving traction section, from the roller to the receiving traction for drying, usually increase the distance, due to dry heating, extension of the car changes, resulting in material expansion and contraction, which are caused by overprinting reasons. Therefore, as with the feeding traction part, a better and accurate tension control is required. In addition, materials with extremely low tension and low tension are also susceptible to electrostatic adhesion due to the wind blown by the dryer, which causes minor changes. These must also be considered.

The tension control at the winding section (between the take-up pulling roller and the receiving shaft) is also an important issue.
This part is to send the printed materials as final products to the next process of compounding, slitting, and bag making. At the same time, care should be taken to ensure that evenly coiled products are free from scratches and wrinkles. It is worth noting that in plastic films and aluminum foils, care must be taken to prevent thickness variations. This is not just a problem of how much tension is taken up, but it is important to adjust the tension while the diameter of the coil is increasing, whether it is adjusted with a constant tension or with a dimensional tension.

There are generally two types of drive methods in the winding section:
(a) surface-driven;
(b) Center-driven.
Although the center-driven type generally uses more, the surface winding type also has good properties, and the condition is good when the roller peripheral speed is constant. Therefore, it is used in some industries. Compared with the receiving department, the receiving department should pay more attention to taper tension and inertia compensation error.
The center drive is to take up the material directly on the paper tube. In the past, the friction control method was used. Now the height, the automatic, and the high-performance electrical control method are adopted. The take-up reeling method is as follows:
1 use magnetic powder clutch;
2 use torque motor;
3 Use a DC motor.
The magnetic powder clutch can obtain excellent performance at a low price; the torque motor cannot be used when the coil diameter changes greatly; the DC motor is often used for large capacity and automatic paper feed.

Third, taper tension

Under normal circumstances, the use of constant tension to take up the material, with the increase of the coil relative to the material inside the torque becomes larger, resulting in slipping that is the coiling shrinkage. Further, due to the shrinkage of the material and the release of air toward the center, the material is squeezed or laterally extruded, resulting in a so-called bamboo shoot phenomenon. Near the core of the winding ridges occur, making the surface uneven. To solve these problems, the tension should gradually decrease when the roll diameter gradually becomes large, that is, taper tension is used. The initial tension at the winding determines the tension at the end of the coiling. The degree of reduction is called taper. A taper of 10% to 50% is generally used. For example, with a 30% taper; that is, starting with a tension of 10kg, the tension from 7kg ends, and the rate of change at this time is different depending on the properties of the material and the mechanism. Manufacturers should test to make various types of test curves so that the control can be used in production. Adjustment.

Fourth, inertia compensation

The so-called inertia compensation is to compensate for the change in tension caused by the inertia of the coil when the machine is started or when the speed is changed during the operation or when the speed reaches a predetermined speed. In practice, the non-linear relationship between the moment of inertia and the diameter is proportional to the density of the material and the width of the material. The printing manufacturer applies the formula to calculate the average value and accurately calculates the compensation curve to guide the production.
At present, the application of advanced tension control devices in the printing press industry in China is not yet widespread. With the development of the reform and opening up, various printing machinery and packaging machinery will adopt more tension control methods to improve product quality.