Paper, cellophane, plastic film, aluminum foil, etc. used for packaging of food, medicine, clothing, clothing, cigarettes, etc., are often printed on a gravure web press. In recent years, diversification of printing and high-speed printing, in particular, the dimensional accuracy requirements of the next process equipment, and in order to save materials, improve the yield and so on. Tension control methods are widely used. At present, the use of tension control in foreign countries tends to be mature and its use is very common.
Try to analyze the tension control principle and characteristics.
First, the characteristics of gravure
At present, the main applications of printing are relief printing, lithographic printing, and gravure printing. The gravure printing is characterized in that the graphics and text are partly concave, and the printing surface is completely immersed in the ink tray. The excess ink is then scraped off with a scraper to transfer the residual ink to the printed material. The method is gravure printing.
Gravure printing features:
The ink in the concave area of ​​the plot can transfer enough ink to reach the layered printed matter. The shade of the printed matter is represented by the depth of the edition. The printed material has a three-dimensional effect, and the fine lines can be clearly printed out. Therefore, various plates can be freely selected and used.
Second, gravure tension control
Before describing this issue, understand what tension control is.
At present, the use of a gravure press requires a tension control principle that has two main parts:
The first part is from the feed to the feed roller. Overprinting error, that is, poor accuracy of overprinting between multi-color printing. The positional relationship of the printing press mechanism is correct. The error is caused by the elongation or shortening of the material. A certain amount of elongation is controlled within the allowable range. 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.
Traction roller action pulls the printing material at a certain speed and tension from the feed shaft and feeds it to the printing section. This part of the tension is greater than the tension of the printing section. The tension is usually controlled by the brake on the feed shaft. As we all know, the work volume gradually becomes smaller at the same time. In recent years, the speed has been increased, the use of materials has been diversified, and the roll diameter has changed greatly. Manual mechanical brakes used in the past have not been able to reach high quality prints. The use of automatic control methods, especially the use of tension measurement tables for data management has become 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. Gradually there are other forms. The magnetic powder brake uses the magnetic powder as the friction medium current to generate the braking force, and its torque-current characteristic is linear. being widely used. Pneumatic brakes are used by printing machines, but their torque characteristics are non-linear and cause large wear. Also gradually replaced by large-capacity magnetic powder brakes.
Feed tension control issues, feed tension is determined by the following factors:
Uneven thickness causes tensile changes;
Material slipping and even being caught slow;
Roll is not installed, material and shaft eccentric;
Turning device rotation produces a change in peripheral speed;
Pressure roller and knife reaction force when receiving 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 the mechanical structure, it must be noted that the inertia of the guide roller and the manufacturing error between the feed shaft and the traction roller are both the cause of the increase in the tension, and especially the low tension of several kilograms or less cannot be ignored.
The tension between the following traction roller and the plate roller (traction part) is changed and controlled.
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 feeding situation, the diameter does not change, and the tension is controlled by other principles.
Printed materials have the same elasticity and plasticity as general objects, and printing processes are performed within the limits of elasticity. The tension must be controlled by the law of elasticity. This is to understand the characteristics of different materials.
In use to the same elongation can be determined by the ratio of the width and thickness of the material tension.
There are two ways to fit tension:
1 Make the diameter of the pressure roller slightly change;
2 Correct the number of compression roller revolutions.
The time taken for the printed material to reach the set tension from the tension-free state is to delay the response between the distance between the rollers and the speed reduction value as a time constant. This form is usually called the tension control.
The basic form of the gravure printing machine feeds the tension between the paper traction and the plate roller, between the plate roller, between the plate roller and the receiving traction.
There is also a pendulum roller type tension control method. The principle is that the tension changes with the change of weight and change of the weight position. Under any condition, the pendulum roller has the effect of absorbing the tension change. The pendulum roller generally uses manufacturing error, small inertia, light weight and large diameter. Rollers (such as hollow aluminum tubes) usually use this method to produce a short-term tension variation between the feed and the pulling roll, which stabilizes the small tension changes. This method can also reduce the loss of the paper.
Then describe the receiving hauling section, drying from the roller to the receiving traction, usually increasing the distance, drying and heating, extension of the car changes, causing the material to expand and contract, which are caused by overprinting reasons. As with the feeding traction, better tension control is required. In addition, ultra-thin paper and low-tensile materials are also blown by the dryer and are susceptible to electrostatic adhesion, which causes minor changes. These must also be considered.
The tension control in the winding section (between the take-up pulling roller and the receiving axis) is also an important issue.
This part is to send the printed material as the final product 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 of scratches and wrinkles. The value of attention is plastic film and aluminum foil, pay attention to prevent thickness changes. This is just the problem of how much tension is taken up. The important thing is that the diameter of the coil is increased and the tension is adjusted at the same time. It is adjusted with a constant tension and adjusted with dimensional tension.
There are generally two types of winding unit drive methods:
(a) surface-driven;
(b) Center-driven.
Although the center-driven type is generally used more, the surface winding type also has good properties, and the condition is good when the peripheral speed of the roller is constant. Some industries use more. Compared with the receiving department, the receiving department should pay more attention to taper tension and inertia compensation error.
The center drive is the direct coiling of material on the paper tube. In the past, the friction control method was used. The current method is the height, automatic, and high-performance electrical control method. The take-up reeling method is as follows:
1 use magnetic powder clutch;
2 use torque motor;
3 Use a DC motor.
Magnetic powder clutches should be able to obtain excellent performance in price; torque motor winding diameter change is not easy to use; large capacity and automatic take over more use of DC motors.
Third, taper tension
Under normal circumstances, the use of constant tension to take up the material, increase the material inside the material volume phase torque increases, resulting in slipping or coiling shrinkage. Further shrinkage of the material and release of air toward the center pressure increase the material being squeezed or laterally extruded, resulting in the phenomenon known as bamboo shoots. Near the core of the roll, ridges appear, 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 during winding determines the tension at the end of the coiling. The degree of reduction is called taper. Generally use 10%-50% taper. For example, with a 30% taper; that is, starting with a tension of 10 kg, the tension from 7 kg ends. At this time, the rate of change material and the mechanism performance are different. Manufacturers should test to make various types of test curves, and adjust the control in production.
Fourth, inertia compensation
The so-called inertia compensation is to absorb the tension when the machine starts. When the speed changes from stationary to a fixed speed during the operation, the inertia of the coil causes a change in the tension. 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, China's printing press industry is not yet widely used in advanced tension control devices. The situation of reform and opening up has been developed, and various types of printing machinery and packaging machinery will adopt more tension control methods to improve product quality. Therefore, in gravure printing, the principle of tension control must be understood, and it will be more handy in application.
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