Plastic film is more and more widely used in packaging industry, from food packaging to commodity packaging, as well as milk and beverage soft packaging, are inseparable from plastic film. This has opened up a broader market for packaging and printing. As one of the three printing methods, gravure printing occupies the main market in plastic film printing due to its printing characteristics, especially the gravure rotary printing machine. In recent years, due to the diversification of printing and the high-speed printing, especially the requirements of the next process equipment for dimensional accuracy and product quality are more and more strict. In order to save materials and improve the yield, higher requirements are put forward for gravure printing technology. Therefore, in plastic film printing, operators must be very familiar with the characteristics of plastic film Mastering the printing skills, especially the control and technical requirements of the press tension, is of great importance to ensure the printing quality of plastic film. Therefore, it is necessary to be familiar with the characteristics and reasonable control of the tension in gravure printing.
Main characteristics of plastic gravure printing
Gravure printing is that the image and text parts are concave, the whole page is immersed in the ink tray, and then the surplus ink is scraped off with a scraper, and the remaining ink in the concave part is transferred to the printed matter. The main feature of gravure printing is that the ink accumulated in the concave part of the layout can transfer enough ink to get hierarchical printed matter. The shade of the printed matter is expressed by the depth of the plate. Therefore, the printed matter has a strong three-dimensional sense, which can clearly print small lines, distinguish layers and vivid images, and can freely and widely select and use various plate materials. In view of this, in plastic film printing, although flexo printing and screen printing also occupy a certain share, but from the present point of view, because gravure printing has been widely used in packaging printing, it has become the main way of plastic film printing, especially gravure rotary printing. To sum up, the main characteristics of plastic gravure printing are as follows: (1) the ink amount of gravure printing is determined by the depth of the concave image and text part of the printing plate. When printing, the depth of the concave image and text part is relatively fixed, so the ink amount is relatively stable; (2) The way of intaglio printing is that the plate cylinder is directly immersed in the ink trough to receive the ink, or the ink is coated on the plate cylinder by the ink transfer roller, which saves many ink transfer rollers and adjustment devices of ink amount; (3) the ink transfer amount of gravure printing is larger than that of offset printing and flexographic printing, so the product has thick ink layer and strong three-dimensional sense; (4) Gravure printing is mostly rotary printing, with high speed and strong ability; (5) gravure printing has a wide range of substrate materials, which is easy to extend and deform such as plastic film and composite materials; (6) gravure printing uses volatile drying, which is suitable for non absorbable plastic film printing.
How to reasonably control the tension of gravure printing
In gravure printing of plastic film, because of the high shrinkage variability of the film, and tension is one of the most important factors to control the shrinkage. In order to ensure the quality of printed matter, it is very important to test and control the tension. Plastic film surface tension (surface energy) is measured by testing the ink available din

ISO 8296, ASTM D 2578-99a method, and dyne test pen. Both DINISO 8296 method and ASTM D 2578-99a are used to brush test ink (such as sofial test ink) on the film to be tested. Dyne pen can be used as a rapid test tool for surface energy after corona treatment, but it is not suitable for system test of printed or coated surface. When the test pen marks a line on the corona treated surface, if it is continuous, it indicates that the surface energy of the material is not less than 38mn / m. if the line is discontinuous, it means that the surface energy of the material is less than 38mn / m, and the treatment is insufficient or even untreated, which does not meet the requirements of printing processing. In gravure rotary printing, the tension of qualified plastic film should be controlled mainly in two parts: from feeding shaft to feeding traction roller and from receiving traction roller to each part after receiving.

(1) Tension control between feeding shaft and feeding traction roller: poor tension control in this part will cause overprint error, that is, the overprint accuracy of multicolor printing is poor. If the position relationship of the printing machine mechanism is correct, the error is caused by the expansion and contraction of the material. The tension is the main factor causing the expansion and contraction of the material. If the elongation is controlled within the allowable range, the tension control is the most effective. Because the traction roller drives the plastic film out from the feed shaft at a certain speed and tension, and then it is sent to the printing section. The tension of this part is greater than that of the printing section. Generally, the brake connected to the feed shaft is used to control the tension. In plastic film printing, with the continuous printing, the film diameter on the feed shaft gradually becomes smaller, so the tension is also reduced. In recent years, due to the high-speed printing and the diversification of printing materials, the tension control is more complex. Different film materials have different shrinkage, so the tension should be controlled according to the materials used. For example, PE, CPP and other films have a large expansion rate, which is easy to deform, so the tension should be relatively small, while pet, OPP and other films have small expansion ratio, so the tension can be correspondingly larger. In addition, the control of tension should be adjusted according to the thickness and internal quality of the film. If the thickness is large, the tension on both sides of the film is not the same and the unevenness is poor, the tension can be increased, otherwise the tension can be reduced. Because the thickness and flatness of the film will make the coil diameter change greatly, the tension should be adjusted according to different materials. The following factors should be paid attention to in the tension control of feeding: tensile change caused by uneven thickness; material slippage and coupling become slower; the coil is not properly loaded, and the material and shaft are eccentric; the circumferential speed change caused by the rotation of the turnover device; the reaction force of the pressing roller and the cutting knife when receiving materials; the roundness of each roller is not enough. Most of the tension changes occur in a short period of time, and all the changes are transmitted to the printing department. From the point of view of mechanical structure, it is necessary to pay attention to the inertia and manufacturing error of the guide roller between the feeding shaft and the traction roller, which are the reasons for the increase of tension, especially the low tension less than several kilogram can not be ignored.
Because different film materials have different elasticity and plasticity, the tension control in printing process should be carried out within the elastic limit according to the law of elasticity. Therefore, it is necessary to understand the elasticity and plasticity of different film materials in order to control the tension accurately. In order to obtain the same elongation in use, the tension can be determined according to the ratio of the width and thickness of the material. Generally, the tension can be adjusted and corrected by adjusting and correcting the pressing roller. If the diameter of the pressing roller is slightly changed and the rotation number of the pressing roller is corrected, the appropriate tension can be obtained. The traditional tension control generally uses manual mechanical brake, which has the characteristics of low price and simple operation, but it is not accurate to grasp the torque value and can not automatically adjust and control the tension. Therefore, it is difficult to print high-quality printing materials and is gradually eliminated by other forms. There are also air brakes, although more used in some printing machinery, but it has the nonlinear torque characteristics and wear and other adverse factors, has been gradually replaced by large capacity magnetic powder brake. Magnetic powder brake uses magnetic powder as friction medium to generate braking force through current. Because of its good linearity of torque current characteristics, it has become a widely used brake. The use of tension automatic control mode, especially the use of tension measurement table data management control tension, to improve the quality of printed matter has a great role.
(2) Tension control from the drawing roller to each part after receiving: for the tension control from the drawing roller to each part after receiving, pay attention to the tension control for the factors such as the tightness, evenness and reliability of the receiving coil.
In order to dry from the plate roller to the receiving traction, the distance is usually increased. Due to the drying and heating, the extension car changes, resulting in material expansion and contraction changes. These are the reasons for inaccurate overprint. Therefore, as with the feeding traction unit, better and more accurate tension control is needed. In addition, the extremely thin and low tension materials are also blown by the wind of the dryer, which is easy to be attached to static electricity, and produce small changes, which must 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 print the material as the final product, to composite, cutting, bag making and other next process. At the same time, attention should be paid to avoid scratching and wrinkling. It is worth noting that in the plastic film materials, attention should be paid to prevent the change of thickness. This is not only the problem of how much tension to use, but also the important thing is to adjust the tension when the winding diameter increases. During the operation of plastic film, the winding tension is required to be stable. With the gradual increase of winding diameter, the winding tension should also be increased correspondingly. At the same time, attention should be paid to the jitter range of the winding, because the winding tension is adjusted by the friction plate on the rolling bearing at the end of the winding shaft. If the shaking range is large, the film shrinks and the tension value increases. Generally, the control tension value can be realized by magnetic powder brake or manual adjustment. In addition, it should be noted that the longer the coil is, the easier it is to stretch, and the tension value should be changed accordingly. Constant and stable tension is very important. Compared with the feeding part, the receiving part should pay attention to the taper tension and inertia compensation error.

The so-called taper tension refers to the fact that the moment relative to the inner material increases with the increase of the coil when the constant tension is used for coiling and receiving, resulting in slippage, that is, coiling shrinkage. In addition, due to the shrinkage of the material and the increase of pressure on the center due to the release of air, the material is squeezed out or squeezed out laterally, resulting in the so-called bamboo shoot phenomenon. Crepe is produced near the coil core to make the surface uneven. In order 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 final tension. The degree of reduction is called taper. Generally, a taper of 10% - 50% is used. For example, 30% taper is used; that is, starting with 10kg tension, then ending with 7kg tension. At this time, the change ratio is different according to the performance of materials and mechanisms. Manufacturers generally use tests to make various types of test curves for comparison and adjustment in production. The so-called inertia compensation is to absorb the tension change caused by the inertia of the coil when the speed changes or when the static speed reaches the predetermined speed during the machine starting and operation. 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 use of tension control abroad has been very mature, there is a set of good technical methods, the use is very common. However, in China, the application of advanced tension control device in printing machine industry is not widespread. With the development of reform and opening up situation, all kinds of printing machinery and packaging machinery will adopt tension control method to improve product quality.
Troubleshooting related to tension
The tension of plastic film directly affects the printing quality. If it is not controlled properly, it will bring quality problems to printed products and even cause waste products. In summary, the tension affects the quality of printed matter and causes the following failures.
(1) The overprint is not accurate. Overprint quality is the most intuitive index of printed matter. There are many factors affecting the inaccurate overprint of plastic film, including the objective factors such as mechanical equipment, ink suitability, environmental temperature, etc., and the subjective factors such as the quality awareness of operators. That is to say, in addition to the film material thickness uniformity is not good; the imprint roller deformation; plate roller design is unreasonable, processing accuracy is poor; printing speed change; drying temperature is too high; guide roller sticking and other factors, the change of printing tension is also one of the main factors. We should pay attention to observe and analyze the phenomenon of overprint inaccuracy and take corresponding measures. If the longitudinal (movement direction) register of plastic film is not accurate, it may be caused by the change of tension during film traction, which may be caused by the change of longitudinal printing registration. The reasons are: improper adjustment of drum brake of Uncoiler device; excessive drum material or too large diameter of cylinder material; improper setting of three roller speed; decrease of surface friction force of three rollers; improper setting of roller speed; When the tension of rewinder changes, it is necessary to check and adjust the control devices to ensure the tension balance and stability during traction. The main reason for the shrinkage and displacement of the longitudinal pattern size during printing is that the shrinkage allowance is insufficient. Due to the influence of plastic film tension and heating and drying, the longitudinal size of the film is larger than that of the printed picture and text, so that the image and text can not be printed in a certain position during printing. The solution is to keep appropriate shrinkage size in plate making and fully reduce film tension. In a word, overprint is closely related to tension. Therefore, attention should be paid to the change of tension in printing, and the tension should be adjusted and controlled to ensure the stability of tension in the whole printing process. In order to avoid or reduce the loss caused by overprint, we should not only ensure that the tension between unwinding and rewinding of substrate is consistent, but also pay attention to the trend of overprint inaccuracy, and then take corresponding solutions.
(2) Poor fastness of printing ink. The reason of poor printing fastness of plastic film is that the affinity between the printing ink and the material to be printed is not good, and the ratio of mixed solvent is not up to the requirements; If the printing environment temperature and relative humidity can not meet the requirements and produce static electricity, the room temperature of the workshop should be controlled at 18 ~ 23 ℃ and the relative humidity should be controlled between 50% ~ 60% to prevent the influence of static electricity and keep the environment clean. Besides, the main reason is that the surface tension of plastic film can not meet the requirements, including poor printability of the film itself and poor corona treatment The surface tension can not meet the printing requirements. Film surface tension and surface wet force are very clear standards for accurately measuring the adhesion of printing ink and other materials on the surface. According to practical experience, the surface tension of plastic film can reach the viscosity requirement when the surface tension is controlled at 39 ~ 42mn / m, while many white pages (no printing content) will be produced when the surface tension is below 37Mn / m, and the viscosity is not good when it is below 35MN / m After corona treatment, the surface tension of plastic film is tested to see if it meets the requirements. If the surface tension is still unqualified after treatment, the film material should be replaced. In addition, attention should be paid to the adjustment and control of tension in printing to ensure its stability in line with the printing requirements.
(3) Adhesion and back rub dirty. The main performance is that the printing surface adheres to the back of the other layer between the upper and lower layers of the winding film, so that the ink on the printing surface is transferred to the back of the other layer. This fault may be caused by excessive winding tension. Attention should be paid to adjusting and reducing the winding tension. In addition, it may be caused by the slow drying speed of the ink and the residual solvent. Therefore, it is necessary to adjust the control drying device, confirm the temperature of the dryer and other control factors, add anti fouling agent in the ink or use the appropriate ink formula to improve the drying speed of the ink, pay attention to the suitability and dosage of the solvent, or use the appropriate fast drying solvent.
(4) The winding is misplaced. That is, the film winding has a lateral deviation from the center of the press. This fault may be caused by the failure of the horizontal paper winding alignment device. Check the device first. In addition, in addition to the uneven thickness of the material, the material should be replaced; if the film is excessively heated, the temperature of the winding paper should be reduced; if the center of the printing machine and the roller are not found correctly, the alignment of the center of the printing machine and each drum should be checked. There are more ink, plastic scraps or foreign matters adhered on the drum. Besides, these dirt and foreign matters adhered to the drum should be removed, and the winding force of the winding paper is too high Low is also the main cause of failure, so pay attention to adjust and control the winding tension.
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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