Plastic film has become more and more widely used in the packaging industry. From foods to the packaging of daily commodities, as well as the soft packaging of milk and beverages, etc., are inseparable from the plastic film. This opened up a broader market for packaging printing. Gravure printing, which is one of the three major printing methods, occupies a major market in plastic film printing due to its printing characteristics, especially gravure web press printing. In recent years, due to the diversification of printing and the high speed of printing, especially the requirements for dimensional accuracy of the next process equipment have become higher and higher, the requirements for product quality have become more and more stringent, and in order to save materials and improve the yield, etc. The gravure printing technology put forward higher requirements. Therefore, in the plastic film printing, the operator must be very familiar with the characteristics of the plastic film, fully grasp the printing techniques, in particular to master the control and technical requirements of the press tension, to ensure that the plastic film The printing quality is of great importance. Therefore, it is necessary to be familiar with the characteristics and reasonable control of the tension in gravure printing.

The main features of plastic gravure printing

Gravure printing is a graphic part of the concave, the layout completely immersed in the ink tray, and then use the scraper to scrape off the excess ink, the ink remaining in the concave part of the transfer to the printed matter. The main feature of gravure printing is that the ink accumulated in the concave part of the layout can transfer a sufficient amount of ink to obtain a layered printed matter. The shade of the printed matter is expressed by the depth of the printing plate, so the printed material has a strong three-dimensional effect and can be printed clearly and finely. The lines, layers, and images are realistic and can be freely selected and used. In view of this, in the printing of plastic films, although flexo printing and screen printing also occupy a certain share, from the current point of view, because gravure printing has been widely used in packaging printing, it has become the main printing of plastic films. Way, especially gravure rotary printing. To sum up, plastic gravure printing mainly has the following characteristics: (1) The amount of gravure printing ink is determined by the depth of the recessed portion of the printing plate graphic part. The depth of the graphic part of the printing is concave when the printing is relatively fixed, therefore, the ink The amount is also relatively stable; (2) Inking method for gravure printing is that the plate cylinder is directly immersed in the ink tank to accept the ink, or the ink transfer roller applies the ink to the plate cylinder, eliminating many transfer rollers. And ink volume adjustment device; (3) Gravure printing ink transfer volume than offset printing and flexographic printing, so the product graphic thick ink layer, strong three-dimensional sense; (4) gravure printing is mostly rotary printing, fast, Strong ability; (5) Wide range of printing materials for gravure printing, good adaptability for plastic film and composite materials, etc. which are easily deformed; (6) Volatile drying for gravure printing, suitable for non-absorbable plastic film printing .

How to reasonably control the tension of gravure printing

In the plastic film gravure printing, because the shrinkage variability of the film is large, and tension is one of the most important factors affecting the control of the shrinkage, it is very important to ensure the quality of the printed matter and test and regulate the tension. The surface tension of plastic film (surface energy) is determined by testing the ink using DIN ISO 8296, ASTM D 2578-99a, and dyne test pens. Both DINISO 8296 and ASTM D 2578-99a apply a test ink (eg, Sofial test ink) to the film to be tested. Dyne test pens can be used as a quick test tool for measuring the surface energy after corona treatment, but not Suitable for system testing of printed or coated surfaces. When the test pen draws a line on the corona-treated surface, if it is continuously lined, it means that the surface energy of the material is not less than 38mN/m, such as intermittently not connected, indicating that the surface energy of the material is less than 38mN/ m. Insufficient or even untreated, not meeting the printing process requirements. Qualified plastic film should mainly control the tension of the two parts in the gravure printing, ie from the feed shaft to the feed traction roller and from the take-up haul rollers to the receiving parts.

(1) Tension control from the feed shaft to the feed drawing roller: This part of the tension control is not good enough to cause overprinting errors, that is, overprinting accuracy between colors of multi-color printing. If the position relationship of the printing press mechanism is correct, the error is caused by the expansion and contraction of the material. Tension is the main factor that causes the expansion and contraction of the material. To control a certain amount of elongation within the allowable range, the tension control is the most effective. Since the pulling roller drives the plastic film at a certain speed and tension from the feed shaft and feeds it into the printing section, the tension in this part is greater than the tension in the printing section. Generally, the tension is controlled by the brake attached to the feed shaft. . In plastic film printing, as printing continues, the diameter of the film roll on the feed shaft gradually becomes smaller, so the tension is also reduced accordingly. In recent years, due to the high speed of printing and the diversification of printing materials, the control of tension has become more complicated. Different film materials have large differences in shrinkage, so the tension should be controlled according to the materials used. For example, PE, CPP and other films have large expansion and contraction rates, and they are easily deformed. Therefore, the tension should be relatively small, while the PET, OPP, and other expansion ratios are small and the tension can be correspondingly larger. In addition, the tension control must be adjusted according to the thickness and inherent quality of the film. Thickness, uneven film on both sides of the film, and unevenness of the film, the tension can be greater, on the contrary can reduce its tension. Because the thickness and flatness of the film will change the diameter of the roll, it is necessary to properly adjust the tension according to different materials. Tension control of the feed should pay attention to the following factors: Stretching changes due to uneven thickness; slippage of the material and even being caught slow; the roll is not installed, the material and the shaft are eccentric; the turning device generates the rotation in the week Change in speed; the reaction force of the press roller and cutter when picking; the roundness of each roller is not enough. Most of the tension changes occur in a short time, and all changes are transmitted to the printing department. From the mechanical structure point of view, it must be noted that the inertia of the guide roller between the feed shaft and the traction roller and manufacturing errors, which are the causes of the increase in tension, especially the low tension of several kilograms or less must not be ignored.

Due to the different elasticity and plasticity of different film materials, the control tension during the printing process must be performed 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 accurately control the tension. 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. Generally adjusting the tension can be used to adjust and correct the pressure roller method, such as a slight change in the diameter of the pressure roller, the number of rotations to correct the pressure roller, etc., can get the right tension. The traditional tension control generally uses a manual mechanical brake, which has the characteristics of low price and simple operation. However, grasping the torque value is inaccurate and can not automatically adjust the control tension. Therefore, it is difficult to print high-quality printed products and gradually be eliminated by other forms. There are also pneumatic brakes. Although they are used in some printing machines, they have non-linear torque characteristics, large wear and other undesirable factors, and they have gradually been replaced by large-capacity magnetic powder brakes. Magnetic powder brakes use magnetic powder as a friction medium to generate braking force through current, and because of their good linearity of torque-current characteristics, they have become widely used brakes. The use of automatic tension control methods, in particular the use of tension gauges with data management to control tension, has a great effect on improving the quality of prints.

(2) Control the tension of each part after receiving the pulling roller to receive the material: For each part from the receiving pulling roller to the receiving material, the tension of the receiving material should be controlled, such as tightness, neatness and reliability.

Between the roller and the receiving traction, usually the distance is increased for drying. Due to the dry heating, the extension car changes, causing the material to expand and contract. These are the reasons for overprinting. Therefore, as with the feeding traction part, a better and accurate tension control is required. In addition, extremely thin and low tension materials are also susceptible to electrostatic adhesion due to the wind blown by the dryer, resulting in minor changes that 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 film materials, care should be taken to prevent thickness variations. This is not only the problem of how much tension is to be taken up, but the important thing is to adjust the tension while the roll diameter is increased. During the operation of the plastic film, the tension of the winding is required to be stable, and the diameter of the winding is gradually increased. The roll tension should also increase accordingly. 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 winding shaft end. If the shaking range is large, the film shrinks and the tension value increases. The general control tension value can be achieved by magnetic brake or manual adjustment. Another thing to note is that the longer the roll, the easier it is to stretch, and the tension value should be changed accordingly. A constant and stable tension is very important. Compared to the receiving section and the receiving section, the receiving section should pay attention to taper tension and inertia compensation error.

The so-called taper tension, that is, under normal circumstances when using constant tension to take up the material, with the increase of the coil relative to the material inside the moment becomes larger, resulting in slipping that is the take-up shrinkage. There is also the so-called bamboo shoot phenomenon due to the shrinkage of the material and the pressure of the air released toward the center to increase the material to be squeezed or to be laterally extruded. 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 larger, that is, taper tension is used. The initial tension at the winding determines the tension at the end of the take-up. The degree of reduction is called taper. Generally use 10% to 50% taper. For example, with a 30% taper; that is, starting with a tension of 10 kg, the tension from 7 kg ends, and the rate of change at this time is different depending on the properties of the material and the mechanism. Manufacturers generally use the test to make various types of test curves for production. Adjust it. The so-called inertia compensation is to compensate for the change in tension caused by the inertia of the roll when the speed of the machine is changed or when the speed is changed from the standstill to the predetermined speed during the start-up and operation of the machine. 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 use of tension control in foreign countries is very mature. It has a good set of technical methods and is used very commonly. However, in China, the application of advanced tension control devices in the printing press industry 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.

Exclusion of tension-related faults

The tension of the plastic film directly affects the printing quality. If it is not properly controlled, it will bring quality problems to the printed matter and even result in waste products. To sum up, tension affects the quality of prints and the resulting failures are mainly the following points.

(1) Overprinting is not allowed. Overprint quality is the most intuitive index of printed matter. The influencing factors of overprinting of plastic film are many, including the impact of objective factors such as mechanical equipment, ink suitability, and ambient temperature, as well as subjective factors such as operator's quality consciousness. That is, in addition to the film thickness uniformity of the film material is not good; impression pressure roller deformation; layout roller design is not reasonable, the processing accuracy is poor; printing speed changes; drying temperature is too high; guide roller sticky and other factors, the printing process changes in tension during operation It is also the main influence factor. Pay attention to observe and analyze the phenomenon of overprinting and take appropriate measures. If the longitudinal direction of the plastic film (motion direction) is misaligned, it may be caused by a change in the longitudinal printing register caused by tension fluctuations in the film pulling. The reasons are: improper modulation of the reel brake of the unwinding device; excessive reeling material, Or the diameter of the material is too large; three roller speeds are set improperly; the surface friction of the three rollers is reduced; the tension of the rewinding machine changes, etc. At this time, check and adjust the control devices to ensure that the tension is stable and stable when pulling. The shrinkage displacement of the longitudinal pattern during printing is mainly due to insufficient contraction margin. Due to the influence of the tension of the plastic film and heat drying during printing, the longitudinal dimension of the film is larger than the shrinkage of the printed graphic, thereby making the image when printed. Can not print