(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. The text cannot be printed in a definite position. The solution is to make proper size shrinkage when making plate; fully reduce the film tension. In short, overprinting is not allowed to have a very close relationship with tension. Therefore, we must always pay attention to changes in tension during printing, adjust and control the tension, and ensure the stability of the entire printing tension. In order to avoid or reduce the losses caused by overprinting, it is necessary to ensure that the unwinding and winding tension of the substrate are the same, and the direction of overprinting should be taken into account, and corresponding solutions should be taken.

(2) Printing ink fastness is bad. Reasons for poor printing fastness of plastic film are: In addition to the poor affinity of the printed ink and the printed material, the mixed solvent ratio does not meet the requirements, the ink should be exchanged and the solvent ratio adjusted; the printing environment temperature and relative humidity cannot meet the requirements. Static electricity, should try to control the workshop room temperature at 18 ~ 23 °C, relative humidity controlled at 50% ~ 60% to prevent static electricity, and keep the environment clean, air circulation, the main reason is that the surface tension of the plastic film is not The requirements, including the poor printability of the film itself, and bad corona treatment, so that the surface tension can not meet the printing requirements. The surface tension of the film and the surface wet force are very clear standards for accurate determination of the adhesion of printing inks and other materials on the surface. According to practical experience, the surface tension of the plastic film can be controlled at 39 to 42 mN/m to meet the viscosity requirements, and the surface When the tension is below 37mN/m, it will cause many white pages (no printing content). The viscosity is not good below 35mN/m. At this time, it is necessary to measure the surface tension of the plastic film after corona treatment. If it is still unsatisfactory after processing, it is necessary to replace the film material. In addition, attention should be paid to adjusting the tension during printing to ensure that it is stable within the scope of the printing requirements.

(3) Adhesions and back stains. This is mainly manifested in that the printing surface between two upper and lower films that are wound up is adhered to the back surface of another layer, and the ink on the printing surface is transferred to the other layer. The possible cause of this failure is caused by excessive winding tension. Attention should be paid to adjusting the winding tension. In addition, it may be due to the ink drying speed is too slow and there are solvent residues, etc., you should adjust the control drying device, confirm the temperature of the dryer and other control factors, add anti-smelling agent to the ink or switch to the appropriate ink formula , Increase the drying speed of the ink, pay attention to the applicability and amount of the solvent, or use a suitable quick-drying solvent.

(4) Roll misplaced. That is, film winding has a lateral deviation from the center of the printing press. This failure may be due to a malfunction of the transverse roll registration device. The device may be checked first. In addition, caused by this failure in addition to uneven material thickness should be replaced materials, excessive heating of the film should reduce the temperature of the paper roll, the center of the press and the drum is not allowed to find the center of the printing press and the drum with the same alignment, the drum sticky With more ink, plastic scraps or foreign objects to remove the dirt and foreign matter adhered on the roller, the low roll-up force of the rolled paper is also a major factor causing the malfunction, so pay attention to adjust the control winding tension.

(Author/Qi Cheng Wu Yanfen)

Source: China Packaging News

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