Fourth, the gravure drying characteristics Gravure due to ink thicker, so the problem of dryness is always there. The printing feature of the gravure is wet and wet. Although there is a long drying path, the ink of the former color group is not dried when the gravure is printed in the gravure, but only a layer of film is formed on the surface of the ink layer to prevent the latter color. The group of inks sticks the previous color ink when they are stacked.
Gravure products are generally not dry for the following reasons.
(1) The gravure network has a deep hole, sometimes reaching a depth of 60 μm, and the amount of ink is large, and drying may be difficult.
(2) In order to ensure the transfer of high-gloss small dots, a slow drying agent must be added to the gravure printing process, which also affects the drying effect of the ink.
(3) The overall balance of the mixed solvent is not easy to grasp, resulting in a large amount of slow drying agent.
(4) In some printing processes, the last color is printed in white ink, and then the rewinding is performed. The distance between the drying and rewinding stations is too short and drying is difficult. And the white ink bottom requires the ink to have a good hiding power, and can only thicken the ink layer. In general, white ink is the largest of all colors.
(5) When the film is printed, the temperature of the drying device cannot be too high, and the drying tunnel cannot be too long, otherwise the film will be stretched and deformed, affecting the overlay accuracy and product length. The commonly used method is the use of low temperature, large air volume, the use of volatile methods in the drying process, abandoned the evaporation.
(6) Solvent volatilization is dependent on the environmental conditions. When the ambient air pressure is relatively low, it is difficult to effectively control the drying performance with a mechanical exhaust device.
(7) There is a boundary layer between the ink layer on the surface of the printed matter and the atmosphere (the foreign counterpart is called "Boundary layer", and some people translate it as "mirror film"), which is a layer containing hot air and Mixture of solvent vapors, high temperature. When the ink is dried at a high temperature, the liquid solvent therein evaporates and moves to the boundary layer. If there is no solvent in the boundary layer, the solvent vapor can be easily transferred to the atmosphere. However, as the concentration of solvent vapor in the boundary layer continues to increase, the evaporation rate of the solvent will decrease, and when the solvent vapor in the boundary layer is saturated, the evaporation process will stop. The traditional gravure drying process does not pay attention to the existence of the boundary layer, and neglects the role of evaporation in drying. Although the industry has made some improvements to the curve of the drying path of the gravure printing machine, the wind resistance is reduced and the air volume is increased, but due to this layer Without the presence of a boundary layer of air flow, some of the remaining solvent in the ink is still entrapped in it.
At present, the commonly used method of gravure printing to solve the problem of poor drying is:
1 Add fast-drying solvent to the ink;
2 Slow down the printing speed to extend the time of the prints in the drying tunnel;
3 Improve the drying curve of the gravure printing machine, reduce the wind resistance, increase the air volume, and even increase the length of the drying tunnel.
Due to the fact that the previous industry did not pay much attention to the drying process of the gravure printing process, some theoretical understanding of the drying process was insufficient, such as understanding of the boundary layer. Many gravure printing machine drying machines return hot air containing solvent vapors directly to the drying tunnel for the second time, which is called hot air circulation. It is not known that this hot air containing a large amount of solvent components inhibits evaporation of the solvent. Therefore, for the new standards proposed by the State in terms of solvent residue control, gravure printing cannot be absolutely controlled under all climatic conditions. In terms of packaging and printing of foods and medicines that are related to people's health, this will be a top priority. Damocles sword.
The gravure process has a bottleneck in the improvement of the drying device. Since the main printing material used in the gravure printing flexible packaging industry is plastic film
In order to prevent the film from affecting registration due to thermal tensile deformation, the temperature setting of the gravure dryer drying device is not very high, and the focus is on increasing the air volume and increasing the wind speed. However, increasing the air volume needs to increase the diameter of the air supply spout. According to the basic principles of fluid mechanics, only the air outlet is large enough to have a large amount of air intake, but the caliber is amplified and the air pressure naturally decreases, and the wind speed cannot be improved. Therefore, improvement of the air volume can only be achieved by improving the resistance of the drying tunnel, using a V-shaped or porous plate nozzle, or adjusting the angle between the outlet direction of the nozzle and the forward direction of the printed material. The results of the above improvements in practical applications are not ideal. Theoretically, the calculated wind speed can reach 40-50 m/s, but even the gravure printer manufacturer's own measurement is only 20 m/s or more. In contrast, the United States patented PCMC's compressed air drying method has not been deliberately improved on the drying tunnel curve, and its nozzle wind speed can reach 330 meters/second. It is arranged on a gravure press and the maximum printing speed (not the mechanical speed) can reach 600 meters/minute.
When the gravure printing machine's drying device can not reach the ideal requirements, the most common method is to lengthen the drying path length or reduce the printing speed, so that the printing material stays in the drying tunnel for a long time. The biggest drawback of this method is to affect the registration. Accuracy and printed reprint length.
In addition, the energy consumption of a gravure drying device is also a problem. Although many gravure printing machines can use four kinds of heating methods such as electricity, steam, hot oil and natural gas for users to choose, in order to achieve the purpose of saving energy costs. Apart from the fact that individual electric heating devices can adjust the heating power according to the need of printing different layouts, most of the heating devices use centralized heating and separation of color groups. Even if some color groups do not require heat or do not need much heat, the total energy is The loss is still high. In general, the ratio of the driving power of the gravure printing machine to the heating power is basically 1:1, and some of them are even upside down.
Fifth, the relationship between the characteristics of the gravure and the finished product rate
The above-mentioned characteristics of gravure printing are related to the high loss in the production process.
Here are some examples. A listed gravure printed flexible packaging company with sales of more than 400 million yuan in 2007 has classified the loss of OPP labels produced by it, excluding the sideline during printing, with a loss of 5.4%. This type of label product is printed on the pearl film and can be shipped after being cut into rolls. Therefore, the loss of 5.4% is mainly due to the change of the roll loss and the print quality problems such as misregistration and knife printing. . Let's look at the loss data of another gravure flexible packaging company with an annual sales of nearly 60 million yuan: the loss requirement for the printing process (8 to 10 colors) is 6%, the compounding process is 1.2%, the slitting process is 0.3%, and the bag making process The process is 2%. This is an indicator issued by the company's management. In fact, the loss of the printing process can not fully meet the requirements. Although there are differences in the management of gravure printing companies, the quality loss in the printing process is roughly the same (without side-strips). After more than 10 years of research, the loss of 6% to 7% of the gravure printing process seems to have dropped to the extreme, and it is difficult to reduce it. There is a bottleneck in the improvement of gravure yield. On the other hand, with the use of a satellite flexo printer to print films, according to the author's understanding, a flexo printing company that does not have any special management features only 1.5% of the flexo printing process. The data shows that the characteristics of the gravure printing process itself are related to its defect rate of finished products.
In order to increase the production rate, gravure printing companies have indeed done their utmost efforts, such as widening the printing width, the author once visited a gravure printing company, and the widths of 4 to 5 gravure printing machines are all at 1600 mm, in order to save money. Edge wire loss to improve yield. In fact, the wider the width of the gravure web, the harder it is to control the width of the printing material at both ends, and the greater the registration error during printing. Although the gravure printing companies continuously carry out equipment updates and strengthen training, the improvement in the yield rate is not effective. This is where the high raw material prices are today, it is an insurmountable obstacle to the operators of gravure printing companies.
The above-mentioned quality problems of gravure printing are related to the defects inherent in the printing process, but unfortunately, many of the peers have not realized this point. For such large losses in the printing process, they are often blamed on the frontline operators. Skills are not good or the sense of responsibility is not strong. When formulating the printing process, the management department was unable to elaborate and only hoped for the on-site processing capability of the first-line gravure captain. However, due to the limitations of the knowledge level, the front line operators could only rely on some incomplete experience in the past. Therefore, the embarrassing situation of current control of gravure printing has been formed: the above gives up leadership, and there is nothing to follow. The flexible packaging industry generally achieves a yield of less than 90%, and some flexible packaging companies have even lower yields.
Since the existence of gravure printing quality problems has its inevitability, a thorough solution to the problem is only to learn from other processes and improve or modify the gravure printing process itself. Of course, the gravure printing process is more mature than the flexographic printing process, but the yield of the flexographic printing process is obviously 3 to 5 percentage points higher than that of the gravure printing process. Should gravure printing not learn useful experience from flexographic printing? ? As an example, the author once saw a sample print, which was originally a proof sheet of a sheetfed offset printing. Because ink saturation is not enough, the contrast between highlights and shadows cannot be opened. Foreign counterparts eventually use flexo printing instead of offset printing. The same 175 lines/inch screen was used, and the highlights area adopted the latest flexo printing technology and was approved by advertising users. The offset printing industry dares to use flexo printing to improve its own shortcomings. Can't we give some inspiration to our gravure counterparts?
ZHONGSHAN GRUWILL HARDWARE CO.,LTD , https://www.gwwirebasket.com