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Application of flexible packaging gravure printing equipment

according to incomplete statistics of insiders, the output value of flexible packaging gravure printing industry is about 1/5 of the total output value of the printing industry, second only to offset printing, and the development prospect is uncertain. Gravure printing will still become a hot investment. The printing industry is an ancient and dynamic industry. In the development process of the printing industry in China and even the world, printing has always been closely related to the progress of human social civilization and the inheritance of human material life and culture, and gradually moves towards a very important role. With the rapid development of modern science and technology, modern printing is at its peak, and the printing industry is booming. Gravure printing, as the elite of the printing industry, has attracted worldwide attention after decades of development, and has become an indispensable part of the national economy and human life

China's flexible packaging gravure printing industry should be said to have developed vigorously since the 1990s. Its development process can be described as overgrown with weeds when flowers are in full bloom, and the growth process is winding and yet firm. The development of China's flexible packaging gravure printing industry today has a very obvious division of development regionality and scale hierarchy. The more representative packaging printing bases are Beijing Tianjin, Yangtze River Delta, Pearl River Delta and so on. These enterprises have relatively advanced equipment, strong technical and economic strength, high product quality and occupy a large market share of soft bags. These conditions have enabled them to develop rapidly in recent years, and the investment scale and production scale are growing day by day. At the same time, it is these enterprises that constantly guide China's flexible packaging printing industry to mature, and promote the standardization and standardization of the development of the industry

however, as a manufacturer of flexible packaging equipment, with the development and maturity of flexible packaging printing industry, it is gradually facing new tasks and challenges. Although China's flexible packaging equipment has made a lot of progress after years of development, and even some manufacturers have made equipment that can make foreign manufacturers feel pressure, this does not mean that China's flexible packaging equipment has been able to meet the basic needs of the domestic flexible packaging printing industry. There are many domestic manufacturers of flexible packaging equipment, including Shaanxi made by beiren in Shaanxi, Zhejiang made by Xinda in Ningbo and Guangdong made by Songde. Among the numerous domestic equipment suppliers, each enterprise has different business philosophy, different purposes, and great differences in production capacity and design technology level. Manufacturers of flexible packaging equipment need continuous guidance and support from flexible packaging printing enterprises in order to achieve rapid, sustainable and healthy development

however, many manufacturers do not have the ability to manufacture high-performance equipment at all, but these manufacturers push their shoddy equipment to the flexible packaging printing enterprises at low prices, resulting in low-grade products, slow development of enterprises, and difficult operation, resulting in a double waste of social wealth and investment capital. On the other hand, this behavior caused high-performance flexible packaging printing equipment manufacturers and flexible packaging printing enterprises with an annual output of 3.6 million tons of plastic packaging materials in China to be in an awkward position and step by step to give way. As a result, the whole industry was in a chaotic competition situation, and the industry would stagnate. Some flexible packaging printing equipment manufacturers and foreign equipment agents are committed to guiding the market to advance development and promoting the progress of the industry. At present, some flexible packaging printing equipment manufacturers have invested a lot of money to develop new products, such as shaftless gravure printing machines. This is originally a measure conducive to the technological progress of the industry, but these enterprises are eager to bring their products to the market, often without considering their own defects in equipment design and manufacturing and technical problems that have not yet been overcome. With half the effort, the equipment demand market retreats from domestic equipment and buys foreign equipment, resulting in the waste and loss of limited capital in the industry

with the development of the industry, the performance requirements of flexible packaging printing enterprises for equipment will become mature. What they need is equipment that produces high-quality products and has high cost performance, such as medical rubber glove samples, rather than equipment with high production costs or equipment investment under the same production capacity. Then manufacturers of flexible packaging printing equipment should consider whether their equipment technology is too backward, or the redundancy cost of a single machine is too high, or there is excessive configuration. And a good flexible packaging printing equipment, in addition to producing high-quality flexible packaging products, should also pay attention to energy consumption and environmental pollution in the production process. In view of the above statement, combined with the current problems of ink drying and product solvent residue commonly existing in the flexible packaging industry, this paper analyzes, in order to provide some enlightenment for equipment purchase or design and manufacturing, in order to make progress

many upholstered products may have encountered problems such as the ink cannot be dried, and the solvent residue of the product exceeds the standard. For example, when the solvent residue of some soft packaging products is required to be less than 5mg/m2, or even 3mg/m2, some machines will fail to open high-speed printing or there are many types of unqualified products. The content of volatile solvent in gravure printing ink is about 58%, and about 6g/m2 of solvent needs to be volatilized in the ordinary printing process,. If the machine is printing at the speed of 120m/min, the printing surface width is 1000mm. According to this calculation, the volatile solvent required per second is about 12g. If the solvent is 13 # resin solution, 14% xylene and 13% isopropanol, the power required to completely volatilize the solvent is not more than 9 4kw (including the energy consumed by the temperature rise part of ink and film). Then the heating power of the printing press we actually use is often greater than 9 Why does 4kw still have problems such as unable to dry ink or excessive solvent residue. To illustrate this problem, we describe the drying process of gravure printing machine as follows:

Room temperature air is first drawn into the hot air circulation system for heating, and then enters the fan for pressurization and is pumped into the oven. In the oven, the hot air is evenly diffused and ejected at a high speed from the nozzle or orifice. Suppose that at this time, the newly printed film enters and passes in front of the nozzle at a speed of 2m/s. at this time, the film and the ink are heated at the same time, and the solvent begins to volatilize. The total heating time of printing in the oven is: effective feeding length/printing speed in the oven. In this process, when the high-speed hot air meets the wet material film, it will exchange heat and moisture with the ink surface, that is, the ink gets heat and loses solvent, and the air loses heat and mixes with solvent vapor, There are sensible heat exchange and latent heat exchange in this process (sensible heat refers to the heat obtained or lost only due to temperature change in the absence of phase transition, and latent heat refers to the heat obtained or lost during phase transition, such as liquid gas phase transition). After the ink comes into contact with the drying air, a saturated solvent air layer with a temperature equal to that of the ink surface will be formed close to the surface of the ink layer, and the saturation concentration of the solvent in this layer of air depends on the temperature of this layer of air: generally, the temperature of this layer of air will be lower than that of the drying air, while the volatile solvent in the air far away from the ink layer has not reached saturation, In other words, there is a concentration gradient difference of volatile solvent in the direction perpendicular to the film. This gradient difference is the driving force of solvent volatilization, that is, the driving force of moisture exchange. At the same time of moisture exchange, heat exchange will occur, that is, air will transfer heat to the ink layer, and the solvent in the ink layer volatilizes and then brings heat back to the air. Of course, the heat brought back to the air by volatile solvent is latent heat, which will not raise the air temperature, but it will lower the temperature of the ink layer

in the above process, we described the solvent evaporation and drying process in the ink, and also noticed two kinds of exchange: wet exchange and heat exchange

① in the wet exchange, it occurs due to the concentration gradient difference

② in the heat exchange, it occurs due to the temperature difference

③ in the wet heat exchange, there is latent heat energy exchange at the same time

④ the saturation concentration of the solvent depends on the temperature of the mixture and the air

so in order to complete this process, we should carry out the following work:

① we should constantly reduce the content of volatile solvent in the dry air, That is, it needs to exhaust

② it is necessary to increase the temperature of the air on the surface of the ink to increase the saturation concentration of the solvent, that is, the heat is transferred to the ink layer

in addition, there is a very key factor affecting the moisture and heat exchange, namely, the drying wind speed v. In the later stage, with the effective solution of Wharton's capacity bottleneck, the drying wind speed V has the same direction change relationship with the heat exchange coefficient and moisture exchange coefficient. That is to say, when we increase the drying wind speed, we also improve the drying capacity. In addition, there is another point in the ink drying process: when the solvent concentration in the air is close to saturation, we can increase the pressure difference between the ink surface and the solvent components in the mixed air by increasing the air temperature to promote the ink drying. The difference between the air temperature and the ink surface temperature is also in direct proportion to the evaporation rate of the solvent, which is why we can appropriately increase the drying temperature when we feel that the drying capacity is insufficient, but this temperature should not be too high

after we have mastered the above points, as the manufacturer of gravure printing machine, we must reasonably design the drying system to achieve these requirements, that is, design and calculate the wind speed, air volume, air temperature, oven feeding length, exhaust, heat exchange efficiency, air duct oven structure, air outlet uniformity, and pipe resistance. There are countless manufacturers of gravure printing machines in China, and only a few manufacturers really calculate and optimize the design. Most of them only make a rough estimate, which leads to many deficiencies and waste in printing production. So how should the drying system of a machine meet the requirements to be more qualified. Through many tests of actual working conditions, the machine with a width of 120m/min and 1000mm is summarized as follows:

① the wind speed reaches 35m/s-45m/s, and the air volume is 2500-3000 (m3/h)

② the air pressure in the static pressure chamber in the oven reaches more than 700pa

③ the total pressure of the fan reaches 1900pa-2200pa, and the static pressure reaches 1400pa-1700pa

④ the total heating power of the monochrome group drying is about 15kw, The maximum is no more than 24kw

⑤ the return air pressure of the oven is no more than - 20Pa

the above points are only some superficial parameters. In practice, it is not possible to meet the drying requirements by reaching the above parameters, which has a great relationship with the pipe matching of the hot air system, the structure of the oven and the exhaust system. For example, the static pressure chamber design structure of the oven, whether the air inlet is smooth, whether the nozzle design is reasonable, and whether the exhaust system can exhaust effectively

through the above description, now we can understand why the heating power of the usually used flexible packaging equipment (such as gravure printing press, compound machine, etc.) is greater than the power required for the complete drying of ink. In fact, this is because of the air volume, wind speed and air temperature used by the drying hot air system. For example, under the same working condition, if we want to heat the dry air to the temperature we set, such as 60 ℃, when the drying wind speed is different, the heat conversion efficiency of the system is different, the total circulating air volume is different, and the power consumed by the heated air is different. Of course, the ink drying efficiency is also different, but the power consumed by the complete drying of the ink layer is unchanged. Therefore, an optimized hot air system will consume less energy on the premise of achieving the ideal drying effect in actual production

in addition to the above statements, I must also specifically refer to

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