The gravure printing press occupies an extremely important position in the printing and packaging industry, and the design of the drying device is one of the core technologies of the gravure printing technology. The drying effect of the drying device is the main factor that restricts the speed and printing quality of the gravure printing machine, and is the key to affecting the performance of the whole machine. It is also a technical bottleneck that the manufacturers of gravure printing machine equipment need to break through.
The drying device of the gravure printing machine is mainly composed of a hot air system and a drying box. The fan and the heater in the hot air system are the power part of the energy source. The fan provides the kinetic energy required for the air flow and the heater needs the air to reach the specified temperature. The heat and air pass through the fan and the heater successively, have a certain wind speed and the temperature required for drying the printed products, and are transferred to the drying box through the pipeline of the hot air system. The drying box receives the hot air input from the hot air system, and blows the hot air to the surface of the film at a high speed through the air nozzle to complete the drying of the printed products. The dried exhaust gas is discharged to the atmosphere through the exhaust duct or used for secondary heat energy utilization. . During the whole process of hot air transfer, whether the structure of the drying system is reasonable directly affects the quality of the drying effect. Therefore, to improve the drying efficiency of the drying device, we must start by optimizing the structure of the drying device and try to eliminate or reduce the impact of the hot air transmission process. The structural design of the energy transmission ensures that the energy passes through the air nozzle and is ultimately effectively and uniformly transferred to the surface of the material film to achieve the purpose of efficient drying.
I. Optimization of the piping layout of the hot air system
The heater is set after the fan, and before the air inlet of the drying box, the fresh air is directly heated by the fan and heated by the heater and directly enters the drying box, which shortens the transmission path and reduces heat loss; and the heater blows the fan at a higher speed. The decompression and homogenization ensures that the hot air enters the drying box smoothly at a low speed, reduces the impact force of the hot air entering the drying box, effectively reduces the kinetic energy transmission affected by adverse factors such as eddy current, and ensures that the air nozzle of the drying box is relatively High wind speed. The schematic diagram of the optimized hot air system is shown in Figure 1.
Second, the optimization of the layout of the inlet and outlet of the drying box and the size of the air outlet
Set the upper part of the drying box to take in air and lower the air. The hot air goes up and down, so that the hot air and the printing layer achieve reverse heat exchange; and the relative humidity of the solvent at the exit of the oven is reduced, which is beneficial to improve the drying efficiency and reduce the solvent residue.
Increase the size of the air inlet and outlet of the drying box, and the ventilation area of the air inlet is increased to about 4 times that of the conventional printing machine drying box, which means that the wind speed when the hot air enters the drying box is reduced to about 1/4. From the corresponding relationship between pressure and wind speed P =, the wind pressure entering the drying box has been reduced to 1/16 of the original structure, which greatly improves the original ventilation box's small air inlet area, large inlet wind speed, and hot air entering at a higher speed. The drying box hits the windward side, and the hot air that enters after it bounces back will form the unfavorable factor of vortex, which eliminates the internal energy consumption of the hot air itself and ensures the efficient transmission of energy.
3. The static pressure cavity of the drying box and the air duct of the tuyere are designed with a certain slope in the width direction
The static pressure chamber and the air duct on the inlet side of the drying box are wide, and the static pressure chamber and the air duct on the operation side (the far end of the air inlet side) are narrow. The dynamic pressure difference in the width direction further improves the uniformity of the wind speed of the wind nozzle in the width direction, which can effectively improve the printing quality of the printed product. The specific structure is shown in Figure 2:
Fourth, improve the design of the return air pipe of the drying box
The return air duct of the drying box is formed by the adjacent air nozzle and the baffle between the air nozzles. The slender holes are uniformly arranged on the upper and lower parts of the baffle close to the air nozzles as the air return openings. After full heat exchange, it enters the return air pipe from the return air port and is finally discharged through the exhaust air pipe. Compared with the traditional dry box near the middle of the inner tank and the empty return air around the inner tank and the dry box, the return air is more uniform and stable, which improves the phenomenon of insufficient and uneven heat exchange between the hot air and the film. The specific structure is shown in the figure 3 shows:
The above points are the main part of the improvement of the drying device of the gravure printing machine. There are still many other improvements. For example, the design and production process of the air nozzle can be made more uniform, and the reasonable design and calculation of the air duct size can be reduced. Pressure loss and heat dissipation of hot air transmission, reasonable configuration of fan power and heating power can reduce excess energy consumption.
In response to the above improvement measures, we performed performance verification experiments on the terminal platform. Through testing, the wind speed of the air outlet of the new drying device is increased by more than 70% compared with the original drying device. The uniformity of the wind speed in the width direction is controlled within ± 1m / s, and the heat loss is reduced by about 30% compared with the original device. It satisfies the drying requirements of high-speed printing very well and lays a certain foundation for the improvement of the performance of the gravure printing machine.