This article analyzes the structure of the closed ink chamber used in the central impression flexo printing machine, and puts forward the reasons and countermeasures for the ink chamber pressure control, ink blade, and ink block. The common ink splash problem in printing were analyzed in detail, and corresponding suggestions for improvement were put forward.
Flexographic printing was originally called letter printing press, which originated in the United States in the 1920s. Since its development, its ink transfer system has undergone several technical improvements, from the earliest two-roller ink transfer system, to the later single doctor blade ink transfer system, and then to today's closed chamber doctor blade system.
1. What is a closed chamber doctor blade system?
Although two-roller ink transfer and single doctor blade ink transfer system are still widely used in today's corrugated boxes, narrow-width labels and even medium-width unit type flexo printing machines, the closed chamber doctor blade system has become the standard configuration of the high speed and high quality printing machines represented by CI flexo printing machines.
The two-roller ink transfer system mainly includes a rubber fountain roller and an anilox roller. The rubber fountain roller scrapes off the excess ink on the surface of the anilox roller. In this case, the amount of ink transferred from the anilox roller to the plate roller is largely determined by the pressure between the rubber fountain roller and the anilox roller. It is basically impossible to achieve precise ink transfer, and ink splash will occur as the printing speed increases.
The emergence of the reverse single doctor blade system solves the problem of precise ink transfer. The doctor blade can evenly scrape off the ink on the surface of the anilox roller, leaving only the ink in the mesh and transferring it to the printing plate. Unlike the pressure of the positive doctor blade on the anilox roller, the pressure on the surface of the anilox roller will change significantly with the increase of printing speed. The pressure of the reverse doctor blade on the surface of the anilox roller is relatively more stable, and the correlation with the change of printing speed is not obvious, so it can ensure more stability. However, after the speed is increased to a certain extent, the problem of ink splash is still more prominent. In addition, the design of the open ink tank will also cause the volatilization of the solvent to be more obvious. The resulting changes in the pH and viscosity of the ink will cause the printing quality to fluctuate.
Compared with the two-roller ink transfer system and the single doctor blade system, the advantages of the closed ink doctor system are obvious. First of all, the reverse blade in the closed chamber doctor blade system ensures high-precision printing quality. Secondly, the ink is in a closed ink circulation system, and the solvent is not easy to volatilize. The performance indicators of the ink are relatively more stable during the printing process, so the printing quality is more stable, and it saves solvents and improves the operating environment. Finally, the ink is in a closed ink chamber, and the ink will not be thrown out when the machine is running at high speed. This is also one of the key factors that modern high-speed flexographic printing presses can run stably to 600 meters or even 800 meters per minute. Of course, the closed chamber doctor blade system has other advantages such as saving ink and avoiding environmental dust from entering the ink circulation system.
The basic structure of a closed chamber doctor blade system includes an ink chamber, two upper and lower blades, and ink blocks at both ends. Among them, the reverse blade plays a role in scraping ink, which directly affects the printing quality. The positive blade only plays a role of sealing ink, and has no direct effect on the printing quality. The ink block also functions as a seal to prevent ink from flowing out of the end surface of the ink chamber.
2. Types of Close Chamber Doctor Blade System
There are currently several closed chamber doctor blade systems with different structures on the market. According to whether the ink is completely in the closed ink circulation system, it can be divided into a fully closed chamber doctor blade system and a semi-closed chamber doctor blade system.
The fully closed chamber doctor blade system generally has an ink inlet at the bottom of the ink chamber, and an ink outlet on the left and right ends of the top. The ink inlet is fed into the ink chamber by the ink pump, and the ink outlet is connected to the ink return pump or reflows by gravity. The ink flows back to the ink tank through the ink tube.
The ink inlet of the ink chamber of the semi-closed chamber doctor blade system is the same as that of the fully closed chamber doctor blade system, but the ink cartridges are installed at the left and right ends of the ink chamber (there are also some semi-closed chamber doctor blade systems that do not need to be installed back to the ink cartridges, only two overflows are provided In the working state, the ink in the ink chamber will flow out from the ink tank to the open ink tank when it is higher than a certain level, and then flow back from the open ink tank to the ink tank. The appearance of semi-closed chamber doctor blade system is usually to solve the problem of ink pressure accumulation in the ink chamber of the fully enclosed chamber doctor blade system. However, because the ink circulation system is half open, it is also easy to cause the drawbacks of solvent volatilization, ink viscosity changes, and easy introduction of external dust and other impurities. At present, there have been other methods or devices on the market to eliminate the pressure accumulation in the ink chamber, and it is not necessary to use a semi-open ink circulation system to solve this problem.
3. Pressure Control of Close Chamber Doctor Blade System
According to the pressure control mechanism of the ink chamber of the closed chamber doctor blade system, it can be divided into pneumatic, mechanical pressure and hydraulic doctor blade systems.
The pneumatic doctor blade system is the most common structure in the market. This structure is relatively simple to install and maintain. After the pressure is adjusted, the cylinder can automatically compensate according to the wear condition of the scraper within a fixed stroke range, and keep the pressure of the scraping system on the anilox roller constant. However, since printing plants seldom equip the printing press with an independent compressed air source, if other large-scale air-using equipment in the same air path is turned on, it will cause an instantaneous change in air pressure, resulting in a change in the blade pressure and a corresponding change in the printing color density, or vibration results in more obvious ink bars, and the higher the machine speed, the greater the risk.
The mechanical pressure doctor blade system uses a special mechanical structure to stably maintain the blade pressure imposed onto the anilox roller, and will not cause pressure drop or jitter even at very high operating speeds. However, the system cannot realize automatic compensation after the blade is worn out. It is necessary to manually adjust the ink chamber position at regular intervals to keep the stable pressure. If the blade wear is large and the ink chamber is not adjusted forward, the risk of ink ejection is likely to occur.
The hydraulic doctor blade system uses a hydraulic station or a pneumatic-hydraulic conversion device to use a hydraulic cylinder to provide the blade pressure. Since the hydraulic system is more stable relative to the air pressure change, the shortcomings of rapid air pressure change can be eliminated, the risk of ink chamber jitter can be reduced, and the blade wear can be continuously compensated. This is an ideal ink chamber pressure control method, but its cost is higher.