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The water delivery mechanism is a unique mechanism in offset printing and a very important part of offset printing. It can be said that for the offset printing machine, whether it is the design or use of the dampening device, it is very important to finally be able to print high-quality prints.
At present in China's more common printing presses, the more common dampening mechanism can be divided into two types of contact and non-contact. There are two types of contact type, intermittent and continuous. Non-contact types are also available: brush roller type, centrifugal type, rollerless dampening type and other forms. Below we give a brief introduction to these types of wetting methods:
(1) Contact dampening device: This device refers to a dampening solution in which the dampening liquid in the water hopper is conducted to the surface of the printing plate through various water rollers for wetting.
a) Intermittent: refers to a method of wetting that the water bucket roller does not use continuous rotation, but relies on the intermittent rotation of the water bucket roller to intermittently transfer the fountain solution to the printing plate.
b) Continuous type: Compared with the intermittent type, the continuous dampening mechanism removes the reciprocating water transfer roller, so that the dampening solution can continuously supply the printing plate.
(2) Non-contact dampening device: This device directly sprays or coats the printing plate without passing through the water roller during the transfer process from the fountain solution in the water bucket to the printing plate. There is no real contact between the parts in the mechanism and the printed version. This type of dampening method has very little water on the surface of the water roller, so some people call it a dampening method without water retention or a continuous dampening method.
1. Water supply part: It consists of water bucket 1, water bucket roller 2, and water supply roller 3. Its function is to supply water quantitatively to the uniform water portion as required for printing.
2. Water leveling part: there are water rollers 4. Its function is to smooth the water in the circumferential direction and the axial direction and deliver it to the water roller.
3. Water landing part: composed of two water landing rollers 5, whose function is to evenly coat the surface of the printing plate with water.
The water bucket roller of this dampening device is intermittently controlled by a ratchet and a pawl.
The disadvantage of this type of mechanism is that due to the inertia generated by the reciprocating swing of the water transfer roller and the water string roller, the pressure between the water rollers is uneven, and at the same time due to the principle of intermittent water transfer drive (such as the transmission principle of ratchet and pawl) , Resulting in an ineffective amount of water control. In addition, due to the relatively large water storage capacity of the water roller velvet itself, the response to the water volume control is relatively slow. Even if very precise water volume control is performed, the amount of water transferred from the printing plate to the paper surface is relatively large, which causes the final quality control. It has a great impact; moreover, the water roller velvet used by the agency is often contaminated with ink or linting, so the water roller velvet must be replaced frequently, which is difficult to meet the requirements for high-quality and high-speed printing of.
The water bucket rollers of PZ4880-01 and J2204 produced by Beiren are realized by continuous motion driven by direct current. The water transfer roller is also a reciprocating motion, and its rotation is realized by its own friction with the water bucket roller and the water string roller. The movement mechanism is basically the same as 08, 05, 03 (see Figure 1).
The continuous dampening device can be directly wetted by the water roller alone, or it can pass the fountain solution to the first inking roller, and at the same time pass the fountain solution and ink to the printing plate (that is, the famous Dagren mechanism ).
(1) The water roller alone directly wets the printing plate
Figure 4 is the ROLANDMATIC alcohol dampening device designed by the German MAN ROLAND company. The device is composed of a water bucket roller 1, a measuring roller 2, a water string roller 3, a water injection roller 4, and the like.
The outstanding advantages of the continuous dampening method are stable water supply, easy control, wide application range, good print quality, and low cost, so it can be said to be the mainstream of today's printing.
Continuous dampening device can use ordinary dampening solution, but now more alcohol dampening solution is used. At present, alcohol fountain solution is widely used at home and abroad, such as flat offset presses in Germany, the United States, Japan and other countries are generally used. Alcohol fountain solution is essentially adding 10-20% alcohol to ordinary fountain solution, which causes the surface tension of the fountain solution to be significantly reduced, and the contact angle of the fountain solution is also significantly reduced compared to ordinary fountain solution. In other words, wetting performance has been greatly improved. Therefore, the amount of fountain solution can be greatly reduced.
The water bucket roller 1 is driven by a separate DC motor, and its surface linear velocity is 20% -50% lower than that of the plate cylinder. The metering roller 2 is driven to rotate by the water bucket roller 1, and a very thin water film is formed between the two. Therefore, the water bucket roller transfers the water film evenly on its surface to the printing plate by means of the speed difference from the first inking roller 3 to wet the printing plate. The inking roller 3 plays the dual role of the plate water roller and the plate ink roller. It is worth mentioning that, because the inking roller 3 is constantly accumulating ink, the alcohol content in the fountain solution is generally as high as 20% to 30%, so as to effectively reduce the scumming faults that are easy to occur. Japan's Mitsubishi has models that use this method of dampening.
Figure 6 shows the schematic diagram of the patented and unique ALCOLOR alcohol dampening device of Heidelberg, Germany.
There are five water rollers in this device. In the picture, 1 is a water bucket roller made of rubber material, 2 is a metering roller polished with a chrome layer, and 4 is a string water roller without a polishing but also with a chrome layer. 5 is the middle stick of nylon material, and the continuous inking device and water delivery device play an important pivotal role. 6 is the first inking roller.
The dampening liquid required for the printing plate is mainly supplied by the water injecting roller 3, and the dampening liquid required for the ink transfer device to achieve water and ink balance is transmitted through the intermediate roller 5, the inking roller 6, the intermediate roller 5, the water injecting roller 3 and The surface speed of the water leveling roller 4 and the plate cylinder are the same.
The water bucket roller 1 is directly driven by the motor, and is adjusted by the speed compensation of the electronic control circuit, and then transmitted to the shaft end gear transmission metering roller 2, so the metering roller always rotates at the same speed as the water bucket roller. An extremely thin wetting film is formed (the thickness of the wetting film can be controlled by a stepless motor). The water roller 4 and the water roller 3 are driven and rotated by the plate cylinder. The rotation speed of the water roller is faster than the metering roller 2. When the two are in contact, the wetting film is elongated and thinned, and can penetrate into the ink of the water roller, and then wetting the printing plate.
The working procedure of the device is:
1. Non-working position: the metering roller 2 is separated from the water application roller 3, and both the water application roller 3 and the ink application roller 6 are separated from the printing plate. The ink is transferred from the inking roller 6 to the water roller 3 and the water leveling roller 4 through the intermediate roller 5, and at the same time, the water bucket roller 1 and the metering roller 2 also operate to prepare the liquid supply. At this position, the dampening device can be cleaned by the inking device.
2. Pre-wetting position: the metering roller 2 is in contact with the water-feeding roller 3, the intermediate roller 5 is also in contact with the ink-filling roller 6, and the fountain solution is transferred to the rollers with ink 3, 4, 5, 6 to make the ink delivery device The ink roller in the absorbs a certain amount of water, so as to achieve water-ink balance. After the water roller 3 is in contact with the printing plate and the printing plate is pre-wet 6 times, the formal printing stage can be performed.
3. Official printing position: Both the water roller 3 and the ink roller 6 enter the working state, that is, water and ink are fed to the printing plate at the same time. The inking roller 6 comes into contact with the printing plate and enters normal printing. After passing through the pre-wetting position stage, water and ink quickly reach equilibrium on the printing plate, and prints that meet the quality requirements can be obtained at the beginning of printing.
In addition, this device is also provided with an air blowing rod on the ink roller, which will volatilize the local excess of the wetting liquid, so as to realize the control of the size of the local dampening liquid.
1. Water bucket roller 2. Scraper 3. Adjustment screw 4. Water shield 5. Water leveling roller 6. Heavy roller 7. String water roller 8. Water roller 9. Water bucket 10. Plate cylinder
The device is more convenient for the adjustment of the water supply amount. During the overall adjustment, the rotation speed of the water bucket roller drive motor is changed. During the partial adjustment, the position of the water shield can be adjusted, so the size of the water supply amount can be controlled more accurately.
Figure 8 shows a brush wetting device designed and used by Harris Corporation in the United States. The brush roller 2 is always in contact with the water bucket roller 1 and rotates at the same speed. Therefore, the water adsorbed on the surface of the water bucket roller is splashed by the brush roller onto the string water roller 3, and passed to the printing plate through the water roller 4 5. The rotation speed of the water bucket 1 determines the amount of water output.
The brush roller water-feeding dampening device adopts a separate transmission, and the dirt such as dampening liquid, paper wool, powder spraying and emulsified ink on the printing plate is not easy to return to the water bucket, thus keeping the dampening liquid clean Sex. But its water leveling effect is not particularly ideal.
Since the spray angle can be adjusted, a row of adjusting screws 4 along the axis of the chrome roller 1 is used to adjust the gap between the rubber Blade and the chrome roller, so that the water supply amount of each section along each direction of the chrome roller axis can be adjusted more accurately. The device is additionally provided with a water tank, and the water supply hopper 2 is continuously supplied with water by a Water Pump.
1. Rotor 2. Water 3. Return water 4. Water supply 5. Water transfer baffle 6. Plate cylinder
Figure 10 shows another centrifugal spray dampening device used in the Roland offset press: the turbine dampening device.
The characteristics of the above two centrifugal spray devices are basically similar to the brush dampening device. The water supply part and the water contact part are not in direct contact, the fountain solution will not flow backward, and the water supply amount can be controlled by adjusting the rotation speed of the water bucket roller. The uniformity of the air spray on the printing plate is better than that of the brush roller. However, due to its relatively high cost, the wetting effect is not very significant, so it has not been promoted more.
3. Roller-free wetting device The roller-less wetting device generally works through the principle of spraying, ultrasonic, electrostatic, and condensation through an air scraper or nozzle system.
The more well-known roller-free wetting devices include Mulun dampening device and Luotaixun dampening device.
Compared with other types of dampening devices, the above two devices are rarely used. In fact, the amount of non-contact dampening devices used is not much. So I won't introduce too much here.
Appendix: Schematic diagram of dampening mechanism of each model of Heidelberg.
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