Analysis of Continuous Change Supply of Offset Wet Water

1, the principle of printing and ink balance

Lithography uses moist water to keep the non-printed part clean. This is the principle and method that has been used for more than two hundred years. How much water supply directly affects the quality of lithographic printing, too little water can not keep the non-printed part clean, small and fine anti-white words will get rid of or dragging the edges of the word, and even the whole film of ink spots, and vice versa. For a long time, the ink became lighter, the full version had white spots, or the entire white water mark was in the printed area, and even the last line and text disappeared first, and the printed ink had a feeling of being completely washed away.

UV ink drying is more affected by water. Over-emulsified ink layer will be cured by irradiation and other problems. Even if it is generally non-forced drying ink, it is not easy to dry under too much water and excessive emulsification. In fact, the ink on the ink supply system of the typographic printer will also cause too much water in the ink due to too much water for a long time, which will cause the viscosity of the ink to drop, and the ink on the top will not move down. The ink only transfers a small part to the lower layout ink roller to the printing plate, resulting in serious ink color reduction, even hard steel roller, copper roller will have deinking phenomenon, and the printing quality can not be used.

2, water consumption analysis

Let's first discuss where is the water on the typographic machine? The first is the consumption of ink emulsification, the second is the consumption of evaporation, and the third is the consumption of paper.

The first ink emulsification can only be between 15 and 20%. If 8000 four-color prints are finished per hour, three kilograms of ink and five liters of moist water are used, meaning emulsified water, only 0.45- 0.75 liters may be consumed with the ink consumed on the plate, so this amount is 9-15% of the total moisture consumption, the third paper absorbs, if 8000 sheets are calculated with a basis weight of 100g/M2, and 342 kilograms 0.4% more paper. For the weight of water, the extra water content of 1.37 kg of water is consumed from the wet system. The second evaporation consumes about 2.88 to 3.18 liters of potash, and more than 50% of the moist water is used.

In addition to layout evaporation, the blanket surface, pressure roller surface, and ink supply system will evaporate more or less. Although the printing plate area does not change due to the speed of printing, the ink supply system of the printing press is accelerated due to the speed increase. As the temperature rises, the evaporation rate increases. However, if the ink supply system is equipped with a constant-temperature cooling water device, this will not happen.

The faster the printing speed, the greater the wind disturbances on the drum surfaces and the more evaporation, and the biggest problem in the management of moist water is that there are too many variables involved in evaporation water changes (including the above-mentioned high-speed ink roller heating The combination of wind, wind disturbances, atmospheric temperature, humidity changes, and the total combination of wind direction in the printing room, etc.) creates a logical pattern of how much water supply is extremely complex and variable, which is the focus of this article.

3, environment and typographic conditions

Many operators of typographers cannot understand these internal mechanical configurations and control and changes in external conditions, so they can still be produced under certain conditions, but as long as internal and external factors arise, they can no longer be strained. , resulting in poor quality, even unsuitable use of poor printing conditions.

A. The best conditions

The best condition is the constant temperature and humidity of the factory, and the printer itself also has a constant temperature ink roller and wet water device. The printer operators are faced with different factors, such as the absorption of water of different sizes and different paper, and the ink consumption of the printing ink. The emulsification, and the amount of water consumed by the high and low printing speeds. Of course, changes in size and speed of printing can be used for predictable arithmetical problems. As for paper, paper, white paper, molded paper, and doline paper are more absorbent, micro-coated and light-coated, followed by snow. Paper is next. Glossy coated paper is very hygroscopic. Mirrored paper, full paper, and plastic sheets are barely absorbed. Therefore, the water consumption may be a quarter-to-quarter gap, and the problem is not obvious.

B, printing room has constant temperature and humidity conditions

The printer room has a constant temperature and humidity, but the printer only has a constant temperature for the water and does not cool the thermostat to the ink. In this case, the temperature of the printing press to the ink system increases at high speed, but is cooled by the moist water. Under evaporation heat absorption, the long-term temperature will increase from 5 to 10°C, which will affect the evaporation of water. At the same time, since the ink will soften after the temperature rises, it will be easier to dirty the plate, so the water will increase. Therefore, it is necessary to increase the water compensation setting at the time of high-speed printing. Others work under the condition of A.

C. The printing room has air-conditioning and no humidity conditions

There is air-conditioning in the printing room, but there is no constant-humidity addition and dehumidification device, and the printing press has only a cooling and constant temperature device for moist water. Basically, the adjustment condition of A+B is similar, but in the dry season in autumn and winter, the wet water should be increased. On the other hand, wet weather such as plum rain and summer rain in the northwest of China may account for only 50% to 20% of evaporative water consumption. Therefore, adjustments must be made in large quantities, mainly due to the fact that the humidity in the air causes the vaporization of moisture to cause printing. Machine surface, blanket, pressure roller, and ink roller surface moisture causes change.

D, printing room does not have air condition

The printing room is open without air conditioning. The printing machine only wets the water and has a cooling thermostat. In this environment, it can be said that it is not conducive to long-term work. The temperature rise during printing is about 5 to 10° C. for an hour and another hour. Two hours, if it is possible to reach 15°C outside in the summer, the cooling effect of moist water is not significant, and if it is in autumn and winter, the increase can be reduced. Imagine 27 to 28 degrees Celsius in the summer morning. Once it rises to 42 to 43 degrees Celsius, plus heat dissipation from the motor, pump, and cooling water, it will cause serious problems on the motor side. The plate will have different heat on the left and right sides. Different evaporation ratios occur, and the motor side is more prone to dry spots.

However, openness is not only caused by a single temperature increase, but also by factors such as near-window drying, uneven drying, and disturbed sunlight. It is not easy to do quality uniformity because the operator must be fine because of temperature rise, wind disturbance and Changes in the external temperature caused by changes in order to do a good job in normal production.

E, no air conditioning low speed feeder conditions

There is no air-conditioner printing room, plus printing conditions with only a shallow tray sink and no temperature-cooling control. At present, there are few, but there are still some low-speed feeders, which are fixed at 3,000 rpm, but also because of the slow speed. Speed ​​printing, so the temperature increase is relatively small, so it should be easier.

However, the surface of such printing cylinders is large, and the models are relatively open. In particular, the distance from the wet water roller to the ink roller is wide, and the distance from the last ink roller to the blanket cylinder is also long. Therefore, the cause of the change is also more. Power saving and heat dissipation often cause the printing press to move to the window under such conditions, but it often causes changes in morning, afternoon, and evening sunshine, and there is a difference in color temperature and illuminance. At the same time, the wind blows out of the window. Because it is also another unpredictable failure factor.

In addition to the environmental factors, materials, printing quantity, printing machine conditions, printing speed, and other factors mentioned above, the adjustment mechanism for the water supply volume of the printing press itself and the continuous or intermittent water supply mode will produce water at the same scale due to different speeds. There are inconsistencies.

4, various water supply mode classification

A, scale type water supply

A section of water supply adjusted by the scale or number of teeth is a water supply method that adopts a transfer water roller. This is only suitable for use in a low-speed type non-variable stencil printer, and processing for increasing or decreasing teeth is performed in response to changes in the external environment and paper. When the two tooth scales are insufficient to increase the amount of water and increase the amount of water on the tri-tooth scale, too many blemishes are still available. If you use a variable speed lithography machine, most of the low-speed water is insufficient and the high-speed water is too much. The reader will have doubts that the high-temperature temperature rises to increase the evaporation of water, and how high the amount of water at high speed will be?

There are two factors that increase the amount of water: one is the transfer of the water roller, the slow speed of printing at low speed, inertia is small, so the inertial force when it is in contact with the sink roller is very small, drag only half gear, otherwise the speed from 3000 to 6000 When the original three-toothed scale was set, the inertia may become larger and the sink roll may be dragged by 1.5 teeth. Then, the moisture content is more than 30%. Originally, the printing speed is doubled and the water is doubled. The printing surface has as much evaporated water. The moisture content is already too large, and now it has increased by 30% due to inertia. Therefore, some mechanical stones are designed to make contact with the front roller deceleration function to create a gentle inertial effect.

Another factor is the sink roller. The water film on the low speed running sink roller is thin. The water film with high speed due to the running of the sink roller is thicker, and at least more than 60% when the speed is doubled. It is also an additional cause of control, and one of the main causes of the increase in water content during high-speed printing.

B. Independent motor control water supply

Continuous water supply, two-stage independent motor control water supply method, if in a good constant temperature and humidity conditions, there is a cooling system for ink, water supply, then a section of water supply is almost no need to adjust, because it is used for school edition For quantitative water supply, some printing presses have recently increased the school edition to 7,000 sheets per hour, which is not a quarter of the 6000 maximum printing speed of 4,000 sheets. This is also the reason for the large adjustment gap, which makes the printing unsmooth in case of paper feeding. Feida still "slow" at nearly half the speed. This two-stage high-speed section is often adjusted on the discharge plate with a rotary knob or control panel in a digital manner. If there is a dry spot, just turn the knob to the end to provide quick water supply. Just focus on the high-speed water supply button.

C, curve setting water supply

Continuous water supply, speed tracking curve setting method is similar to the above B type, except that the printer can set H, M, L and other curves of the relationship between speed and water supply, and the difference between H high and low speed water supply is the largest. The low-speed water supply starts at a low point, and its highest point may be only 1.5 times the water supply at a low point. The operator selects the appropriate curve to perform the water supply according to the paper and weather conditions (humidity is the main factor), so when the speed changes, it is the most It can increase or decrease the water supply by following the curve. It is one of the most ideal ways to use the program.

D, perceived water supply

The perceptual water supply method is also based on continuous water supply. However, infrared or other methods are used to sense the thickness of the water in the printing plate surface, and the water supply to the drive motor for continuous water supply is adjusted. Although it is ideal, it has not yet been widely used. .

5. Demand and Causes of Safe Water Supply

Since the logic of safe water supply cannot be eliminated in the driest zone, it may be necessary to maintain enough water for the “dryest” zone to keep the printing plate clean. More than 40~50% of the amount of water, so if you want to dry too dry can not produce spots, too wet place and can not have too much emulsification to make the ink drop significantly or produce water droplets white spots, this will depend on maintaining the water roller pressure uniformity, system The appropriate amount of small roller adjustment, the surface of the water roller as far as possible not stained with ink, so that the printing plate from left to right are very clean and even water.

In the old-fashioned water supply system, sometimes the water was not enough, the operator had an emergency remediation method for adding water to the area. In the past, the sponge was used to sway the water, and then the plastic oiler was used to make up the water, but the water was often lost between the sprinkler and the water droplets. Make water or acid-containing water splashed on the bearing of the water roller or the bearing housing, let this part of the machine