The quality of offset printing products is inseparable from whether ink and water maintain a certain balance. In the printing process, the water supply volume is too large or too small is often easy to occur, because the size of the printing plate moisture is manifested from the printing plate dirty. However, the so-called dirtying is not only caused by the small amount of water. The operator often incorrectly increases the amount of water supply, causing the ink and water to lose balance, forming a vicious circle. What are the printing consequences of excessive moisture?
1. Dilute the ink color to make the ink color lighter and dull, which affects the color of the printed product. If the method of increasing the amount of ink is used to save the ink color, it will cause large ink and large water, making the image thick, blurry, and unclear.
2. Cause the ink roller to deink. In severe cases, there will be accumulated moisture on the entire ink roller, so that the plate image and text part cannot get the normal ink supply, causing the plate image and text to "bloom".
3. Affect the drying of the blot. If excessive water is mixed into the ink, it will inhibit the drying of the ink. Coupled with the absorption of more moisture in the paper, the product will dry more slowly.
4. The paper fiber absorbs too much water, which will cause deformation, resulting in inaccurate overprinting. It will also make the paper texture loose, easy to fluff, and pile up on the blanket or printing plate, which seriously affects the quality of the printed product.
5. The emulsification of the ink causes the ink to lose its viscosity. After the ink is emulsified, it cannot play a protective role on the printing plate's graphics and text, which is easy to cause "pattern", and at the same time will reduce the printing plate's durability.
Through the above analysis, it is shown that the mastery of water-ink balance is a key link in offset printing. In addition to experience, the operator is more important to master the correct use and adjustment of the water delivery mechanism of the offset press.
In order to adapt the water supply of the water delivery mechanism to the requirements of printing production, the water delivery mechanism is provided with a device for controlling moisture and adjusting the amount of water. This can be divided into three parts according to their working nature: one is the water supply device; the other is the water leveling device; the third is the water supply device.
The water supply device is mainly composed of a water tank, a water bucket roller and a water transfer roller. The water tank is used to store water, and there is a water bucket roller (also called water outlet roller) on the upper part. The lower half of the hopper roller is immersed in the hopper solution, and they do not contact or support each other. In addition to adding water by an automatic water dispenser during operation (many manufacturers have not used it), water can also be added manually. Manual water addition requires the operator to constantly add the previously prepared solution to the water tank. Since the water supply in the bucket roller is related to the height of the bucket solution, to make the water supply even, the bucket solution in the sink must maintain a certain water level. The operator should not be anxious when adding the water bucket solution, and slowly add the water bucket solution to the water tank, and cannot stir the water tank solution. If it oscillates, one causes the local moisture to suddenly increase during normal printing, which affects the quality of the product; the other is that the bucket solution will sprinkle and drip onto the front part under the sink, causing abnormal paper feed and clutch pressure Dropped on the electric teeth of the front gauge to form a conductive); third is dropped on the printed product, causing water droplets on the paper, and it is easy to appear double sheets when printing the next color.
The water transfer roller, also known as the draft roller, is a rubber roller that is installed between the water bucket roller and the string water roller and swings back and forth to transfer moisture. Its surface is covered with water roller fleece. The adjustment requirement of the water transfer roller is that it should be parallel to the water bucket roller and have appropriate pressure when draught. It is adjusted by the tension springs on both sides. When adjusting, the tension of the tension spring must be consistent to ensure that the water transfer roller works parallel to the water bucket roller when draught. The water supply depends on the cam, and the adjustment must put the water transfer roller against the string water roller with proper pressure. There are an adjusting screw and a positioning backing on each side below the water roller. The adjustment also requires the water transfer roller and the water string roller to remain parallel. The water transfer roller also has a draught clutch device. When opening and closing the press, the normal draught feed water swings back and forth through the 0.7V iron suction of 220V. Stop swinging back and forth when the pressure is off, that is, no longer lean on the water outlet roller. The buttons for water supply opening and water supply stop on the operation table are used to control this iron suction, and they work according to the production situation. Sometimes when we print normally, we will find that the moisture on the press surface becomes larger after the pressure is removed. why? This is because after the iron suction stops working, the pulling force of the top claw tension spring beside the iron suction is not enough, which causes the drum to still be draught from the pressure water transfer roller. The strength of the tension spring is not enough, causing the suction iron to retract one or two beats, which is why the moisture on the layout becomes larger. Solving this problem is very easy to do, just change a new tension spring or shorten the old tension spring by one or two knots.
The water leveling device is mainly a string of water rollers. Generally made of corrosion-resistant metal materials. In order to make the water on the water roller even, the water roller not only has rotary motion, but also axial motion. The surface speed of the water roller is equal to the surface speed of the printing plate, so as to keep the plate water roller moving at the same speed. The axial tandem motion of the water string roller is adjusted by an eccentric connecting rod, which is relatively simple and generally does not need to be adjusted during the printing process.
The water supply device may be called a printing plate water roller or a plate water roller. Generally, there are two, and its surface is covered with water roller fleece with better water absorption like the water transfer roller. The function of the plate water roller is to evenly transfer the aqueous solution from the water transfer roller and the water roller to the printing plate. Its surface speed must be equal to the speed of the printing plate surface, and no axial runout and radial runout are allowed, otherwise it will easily cause damage to the printing plate. The requirements for adjusting the damping water roller are relatively high, and adjustments must be made in accordance with the steps. First adjust the eccentricity, so that the pressure between the plate water roller and the water roller is just right. Then adjust the cone head to make the pressure between the plate water roller and the printing plate appropriate. The above method is to explain that when adjusting the eccentricity, not only will the pressure between the plate water roller and the string water roller be changed, but also the pressure between the plate water roller and the plate will be affected.
When adjusting the cone head, only the pressure between the plate water roller and the printing plate is changed, while the pressure between the plate water roller and the string water roller is unchanged. When adjusting the damping water roller, the lower water roller should be adjusted first, and then the upper water roller. The pressure of the upper water roller is lower than that of the lower water roller, because it can use the upper water roller to collect the excess water from the plate. This is an experience from practice. When adjusting the pressure of the plate water roller, pay special attention to the adjustment eccentric positions at both ends. If the difference is too large, the position of the plate water roller and the printing plate and string water roller will not be parallel. . Therefore, when adjusting the damping water roller, the method of correcting the center distance must be used to measure the tightness of both ends and the middle. Therefore, when adjusting the water roller, it is necessary to use the method of correcting the center distance to measure the three points on both ends and the middle to prevent the contact pressure between the water roller and the printing plate from being too heavy, otherwise the friction of the printing plate will increase. It is greatly abraded, which makes the printing plate text "pattern", the printing resistance is reduced, and it will cause the water roller and the printing plate to jump when contacting, making the jumping part dirty, and the re-falling part has a "water bar "(That is, the white bar). However, the contact pressure between the plate water roller and the printing plate can not be too light, which is easy to cause insufficient water supply on the plate surface and cause soiling. If the water supply is increased improperly, it will cause excessive moisture on the layout. So how to determine whether the contact pressure is in the best adjustment position? According to experience, it is to identify whether there is a jump phenomenon during the operation by pressing the plate water roller and the printing plate together, starting the machine, and then touching the bracket seat of the plate water roller by hand. Because the printing plate cylinder has a printing surface and a gap, it is easy to feel the lightness of the contact pressure of the plate water roller. Under normal circumstances, there is a slight sense of beating out. Under normal circumstances, the feeling of a slight beating is the most suitable. If the beating is very serious, it means that the pressure is too medium, and you should adjust it again.
After adjusting the water delivery mechanism, the following work needs to be done: first, the liner of the printing plate should be compounded; second, the aqueous solution in the sink must be free of impurities; third, the water roller velvet should be changed frequently; fourth, the water outlet roller and string water The roller must be cleaned frequently and must be kept clean.
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