Lithography is based on the principle of incompatibility between oil and water. The graphic part of the printing plate can be oleophilic and hydrophobic, and the blank part should be filled with water before it can be selectively adsorbed by hydrophilic and oleophobic. In addition, the characteristic of offset printing is the use of indirect printing, and the use of dots to make color reproduction. It is very important to know whether the ink and ink in the offset printing is proper, the normal transfer of the imprint, the depth of the ink color, the accurate color registration, and the dryness and stickiness of the printed matter. Therefore, the ability to correctly control and control the water-ink balance is the key to ensuring stable print quality. As a skilled printing worker, you should not only understand the relationship between water and ink balance, but also master the techniques and essentials of water and ink balance.
1 Relationship between ink and wash
Water-ink balance refers to adjusting the supply of fountain solution at a certain printing speed and printing pressure, so that the volume ratio of fountain solution contained in the emulsified ink is controlled at 15% to 26%, forming a slight degree of water-in-oil ( W / O) type emulsified ink, with the smallest amount of liquid supply to compete with the ink on the printing plate.
The most important thing in water-ink balance is the control of fountain solution and ink. The fountain solution is composed of water and surfactant. The fountain solution can fully wet the plate surface, has the cleaning ability and a stable pH value, to ensure that the plate surface will not be dirty.
1) The role of fountain solution
â‘ Form a uniform water film on the blank part of the printing plate to prevent the ink on the graphic from wetting into the blank part and prevent the dirty version.
â‘¡The electrolyte in the fountain solution reacts with the metal plate base exposed by abrasion to form a new hydrophilic layer continuously to maintain the hydrophilicity of the blank part of the plate surface.
â‘¢Control the temperature of the printing ink. As the speed of the new printing press is getting faster and faster, too fast printing speed will increase the temperature of the ink during high-speed transmission, and the fluidity of the ink will increase, resulting in an increase in the dot. Therefore, the high-speed machine puts forward new requirements on the fountain solution, that is, when the fountain solution passes through the plate surface and contacts the inking roller, the temperature of the inking roller and the ink can be reduced.
â‘£Do not enlarge or shrink the text. The fountain solution must have the function of stabilizing the relative position of the graphic part and blank part of the printing plate to ensure that the graphic part will neither expand nor shrink during the entire printing process.
The ink is generally composed of pigments, fillers, binders and auxiliary additives. When selecting the ink, the rheological properties and optical properties of the ink should be fully considered.
The rheological properties of the ink is the main basis for judging whether the ink has good printing performance. The rheological properties of the ink mainly depend on the ink binder, of course, it can also be appropriately adjusted by auxiliary additives. The evaluation of ink rheological properties can be reflected by the main indicators of ink rheological properties, namely the viscosity, viscosity, thixotropy, yieldability, viscoelasticity and fluidity of the ink. In actual production, only by mastering these performance indexes can we know the printing properties of various types of inks, so as to achieve the purpose of selecting inks that meet the printing requirements.
The ink and water must exist on the printing plate at the same time to maintain balance. The purpose is to maintain the maximum ink load in the graphic printing area, make the ink color bright, saturated, and the dots clear and smooth, but also keep the printing area highly clean and tidy, which requires Pay attention to adjust the relationship between water supply and ink supply in production.
In printing, the ink color of the printed product is not only related to the ink volume of the ink fountain, but also directly affected by the amount of water supply and the balance of water and ink on the layout. As the water content of the plate increases, the color of the blotting ink gradually fades and becomes lighter. Conversely, if the water supply is reduced, the water content on the surface will be reduced, and the ink color will be relatively darker than before. Therefore, the increase and decrease of the ink supply and the water supply of the layout will change the depth of the ink color. In the printing process, it should be adjusted according to the actual situation. You should not rely solely on reducing the amount of water supply to improve the ink color, because the water content is too small, losing the balance of water and ink, and it is easy to make the blank part dirty. However, it is necessary to prevent the occurrence of large-scale practices. When the amount of water is excessive and the ink color becomes lighter, you must not ignore the actual situation and mistakenly believe that the amount of ink supply is small. Instead of reducing the amount of water supply, you must increase the amount of ink frequently. Such a reciprocal increase in the amount of ink supply and water supply results in a vicious circle, which eventually causes all the disadvantages of water and ink to appear, making printing impossible.
In the actual production, experienced printing workers often control the water supply volume to the smallest possible range on the premise that the printing plate is not dirty, and keep the water supply volume and ink supply volume in a relatively stable state. In this way, it can ensure that the ink color of the printed product is consistent before and after, and the printing operation is stable. Promote the practice of using small water and thick ink. The small water here refers to the minimum amount of water on the premise that the blank part of the layout is not dirty; the so-called ink thickness is also based on the small water, which causes excessive ink emulsification. The ink layer cannot be thick. [next]
2 Conditions for producing water-ink balance
In the printing process, in addition to proper control of the ink and ink, the stoppage, temperature and humidity changes in the printing shop, the running speed of the printing machine, the type of ink and paper, and the quality of the plate material will also directly affect the ink and ink balance.
1) The machine must run at a constant speed. In production, the machine must maintain normal speed and run at a uniform speed. The speed of the machine will suddenly change, which may cause the water-ink balance to lose control. The faster the machine speed, the smaller the water squeeze and drip, the smaller the water consumption, and the larger the water consumption.
2) The temperature and humidity of the printing workshop should be constant. In principle, the workshop is required to have constant temperature and constant humidity conditions, because changes in temperature will cause changes in ink viscosity and fluidity. The workshop temperature is high, the moisture evaporates quickly, the water consumption is large, and vice versa. The humidity in the workshop is high, the water evaporation is slow, the water consumption is small, and the contrary is large, so that the ink and ink are out of balance. Therefore, it is best to install air conditioners and humidifiers in the workshop to control the temperature and humidity within the specified range.
3) Ink properties. The water resistance of the ink determines the water content of the ink. Water resistance is strong, the water consumption is small, the water consumption is small; on the contrary, the water consumption is relatively large. Different brands of ink have different quality and different water resistance.
4) The nature of the paper. The paper is rough, the texture is loose, the smoothness is low, the paper absorbs more moisture, and the amount of ink used is larger. The paper has a dense texture and high smoothness, so the water consumption is small and the ink consumption is small.
5) Printing plate category. Different printing plates have different sizes of trachoma thickness. The surface of the plate is thick, and the water stored is more. The water layer of the plate is relatively thick, and the water consumption is relatively large, and vice versa. Therefore, the grit on the surface of the printing plate must be kept uniform and fine, with a certain physical strength, so that the graphics and blank parts have a solid attachment foundation and good water storage conditions.
6) Layout graphic area. Large graphic area, large amount of ink, large consumption of water and ink, large water consumption; on the contrary, the amount of water and ink consumption is small.
7) Fountain solution. The pH value of fountain solution should generally be controlled at about 4.5 to 5.5, and it must be kept constant to avoid large-scale fluctuations. Therefore, when adding the wetting powder, an appropriate amount should be used to avoid putting too much or too little.
8) Adjust the pressure correctly. The printing press must have precise cylinder pressure, ink roller pressure and water roller pressure. These three pressures directly determine the water-ink balance. [next]
3 Identification of moisture content
Before printing, according to various conditions, the number of people who can only roughly estimate the water consumption is small. In actual printing, various methods must be used to identify the size of the water. The most commonly used method is to identify the amount of moisture by the operator visually inspecting the intensity of the reflected light on the layout. Because a certain thickness of water film can fill the plate sand day, reduce diffuse reflection and increase the amount of reflected light. However, this method also has certain limitations. It is affected by the ratio of the graphic area of ​​the layout, the influence of the printing plate material, the influence of the light intensity and the angle of observation, etc. Therefore, in daily printing, it is often judged from the following points whether the moisture is too large:
â‘ Ink shoveled with an ink knife, the ink color lightens the hair cream, and the ink color is thick and dull, indicating that the water content is too large.
â‘¡ Use an ink knife to shovel the ink on the ink roller. There are small water drops on the ink knife, and there is also water in the ink fountain.
â‘¢ The layout often appears dirty, or the moisture on the layout is still dry after a long shutdown.
â‘£The imprinted dots are empty, the imprint of the bite is wavy, and the ink is dull and dull.
⑤The printed sheet is curled and weak, and the paper collection is uneven.
â‘¥ There are water shadows or water drops on the trailing edge of the blanket roller.
⑦ There is a big difference between the ink color of the printed product after the shutdown and the printed product before the shutdown.
⑧When adding ink on the ink roller, because of the presence of water film on the surface of the ink roller, it is not easy to make the ink even, and the ink roller will slip when the ink is transferred to the hem.
4 The effect of improper mastery of water and ink balance on print quality
In the printing process, improper control of ink and ink will have a great impact on the quality of the print, resulting in inaccurate overprinting, color restoration distortion, and "pattern", "pasted" and other phenomena, as follows:
1) Affect overprint accuracy
Due to the excessive water content of the plate, the paper absorbs excessive moisture and stretches after transfer. When the next color is ready for overprinting, it shrinks due to the moisture emitted from the four sides of the paper during storage. Not allowed.
2) Cause excessive emulsification of ink, hinder the normal transmission of ink, and affect product quality
In the printing process, due to the excessive moisture on the plate surface, the moisture on the plate surface is squeezed into the inside of the ink under the action of mechanical pressure and the ink roller, causing excessive emulsification of the ink. Excessive moisture will also accumulate on the ink surface. In severe cases, there will be a water layer between the ink roller and the ink roller, the ink roller and the plate surface, and between the plate surface and the blanket. When the water layer reaches a certain level, it will seriously affect the normal transfer of ink, thus affecting the quality of the product.
3) The ink color of the printed product becomes lighter and the gloss of the printed product decreases
Due to excessive water content on the printing surface, they are often distributed on the surface of the ink roller in the form of fine beads, and are dispersed in the ink by mechanical force, thereby reducing the number of pigment particles per unit area and reducing the color saturation of the ink. The emulsification value increases and the ink color becomes lighter. Not only the ink color of the product is gray, but also the printing plate "pattern", "floating dirt" and other phenomena have occurred one after another, causing printing failure. At the same time, when the ink contains excessive moisture, the ink film of the printed product cannot fully oxidize the conjunctiva. The surface is rough and not smooth, making the ink film dull and dull, and the gloss of the printed product is reduced, which affects the color and color of the printed dots. Restore effect.
4) The paste version is dirty, which affects the product quality
In addition, when the plate ink is heavy, the amount of ink on the plate is too large, and the ink layer is too thick. Under the action of the squeezing force, the graphic dots spread and the paste plate becomes dirty, causing quality problems in the printed product. [next]
5 Precautions for maintaining water and ink balance in daily production
In daily operations, in addition to correctly grasping the actual situation of the size and distribution of the layout, the nature of the printing paper, the performance of the ink, etc., special attention should be paid to the following points:
â‘ Before starting, you must add enough water evenly on the dry water roller.
â‘¡ When the machine is idling, the water supply and ink supply should be stopped immediately to avoid the water and ink being unbalanced due to the large water and ink.
â‘¢ The new water-fleece cover should be tight and flat, and the two ends should be sewn firmly. Before using the machine, the water roller must be fully wetted. When printing, the water content must be kept to a minimum. Newly replaced water rollers tend to have a higher water content than old water rollers, which can easily cause excessive moisture failure.
â‘£ Never adjust the ink too thinly when adjusting ink. If the ink is too thin, the fluidity is too large, and the viscosity and cohesion naturally decrease, resulting in a decline in water resistance, which can easily cause serious emulsification, and the ink will spread under the pressure of the roller. In serious cases, it will produce paste plates, which will affect product quality. Therefore, it is better to control the amount of ink-adjusting oil below 10%.
⑤ In the printing process, the ink tends to adhere to the surface of the water roller, and the thicker the thicker, the larger and larger the area. After stopping, the ink will dry and consolidate quickly. If it is not removed in time, it will cause uneven water supply, make the ink unbalanced, and affect the service life of the printing plate and water roller, so it should be paid enough attention.
â‘¥ Correctly adjusting the pressure of water roller and ink roller is the prerequisite for ensuring the balance of water and ink. If the pressure is not adjusted properly, it will often cause poor water and ink delivery.
In short, in the printing process, the operator must insist on three-time operation, that is, check the print sample and observe the water content of the plate, dip the ink in the ink fountain, wash the blanket and printing plate, and control the moisture of the plate to a minimum (the blank part does not hang Dirty), so that the ink is always in a normal and stable state. Only in this way can the quality of the entire printing process be stabilized and the printing work be smooth.
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