Offset ink balance principle and control method
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1 What is ink balance?
Offset printing is based on the principle of ink and water repelling (incompatible), so that the ink and water exist on the printing plate at the same time and maintain balance with each other to achieve the correct transfer of images and texts, and obtain a printing effect with rich layers, clear images and vivid colors. Ink balance is the basis of offset printing.
The offset printing plate is divided into a blank portion of hydrophilic oleophobic and a graphic portion of oleophilic hydrophobic. In the printing process, the ink is supplied after the water is supplied, and the blank portion and the image portion selectively adsorb water and ink, respectively, and then form a uniform water film and ink film. These water film and ink film are fused together under a certain printing speed and pressure through the roller and the ink roller to emulsify the ink. There are two kinds of emulsified states of the ink: when the water is large, the hour is formed into an "oil-in-water (O/W)" type; when the water is small, the "water-in-oil (W/O)" type is formed. When the water and ink reach the desired supply amount, the ratio of the volume of water to the ink in the emulsified oil film is between 15% and 26%, forming a slight degree of water-in-oil "(W/O)" type ink, thereby The amount of water on the plate and the amount of ink compete with each other, and at this point, the ink balance is reached.
In the actual printing process, since the water film and the ink film are constantly affected by many variables, it is impossible to maintain a strict ink-and-wash balance. Balance is only a relative concept and a dynamic balance.
The ink and water balance is just right, the ink can be ideally transferred, so that the dots are lightly smeared, neat, the image is clear, the layers are rich, the colors are bright and saturated, and the printing accuracy is ensured, and the drying of the printed matter is ensured, and at the same time, it is ensured. The blank part is clean and tidy.
2 The adverse effects of ink imbalance on printing
The effects of ink balance imbalance on printing can be summarized into the following four cases:
1) Water small ink hour: the ink supply quantity can not meet the requirement of quantity. At this time, because the water quantity of the printing plate is not enough to form the water film, the blank part of the print does not have enough ink repellent ability, and can not compete with the ink, so it is easy to absorb the ink. The ink on the roller causes the blank portion to be dirty. At the same time, because the ink supply amount is also small, the amount of ink absorbed in the graphic part is insufficient, which causes the print to be faint and dull, and the image is not strong and lacks luster.
2) When the water is small, the water is not enough, and the water film on the ink surface cannot form an ink-repellent water film to resist the ink. The ink on the ink roller is easily adsorbed to the blank non-image area, resulting in stickiness; the dots are not clear. The expansion of the dot and the dot is serious, resulting in the loss of the image, and the text is easy to paste, especially the small white text; because the water is small, the water content of the different graphic areas is not the same, and it is easy to cause the ink color unevenness.
3) Water and ink hour: The moisture of the plate is too large, not only the plate water film is too thick, but also the water will spread along the ink roller. After the roller and the ink roller are squeezed each other, the ink emulsification accelerates, forming an oil-in-water ( O/W) "type emulsified state, thus hindering the normal transmission of ink. The image area does not get saturated ink, even without ink, the graphic ink color gradually becomes lighter, the imprint is not strong, the dots are real, gray, tarnished, the image is illusory, lacking layers. In addition, due to the large amount of water, the paper will cause deformation and elongation after moisture absorption, which will affect overprinting.
4) When the water is large and large: Due to the excessive moisture in the layout, the water is continuously distributed to the ink roller, the ink is excessively emulsified, the viscosity and cohesion of the ink are lowered, and the ink transmission is hindered. Although the amount of ink is large, it accumulates on the surface of the ink roller, and the amount of ink on the surface is reduced, resulting in a lighter color, a lighter color, an old color, a dull image and a layer. At this time, even if the ink supply amount is increased, the effect will not be improved, and at the same time, the water roller will be sticky and the surface will be "floating"; due to the large moisture in the ink, the performance of the ink will change, especially the drying performance is deteriorated, and the blotting is performed. Drying becomes slow and easily causes the print to become dirty and dirty.
The correct ink-and-water balance condition is to achieve the ink-and-wash balance with the minimum amount of water while ensuring that the density of the multiple primary colors reaches the standard.
3 How to control ink balance
Controlling the ink-and-water balance In addition to the offset control, the offset is controlled to the minimum according to the correct ink-and-water balance conditions. The following points should also be noted:
1) Put the quality of the printing plate off. The printing plate is the basis for printing and a prerequisite for ink and water balance. It is necessary to select a PS plate with fine and uniform sand and a certain mechanical strength, so that the plate has a solid graphic foundation and a blank foundation, keeping the lipophilic and hydrophilic relatively stable, and improving the printing durability of the printing plate.
2) Manage the water - dampening solution. Water management is very important. For common type of fountain solution, chemical components such as phosphoric acid, ammonium nitrate, ammonium dihydrogen phosphate, ammonium dichromate, citric acid and gum arabic are usually added; alcohol dampening solution is also added with ethanol or isopropanol. Due to the addition of acidic substances, the dampening solution is weakly acidic, and its acidity has a great influence on the total wetting property of the dampening solution, so the pH of the dampening solution must be strictly controlled. The general control range is between 4.5 and 5.5. What is the best value depends on other factors. For example: the pH of the paper, the amount of ink on the layout, and the performance of the ink. Ethanol, the main ingredient of alcohol, can reduce the surface tension of dampening solution and improve surface activity, which is beneficial to ink balance.
3) Control the ink. The key influence of offset ink on ink balance is the viscosity, fluidity, water resistance, particle size, etc. of the ink, especially viscosity and fluidity, which must be controlled. The ink with high viscosity and low fluidity is also strong in water resistance. Maintaining proper viscosity and fluidity can maintain a relatively stable ink-to-ink balance. If the ink is too thin and fluid, its viscosity and cohesion are inevitable. Decreased, water and water repellent properties are reduced, and it is easy to emulsify excessively; if it is to accelerate the drying of the ink, adding excessive dry oil will also increase the viscosity of the ink and enhance the adhesion. It is required to dilute the ink fountain when offset printing, in order to improve the fluidity of the ink and maintain good transfer performance, thereby facilitating the balance of ink and water. Because different inks have different properties, different prints have different ink performance requirements, so it is necessary to select inks with performance requirements.