Offset ink balance principle and control method

Offset is the use of ink and water repulsion (incompatibility) principle, so that the ink and water at the same time on the printing plate exist and maintain mutual balance to achieve the correct transfer of the graphic and obtain a rich layer, clear images, colorful printing effect. The ink balance is the basis of offset printing.

Offset printing plates are divided into hydrophilic oleophobic blanks and lipophilic hydrophobic graphic parts. In the printing process, ink is supplied first after the water is supplied, and the blank part and the graphic part selectively adsorb water and ink, respectively, and then a uniform water film and ink film are formed. The water film and the ink film are fused together at a certain printing speed and pressure, and squeezed between the drum and the ink roller to emulsify the ink. There are two types of ink emulsification: when the water is large in ink, an "oil-in-water (O/W)" type is formed; when the water is small, a "water-in-oil (W/O)" type is formed. When water and ink reach a desired supply, the ratio of water to ink in the emulsified oil film is between 15% and 26%, resulting in a slight degree of water-in-oil (W/O) type ink. The amount of water on the plate and the amount of ink compete with each other. At this time, the ink balance is achieved.

In the actual printing process, since water film and ink film are constantly affected by many variable factors, it is impossible to maintain strict ink-water balance. Balance is only a relative concept and a dynamic balance. (China Gold Packaging Network, a professional metal packaging site)

The balance of ink and ink is just right, and the ink can be transferred in an ideal way, so as to achieve dot-dip, neat, clear images, rich layers, colorful and saturated printing effects, and to ensure the accuracy of overprinting and to promote the drying of printed products, while ensuring The blank part is clean and tidy.

2 The unfavorable effect of ink-ink imbalance on printing

The impact of ink balance imbalance on printing can be summarized in the following four situations:

1) Small water ink hours: Insufficient supply of ink, because the amount of water in the printing plate is not sufficient to form a water film, the blank part of the print does not have sufficient ink repellency, and it cannot compete with ink, so it is easy to absorb ink. The ink on the roller causes the blank part to be dirty; at the same time, since the amount of ink supply is also small, the amount of ink absorbed by the graphic part is insufficient, which will cause the imprint to be dull and dark, and the graphic is not strong and lacks luster.

2) When the ink is small: When the water supply is insufficient, the blank part of the layout cannot form a water-repellent ink film to resist the ink. The ink on the ink roller can be easily adsorbed to the blank non-graphic area, resulting in stickiness; the dots are not clear And the proliferation of outlets is serious. As a result, the images are lost and the text is easy to paste, especially against small and medium-sized texts. Because the water is small, the degree of water shortage in different graphic areas is different, and it is easy to cause uneven layout.

3) Large water ink hours: The water content of the layout is too large, not only the water film of the printing plate is too thick, but also moisture will spread along the ink roller. After the rollers and the ink roller are pressed against each other, the ink emulsification is accelerated, forming a "water-in-oil" ( O/W) type of emulsification, which hinders the normal transfer of ink. The text area is not saturated ink, not even ink, the graphic ink is gradually lighter, the print is not strong, the network point is empty, dark, lost luster, the image is false, lack of hierarchy. In addition, due to the large amount of water, the paper will cause distortion after the moisture absorption, affect the overprint.

4) When water and ink are big: When the water content on the plate is excessive, the moisture spreads continuously to the ink roller, the ink emulsifies excessively, the ink viscosity and cohesion force decrease, and the ink transmission is hindered. Although the amount of ink is large, many of them accumulate on the surface of the ink roller, and the amount of ink on the plate surface is reduced. This causes the ink to gradually become lighter, the color is lighter, the color is old, and the image is dull and dark. (China Gold Packaging Network, a professional metal packaging site) At this time, even if the increase in the amount of ink supply will not improve the results, and at the same time there will be water roller dirty and layout "floating" phenomenon; due to the larger moisture in the ink, Caused by changes in its performance, especially drying performance deterioration, imprint drying becomes slow, easily lead to prints dirty, dirty.

The correct ink-water balance condition is to realize the ink-water balance with the minimum amount of water under the premise of ensuring that the density of multi-primary solids reaches the standard.

3 How to control the ink balance

Controlling Ink Balance In addition to controlling the amount of water to the minimum based on the correct ink balance conditions during the operation of the offset press, the following points should also be noted:

1) Put the quality of the printing plate. Printing plates are the basis for printing, and are also a prerequisite for ink-water balance. It is necessary to select a PS plate with dense, uniform sand grain, and a certain mechanical strength, so that the plate has a solid graphic basis and a blank basis, maintains a relatively stable lipophilic and hydrophilic, and improves the printing plate's resistance to printing.

2) Manage water - fountain solution. Water management is very important. For common fountain solutions, chemical components such as phosphoric acid, ammonium nitrate, ammonium dihydrogen phosphate, ammonium dichromate, citric acid, and gum arabic are usually added; ethanol or isopropyl alcohol is also added to the alcohol fountain solution. Due to the addition of acidic substances, the fountain solution is weakly acidic, and its acidity has a great influence on the total wetting properties of the fountain solution. Therefore, the pH of the fountain solution must be strictly controlled. The general control range is between 4.5 and 5.5, which is the best value and depends on other factors. For example: the pH value of the paper, the size of the ink on the layout, and the performance of the ink. Ethanol, the main component of alcohol, can reduce the surface tension of fountain solution and improve the surface activity, which is beneficial to the balance of water and ink.

3) Control the ink. Offset inks have a key influence on the ink balance in terms of ink viscosity, fluidity, water resistance, particle size, etc., especially viscosity and fluidity, and must be controlled well. High-viscosity, low-flow inks are also strong in water resistance. Maintaining proper viscosity and fluidity can maintain a relatively stable ink-water balance. If the ink is adjusted to be too thin and fluidity is large, its own viscosity and cohesion are inevitable. If it is decreased, the water resistance and water repellency will be reduced and the emulsification will be excessive; if the ink is dried to accelerate the addition of excessive dry oil, it will also increase the viscosity of the ink and enhance the adhesion. When offset printing is required to stir ink fountains, it is also to improve ink flowability and maintain good transfer performance, which helps maintain the ink balance. Because different inks have different properties, different inks have different requirements for ink performance. Therefore, inks that meet the requirements for performance must be selected.