Offset ink balance principle and control method

1 what is the ink balance

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 image, 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 portion and the graphic portion 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 in ink, 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 just a relative concept and it is a dynamic balance.

The balance of ink and ink is just right, and the ink can be transferred in an ideal way to achieve dot-dyeing, orderliness, clear images, rich layers, colorful and saturated printing effects, and to ensure the accuracy of overprinting and to promote the drying of prints 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, due to the insufficient amount of water in the printing plate to form a water film, the blank part of the print does not have enough ink repellent ability and can not 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 supplied is also small, the amount of ink absorbed by the graphic part is insufficient, which will result in the imprint being dark and dull, 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 is easily adsorbed to the blank non-image area, resulting in sticky; the dots are not clear And the proliferation of outlets is serious, resulting in the loss of image layers, and the text is also prone to scumming, especially against small and white texts. Due to the small water, the lack of water in the graphic areas with different structures is not the same, and it is easy to cause uneven layout ink.

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 the moisture will spread along the ink roller. After the rollers and ink rollers are pressed against each other, the ink emulsification speeds up, forming a "water in oil ( O/W) type of emulsification, which hinders the normal transfer of ink. The graphic area is not saturated with ink, or even inked. The graphic ink becomes lighter, the print is not strong, the dots are dark, dark, dull, and the images are false and lacking in level. 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 viscosity and cohesive force of the ink 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 flat. At this time, even if the ink supply is increased again, the effect will not be improved. At the same time, there will be a phenomenon that the water roller sticks dirty and the surface of the plate will “float”; due to the large amount of moisture in the ink, the performance of the ink will change, especially the drying performance will deteriorate. Drying becomes slow, which can easily cause dirty and dirty prints.

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.

3How 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 ordinary fountain solutions, chemical components such as phosphoric acid, ammonium nitrate, ammonium dihydrogen phosphate, ammonium dichromate, citric acid, and gum arabic are usually added; alcoholic 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 the fountain solution and improve the surface activity, which is beneficial to the ink-water balance.

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, which must be controlled. 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 drying, adding an excessive amount of dry oil 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.

Reprinted from: Screen Printing Industry