Printing balance of offset fountain solution (middle)

1) Ink balance concept

The mixing curves of the three different inks, the ordinate is the amount of water mixed in the 100g ink measured after mixing the wetting liquid and the ink, and the abscissa is the mixing time of water and ink. Curve A indicates the ink that is most likely to be mixed with water. Since the water content in the ink is too high, the ink head is too short, and the ink is difficult to output from the ink fountain of the printing press, so it is not suitable for offset printing; the curve C indicates that the ink is not easily mixed. Water ink, the ink is almost repellent to the water, the fountain solution squeezed into the ink in the printing quickly separates from the ink, attaches to the surface of the ink and forms a water film, which hinders the ink The transfer to blankets, experience shows that this ink is very poor transfer performance, and may make the print hair, it can not be used for lithography; only the curve B ink can make the print smoothly, experiments proved that when the curve B The moisture content of the ink reaches 26% (ie, when 100g of ink contains 26g of water), the printing quality is best.

2) Factors affecting ink emulsification in fountain printing

1 wetting agent

The dampening solution contains a certain amount of wetting agent. The wetting agent is also a kind of surfactant, which not only can reduce the surface tension of the fountain solution, but also accelerate the ink emulsification. Gum arabic is a kind of colloid with excellent hydrophilicity. Although it is mainly used to rub the surface of the printing plate to protect the printing plate, a small amount of gum arabic is often added in the fountain solution. Its purpose is to improve the fountain solution. Degree of wetting, but it also increases the emulsification of the ink while increasing the degree of wetting of the dampening plate.

2 fountain solution acidity

The fountain solution should have some weak acidity, which is determined by the process technology. Due to the physical and chemical effects of the printing, calcium salts are deposited between the fountain solution, the ink roller, the printing plate, and the blanket. Excessive calcium salts may cause ink transfer difficulties and emulsification imbalance. The calcium salt is mainly derived from the paint on the surface of the paper, the hardness of the water, and the pigment of some inks (such as red ink). The excessively acidic dampening solution accelerates the dissolution of the ink pigments and the dissolution of the paper coating.

3 fountain solution dosage

Excessive fountain solution accelerates the emulsification of the ink from the “oil-in-water” state to the “oil-in-water” emulsification state. When this happens, the emulsification of the ink and the balance between the ink and the fountain solution will be affected and the balance will be lost.

3) Measures to ensure ink and water balance

1 During the printing process, the printing plate must have a solid graphic foundation and a blank basis to maintain the stability of oleophilic and hydrophilic.

2 Normal fountain solution has large surface tension, poor wettability, and a large amount of supply; alcohol fountain solution has a low surface tension and good wetting properties. It can achieve ink and water balance with less fountain solution. The surface tension of the non-ionic surfactant fountain solution is also low, and the fountain solution supply should be less than the ordinary fountain solution.

3 Grasp the principle of less water and thicker ink. Prerequisite that the blank part of the layout is not stained, try to apply the minimum amount of water to achieve good ink transfer. Water is over-emulsified and the ink layer cannot be thick. From the point of view of the transfer process of the offset printing ink, three times of ink mixing and emulsification occurred in one water supply and ink supply, and it is impossible to maintain a strict boundary between water and oil. Therefore, the balance of water and ink in offset printing can only be a relative concept, and the perfectly ideal ink-water balance does not exist. As long as the best balance is achieved, the ideal print can be printed.

4 For smooth paper, the water supply can be slightly smaller, rough paper can be slightly larger; when the machine is running fast, the amount of water can be slightly smaller, and the amount of water can be larger at low speed operation.

5 The environment temperature and humidity can not be ignored. During the printing process, the layout moisture is distributed in two forms: direct and indirect. The layout moisture meets the needs of ink and water balance during printing, and most of them are distributed to the space. The higher the ambient temperature, the faster the emission will be.

6 Control the pH of the fountain solution at about 4.5 to 5.5. In addition, since the pH value of offset printing paper surface has a great influence on the pH value of the fountain solution, it is better for paper.

The pH is measured. When the pH of the paper is too low, the pH of the fountain solution should be slightly raised; conversely, if the pH of the paper is too high, the pH of the fountain solution should be appropriately reduced. According to the relevant information and practice proved that when the pH value of the paper is 9, the pH of the fountain solution is 4; when the pH of the paper is 8, the pH of the fountain solution is 5 as well.

7 Grasp the pressure of the water and ink roller. If the pressure is too small, the water supply and ink supply will be poor. If the pressure is too high, the plate will be damaged.

3 balance between fountain solution and printing plate

1) The effect of the amount of fountain solution on the printing plate.

The balance between the ink and the fountain solution does not mean that the relationship between the printing plate and the fountain solution is balanced. When the amount of the fountain solution is too large, the ink will be over-emulsified and there will be too much fountain solution on the surface of the printing plate. This is a This is a very obvious situation. However, if the fountain solution is only too high, the ink will not be emulsified immediately. This is because the more dampening solution is transferred to the ink roller after adsorption by the printing plate. At this time, the amount of fountain solution on the printing plate is too much, and the amount of printing plate and fountain solution normally needed is out of balance. The surface of the printing plate will show a phenomenon of splashing at this time. If the amount continues to increase, the problem of over-emulsification of the ink occurs.

The use of a small amount of fountain solution can also cause the imbalance between the printing plate and the fountain solution. The most obvious is that the inorganic salt layer on the surface of the printing plate is not stably enriched and protected, and the metal aluminum is exposed, resulting in a dirty plate phenomenon. .

A lot of experience in experiments and productions has proved that when the normal printing, the ink layer thickness of the printing plate graphic part is 2~3μm, and the blank water film thickness is about 0.5~1μm, the ink contains the volume of the fountain solution. The score is 15 to 26% and the maximum is no more than 30%. Basically, the ink and ink balance of offset printing is achieved.

2) Effect of acidity of fountain solution on printing plate

From the viewpoint of the composition of the fountain solution, mainly acids and salts, in the printing process, due to chemical reaction results, the inorganic salt protective layer on the surface of the printing plate can be compensated in time. During the printing process, the surface of the printing plate is rubbed by the water roller, and the inorganic salt protective layer on the printing plate surface will be damaged to varying degrees, and the acid and salt substances in the fountain solution need to be supplemented. If the dampening acidity is too strong or too weak, this compensation will not work properly. This situation is actually the imbalance between the printing plate and fountain solution. (To be continued)