Drying and Printing of Ink in Offset Printing (I)

The quality of an offset product depends not only on the printing plate, the machine, the paper, the fountain solution, the ink, and the technical control of these conditions, but also on the drying mode and drying speed of the ink. A printed matter is often good for printing, but due to the ink drying too quickly or drying too slowly, the overall quality of the printed matter is finally seriously affected, or the normal production is affected, and even becomes a waste product.

In the printing process, there are many ways to dry the ink. The drying of the ink is a complex physical and chemical process. It is generally believed that the correct addition of dry oil is the key to solving this problem. In fact, the correct placement of dry oil in printing is only the control of ink. Drying on the one hand, the final decision on the drying speed lies in the mastery of the technical process during the printing process and the control of various physical and chemical factors. The temperature of the environment, the emulsification value of the ink, the pH value of the paper and fountain solution, etc. are all related to the ink. The drying is closely related. Only by fully understanding and understanding the principle of ink drying and the factors that affect the ink drying, accumulating experience, mastering the rules, and improving the operation level, can we control the timely drying of ink and ensure the quality of printed products.

First, the ink drying method

The ink is transferred to the surface of the paper to form a liquid film layer. The process of the film layer becoming a solid film layer after a series of physical or chemical changes is called a drying process. Different ink types have different drying processes and mechanisms. The drying speed of the ink is related to the drying mode. The drying mode of the ink depends on the connecting material of the ink. In most cases, the drying of the ink is mainly based on a certain drying method, accompanied by others. Drying method to complete, the ink drying on paper mainly in the following ways:

1. Infiltration drying

During the drying process of the ink, if part of the ink is penetrated into the interior of the paper and the other part of the ink is fixed on the surface of the paper together with the pigment, the drying of the ink is accomplished by the permeation of the ink and the absorption of the paper. The drying method is called osmotic drying and it belongs to physical drying. Infiltration includes pressure infiltration and free infiltration: When the printing plate comes into contact with the paper, pressure infiltration occurs. After the printing, under the action of the capillary force of the paper, free penetration occurs. After a certain time, when the capillary force of the paper and the pigment particles of the ink When the capillary force is almost equal to the tension of the connecting material, the permeation stops. During the printing process, pressure penetration dominates. Both pressure and free penetration are related to the distribution of the capillary pore size of the paper and the distribution of the size of the ink pigment. The viscosity of the ink is small or the flowability is large, and the paper is porous. If it is loose, it will have a long penetration time and a large amount of penetration. The longer the time of infiltration, the greater the amount of infiltration; the faster the ink dries up, the less the amount of infiltration. The osmotic drying has the characteristics of fast drying speed, but due to the large amount of ink absorbed into the fibrous tissue of the paper, the imprinted ink layer is dull, there is no gloss, the phenomenon of ink pulverization and the like, seriously affect the product quality, so in the offset printing, the printing product Mainly rely on oxidation conjunctiva drying.

2. Oxidative conjunctiva drying

After the dry vegetable oil is used as the connecting material, after the oxygen in the air is absorbed, an oxygen polymerization reaction takes place to dry the surface of the substrate. The oxygen polymerization causes the ink to change from a liquid state to a solid state to form a glossy, rubbing-resistant film. The process of a solid ink film, known as the drying of the oxidized conjunctiva of the ink, is a chemical drying process. Drying of the oxidized conjunctiva is the most important form of drying in offset printing.

3. Volatile drying

In the drying process of the ink, the solvent in the binder evaporates, and the remaining resin and the pigment form an imprint on the paper. The drying of the ink is accomplished by the volatilization effect of the solvent from the film layer. Such an ink drying form is called Volatile drying, it is a physical drying process.

Second, the impact of the drying factor

Ink drying should be timely, to meet the needs of production and quality, ink drying speed, and the impact of many factors, and the external conditions, raw material properties, process operations and other factors are closely related, including the following aspects:

1. The effect of desiccant

The drying of the ink layer on the paper surface is mainly a process of physical and chemical changes. In particular, the process of oxidizing the conjunctiva of the connecting material takes a long time and cannot meet the production requirements. Therefore, a desiccant is added to the ink and the dry oil is added. Accelerate the drying of the print.The desiccant can make the binder in the ink accelerate the absorption of oxygen in the air to promote the molecular polymerization, and can accelerate the polymerization reaction, which greatly promotes the drying of the imprint.

2. The influence of pigment

Different pigments have different effects on ink drying. Some pigments can promote drying, some slow drying, and others have no obvious effect on ink drying. The pigments that promote drying include iron blue and chrome yellow, which delay the drying of carbon black, lead oxide, etc. Most organic pigments have no significant effect on the drying of the ink. In the same type of ink, the drying speeds of red ink, black ink, blue ink, and yellow ink are accelerated in turn.

3. Influence of temperature and humidity

When the temperature rises, the movement speed of the material molecules increases, the intramolecular energy increases, the reaction rate of the oxidized conjunctiva also accelerates, and the drying speeds up. In general, the drying speed is doubled for every 10 degrees of temperature increase. The drying process is an exothermic reaction, and the heat released when the sheet is dried, in turn, also promotes the drying of the ink.

The effect of humidity on drying is related to the pigment in the ink. When the humidity of certain inks is up to %&', the drying of the ink is not affected. However, most of the pigment inks are affected by humidity, the relative humidity of the air is high, the speed of the oxidative conjunctiva is correspondingly slowed, and the drying speed is also slowed down.


(to be continued)