Ink drying and printing

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 <br> Ink transfer to the surface of the paper to form a liquid film, the film through a series of physical or chemical changes and become a solid film process known as the 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 In the drying process of the ink, if part of the ink is penetrated into the interior of the paper, and another part of the binder 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. This drying method of the ink 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 with dry vegetable oil as the connecting material of the ink, after absorbing oxygen in the air, the oxygen polymerization reaction occurs, dry solid on the surface of the substrate, the use of oxygen polymerization reaction to make the ink from the liquid to solid state, forming The process of gloss, abrasion resistance, and firm ink film is called oxidative conjunctiva drying of the ink. It is a chemical drying process. Oxidative conjunctiva drying is the most important dry form in offset printing.

3. Volatile drying In the drying process of the ink, the solvent in the binder evaporates, and the remaining resin and pigment form the imprint on the paper. The drying of the ink is accomplished by the volatilization of the solvent from the film layer. The ink drying form is called volatile drying and 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. Influence of pigments 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 is accelerated, the intramolecular energy can be increased, the reaction speed of the oxidation conjunctiva is also accelerated, and the drying is accelerated. 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.

4. Effect of paper

Paper has a great influence on the ink drying, mainly in:
(1) Acidity of the paper The acidity of the paper surface reacts with the metal salt in the desiccant to disable the desiccant, prevent or destroy the oxidative polymerization reaction, slow the drying of the blot on the paper surface, the smaller the PH value of the paper, The slower the drying speed.
(2) The structure of the paper surface is rough, the structure is loose, and the paper with a high permeability and a small degree of sizing promotes drying, and vice versa. This is reflected in the osmotic drying method. The ink on the smooth and tough paper surface is thicker than the surface rough and loose paper, it has less contact with the air, the oxidized conjunctiva is slow, so the surface smooth and tough paper imprint slow drying.
(3) Water content of paper The moisture content of the paper has a certain degree of influence on the penetration of the ink and on the oxidized conjunctiva. The moisture content of the paper is high and the penetration of the ink into the paper is reduced. At the same time, the moisture can also cause the oxidized conjunctiva of the ink to be blocked.

5. Effect of fountain solution The stronger the acidity of the fountain solution, the longer the drying time of the ink. This is because ink emulsification inevitably exists during the offset printing process. The acid in the fountain solution reacts with the desiccant. In response, the desiccant fails to dry the ink. Generally speaking, the pH of the fountain solution should be maintained between 4.5 and 6. The stacking has little effect on the drying of the ink.

6. Influence of additives

In the printing process, in order to make the ink have good printing adaptability, it is necessary to add various additives in the ink. In fact, the additive has a certain influence on the drying of the ink. For example, both Weili oil and white oil have the property of inhibiting ink drying and slow drying of the imprint. The main component of Willi oil is aluminum hydroxide. Aluminum hydroxide will absorb the desiccant and make it lose its function. At the same time, the aluminum hydroxide will increase. Ink emulsification, white oil is also very easy to cause the emulsification of the ink, affecting the oxidation of the ink conjunctiva.

In the printing process, in order to reduce the viscosity of the ink, non-drying auxiliary materials such as de-bonding agents are added. The auxiliary materials themselves are dried very slowly. A small amount of the additives does not significantly affect the drying of the ink. If the ink is used excessively, the wax in the de-bonding agent is used. The substance has a small specific gravity and floats on the surface of the imprint and blocks the oxidized polymeric conjunctiva of the binder.

7. Influence of other factors

The smaller the area occupied by layout images, the greater the emulsification of the ink and the less drying of the ink. At this time, it is necessary to control the moisture of the layout, and appropriately increase the amount of desiccant to ensure timely drying of the ink. If the ink layer is too thick, the contact area between the ink layer and the air will be reduced, and the contact between the ink and the paper will also be reduced, and the oxidized conjunctiva and infiltration will be affected.

The stacking of printing products has a lot to do with drying. If the area of ​​the printed sheets is large, the paper surface is smooth, and the stacking height is high, the air is difficult to contact with the ink. If the oxygen is not added in time, the drying will be slow, and the middle of the printed sheets will be even worse.
Whether the printed sheet is exposed or not has an effect on the drying speed of the ink.

Third, the ink drying speed unfavorable degree of harm

1.Hazards caused by ink drying too quickly:
(1) The ink is dried too fast and the viscosity of the ink is increased, the flowability is poor, the paper is pulled, and even when the paper is severe, the paper is torn;
(2) The ink accumulates in the graphic part of the printing plate and the eraser, making the plate graphic dirty;
(3) The ink conjunctiva on the surface of the printing plate and the ink roller is dry and solid, changing the original wetting properties to weaken the lipophilicity, the printing plate is not inked, the ink roller is not under ink, and the graphic is white and pale;
(4) The ink film is dry and the surface is very smooth, loses the adsorption force, and the phenomenon of crystallization occurs, so that the back color is not overprinted.

2. Damage caused by ink dripping or slow drying:

(1) The next color overprint cannot be completed, and the next process such as cutting cannot be performed;
(2) The back of the printed product is dirty;
(3) The amount of ink permeation increases, resulting in powdering, the edges of the picture are blurred, and the picture text is dull.

Fourth, the ink drying control

1. The correct use of dry oil The correct addition and placement of dry oil is one of the important means to control ink drying, according to the nature of dry oil itself, the temperature and humidity of the environment, the structural properties of the paper, the type of oil, the thickness of the ink layer , the amount of additives added, the number of print and other factors to determine the type and amount of dry oil, add too much dry oil, will bring the ink texture thicker, ink roller drying, the ink is diluted, ink emulsification plus Problems such as big printing make it difficult to carry out printing. Actually adding excessive amounts of dry oil can cause unpleasant conditions. In the case of excessive dry oil, the surface of the ink layer will soon dry and form a closed state. The internal drying is extremely slow. However, when the amount of dry oil is low, the printed products cannot be dried within the required time, which will delay the production speed. The printed products are likely to be dirty and seriously affect the printing quality.

2. Strengthen the inspection and management of raw materials The inspection of raw materials is one of the important ways to improve the quality of products. It is necessary to test the printing performance of inks and papers, especially the drying performance. It is estimated that dry oil can be added and discharged, and during the production process, Do not change the viscosity and fluidity of the ink for convenience.

3. Observe the various signs of printing. Ink drying conditions In the printing process, various signs can be shown. Some unfavorable factors also have warning signs. If you can find and take measures, you can prevent the problems of drying too quickly or too slowly. Ensure smooth production.

4. The influence of the pH of the dampening solution and the pH of the plate dampening solution on the drying is as described above. Moisture in the layout will increase the emulsification of the ink, slow the drying, and print with the minimum moisture, which is one of the technical measures to ensure the product is dried in time.

Source: China Printing Chemicals Network