Over-emulsification of ink and its various preventive measures

The incompatibility between ink and fountain solution is the basic principle of offset printing, but in the actual printing process, the complete incompatibility between the two cannot be achieved because the printing process is dynamic, and the water and the ink are affected by rollers, water rollers, Ink roller extrusion, under the action of mechanical force, water and ink will inevitably produce mutual infiltration, dispersion, so emulsification is inevitable, there are two types of emulsification: when the water is large ink hours, the formation of "oil in water (O / W) "When the water is small, the formation of "water in oil (W/O)". Practice has shown that a slight, moderate emulsification of the ink is beneficial to offset printing: when the ratio of the volume of water to the ink in the emulsified ink film is between 15% and 26%, the repulsion and compatibility between the ink and the water ( Dispersion) to achieve a relatively balanced state, ink transfer, printing effect is more ideal, so in the printing process, the fountain solution and ink to maintain a more reasonable supply (of course, this supply contains quality factors), so that ink Being in a relatively balanced state is very important, which is a prerequisite for improving the printing quality.

First, the impact of ink emulsion on printing

If the ink emulsification is just right, it means that the ink is in a relatively balanced state, which creates a good printing condition for the offset printing. The graphic ink can realize the transfer of the ink, so that the dots are clean and tidy, the images are clear, the layers are rich, and the colorful and saturated printing effect. , and to ensure the accuracy of overprinting and promote the drying of the print, while ensuring that the blank part of the non-printing area is clean and tidy. Therefore, whether or not the emulsification of the ink can be correctly controlled and controlled is the key to ensuring the balance of the ink and the stable printing quality. If the ink is not properly mastered and the ink is over-emulsified, it will inevitably cause the ink and ink to become unbalanced and have a bad influence on the printing quality. In practice, if the ink is not properly mastered, it is easy to over-emulsify the ink, which is a great harm to the printing quality.

Excessive emulsification of ink means that there are too many fine droplets dispersed in the ink, which exceeds a certain proportion, and the properties of the ink change immediately. For example, the color of the ink becomes lighter, the color saturation decreases, the viscosity decreases, and the adhesion weakens. Process and print quality are not stable. For example: the gray balance is broken, the color is not restored, the color of the printed product is dark, the level is unclear; the ink transferability deteriorates, the ink color suddenly changes, the difference between the ink before and after the product is large, and the process troubles such as floating dirty and paste version are prone to occur. The drying performance of the print is greatly slowed down, and the back side is likely to be dirty. Therefore, to control the printing quality, it is necessary to prevent the occurrence of over-emulsification of the ink.

Second , the cause of excessive ink emulsification and prevention

There are many factors that cause ink over emulsification. There are mainly paper, fountain solution, ink, diluting agent and desiccant. Therefore, to avoid excessive emulsification of ink, it is necessary to prescribe the right medicine for different influencing factors and prevent it from occurring.

1. Paper factor

As paper, such as certain offset papers, coated papers, and whiteboard papers, the surface sizing, paint, or other fillers contain more or less of the surface-active substances, they have a certain degree of water solubility, if combined with paper. Insufficient, wetted by the fountain solution during the printing process and squeezed and peeled off by the blanket, these surface active substances fall off and are transmitted to the ink roller and water bucket through the printing plate, and are fused with the ink to make the viscosity of the ink. Greatly reduced, water absorption increases, water resistance is affected, causing ink emulsification. The greater the water content of the plate, the more fillers are dissolved and the emulsification is more severe; the sizing agent is not good, the paper is loose, and the more serious the paper is, the more likely the emulsification is.

Preventive measures: 1 to use better quality paper, fundamentally eliminate the paper caused by excessive emulsification of ink; 2 wash rubber, printing plates, timely removal of falling paper powder, paper hair; 3 add a small amount of additives in the ink, And reduce the layout of water and reduce the pressure of the water roller to reduce the paper off powder.

2. Fountain solution factors

The dispersion of the fountain solution is one of the conditions that cause the ink to emulsify. The water film on the surface of the printing plate, under the action of the mechanical force, forms tiny droplets and diffuses into the ink, causing the ink to emulsify, so the amount of the fountain solution is If it is too large, the water film thickness is too thick, and the moisture spreading along the ink roller increases. After the rollers and the ink roller are pressed against each other, the ink emulsification is accelerated and over-emulsified. The greater the amount of water, the thicker the water film, the smaller the PH value, the more serious the over-emulsification.

Preventive measures: 1 control the amount of dampening fluid, to ensure that the plate is not dirty, the print meets the level, density, color and other quality requirements, reduce the amount of ink, reduce the amount of water; 2 control the fountain solution ratio, PH The value should not be too small, it is best to use alcohol fountain solution.

3. Ink factor

The viscosity of ink is the main factor for emulsification, and it can produce different degrees of emulsification regardless of the viscosity. The viscosity of the ink is large, its molecular cohesion is large, the water resistance is strong, and the emulsification value is small; conversely, if the ink has small viscosity, strong fluidity, small cohesion between molecules, and poor water resistance, it is easy to emulsify. Under normal circumstances, the ink viscosity is inversely proportional to emulsification, and fluidity is directly proportional to emulsification.

The difference in water repellency of the pigment itself also causes changes in the emulsification value of the ink. For example, the black ink and the red ink have better water resistance than the chrysanthemum and peacock blue inks, and the emulsification value is also small.

Preventive measures: 1 selection of inks with good water resistance; 2 for easily emulsified ink, can be properly added to the No. 0 varnish, increase the viscosity of the ink, try not to reduce the viscosity of the ink by adding No. 6 varnish; 3 Use new ink.

4. Dilute and desiccant factors

The commonly used diluting agents include dilute agents, white oil, white ink, and virial oil. Among them, the dilute agent is an amphoteric oxide, which has strong hygroscopicity, a large emulsification value, and a large white oil emulsification value. Easy to cause the emulsification of the ink, if you add these diluting agents in the ink, you must make the ink emulsification accelerated; driers such as white dry oil, red dry oil, the emulsification value is larger, adding the appropriate amount of driers can speed up the ink drying, However, if it is added in excess, the drying speed of the ink will be reduced, and at the same time, the ink emulsification will be deteriorated.

Preventive measures: According to the specific circumstances to add diluting agent or desiccant, can not blindly random, try to control within a certain amount, generally about 2% -3%, can not exceed 5%.

5. Excessive emulsification caused by other factors

Other factors such as shop temperature, humidity, vehicle speed, and the pressure of the water roller and the ink roller will also have a certain influence on the ink emulsification. For example, the higher the shop's temperature, the higher the surface temperature of rollers and ink rollers, the thinner the ink, and the greater the fluidity, the lower the viscosity and the lower the water resistance. If the vehicle speed is high, the interaction force is large. These factors affect the ink. The over-emulsification plays a strong catalytic role, the greater the temperature of the workshop, the more easily the ink is emulsified.

Preventive measures: 1 Workshop to install air-conditioning as much as possible, the workshop temperature is controlled between 20 °C -26 °C, the humidity is controlled between 53% -65%; 2 pay attention to adjust the pressure of the water roller, ink roller, make it in the appropriate range Within, to ensure smooth water delivery, infusion of ink; 3 when the idling stop, should shut down the water supply, ink supply, to avoid large ink ink accelerated ink emulsion.

There are many factors that affect the emulsification of ink, and many times, the cause of excessive emulsification of the ink is often a combination of several factors, such as: graphics, small area, less ink consumption, ink prone to prolonged contact with water Emulsification, if the ink is poor in water resistance, the amount of water is too large, the ink is more likely to emulsify. Therefore, for the problem of ink emulsification, we should analyze and eliminate them one by one in various aspects. We must establish the idea of ​​prevention beforehand and focus on prevention. In addition to the good quality of raw materials, we must also develop good work habits and regulations. Process operations, such as: do a good job of "Sanping" and "Sanqin," timely find and solve problems, and pay attention to the accumulation of experience, try to grasp the appropriate ink emulsification, control the balance of ink and wash, printed outlets are clear, structured, color Bright, high-quality prints.