Metal printing belongs to the special printing industry in China, but with the rapid development of packaging printing and the wide application of metal packaging products, the application of metal printing has become increasingly widespread. However, due to the slow start of metal printing in our country and the lack of attention, the quality of metal printing is not satisfactory.
The metal printing (tinplate printing) process utilizes the principle of non-mixing of ink and water, that is, it is hydrophilic and oil-repellent on the blank part of the PS plate, and the graphic part is hydrophobic and water-repellent. When printing, the plate is wetted to make the blank part adsorb first. Water film, and then give the ink on the printing plate, the graphic part of the ink printing. As the appearance of the printing requires fine, overprint precision, anti-counterfeiting, low waste rate, etc., the use of conventional wetting, poor quality prints, it is difficult to achieve the above requirements. Especially in the tinplate lithographic printing process, since the tinplate has no moisture absorption to the blanket cylinder, the moisture easily stays between the rollers. If an ordinary dampening system is used, it is difficult for the operator to grasp the ink-water balance in the iron printing process. This will cause the following drawbacks:
1. Too much water in the ink will cause the ink to emulsify, the ink is light, the dots are not clear, at the same time, too much moisture will hinder the normal transfer of the ink between the ink rollers; 2. Too much water on the plate, before and after printing is interrupted, the ink color The gap is large and affects the color consistency of the product. 3. The water volume is too small to make the printing paste and the water glue sleeve dampening particularly obvious; 4. The intermittent water conveyance of the traditional water rubber sleeve makes it easy to unevenly distribute or generate water .
Therefore, advanced iron printing machines often use alcohol moisturizing devices, such as: the famous British manufacturer of printing machine CRABTREE (Coli), Fuji Corporation of Japan are using alcohol to meet the requirements of metal printing, but the price is relatively high.
First, the principle of alcohol wetting
1. Alcohol fountain solution
The main ingredient in alcoholic fountain solution is alcohol, plus appropriate amounts of phosphoric acid and hydrophilic colloids. Alcohol, also known as ethanol, is an organic solvent with a short hydrocarbon chain and is a polar molecule. Because of the hydrogen bonding of the molecules, it can form azeotropic mixtures with water and hydrogen bonds with water molecules. As a result of the association of the hydrogen bonds of the alcohol molecule and the water molecule, the surface tension of water can be significantly reduced, the affinity of the solution to the plate surface is increased, and the erosion of the blank portion by the ink is repelled. Alcohol type fountain solution, the ethanol content is generally used formula is generally 13% to 18% isopropyl alcohol, 1% to 2% a small amount of gum arabic, 2% to 3% fountain solution, 77% to 84% water Mixed composition. See Table 1[5].
2. The principle of alcohol dampening
The surface of the printing plate is composed of a metal salt layer, a metal oxide, and a fat sensing layer. The metal salt layer and the metal oxide layer and the sensing layer divide the entire printing plate into hydrophilic and lipophilic surface areas. Therefore, water and ink can selectively adsorb on the hydrophilic and oleophilic domains, so that part of the graphic image is lipophilic and adsorbs ink, and the blank part forms hydrophilic water film [4].
Table 1 The formula of several alcohol dampening fluids
Tab.1 Formulae of alcohol wetting liquid
The theory of selective adsorption of fountain solution on printing plate is as follows:
1. Surface energy theory
It is known from human physics that the surface layer of a liquid has characteristics that are not internal to the molecule due to the mutual attraction between molecules and molecules. The relationship between the solid surface and its internal molecules is exactly the same as that of the liquid, but the shape of the solid surface is certain, and the effect of interfacial tension is the adsorption of gas or liquid molecules on the solid surface [6].
The force per unit length of the liquid surface boundary is surface tension and the unit is N/m. Due to the presence of surface tension, the liquid has the tendency to automatically shrink its surface into a spherical shape when no external force is applied.
The surface tension of the printing ink is generally (3.0 ~ 3.6) × 10-2N / m, and the surface tension of the fountain solution is (4 ~ 7) × 10-2 N / m, the ink can be easily printed in the plate The surface is adsorbed and spread on the surface, but the water cannot spread on the surface of the lipophilic base because the spread of the ink on the lipophilic base forming the graphic text can lead to a decrease in the energy of the system. From the energy data, this is a release of energy. the process of.
2. Polarity theory.
In the molecule, the number of positive and negative electrons is equal, and the entire molecule is neutral. However, for positive and negative charges, the charge center may converge at one point or at different points. When the positive and negative charge centers are divided into two points, the whole molecule has polarity; if the positive and negative charge centers are at the same point, The entire molecule does not have polarity; if the positive and negative charge centers are at the same point, the entire molecule has no polarity. Precisely because of the existence of such polar phenomena and the law of similar attracting between substances, that is, molecules adsorbing polar molecules, non-polar molecules adsorb non-polar molecules, so when the fountain solution with strong polarity passes through the same When the plate is strongly polar, it can be adsorbed by the plate, and the molecules in the graphic part (photosensitive compounds) are non-polar molecules and do not adsorb the alcohol solution, thereby realizing the selective adsorption of the fountain solution.
3. Alcohol consumption and moisturization
From FIG. 1 , the contact angle of the water droplet on the solid surface is larger than the contact angle θ between the alcohol solution and the solid surface. The size of the contact angle is related to the amount of alcohol solution added to the water. When the angle θ is small, the larger the contact area between the same volume of droplets and the solid surface, the better the wetting effect.
figure 1
It can be known from the Yang-style wetting equation: cos z = (bc)/a
Where: z is the contact angle; a is the surface tension of the liquid; b is the surface tension of the solid; c is the interfacial tension between the liquid and the solid.
In the wetting equation, b is determined by the type of solid and is a constant. A and B are proportional to the contact angle. To make a certain liquid wet the surface of a liquid well, the antenna should be reduced. The specific method is as follows:
1 reduce the surface tension of the liquid;
2 Reduce the interfacial tension between liquids and solids.
The relationship between alcohol concentration and surface tension is shown in Figure 2. As can be seen from the figure, the surface tension decreases as the alcohol concentration increases. That is to say, adding alcohol in a certain proportion in water can reduce the surface tension of water, change the wetting state of water on the plate, and reduce the amount of water on the plate surface.
However, this is not to say that the greater the content of alcohol is, the better it is. Generally, the actual amount of alcohol in the fountain solution is between 6% and 15%.
figure 2
4. Alcohol wetting characteristics
Because alcohol can greatly reduce the surface tension of the fountain solution and improve the film formation of the fountain solution on the printing plate, the degree of emulsification of the ink is low, which is beneficial to the transmission of the ink and the reproduction of colors. The characteristics are as follows [2] :
1 Can significantly reduce the surface tension of water, reduce the water supply, maintain the stability of fountain solution, easy to achieve the same ink. Due to the volatility of alcohol, the emulsification value of the ink is greatly reduced, and the print can be brightly colored and glossy.
2 The alcohol-based fountain solution can use a fleece-free water roller, so the product quality is not affected due to aging of the water roller, and the production cost is reduced at the same time.
3 The time required to reach the water and ink balance is less and the amount of ink is stable, so the auxiliary time can be shortened.
However, because alcohol is volatile, it has a certain impact on human health and environmental protection.
Two, tinplate performance analysis
1. Tinplate printing features
The principle of tinplate printing is similar to that of lithographic offset printing, but due to the special nature of the material, the structure of the equipment should be correspondingly structured to suit the needs of tinplate printing [1].
Tinplate printing has the following features:
1Because the surface of the tin plate is a tin plating layer, it has a glittering color effect. After the background color printing, the printed picture has the characteristics of bright colors, rich layers, and good visual effects.
2 The tinplate has good mechanical properties, processing and forming properties. Its printed products can realize novel and unique design, and can be used to manufacture various shaped containers, cans, boxes and other packaging containers.
3 The good performance of tinplate and the good abrasion resistance and durability of its printing ink are conducive to the realization of the use value and artistic unity of the goods.
Third, the application of alcohol dampening system in tinplate printing
Alcohol wetting liquid is usually prepared by adding ethanol or isopropyl alcohol in common wetting fluid. The effect of isopropanol is similar to that of ethanol, but its rate of volatilization is slower than that of ethanol. Ethanol improves the spreading performance of the wetting fluid on the printing plate, greatly reducing the amount of wetting fluid, so it also reduces the water and the severe emulsification of the ink. Since ethanol has a large latent heat of evaporation, it can carry a lot of heat when volatilized, so that the layout temperature is reduced, the dot increase value decreases, non-graphical parts are not easy to stain dirty. Therefore, the use of alcohol wetting fluid can print high-quality fine offset products, which has been highly praised by people in the printing industry in recent years.
However, the volatilization rate of ethanol is faster, and if it is not controlled, the wetting liquid will decrease due to the concentration of ethanol. As the surface tension rises and the wettability decreases, it is necessary to replenish the consumed ethanol in time. At the same time, in order to reduce the volatilization of ethanol, the temperature of the wetting liquid should be controlled to be below 10°C (preferably 4 to 9°C). Therefore, an offset printing machine using an alcohol wetting liquid generally adopts the auxiliary equipment for circulating cooling of the alcohol wetting liquid shown in FIG. 3 and automatically adding ethanol. The water fountain roller and the metering roller are independently driven by the DC speed regulating motor, while the water roller and the axially moving bridge roller are driven by the plate cylinder gear.
image 3
1. Alcohol wetting water supply device
1 fountain solution pre-mixing device: mix isopropyl alcohol, water and additives;
2 damper circulator: convey fountain solution to the freezing fountain device;
3 Low-temperature freezing equipment: The temperature of the fountain solution is kept at about 5°C, the main purpose is to prevent the isopropyl alcohol from evaporating and the ink emulsification and other failures;
4 Bucket: A good insulation layer is needed to keep the fountain solution at about 5°C.
2. Water control method
This system water volume control is adjusted by three aspects of pressure, inclination and speed to achieve ink-water balance.
3. Advantages of Alcohol Wetting System
Due to the small amount of dampening solution used in the alcohol dampening system, the working temperature of the layout is low, and the concentration is kept constant to improve the printing quality. In addition, the tinplate printing has the following features:
1 The proportion of rotation of each water roller of the alcohol dampening system is different (when the transmission ratio of the roller and the dampening roller is 1:0.8), the amount of the dampening fluid does not change even if the pipeline speed changes. Consistent, so that the ink balance to achieve print quality requirements, in addition to this combination of ratios, also has the role of protecting the PS plate surface.
2 The swinging roller W4 can swing and traverse between the dampening roller and the ink roller. Adjusting it can make the dampening plate on the dampening roller more uniform, and can also remove the water line and the printing ghosts and inks on the printing surface. Other product defects.
3 When the fan is working, it can not only reduce the operating temperature of the ink system, but also can also take the water in the ink path, which can maintain the printability of the ink and reduce the ink emulsification to play a certain role.
4 The alcohol fountain system continuously delivers water to avoid the uneven distribution of dampening fluid and the water plumbing failures caused by intermittent effluent.
4. Conclusion
It can be seen that alcohol wetting plays an important role in metal printing. Controlling the amount of alcohol and improving the effect of alcohol moistening can greatly improve the quality of metal prints. Therefore, alcohol dampening has a wide range of application prospects in the packaging and printing industry.