The traditional water-based offset printing is to control the balance of water and ink to achieve the purpose of ink affinity in the graphics and text, and hydrophilicity in the non-graphic blank portion, so as to complete the printing process. With the participation of fountain solution, it is also playing the role of half angel and half devil.
Some readers may be wondering why angel and devil are used to interpret the fountain solution in water offset printing. In fact, in actual production, the control of the amount of fountain solution is very important. Impact. For example, the phenomenon of frequent dusting, fluffing, and even "peeling" of the paper during the printing process that has to stop the rubber roller to scrub, which certainly has a certain relationship with the performance of the paper, but we must also consider the fountain solution Whether the dosage is too large or the PH value is too acidic affects the reduction of the paper surface strength. Furthermore, a very important reason for the phenomenon of "dirt back dirt" from time to time in the printing production process is that the amount of fountain solution is excessive or the proportion of fountain solution is improper, and the acidity is too strong to make the ink dry too slowly. In many cases, the colors of the printed matter are exquisite and the overprinting is accurate, because the back side is partially smudged, and it cannot be properly resolved. Only the printed matter is scrapped. Therefore, the traditional water-based offset printing is inseparable from the fountain solution, but if it is improperly controlled and has an adverse effect, it is indeed a headache.
The fountain solution is mainly composed of water, isopropyl alcohol or alcohol substitutes and water bucket liquid, which through their respective functions jointly complete the role of fountain solution in traditional water-based offset printing. A brief introduction is given separately.
"Clean" water
The main component of fountain solution is water, but it is not our commonly used tap water. Since tap water contains more calcium and magnesium ions, it will generate calcium saponification after chemical reaction with the ink resin, and it has strong adsorption to rubber materials such as water roller and ink roller. If contacted for a long time, the surface of these rollers will be crystallized, resulting in poor water transfer and ink transfer. If not handled in time, the rollers will be scrapped. Because of this, we often see filters installed in many printing shops or simply buy bottled water for use. It is particularly emphasized here that if you use bottled water, you must ensure that the water quality has filtered out the calcium and magnesium ions well. If a filter is used, it must be ensured that the filter has good ion filtering function, and the filter element must be replaced regularly.
We know that "clean" water must be used in fountain solution, but what kind of standard can be called "clean"? In fact, the detection indicators of filtered water mainly include conductivity, PH value and solid matter content (PPM) value. It must be tested by a water quality analyzer before adding alcohol and water. The author suggests that the conductivity of water should be less than 400 μs / cm, the PH value should be 6.8 to 7.1, and the PPM value should be less than 300. Generally speaking, the measured value only represents the result at that time, because the quality of water changes at any time, so the qualified water quality should be used as soon as possible to avoid the waste of the water quality due to the change.
No more no less alcohol
In order to prevent the occurrence of dirt on the back side during the printing process, as long as the ink supply is guaranteed, the less water is used, the better. The usual measure is to add a certain amount of alcohol to the fountain solution. We can understand in this way that the same volume of water will have a contact area when it comes into contact with the printing plate, but when it contains a certain amount of alcohol, its contact area with the printing plate will increase, thereby reducing the amount of printing The total amount of water required. The more alcoholic substances are put in, the better the effect of fountain solution will be? This is not the case. Under normal circumstances, the amount of alcohol accounts for 8% to 10% of the fountain solution composition. It is enough to add too much, but it will adversely affect the printing effect. It can be mainly attributed to the following points.
1. Excessive addition of alcohol can cause abnormal ink emulsification, which makes it difficult for the inking roller to inking
2. Too much alcohol can damage the adhesive in the ink, dissolve the protective film of the pigment in the metal ink, and reduce the vividness and gloss of the color of the printed matter.
3. Too much alcohol can damage the paper coating, reduce the surface strength of the paper, and cause powder and hair loss or even "peeling".
4. Alcohols are volatile organic compounds. Excessive use can harm the environment and human health.
5. Alcoholic substances are also part of the printing cost. Excessive use is not conducive to reducing production costs.
In addition, the author needs to point out that although isopropyl alcohol (IPA) and industrial alcohol are both alcoholic substances, due to the slow evaporation rate of isopropyl alcohol, it has a stronger ability to reduce the surface tension of the solution, so it is a dampening solution The best choice. Since the price of isopropyl alcohol is higher than that of industrial alcohol, it is inevitable that the industrial alcohol will be mixed with isopropyl alcohol and then sold as pure isopropyl alcohol. Here provides a simple and convenient detection method for readers to screen. The appearance of isopropyl alcohol itself should be clear and pure, which is the first step of the test. Then use a cup to mix water and isopropyl alcohol in the ratio of 1: 1. After standing for 30 to 45 minutes, observe the clarity of the liquid: good isopropyl alcohol should continue to maintain a clear and transparent appearance, if it becomes cloudy Consider its composition. Of course, there is also a more professional test, that is, the purity content is detected by an isopropanol refractometer, but the equipment is not easily available on the market, and can be replaced by an alcohol refractometer.
"Harmonious" Water Bucket Liquid
The water bucket liquid is actually a general term after mixing a variety of fountain solution additives, mainly including acidic electrolytes, buffers, surfactants, hydrophilic colloids, preservatives, bactericides, defoamers, etc. Why add water bucket liquid? Here may be a common sense in life. If you have oil on your hands, it is very difficult to wash off the oil if you rinse it with clean water. However, if you use substances such as soap and hand sanitizer, it will become very easy. The reason is that oil and water are mutually exclusive substances, but the two insoluble substances can be indirectly made miscible through the intermediate medium. The same is true for the balance of ink and water in printing. The ink and water itself are not miscible, because the acidic electrolyte in the bucket fluid provides an acidic environment (pH value of about 4.8 ~ 5.5), so that the ink and water can be miscible, forming a "water-in-oil" dispersion system, that is, ink Of emulsification. When the volume of water accounts for about 15% to 26% of the ink-water mixture, the ink and fountain solution are in a state of mutual dissolution. In addition, other added components in the water bucket liquid can inhibit the volatilization of isopropanol, control the surface tension and PH value of the fountain solution, and clean, protect and repair the printing plate.
Although the water fountain is also an essential part of the fountain solution, the harm of too much water fountain to the printing production is also huge. There are mainly the following points.
1. Excessive acidity of fountain solution will destroy the surface strength of the paper, resulting in the phenomenon of powder and fluff.
2. Excessive acidity of the fountain solution will also lead to plate-off or reduced printing durability of the plate.
3. Too much acidity of fountain solution will make the ink drying slower, so that the back of the printed matter will be smudged.
"Suitable" deployment
After the water, alcoholic substances and water bucket liquid are available, the most important task is to properly distribute them. According to the author's experience, mixing 4% water bucket liquid + 10% isopropyl alcohol + 86% filtered water is the most suitable. In addition, the temperature of the water tank should be set at about 8 ° C, and the temperature of the water tank should be set at 10 ° C to 11 ° C. The temperature can also be slightly increased by 1 ° C to 2 ° C to reduce the volatilization of isopropanol.
With the continuous breakthroughs in technology, the alcohol-free fountain solution has been widely used in practical production. While we fully understand the relevant knowledge and skills of traditional fountain solution and better print, we also look forward to more environmentally Alcohol fountain solution brings more surprises.
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