The function of the offset printing wetting system is to transfer the water-based solution to the printing plate, so that the non-graphic part of the printing plate rejects ink. Isopropyl alcohol (IPA) is the most commonly used additive in fountain solution. IPA can reduce the surface tension of fountain solution and increase the viscosity of fountain solution; IPA volatilization can also play a "cooling" role. The alcohol-based fountain solution also has the advantage of making the operation of the offset printing process more "loose", which can make up for the problems caused by factors such as outdated printing equipment, poor maintenance, and lack of operator skills.
The original reason for canceling alcohol-based fountain solution was a severe shortage of IPA supply; today's main driving force comes from environmental protection, health and safety. Using alcohol-free (accurately speaking should be no IPA) printing, simple adjustment of ink and water, good printing quality, clear dots, and the ink is rarely emulsified. In the early days of non-alcohol printing, the printing house had worked hard to find products with IPA characteristics that could replace IPA. They can be called the first generation "alcohol substitutes". Today, the second generation of advanced formula is already a well-deserved "unfermented" formula. In this article, "alcohol-free" can also refer to the replacement formula of unleavened (IPA) and alcohol (IPA).
In terms of the effect on the viscosity, surface tension, PH value and conductivity of the fountain solution, alcohol substitutes were asked about IPA. Commonly used alcohol substitutes are mainly composed of one or more glycol and glycol ether family chemicals and other additives. Initially these products can only reduce the amount of IPA; now they can completely replace the IPA in the fountain solution and have better operating performance. Unleavened products are not universal, so printers must contact suppliers closely to obtain the best fountain solution.
It is said that web printing companies tend to use single-component alcohol-free products, while sheet-fed printing companies prefer to use composite alcohol-free products. In comparison, compound alcohol-free products are more flexible. Affected by the "integrated system" way of thinking, general fountain solution products often add all the substances that need to be added. This fountain solution is generally considered to be the most reliable. It is worth noting that the concentration of alcohol substitutes in fountain solution is much lower than that of IPA. To investigate the performance of fountain solution, we generally start with the following aspects:
1. Viscosity
Viscosity is a measure of fluid resistance to flow. The concentration of alcohol substitute in the fountain solution is much lower than that of IPA, so the alcohol substitute has almost no effect on the viscosity of the fountain solution. Due to the low viscosity of the fountain solution using alcohol substitutes, in continuous flow contact type wetting systems, there is very little fountain solution on the water transfer or metering rollers. The supply of plate solution. One way to compensate for the loss of viscosity is to cool the fountain solution, but excessive cooling of the fountain solution can also have a negative effect.
2. Surface tension
Quickly wetting the printing plate and forming a water film are the two key requirements for the offset fountain solution, which is mainly determined by the surface tension of the fountain solution. The surface tension of pure water is 72 dyne / cm. In the fountain solution, the alcohol with a concentration of (10-25)% can reduce the surface tension of the fountain solution to (35-45) dynes / cm, so that the fountain solution can be quickly in the form of a continuous film The supplementary film on the printing plate is opened. Alcohol-free fountain solution uses surface active agent and solvent to reduce surface tension. Due to its polar molecular orange structure, surfactants will accumulate at the interface. When the printing press is running at high speed, the interface changes rapidly, and the surfactant must also be transferred quickly to compensate for the new interface. The content of surfactant in the fountain solution is very important in the printing process. When the content is too large, it will cause the emulsification of the ink.
It is generally believed that a fountain solution with a low surface tension is more suitable for high-speed printing, but there are reports to the contrary. Today's alcohol replacement pairs achieve the same low surface tension as IPA.
3. Printing brightness and gloss
Alcohol will attack the ink, make the ink tarnish and affect the color. At the same time, the printing machine's ink wont must maintain a thicker ink layer to achieve a qualified color. When using alcohol substitutes, it will not dilute the ink at the correct concentration, so the amount of ink used and the amount of water used are less. The result is that the dots are enlarged and the dots are clearer.
4. The adhesion of fountain solution and ink film
The ink must be able to adhere to a certain amount of fountain solution to form an emulsion. Under ideal conditions, the fountain solution is quickly adhered by the ink and reaches a balanced operating condition. Under balanced operating conditions, small dampening droplets diffuse into the ink, forming a "water-in-oil" emulsion locally. The pressure between the printing plate and the blanket helps emulsification to occur. The amount of fountain solution in the emulsion can affect the density, dryness, viscosity, viscosity, and ink transfer performance of the ink. Excessive emulsification will disrupt the balance of water and ink, resulting in thinner ink layers and incomplete images, causing "snowflake" to appear on printed products.
5. Conductance and pH
Conductance is a measure of the electrical conductivity of a material; pH is a measure of the concentration of hydrogen ions in water. Consistency of conductivity and pH is very important for fountain solution.
The conductance of pure water is close to 0 μm, and it is a good conductor of electricity. The conductance of water is proportional to the amount of ions in the water, so conductance is also used as an approximate measure of water quality. Before the printing press is used, the conductivity of the fountain solution needs to be measured. Abnormal changes in conductance are caused by any impurities. During the printing process, due to the contamination of the ink and paper, the increase in dampening wave conductance is normal. According to the GATF approach, during the printing process, the battery is measured every 4 hours and the conductivity of each new batch of fountain solution is also measured. When replenishing water, the conductance change after replenishing water should be less than 50 μm. If the conductance change reaches 200 μm, it must be treated before it can be used.
When measuring conductivity, the pH value is generally measured at the same time. The pH value of the fountain solution should generally be 4.4 to 5.0. If the pH value is too high (above 5.5), the printing plate will become dirty; if the pH value is too low (below 3.5), it will cause the printing plate to bloom, the ink is emulsified in the fountain solution, and the ink roller will deink. The drying time of the ink layer is longer. For high-quality printing, it is necessary to keep the pH value of the fountain solution used, otherwise there will be a series of problems, paste plate (the ink is emulsified in the fountain solution), the printing plate is blooming (the graphic on the printing plate Partially uninked), victory (the ink adheres to the position of the printing plate), ink roller deinking (the ink roller is not attached to the ink), or the ink has not been safely dried on the paper.
In order to ensure the stability of the pH value, all dampening Poli added buffer. When the concentration of fountain solution changes, the pH value may not change much, but its conductivity continues to increase. Therefore, the conductivity is more suitable than the pH value to determine the change in solid content in the solution. However, attention to small changes in pH can help avoid many printing problems.
6. The use of alcohol-free fountain solution for alcohol replacement has two purposes:
One is to use safe materials to manufacture high-quality products, and the other is to select and use unfermented fountain solution that produces good results. Printing is an interactive process, and the industry cannot expect to change some links (such as removing alcohol) without making comprehensive adjustments.
When using a new fountain solution (or any other new material), the operator can test before applying. The printing unit itself is the best laboratory. Under controlled conditions (that is, other conditions remain stable except for the test items), testing on the printing unit can obtain the complete information needed to solve the actual problems of the printing process. When testing, it is best to follow the manufacturer's technical specifications. Within the tolerance range of the technical specifications, generally start from the low end, and all the original materials of the test process should be saved.
7. Mechanical adjustment
The structure, control system, wetting system, configuration, etc. of different printing machines are not questioned, and the adjustment method is also difficult to unify. The requirements for using alcohol-free fountain solution are stricter than when using IPA fountain solution, so some problems that can be covered when using IPA fountain solution will appear. It is recommended to reduce the use of alcohol to about 8%. If there are still problems, it means that IPA has indeed covered some notable problems.
The key to ensuring the success of unleavened printing is to correctly select the hardness of the water roller of the wetting device and adjust the roller gap. When using alcohol-free printing, the hardness of the metering roller is slightly lower than when using alcohol fountain solution. Generally, the hardness of the metering roller is (25-30) Shore, which will become harder after being used on the machine. When using alcohol-free fountain solution, it is recommended to reduce the hardness of the metering roller to (8 ~ 22) Shore, because the soft roller is easy to absorb water, and can increase the gap between the metering roller and the chrome roller, but does not increase the printing pressure. Make the adjustment range of the operator more relaxed. Another problem encountered when using standard hardness metering rollers is that water forms streaks (or ridged watermarks) on the pavilion rollers and metering rollers, and at the same time causes print marks on the circumference. The use of soft metering rolls increases the roll gap and can often flatten the water film on the chrome rolls. Another reason for the formation of ridged water marks is the speed difference between the two rollers.
The use of Diol fountain solution also needs to adjust the gap between the winding roller and dampening roller. When using IPA fountain solution, the gap is generally (8 ~ 10) mm wide, and when using alcohol-free fountain solution, the roll gap should be reduced to (3 ~ 5) mm. In a typical IPA wetting device, the metering roller and the chrome roller are driven by gears through separate motors, while the plate ink roller is driven by the printing press. If the plate ink roller starts to drive the chrome roller, the speed of the chrome roller increases and the wetting control is lost. When the roller of the wetting system accelerates beyond the normal state, water tends to accumulate at both ends of the roller, causing splashing; the speed of the roller is too fast, and the motor of the wetting system will be improperly stressed, causing circuit breakage and motor failure.
The adjustment of the metering roller speed is also a key issue. In order to measure the alcohol-free fountain solution, a relatively high speed of the dampening roller is often required, and the high conditions will focus on the ejection and the control range is inaccurate.
The metering roller of some printing machines will be designed as a spindle type, the purpose is to make the fountain solution evenly distributed on the printing plate, some unfermented fountain solution is best to use the spindle to the metering roller. If the middle part of the plate roller is too wet and the two ends are too dry, the metering roller must usually be adjusted. If there is not enough dampening solution in the center of the plate roller, the metering roller should be parallel to the chrome roller. The optimal angle of deviation adjustment varies with the alcohol substitute used and should be determined by experiment. It can be determined whether the ink-water balance is good by observing the dirty line formed at the bend of the printing plate mouth. After the printing machine enters a stable state, the printing machine is stopped to evaluate the dirty line. When the ink-ink balance is ideal, a thin and even dirt line will be produced on the width of the printing plate. If the printing plate is too dry in a certain part, the width of the dirty line increases, and if it is too wet, the dirty line is eliminated.
8. Possible problems
Switching to alcohol-free fountain solution is likely to encounter some unprecedented problems. Problems caused may include:
Metering roller absorbs ink
Alcohol substitutes can make the metering roller sensitive to ink. The ink first adheres to the chrome ravioli and then passes to the metering roller. The solution is to contact the printer manufacturer or use a specially formulated mixture to corrode the roller. The hydrophilicity of the metering roller is maintained by coating with cloth gum or paint. Some organizations believe that the sensitivity of the metering roller is due to the contamination of the fountain solution, because only 1% of the blanket cleaning agent on the metering roller is enough to destroy the satisfactory fountain solution.
Ridge watermark
As mentioned earlier, when using alcohol substitutes, if the hardness of the metering roller is (25-30) Shore, ridged watermarks may appear, and thus affect the print quality. The solution is to use a softer metering ravioli. Some manufacturers have also suggested that metering rollers with long fiber surfaces are more hydrophilic. It is very important to maintain the correct concentration to determine the correct fountain solution concentration, especially during the initial formulation. Most alcohol substitutes will slowly release from the plate solution. Since there is no method to accurately adjust the concentration of the alcohol substitute, the use of unfermented fountain solution needs to be changed at least once a week. To make the fountain solution too cool Cooling is very important when increasing the printer speed, but the operator sometimes makes the fountain solution too cool. This will increase the viscosity of the ink and cause problems such as paper roughening and ink stacking. The correct temperature depends on the conditions of the printing workshop in the first place, and the dampening liquid refrigeration unit should be adjusted to (10 ~ 13) ℃. Or as recommended by the supplier.
Organic growth
If organic matter grows in the fountain solution circulation system, then the circulation system should be cleaned at least once a week, including the drain bucket. It can be rinsed with ordinary water; the mold eliminator is also used to prevent the growth of organic matter; it can also be used to rinse the circulation system with an aqueous solution containing (1-5%) bleach at the end of the working week, but before refilling the fountain solution , To ensure that the circulation system has been cleaned, and no detergent will remain.
Ink roller deinking
Using different alcohol-free fountain solutions without thoroughly cleaning the circulation system may cause ink roller deinking. At this time, the method of plating the ink roller with copper can be used for the old printing press equipped with steel ink roller. If this problem still occurs on a printing machine equipped with nylon or Teflon-coated tandem rollers, you can remove the ink with a solvent first, and then rinse the ink set with hot water to solve the problem.
Blistering
When the fountain solution foams, the accumulated foam will move to the edge of the metering roller and splash onto the paper; the foam will also hinder the flow of water to the rotating bucket roller. IPA itself is a good defoamer, and when using alcohol-free fountain solution, a small amount of defoamer can be used, but it should be noted that the use of catalytic accelerator dryers should avoid the use of silicone resin defoamers, called it Expensive catalytic beds may be "poisoned". In addition, there are some mechanical devices to avoid foaming.
Paste version
If the amount of alcohol substitute is too small, the dampening is sufficient, and sometimes the fine part of the printing plate will be blurred. But some other problems can also lead to the phenomenon of pasting.
Sedimentation on the metering roller
Sometimes, alkali formation occurs on the metering roller. The sediment on the metering roller may be the filler calcium in the paper, or it may be salt or gum in the fountain solution when the pH is too high. These white precipitates make the metering roller ink-friendly. The solution to this problem is to move the metering roller away from the chrome roller, clean it with a suitable cleaning fluid and reduce its sensation. Paper fluff caused by thin ink layer
Some alcohol substitutes require a lower water-ink balance point, and the thickness of the ink layer should be reduced to 0.15 μm, which deviates from the normal (02 to 0.4) μm. The thin layer of ink exerts a large pulling force on the paper, which causes the phenomenon of paper rushing. However, there are some alcohol-free fountain solutions that rarely cause paper fluffing when the ink layer is thin.
In general, good condition and cooperative spirit are the key to successful alcohol-free printing. It is also essential to carefully consider the compatibility of the consumables used in the entire printing system and actively adopt the manufacturer ’s laws.
Fungho Industries (Group) Co., Limited , https://www.funghogroup.com