Alcohol-free fountain solution
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The function of the offset wetting system is to transfer the water-based solution to the printing plate to reject the non-image portion of the printing plate. Isopropanol (IPA) is the most commonly used additive in fountain solutions. IPA reduces the surface tension of the dampening solution and increases the viscosity of the dampening solution; IPA volatilization also acts as a "cooling". The alcohol-based fountain solution also has the advantage of making the operation of the offset printing process more "loose", and can compensate for various problems caused by obsolete printing equipment, poor maintenance, and lack of skill of the operator.
The original reason for the elimination of alcohol-based fountain solutions was a serious shortage of IPA supply; today's main drivers are environmental, health and safety. It is printed with no alcohol (accurately, it should be IPA-free), the ink adjustment is simple, the printing quality is good, the dots are clear, and the ink is rarely emulsified. In the early days of alcohol-free printing, printers were looking for products that had IPA characteristics and could replace IPA. They can be referred to as the first generation of "alcohol substitutes." Today, the second generation has become an advanced formula that is well-deserved "unleavened". As used herein, "alcohol-free" can refer to both an unleavened (IPA) formulation and an alcohol (IPA) alternative formulation.
In terms of the effect on the viscosity, surface tension, pH and conductivity of the fountain solution, an alternative to alcohol was asked for IPA. Commonly used alcohol substitutes consist primarily of one or more chemicals of the ethylene glycol and glycol ether family and other additives. Initially these products only reduced the amount of IPA; now they are able to completely replace the IPA in the fountain solution and have better operational performance. Unfermented products are not universal, so printers must work closely with suppliers to get the best dampening solution.
It is said that web printing manufacturers tend to use one-component alcohol-free products, while sheetfed printers prefer to use composite non-alcohol products. In comparison, composite alcohol-free products are more flexible. Affected by the “integrated system” way of thinking, general-purpose fountain solutions often incorporate all the substances that need to be added. This dampening 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 IPA. Investigate the performance of dampening solution, generally starting from the next few aspects:
1. Viscosity
Viscosity is the measure of fluid resistance to flow. The concentration of the alcohol substitute in the dampening solution is much lower than that of IPA, so the alcohol substitute has little effect on the viscosity of the dampening solution. Due to the low viscosity of the dampening solution using the alcohol substitute, in the continuous flow contact type wetting system, the dampening solution on the water regulating roller or the metering roller is small, which requires increasing the speed of the dampening roller to ensure the moisturizing The supply of the plate solution. One way to compensate for viscosity loss is to cool the fountain solution, but the excessive cooling of the fountain solution also has a negative effect.
2. Surface tension
Rapid wetting of the plate and formation of a water film are two key requirements for offset fountains, which are primarily determined by the surface tension of the fountain solution. The apparent tension of pure water is 72 dynes/cm. In the dampening solution, the concentration of (10~25)% alcohol can reduce the surface tension of the dampening solution to (35~45) dynes/cm, so that the dampening solution can be quickly in the form of continuous film. The auxiliary film on the plate was opened. Alcohol-free fountain solutions use surface active agents and solvents to reduce surface tension. Due to its polar molecular orange structure, the surfactant will accumulate at the interface. When the press is running at high speed, the interface changes rapidly and the surfactant must be transferred quickly to compensate for the new interface. The amount of surfactant in the fountain solution is quite important during the printing process, and when the amount is too high, the ink is emulsified.
It is generally believed that dampening solutions with low surface tension are more suitable for high speed printing, but there are also reports on the contrary. Today's alcohol substitutes achieve the same low surface tension as IPA.
3. Printing brightness and gloss
Alcohol erodes the ink and tarnishes the ink and affects the color. In the same process, the ink of the printing press must maintain a thicker ink layer to achieve a acceptable color. When using an alcohol substitute, the ink will not be diluted at the correct concentration, so the amount of ink used and the amount of water used are small, and the result is that the dot enlargement is small and the dots are clearer.
4. Adhesion between fountain solution and ink film
The ink must be capable of adhering a certain amount of fountain solution to form an emulsion. Under ideal conditions, the fountain solution is quickly adhered to the ink and reaches equilibrium operating conditions. Under balanced operating conditions, small dampening droplets diffuse into the ink, forming a "water-in-oil" type emulsion locally. The pressure between the plate and the blanket helps the emulsification to occur. The amount of fountain solution in the emulsion can affect the density, drying, tack, viscosity, and ink transfer properties of the ink. Excessive emulsification destroys the ink-and-wash balance, causing the ink layer to become thinner and the image to be incomplete, causing "snow" on the print.
5. Conductance and pH
Conductance is a measure of the conductivity of a material; pH is the amount of hydrogen ion concentration in water. It is important that the conductance and pH remain consistent for the fountain solution.
The conductance of pure water is close to 0 micrometers, which 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. The conductance of the fountain solution needs to be measured before the press is used. Abnormal changes in conductance are caused by any impurities, and during the printing process, the increase in dampening wave conductance is normal due to contamination of the ink and paper. According to the GATF, the electricity is measured every 4 hours during the printing process, and each new batch of fountain solution also needs to measure the conductance. When hydrating, the conductance change after filling water should be less than 50 micrometers. If the conductance changes by 200 micrometers, it must be processed before use.
When measuring the conductance, the pH is generally measured simultaneously. The pH of the dampening solution should generally be 4.4 to 5.0. If the p H value is too high (above 5.5), the plate will be dirty; if the pH is too low (below 3.5), the plate will be embossed, the ink will be emulsified in the dampening solution, and the ink roller will be deinked. The drying time of the ink layer is lengthened. For high-quality printing, the pH of the dampening solution used should be kept optimal, otherwise there will be a series of problems, such as paste (ink emulsified in dampening solution), printing plate (printing on the plate) Partially uninked), winning (ink attached to the image of the printing plate), ink roller deinking (ink roller does not adhere to the ink), or the ink is not safely dried on the paper.
To ensure a stable pH, all dampening waves are buffered. When the concentration of the dampening solution changes, the pH value may not change much, but its conductance continues to increase. Therefore, the conductance is more suitable than pH to determine the change in solids content in the solution. Once, pay attention to small changes in pH to help avoid many printing problems.
6. There are two purposes for using alcohol-free fountain solutions for alcohol substitutes:
One is to use high-quality products to make high-quality products, and the other is to choose and use a non-enriched fountain solution that produces good results. Printing is an interactive process, and the industry cannot expect to change one of the links (such as removing alcohol) without making full adjustments.
When using a new dampening solution (or any other new material), the operator can test it before applying it. The printing unit itself is the best laboratory. Under controlled conditions (ie, other conditions remain stable except for the test items), testing on the printing unit provides the complete information needed to solve the actual problems of the printing process. When testing, it is best to follow the manufacturer's specifications. Within the tolerances of the technical specifications, generally start from the low end, and all the original data of the test process should be saved.
7. Mechanical adjustment
The structure, control system, wetting system, configuration, etc. of different printing presses are not required, and the adjustment method is difficult to unify. The use of alcohol-free fountain solutions requires more stringent equipment than with IPA fountain solutions, so some of the problems that can be masked with IPA fountain solutions are apparent. It is recommended to reduce the use of alcohol to around 8%. If there are still problems, it means that IPA does cover up some notable problems.
The key to ensuring the success of unleavened printing is to properly select the hardness of the water roller of the wetting unit and adjust the nip. 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 to 30) Shore, which will become harder after use. When using alcohol-free fountain solution, it is recommended to reduce the hardness of the metering roller to (8 to 22) Shore, because the soft roller is easy to absorb water, and the nip between the metering roller and the chrome roller can be increased without increasing the printing pressure. Make the operator's adjustment range more relaxed. Another problem encountered when using standard hardness metering rolls is the formation of streaks (or ridges of water) on the pavilion and metering rolls, and the resulting print streaks in the week. The use of a soft metering roll increases the nip, often flattening the water film on the chrome roll. Another reason for the formation of ridged water lines is the difference in speed between the two rolls.
The use of the ground alcohol fountain solution also requires adjustment of the gap between the winding roller and the dampening roller. When using IPA fountain solution, the gap is generally (8 ~ 10) mm wide, and when using alcohol-free fountain solution, the nip is reduced to (3 ~ 5) mm. In a typical IPA wetting device, the metering roller and the chrome roller are geared by a separate motor, while the plate cylinder is driven by the printer. 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 normal, water tends to collect at both ends of the roller, causing splashing; the speed of the roller is too fast, and the motor of the wetting system is improperly stressed, causing disconnection and motor failure.
The adjustment of the metering roller speed is also a key issue. In order to meter alcohol-free fountain solutions, it is often necessary to have a higher rotational speed of the dampening roller, and the high conditions will result in indirect ejection and inaccurate control.
The measuring rolls of some presses are designed as spindles in order to distribute the dampening solution evenly over the plates. Some unleavened dampening solutions are best used with spindle-to-counting rolls. If the middle of the plate roll is too wet and the ends are too dry, it is usually necessary to adjust the metering roll; if there is not enough dampening solution in the center of the plate roll, the metering roll should be parallel to the chrome roll. The optimum angle for the deviation adjustment varies with the alcohol substitute used and should be determined experimentally. By observing the viscous line formed at the bend of the plate mouth, it can be determined whether the ink balance is good, and the printer downwell is evaluated for the dirty line after the printing machine enters a stable state. When the ink balance is ideal, a fine and uniform dirty line is produced on the width of the printing plate. If the plate is too dry in a certain part, the width of the dirty line will increase, and if it is too wet, the dirty line will be eliminated.
8. Possible problems
Switching to alcohol-free dampening solution is likely to encounter some problems that have never been seen before. The problems caused may include:
Metering roller ink absorption
Alcohol substitutes make the metering roller sensitive to ink. The ink is first adhered to the chrome crucible and then passed to the metering roller. The solution is to contact the printer manufacturer or use a specially formulated mixture to etch the rolls. The hydrophilicity of the metering rolls is maintained by the application of a gum or coating. Some agencies believe that the sensitivity of the metering roller is due to contamination of the fountain solution, as the metering roller can be used to destroy a satisfactory fountain solution as long as it is stained with 1% blanket cleaner.
Ridge water pattern
As described above, when an alcohol substitute is used, if the hardness of the metering roll is (25 to 30) Shore, ridge water marks may occur, and thus the quality of the print is affected. The solution is to use a softer metering. There are also some manufacturers suggesting that the long fiber surface metering rolls are more hydrophilic. It is important to maintain the correct concentration to determine the correct dampening solution concentration, especially when initially formulated. Most alcohol substitutes will slowly emanate from the plate solution. Since there is no way to determine the concentration of the alcohol substitute, the unleavened lotion should be replaced at least once a week. Cooling the fountain solution too much Cooling is important when increasing the speed of the printer, but the operator sometimes overcools the fountain solution. This will increase the viscosity of the ink and cause problems with paper plucking and stacking. The correct temperature pair depends first on the conditions of the printing year workshop, while the fountain solution refrigeration unit should be adjusted to (10 ~ 13) °C. Or as recommended by the supplier.
Organic breeding
If organic matter is present in the fountain solution circulation system, the circulation system should be cleaned at least once a week, including the discharge bucket. It can be rinsed with normal water; the mold remover 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 during the week of work, but before refilling the dampening solution Make sure that the circulatory system is clean and that no detergent remains.
Ink roller deinking
Ink roller deinking can occur with different alcohol-free fountain solutions without thorough cleaning of the circulation system. At this time, the method of copper plating the ink feed roller can be applied to an old printing press equipped with a steel ink roller. If this problem occurs on a press equipped with nylon or polytetrafluoroethylene coated rolls, it is possible to remove the ink with a solvent and then rinse the ink with hot water.
Foaming
When the dampening solution is foaming, the accumulated foam will move to the edge of the metering roller and splash onto the paper; the foam will also block the flow of water onto 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 accelerated dryer should avoid the use of silicone defoamer, called it It is possible to "poison" expensive catalytic beds. In addition, there are some mechanical devices to avoid blistering.
Paste
If the amount of alcohol substitute is too small, the dampening is sufficient, and sometimes the fine part of the plate may be pasted. But other problems can also lead to the occurrence of paste.
Sediment on the metering roller
Sometimes, alkalizing occurs on the metering roller. The deposit on the metering roller may be the filler calcium in the paper, or it may be the precipitation of salt or gum in the fountain solution when the pH is too high. These white precipitates make the metering rolls inactive. The solution to this problem is to leave the metering roller off the chrome roll, clean it with a suitable cleaning solution and desensitize it. Paper fluff caused by thin ink layer
Some alcohol substitutes require a lower ink balance point, and the thickness of the ink layer is reduced to 0.15 μm, which deviates from the normal (02 to 0.4) μm. The thinner ink layer has a large pulling force on the paper, and the paper catches up. However, some alcohol-free fountain solutions rarely cause paper fluffing when the ink layer is thin.
In general, good state and collaborative spirit are the key to successful alcohol-free printing. It is also essential to carefully consider the compatibility of the consumables used throughout the printing system and to actively adopt the manufacturer's laws.