The most frequently asked questions from the printing industry sales staff, technical service representatives and printing staff in the sheetfed printing process are mainly around the drying of ink on coated and uncoated paper. To find the answers to these questions, you must understand the ink and the drying process of the sheet of ink.
Ink drying process
The ink drying process includes two steps. The first step, oxidation, is a process through which oxygen combines with the oil in the ink and the ink to form a solid. The second step is absorption. Through this process, the solvent penetrates into the paper and allows oxygen to come into contact with the oil or ink. If the solvent does not penetrate into the paper or coating fast enough, it will reduce the reaction speed of oxygen with oil and ink, eventually extending the drying time.
With this in mind, let us further focus on the drying process. When the ink is initially printed on the substrate, the solvent in the ink will penetrate into the fissures of the substrate's fibers or coating. The pigment remains in the dry oil part, which fixes the pigment on the printing surface. However, this is not completely dry. The ink film becomes very viscous due to the loss of solvent. In this way, the ink loses fluidity and stops there to prepare for conversion. As the solvent is lost, oxygen reacts with the oil, and the resin begins to dry. At this stage, the ink surface is fixed and will not be transferred to another paper in the stack. However, the ink located in the center of the ink film still maintains a certain liquid state. Depending on the composition of the ink, the thickness of the printed ink film, the nature of the paper or other material, and the environmental conditions, it takes two minutes to more than one and a half hours to completely fix the ink.
After the printing is completed, oxidation occurs in the alkyd resin or dry oil, and may also occur in the resin. This polymerization results in a three-dimensional, network structure of chemical binders. Oxygen reacts with all the oil, resin, etc. in the ink to convert the liquid material in the ink film to a solid, so that the ink is completely dried. If you leave linseed oil open, the same reaction will occur. Oxygen will react and cross-link with flaxseed oil, eventually hardening.
A paper or coating with a very tight surface will cause the solvent penetration rate to be very slow. Therefore, there will be residual solvent in the ink, and will interfere with the reaction between oxygen and oil and ink. Of course, this will also increase the ink drying time.
In order to speed up the drying, "drying agent" is used in the ink to accelerate the oxidation process. The two most widely used and longest-used desiccants in industry include the following:
â— Cobalt: can accelerate the surface drying process (fixation). Because cobalt is blue, it turns brown during drying, and it is easy to fade white. Moreover, it is easily soluble in organic acids, so acidic fountain solution with a PH value that is too low (below 4.0) affects it.
â— Manganese: It can accelerate the internal drying process and is a more powerful desiccant than cobalt. This desiccant is brown, but has less effect on white than cobalt, and does not easily penetrate into the fountain solution.
A mixture of these desiccants is generally used during printing operations to accelerate both surface and internal drying to complete the drying process.
According to some of the following steps, the printer can overcome the ink drying problem:
1. Discuss with the ink supplier before printing, including the paper used. The best solution to the drying problem is prevention. All papers are different. For example, the drying of ink on newsprint is completely different from that on paper like Springhill Opaque Smooth.
2. When printing on paper with dense surface, avoid using soybean oil-based ink. Soybean oil is a semi-dry (slow) oil.
3. Keep the PH value of fountain solution in the range of 4.0 to 5.0. The higher the acidity (the lower the PH value), the slower the ink drying process.
4. When printing a large area on the ground, the overprinting adopts the ink adjusting agent containing desiccant or the transparent size.
5. Use the recommended amount of desiccant for the ink being used. Too much desiccant will plasticize the ink so that it does not dry.
6. Mixing additional desiccant in the fountain solution can improve the drying performance of the ink. Graf O Siec is an example of an additional desiccant, which can be added in an amount of 1% to 3%.
7. Make the paper stack lower at the receiving end of the printing press.
8. Use large particles of powder to help the printed paper separate, and allow more oxygen to enter and react with oil and resin.
9. Let the supplier increase the strength of the ink, so that it can be printed with a thinner ink layer, which can be fixed and dried more easily.
10. Because the temperature of the printing press or paper may be too low and will delay the drying of the ink, pre-treat the paper before printing.
11. Remember that the ink made of Reflex Blue prolongs the drying time. Discuss with ink suppliers.
12. There should be sufficient drying time before operation or replacement work.
13. Use infrared drying to accelerate ink drying.
14. Remember that the ink used for film printing will be fixed and hardened on the tough film within a few hours.
Moreover, the best course of action is to discuss the job and the paper to be used with the ink supplier. Some ink drying problems are the result of an inappropriate carrier, which leads to a low oxidation process or lack of available catalyst. The rate of oxidation can be increased by incorporating more oil or alkyd resin in the catalyst, or by using completely dry materials, such as linseed oil or tung oil, instead of semi-dry oils, such as soybean oil.
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