The most frequently asked questions from printing industry sales personnel, technical service representatives, and printing staff for sheetfed printing processes are mainly around the drying of ink on coated paper and uncoated paper. To find the answers to these questions, you must understand the ink and sheet-fed ink drying process.
Ink drying process
The ink drying process includes two steps. The first step, oxidation, is a process by which oxygen forms a solid in combination with the oil in the ink and the varnish. The second step, absorption, through which the solvent penetrates the paper and allows oxygen to come in contact with oil or varnish. If the solvent penetrates into the paper or the coating is not fast enough, the reaction rate of oxygen with the oil and the varnish will be reduced, eventually lengthening the drying time.
With this in mind, let us pay more attention to the drying process. When the ink is initially printed on the substrate, the solvent in the ink penetrates into the gaps in the fiber or coating of the substrate. The pigment remains in the dry oil section, which fixes the pigment on the printing surface. However, this is not completely dry. Due to the loss of the solvent, the ink film becomes very viscous, so that the ink loses its fluidity and stops there for conversion. With the loss of solvent, oxygen reacts with the oil and the resin begins to dry. At this stage, the surface of the ink is fixed and does not transfer to another sheet of paper. However, the ink at the center of the ink film still maintains a certain liquid state. Depending on the composition of the ink, the thickness of the printing ink film, the nature of the paper or other materials, and the environmental conditions, it takes two minutes to more than one and a half hours to fully fix the ink.
After the printing is completed, oxidation occurs in the alkyd resin or drying oil, and oxidation may also occur in the resin. This polymerization results in a three-dimensional, network-like structure of the chemical binder. Oxygen reacts with all the oil, resin, etc. in the ink to convert the liquid material in the ink film into a solid, so that the ink is completely dry. The same reaction occurs if you leave flaxseed oil open. Oxygen reacts with linseed oil and cross-links and eventually hardens.
A paper or coating with a very tight surface can cause the solvent to penetrate very slowly. Therefore, there will be solvent residues in the ink and will interfere with the reaction between oxygen and oil and varnish. Of course, this will also increase the ink drying time.
In order to speed up the drying, "dryer" is used in the ink to accelerate the oxidation process. The two most used and longest drying agents in industry include the following:
â— Cobalt: It can accelerate the surface drying process (fixed). Because cobalt is blue, it turns brown during drying and tends to discolor white. Also, it is easily soluble in organic acids, so an acidic fountain solution with a too low pH (less than 4.0) has an effect on it.
â— Manganese: It can accelerate the internal drying process and is a more powerful desiccant than cobalt. This desiccant is brown, but its effect on white is smaller than that of cobalt and it is not easy to infiltrate the fountain solution.
Mixtures of these desiccants are typically used during printing operations to simultaneously accelerate both surface and interior drying to complete the drying process.
According to some of the following steps, the printer can overcome ink drying problems:
1. Discuss work with the ink supplier prior to printing, including the paper used. The best solution to the drying problem is prevention. All paper is different. For example, ink drying on newsprint is like in Springhill
Opaque Smooth Drying on a type of paper is completely different.
2. When printing on dense-surfaced paper, avoid using soybean oil-based inks. Soybean oil is semi-dry (slow) oil.
3. Keep the pH of the fountain solution in the range of 4.0 to 5.0. The higher the acidity (lower pH), the slower the drying process of the ink.
4. When printing a large area on the ground, the overprinting uses a varnish containing desiccant or a transparent size.
5. Use the recommended amount of desiccant for the ink you are using. Too much desiccant plasticizes the ink so that it does not dry.
6. Adding additional desiccant to 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 stack lower at the take-up end of the press.
8. Use large particle dusting to help separate the printed paper and allow more oxygen to enter and react with the oil and the 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 more easily fixed and dried.
10. Pre-press the paper before printing because the temperature of the press or paper may be too low and will delay ink drying.
11. Remember that inks made with Reflex Blue extend the drying time. Discuss with the ink supplier.
12. Have sufficient drying time before running or changing work.
13. Use infrared drying to accelerate ink drying.
14. Keep in mind that inks used for film printing are fixed and hardened on tough films within a few hours.
Moreover, the best course of action is to discuss the job with the ink supplier and the paper to be used. Some ink drying problems are the result of inadequate carriers, which result in low oxidation processes or lack of available catalyst. Oxidation rates can be increased by incorporating more oil or alkyd resin in the catalyst, or by completely drying the material, such as linseed oil or tung oil, instead of a semi-dry oil, such as soybean oil.
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