Several tips for drying inks
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The most frequently asked questions from print industry salespeople, technical service representatives, and the printing staff of the sheet-fed printing process are primarily the drying of the ink on coated and uncoated paper. To find answers to these questions, you must understand the ink drying process for inks and sheetfed.
Ink drying process
The drying process of the ink consists of two steps. The first step, oxidation, is the process by which oxygen combines with the oil and varnish in the ink to form a solid. In the second step, absorption, through this process, the solvent penetrates into the paper and allows oxygen to come into contact with the oil or the varnish. If the solvent penetrates into the paper or the coating is not fast enough, it will reduce the reaction speed of oxygen with oil and varnish, and ultimately extend the drying time.
With this in mind, let's take a closer look at the drying process. When the ink is initially printed on the substrate, the solvent in the ink penetrates into the gaps in the fibers or coating of the substrate. The pigment remains in the dry oil portion, which holds the pigment on the printing surface. However, this has not been completely dried, and the ink film becomes very viscous due to the loss of the solvent, so that the ink loses fluidity and stops there 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 does not transfer to another sheet of paper in the stack. However, the ink at the center of the ink film remains in 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 between two minutes and more than one and a half hours for the ink to completely settle.
After the printing is completed, oxidation occurs in the alkyd resin or the dried 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 of the oil, resin, and the like in the ink to convert the liquid material in the ink film into a solid such that the ink is completely dried. The same reaction can occur if you leave the linseed oil open. Oxygen will react with the linseed oil and cross-link and eventually harden.
A paper or coating with a very tight surface will allow the solvent to penetrate very slowly. Therefore, there is solvent residue in the ink and it interferes with the reaction between oxygen and oil and varnish. Of course, this will also increase the ink drying time.
To speed up drying, a "drying agent" is used in the ink to accelerate the oxidation process. The two most widely used and long-lasting desiccants in the industry include the following:
●Cobalt: Accelerates the surface drying process (fixed). Because cobalt is blue, it turns brown during drying and tends to fade white. Also, it is readily soluble in organic acids, so acidic fountain solutions with too low a pH (less than 4.0) have an effect on it.
● Manganese: It accelerates the internal drying process and is a more powerful desiccant than cobalt. This desiccant is brown, but has a smaller effect on white than cobalt and does not easily penetrate into the fountain solution.
These desiccant mixtures are typically used during the printing operation to simultaneously accelerate the 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 work with the ink supplier before printing, including the paper used. The best solution to the problem of drying is prevention. All papers are different. For example, the drying of ink on newsprint is completely different from drying on paper like Springhill Opaque Smooth.
2. Avoid using soy oil based inks when printing on dense surface paper. Soybean oil is a semi-dry (slow) oil.
3. Keep the pH of the dampening solution in the range of 4.0 to 5.0. The higher the acidity (the lower the pH), the slower the drying process of the ink.
4. When printing large areas of the field, the overprinting uses a varnish containing a desiccant or a 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 out.
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 from 1% to 3%.
7. Make the paper stack lower at the delivery end of the press.
8. Use large particles of dust to help separate the printed paper and allow more oxygen to enter and react with the oil and resin.
9. Allow the supplier to increase the strength of the ink so that it can be printed with a thinner ink layer that can be more easily fixed and dried.
10. Because the temperature of the press or paper may be too low and delay drying of the ink, pre-treat the paper before printing.
11. Remember that inks made of Reflex Blue extend drying time. Discuss with the ink supplier.
12. Allow sufficient drying time before running or replacing 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 inadequate carriers which result in a low oxidation process or a lack of available catalyst. The rate of oxidation can be increased by combining more oil or alkyd resin in the catalyst, or by the use of a completely dry material, such as linseed oil or tung oil, instead of a semi-dry oil, such as soybean oil.