Paper smoothness Paper smoothness affects the overall appearance of the ink film and the print sheet. There are two kinds of defects in smoothness. The former refers to defects that can be seen with the naked eye, such as severe uneven thickness or wrinkles. These defects are sometimes caused by the structure of the fibers. Small flaws are flaws that are invisible to the naked eye, such as dents and holes that can damage the dots and affect print quality.
Dust particles Adhering to the surface of the paper or dust on the web edge during printing can also affect printability. If no ink filter is installed on the flexographic printing press, dust particles will accumulate in the ink and be carried on the printing plate. This phenomenon can be eliminated by installing an ink filter in front of the ink pump. In addition, the dust particles may also adhere to the underside of the doctor blade, causing streaks on the printed sheets; sometimes the dust particles may adhere to the ink holes of the anilox roller, resulting in uneven ink transfer. Therefore, the ink filter should be installed as soon as possible to effectively control the dust particles.
Visual effects In the relationship between paper and printing effects, you should also pay attention to the effect of the paper's ink and coloring properties on the ink film. Obviously, the color of the paper has a certain influence on the visual effect of the printed sheet. Because the substrates are not the same, the same ink will produce very different visual effects, and even make the tone change. The same kind of job is printed on different types of paper, and the quality of the print is different. It often makes people feel disappointed.
Chemical composition The chemical composition of paper has a great influence on printability. Excessive acid content in the paper slows down the drying of the ink. Excessive acid content in the paper slows down the drying of the ink. This is evident in regions with high relative air humidity and high moisture content. In general, uncoated paper with a pH of 5 or 5.5 (or slightly higher) and a pH of 7-8.5 The coated paper will not cause problems in the ink drying, unless the relative humidity is high, such as in Southeast Asia, the relative humidity of more than 65%, depending on the circumstances, can be used hot fast drying method. Understanding the moisture content of the paper and keeping it at the required level is a basic guarantee for the production of quality prints. Paper fibers, like sponges, absorb or release water molecules according to the humidity of the printing environment.
The paper fiber absorbs or releases water. The state of the molecule is called equilibrium. Different grades of substrates have different balance points. For example, at phase humidity, the absolute humidity of bleached paperboard can reach 6%, while the absolute humidity of unbleached kraft paper can reach 7.5%. Dry webs tend to break on the press because many flexo printers pull the web over the printing unit. The normal humidity of the paper is 5-7 %. High humidity can affect the paper's ink performance. When the humidity is too low, the ink layer cracks when the printed sheet is folded. Paper fibers absorb moisture from the air and become large in volume and generate internal stresses. Release of internal stress can cause edge warpage.
Water based inks, solvent based inks and UV inks can be used in flexographic printing of printing inks. The quality of water-based inks has been improved and harmless to the environment. Therefore, more attention has been paid to flexographic printing. Water-based inks today have almost as much print quality on paper as solvent inks. The water-based ink does not contain volatile organic solvents. It is proved to be a more suitable flexo ink with the advantages of abrasion resistance, printability, ink layer characteristics, and dot gain value. (To be continued)