Rheology and printability of inks

First, printability and rheology

The so-called rheology is a branch of research on the deformation and mobility of substances. The printing ink is pressed onto the ink roller on the printing press, passed to the printing plate, and then transferred to the paper. But in this process, there are all kinds of deformations and flows. Obviously, the rheological properties of printing inks play an important role in printability. Recently, Moyer and Myers discussed the rheology of printing inks from this point of view. We also intend to generalize the rheology of printing inks from the same point of view.

The behavior of the ink during the printing process will be described in detail in the following "ink transfer" and "printing ink". However, after the behavior of the ink is clarified, it is easier to understand the rheology of the ink. For this purpose, here we briefly introduce the behavior of the ink, explore the behavior of the ink as a rheological object, and then discuss the rheological properties of the ink.

Second, the rheological problems in the printing process

As long as the behavior of the ink on the printing press is examined, the reason why these tests can evaluate the printability of the ink can be understood. From the perspective of rheology, the behavior of the ink is divided into the following stages; 1 the process of transferring from the ink tank to the printing machine ink fountain; 2 the process of transferring from the ink fountain to the ink roller; 3 the ink roller, the ink is uniform The open process and the transfer process from the ink roller to the ink roller (this process is via several ink rollers to the plate cylinder), 4 inks are transferred from the plate cylinder to the paper, 5 ink to the paper is printed on the paper Infiltration, deformation of the formed ink film. The rheological processes at each stage are governed by slightly different properties, so talk about it in some detail.

1 is poured, or scraped with a shovel, so it is a matter of mobility or plasticity. 2 is the ink from the wide place into the gap between the slit, and the ink's shear fluidity, too hard, of course, trouble, but the viscosity is too small; normal hiding power of the ink will adhere well to the ink roller on. 3 is the ink is squeezed between the ink rollers (here because the ink roller is rotating at the same speed, so the shear force is not too much), with a uniform film evenly open to the surface of the ink roller. The pigment particles contained in the ink are crushed and the secondary agglomeration is destroyed to become a first-order particle evenly dispersed. This is repeated, and finally evenly spread on the plate cylinder to obtain a uniform ink film. This is an important process that does not produce depth in printing.

In the process of 3, while the ink shows fluidity, there is also the effect of internal structure destruction caused by the shear force caused by the particles. The shearing force does not work continuously, but only works when the ink enters the gap between the rollers (1/10 sec up and down). The remaining time (1 sec up and down), the ink is not attached to the surface of the ink roller without force. In this way, the problem of fluidity occurs when force is intermittently applied. Of course, if the hiding power of the ink is not good, the ink will not adhere to the ink roller, and therefore only a non-uniform film can be obtained.

When the ink is transferred between the ink rollers and the ink is transferred to the paper during the next step 4, initially, the ink layer is gradually crushed, forming a gap between the ink roller and the ink roller (or paper). The thin ink film, followed by the ink roller continues to rotate. As the slit gap expands, the ink film is stretched and then broken. This process is a problem to be studied. The resistance at this time is also related to the power to determine the rotation of the ink roller, and is referred to as the viscosity property. It is obvious that the length of the ink sticking thread mentioned in Japanese Industrial Standards is related to this. The viscosity-related rheological phenomena include the compression and elongation of ink and the breakage of wire drawing. The mechanism of the fracture is related to the formation of voids inside the ink and the viscoelastic deformation and destruction of the ink film. Moreover, because these phenomena are performed at a very fast speed on the printing press, if the deformation of the corresponding velocity is not studied, the exact rheological explanation for this phenomenon cannot be obtained.

The process of 5 was started from the 4th. When the ink is pressed between the ink roller and the paper, a part of the ink is pressed between the fine slits of the paper. Then, when the ink film breaks, the ink penetrates the gap between the fibers of the paper by surface tension. Of course, there are only cases where the link material penetrates. On the other hand, in the process, the solvent evaporates leaving a thin film of bright oil. Also, dry oil is hardened after oxidation. In any case, a film with a certain hardness must be formed in a short time to prevent the ink from bleeding during the image formation. In the final six process, the formed film has the problem of deformability and strength. Although it is unlikely to produce distortion, it still requires occasional bumps and strength that cannot be wiped off. When the prints are stacked together, in order not to be adhered to the ink film on the back side of the paper, the ink needs to be rapidly hardened and have sufficient strength after film formation.

In addition, as a special case, in gravure printing, the transfer of ink is mainly accomplished by penetrating the paper, so the fluidity of the ink is particularly important. Also, press inks are used in high-speed printers. Therefore, there is also a problem of ink flow in the tubes that transport ink to the printer.

From the above situation, the study of the rheological properties of ink is related to the fluidity of the ink, especially its viscosity, plasticity and consistency, the viscosity at a large shear rate and its time dependence, and the behavior of viscoelasticity, The breakage of the ink film is a phenomenon that should be specially considered.