1. Printability and Rheology
The so-called rheology is a branch of studying the deformation and fluidity of matter. The printing ink is pressed onto the ink roller on the printing machine, transferred to the printing plate, and then transferred to the paper. But in this process, there are various 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 summarize the rheology of printing inks from the same point of view.
The behavior of the ink in the printing process will be described in detail later in "Ink Transfer" and "Printing Ink". But after the behavior of the ink has been clarified, it is easier to understand by examining the rheology of the ink. To this end, here is a brief introduction to the behavior of ink, exploring the behavior of ink as an object of rheology, and then discussing the rheological properties of ink.
2. Rheology 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; ①The process of transferring from the ink tank to the ink fountain of the printing press; ② The process of transferring from the ink fountain to the ink roller; ③ On the ink roller, the ink is evenly distributed The process of opening and the transfer process from the ink roller to the ink roller (this process reaches the plate cylinder through several ink rollers), ④ the ink is transferred from the plate cylinder to the paper, ⑤ the ink printed onto the paper Infiltration, ⑥ the deformation of the ink film formed. The rheological process at each stage is dominated by slightly different properties, so I will discuss it in detail.
â‘ It is poured or scraped with a shovel, so it is a problem of fluidity or plasticity. â‘¡The ink extends from the wide place to the narrow gap, which is related to the shear fluidity of the ink. It is too hard and troublesome, but the viscosity is not too small; usually the ink with good hiding power will adhere well to the ink roller on. â‘¢ The ink is squeezed between the ink rollers (here, because the ink rollers rotate at the same speed, so the shearing force is not too large), spread out to the surface of the ink rollers with a uniform film. The pigment particles contained in the ink are crushed, the secondary aggregation is destroyed and the primary particles are uniformly dispersed. This is repeated, and finally spread evenly on the plate cylinder to obtain a uniform ink film. This is an important process that does not cause shade in printing.
In the process of â‘¢, while the ink exhibits fluidity, there is also the effect of the destruction of the internal structure formed by the particles due to the shear force. The shearing force does not work continuously, only when the ink enters the gap between the ink rollers (up and down 1 / 10sec). In the remaining time (up and down 1sec), the ink adheres to the surface of the ink roller without force. In this way, there is a problem of fluidity when the 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, so only an uneven film can be obtained.
When the ink is transferred between the ink rollers, and when the ink is transferred to the paper in the next step â‘£, at the beginning, the ink layer is gradually crushed, forming between the narrow gap between the ink roller and the ink roller (or paper) The thin ink film, and then the ink roller continues to rotate. With the expansion of the gap, the ink film is stretched to form a filament, and then broken, this process is a problem to be studied. The resistance at this time is also related to the power that determines the rotation of the ink roller, and is called the property of viscosity. Obviously, the ink sticky wire length mentioned in the Japanese industrial standard is related to this. Rheological phenomena related to viscosity include the flow caused by the compression and extension of ink and the breakage of wire drawing. The mechanism of fracture is related to the way in which voids are formed inside the ink, and the viscoelastic deformation and destruction of the ink film. And because these phenomena are carried out at a very fast speed on the printing press, it is impossible to get a precise rheological explanation for this phenomenon without studying the deformation at the corresponding speed.
The process of ⑤ started from the time of ④. When the ink is pressed between the ink roller and the paper, a part of the ink is pressed into the gap between the paper. Then, when the ink film breaks, the ink penetrates the gap between the paper fibers by surface tension. Of course, there are also cases where only the linking material penetrates. On the other hand, during this process, the solvent evaporates, leaving a thin film of bright oil. Also, the 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 imaging. In the last ⑥ process, it is the deformability and strength of the formed film. Although it is unlikely to be deformed, it is necessary to occasionally rub the adhesion and strength that cannot be wiped off. When stacking prints together, in order not to stick the ink film on the back of the upper paper, the ink needs to be hardened quickly and have sufficient strength after film formation.
In addition, as a special case, in gravure printing, the transfer of ink is mainly done by penetration into the paper, so the fluidity of the ink is particularly important. In addition, news inks are used in high-speed printers, so there should be a problem of ink flow in the tubes that deliver ink to the printers.
From the above situation, the study of the rheological properties of the ink is related to the fluidity of the ink, especially its viscosity, plasticity and consistency, the viscosity and its time dependence at a large shear rate, and the behavior of viscoelasticity, Ink film breakage, etc., are phenomena that should be considered in particular.
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