Ink is a kind of paste-like adhesive with certain fluidity. Viscosity, flexion value, thixotropy, fluidity, dryness, etc. all determine the performance of the ink.
Viscosity: It is a property that prevents the flow of fluid substances. It is a measure of the ability of fluid molecules to hinder relative motion between molecules, that is, the resistance of fluid flow. The viscosity of the ink has an important relationship with the transfer of the ink during the printing process and the properties and structure of the paper. If the viscosity of the ink is too large, the transfer of the ink during the printing process is not easy to be uniform, and the phenomenon of fuzzing the paper occurs, which makes the layout flowery; Too small, the ink is easy to emulsify, get dirty, and affect the quality of printed matter. In the printing process, the requirement of ink viscosity depends on factors such as the printing speed of the printing press, the degree of public software on the paper structure, and changes in temperature and humidity in the surrounding environment.
Yield value: refers to the minimum moving stress required to make the liquid begin to flow. If the ink yield value is too large, the fluidity will be poor and it is not easy to open, and if the yield value is too small, the dots during printing are prone to halo and cause the printed matter to be unclear. The size of the yield value is related to the structure of the ink, has a direct impact on the fluidity of the ink, and is an important inspection index for the quality of offset and gravure inks. Generally speaking, the pigment particles in the ink increase and the number increases, and the force and viscosity that hinder the flow of the liquid will also increase. For example, the inks of letterpress printing and lithography used in packaging printing contain a large amount of pigments, and their forces must be greater than a certain value to flow. Therefore, the force of the ink must be greater than a certain value to flow, and this value is the yield value of the ink.
Thixotropy: It refers to the phenomenon that the ink changes from thick to thin with the stirring action when it is stirred by external force, and then returns to the original consistency after the stirring action stops. Due to the thixotropy of the ink, after the ink is subjected to the rotation of the printing press on the ink roller, it will increase the fluidity, increase the ductility, and make the ink easy to transfer; when the ink is transferred to the paper after printing, it loses the effect of external force , The ink changes from thinning to thick without overflowing around, forming a good print. If the thixotropy of the ink is too large, the ink in the ink fountain will not be easy to rotate, which will affect the ink transfer function of the ink roller.
Fluidity: It means that the ink will flow like a liquid under its own gravity, which is determined by the viscosity, yield value and thixotropy of the ink, and it is also closely related to temperature. The fluidity of the ink is related to whether the ink can be poured from the container, transported from the ink storage tank to the ink fountain of the printing press, smoothly transferred from the ink fountain, good distribution on the printing press, transfer to the layout and transfer On the substrate, it also affects the effect of printing.
The length of the ink filament: refers to the degree to which the ink is initially stretched into a filament without breaking. Its length is related to the thixotropy, yield value and plastic viscosity of the ink. Ink with short ink filaments is an ink with good printing performance in offset printing and letterpress printing, and will not cause ink flying in the printing process. At the same time, the ink layer on the printed matter is also uniform and thick. The length of the ink filament is a common method to measure the performance of ink.
Drying of ink: Drying of ink refers to the process of changing from a liquid or a paste to a solid film after the ink adheres to the printed matter to form an imprint. This process is the transition from a liquid or paste to the edge material in the ink Solid and finished. Because the binders used in various inks and their formula ratios are different, the drying process of the inks is also different. After the ink is transferred from the printing plate to the surface of the printed product, a part of the connecting material in the ink penetrates, and the solvent in the connecting material also begins to volatilize. Some of the connecting materials begin to produce chemical or physical reactions, thereby causing the imprinting ink on the surface of the substrate. The layer gradually increases its viscosity and hardness, eventually forming a solid film. Generally, relief printing inks are mainly permeable drying, lithographic offset printing inks are mainly oxidized conjunctival drying, and gravure printing inks are mainly volatile drying because they use more volatile solution agents as binders.