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What are the main properties of printing inks?
Release time:
2022-06-23 18:04
Performance
Ink is a kind of slurry adhesive with a certain fluidity. The viscosity, yield value, thixotropy, fluidity, and dryness 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 interact with each other and hinder the relative motion of their 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, 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 pulling the paper occurs, making the layout flowery; if the viscosity is too small, the ink is easy to emulsify and get dirty, affecting the quality of the printed product. The requirements for the viscosity of the ink during the printing process depend on factors such as the printing speed of the printing press, the degree of public software on the paper structure, and the changes in temperature and humidity in the surrounding environment.
Yield value: It refers to the minimum moving stress required for the liquid to start flowing. If the yield value of the ink is too large, the fluidity will deteriorate and it will not be easy to open. If the yield value is too small, the dots during printing will easily smudge and cause unclear prints. The size of the yield value is related to the structure of the ink, and has a direct impact on the fluidity of the ink. It is an important test indicator for the quality of offset and gravure inks. Generally speaking, as the particles of the pigment in the ink increase in size and quantity, the force and viscosity that hinder the flow of the liquid will also increase. For example, the inks used in letterpress printing and flat printing for packaging printing contain a large amount of pigments, and the force they are subjected to must be greater than a certain value in order to flow. Therefore, the force to which the ink is subjected must be greater than a certain value in order to flow, and this value is the yield value of the ink.
Thixotropy: It refers to the phenomenon that when the ink is stirred by external force, it changes from thick to thin with the stirring action, and returns to its original consistency when the stirring action stops. Due to the thixotropy of the ink, the ink on the ink roller will increase its fluidity and ductility after being rotated by the printing press, making the ink easy to transfer; when the ink is transferred to the paper through printing, the ink changes from thin to thick without overflowing to the surroundings due to the loss of external force, forming a good imprint. If the thixotropy of the ink is too large, the ink in the ink fountain will not rotate easily, which will affect the ink transfer function of the ink roller.
Fluidity: refers to the ink flowing like a liquid under its own gravity, which is determined by the viscosity, yield value and thixotropy of the ink, and 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 tank to the ink fountain of the printing press, smoothly transferred from the ink fountain, well distributed on the printing press, transferred to the layout and transferred to the substrate, and also affects the printing effect.
The length of the ink thread: refers to the degree to which the ink is initially stretched into a thread without breaking. Its length is related to the thixotropy, yield value and plastic viscosity of the ink. Ink with short ink threads is an ink with good printing performance in offset printing and letterpress printing. It will not cause ink flying during the printing process. At the same time, the ink layer on the printed product is also uniform and thick. The length of the ink thread is a common method to measure the performance of the ink.
Ink drying: Ink drying refers to the process of ink changing from liquid or paste to solid film after it adheres to the printed matter to form a print. This process is completed by the change of the border material in the ink from liquid or paste to solid. Because the binders and their formula proportions used in various inks are different, the drying process of ink is also different. After the ink is transferred from the printing plate to the surface of the printed matter, part of the binder in the ink penetrates, and the solvent in the binder also begins to evaporate. Some binders begin to produce chemical or physical reactions, so that the imprinted ink layer on the surface of the substrate gradually increases its viscosity and hardness, and finally forms a solid film layer. Usually, letterpress printing ink is mainly dried by penetration, offset lithography ink is mainly dried by oxidation film, and gravure printing ink is mainly dried by volatility because it uses a highly volatile solution as a binder.
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