The important influence of ink rheology on printing quality is discussed

The important influence of ink rheology on printing quality is discussed

2024-05-11

The rheological properties of the ink include the viscosity, adhesion, yield value, thixotropy, fluidity and fluidity, and transferability of the ink. In the printing work, the rheology of the ink has an important impact on the quality of the printed product. The following are to talk about the impact of these rheological properties on the printing quality of ink.
First, viscosity. Viscosity is a measure of the ability to hinder the relative movement of molecules caused by the mutual attraction of fluid molecules, which shows a property of preventing flow in the fluid and the degree of resistance to flow. The motion resistance caused by the mutual attraction between fluid molecules in the process of fluid flow is called internal friction. Viscosity is an important indicator of ink rheology, in the printing process of a certain viscosity is to ensure that the ink support transfer and uniform transfer of the main conditions, it is related to the printing process can proceed smoothly, and directly affect the uniformity of ink, clarity and gloss. Improper control of ink viscosity in printing will bring a variety of failures to printing, as listed below.
1.When the viscosity of the ink is too large, the drawing property is strong, during the ink transfer process, the wire head is too long when it splits between the ink rollers, and the broken end of the wire head is easy to form the ink flying into the air, forming the flying ink phenomenon in printing, and the flying ink phenomenon is more obvious in high-speed printing.
2.the viscosity of the ink is too large, it is easy to cause the depowder of the paper, drawing or delamination stripping phenomenon. This phenomenon is particularly obvious when the structure of the printing paper is loose and the surface strength is not high.
3, when the ink is transferred and transferred between the ink rollers, the viscosity is too large to make the ink transfer difficult. Excessive ink viscosity will lead to uneven distribution of ink, insufficient ink transferred to the plate or printed matter, uneven ink, and the phenomenon of bare print graphic base.
Other effects of excessive viscosity on the printing process, such as the increase in the amount of ink, the printing ink layer is too thick, the drying speed is slow, and the back of the printing matter is easy to stain or adhere; In flat paper printing, it is also easy to produce the phenomenon that the paper is sucked into the ink roller.
5.the viscosity of the ink is too small, its fluidity increases, and it is easy to produce ink emulsification in the offset printing, resulting in dirty prints; The ink with too small viscosity is easy to produce spreading phenomenon on the paper, resulting in a large imprint area, a decline in clarity, and reduce the firmness of the combination of the imprinted dry conjunctiva and the substrate surface and the gloss of the imprint.
6.The viscosity of the ink is too small, often can not drive the ink particles in the larger pigment transfer together, so that these pigment particles gradually deposited on the ink roller, rubber cloth or printing plate, forming a stack plate phenomenon.
Second.Yield value. The yield value of the ink refers to the minimum force required when the ink fluid is deformed and begins to flow. The ink does not flow immediately when it begins to deform under the shearing stress, but when the shearing stress gradually increases, the plugging phenomenon occurs. Plug flow refers to a displacement phenomenon in which the fluid partially deforms under the force and generates laminar flow. As the shear stress increases, the plug flow decreases. When the shear stress reaches a certain value, the plug flow disappears and the flow of the fluid becomes laminar flow. The yield value of the ink not only affects the fluidity and transferability of the ink, but also reflects the viscosity and soft hardness of the ink constitution. In the use of ink, the flow of ink with large yield value is small, and the ink body is hard. In the process of ink delivery, from the ink hopper to the ink leveling roller, and then to the plate ink roller and the plate surface, the ink is not easy to open and ink is not easy to ink because the yield value is too large. If the yield value is too small, the ink flow is large, and the printing spread and the ink concentration is not high are easy to produce on the screen printing and the text line printing. The yield value of the ink should be reasonably determined according to the requirements of printing, and the yield value of the offset ink of flat paper in the general offset printing is between 1000-3000, and the offset ink of web paper is between 200-1500.
Three, thixotropy. Thixotropy means that the fluidity of the ink increases under the action of external agitation, and the fluidity gradually decreases after stopping the agitation. The ink suitable for offset printing should have good thixotropy. In printing, due to the extrusion pressure and shear force between the ink rollers, the viscosity of the ink decreases, the fluidity increases, and the ductility increases, which is conducive to the smooth and uniform transfer of the ink and the transfer rate of the ink is improved. When the ink is transferred to the substrate surface, the external force disappears, the ink fluidity becomes smaller, gradually thickens from thin, and the viscosity rises, so as to ensure the clarity of the ink impression, especially the dot print, which can avoid the phenomenon of dot expansion. If the thixotropy of the ink is too large, it will cause the ink supply is not smooth, and the ink supply interruption phenomenon will occur in serious cases, affecting the ink supply amount and the ink equalization of continuous printing.
Fourth, mobility and liquidity. Fluidity refers to the ability of the ink to expand its diameter when it flows under pressure. It indicates the degree of thinness and thickness of the ink and the degree of slack and tightness of the ink structure. Fluidity is the comprehensive performance of ink viscosity, yield value, thixotropy and fluidity. Under normal circumstances, when the viscosity of the ink is high, the yield value is large, and the thixotropy is large, the fluidity is small, and the fluidity is large. The size of the fluidity must be adapted to the printing, the flow of the ink is too large, and the ink is large, which will have many adverse effects on the printing. The specific performance is summarized as follows.
1, the flow is too strong, the printing under the printing pressure to produce spreading phenomenon, transferred to the surface of the paper printing shape size expansion, especially in the network plate, line plate printing, affecting the clarity of the print, level and color performance. After printing, the ink layer becomes thinner, the color brightness of the printed matter decreases, and the hue becomes lighter.
2, after printing, the dilution is large, increasing the penetration of the binder, and serious ink permeability is too strong. The flow of ink is too large is often caused by the excessive content of oil in the binder, so that the ink will produce a lubrication phenomenon in the transfer process, so that the edge of the plate image slide marks. The sliding phenomenon between the inking roller and the printing plate will cause the radial prickly spread on the edge of the printing plate, which is reflected in the printed matter and is called the shielding phenomenon.
3, flow too large ink offset printing is prone to emulsification, resulting in printed matter floating dirty phenomenon; The flow is too small, which is manifested by the large consistency and viscosity of the ink, and some drawbacks caused by the excessive viscosity of the ink are easy to appear in printing. Different kinds of inks have different flow, which is made according to printing requirements, the flow value of the commonly used flat paper offset ink is 28-36mm, and the web offset ink is 30-40mm.
Fifth Transferability. The transferability of printing ink refers to the ability to transfer the amount of ink from one surface to another surface when the ink layer is separated between two contact surfaces during the ink transfer process. The transferability of ink can be expressed by the transfer rate. The transfer rate of ink refers to the ratio of the amount of ink transferred to the surface of the paper to the amount of ink on the plate, expressed as a percentage.
The printing process is essentially the transfer process of ink. After the ink is output from the ink transfer roller, it is transferred to the inking roller through the ink leveling roller, then transferred to the plate, then transferred to the blanket, and finally transferred to the substrate to complete the printing process. The transfer of ink is displayed in the entire printing process, which is related to the ink distribution when the ink layer is separated during the transfer process, and determines the amount and uniformity of the ink amount transmitted to the surface of the substrate after multiple transfers, thus affecting the thickness of the ink film on the printing product, the fullness of the ink color, the brightness of the color and the glossiness. When the ink transfer is poor, the amount of ink transferred to the printing plate or rubber blanket is insufficient, which is easy to cause the graphic foundation of the printed matter to be exposed and the formation of flower plate phenomenon; Or cause uneven ink color between ink rollers, ink accumulation is too thick, resulting in thick ink color, flying ink, ink roller surface dry conjunctiva and other failures.