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The rheological properties of ink include ink viscosity, adhesion, yield value, thixotropy, fluidity and fluidity, transferability and other properties.
1. Viscosity Viscosity is a measure of the ability of fluid molecules to attract each other and hinder the relative movement of molecules. It shows a property that prevents flow in a fluid and the degree to which it resists flow.
In the process of fluid flow, according to the structure and nature of the fluid, there will be a phenomenon that the flow is difficult when the fluid is in a viscous state, and the flow is smooth when the fluid is in a dilute state. The resistance to movement caused by attraction is called internal friction. This property of fluid is called viscosity.
Viscosity is an important indicator of ink rheology. A certain viscosity in the printing process is the main condition to ensure ink transfer and uniform transfer, because it is related to the smoothness of the printing process and directly affects the uniformity of the ink color , Clarity and gloss.
Improper control of ink viscosity during printing will bring various failures to printing. as follows:
(1) When the viscosity of the ink is too large, the viscosity is too large and the stringiness is strong. During the ink transfer process, the filament head is too long when split between the ink rollers, and the broken filament end is likely to form ink flying into the air, forming a flying ink phenomenon in printing. The flying ink phenomenon is more obvious when printing at high speed.
(2) If the viscosity of the ink is too large, it is easy to cause de-powdering, pulling or peeling of the paper. This is caused by the viscosity of the ink exceeding the surface strength of the paper under certain printing conditions. This phenomenon is more 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 excessive viscosity makes the ink transfer difficult. Because the ink transfer rate is inversely proportional to the viscosity, too large viscosity will cause uneven distribution of the ink and transfer to the printing plate or The amount of ink on the printed matter is insufficient, the ink filaments are uneven, and there is a phenomenon that the graphic foundation of the printed matter is exposed.
(4) If the viscosity of the ink is too large, it will have some other effects on the printing process. If the amount of ink used in the printing process increases, the ink layer of the printed matter is too thick, and the drying speed is slowed down, which is prone to staining or sticking on the back of the printed matter. In flat paper printing, it is also easy to cause the paper to be caught in the ink roller.
(5) The viscosity of the ink is too small, the fluidity increases, and the appearance is that the ink is thin. Lithographic offset printing is prone to ink emulsification, which causes the printed matter to become dirty.
(6) Ink with too low viscosity is easy to spread on the paper, resulting in a large footprint and reduced clarity, and it reduces the firmness of the blot and the substrate surface after drying and conjunctiva and the gloss of the blot.
(7) If the viscosity of the ink is too small, the pigments with larger particles in the ink cannot be transferred together, so that these pigment particles are gradually deposited on the ink roller, blanket or printing plate to form a stacking phenomenon.
2. Yield value The yield value of ink refers to the minimum force required when the ink fluid deforms and starts to flow.
The ink starts to deform under the action of shear stress and does not immediately flow, but the plug flow phenomenon occurs when the shear stress gradually increases. The plug flow refers to a displacement phenomenon in which the fluid partially deforms under the force and produces laminar flow. The shear stress increases and the plug flow decreases. When the shear stress reaches a certain value, the plug flow disappears and the fluid flow is in a laminar state. At this time, the ink flow is in a viscous state, and the flow characteristics of plastic fluid appear.
The relationship between yield value and printing:
The ink yield value not only affects the fluidity and transferability of the ink, but also reflects the viscosity and softness of the ink body. In the use of ink, the ink with a large yield value has a low fluidity and the ink body is hard. During the ink transfer process, from the ink fountain to the ink leveling roller, to the plate ink roller and the surface of the printing plate, the yield value will be too large. Ink is not easy to spread, not easy to ink and other phenomena. If the yield value is too small, the fluidity of the ink is large, and the screen printing and text line printing are easy to produce imprinting and low ink density. Therefore, the yield value of the ink should be reasonably determined according to the printing requirements. The yield value of offset paper ink ranges from 1000-3000, while the offset ink of web paper ranges from 200-1500.
3. Thixotropy Thixotropy refers to the performance of the ink to increase the fluidity under the action of external stirring, and the fluidity gradually decreases and returns to its original state after the stirring is stopped.
The relationship between thixotropy and printing:
The ink suitable for offset printing should have good thixotropy. During printing, due to the squeezing force and shearing force between the ink rollers, the viscosity of the ink is reduced, the fluidity is enhanced, and the ductility is increased. It is conducive to the smooth and uniform transfer of the ink and improves the transfer rate of the ink. When the ink is transferred to the surface of the substrate, the external force disappears, the fluidity of the ink becomes smaller, and the viscosity gradually increases from thin to thick, so as to ensure the ink mark Sharpness, especially dot prints, can avoid dot expansion and other phenomena. However, if the thixotropy of the ink is too large, it will cause poor ink supply. In severe cases, the ink supply will be interrupted, affecting the amount of ink supply and ink uniformity of continuous printing. Therefore, in the printing operation, the ink should be stirred frequently in the ink fountain to increase the fluidity of the ink and ensure the normal ink supply during the printing process.
4. Fluidity and fluidity Fluidity refers to the ability of the ink to expand in diameter when it flows under pressure. It indicates the thinness and thickness of the ink and the looseness and tightness of the ink structure.
Fluidity is the comprehensive performance of the viscosity, yield value, thixotropy and fluidity of the ink. Generally, when the viscosity of the ink is high, the yield value is large, and the thixotropy is large, the fluidity is small, otherwise the fluidity is large.
The size of the fluidity must be compatible with the printing. The fluidity of the ink is too large, which shows the greatness of the ink thinness, and the impact on the printing:
(1) The degree of fluidity is too strong, and the imprint is spread under the action of printing pressure. The imprint size transferred to the surface of the paper is enlarged, especially in the printing of screen and word lines, which affects the clarity, gradation and color of the printed matter. Performance.
(2) The ink layer becomes thinner after printing, the vividness of the printed matter decreases, and the hue becomes lighter.
(3) The thinness after printing is large, which increases the penetration of the binder, and in serious cases, various disadvantages of excessive ink penetration occur.
(4) Excessive fluidity of ink is often caused by too much oil content in the binder. Such ink will produce a lubrication phenomenon during the transfer process, which will cause sliding marks on the edges of the printing plate. The sliding phenomenon between the inking roller and the printing plate will cause radial thorn-like spreading on the edges of the printing plate and text to be reflected on the printed product, which is called the edge-shielding phenomenon.
(5) Emulsions are prone to occur in ink offset printing with excessive fluidity, which causes the printed matter to become dirty. If the fluidity is too small, the ink has high consistency and high viscosity. It is easy to produce some phenomena when the viscosity of the ink is too large. Different types of ink have different fluidity, which is made according to the printing requirements.
Commonly used sheet-fed offset printing ink has a flow value of 28-36mm, and web offset printing ink is 30-40mm.
5. The transferability of transferable printing ink refers to the ability to transfer the amount of ink from one surface to the other when the ink layer is separated between the 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 and the amount of ink on the printing plate, expressed as a percentage, the calculation formula is as follows:
Z = × 100%
In the formula: Z-ink transfer rate,%
X- the amount of ink on the printing plate before printing, g / m2;
Y- the amount of ink on the paper after printing, g / m2
The level of ink transfer rate indicates the amount of ink transfer.
The relationship between ink transferability and printing:
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, and then transferred to the printing plate, then transferred to the blanket, and finally transferred to the substrate To complete the printing process. Therefore, the transferability of the ink is shown in the entire printing process, which is related to the distribution of the amount of ink when the ink layer is separated during the transfer process, and determines how much ink is transferred to the surface of the substrate after multiple transfers. And the degree of uniformity, thus affecting the thickness of the ink film on the printed product, the fullness of the ink color, the vividness of the color and the gloss. The ink with good transferability is said to be evenly distributed when the ink film is separated in the ink roller of the printing press. After multiple passes, it can be increased to the uniformity of the ink layer on the surface of the printing plate, the density of the ink color and the degree of fullness. When the ink transfer is poor, the amount of ink transferred to the printing plate or blanket is insufficient, which is easy to cause the basic image of the printed product to be exposed, forming a pattern phenomenon, or causing uneven ink color between the ink leveling rollers, and the accumulated ink is too thick, resulting in ink color density , Fly ink, ink roller surface dry film and other faults.
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