The rheological of ink include the ink viscosity, adhesiveness, yield value, thixotropy, fluid degree & fluidity, and transferability. The improper control of printing performance will lead to various faults in printing.
Viscosity refers to the measurement of the relative motion ability between molecules caused by the attraction of fluid molecules. It shows the nature of the flow in the fluid and the extent of its resistance to flow. In the process of fluid flow, this phenomenon will appear according to the structure and properties of fluid: It is difficult to flow when the fluid is sticky, when the fluid is thin, the flow is easy. The reason for this phenomenon in fluids is that: In the flow, there is the motion resistance caused by the attraction of fluid molecules, which is called internal friction.
This property of fluid is called viscosity. The viscosity is an important measure of the rheology of the ink in the printing process, the ink viscosity is to ensure the main conditions of transfer and uniform transfer support, as it relates to the printing process can be carried out smoothly, and directly affect the uniformity, clarity and gloss imprinted ink.
(1) when the viscosity of ink is too large, the wire drawing is strong. In the ink transfer process, the division between rollers wire head is too long, the ends of the broken wire head is easy to form ink flying into the air, is the formation flying phenomenon of the printing, ink flying phenomenon is more obvious in high speed printing.
(2) the ink viscosity is too large, it is easy to cause the powder brushed or delamination phenomena in removing paper. This is caused by the adhesive force of the ink which exceeds 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 ink pass and transfer between ink rollers, excessive viscosity makes ink transfer difficult, because the ink transfer rate and viscosity are inversely proportional, so the viscosity is too large, resulting in uneven distribution of ink, the ink transferred to the plate or printed matter is not enough, and the ink is uneven.
(4) the viscosity of the ink is too large, the printing process will also have some other effects. If the amount of ink increases during printing, the ink layer is too thick and the drying speed slows down, It is easy to appear soiled or stuck on the back of the printed material. In the flat paper printing is also prone to the phenomenon of being involved in roller.
(5) the viscosity of ink is too small, the fluidity is increased, and the appearance of the ink is diluted. It is easy to produce ink emulsification in offset printing, resulting in printing dirty.
(6) the ink with too small viscosity is easy to spread on the paper, resulting in a large area of imprinting and a decrease in clarity, in addition, the firmness and the glossiness of the imprinted surface after drying are reduced.
(7) the ink viscosity is too small, often cannot drive along with the larger particles in the pigment ink transfer, the pigment particles gradually deposited on rollers, blanket or plate, forming a pile of version.
2. yield value
The yield value of an ink is the minimum force required for the ink to deform and start to flow. Under the action of shear stress, the ink begins to deform and does not flow immediately, but the slug flow occurs when the shear stress increases gradually, slug flow is a displacement phenomenon of laminar flow caused by partial deformation of fluid under force, with the increase of shear stress, slug flow decreases, when the shear stress reaches a certain value, the slug flow disappears, the flow of the fluid is laminar. At this point the flow of the ink is viscous and the flow characteristics of the plastic fluid appear.
The relationship between yield value and printing:
The yield value of ink not only affects the fluidity and transferability of ink, but also reflects the stickiness and hardness of the ink. the ink flow yield value is small, the ink is hard, in the ink transmission process, from the ink to the ink roller, and then to the version of roller and plate surface, it will produce these phenomena: because the yield value is too large, the ink is not easy to be evenly opened, and the ink is not easy to lower.
Ink with small yield value has high mobility, easy to produce the phenomenon of higher concentration spreading and ink blot for screen printing and text line printing. therefore, the yield value of ink should be determined according to the requirements of printing, in general offset printing, the yield value of flat offset ink is between 1000-3000, while the web offset ink is between 200-1500.
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