The Effect Of Ink Rheology On Printing Quality

In the printing work, the ink rheology has a certain influence on the quality and level of printed products. Then the following will analyze the properties of various rheological properties of the ink, and obtain its specific impact on the printing work.

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 to hinder relative movement between molecules. It exhibits a property of preventing flow in the fluid and the degree of its own resistance to flow.

In the process of fluid flow, according to the structure and properties of the fluid, it will be difficult to flow when the fluid is in a viscous state, and flow smoothly when the fluid is in a thin state. The reason for this phenomenon is that there are interactions between fluid molecules in the reflow. The movement resistance caused by attraction is called internal friction. This property of fluids 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 support and uniform transfer, because it is related to the smooth progress 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 many kinds of 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 ends are too long when split between the ink rollers, and the broken ends of the filament ends are likely to form ink flying into the air, resulting in ink flying phenomenon during printing, which is more obvious in high-speed printing.

(2) Excessive viscosity of the ink can easily cause the phenomenon of paper powdering, napping or delamination. 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, excessive viscosity will make ink transfer difficult, because the ink transfer rate is inversely proportional to the viscosity, so excessive viscosity will result in 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 the graphic foundation of the printed matter is exposed.

(4) Excessive viscosity of the ink will have some other effects on the printing process. If the amount of ink used in the printing process is increased, the ink layer of the printed matter is too thick, and the drying speed is slowed down, and it is easy to stain or stick on the back of the printed matter. In flat paper printing, it is easy to cause the paper to be rolled into the ink roller.

(5) The viscosity of the ink is too small, the fluidity is increased, and the appearance of the ink is expressed as the thinness of the ink. In lithographic offset printing, ink emulsification is prone to occur, causing dirty prints.

(6) Ink with too low viscosity is prone to spreading on the paper, resulting in a large print area and reduced clarity, and it reduces the firmness of the print and the surface of the substrate after the print is dried and the conjunctiva and the gloss of the print.

(7) If the viscosity of the ink is too small, it is often unable to drive the pigments with larger particles in the ink to transfer together, so that these pigment particles are gradually deposited on the ink roller, blanket or printing plate, forming a pile-up phenomenon.

2. Yield value

The ink yield value refers to the minimum force required when the ink fluid deforms and starts to flow.

When the ink begins to deform under the action of shear stress, it does not flow immediately, but the phenomenon of plug flow occurs when the shear stress gradually increases. Plug flow refers to a displacement phenomenon in which the fluid partially deforms under the applied force and produces laminar flow. As the shear stress increases, the slug flow decreases. When the shear stress reaches a certain value, the slug 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 a plastic fluid appear.