Ink common sense and skills (in)

3. Ink characteristics

The ink is a pasty paste that is colored and has a certain degree of fluidity and can be printed and dried on the substrate. Therefore, color, rheological properties, and drying performance are the three most important features of inks.

(1) Viscosity

Viscosity is a property that prevents the flow of a fluid and is a measure of the ability of a fluid molecule to interact and create a barrier to the relative motion of its molecules, ie, the resistance to fluid flow.

The viscosity of the ink and the transfer of ink during the printing process are related to the nature and structure of the paper. The ink viscosity is too large, the ink transfer during the printing process is not easy to uniform, and the phenomenon of paper pulling is generated, so that the printing on the printing surface; the viscosity is too small, the ink Easy to emulsify, dirty, affect product quality.

The viscosity of the ink is related to the viscosity of the vehicle, the amount of pigments and additives used, the particle size of the pigments and additives, and the dispersion of the pigments and additives in the vehicle.

The requirements for the viscosity of the ink in the printing process are related to the printing speed of the printing press, the softness of the paper structure, and the change of the ambient temperature.

(2) Yield value

The minimum shear stress required to start the flow of liquid is called the yield value.

An ink with a large yield value has poor flow properties and is not easy to open. The ink whose yield value is too small is liable to cause haziness and unclear printing dots.

The yield value is related to the structure of the ink, and the size of the yield value has a direct effect on the flow of the ink. It is an important test indicator for the quality of offset and gravure printing inks.

(3) Thixotropy

The ink is stirred by the external force. It will be thickened by the action of stirring. After standing, the ink will return to its original consistency, called thixotropy.

Since the ink has thixotropy, when the ink is mechanically rotated on the ink roller, its fluidity increases, its ductility increases, and the ink is easily transferred. When the ink is transferred to the paper after printing, the effect of external force is lost, and the ink thickens from dilute and does not overflow to the surroundings, forming a good print. However, if the thixotropy of the ink is too large, the ink in the ink fountain is not easily rotated, which affects the ink transfer of the ink roller.

(4) Liquidity

The ability of ink to flow like a liquid under its own gravity is called the fluidity of the ink.

The fluidity of the ink is related to whether the ink can be poured from the container, conveyed from the storage tank to the ink fountain of the printing machine, smoothly transferred from the ink fountain, well distributed on the printing machine, transferred to the layout, and transferred to the printing. On the object, it also affects the effect of printing.

The fluidity of the ink is determined by the viscosity, yield value, and thixotropy of the ink, and is also closely related to the temperature.

(5) Length of ink

The process in which the ink is stretched into a filament without breaking is called the filament length.

The ink with short silk ink is good ink with good printing performance in offset printing and letterpress printing, because it will cause ink flying phenomenon. At the same time, the printing ink is uniform and thick, and the ink length is commonly used to measure ink performance. Bad.

The length of the ink is related to the thixotropy, yield value, and plastic viscosity of the ink.

(6) Drying of ink

After the ink adheres to the print to form an imprint, it must be changed from a liquid or pasty into a solid film. This process of change is usually referred to as drying of the ink.

The drying of the ink is accomplished by changing the binder in the ink from a liquid or paste to a solid. The binders used in various inks and their blending ratios are different. Therefore, the drying process for forming the ink from the different materials is also different. After the ink is transferred from the printing plate to the surface of the printed product, part of the connecting material in the ink penetrates. At the same time, the solvent in the connecting material begins to volatilize, and some of the connecting materials react chemically and physically to make the surface of the substrate imprinted. The ink layer gradually increases its viscosity and hardness, eventually forming a solid film layer.

In general, letterpress printing inks are mainly osmotic drying. Lithographic offset inks are mainly dried by oxidized conjunctiva. Gravure printing inks use a strong solvent as the connecting material, so it is mainly volatile drying.