Printing ink viscosity adjustment

Ink is one of the main materials for printing. It brings people a colorful world through the selective absorption of light. Ink is a stable colloidal dispersion system consisting mainly of pigments, binders and various additives. The printing suitability of ink has always been the object of concern in the printing industry. Its quality directly affects the printing process and the quality of printed products. A typical aspect of ink printability is the viscosity of the ink.

First, the viscosity of the ink

1. Definition of viscosity

The ink viscosity is a physical quantity used to measure the ink viscosity. Usually the ink is dispersed by fine pigment particles in the binder to form a colloidal dispersion system. In the interior of the system, due to the intermolecular forces, there is cohesion between them. When the ink flows under external force, the system will produce a resistance, which we call internal friction. The internal friction exhibited by the ink in the flow is the viscosity of the ink. The ink viscosity does not make much sense on its own, but as an important indicator of the rheological properties of an ink, it reflects the printability of the ink from one aspect.

2. Viscosity manifestation

Viscosity is generally classified into Newtonian viscosity, plastic viscosity, apparent viscosity, differential viscosity, etc., which are not comparable to each other, but as an important indicator of ink rheological properties, they are comparable in the same expression method. The viscosity of the same type of expression means that the fluidity is small when it is large and the fluidity is large when it is small. In practical applications, a viscometer is generally used to detect the ink's plastic viscosity.

Second, the ink viscosity and ink printability

The printability of the ink depends mainly on the ink's transmissibility and the dot's imageability. In the printing process, the ink is continuously split and transferred between the ink roller and the ink roller, between the ink roller and the printing plate, between the printing plate and the blanket, and between the blanket and the substrate. The moment the ink layer is split, The cohesive force of the ink system creates a resistance between ink molecules that prevents the ink layer from splitting. If the viscosity of the ink is too high, the resistance generated between the ink molecules will also be very large, resulting in the difficulty of ink layer splitting, webbing pick-up of the substrate or even peeling phenomenon, greatly reducing the service life of blankets and printing plates and other consumables. When printing with UV inks in production, ordinary printing plates and blankets can be easily damaged because UV inks have a particularly high viscosity.

Moreover, inks with large viscosity inks are long and can easily be pulled into filaments. During the transfer process, the ink filaments often break into several joints and splash around, resulting in flying ink. If the viscosity of the ink is too high, printing failures such as ink blocking and uneven ink transfer may occur.

The ink whose viscosity is too low has a small cohesive force in the system. The dampening solution can easily invade the inside of the ink during printing, emulsify the ink, and cause floating and dot enlargement. When the ink is transferred, the ink is not easily adhered to the surface of the ink roller, resulting in poor ink transfer and printing failures such as a stack plate, a stack of rubber, and an ink roller.

It can be seen that the ink viscosity is closely related to the printability of the ink. Appropriate ink viscosity is a prerequisite for a good printability of the ink, and the viscosity of the ink must be controlled during production.

Third, the ink viscosity adjustment

Ink is a colloidal dispersion system. The main factors affecting the viscosity are the temperature and pressure of the environment, the volume ratio of the ink and the binder in the ink composition and structure, the size and shape of the pigment particles, the presence of the surfactant, and the touch of the ink. Degeneration and so on.

1. Adjust the composition and structure of the ink

According to some empirical rules in the ink study, if the concentration of solid particles such as pigment particles in the ink is expressed as a percentage, the plastic viscosity increases exponentially with the solid concentration. If the concentration of solid particles is constant, the smaller the particles, the greater the plastic viscosity. Materials commonly used in production to adjust the viscosity of inks include detackifiers, No. 0 varnish and No. 6 varnish.

No. 6 varnish oil has low viscosity and good fluidity. The ink with too high viscosity can be miscible with the binder, increase the volume percentage of the binder, and relatively reduce the volume percentage of solid particles, thereby reducing the viscosity of the ink. the goal of. The addition of a detackifier to an ink with a too high viscosity can shorten the length of the ink, change the structure of the binder, and reduce its viscosity. Whether to add a detackifier or add No. 6 varnish during use depends on the specific circumstances. The volatilization of the solvent in the ink not only increases the viscosity of the ink but also increases the consistency of the ink. Adding No. 6 varnish at this time can adjust the viscosity and consistency of the ink at the same time. If the viscosity of the ink is high due to its own structure, it may be better to add an appropriate amount of detackifier. Because this will not change the thinness of the ink, it is beneficial to the ink transfer and the formation of dots.

No. 0 varnish is added to the ink with too low viscosity, and according to experience, the percentage of the linking material will be increased, and the viscosity should be lower, which is not the case. Because the viscosity of No. 0 varnish is extremely large, it can hardly flow by itself. Adding it to the ink will not only promote the flow of the ink, but will also act as a barrier, which will increase the internal friction of the ink system and make the viscosity of the ink. Increased.

2. Using thixotropy of ink

The composition and structure of the offset ink itself determine its greater thixotropy. The thixotropy fluid not only has pseudoplasticity, but also the shear stress and apparent viscosity decrease with time at constant temperature and constant shear rate. That is, the apparent viscosity of the thixotropic fluid decreases with the continuous stirring.

From the production, it is known that an ink with an excessively high viscosity easily causes a printing failure of ink blocking in the ink fountain, and an ink agitator is often used to solve the failure. In fact, an ink stirrer is an example of using thixotropy of ink to change its viscosity. Because the ink is a thixotropic fluid, the stirrer stirs back and forth in the ink fountain, and the ink undergoes thixotropic phenomenon, which reduces the apparent viscosity, so that the ink is smoothly transferred from the ink fountain to the dispensing stroke. When the ink reaches the substrate, the mechanical action of the outside world is gone, and the apparent viscosity rises again. This ensures that the ink does not overflow to the surroundings, making the outlets clear.

In summary, the ink viscosity is an inherent property of the ink itself, and it has a decisive influence on the printability of the ink. The ink must have a good printability and firstly it must have an appropriate viscosity. In the production operation, it should be given a high degree of attention, through appropriate methods to keep it within a certain range to meet the needs of printing. (Author/Liu Binhui)