Ink is important material for printing. Its performance directly has an impact on printing quality and production efficiency. Let's talk about ink viscosity.

Rheology

Rheology is the science of deformation and flow of materials. Ideal elastic bodies and ideal viscous materials do not actually exist. The deformation laws of many materials, such as paper and ink, are complex.

Ink is pressed evenly onto ink roller on printer. It is transferred to screen, then to rubber blanket, and finally to paper. In this process, there are deformations and flows. When ink is deformed by force, it will show some features of elastic deformation and viscosity. This is called viscoelasticity.

The deformation of this viscoelastic object is not only related to the stress, but also to the development speed of these deformations. The rheological properties of printing ink play an important role in printability.

Ink viscosity and printing

Viscosity is a measure of the ability of fluid molecules to hinder their relative motion due to mutual absorption. It is an indicator of the resistance (or internal friction) to fluid flow.

During the flow of a fluid, depending on its structure and properties, it may be difficult to flow when it is viscous, but it may flow easily when it is thin. The reason is that there is a resistance to movement caused by the mutual attraction between fluid molecules during the flow, which is called internal friction. This property of fluid is called viscosity.

Viscosity is an important indicator of the rheological properties of ink. During printing, a certain viscosity is the main condition to ensure the support and uniform transfer of ink. It is related to whether the printing can proceed smoothly and directly affects the uniformity, clarity and glossiness of the printed color.

Improper control of ink viscosity during printing will cause some problems.

If the ink viscosity is too high, and the drawing property is too strong, during ink transfer process, thread head will be too long when it splits between the ink rollers. The broken thread head end is easy to cause ink to fly into the air. The flying ink is more obvious during high-speed printing.

Too high viscosity of ink is easy to cause the paper to delaminate. Because ink viscosity exceeds the surface strength of the paper under certain printing conditions. This issue is more obvious when the structure of paper is loose, and the surface strength is not high.

Too high ink viscosity will also have some other effects on printing. For example, the amount of ink used in printing increases, the ink layer of the printed product is too thick. The drying speed slows down, and the back of the printed product is prone to staining or sticking. In flat paper printing, it is also easy for the paper to be rolled into the ink roller.

Too low viscosity of the ink will increase fluidity. The ink will be too thin. Ink emulsification is easy to occur in offset printing, causing the printed product to be dirty.

Ink with too low viscosity tends to spread on paper, resulting in a large print area and reduced clarity. It also reduces the firmness of the print's bond with the substrate surface after it dries. Gloss of the print can be bad.