Screen printing inks should meet requirements for storage, printing, durability, economics, and security, but in terms of technical performance, printability and application suitability are the main factors.
First, the printability of printability is also called the operating characteristics, in general, the ink should be able to successfully print from the printing plate to the substrate on the printing, not blocking the screen on the screen, drying fast on the substrate, The print has good reproducibility.
The influencing factors of printability are complex and there are about 60 variables. Here are a few key features.
Viscosity is a property of the ink that hinders its relative flow inside. The viscosity unit is Pas (seconds). The viscosity of silk screen printing ink is medium and high in all types of ink (see 4-4). Viscosity has a great influence on the printing effect. If the viscosity is too large, the ink transfer will be difficult, the imprinting integrity, adhesion fastness and glossiness will be deteriorated, and even the problems such as wire drawing, mesh defects, and ink blocking will be caused. If the viscosity is too small, the imprinting will be enlarged and the definition will be poor. Poor, easy to dirty plates and prints.
Comparison of ink viscosity Table 4-4

Ink type viscosity (Pa·s)
Letterpress ink 10-100
Aniline ink 0.1-0.2
Gravure ink 0.05-0.20
Silk screen printing ink 1-100
Offset printing ink 100

Generally, the viscosity of the ink prepared is greater than 4 Pa·s, and the viscosity is adjusted depending on the number of screen meshes, the size of the pattern, the printing speed, the performance of the printing surface, and the printing temperature, and it is allowed to change within the range of 1-100 Pa·s.
Some screen printing inks may have a change in their viscosity during the printing process, such as the volatilization of solvents, making the ink thicker and the viscosity becoming larger. Some inks have different viscosity depending on the shear rate. There are 3 cases, as shown in Figure 4-2 (abbreviated), where curve a represents Newtonian fluids, such as low viscosity (less than 10 Pa·s) binders and liquid inks. Viscosity is a constant, not affected by the shear rate; curve b pseudoplastic fluid, viscosity decreases with increasing shear rate, also known as shear thinning fluids, such as cellulose solutions, emulsions, and most screen printing inks. The leveling properties of these inks are good, that is, the viscosity increases with the increase of the shear rate. Also known as shear thickening fluids, some high solid content (pigment and filler) inks are prone to shear expansion. The viscosity of some inks decreases with the increase of the shear rate or shear time or both, and after the shearing stops, the viscosity can recover, and this phenomenon is called thixotropy.
The relationship between viscosity change and printability is that the ink on the plate, the more stable the better the viscosity, but after the transfer to the print, the viscosity becomes faster and better. Thixotropy is unfavorable to the former and beneficial to the latter, so proper thixotropy is desirable, and shear thickening is not good for printing.
The adjustment of the viscosity, addition of solvents, diluents or plasticizers, can reduce the viscosity, add fillers, pigments, silicides and viscous transparent bases, and can increase the viscosity.
The minimum shear stress required to make the ink begin to flow is called the yield value. The yield value is too large, the ink is hard, it is difficult to open, the inconvenience of inking, and the leveling property are poor; the yield value is too small, and the reproducibility of printed thin lines or dots is poor. Silk screen printing ink layer is thick, so the yield value can not be too small, usually between 10-2-10-3N/cm2. When printing fine lines, the yield value should be high.
Fluidity In flat and convex printing, the concept of fluidity is often used to describe the degree of ink flow. It measures the diameter of a certain amount of ink spread in a specified time under the pressure of a parallel plate, and is used as the degree of fluidity. Numerical value. For silk screens, the concept of liquidity is more convenient. The fluidity is the result of the ink's viscosity, yield value, and thixotropy comprehensive reaction, indicating the flow properties of the ink itself without external force. Screen printing ink is too fluid, the transfer of the ink is difficult to control, there will be self-leakage (referring to the ink on the screen, no pressure will leak on its own), paste back (refers to the screen version of the ink spilled to the printing version Face-to-face), as well as ink and other disadvantages along the squeegee direction. The liquidity is too small, thin lines are difficult to print, the gloss of the prints is poor, and even the network is short of ink. Appropriate fluidity should be: The ink does not leak on the screen due to its own weight, and the surface of the blot should not reveal knots. (To be continued)