Several properties of screen printing inks

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.

Printing suitability is also called job characteristics. In general, the ink should be able to print from the printing plate to the substrate, and the screen should not be blocked on the screen. It will dry quickly on the substrate and the print will have good reproducibility.

The influencing factors of printability are complex and there are about 60 variables. Here are a few key features.

Viscosity
It is a characteristic that hinders the relative flow inside the ink. The viscosity unit is Pas (seconds). The viscosity of screen printing inks is among medium and high levels in all types of inks. 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.

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 kinds of situations, such as low viscosity (less than 10 Pa·s) binder and liquid ink, the viscosity is a constant, not affected by the shear rate; pseudoplastic fluid, viscosity The shear rate increases and decreases, 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.

Yield value
The minimum shear stress required to start the flow of ink 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 commonly used to describe the degree of flow of ink. It is the measurement of the diameter of a certain amount of ink spreading in a specified time under the condition of pressure of a parallel plate, and is used as the numerical value of fluidity. 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, the thin line is difficult to print, the glossiness of the printing is poor, and even the proper fluidity of the ink in the ink is due to: the ink does not leak due to its own weight on the screen, and the surface of the blot should not reveal the knot.

The empirical judgment method of fluidity is to provoke the ink with a dipstick knife, and then let it flow down and pull it into ink. If the ink is long, the fluidity is good, and vice versa.

Surface Tension
The surface tension of the ink is related to the transfer properties of the ink and the stability of the print on the printing surface. This relationship is even more pronounced when printed on glossy materials, especially plastic surfaces. For example, when the surface tension of the ink is greater than the surface tension of the printing surface, the imprinting will shrink, and even fisheye-like holes will appear. If the surface tension of the printing ink and the ink is printed, the printing will be blocked. These conditions can be adjusted by adding surfactants, microcrystalline wax, silicide, etc., so that the surface tension of the ink is equal to or less than the surface tension of the printing surface to obtain a good printing effect.

Fineness
Ink fineness refers to the degree of dispersion of solid powders such as pigments and fillers in the binder. The fineness of the ink is not the same as the fineness of the solid powder, which refers to the basic particle size of the powder, which is small, see Table 4-5. In general, the basic particle size of the pigment is less than 1 μm. In fact, the pigment is aggregated with several elementary particles (up to 50 μm in size) and added to the binder of the ink. After grinding, the aggregates disperse and the dispersion becomes better. The smaller the ink particles (fineness). The fineness of general ink is about 5μm, ultrafine up to 1-2μm. Fineness affects the sheen of the ink film and the rheology of the ink.

In terms of the printability of silk screen printing, the fineness should be compatible with the aperture of the printing plate mesh, ie, the ink particle size should be less than or equal to 1/3 of the screen size.

Dryness
Refers to the drying speed of the ink. It has two requirements: the slower the ink dries on the plate, the faster it will dry on the printing surface. Fast drying on the plate will affect the ink viscosity, rheology and the uniformity of the imprint, and even cause blocking of the network, also known as netting, the time for the ink to start netting should not be less than 5 minutes, the slow drying on the print, then affect Production speed or increase drying costs.

It should be said that online slow-drying and imprinting fast-drying inks are the ideal drying performance, resulting in the use of inks such as photo-setting, thermosetting and hot-inking.

Viscoelasticity
Viscoelasticity refers to a kind of mechanical property that the material has both the solid elasticity and the liquid viscosity (plasticity) under the external force. Polymers are almost all viscoelastic, so most also have this property. Viscoelasticity is closely related to the splitting of the ink during printing, the drawing conditions, and the smoothness of the surface of the print.

The difference in viscoelasticity of the material can be described by the relaxation time. Simply put, when the external force is less than a certain time, the material shows elasticity. When the external force is longer than this time, the material will change. This time is called relaxation time Ï„. The relationship between Ï„ and material properties is:

When τ → ∞, the object is completely solid and only elastic;

When τ → 0, the object is a liquid and it is only sticky;

<τ <∞, the object is viscoelastic.

The value of Ï„ reflects the size of the intermolecular binding force of the object. Therefore, the Ï„ value of the linear structure polymer is small, and the Ï„ value of the polymer is higher in the network structure or the cross-linking point.

Relaxation time is very useful to adjust the printing speed, especially the separation speed between printing plate and printing surface, such as the separation speed should not be less than Ï„, otherwise the ink transfer is elastic and the transfer rate is low. There is no sticky ink with elasticity, which is difficult to transfer when printing; there is no elasticity only with sticky ink, or drawing is serious, or the flow is out of control, and no good print can be obtained.

The elasticity provided by the viscoelasticity allows the split ink to snap back quickly, ensuring a good impression.

Reprinted from: China Printing Network