Several properties of the suitability of screen ink printing
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Screen printing inks should meet the requirements of storage, printing, durability, economy and safety, but in terms of technical performance, it is mainly printing suitability and applicability.
Printing suitability is also called work characteristics. In general, the ink should be able to print from the printing plate to the substrate smoothly during printing. The net is not blocked on the screen, and the substrate is dried quickly and the print reproducibility is good.
The factors affecting printability are complex, with about 60 variables, and here are a few key features.
Viscosity
It is a characteristic that the inside of the ink hinders its relative flow. The unit of viscosity is Pas (Pa·s). The viscosity of screen printing inks is among the various types of inks and is of high level. Viscosity has a great influence on the printing effect. If the viscosity is too large, the ink transfer will be difficult, the integrity of the print, the adhesion fastness and the gloss will be deteriorated, and even the defects such as wire drawing, mesh lacking ink and blocking the net will be caused; if the viscosity is too small, the print will be enlarged and the definition will be sharp. Poor, easy to print plates and prints.
Generally, the preparation viscosity of the ink is more than 4 Pa·s, and the viscosity is appropriately adjusted depending on the mesh size, the pattern size, the printing speed, the printing surface performance, and the printing temperature, and is allowed to vary within the range of 1-100 Pa·s.
Some screen printing inks may change in viscosity during printing, such as evaporation of the solvent, thickening the ink, and increasing the viscosity. Some inks vary in viscosity depending on the shear rate. There are three cases, 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 is increased and reduced, also known as shear thinning fluids, such as cellulose solutions, emulsions, and most screen printing inks. This kind of ink has good leveling property, that is, the viscosity becomes larger as the shearing speed increases. It is also called shear thickening fluid, and some inks with high solid content (pigment and filler) are prone to shear expansion. Some ink viscosity decreases with increasing shear rate or shear time or both, and after shearing stops, the viscosity recovers, which is called thixotropic.
The relationship between viscosity change and printability is that the ink is more stable on the printing plate, but the viscosity becomes as fast as possible after transfer to the printing. Thixotropy is detrimental to the former and beneficial to the latter, so proper thixotropy is desirable, and shear thickening is detrimental to printing.
The viscosity is adjusted, solvent, diluent or plasticizer can reduce the viscosity, add filler, pigment, silicide and viscous transparent base to improve viscosity.
Yield value
The minimum shear stress required to start the ink flow is called the yield value. The yield value is too large, the ink is hard, it is difficult to open, the ink is inconvenient, and the leveling property is poor; the yield value is too small, and the printing fine line or dot reproducibility is poor. The silk screen ink layer is thicker, so the yield value should not be too small, usually between 10-2-10-3 N/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 ink flow. It is the diameter of a certain amount of ink laid out within a specified time as a value of fluidity under the condition of parallel plate pressurization. For silk screen printing, the concept of liquidity is more convenient. Fluidity is the result of the viscosity, yield value and thixotropic reaction of the ink, indicating the flow properties of the ink itself without external force. The fluidity of the screen printing ink is too large, the transfer of the ink is difficult to control, and there will be self-leakage (referring to the ink on the screen, which will leak by itself without pressure), and the paste back (refers to the ink on the screen leaking to the printing of the plate) Face-back), as well as inks along the direction of the ink. The fluidity is too small, the printing of fine lines is difficult, the gloss of the imprint is poor, and even the proper fluidity of the net lack of ink should be: the ink does not leak on the screen due to its own weight, and the surface of the imprint should not be exposed.
The empirical method of liquidity is to use the ink-adjusting knife to pick up the ink, and then let it flow down by itself and pull it into a filament. 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 more pronounced when printed on glossy materials, especially on plastic surfaces. For example, when the surface tension of the ink is larger than the surface tension of the printing surface, the blot will shrink, and even a fisheye-like small hole may appear. If the surface tension of the ink is printed, the printing may be blocked. These conditions can be adjusted by adding a surfactant, a crystallite wax, a silicide or the like so that the surface tension of the ink is equal to or smaller than the surface tension of the printing surface to obtain a good printing effect.
Fineness
The ink fineness refers to the degree of dispersion of a solid powder such as a pigment or a filler in a binder. The fineness of the ink is not the same as the fineness of the solid powder. The latter refers to the basic particle size of the powder, which is small, as shown in Table 4-5. Wherein the general pigment has a basic particle size of less than 1 μm. In fact, the pigment is adhered to a plurality of elementary particles in an aggregate state (up to 50 μm in size), added to the binder of the ink, and the aggregate is dispersed and dispersed as well. The smaller the ink particles (fineness). Generally, the fineness of the ink is about 5 μm, and the fineness is up to 1-2 μm. Fineness affects the gloss of the ink film and the rheology of the ink.
In terms of the printability of the silk screen, the fineness should be compatible with the aperture of the screen mesh, that is, the ink particle size ≤ 1/3 mesh aperture.
Dryness
Refers to the drying speed of the ink. It has two requirements: the ink is dried on the printing plate as slowly as possible, and the faster it is printed on the printing surface, the better. Drying on the plate will affect the viscosity, rheology and uniformity of the ink, and even cause the netting, also known as netting, the time for the ink to start netting should not be less than 5 minutes, and the drying on the printing is slow, which will affect Production speed or increased drying costs.
It should be said that the slow drying and fast drying inks on the Internet are the ideal drying performance, and the inks such as light solid, thermosetting and hot stamping are produced.
Viscoelasticity
Viscoelasticity refers to a mechanical property in which a substance combines the elasticity of a solid with the viscosity (plasticity) of a liquid under the action of an external force. Most of the polymers have viscoelastic properties, so most of them also have this property. The viscoelasticity is closely related to the splitting of the ink during printing, the drawing condition, and the smoothness of the surface of the imprint.
The difference in viscoelastic properties of the material can be described by the relaxation time. Simply put, when the external force acting time is less than a certain time, the material exhibits elasticity. When the external force acts for more than this time, the material will flow. This time is called the relaxation time τ. The relationship between τ and material properties is:
When τ→∞, the object is completely solid and only elastic;
When τ→0, the object is liquid and only sticky;
When <τ<∞, the object is viscoelastic.
The magnitude of the τ value reflects the amount of binding force between the molecules of the object. Therefore, the τ value of the linear structure polymer is small; and the τ value of the polymer having a large network structure or a cross-linking point is large.
The relaxation time is very useful for adjusting the printing speed, especially the separation speed between the printing plate and the printing surface. For example, the separation speed should not be less than τ, otherwise the ink transfer is elastically split and the transfer rate is low. There is no sticky, only elastic ink, which is difficult to transfer when printing; no elastic, only viscous ink, or severe drawing, or rheological loss, can not get a good imprint.
The appropriate elasticity provided by viscoelasticity allows the split ink to rebound quickly, ensuring a good imprinting effect.